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Biochimica et Biophysica A
Review
Function of prokaryotic and eukaryotic ABC proteins in lipid transport
Antje Pohla,b, Philippe F. Devauxb, Andreas Herrmanna,*
aHumboldt-University Berlin, Institute of Biology, Invalidenstr. 42, D-10115 Berlin, GermanybInstitut de Biologie Physico-Chimique, UMR CNRS 7099, 13 rue Pierre et Marie Curie, 75005 Paris, France
Received 2 August 2004; received in revised form 8 November 2004; accepted 16 December 2004
Available online 31 December 2004
Abstract
ATP binding cassette (ABC) proteins of both eukaryotic and prokaryotic origins are implicated in the transport of lipids. In humans,
members of the ABC protein families A, B, C, D and G are mutated in a number of lipid transport and metabolism disorders, such as Tangier
disease, Stargardt syndrome, progressive familial intrahepatic cholestasis, pseudoxanthoma elasticum, adrenoleukodystrophy or
sitosterolemia. Studies employing transfection, overexpression, reconstitution, deletion and inhibition indicate the transbilayer transport of
endogenous lipids and their analogs by some of these proteins, modulating lipid transbilayer asymmetry. Other proteins appear to be involved
in the exposure of specific lipids on the exoplasmic leaflet, allowing their uptake by acceptors and further transport to specific sites.
Additionally, lipid transport by ABC proteins is currently being studied in non-human eukaryotes, e.g. in sea urchin, trypanosomatides,
arabidopsis and yeast, as well as in prokaryotes such as Escherichia coli and Lactococcus lactis. Here, we review current information about
the (putative) role of both pro- and eukaryotic ABC proteins in the various phenomena associated with lipid transport. Besides providing a
better understanding of phenomena like lipid metabolism, circulation, multidrug resistance, hormonal processes, fertilization, vision and
signalling, studies on pro- and eukaryotic ABC proteins might eventually enable us to put a name on some of the proteins mediating
transbilayer lipid transport in various membranes of cells and organelles.
It must be emphasized, however, that there are still many uncertainties concerning the functions and mechanisms of ABC proteins
interacting with lipids. In particular, further purification and reconstitution experiments with an unambiguous role of ATP hydrolysis are
needed to demonstrate a clear involvement of ABC proteins in lipid transbilayer asymmetry.
D 2004 Elsevier B.V. All rights reserved.
Keywords: ABC protein superfamily; Flippase; Cholesterol; Lipid asymmetry; Lipid exposure; Molecular mechanism
1. Introduction
The ATP binding cassette (ABC) protein superfamily
comprises transporters for a whole variety of organic and
inorganic compounds. In 1992, Higgins and Gottesmann [1]
1388-1981/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.bbalip.2004.12.007
Abbreviations: ABC, ATP binding cassette; APLT, aminophospholipid
translocase; Cer, ceramide; FA, fatty acid; GlcCer, glucosylceramide; HDL,
high density lipoprotein; LTC, leukotriene C; nbd, [N-(7-nitrobenz-2-oxa-
1,3-diazol-4-yl)amino]; NBD, nucleotide binding domain; PAF, platelet
activating factor; PC, phosphatidylcholine; PE, phosphatidylethanolamine;
PS, phosphatidylserine; PXE, pseudoxanthoma elasticum; SL, spin labeled;
SM, sphingomyelin; TM, TMD, transmembrane (domain); VLCFA, very
long-chain fatty acids; X-ALD, X-linked adrenoleukodystrophy
* Corresponding author. Tel.: +49 30 2093 8860; fax: +49 30 2093
8585.
E-mail addresses: [email protected] (A. Pohl)8
[email protected] (A. Herrmann).
pointed out that ABCB1 (MDR1 Pgp) behaved like a
bflippaseQ which would be able to transport amphiphilic
molecules (potentially also lipids) from the inner to the outer
leaflet of the plasma membrane. Since then, there have been
many indications for lipid transport mediated by ABC
proteins in cellular membranes, and a few reports on lipid
transport by purified ABC proteins in reconstituted systems.
Lipid transport by human ABC proteins has been the subject
of several review articles in the past [2–5]. In the following,
we will first give a general introduction on lipid transbilayer
movement and assays used for its determination, before
reviewing data accumulated on the involvement of eukary-
otic and prokaryotic ABC proteins in lipid transport. We
will then discuss their putative role in lipid transbilayer
transport and exposure, and show models proposed for
mechanisms of lipid transbilayer transport by ABC proteins,
cta 1733 (2005) 29–52
A. Pohl et al. / Biochimica et Biophysica Acta 1733 (2005) 29–5230
before relating some concluding remarks. Considering the
broad span of disciplines contributing to this topic, it
appears to be important in this article to draw a clear line
between the transport of endogenous lipids and lipid analogs
carrying a reporter group or other modifications. Similarly,
phenomena which appear to be related to the action of a
particular protein need to be distinguished from those for
which a specific transport has been proven unequivocally.
On the one hand, most attempts in reconstituted systems
have revealed only very small effects of ABC proteins on
lipid transport so far, with an influence of ATP hydrolysis
hardly significant.
In experiments on whole cells, on the other hand, the
involvement of various transport steps must be taken into
account, complicating transport rate quantification, and
hence the evaluation of their physiological significance.
2. Lipid transbilayer movement
Lipids form a vast group of chemically different
amphiphilic or hydrophobic substances containing a sub-
stantial portion of aliphatic or cyclic hydrocarbon. Abundant
Fig. 1. Lipid substrates of ABC proteins (examples). Hydrophilic parts are indicate
(in PE), choline (in PC) or serine (in PS). Examples for the sphingolipid moiety Y a
galactose (in GalCer) or lactose (in LacCer). The fatty acid shown is oleic acid, th
[218].
lipid classes are (glycero-) phospholipids, sphingolipids,
steroids, lipopolysaccharides and triacylglycerols (for some
examples see Fig. 1). The phospholipids phosphatidylcho-
line (PC), phosphatidylethanolamine (PE), phosphatidylser-
ine (PS) and phosphatidylinositol (PI), the sphingolipid
sphingomyelin (SM) and the steroid cholesterol are the
major lipids found in mammalian membranes [6,7].
The lipid transbilayer distribution in the eukaryotic
plasma membrane is asymmetrical, generally with the
majority of PC and sphingolipids in the exoplasmic leaflet,
and the aminophospholipids PE and PS mainly in the
cytoplasmic leaflet (reviewed in [8]). This asymmetry raises
several questions about the biological mechanisms by which
it is established (Fig. 2A), and about the putative biological
functions coupled to it.
2.1. Spontaneous transbilayer movement
The rate of spontaneous transbilayer movement (flip-
flop) in a pure lipid membrane differs for each lipid,
depending on its structure (headgroup, backbone) and its
environment (reviewed in [9]). Small, uncharged lipids
(including cholesterol) and negatively charged lipids in their
d by blue clouds. Examples for the phospholipid moiety X are ethanolamine
re hydrogen (in ceramide), phosphorylcholine (in SM), glucose (in GlcCer),
e steroid shown is cholesterol. Lipid A structure as predominant in E. coli
Fig. 2. Transbilayer lipid movement in biological membranes and models for function of lipid transporting ABC proteins (A). Lipids can move across a lipid
bilayer by passive flip-flop or mediated by proteins. Protein-mediated transbilayer movement could be energy-independent, unspecific and bidirectional (e.g.
scramblase), or energy-dependent and monodirectional (e.g. inward transport by P-type-ATPases, and outward transport by ABC proteins). The lipid specificity
varies among transporters (for details, see text). The number of lipids indicates the efficiency of lipid movement at a qualitative level. (B) Models for ABC
protein function in lipid transport and/or exposure to an acceptor A (for details, see text).
1 (Typically, the abbreviation for [N-(7-nitrobenz-2-oxa-1,3-diazol-4
yl)amino] is written in capital letters (NBD). However, in order to avoid
confusion with the abbreviation for nucleotide binding domain, this style o
abbreviation was chosen).
A. Pohl et al. / Biochimica et Biophysica Acta 1733 (2005) 29–52 31
protonated form can flip across pure lipid bilayers within
seconds or minutes. In contrast, lipids with highly polar
headgroups move only slowly from one leaflet of a lipid
bilayer to the other (half-times of the order of hours to days
[10,11]). Cholesterol has been shown to decrease the
transbilayer movement of phospholipids [12,13].
2.2. Energy-independent and energy-dependent flippases
In eukaryotic cells, most lipids are synthesized asym-
metrically in the membranes of the endoplasmic reticulum
(ER) and the Golgi system from which they reach, e.g. via
vesicle traffic, the plasma membrane or other organelles
(reviewed in [9]). Because lipid movement across cellular
membranes is essential for cell growth and survival, lipid
transporting proteins (flippases) are required for efficient
transbilayer lipid movement. Although some proteins have
been identified as candidate lipid transporters over the last
years (reviewed in [9,14,15]), the protein vehicles respon-
sible for many lipid transport phenomena have not been
identified yet (Table 1).
In the mammalian ER, energy-independent flippases were
shown to mediate rapid bidirectional, rather unspecific
phospholipid flip-flop (half times of the order of minutes or
less) to ensure balanced growth of this membrane [16–18].
Similarly, rapid protein-mediated flip-flop has been demon-
strated for phospholipids and glucosylceramide (GlcCer, a
precursor for complex glycosphingolipids) in the Golgi [19].
Lipid flippase activity was also found in the bacterial plasma
membrane [20,21].
Already in 1980, it was shown that the mere presence of
membrane proteins facilitates lipid flip-flop [22]. Thus, it is
not clear whether a dedicated flippase or a family of
selective flippases is involved in bidirectional lipid move-
ment across ER and Golgi membranes [23,24].
Energy-independent, bi-directional transbilayer move-
ment of all major phospholipids has equally been shown in
the eukaryotic plasma membrane. This flip-flop (half time of
the order of 1 min) [25,26], activated by cell stimulation and
the subsequent increase in intracellular calcium, has been
ascribed to the lipid scramblase protein. Effort has been made
to isolate and clone the potential scramblase PLSCR1 [27].
Experiments with fluorescent (nbd1) phospholipids in the
rabbit intestine brush border have indicated ATP- and Ca2+-
independent transbilayer movement of monoacyl phospho-
lipids and lipids with short chains, supposedly constituting a
mechanism for the intestinal uptake of phospholipid
digestion products as lysolipids [28].
While energy-independent flippases allow lipids to
equilibrate rapidly between the two bilayer leaflets, energy-
dependent flippases are responsible for a net transfer of
specific lipids to one leaflet of a membrane. In the plasma
membrane of eukaryotic cells, spontaneous flip-flop of
phospholipids is limited, possibly due to the high cholesterol
content, allowing the generation of a stable asymmetric lipid
-
f
Table 1
ABC proteins involved in lipid transport
Organism Family Name Trivial name Involvement Lipid analogs Endog. lipids
Human ABCA ABCA1 ABC1 macrophage lipid homeostasis SL-PS cholesterol
phospholipidsHDL deficiencies
Tangier disease
phagocytosis
ABCA2 ABC2 macrophage lipid homeostasis? steroids?
neural development?
ABCA3 ABC3 lung surfactant synthesis? PC?
ABCA4 ABCR
Rim
dark adaptation N-retinylidene-PE
Stargardt disease
ABCA6 macrophage lipid homeostasis?
ABCA7 ABCX hematopoiesis? cholesterol
phospholipids
ceramide
macrophage lipid homeostasis?
keratinocyte differentiation?
ABCA9 hematopoiesis?
macrophage lipid homeostasis?
ABCA10 macrophage lipid homeostasis?
ABCB ABCB1 MDR1 Pgp detoxification C6-nbd-PC
C6-nbd-PE
C6-nbd-PS
C6-nbd-SM
C6-nbd-GlcCer
steroids
cholesterol
GlcCer, PS,
SM, PAF
multidrug resistance
adrenal secretion
dendritic cell migration
ABCB4 MDR2/3 Pgp bile formation C6-nbd-PC PC
progessive familiar
intrahepatic cholestasis
ABCC ABCC1 MRP1 detoxification C6-nbd-SM
C6-nbd-GlcCer
C6-nbd-PC
C6-nbd-PS
multidrug resistance?
LTC inflammatory response
ABCC6 MRP6 pseudoxanthoma elasticum
lipid transport and metabolism
ABCD ABCD1 ALDP beta oxidation VLCFAs?
X-Adrenoleukodystrophy
peroxisome biogenesis
ABCD2 ALDRP
ALDL1
beta oxidation VLCFAs?
ABCD3 PMP70
PXMP1
beta oxidation VLCFAs?
peroxisome biogenesis
ABCD4 PXMP1L
P70R
PMP69
beta oxidation VLCFAs?
ABCG ABCG1 WHITE macrophage lipid homeostasis cholesterol, PC
ABCG2 BCRP1
MXR1
ABCP
detoxification multidrug resistance bodipy-Cer
C6-nbd-PS
C6-nbd-PC
steroids? PS
ABCG5 WHITE3 bile steroid secretion cholesterol
steroidssitosterolemia
ABCG8 WHITE4 bile steroid secretion cholesterol
steroidssitosterolemia
Sea urchin ABCA SuABCA sperm acrosome reaction? phospholipid?
cholesterol?
Leishmania ABCA LtrABC1.1 parasite–host interaction vesicular transport C6-nbd-PC
C6-nbd-PE
C6-nbd-PS
ABCB LPgp-lp multidrug resistance bodipy-PC
alkyl-lyso-
phospholipids
Arabidopsis PMP
(ABCD)
Ped3p COMATOSE
CTS PXA1
beta oxidation FA CoAs?
Yeast PDR/CDR Pdr5p drug resistance C6-nbd-PE PE, steroids
Cdr1p drug resistance C6-nbd-PC
C6-nbd-PE
C6-nbd-PS
PE
A. Pohl et al. / Biochimica et Biophysica Acta 1733 (2005) 29–5232
Organism Family Name Trivial name Involvement Lipid analogs Endog. lipids
Cdr2p drug resistance C6-nbd-PC
C6-nbd-PE
C6-nbd-PS
phospholipid?
Cdr3p C6-nbd-PC
C6-nbd-PE
C6-nbd-PS
phospholipid?
MDR
(ABCB)
Ste6p pheromone transport lyso-PC?
drug resistance?
MRP/CFTR
(ABCC)
Yor1p drug resistance C6-nbd-PE
efflux organic anions?
ALDp
(ABCD)
Pat1p Pxa2p peroxisomal lipid transport VLCFAs?
Pat2p Pxa1p peroxisomal lipid transport VLCFAs?
E. coli ABCB MsbA lipid A transport lipid A
phospholipids
L. lactis ABCB LmrA drug resistance C6-nbd-PE lipid A
lipid A transport?
F. novicida ? ValA lipid A transport? lipid A-linked
polysaccharide
Substrates confirmed in reconstitution experiments are in bold print. Question marks indicate functions or substrates for which an involvement has not been
proven experimentally.
Table 1 (continued)
A. Pohl et al. / Biochimica et Biophysica Acta 1733 (2005) 29–52 33
distribution between the two leaflets. The depletion of
cholesterol leads to an enhanced spontaneous flip-flop of
phospholipid analogs in the red blood cell membrane [12].
PE and PS are subject to an efficient and rather rapid
energy-dependent inward transport from the outer to the
inner leaflet mediated by a protein, the aminophospholipid
translocase [29,12]. Some cell types display similar trans-
port of PC across the plasma membrane and may contain
either a PC-specific translocase in addition to the amino-
phospholipid translocase, or an inward translocase of
different specificity which transports both aminophospholi-
pids and PC (see [15]). The aminophospholipid translocase,
not yet identified on the molecular level, very likely belongs
to the novel DrS2p P-type-ATPase subfamily with over a
dozen members in eukaryotes from yeast to plant cells
[30,31,15]. Two members of this family, Dnf1p and Dnf2p,
have been shown to be essential for ATP-dependent
transport of fluorescent analogs of PS, PE, and PC from
the outer to the inner leaflet of the yeast plasma membrane
[32].
Recently, it was reported that the fluorescent analog nbd
PS is a preferred substrate of Drs2p localizing to the trans-
Golgi-network [33].
However, it is still unclear what permits the accumulation
of the majority of PC and sphingolipids in the mammalian
outer plasma membrane leaflet. ABC proteins were sug-
gested to be involved in the outward transport of phospho-
lipids [34]. Alternatively, it was proposed on theoretical
grounds that the inward transport of aminophospholipids,
together with passive fluxes, would be sufficient to
accumulate choline containing lipids in the outer leaflet
[35].
Changes in the distribution of a lipid species between the
membrane leaflets can influence membrane curvature and
fusion competence, protein association and activity, as well
as various biochemical pathways (reviewed in [9]). Because
of the low compressibility of lipid monolayers, the inward
and outward transport of lipids requires a subtle balance. In
the absence of a compensatory flux, unidirectional lipid
transport by energy-coupled transporters might create an
area imbalance between the two membrane leaflets, building
up surface tension eventually relaxing by membrane
budding or invagination [36,37] (see also Section 6). This
phenomenon was suggested to be a molecular motor for the
first stage of endocytosis [38,39]. The balance between
inner and outer membrane leaflet has been generally
overlooked when the activity of potential lipid transporters
was assessed in large unilamellar vesicles (LUVs), where
surface tension generated by lipid transport could be
sufficient to eventually block the transporter itself (dis-
cussed in [37,39,40]).
2.3. Determination of lipid transbilayer distribution
The techniques used to determine the transbilayer
distribution of lipids have been described and critically
evaluated in the literature [7,41–46]. In Fig. 3, some assays
for lipid analogs as well as for endogenous lipids are shown
schematically. Since methods for the rapid quantification of
endogenous lipids are still very limited, spin-labeled (SL) or
fluorescent lipid analogs are frequently employed to
determine lipid transbilayer distribution. The relevance of
such probes has been discussed by Devaux et al. [42]:
Briefly, bulky reporter moieties and a short fatty acid chain
replacing one of the natural long chains to facilitate
membrane incorporation may affect lipid polarity and cause
perturbations, somewhat modifying the absolute values of
the transbilayer distribution of a lipid. However, compar-
Fig. 3. Assays for the detection of lipid transbilayer distribution. (A) BSA-extraction, dithionite and ascorbate assays for fluorescent and spin-labeled lipid
analogs. The short-chain lipid analog precursor integrates into the outer membrane leaflet (1), crosses the plasma membrane (e.g. by passive flip-flop) (2) and
distributes to different intracellular membranes (e.g. by monomeric transport) (3). ER or Golgi enzymes convert part of the lipid analog precursor to the lipid
analog of interest (4), which can distribute back to the plasma membrane, where it becomes available to outward transport by transporter proteins (5). Lipid
analog is extracted from the outer plasma membrane leaflet by BSA (6), followed by the separation of cells and media. In a variant of this assay, the cells can be
directly labeled with the lipid analog of interest, if it is able to reach the inner plasma membrane leaflet by passive flip-flop or active transport, analog remaining
on the outer leaflet being removed by BSA extraction prior to outward transport incubation. Alternatively to BSA extraction, fluorescene of lipid analogs can be
quenched using dithionite, and the spin-label signal be reduced using ascorbate (7). (B) Phospholipase/sphingomyelinase assay for endogenous lipids.
Endogenous lipid is synthesized in the presence of radioactive precursors in the cell (I) and localizes to the various cellular membranes due to vesicular or
monomeric transport (II), to modification, and to the presence of transporter proteins (III). Phospholipase A2 treatment converts lipids in the outer plasma
membrane leaflet to lysolipid and fatty acid (IV). Lipid products are then analyzed by chromatography and can be compared to samples untreated with enzyme.
An analogous technique is used for SM, employing sphingomyelinase. (C) Chemical modification assays for endogenous lipids. Endogenous lipids present on
the outer plasma membrane leaflet are typically modified on the level of the headgroup. Reagents frequently used for modification are trinitrobenzene sulfonic
acid (TNBS, specific for PE) and fluorescamine. (D) Antibody, peptide or protein binding assay for endogenous lipids. Specific antibodies, respectively
peptides (e.g. Ro09-198, binding to PE [45,46]) or proteins (e.g. Annexin V, binding to PS) with affinity for a particular lipid headgroup, bind to endogenous
lipids present on the outer plasma membrane leaflet; the amount of bound antibody or binding protein is quantified.
A. Pohl et al. / Biochimica et Biophysica Acta 1733 (2005) 29–5234
isons between lipids with different headgroups, but with the
same fatty acid chains, can be very revealing about the
specificity of the lipid transport. Recently, various choles-
terol analogs have been studied showing large variations in
the potential to mimic endogenous cholesterol [44]. Never-
theless, new assays will have to be developed to rapidly
determine the transbilayer distribution of endogenous lipids.
3. General features of the ABC protein superfamily
The ATP-binding cassette (ABC) protein superfamily
comprises a large number of transporters, channels and
regulators in pro- and eukaryotes [reviewed in [47–49]].
Their functions range from the acquisition of nutrients and
the excretion of waste products to the regulation of various
cellular processes. Generally, ABC proteins are low
capacity, but high affinity transporters, able to transport
substrates against a concentration gradient of up to 10 000
fold. Hydrolysis of ATP is required for substrate transport.
ABC proteins are mainly either import or export pumps,
bidirectional ABC proteins appear to be rare exceptions
[50]. Import pumps seem to be limited almost exclusively to
prokaryotes. Typically, ABC proteins are relatively specific
for a particular set of substrates (the multispecific ABCB1
(MDR1 Pgp) probably represents an exception in this
respect). Substrates can be amino acids, sugars, inorganic
ions, peptides, proteins, lipids and various organic and
inorganic compounds. ABC proteins consist of nucleotide
binding domains (NBDs), and of transmembrane (TM)
domains (TMDs) of usually six alpha-helices. NBDs and
TMDs can occur as separate proteins (frequently found in
prokaryotes) or fused together. The transmembrane domains
vary considerably between different ABC proteins, whereas
the nucleotide binding domains are highly conserved (e.g.
Walker motifs). The substrate specificity is believed to be
determined by the transmembrane domains, including the
loops connecting the individual helices [47].
4. Eukaryotic ABC proteins
Structurally, eukaryotic ABC proteins typically possess
two nucleotide binding domains and two TM domains,
probably representing the minimal functional unit (full-size
protein). In some ABC proteins, deviating organizations of
the domains can prevail [49] (Fig. 4), e.g. half-size proteins
with one NBD and one TM domain each, thought to require
Fig. 4. Domain organization of human ABC proteins involved in lipid transport. Transmembrane domains (TMDs) are shown as membrane-spanning helices,
nucleotide binding domains are marked NBD. Intracellular domains (ICDs) and posttranslational modifications (e.g. glycosylations) are not shown.
A. Pohl et al. / Biochimica et Biophysica Acta 1733 (2005) 29–52 35
dimerization in order to be functional. ABC proteins with
nucleotide binding domains only (families E, F) are likely
not directly involved in membrane transport, and are
thought to have regulatory functions.
4.1. Human ABC proteins
The 49 human ABC proteins currently known can be
classified into 7 families (A–G) according to sequence
similarity [49,51]. An overview can be found on Michael
Mqller’s website (http://nutrigene.4t.com/humanabc.htm).
Several human ABC proteins found to be mutated in lipid-
linked diseases (families A, B, C, D, andG)were suggested to
be involved in lipid transport. Although diseases due to
complete loss of function of single ABC proteins do not
appear to have a very high incidence in the population,
polymorphisms in ABC genes are likely to play a role in
medically relevant phenomena. At present, direct transport of
lipid substrates has only been shown for a small number of
human ABC proteins. The ABC proteins identified in
mammals so far (e.g. in mouse, rat, pig) are highly similar
to members of the human ABC protein families, and will
therefore not be discussed separately in this work.
4.1.1. Human ABCA (ABC1) family
Out of the 12 ABCA family members, all of which are
full-size proteins, 8 are assumed to transport lipophilic
substrates. For a number of these, an involvement in lipid
transport was deduced from lipid dependent expression, and
homology to ABCA1, but has not yet been confirmed
experimentally.
ABCA1 (ABC1), studied intensely with regards to lipid
transport, is expressed in numerous tissues such as the
trachea, lung, adrenal gland, spleen and uterus [52], in some
of which its expression is steroid-dependent [53]. ABCA1-
GFP chimeras localize to the plasma membrane and to
intracellular vesicles in transfected HeLa cells [54]. ABCA1
has been implicated with the transport of cholesterol and
phospholipids (see Section 6).
In mammals, the majority of cholesterol is synthesized de
novo in the liver, and delivered to peripheral cells by
lipoproteins. Since peripheral cells are unable to degrade
cholesterol, any surplus of cholesterol must either be stored
in the cytosol in the form of esters, or released from the cell.
ABCA1 mutations can be responsible for some cases of
familial high density lipoprotein (HDL) deficiency, e.g.
Tangier disease [55–57], characterized by impaired efflux
of cholesterol and phospholipids from peripheral cells onto
apolipoproteins such as apoA-1. Cholesterol accumulation in
macrophages and apolipoprotein degradation lead to tissue
deposition of cholesterol esters and increase the risk of
arteriosclerosis in the patients. While Tangier cells typically
fail to bind nascent apoA-I [58,59], the expression of ABCA1
in cultured cells has been found to enhance the binding of
apoA-I to the plasma membrane [60], and to increase the
efflux of cellular phospholipid and cholesterol to this
apolipoprotein [61]. Both sequential and parallel mechanisms
have been proposed for the transport of phospholipids and
cholesterol by ABCA1 (see also Section 6).
ABCA1 has been suggested to directly transport the
aminophospholipid PS (typically restricted to the cytoplas-
mic leaflet of mammalian plasma membranes) to the
A. Pohl et al. / Biochimica et Biophysica Acta 1733 (2005) 29–5236
exoplasmic leaflet [61], where the presence of PS facilitates
apoA-1 binding, leading thus indirectly to cholesterol efflux
[61–63]. Indeed, the exposure of endogenous PS (detected
via prothrombinase activity and binding of Annexin V)
upon Ca2+ induced stimulation was found to be low in red
blood cells, resp. thymocytes, derived from mice lacking
ABCA1, and could be partially restored upon ABCA1
transfection [61]. A recent study on apoptotic murine cells
confirmed elevated levels of exposed PS to cause a strong
increase in apoA-I binding, although not sufficient to trigger
phospholipid and cholesterol efflux to apoA-I [64].
In contrast, Wang et al. [65] recently proposed direct
transbilayer transport of both cholesterol and phospholipids
via ABCA1, following experiments in which phospholipid/
apoA-I particles made by ABCA1 were unable to stimulate
passive cholesterol efflux when added to a second set of cells.
In addition to a role in lipid loading of apolipoproteins,
ABCA1 has been implicated with PS exposure on the outer
plasma membrane leaflet of apoptotic cells and phagocytiz-
ing macrophages [66]. In the absence of apoliposomes, Ca2+
induced externalization of spin-labeled (SL) analogs of PS,
but not of PC, was reduced in mice lacking ABCA1 [61].
The large ABC protein ABCA2, found in high levels in
human brain (possibly in oligodendrocytes), colocalizes
with a lysosomal/endosomal marker [67]. Vulevic et al.
[67] have associated ABCA2, which contains a signature
motif for lipocalins (protein family binding small hydro-
phobic molecules), with the transport of steroids and lipids
due to colocalization with an analog of the steroid
estramustine, and increased estramustine resistance upon
ABCA2 gene overexpression.
Kaminski et al. suggested a role for ABCA2 in macro-
phage lipid metabolism and neural development, after
having found an induction of ABCA2 mRNA during steroid
loading [68]. Based on these observations and the unique
expression profile, Schmitz and Kaminski [69] inferred a
role of ABCA2 in transbilayer lipid transport of neural cells.
ABCA3 is expressed exclusively in type II epithelial lung
cells expressing the gene surfactant protein A. It is
hypothesized to play a role in the formation of pulmonary
surfactant [70], a mixture containing PC and various
surfactant proteins, which reduce surface tension on the
surface of the alveoli. Mutations in ABCA3 were found in
several cases of infants with fatal surfactant deficiencies
[71]. ABCA3 is localized in the plasma membrane and in
the limiting membrane of lamellar bodies, in which
surfactant is stored prior to exocytotic delivery into the
alveolar space. Mulugeta et al. have therefore speculated
that ABCA3 might transport PC into or other lipids out of
lamellar bodies, which are highly enriched in PC [72].
ABCA4 (ABCR or Rim protein) [73] is localized in the
photoreceptor outer segment disc membranes of the retina
[74,75]. Reconstitution studies, in which its ATPase activity
was stimulated by retinal, lead to the hypothesis that
ABCA4 may function as an active retinoid transporter
[76]. ABCA4 appears to be highly substrate-specific, being
involved in dark-adaptation through the transport of the
lipid product all-trans-N-retinylidene-PE across the disc
membrane following the photobleaching of rhodopsin. This
transport allows all-trans-retinal to be reduced to all-trans-
retinol on the surface of the disc membrane. Thus, ABCA4
mediated transport is an important step in the recycling of
all-trans-retinal to 11-cis-retinal for the regeneration of
rhodopsin. Studies on the ATPase activity of reconstituted
ABCA4 strongly suggested PE to be required to couple the
binding of retinoids to ABCA4 ATPase activity [76].
Defective ABCA4 can cause the degeneration of the macula
lutea and consequent deterioration of vision (Stargardt
disease), presumably through the accumulation of the
lipofuscin fluorophore N-retinylidene-N-retinyl ethanol-
amine (A2E) [77,78].
ABCA6 shows expression in the lung, heart, brain, liver
and ovaries. As ABCA6 expression is suppressed by
cholesterol loading [79], the protein was attributed a
potential role in macrophage lipid homeostasis.
ABCA7 is expressed mainly in myelo-lymphatic tissues.
The protein is localized either in the plasma membrane or
the endoplasmic reticulum [80], and was implicated with the
specification of hematopoietic cell lineages during develop-
ment [81]. As the loading of macrophages with steroids
increases, and unloading decreases the expression of the
ABCA7 gene, Kaminski et al. proposed ABCA7 to be
involved in macrophage transbilayer lipid transport [82].
ABCA7 overexpression in HeLa cells increased the
exposure of endogenous ceramide (Cer) on the outer plasma
membrane leaflet, and raised levels of endogenous PS [83].
As ABCA7 is upregulated during keratinocyte differentia-
tion, this led to speculations about a regulator role for
ABCA7 in lipid transport during terminal keratinocyte
differentiation. Very recently, apolipoprotein-mediated
release of cellular cholesterol and phospholipids was
reported from HEK293 cells transiently or stably expressing
ABCA7 [80,84]. Furthermore, the transfection of GFP-
tagged ABCA7 of L929 cells triggered apolipoprotein-
mediated assembly of cholesterol-containing HDL. In
contrast, Wang et al. [65] reported ABCA7-mediated efflux
of phospholipid and SM, but in contrast to ABCA1 not of
cholesterol, from HEK293 cells.
ABCA9, highly homologous to ABCA6, shows ubiqui-
tous expression, the highest levels being found in the heart,
brain and fetal tissues. ABCA9 is induced during macro-
phage differentiation. Different from ABCA7, the expres-
sion of ABCA9 in macrophages decreases upon steroid
loading of the cells. Piehler et al. suggested that ABCA9
might act on monocyte differentiation and macrophage lipid
homeostasis [85].
ABCA10, as ABCA9 highly homologous to ABCA6, is
ubiquitously expressed, with high gene expression levels in
the heart, brain, and the gastrointestinal tract. As its gene
expression in macrophages is suppressed by cholesterol
loading, Wenzel et al. hypothesized on the involvement of
ABCA7 in macrophage lipid homeostasis [86].
A. Pohl et al. / Biochimica et Biophysica Acta 1733 (2005) 29–52 37
4.1.2. Human ABCB (MDR/TAP) family
The 4 full-size and 7 half-size proteins of the ABCB
family show highly varied specificities (e.g. amphiphilic
compounds, peptides, iron, phospholipids, bile salts) [51].
The proteins associated with antigen processing ABCB2, 3
(TAP1, 2), and the bile salt export pump ABCB11 (BSEP)
are ABCB protein family members.
ABCB1 (Multidrug Resistance 1 P-glycoprotein, MDR1
Pgp) [87] is a full-size protein with two transmembrane
domains and two nucleotide binding domains, which
appears to be functional as a monomer [88]. It occurs in
the apical membrane domain [89] of epithelia with secretory
functions (e.g. adrenal gland, kidney), at the pharmacolog-
ical borders of the body (intestine, blood–brain barrier, feto-
maternal barrier) [90], and in many tumor tissues [91].
ABCB1 has a surprisingly broad spectrum of (mainly
cationic or electrically neutral) amphiphilic substrates [92].
One of its important physiological roles appears to be the
protection of the organism against toxins by exporting these
into the bile, urine, or gut. In tumors, the overexpression of
ABCB1 is one of the principal factors responsible for
multidrug resistance (MDR) against a variety of structurally
unrelated drugs. ABCB1 is involved in other phenomena as
well, in which, interestingly, lipid transport often seems to
be implicated: It was found to mediate the secretion of the
steroid aldosterone by the adrenals [93], and its inhibition
blocked the migration of dendritic immune cells [94],
possibly related to the outward transport of the lipid platelet
activating factor (PAF, see below). Ueda et al. reported
ABCB1 mediated transport of the steroids cortisol and
dexamethasone, but not of progesterone in ABCB1 trans-
fected cells [95]. Inhibition studies have also led to
speculations about the transport of cholesterol by ABCB1
[96] (see also Section 6). Short-chain and long-chain (nbd
and radiolabeled) analogs of PC, PE, PS, SM, and GlcCer
were found to be expelled from ABCB1 overexpressing
cells [97–99]. Among the endogenous lipids, the short-chain
PC PAF [100] is an ABCB1 substrate (ABCB1 antisense
oligonucleotides blocking PAF secretion in human mesan-
gial cells). Other potential substrates are GlcCer (rescued
from cytosolic hydrolysis in the presence ABCB1, and
strongly reduced in the absence of ABCB1) [101] and PS
(as found by Annexin V binding experiments in ABCB1
overexpressing human gastric carcinoma cells) [99]. Inter-
estingly, the exposure of endogenous SM to the outer
plasma membrane leaflet of human myeloblastic cells was
reduced upon ABCB1 inhibition, a possible hint for ABCB1
mediated transport of this lipid involved in signalling [102].
While the inability of the ABCB1 mouse homologs Mdr1a/
1b to restore the transport of PC into the bile of mice lacking
Mdr2 (homologous to human ABCB4) [103] could suggest
that natural long-chain PC is not an ABCB1 substrate, it is
also conceivable that Mdr1a/1b activity was too low in this
system. Multispecific transport of diverse endogenous lipids
via ABCB1 could affect the transbilayer distribution of
lipids, in particular of species normally predominant on the
inner plasma membrane leaflet, such as PS and PE.
Reconstitution experiments with ABCB1 have thus far
yielded ambiguous results: While Romsicki and Sharom
[104] found a low ATP dependent increase in reoriented
short-chain nbd analogs of PC, PE, PS and SM, Rothnie et
al. [40] observed low reorientation of short-chain nbd
analogs of PC, PE, Cer and of short-chain SL analogs of
PC, PE, GlcCer, and SM which was ATP independent.
Interestingly, the activity of ABCB1 was dependent on
cholesterol. Due to the small size of the vesicles containing
the reconstituted protein, lateral pressure (surface tension)
might have prevented substantial transport of lipids via
ABCB1 [105] (see Section 2). The structure and potential
mechanisms of ABCB1 will be discussed in Section 7.
ABCB4 (MDR2/3 Pgp), a full-size protein, is a close
relative of ABCB1 (MDR1 Pgp), sharing 75% of its amino
acid sequence [49]. Unlike ABCB1, ABCB4 is highly
substrate specific, exclusively transporting (short-chain nbd
analogs of) PC, as observed in ABCB4 transfected porcine
cells [97]. The secretion of PC into the bile appears to be the
physiological function of ABCB4 [103] (see also Section 6),
the protein being present in high amounts in the canalicular
membranes of hepatocytes. In mice lacking ABCB4, PC
secretion into the bile is abolished [103], and the trans-
bilayer movement of radioactively labeled endogenous PC
appears to be slightly enhanced in fibroblasts from mice
overexpressing the ABCB4 gene [106]. In some cases of
progressive familiar intrahepatic cholestasis (type III) in
humans, ABCB4 has indeed been found to be defective
[107].
4.1.3. Human ABCC (CFTR/MRP) family
All 13 ABCC family members are full-size proteins.
However, they differ in the number of transmembrane
domains (two for ABCC4, ABCC5, ABCC11 and
ABCC12, while all others possess a third TM domain,
TMD0 (Fig. 4)). The major functions of the ABCC proteins
are, among others, the protection against toxic compounds
and the secretion of organic anions [108]. Examples for
ABCC family members are the cystic fibrosis transmem-
brane conductance regulator ABCC7 (CFTR), defective in
mucoviscidosis, and the sulfonylurea receptors ABCC8,9
(SUR 1, 2), regulating associated potassium channels.
ABCC8 is defective in patients with familial persistent
hyperinsulinemic hypoglycemia of infancy. The ABCC
family members differ in substrate specificity, tissue and
organelle distribution [109].
ABCC1 (MRP1) [110] is located in the basolateral domain
of polarized cells [111], and displays wide tissue distribution
[109]. ABCC1 transports a large variety of toxins across the
plasma membrane, either unconjugated or conjugated with
glutathione, sulfate or glucuronate [112]. While it is unclear
whether ABCC1 contributes significantly to multidrug
resistance in tumor cells [109], it protects particularly
sensitive organs by expelling toxins into the blood (the
internal environment [113], in contrast to the apically located
A. Pohl et al. / Biochimica et Biophysica Acta 1733 (2005) 29–5238
ABCB1 (MDR1 Pgp) which exports toxins into the external
environment). Additionally, ABCC1 mediates the leuko-
triene C (LTC) dependent inflammatory response by the
transport of the arachidonic acid derivative LTC4 [114].
ABCC1 transfected pig kidney epithelial cells showed
increased outward transport of short-chain nbd analogs of
SM, and GlcCer [115], and the transport of short-chain (PS,
PC) and long-chain PC nbd analogs in erythrocytes was
equally attributed to ABCC1 in an inhibition study [116].
Upon reconstitution, ABCC1 transported an nbd PC analog
[117]. However, thus far, no endogenous lipids have been
found to be ABCC1 substrates.
Despite similarities in the substrate spectrum of ABCC1
and several other ABCC proteins, Raggers et al. did not
observe translocation of short-chain nbd lipid analogs by
other ABCC proteins tested besides ABCC1 [101].
ABCC6 (MRP6) is expressed exclusively in the liver and
kidney, where the gene product is localized on the
basolateral domain of the plasma membrane [118]. ABCC6
defects result in pseudoxanthoma elasticum (PXE)
[119,120], a disorder characterized by a calcification of
the elastic fibers of the eye, skin, and vasculature, leading to
decreased visual acuity, characteristic skin lesions, and
peripheral vascular disease. Additionally, frequently found
high plasma triglyceride levels and low plasma HDL
cholesterol in PXE patients suggest an involvement of
ABCC6 in lipid transport and metabolism [121].
The molecular basis of this disease is not solved, and
PXE might be a primary metabolic disorder with secondary
involvement of elastic fibers [122].
The transport of glutathione conjugates upon ABCC6
gene expression in Sf9 insect cells point to a role for ABCC6
in the transport of organic anions [123], thought to confer
low levels of resistance to certain anticancer agents [124].
4.1.4. Human ABCD (ALD) family
All four knownmembers of the ABCD family are half-size
proteins found in peroxisomes, single-membrane organelles
involved in beta-oxidation of long and very long chain fatty
acids, synthesis of bile acids, cholesterol plasmalogens and
metabolism of amino acids and purines. ABCD proteins have
been implicated with the transport of fatty acids (FA),
coenzyme A (CoA), or FA-CoA, although ATP independent
transport of very long chain fatty acids (VLCFA) into
peroxisomes [125] might argue against direct VLCFA trans-
port via ABCD proteins. ABCD half-size proteins can form
not only homodimers (ABCD1–ABCD1), but also hetero-
dimers (ABCD1–ABCD2, ABCD1–ABCD3, ABCD2–
ABCD3) [126]. Although the heterodimers might be func-
tional, the differing tissue gene expression of ABCD
members argues against obligatory heterodimerization [127].
ABCD1 (ALDP) mRNA is highly expressed in human
liver, heart, skeletal muscle, lung and intestine [128].
Defects in ABCD1 result in the inherited neurometabolic
disorder X-linked adrenoleukodystrophy (X-ALD) [129],
characterized by elevated levels of VLCFA in nervous
system white matter and the adrenal cortex [130], leading to
neuron demyelinization, renal insufficiency and testicular
dysfunction. Increased VLCFA levels have been attributed
to impaired peroxisomal beta-oxidation, and transfection
with ABCD1 cDNA was shown to restore beta-oxidation in
X-ALD fibroblasts [131] (see also yeast homolog Pat1p;
Section 4.5). Recently, mitochondrial beta-oxidation was
reported to affect peroxisomal beta-oxidation, and a role of
ABCD1 in the interaction of peroxisomes with mitochon-
dria was suggested [132]. ABCD1 has been implicated with
peroxisome biogenesis, restored upon ABCD1 overexpres-
sion in Zellweger cells (having a defect in the peroxisomal
protein Pex2p) [133].
ABCD2 (ALDRP) is closely related to ABCD1 (63%
amino acid identity). Its mRNA is highly expressed in human
brain and heart [128]. Similar to ABCD1, transfection with
ABCD2 cDNA restored beta-oxidation in X-ALD (ABCD1
defect) fibroblasts [134], and peroxisome proliferation could
be induced upon pharmacologically increased ABCD2
expression in X-ALD cells [135]. In X-ALD fibroblasts,
ABCD2 induction by steroid depletion was found to reduce
the accumulation of VLCFA [136]. While defects in ABCD2
as a cause for X-ALD were considered to be unlikely,
ABCD2 could be a heterodimeric partner for ABCD1 in some
tissues, acting as a modifier gene accounting for the high
phenotypic variability of X-ALD [127].
ABCD3 (PMP70, PXMP1) shows 36% amino acid
identity with ABCD1. The mRNA of its mouse homolog
is highly expressed in the liver, kidney, heart, lung and
intestine [128]. The overexpression of ABCD3 was found to
restore beta-oxidation in X-ALD fibroblasts and to normal-
ize peroxisome biogenesis in Zellweger cells [133] (see also
yeast homolog Pat2p, Section 4.5).
ABCD4 (PXMP1L, P70R, PMP69) shares 25% of
ABCD1 amino acid sequence. ABCD4 mRNA is highly
expressed in human kidney, spleen and testis [128]. As
ABCD2 and 3, it has been suggested to act as a modifier
gene contributing to X-ALD phenotypic variability, possibly
heterodimerizing with the other members of this protein
family [137].
4.1.5. Human ABCG (WHITE) family
The five characterized ABCG proteins are half-size
proteins. In contrast to other proteins, the ABC-domain is
N-terminal, followed by the transmembrane domain (Fig. 4)
[138]. A number of ABCG members are thought to be
involved in the transport of steroids (ABCG1, 5, 8),
additionally, some appear to transport phospholipids
(ABCG1) and toxins (ABCG2).
ABCG1 (WHITE) [139] is thought to be active either as a
homo- or a heterodimer (possibly with ABCG2) [140]. It
shows an ubiquitous gene expression pattern [138] and
localizes primarily to ER and Golgi [140]. ABCG1 derives
its trivial name from its homology to the Drosophila white
protein, which transports guanine and tryptophane as
precursors for eye pigments [141]. ABCG1 itself appears
A. Pohl et al. / Biochimica et Biophysica Acta 1733 (2005) 29–52 39
to serve a different function, presumably in the transport of
phospholipids and steroids out of macrophages, as its gene
expression in macrophages was found to be induced during
cholesterol influx, and suppressed by lipid efflux via HDL.
The inhibition of ABCG1 expression resulted in reduced
HDL-dependent efflux of cholesterol and PC from these
cells [140].
ABCG2 (BCRP, MXR, ABCP) [142] is found in the
placenta, intestinal epithelium, liver canaliculi, breast ducts
and lobules, as well as in veinous and capillary endothelium
[143]. Recent studies suggest the protein to be active as a
homotetramer [144] in the plasma membrane [145]. It is
assumed to prevent the tissue uptake of xenobiotics [143],
transporting various drugs across the plasma membrane in
an ATP dependent process [146]. Transfection studies
proved the overexpression of ABCG2 to induce multidrug
resistance in a previously drug sensitive cell line [147]. In
addition to drug transport, the reduced accumulation of a
short-chain Bodipy analog of ceramide in ABCG2 over-
expressing cells has given hints for a transport of lipid
analogs [146]. Recently, we have found increased outward
movement of short-chain nbd analogs of PS and PC, but not
of PE, and increased exposure of endogenous PS in a human
gastric carcinoma cell line overexpressing ABCG2 [148]. In
Lactococcus lactis transfected with human ABCG2,
ABCG2-associated ATPase activity was significantly stimu-
lated by cholesterol and estradiol, pointing to a possible role
of ABCG2 in the transport of steroids [149].
ABCG5 (WHITE3) and ABCG8 (WHITE4) are encoded
by neighbouring genes [150]. Their mouse homologues are
expressed in the liver and intestine [151]. Very recently,
ABCG5 and ABCG8 have been demonstrated to function as
an obligate heterodimer to promote steroid secretion into the
bile [152–154]. Mutations in either ABCG5 or ABCG8
result in an identical clinical phenotype; and the expression
of both genes is required for either protein to be transported
to the plasma membrane. Defects in ABCG5 and 8 can
result in sitosterolemia, a disorder characterized by
increased uptake of steroids (among them the plant steroid
beta-sitosterol) in the intestine, combined with decreased
biliary excretion, resulting in cholesterol deposits in skin
and tendons, and in premature coronary artery disease
[150,155,156].
The overexpression of human ABCG5 and 8 in mice
promoted bilary cholesterol secretion, reduced cholesterol
absorption, and increased hepatic cholesterol synthesis [157].
Furthermore, no significant sitosterol transport by
ABCB1 (MDR1 Pgp), ABCC1 (MRP1), and ABCG2
(BCRP) was found [158].Taken together, these data
demonstrate a central role of ABCG5 and ABCG8 in in
vivo cholesterol excretion [159] (see also Section 6).
4.2. Sea urchin ABC proteins
SuABCA is a full-size ABC protein found abundantly in
the sea urchin sperm plasma membrane, and is possibly
implicated in the sperm acrosome reaction during fertiliza-
tion. Due to its close homology to human ABCA3,
Mengerink and Vacquier suggested SuABCA to be involved
in phospholipid or cholesterol transport [160].
4.3. Eukaryotic parasite ABC proteins
ABC proteins found in parasites are of particular clinical
relevance due to the occurrence of multidrug resistant
strains. Specific attention will be given here to ABC
proteins found in unicellular eukaryotes of the Trypanoso-
matidae family, causing Leishmaniasis and Trypanosomia-
sis, major and globally widespread parasitic diseases.
In Leishmania spp., three different families of ABC
proteins are known (reviewed in [161,162]), homologous to
the mammalian ABC families ABCA, ABCB, and ABCC,
respectively.
4.3.1. Leishmania ABCA family
LtrABC1.1 [161], a full-size protein containing two
transmembrane domains and two nucleotide binding
domains, is found mainly in the plasma membrane and
flagellar pocket of Leishmania [163]. LtrABC1.1 expres-
sion appears to be uncorrelated with multidrug resistance. In
a Leishmania tropica cell line overexpressing LtrABC1.1,
the accumulation of short-chain nbd analogs of PC, PE, and
PS was found to be reduced [163], regardless of the nature
of the phospholipid head group. Furthermore, LtrABC1.1
overexpression reduced vesicular transport. Parodi-Talice et
al. have suggested a role for LtrABC1.1 in lipid movement
across the plasma membrane and in vesicle trafficking.
Finally, Legare et al. noted the potential importance of
members of the Leishmania ABCA family in the interaction
of the parasite with its host, an engulfing macrophage cell
[162].
4.3.2. Leishmania ABCB family
Leishmania Pgp-like proteins (LPgp-lp), about 37%
identical to mammalian ABCB1s, are full-size ABC
proteins [161]. They have been shown to transfer multidrug
resistance upon transfection [164]. In Leishmania enriettii,
Leishmania Pgp-like protein is located mainly in different
intracellular vesicles [165]. Upon the overexpression of the
LPgp-lp gene in L. tropica, the parasite cells accumulated
lower amounts of a short-chain Bodipy analog of PC and the
antiproliferative alkyl-lysophospholipids miltefosine and
edelfosine than did the controls [166].
4.4. Plant ABC proteins
Ped3p (peroxisome defective protein 3, also designated
COMATOSE, CTS, PXA1), predicted to be a full-size protein
[167,168], is found in arabidopsis glyoxysomes, specialized
peroxisomes occurring in cells of storage organs (endo-
sperms, cotyledons) and senescent organs [168]. Both halves
of Ped3p show significant sequence homology to the human
A. Pohl et al. / Biochimica et Biophysica Acta 1733 (2005) 29–5240
half-size protein ABCD1 (ALDP) [169]. In Ped3p defective
plants, fatty acid beta-oxidation is impaired [168,169],
causing a defect in gluconeogenesis that severely inhibits
seedling germination in the absence of sucrose [170]. As
Ped3p mutants accumulate fatty acyl CoAs, Footitt et al.
proposed the protein to be a transporter of fatty acyl CoAs
with little specificity concerning chain length [169].
4.5. Yeast ABC Proteins
In yeast, the 31 ABC proteins identified so far have been
classified into 6 clusters, including 10 subclusters, according
to predicted topology, binary amino acid sequence compar-
ison and phylogenetic classification. Both half-size and full-
size proteins exist in yeast. Besides I.1 and VI, all
subclusters seem to have human homologues [171].
4.5.1. PDR/CDR family
Pdr5p is a full-size protein located in the plasma
membrane [172] of Saccharomyces cerevisiae, mediating
multidrug resistance. It confers resistance to progesterone
and deoxycorticosterone, which are also inhibitors of Pdr5p
drug transport, suggesting these steroids to be direct
transport substrates of Pdr5p [173]. Pdr5p deletion in S.
cerevisiae leads to an increased accumulation of short-chain
nbd PE [174]. Notably, the depletion of Yor1p and Pdr5p
causes reduced surface exposure of endogenous PE [32].
Cdr1p and Cdr2p, candida drug resistance proteins of
the human pathogenic yeast C. albicans, are full-size
proteins composed of two homologous halves, each
comprising a TM domain and a nucleotide binding domain
[175]. Cdr3p, highly homologous to Cdr1p and Cdr2p,
shows the same domain organization, but appears not to be
involved in drug resistance [176]. In C. albicans strains with
a disrupted CDR1 gene, the (already low) exposure of
endogenous PE on the outer plasma membrane leaflet,
revealed via TNBS labeling, was reported to be further
reduced [177]. However, taking into account the limitations
of the TNBS labeling approach and the very low percentage
of labeled PE, the results have to be taken with caution.
Furthermore, gene disruption may also affect other pro-
cesses involved in PE exposure, for example, intracellular
lipid trafficking. In transfected S. cerevisiae cells, Cdr1p
and Cdr2p elicited in-to-out transport of short-chain nbd
analogs of PE, PC and PS. In contrast and rather excep-
tional, Cdr3p mediated the out-to-in transport of these
analogs [178].
4.5.2. MDR family (ABCB homolog)
Ste6p, a S. cerevisiae full-size ABC protein transporting
the farnesylated dodecapeptide mating pheromone a-factor
[179], was reported to confer resistance against the lyso-PC
analog ET-18-OCH3 (Edelfosine) in transfected yeast cells,
presumably by outward transport of the drug [180].
However, failure to reproduce these results led to retraction
of the original article [181].
4.5.3. MRP/CFTR family (ABCC homolog)
Yor1p is a plasma membrane located [182] full-size
protein in S. cerevisiae. It is related to the human ABCC
family [183]. Besides conferring drug resistance, Yor1p has
been suggested to be involved in the efflux of organic
anions. The deletion of Yor1p in S. cerevisiae resulted in
increased accumulation of a short-chain nbd analog of PE
[174].
4.5.4. ALDp family (ABCD homolog)
Pat1p (also designated Pxa2p) and Pat2p (also desig-
nated Pxa1p) [184,185] are S. cerevisiae half-size proteins
related to human ABCD3 (PMP70) and ABCD1 (ALDP),
respectively. Pat1 and Pat2 are thought to heterodimerize
[185] in the peroxisomal membrane, and are required for the
import of long-chain fatty acids into peroxisomes, Pat1 and
Pat2 deletion causing a partial deficiency in long-chain fatty
acid beta-oxidation [184]. These data also support the
hypothesis that ABCD1 and ABCD3 are involved in
VLCFA transport (see Section 4.1).
5. Prokaryotic ABC proteins
ABC proteins exist in both Gram-positive and Gram-
negative bacteria (reviewed in [48]). Most of them are
concerned with the import of small solutes, depending on
specific binding proteins, but exporters exist as well. In
prokaryotes, ABC proteins can possess either separate NBD
and TM domains, or fused domains.
5.1. Prokaryotic ABCB family
MsbA is a half-size ABC protein [186,187] active as a
homodimer [188] in the Escherichia coli inner membrane. It
is a close bacterial homolog of ABCB1 (MDR1 Pgp) as was
deduced from protein sequence homology [189,190]. It is an
essential ABC protein in prokaryotes, conserved in all
bacteria, with more than 30 orthologs identified today [191].
MsbA plays an important role in the transport of lipid A
from the inner to the outer membrane of Gram-negative
bacteria. Lipid A, a hexa-acylated disaccharide of glucos-
amine unique to Gram-negative bacteria, is a major
component of the outer membrane, representing the hydro-
phobic anchor of lipopolysaccharides on the outside of the
outer membrane. MsbA defects cause the accumulation of
lipid A and phospholipids in the inner membrane, lethal to
E. coli [187,192]. The stimulation of the ATPase activity of
MsbA reconstituted into liposomes by lipid A [193], but not
by short-chain nbd phosphatidylglycerol [23], provides
further indications for specific transport of lipid A by MsbA
[23]. Very recently, newly synthesized lipid A, and possibly
PE, has been shown to accumulate on the cytoplasmic half
of the E. coli inner membrane upon the inactivation of
MsbA in a temperature-sensitive mutant [194] arguing for
acceleration of transbilayer movement by MsbA.
A. Pohl et al. / Biochimica et Biophysica Acta 1733 (2005) 29–52 41
The structure and potential mechanisms of MsbA will be
discussed in Section 7.
LmrA, a half-size protein in L. lactis forming homo-
dimers, extrudes various drugs [195]. Like MsbA, LmrA is
homologous to both halves of human ABCB1 (MDR1 Pgp)
[196,189], which it can complement functionally in human
lung fibroblasts, causing multidrug resistance [189].
Recently, Reuter et al. could show functional substitution
of temperature-sensitive mutant MsbA in E. coli, suggesting
transport of lipid A by LmrA [190].
Reconstituted LmrA has been found to mediate ATP-
dependent transport of fluorescent short-chain nbd PE [197].
However, short-chain nbd PC was not recognized, suggest-
ing headgroup-specificity, different from the low substrate
specificity of ABCB1 despite the high sequence conserva-
tion. It is not known whether LmrA also transports
endogenous phospholipids of L. lactis. In the light of the
rapid, ATP-independent lipid flip-flop mediated by proteins
in the inner membrane of bacteria [20,21,198–200] with
half-times of the order of one minute, it remains open
whether lipid transport by LmrA is of physiological
relevance for L. lactis.
5.2. Val A
ValA is an ABC protein in Francisella novicida with
high homology to E. coli MsbA, able to rescue MsbA
defective E. coli [201]. Due to this finding and decreased
cell surface exposure of a lipopolysaccharide epitope in the
absence of functional ValA, McDonald et al. suggested
ValA to be required for the transport of lipid A molecules
linked to core polysaccharide across the inner membrane.
6. Putative functions in lipid transbilayer transport and
exposure
The studies summarized above document the physiolog-
ical importance of ABC proteins in lipid transport, their
malfunction being able to cause severe diseases. This raises
the question of the specific function of ABC proteins in lipid
transport (Fig. 2B).
Protein-mediated lipid transport can serve essentially two
functions, which may not exclude each other:
(i) Lipid transbilayer transport, establishing, preserving or
perturbing a distinct asymmetric transbilayer lipid
distribution (asymmetric distribution of lipid species
between the two leaflets, or asymmetry in the number
of lipid molecules per leaflet, resulting in an area
difference between the two leaflets)
(ii) Exposure of lipids to acceptors: Besides being mem-
brane constituents, some phospholipids are constituents
of bile and of pulmonary surfactant, and various
steroids act as hormones. In addition to movement
from one membrane leaflet to the other, some lipids
require therefore transport to other membranes, onto
lipoproteins, or into the extracellular lumen.
So far, there is no indication for an involvement of ABC
proteins in the generation of an asymmetric transbilayer
distribution of abundant lipids such as phospholipids or
cholesterol. All available quantitative data indicate that the
activity of ABC transporters in cellular membranes cannot
compete with the inward-directed activity of the flippases
described in Section 2 to affect transbilayer distribution on a
qualitative level. However, the activity of ABC proteins
may not be negligible. Several studies indicate that outward-
directed lipid transport mediated by ABC proteins can
modulate the transbilayer distribution even of abundant
phospholipids inward transported by energy-dependent
flippases in the mammalian plasma membrane. An example
is the enhanced exposure of aminophospholipids on the
outer plasma membrane leaflet of ABCB1 (MDR1 Pgp),
Yor1p, Pdr5p and ABCA1 expressing cells. ABC proteins
may thus affect physiological functions associated with
asymmetric transbilayer lipid distribution. Their outward-
directed lipid transporting activity may also counteract
vesicle budding driven by energy-coupled flippases (see
Section 2.2): While the overexpression of ABC genes in
yeast can lead to endocytosis defects [202,174], the loss of
ABCA1 function in Tangier fibroblasts is associated with
enhanced endocytosis [203], supporting a functional link
between lipid transport and vesicle biogenesis.
Furthermore, ABC proteins may play a role in the
transbilayer distribution of marginal, but physiologically
relevant lipids exhibiting slow passive transbilayer move-
ment. In Section 7, models for lipid transbilayer transport by
ABC proteins will be discussed.
Most examples of ABC proteins implicated in lipid
transport indicate a role not primarily in lipid asymmetry,
but rather in the exposure of specific lipids on the
exoplasmic leaflet, allowing their uptake by acceptors and
further transport to specific sites: The lipopolysaccharide
precursor lipid A has to be delivered from the outer leaflet
of the inner membrane to the outer membrane of bacterial
cells, and PC transported across the canalicular membrane
by ABCB 4 (MDR3 Pgp) is taken up by bile salts in the bile
canalicular lumen. The necessity of appropriate exposure of
membrane-bound lipid to a cognate acceptor can maybe best
be illustrated for cholesterol: Deeply buried in the mem-
brane with the polar OH group facing the aqueous phase and
the alkyl chain oriented toward the bilayer center, choles-
terol has recently been shown to experience rapid flip-flop
in PC vesicles, and membranes of erythrocytes and
presumably most other cells (reviewed in [204]). The
primary function of ABCG5/8 may therefore very likely
not be cholesterol transbilayer transport across the canal-
icular membrane, but rather facilitation of lumenal choles-
terol uptake (e.g. by mixed bile salt and PC micelles),
possibly by pushing it partly into the aqueous phase, as
suggested recently [205].
A. Pohl et al. / Biochimica et Biophysica Acta 1733 (2005) 29–5242
A similar mechanism may be relevant for ABCA1 in the
delivery of cholesterol to apoA-1, although this seems to be
more complicated: Here, PC is involved in the release of
cholesterol from the respective membranes, which has also
been suggested for ABCG5/8 [205].
Taking into account the opening of the central pore of
ABCB1 (MDR1 Pgp) to the lipid phase in the presence of
nucleotide (see Section 7), one could even ask whether ABC
proteins may take up substrates from the same leaflet from
which they are delivered to an acceptor.
While being hypothetical at present, different modes of
lipid exposure by ABC proteins could be imagined, which
may or may not include the transport of these lipids from the
opposite leaflet of the membrane to the side of acceptor
localization (Fig. 2B). In principle, transport and exposure
of the lipid to the acceptor could be a two-step process or be
combined in one step as proposed in the vacuum cleaner
model [1]. In the absence of an acceptor, the lipid might be
released to the exoplasmic leaflet (modifying transbilayer
lipid distribution as a side effect), while the activity of the
ABC protein is likely underestimated in this case.
7. Models for lipid transbilayer transport by ABC
proteins
ABC proteins display variable transmembrane domains
(TMDs), while the nucleotide binding domains (NBDs) are
highly conserved in ABC proteins of diverse origins. It is
therefore assumed that all ABC proteins bind and hydrolyze
ATP in a similar fashion and use a common mechanism on
the NBD level to power the translocation of substrate via the
TMDs [206]. In addition to biochemical data, structures
obtained by X-ray and electron cryo crystallography have
been used to propose models for the mechanism of substrate
transport by ABC proteins.
The main models currently discussed are the tilting
model and the rotating helix model [207]: In the tilting
model (Fig. 5), the TMDs are considered as rigid entities.
The protein possesses a chamber open towards the
cytoplasmic face. The substrate enters the chamber (e.g.
from the cytoplasmic aqueous phase or the inner leaflet of
the bilayer). ATP binding and hydrolysis cause conforma-
tional changes (tilting) of the TMDs, leading to the release
of the substrate into the exoplasmic or periplasmic aqueous
phase now accessible from the chamber.
In the rotating helix model (Fig. 6), the individual
movement of TM helices is taken into account. Conforma-
tional changes lead to the rotation of the TM helices, such as
to reorient the substrate binding site from the inner leaflet of
the lipid bilayer to the aqueous chamber, from where the
substrate is released either into the exoplasmic/periplasmic
aqueous phase or to the membrane outer leaflet. As both
models focus on different aspects, they may not be mutually
exclusive. The order of substrate binding, nucleotide bind-
ing and hydrolysis, and conformational changes, as well as
the interactions between the protein domains are subject of a
current discussion.
In the following, we will summarize information on
substrate transbilayer transport on the basis of structures of
prokaryotic export (MsbA) and import (BtuCD) ABC
proteins, and of the eukaryotic export protein ABCB1
(MDR1 Pgp) in the presence or absence of nucleotide.
7.1. Information derived from MsbA
The X-ray crystal structures of a homodimer of the half-
size ABC protein MsbA (see Section 5.1) from E. coli (4.5
2 resolution) and Vibrio cholera (3.8 2 resolution) obtained
by Chang and Roth [188] and Chang [208] in the absence of
nucleotide have been interpreted to reflect two different
conformational states of MsbA involved in transport of lipid
A, revealing an open (E. coli) and a closed conformation (V.
cholera) [191]. In E. coli MsbA, the TM helices are tilted
by 308 to 408 relative to the normal of the membrane, being
in intermolecular contact on the outer membrane leaflet side.
They form a cone shaped structure with large openings (25
2) towards the inner membrane leaflet at each of the dimer
interfaces. The openings lead into a cone-shaped chamber in
the interior of the TMDs. The cytoplasmic side of the
chamber contains positively charged residues, while the
periplasmic side is essentially hydrophobic. The base of the
chamber facing the cytoplasm is up to 45 2 wide.
The NBDs do not have intermolecular contact and are
separated by about 50 2. An intracellular domain (ICD) is
situated between the TMDs and the NBDs. However,
whether the tertiary structure proposed corresponds to a
native conformation has been questioned by several authors
[207,209,210] (see below).
In V. cholera MsbA, the TM helices are less tilted (108 to308), and the openings towards the inner membrane leaflet
are more narrow (12 2) than in the E. coli structure.
However, the chamber volume remains large enough to
accommodate a lipid A molecule. TMDs and NBDs make
intermolecular contact. Compared to E. coli MsbA, the
alpha-domain of the NBD is rotated about 1208 along the
dimer axis and contacts the opposing NBD.
Chang [191] proposed the following transport model
(Fig. 5): In the open conformation, the NBDs are not
firmly attached to the ICD region connecting TMDs and
NBDs, and can rotate freely relative to the TMD. Substrate
recognition, perhaps by charge–charge interactions, at the
cytoplasmic chamber opening induces nucleotide binding,
changing the conformation of the NBDs and promoting
their dimerization. Dimerization drives trapping of the
substrate in the chamber (closed conformation), from
where it spontaneously flips to the periplasmic side due
to energetically unfavourable interaction of its hydrophobic
chains with the polar part of the chamber. The NBD dimer
cooperatively hydrolyzes ATP, leading to conformational
changes in the NBDs. Relayed through the ICD, the
conformational changes cause the TMDs to open towards
Fig. 5. (A) Backbone structure of E. coli and V. cholera MsbA. Structures of E. coli MsbA (Eco) (Chang and Roth [188]) (left) and V. cholera (VC) (Chang
[208]) MsbA (right) were obtained from the Protein Data Bank, 1JSQ and 1PD4, respectively. The transmembrane domains (TMDs) and nucleotide binding
domains (NBDs) are shown in yellow and green, respectively. The six positively charged residues in the chamber are in blue. (B) Model proposed for lipid
transport via MsbA (tilting model). The protein is in its open conformation (1). Substrate recognition (2) induces nucleotide binding, which promotes NBD
dimerization and trapping of the substrate that flips to the opposite side (closed conformation) (3). ATP hydrolysis causes opening of the TMDs to the outer
membrane side, releasing the substrate (4). ADP and Pi dissociation reset the protein into its initial state. See Section 7.1 for details, and also a model published
by Chang [191].
A. Pohl et al. / Biochimica et Biophysica Acta 1733 (2005) 29–52 43
the periplasmic side, releasing the substrate. Upon the
dissociation of ADP and Pi, the NBDs separate from each
other and the TMDs are reset to the resting state. While
the two MsbA structures favor a tilting mechanism, the
data could also be consistent with the rotation of certain
helices during the catalytic cycle [191]. The subsequent
transfer of lipid A across the periplasmic space and to the
outer leaflet of the outer membrane is not fully understood
[193].
7.2. Information derived from BtuCD
The ABC protein BtuCD mediates the import of vitamin
B12 in E. coli. Although BtuCD has not been implicated
with lipid transport, its structure (resolved in the absence of
nucleotide by X-ray crystallography with a resolution of
3.2 2 [206]) provides important information on the
mechanism of substrate transport. The BtuCD complex is
a heterotetramer consisting of two copies each of the BtuC
(TMD) and BtuD (NBD) subunits. The overall structure
has been described to resemble an inverted portal. The two
TMDs consist of 10 helices each, unlike the 2�6 helices
predicted for a number of other ABC proteins. At the
interface of the TMDs, a cavity opens to the periplasmic
space and spans 2/3 of the predicted lipid membrane. The
cavity is closed to the cytoplasm by the so-called gate. The
cytoplasmic loop between the TM helices 6 and 7 (L loop)
provides most of the interface with the NBD. Due to the
lack of an ICD between the TMDs and the NBDs, the
NBDs are located just below the membrane surface. The
two TMDs, as the two NBDs, are in close contact to each
other.
The following transport mechanism has been proposed
[206]: The substrate (on the periplasmic side) interacts
Fig. 6. ABCB1 (MDR1 Pgp) structure and model proposed for lipid transport (rotating helix model). The protein is in its barrel conformation (left: structure;
right: model) (1). Substrate binding at the inner membrane leaflet induces nucleotide binding, leading to helix rearrangements which open a gap between two
TMDs, enabling the substrate to enter the central pore (2). The substrate moves from the central pore into the outer membrane leaflet or the extracellular milieu
(3). ATP hydrolysis resets the transporter into its initial state (4). See Section 7.3 for details. Electron micrographs of ABCB1 were taken from Rosenberg et al.
[209] (see Figs. 3a and d of the publication) with permission.
A. Pohl et al. / Biochimica et Biophysica Acta 1733 (2005) 29–5244
through the TMD via the connecting L loop with the
nucleotide binding site (this step might be particular for
importers, in which nucleotide binding site and substrate are
located on opposite sides of the membrane). The binding and
hydrolysis of ATP at the nucleotide binding site induce close
contact of the two NBDs, leading to long-range conforma-
tional changes, which force the TMDs to spread and to open
the gate of the translocation pathway at the interface of the
two NBDs. The substrate passes through the pathway open to
the cytoplasmic space either by diffusion or due to forces
applied by the TMDs. The release of ADP and Pi causes the
ABC protein to return into its resting state, including
reorientation of the NBDs. In contrast to the mechanism
proposed for MsbA, NBDs and TMDs remain in contact
throughout the transport cycle. BtuCD structure and compar-
ison to the ATP-bound form of Rad50, a DNA double-strand
break repair enzyme with an ABC-type ATPase domain
[211], suggests a tilting mechanism, with the particularity of
describing substrate import. Locher and Borths have specu-
lated on a common mechanism in im- and exporters,
considering the binding protein as the substrate in the case
of importer proteins [212].
A. Pohl et al. / Biochimica et Biophysica Acta 1733 (2005) 29–52 45
7.3. Information derived from ABCB1 (MDR1 Pgp)
The structure of the full-size ABCB1 (see Section 4.1.2)
monomer was determined in the presence and absence of
nucleotide using electron cryo crystallography (20 2resolution) [209]. In the absence of nucleotide, the TMDs
are approximately parallel and form a barrel surrounding a
central pore appearing to be open at the extracellular face of
the membrane, and closed at the intracellular face. Small
densities protrude into the pore. The binding of nucleotide
leads to substantial reorganization of the TMDs involving
the repacking and rotation of the TM helices within the
membrane: In the presence of nucleotide, the TMDs consist
of three clearly segregated domains, enclosing a central pore
less obviously closed towards the intracellular face, and
additionally open to the lipid phase along one side with a
gap appearing between two domains. This gap, equivalent
to most of the depth of the bilayer, could give substrates
from the lipid bilayer access to the central pore. Due to the
technique employed, data concerning the NBDs are
insufficient for exhaustive characterization.
The following transport model was proposed (Fig. 6)
[209,213]: Substrate binds to the protein from the inner
membrane leaflet. Subsequently, ATP binding leads to
conformational changes. The TMDs part in order to enable
the substrate to enter the central pore, giving it access to the
exoplasmic milieu. ATP hydrolysis resets the transporter
into its initial state as also suggested for BtuCD [212]. This
is in agreement with the finding that the binding and
hydrolysis of ATP can be independent events associated
with distinct conformational states of the transporter
[214,215]. Data from the two structures including substan-
tial TM helices repacking upon nucleotide binding is
consistent with the helix rotation model, while not being
easily reconcilable with the tilting model.
When comparing the different structures and proposed
models of transport, important differences are obvious for
the ABC proteins discussed here. As it is unclear whether
the TMDs of different subfamilies are related to each other
either evolutionarily or structurally [209], TMDs might be
custom-made for the respective substrate. Obviously, the
TMDs for an importer of a hydrophilic molecule (e.g.
BtuCD) must differ from that for an exporter of amphiphilic
substrates (e.g. MsbA, ABCB1), whereas the mechanism of
transport could be a common one. While BtuCD has
sequence similarity to ABCB1 only on the level of the
NBDs, the ABCB family members ABCB1 and MsbA
share sequence similarity in both TMDs and NBDs [210].
However, the lack of a TMD:TMD interface and the large
distance between NBDs in the proposed E. coli MsbA
structure, incompatible with disulfide cross-linking studies
and structural data for ABCB1, lead Stenham et al. [210] to
question the validity of the proposed tertiary structure of E.
coli MsbA. In contrast, the tertiary structure of BtuCD
contains a parallel TMD:TMD interface and an NBD:NBD
interface as found in isolated prokaryotic NBDs.
By rotation of the E. coli MsbA NBDs by 1508 relativeto the cognate TMDs, Stenham et al. [210] have obtained a
structural model for ABCB1 with a consensus NBD:NBD
interface and a parallel TMD:TMD interface, consistent
with crosslinking data and electron crystallography studies.
According to this model, the TMDs surround a chamber
open at the exoplasmic surface and closed at the cytoplas-
mic surface. The rotating helix model appears to be
particularly suited for lipid transport, as the substrate can
access the transport pathway from the membrane, as
suggested to be necessary for ABCB1 mediated transport,
which might also partially explain ABCB1 substrate multi-
specificity [1]. Meanwhile, the various structures discussed
here reflect conformational snapshots of ABC proteins
during the transport cycle, and may not provide a sufficient
basis to unravel the transport mechanism and all involved
conformational intermediates.
It also remains to be established whether MsbA and
ABCB1 can serve as models for other lipid-transporting
ABC proteins not belonging to the ABCB family. Further
structural data from other ABC proteins is required to reveal
whether the TMDs of lipid-transporting ABC proteins are
structurally similar, and function according to the same
principle.
8. Conclusions
A long way from being solely a subject for basic
research, lipid transport reaches far into the clinical domain.
The transport of lipids across a membrane can have a net
secretory function for a cell or an organelle, or concern
mostly the transbilayer distribution of lipids in a membrane.
Cholesterol transport by ABCA1 out of macrophages, PC
transport by ABCB4 (MDR3 Pgp), the transport of fatty
acids into peroxisomes by members of the ABCD (ALD)
family, and steroid secretion by ABCG5 and 8 seem to be
examples for secretion events, where rather high quantities
of lipids must be efficiently transported to other membranes,
onto lipoproteins, or into the extracellular lumen. Here, the
exposure to appropriate acceptors appears to be an essential
step. In a membrane with an asymmetric distribution of the
lipid species across the two leaflets, on the other hand, a
comparatively slow transport of relatively few lipid mole-
cules can be sufficient to help increase or break down this
asymmetry, having for example potential effects on signal-
ling. This could be the case for the transport of PS by
ABCA1 in apoptotic cells, serving as a signal for
phagocytosis. Meanwhile, other proteins existing in the
same membrane may transport lipids in the same or the
opposite direction as ABC proteins. Knock-out studies in
mammals on ABC proteins have yielded unexpected results,
likely due to functional compensation by other ABC
proteins [216]. Similarly, transfection with one ABC gene
has been shown to influence the expression of another ABC
gene [217].
A. Pohl et al. / Biochimica et Biophysica Acta 1733 (2005) 29–5246
In addition, ABC proteins have been proposed to be
transporters, channels and regulators, meaning they can
potentially affect transbilayer transport of lipids directly, or
indirectly through regulation of other proteins.
This makes clear why experimental setups are needed
which quantify lipid transport in a comparable way, verify
its attribution to a particular protein in the absence of a
transporter background, determine its specificity and permit
the distinction between transport events against or with a
concentration gradient, requiring or not the hydrolysis of
ATP, and the presence of acceptor molecules.
It should be noted that ABC proteins implicated in the
transport of lipids can be localized in the plasma membrane,
as well as in intracellular membranes, complicating direct
measurements on cells. Now that all ABC proteins in
humans, and many in other organisms, have been identified,
reconstitution into vesicles large enough to allow lipid
redistribution (e.g. giant unilamellar vesicles (GUVs))
appears to be a promising technique to answer many current
questions. As the number of unequivocally identified lipid
transporter proteins is small, putative lipid bulk transporters
will be of particular interest as models.
However, the identification of lipid transport remains
difficult. Short-chain lipid analogs offer the advantage of
easy integration into and extraction from membranes, but
only reach a certain level of accordance with endogenous
lipids for these same reasons.
Endogenous lipids can be bound by certain antibodies or
lipid binding peptides or proteins such as Annexin V, and
they can be chemically modified (e.g. by TNBS), or
extracted from membranes via lipid transfer proteins. At
the same time, reliable techniques for the quantification of
endogenous lipids on a specific leaflet exist for a few lipid
species only. The combination of results obtained in model
systems with data from cells overexpressing ABC genes
upon stimulation, selection or transfection, as well as from
cells lacking functional ABC proteins upon mutation, knock-
out or inhibition will therefore be indispensable to obtain a
clear picture of the role of these proteins in lipid transport.
At the present state, the ABC protein superfamily can
already be considered to be of great importance in the
transport of lipids in prokaryotic as well as in eukaryotic
organisms, while the map of proteins responsible for lipid
transport, far from complete today, continues to be drawn.
Acknowledgements
The authors thank Thomas Pomorski, Erwin Schneider
(Humboldt University Berlin) and Francisco Gamarro
(Instituto de Parasitologia y Biomedicina, Granada), as well
as David Stroebel and Cecile Breyton (IBPC, Paris) for
critical reading of the manuscript. This work was supported
by grants from the EC research training networks Sphingo-
lipids: Sphingolipid Synthesis and Organisation, and Lipid
flippases—Protein-mediated lipid translocation: Regulation
and physiological significance of transbilayer lipid distribu-
tion, and the COST action D22 Lipid–Protein Interaction.
Note added in proof
A detailed discussion of the mechanism of ABC protein-
mediated transport is presented in the recent view of
Higgins and Linton [219]. First biochemical evidence for
the binding of N-retinylidene-PE to ABCA4 and for
dissociation of this lipids from ABCA4 upon binding and
hydrolysis of ATP has been shown recently [220].
References
[1] C.F. Higgins, M.M. Gottesman, Is the multidrug transporter a
flippase? Trends Biochem. Sci. 17 (1992) 18–21.
[2] P. Borst, N. Zelcer, A. van Helvoort, ABC transporters in lipid
transport, Biochim. Biophys. Acta 1486 (2000) 128–144.
[3] G. Schmitz, W.E. Kaminski, E. Orso, ABC transporters in cellular
lipid trafficking, Curr. Opin. Lipidol. 11 (2000) 493–501.
[4] A. Tannert, A. Pohl, T. Pomorski, A. Herrmann, Protein-mediated
transbilayer movement of lipids in eukaryotes and prokaryotes: the
relevance of ABC transporters, Int. J. Antimicrob. Agents 22 (2003)
177–187.
[5] J. Stefkova, R. Poledne, J.A. Hubacek, ATP-binding cassette (ABC)
transporters in human metabolism and diseases, Physiol. Res. 53
(2004) 235–243.
[6] R.B. Gennis, Biomembranes, in: C.R. Cantor (Ed.), Springer
Advanced Texts in Chemistry, Springer Verlag, New York, 1989,
pp. 20–22.
[7] A. Zachowski, Phospholipids in animal eukaryotic membranes:
transverse asymmetry and movement, Biochem. J. 294 (1993) 1–14.
[8] P.F. Devaux, Static and dynamic lipid asymmetry in cell membranes,
Biochemistry 30 (1991) 1163–1173.
[9] H. Sprong, P. van der Sluijs, G. van Meer, How proteins move lipids
and lipidsmove proteins, Nat. Rev.,Mol. Cell Biol. 2 (2001) 504–513.
[10] R.D. Kornberg, H.M. McConnell, Inside–outside transitions of
phospholipids in vesicle membranes, Biochemistry 10 (1971)
1111–1120.
[11] R. Homan, H.J. Pownall, Transbilayer diffusion of phospholipids:
dependence on headgroup structure and acyl chain length, Biochim.
Biophys. Acta 938 (1988) 155–166.
[12] G. Morrot, P. Herve, A. Zachowski, P. Fellmann, P.F. Devaux,
Aminophospholipid translocase of human erythrocytes: phospholipid
substrate specificity and effect of cholesterol, Biochemistry 28 (1989)
3456–3462.
[13] K. John, S. Schreiber, J. Kubelt, A. Herrmann, P. Mqller, Transbilayermovement of phospholipids at the main phase transition of lipid
membranes: implications for rapid flip-flop in biological membranes,
Biophys. J. 83 (2002) 3315–3323.
[14] D.L. Daleke, Regulation of transbilayer plasma membrane phospho-
lipid asymmetry, J. Lipid Res. 44 (2003) 233–242.
[15] T. Pomorski, J.C.M. Holthuis, A. Herrmann, G. van Meer, Tracking
down lipid flippases and their biological functions, J. Cell. Sci. 117
(2004) 805–813.
[16] W.R. Bishop, R.M. Bell, Assembly of the endoplasmic reticulum
phospholipid bilayer: the phosphatidylcholine transporter, Cell 42
(1985) 51–60.
[17] A. Herrmann, A. Zachowski, P.F. Devaux, Protein-mediated phos-
pholipid translocation in the endoplasmic reticulum with low lipid
specificity, Biochemistry 29 (1990) 2023–2027.
A. Pohl et al. / Biochimica et Biophysica Acta 1733 (2005) 29–52 47
[18] X. Buton, G. Morrot, P. Fellmann, M. Seigneuret, Ultrafast
glycerophospholipid-selective transbilayer motion mediated by a
protein in the endoplasmic reticulum membrane, J. Biol. Chem. 271
(1996) 6651–6657.
[19] X. Buton, P. Herve, J. Kubelt, A. Tannert, K.N. Burger, P. Fellmann, P.
Mqller, A. Herrmann, M. Seigneuret, P.F. Devaux, Transbilayer
movement of monohexosyl sphingolipids in endoplasmic reticulum
and Golgi membranes, Biochemistry 41 (2002) 13106–13115.
[20] S. Hrafnsdottir, J.W. Nichols, A.K.Menon, Transbilayer movement of
fluorescent phospholipids in Bacillus megaterium membrane vesicles,
Biochemistry 36 (1997) 4969–4978.
[21] S. Hrafnsdottir, A.K. Menon, Reconstitution and partial character-
ization of phospholipid flippase activity from detergent extracts of
the Bacillus subtilis cell membrane, J. Bacteriol. 182 (2000)
4198–4206.
[22] W.J. Gerritsen, P.A. Henricks, B. de Kruijff, L.L. van Deenen, The
transbilayer movement of phosphatidylcholine in vesicles reconsti-
tuted with intrinsic proteins from the human erythrocyte membrane,
Biochim. Biophys. Acta 600 (1980) 607–619.
[23] M.A. Kol, A. van Dalen, A.I. de Kroon, B. de Kruijff, Translocation
of phospholipids is facilitated by a subset of membrane-spanning
proteins of the bacterial cytoplasmic membrane, J. Biol. Chem. 278
(2003) 24586–24593.
[24] M.A. Kol, A.N. van Laak, D.T. Rijkers, J.A. Killian, A.I. de Kroon,
B. de Kruijff, Phospholipid flop induced by transmembrane peptides
in model membranes is modulated by lipid composition, Biochem-
istry 42 (2003) 231–237.
[25] E.F. Smeets, P. Comfurius, E.M. Bevers, R.F. Zwaal, Calcium-
induced transbilayer scrambling of fluorescent phospholipid analogs
in platelets and erythrocytes, Biochim. Biophys. Acta 1195 (1994)
281–286.
[26] P. Williamson, E.M. Bevers, E.F. Smeets, P. Comfurius, R.A.
Schlegel, R.F. Zwaal, Continuous analysis of the mechanism of
activated transbilayer lipid movement in platelets, Biochemistry 34
(1995) 10448–10455.
[27] P.J. Sims, T. Wiedmer, Unraveling the mysteries of phospholipid
scrambling, Thromb. Haemost. 86 (2001) 266–275.
[28] Z. Zhang, J.W. Nichols, Protein-mediated transfer of fluorescent-
labeled phospholipids across the brush border of rabbit intestine,
Am. J. Physiol. 267 (1994) G80–G86.
[29] M. Seigneuret, P.F. Devaux, ATP-dependent asymmetric distribution
of spin-labeled phospholipids in the erythrocyte membrane: relation
to shape changes, Proc. Natl. Acad. Sci. 81 (1984) 3751–3755.
[30] X. Tang, M.S. Halleck, R.A. Schlegel, P. Williamson, A subfamily of
P-type ATPases with aminophospholipid transporting activity,
Science 272 (1996) 1495–1497.
[31] E. Gomes, M.K. Jakobsen, K.B. Axelsen, M. Geisler, M.G.
Palmgren, Chilling tolerance in Arabidopsis involves ALA1, a
member of a new family of putative aminophospholipid translocases,
Plant Cell 12 (2000) 2441–2454.
[32] T. Pomorski, R. Lombardi, H. Riezman, P.F. Devaux, G. van Meer,
J.C. Holthuis, Drs2p-related P-type ATPases Dnf1p and Dnf2p are
required for phospholipid translocation across the yeast plasma
membrane and serve a role in endocytosis, Mol. Biol. Cell 14 (2003)
1240–1254.
[33] P. Natarajan, J. Wang, Z. Hua, T.R. Graham, Drs2p-coupled
aminophospholipid trasnlocase activity in yeast Golgi membranes
and relationship to in vivo function, Proc. Natl. Acad. Sci. 101
(2004) 10614–10619.
[34] N. K7lin, J. Fernandes, S. Hrafnsdottir, G. van Meer, Natural
phosphatidylcholine is actively translocated across the plasma
membrane to the surface of mammalian cells, J. Biol. Chem. 279
(2004) 33228–33236.
[35] R. Heinrich, M. Brumen, A. Jaeger, P. Mqller, A. Herrmann,
Modelling of phospholipid translocation in the erythrocyte mem-
brane: a combined kinetic and thermodynamic approach, J. Theor.
Biol. 185 (1997) 295–312.
[36] E. Farge, P.F. Devaux, Shape changes of giant liposomes induced by
an asymmetric transmembrane distribution of phospholipids, Bio-
phys. J. 61 (1992) 347–357.
[37] E. Farge, P.F. Devaux, Size-dependent response of liposomes to
phospholipid transmembrane redistribution from shape change to
induced tension, J. Phys. Chem. 97 (1993) 2958–2961.
[38] E. Farge, D.M. Ojcius, A. Subtil, A. Dautry-Varsat, Enhancement
of endocytosis due to aminophospholipid transport across the
plasma membrane of living cells, Am. J. Physiol. 276 (1999)
C725–C733.
[39] P.F. Devaux, Is lipid translocation involved during endo- and
exocytosis? Biochimie 82 (2000) 497–509.
[40] A. Rothnie, D. Theron, L. Soceneantu, C. Martin, M. Traikia, G.
Berridge, C.F. Higgins, P.F. Devaux, R. Callaghan, The impor-
tance of cholesterol in maintenance of P-glycoprotein activity and
its membrane perturbing influence, Eur. Biophys. J. 30 (2001)
430–442.
[41] E.M. Bevers, P. Comfurius, D.W. Dekkers, R.F. Zwaal, Lipid
translocation across the plasma membrane of mammalian cells,
Biochim. Biophys. Acta 1439 (1999) 317–330.
[42] P.F. Devaux, P. Fellmann, P. Herve, Investigation on lipid asymmetry
using lipid probes: comparison between spin-labeled lipids and
fluorescent lipids, Chem. Phys. Lipids 116 (2002) 115–134.
[43] T. Pomorski, A. Herrmann, A. Zachowski, P.F. Devaux, P. Mqller,Rapid determination of the transbilayer distribution of NBD-
phospholipids in erythrocyte membranes with dithionite, Mol.
Membr. Biol. 11 (1994) 39–44.
[44] H.A. Scheidt, P. Mqller, A. Herrmann, D. Huster, The potential of
fluorescent and spin-labeled steroid analogs to mimic natural
cholesterol, J. Biol. Chem. 278 (2003) 45563–45569.
[45] Y. Aoki, T. Uenaka, J. Aoki, M. Umeda, K. Inoue, A novel peptide
probe for studying the transbilayer movement of phosphatidyletha-
nolamine, J. Biochem. (Tokyo) 116 (1994) 291–297.
[46] U. Kato, K. Emoto, C. Fredriksson, H. Nakamura, A. Ohta, T.
Kobayashi, K. Murakami-Murofushi, T. Kobayashi, M. Umeda, A
novel membrane protein, Ros3p, is required for phospholipid
translocation across the plasma membrane in Saccharomyces
cerevisiae, J. Biol. Chem. 277 (2002) 37855–37862.
[47] C.F. Higgins, ABC transporters: from microorganisms to man, Annu.
Rev. Cell Biol. 8 (1992) 67–113.
[48] J. Young, I.B. Holland, ABC transporters: bacterial exporters-revisited
five years on, Biochim. Biophys. Acta 1461 (1999) 177–200.
[49] I. Klein, B. Sarkadi, A. Varadi, An inventory of the human ABC
proteins, Biochim. Biophys. Acta 1461 (1999) 237–262.
[50] A.H. Hosie, D. Allaway, M.A. Jones, D.L. Walshaw, A.W. Johnston,
P.S. Poole, Solute-binding protein-dependent ABC transporters are
responsible for solute efflux in addition to solute uptake, Mol.
Microbiol. 40 (2001) 1449–1459.
[51] M. Mqller, 49 human ATP-binding cassette transporters, 2003, http://
nutrigene.4t.com/humanabc.htm.
[52] T. Langmann, R. Mauerer, A. Zahn, C. Moehle, M. Probst, W.
Stremmel, G. Schmitz, Real-time reverse transcription-PCR expres-
sion profiling of the complete humanATP-binding cassette transporter
superfamily in various tissues, Clin. Chem. 49 (2003) 230–238.
[53] M. Denis, R. Bissonnette, B. Haidar, L. Krimbou, M. Bouvier, J.
Genest, Expression, regulation, and activity of ABCA1 in human cell
lines, Mol. Genet. Metab. 78 (2003) 265–274.
[54] E.B. Neufeld, A.T. Remaley, S.J. Demosky, J.A. Stonik, A.M.
Cooney, M. Comly, N.K. Dwyer, M. Zhang, J. Blanchette-Mackie, S.
Santamarina-Fojo, H.B. Brewer Jr., Cellular localization and
trafficking of the human ABCA1 transporter, J. Biol. Chem. 276
(2001) 27584–27590.
[55] A. Brooks-Wilson, M. Marcil, S.M. Clee, L.H. Zhang, K. Roomp,
M. van Dam, L. Yu, C. Brewer, J.A. Collins, H.O. Molhuizen, O.
Loubser, B.F. Ouelette, K. Fichter, K.J. Ashbourne-Excoffon, C.W.
Sensen, S. Scherer, S. Mott, M. Denis, D. Martindale, J. Frohlich, K.
Morgan, B. Koop, S. Pimstone, J.J. Kastelein, M.R. Hayden, et al.,
A. Pohl et al. / Biochimica et Biophysica Acta 1733 (2005) 29–5248
Mutations in ABC1 in Tangier disease and familial high-density
lipoprotein deficiency, Nat. Genet. 22 (1999) 336–345.
[56] M. Bodzioch, E. Orso, J. Klucken, T. Langmann, A. Bottcher, W.
Diederich, W. Drobnik, S. Barlage, C. Bqchler, M. Porsch-
Ozcurumez, W.E. Kaminski, H.W. Hahmann, K. Oette, G. Rothe,
C. Aslanidis, K.J. Lackner, G. Schmitz, The gene encoding ATP-
binding cassette transporter 1 is mutated in Tangier disease, Nat.
Genet. 22 (1999) 347–351.
[57] S. Rust, M. Rosier, H. Funke, J. Real, Z. Amoura, J.C. Piette, J.F.
Deleuze, H.B. Brewer, N. Duverger, P. Denefle, G. Assmann,
Tangier disease is caused by mutations in the gene encoding ATP-
binding cassette transporter 1, Nat. Genet. 22 (1999) 352–355.
[58] A. von Eckardstein, A. Chirazi, S. Schuler-Luttmann, M. Walter, J.J.
Kastelein, J. Geisel, J.T. Real, R. Miccoli, G. Noseda, G. Hobbel, G.
Assmann, Plasma and fibroblasts of Tangier disease patients are
disturbed in transferring phospholipids onto apoA-I, J. Lipid Res. 39
(1998) 987–998.
[59] J.F. Oram, A.J. Mendez, J. Lymp, T.J. Kavanagh, C.L. Halbert,
Reduction in apolipoprotein-mediated removal of cellular lipids by
immortalization of human fibroblasts and its reversion by cAMP:
lack of effect with Tangier disease cells, J. Lipid Res. 40 (1999)
1769–1781.
[60] N. Wang, D.L. Silver, P. Costet, A.R. Tall, Specific binding of
ApoA-I, enhanced cholesterol efflux, and altered plasma membrane
morphology in cells expressing ABC1, J. Biol. Chem. 275 (2000)
33053–33058.
[61] Y. Hamon, C. Broccardo, O. Chambenoit, M.F. Luciani, F. Toti, S.
Chaslin, J.M. Freyssinet, P.F. Devaux, J. McNeish, D. Marguet, G.
Chimini, ABC1 promotes engulfment of apoptotic cells and trans-
bilayer redistribution of phosphatidylserine, Nat. Cell Biol. 2 (2000)
399–406.
[62] O. Chambenoit, Y. Hamon, D. Marguet, H. Rigneault, M. Rosseneu,
G. Chimini, Specific docking of apolipoprotein A-I at the cell surface
requires a functional ABCA1 transporter, J. Biol. Chem. 276 (2001)
9955–9960.
[63] N. Wang, D.L. Silver, C. Thiele, A.R. Tall, ATP-binding cassette
transporter A1 (ABCA1) functions as a cholesterol efflux regulatory
protein, J. Biol. Chem. 276 (2001) 23742–23747.
[64] J.D. Smith, C. Waelde, A. Horwitz, P. Zheng, Evaluation of the role
of phospholipid translocase activity in ABCA1-mediated efflux, J.
Biol. Chem. 277 (2002) 17797–17803.
[65] N. Wang, D. Lan, M. Gerbod-Giannone, P. Linsel-Nitschke, A.W.
Jehle, W. Chen, L.O. Martinez, A.R. Tall, ATP-binding cassette
transporter A7 (ABCA7) binds apolipoprotein A-I and mediates
cellular phospholipid but not cholesterol efflux, J. Biol. Chem. 278
(2003) 42906–42912.
[66] D. Marguet, M.F. Luciani, A. Moynault, P. Williamson, G. Chimini,
Engulfment of apoptotic cells involves the redistribution of
membrane phosphatidylserine on phagocyte and prey, Nat. Cell
Biol. 1 (1999) 454–456.
[67] B. Vulevic, Z. Chen, J.T. Boyd, W. Davis Jr., E.S. Walsh, M.G.
Belinsky, K.D. Tew, Cloning and characterization of human
adenosine 5V-triphosphate-binding cassette, sub-family A, transporter
2 (ABCA2), Cancer Res. 61 (2001) 3339–3347.
[68] W.E. Kaminski, A. Piehler, K. Pullmann, M. Porsch-Ozcurumez, C.
Duong, G.M. Bared, C. Bqchler, G. Schmitz, Complete coding
sequence, promoter region, and genomic structure of the human
ABCA2 gene and evidence for sterol-dependent regulation in
macrophages, Biochem. Biophys. Res. Commun. 281 (2001)
249–258.
[69] G. Schmitz, W.E. Kaminski, ABCA2: a candidate regulator of
neural transmembrane lipid transport, Cell. Mol. Life Sci. 59 (2002)
1285–1295.
[70] G. Yamano, H. Funahashi, O. Kawanami, L.X. Zhao, N. Ban, Y.
Uchida, T. Morohoshi, J. Ogawa, S. Shioda, N. Inagaki, ABCA3 is a
lamellar body membrane protein in human lung alveolar type II cells,
FEBS Lett. 508 (2001) 221–225.
[71] S. Shulenin, L.M. Nogee, T. Annilo, S.E. Wert, J.A. Whitsett, M.
Dean, ABCA3 gene mutations in newborns with fatal surfactant
deficiency, N. Engl. J. Med. 350 (2004) 1296–1303.
[72] S. Mulugeta, J.M. Gray, K.L. Notarfrancesco, L.W. Gonzales, M.
Koval, S.I. Feinstein, P.L. Ballard, A.B. Fisher, H. Shuman,
Identification of LBM180, a lamellar body limiting membrane
protein of alveolar type II cells, as the ABC transporter protein
ABCA3, J. Biol. Chem. 277 (2002) 22147–22155.
[73] R. Allikmets, N. Singh, H. Sun, N.F. Shroyer, A. Hutchinson, A.
Chidambaram, B. Gerrard, L. Baird, D. Stauffer, A. Peiffer, A.
Rattner, P. Smallwood, Y. Li, K.L. Anderson, R.A. Lewis, J.
Nathans, M. Leppert, M. Dean, J.R. Lupski, A photoreceptor cell-
specific ATP-binding transporter gene (ABCR) is mutated in
recessive Stargardt macular dystrophy, Nat. Genet. 15 (1997)
236–246.
[74] H. Sun, J. Nathans, Stargardt’s ABCR is localized to the disc
membrane of retinal rod outer segments, Nat. Genet. 17 (1997)
15–16.
[75] M. Illing, L.L. Molday, R.S. Molday, The 220-kDa rim protein of
retinal rod outer segment is a member of the ABC transporter
superfamily, J. Biol. Chem. 272 (1997) 10303–10310.
[76] J. Ahn, J.T. Wong, R.S. Molday, The effect of lipid environment and
retinoids on the ATPase activity of ABCR, the photoreceptor ABC
transporter responsible for Stargardt macular dystrophy, J. Biol.
Chem. 275 (2000) 20399–20405.
[77] J. Weng, N.L. Mata, S.M. Azarian, R.T. Tzekov, D.G. Birch, G.H.
Travis, Insights into the function of Rim protein in photoreceptors
and etiology of Stargardt’s disease from the phenotype in abcr
knockout mice, Cell 98 (1999) 13–23.
[78] N.L. Mata, J. Weng, G.H. Travis, Biosynthesis of major
lipofuscin fluorophore in mice and humans with ABCR-mediated
retinal and macular degeneration, Proc. Natl. Acad. Sci. 97 (2000)
7154–7159.
[79] W.E. Kaminski, J.J. Wenzel, A. Piehler, T. Langmann, G. Schmitz,
ABCA6, a novel A subclass ABC transporter, Biochem. Biophys.
Res. Commun. 285 (2001) 1295–1301.
[80] Y. Ikeda, S. Abe-Dohmae, Y. Munehira, R. Aoki, S. Kawamoto, A.
Furuya, K. Shitara, T. Amachi, N. Kioka, M. Matsuo, S. Yokoyama,
K. Ueda, Posttranscriptional regulation of human ABCA7 and its
function for the apoA-I-dependent lipid release, Biochem. Biophys.
Res. Commun. 311 (2003) 313–318.
[81] C. Broccardo, J. Osorio, M.F. Luciani, L.M. Schriml, C. Prades, S.
Shulenin, I. Arnould, L. Naudin, C. Lafargue, M. Rosier, B. Jordan,
M.G. Mattei, M. Dean, P. Denefle, G. Chimini, Comparative analysis
of the promoter structure and genomic organization of the human and
mouse ABCA7 gene encoding a novel ABCA transporter, Cytoge-
net. Cell Genet. 92 (2001) 264–270.
[82] W.E. Kaminski, E. Orso, W. Diederich, J. Klucken, W. Drobnik, G.
Schmitz, Identification of a novel human sterol-sensitive ATP-
binding cassette transporter (ABCA7), Biochem. Biophys. Res.
Commun. 273 (2000) 532–538.
[83] D. Kielar, W.E. Kaminski, G. Liebisch, A. Piehler, J.J. Wenzel, C.
Mohle, S. Heimerl, T. Langmann, S.O. Friedrich, A. Bottcher, S.
Barlage, W. Drobnik, G. Schmitz, Adenosine triphosphate binding
cassette (ABC) transporters are expressed and regulated during
terminal keratinocyte differentiation: a potential role for ABCA7 in
epidermal lipid reorganization, J. Invest. Dermatol. 121 (2003)
465–474.
[84] S. Abe-Dohmae, Y. Ikeda, M. Matsuo, M. Hayashi, K. Okuhira, K.
Ueda, S. Yokoyama, Human ABCA7 supports apolipoprotein-
mediated release of cellular cholesterol and phospholipid to
generate high density lipoprotein, J. Biol. Chem. 279 (2004)
604–611.
[85] A. Piehler, W.E. Kaminski, J.J. Wenzel, T. Langmann, G. Schmitz,
Molecular structure of a novel cholesterol-responsive A subclass ABC
transporter, ABCA9, Biochem. Biophys. Res. Commun. 295 (2002)
408–416.
A. Pohl et al. / Biochimica et Biophysica Acta 1733 (2005) 29–52 49
[86] J.J. Wenzel, W.E. Kaminski, A. Piehler, S. Heimerl, T. Langmann,
G. Schmitz, ABCA10, a novel cholesterol-regulated ABCA6-like
ABC transporter, Biochem. Biophys. Res. Commun. 306 (2003)
1089–1098.
[87] R.L. Juliano, V. Ling, A surface glycoprotein modulating drug
permeability in Chinese hamster ovary cell mutants, Biochim.
Biophys. Acta 455 (1976) 152–162.
[88] T.W. Loo, D.M. Clarke, The minimum functional unit of human P-
glycoprotein appears to be a monomer, J. Biol. Chem. 271 (1996)
27488–27492.
[89] F. Thiebaut, T. Tsuruo, H. Hamada, M.M. Gottesman, I. Pastan, M.C.
Willingham, Cellular localization of the multidrug-resistance gene
product P-glycoprotein in normal human tissues, Proc. Natl. Acad.
Sci. 84 (1987) 7735–7738.
[90] P. Borst, A.H. Schinkel, J.J. Smit, E. Wagenaar, L. van Deemter, A.J.
Smith, E.W. Eijdems, F. Baas, G.J. Zaman, Classical and novel forms
of multidrug resistance and the physiological functions of P-
glycoproteins in mammals, Pharmacol. Ther. 60 (1993) 289–299.
[91] C. Cordon-Cardo, J.P. O’Brien, J. Boccia, D. Casals, J.R. Bertino,
M.R. Melamed, Expression of the multidrug resistance gene product
( P-glycoprotein) in human normal and tumor tissues, J. Histochem.
Cytochem. 38 (1990) 1277–1287.
[92] J.M. Ford, W.N. Hait, Pharmacology of drugs that alter multidrug
resistance in cancer, Pharmacol. Rev. 42 (1990) 155–199.
[93] E. Bello-Reuss, S. Ernest, O.B. Holland, M.R. Hellmich, Role of
multidrug resistance P-glycoprotein in the secretion of aldosterone
by human adrenal NCI-H295 cells, Am. J. Physiol., Cell Physiol.
278 (2000) C1256–C1265.
[94] G.J. Randolph, S. Beaulieu, M. Pope, I. Sugawara, L. Hoffman, R.M.
Steinman, W.A. Muller, A physiologic function for P-glycoprotein
(MDR-1) during the migration of dendritic cells from skin via afferent
lymphatic vessels, Proc. Natl. Acad. Sci. 95 (1998) 6924–6929.
[95] K. Ueda, N. Okamura, M. Hirai, Y. Tanigawara, T. Saeki, N. Kioka,
T. Komano, R. Hori, Human P-glycoprotein transports cortisol,
aldosterone, and dexamethasone, but not progesterone, J. Biol.
Chem. 267 (1992) 24248–24252.
[96] E. Wang, C.N. Casciano, R.P. Clement, W.W. Johnson, Cholesterol
interaction with the daunorubicin binding site of P-glycoprotein,
Biochem. Biophys. Res. Commun. 276 (2000) 909–916.
[97] A. van Helvoort, A. Smith, H. Sprong, I. Fritzsche, A.H. Schinkel, P.
Borst, G. van Meer, MDR1 P-glycoprotein is a lipid translocase of
broad specificity, while MDR3 P-glycoprotein specifically trans-
locates phosphatidylcholine, Cell 87 (1996) 507–517.
[98] I. Bosch, K. Dunussi-Joannopoulos, R. Wu, S. Furlong, J. Croop,
Phosphatidylcholine and phosphatidylethanolamine behave as sub-
strates of the human MDR1 P-glycoprotein, Biochemistry 36 (1997)
5685–5694.
[99] A. Pohl, H. Lage, P. Mqller, T. Pomorski, A. Herrmann, Transport of
phosphatidylserine via MDR1 (multidrug resistance 1) P-glycopro-
tein in a human gastric carcinoma cell line, Biochem. J. 365 (2002)
259–268.
[100] S. Ernest, E. Bello-Reuss, Secretion of platelet-activating factor is
mediated by MDR1 P-glycoprotein in cultured human mesangial
cells, J. Am. Soc. Nephrol. 10 (1999) 2306–2313.
[101] R.J. Raggers, T. Pomorski, J.C. Holthuis, N. Kalin, G. van Meer,
Lipid traffic: the ABC of transbilayer movement, Traffic 1 (2000)
226–234.
[102] C. Bezombes, N. Maestre, G. Laurent, T. Levade, A. Bettaieb, J.P.
Jaffrezou, Restoration of TNF-alpha-induced ceramide generation
and apoptosis in resistant human leukemia KG1a cells by the P-
glycoprotein blocker PSC833, FASEB J. 12 (1998) 101–109.
[103] J. Smit, A. Schinkel, R. Oude Elferink, A. Groen, E. Wagenaar, L.
van Deemter, C. Mol, R. Ottenhoff, N. van der Lugt, M. van Roon,
M. van der Valk, G. Offerhaus, A. Berns, P. Borst, Homozygous
disruption of the murine mdr2 P-glycoprotein gene leads to a
complete absence of phospholipid from bile and to liver disease, Cell
75 (1993) 451–462.
[104] Y. Romsicki, F.J. Sharom, Phospholipid flippase activity of the
reconstituted P-glycoprotein multidrug transporter, Biochemistry 40
(2001) 6937–6947.
[105] P.F. Devaux, Reconstitution of flippase activity into liposomes, Cell.
Mol. Biol. Lett. 7 (2002) 227–229.
[106] A. Smith, J. Timmermans-Hereijgers, B. Roelofsen, K. Wirtz, W. van
Blitterswijk, J. Smit, A. Schinkel, P. Borst, The human MDR3 P-
glycoprotein promotes translocation of phosphatidylcholine through
the plasma membrane of fibroblasts from transgenic mice, FEBS
Lett. 354 (1994) 263–266.
[107] J.F. Deleuze, E. Jacquemin, C. Dubuisson, D. Cresteil, M. Dumont,
S. Erlinger, O. Bernard, M. Hadchouel, Defect of multidrug-
resistance 3 gene expression in a subtype of progressive familial
intrahepatic cholestasis, Hepatology 23 (1996) 904–908.
[108] P. Borst, R. Evers, M. Kool, J. Wijnholds, A family of drug
transporters: the multidrug resistance-associated proteins, J. Natl.
Cancer Inst. 92 (2000) 1295–1302.
[109] P. Borst, R.O. Elferink, Mammalian ABC transporters in health and
disease, Annu. Rev. Biochem. 71 (2002) 537–592.
[110] S.P. Cole, G. Bhardwaj, J.H. Gerlach, J.E. Mackie, C.E. Grant, K.C.
Almquist, A.J. Stewart, E.U. Kurz, A.M. Duncan, R.G. Deeley,
Overexpression of a transporter gene in a multidrug-resistant human
lung cancer cell line, Science 258 (1992) 1650–1654.
[111] R. Evers, G.J. Zaman, L. van Deemter, H. Jansen, J. Calafat, L.C.
Oomen, R.P. Oude Elferink, P. Borst, A.H. Schinkel, Basolateral
localization and export activity of the human multidrug resistance-
associated protein in polarized pig kidney cells, J. Clin. Invest. 97
(1996) 1211–1218.
[112] J. Wijnholds, Drug resistance caused by multidrug resistance-
associated proteins, Novartis Found. Symp. 243 (2002) 69–79.
[113] J. Wijnholds, E.C. deLange, G.L. Scheffer, D.J. van den Berg, C.A.
Mol, M. van der Valk, A.H. Schinkel, R.J. Scheper, D.D. Breimer, P.
Borst, Multidrug resistance protein 1 protects the choroid plexus
epithelium and contributes to the blood-cerebrospinal fluid barrier, J.
Clin. Invest. 105 (2000) 279–285.
[114] I. Leier, G. Jedlitschky, U. Buchholz, S.P. Cole, R.G. Deeley, D.
Keppler, The MRP gene encodes an ATP-dependent export pump for
leukotriene C4 and structurally related conjugates, J. Biol. Chem.
269 (1994) 27807–27810.
[115] R.J. Raggers, A. van Helvoort, R. Evers, G. van Meer, The human
multidrug resistance protein MRP1 translocates sphingolipid analogs
across the plasma membrane, J. Cell. Sci. 112 (1999) 415–422.
[116] D. Kamp, C.W. Haest, Evidence for a role of the multidrug resistance
protein (MRP) in the outward translocation of NBD-phospholipids in
the erythrocyte membrane, Biochim. Biophys. Acta 1372 (1998)
91–101.
[117] Z. Huang, X. Chang, J.R. Riordan, Y. Huang, Fluorescent modified
phosphatidylcholine floppase activity of reconstituted multidrug
resistance-associated protein MRP1, Biochim. Biophys. Acta 1660
(2004) 155–163.
[118] G.L. Scheffer, X. Hu, A.C. Pijnenborg, J. Wijnholds, A.A. Bergen,
R.J. Scheper, MRP6 (ABCC6) detection in normal human tissues
and tumors, Lab. Invest. 82 (2002) 515–518.
[119] A.A. Bergen, A.S. Plomp, E.J. Schuurman, S. Terry, M. Breuning, H.
Dauwerse, J. Swart, M. Kool, S. van Soest, F. Baas, J.B. ten Brink,
P.T. de Jong, Mutations in ABCC6 cause pseudoxanthoma elasticum,
Nat. Genet. 25 (2000) 228–231.
[120] O. Le Saux, Z. Urban, C. Tschuch, K. Csiszar, B. Bacchelli, D.
Quaglino, I. Pasquali-Ronchetti, F.M. Pope, A. Richards, S. Terry, L.
Bercovitch, A. de Paepe, C.D. Boyd, Mutations in a gene encoding
an ABC transporter cause pseudoxanthoma elasticum, Nat. Genet. 25
(2000) 223–227.
[121] J. Wang, S. Near, K. Young, P.W. Connelly, R.A. Hegele, ABCC6
gene polymorphism associated with variation in plasma lipoproteins,
J. Hum. Genet. 46 (2001) 699–705.
[122] F. Ringpfeil, L. Pulkkinen, J. Uitto, Molecular genetics of
pseudoxanthoma elasticum, Exp. Dermatol. 10 (2001) 221–228.
A. Pohl et al. / Biochimica et Biophysica Acta 1733 (2005) 29–5250
[123] A. Ilias, Z. Urban, T.L. Seidl, O. Le Saux, E. Sinko, C.D. Boyd, B.
Sarkadi, A. Varadi, Loss of ATP-dependent transport activity in
pseudoxanthoma elasticum-associated mutants of human ABCC6
(MRP6), J. Biol. Chem. 277 (2002) 16860–16867.
[124] M.G. Belinsky, Z.S. Chen, I. Shchaveleva, H. Zeng, G.D. Kruh,
Characterization of the drug resistance and transport properties of
multidrug resistance protein 6 (MRP6, ABCC6), Cancer Res. 62
(2002) 6172–6177.
[125] I. Singh, O. Lazo, G.S. Dhaunsi, M. Contreras, Transport of fatty
acids into human and rat peroxisomes. Differential transport of
palmitic and lignoceric acids and its implication to X-adrenoleuko-
dystrophy, J. Biol. Chem. 267 (1992) 13306–13313.
[126] L.X. Liu, K. Janvier, V. Berteaux-Lecellier, N. Cartier, R. Benarous,
P. Aubourg, Homo- and heterodimerization of peroxisomal ATP-
binding cassette half-transporters, J. Biol. Chem. 274 (1999)
32738–32743.
[127] G. Lombard-Platet, S. Savary, C.O. Sarde, J.L. Mandel, G. Chimini,
A close relative of the adrenoleukodystrophy (ALD) gene codes for a
peroxisomal protein with a specific expression pattern, Proc. Natl.
Acad. Sci. 93 (1996) 1265–1269.
[128] J. Berger, S. Albet, M. Bentejac, A. Netik, A. Holzinger, A.A.
Roscher, M. Bugaut, S. Forss-Petter, The four murine peroxisomal
ABC-transporter genes differ in constitutive, inducible and devel-
opmental expression, Eur. J. Biochem. 265 (1999) 719–727.
[129] J. Mosser, A.M. Douar, C.O. Sarde, P. Kioschis, R. Feil, H. Moser,
A.M. Poustka, J.L. Mandel, P. Aubourg, Putative X-linked adreno-
leukodystrophy gene shares unexpected homology with ABC trans-
porters, Nature 361 (1993) 726–730.
[130] M.C. McGuinness, H.P. Zhang, K.D. Smith, Evaluation of pharma-
cological induction of fatty acid beta-oxidation in X-linked
adrenoleukodystrophy, Mol. Genet. Metab. 74 (2001) 256–263.
[131] N. Shinnoh, T. Yamada, T. Yoshimura, H. Furuya, Y. Yoshida, Y.
Suzuki, N. Shimozawa, T. Orii, T. Kobayashi, Adrenoleukodystrophy:
the restoration of peroxisomal beta-oxidation by transfection of normal
cDNA, Biochem. Biophys. Res. Commun. 210 (1995) 830–836.
[132] M.C. McGuinness, J.F. Lu, H.P. Zhang, G.X. Dong, A.K. Heinzer,
P.A. Watkins, J. Powers, K.D. Smith, Role of ALDP (ABCD1) and
mitochondria in X-linked adrenoleukodystrophy, Mol. Cell. Biol. 23
(2003) 744–753.
[133] L.T. Braiterman, S. Zheng, P.A. Watkins, M.T. Geraghty, G. Johnson,
M.C. McGuinness, A.B. Moser, K.D. Smith, Suppression of
peroxisomal membrane protein defects by peroxisomal ATP binding
cassette (ABC) proteins, Hum. Mol. Genet. 7 (1998) 239–247.
[134] A. Netik, S. Forss-Petter, A. Holzinger, B. Molzer, G. Unterrainer, J.
Berger, Adrenoleukodystrophy-related protein can compensate func-
tionally for adrenoleukodystrophy protein deficiency (X-ALD):
implications for therapy, Hum. Mol. Genet. 8 (1999) 907–913.
[135] S. Kemp, H.M. Wei, J.F. Lu, L.T. Braiterman, M.C. McGuinness,
A.B. Moser, P.A. Watkins, K.D. Smith, Gene redundancy and
pharmacological gene therapy: implications for X-linked adrenoleu-
kodystrophy, Nat. Med. 4 (1998) 1261–1268.
[136] I. Weinhofer, S. Forss-Petter, M. Zigman, J. Berger, Cholesterol
regulates ABCD2 expression: implications for the therapy of X-linked
adrenoleukodystrophy, Hum. Mol. Genet. 11 (2002) 2701–2708.
[137] N. Shani, G. Jimenez-Sanchez, G. Steel, M. Dean, D. Valle,
Identification of a fourth half ABC transporter in the human
peroxisomal membrane, Hum. Mol. Genet. 6 (1997) 1925–1931.
[138] G. Schmitz, T. Langmann, S. Heimerl, Role of ABCG1 and other
ABCG family members in lipid metabolism, J. Lipid Res. 42 (2001)
1513–1520.
[139] H. Chen, C. Rossier, M.D. Lalioti, A. Lynn, A. Chakravarti, G.
Perrin, S.E. Antonarakis, Cloning of the cDNA for a human
homologue of the Drosophila white gene and mapping to
chromosome 21q22.3, Am. J. Hum. Genet. 59 (1996) 66–75.
[140] J. Klucken, C. Bqchler, E. Orso, W.E. Kaminski, M. Porsch-
Ozcurumez, G. Liebisch, M. Kapinsky, W. Diederich, W. Drobnik,
M. Dean, R. Allikmets, G. Schmitz, ABCG1 (ABC8), the human
homolog of the Drosophila white gene, is a regulator of macrophage
cholesterol and phospholipid transport, Proc. Natl. Acad. Sci. 97
(2002) 817–822.
[141] G.D. Ewart, D. Cannell, G.B. Cox, A.J. Howells, Mutational
analysis of the traffic ATPase (ABC) transporters involved in uptake
of eye pigment precursors in Drosophila melanogaster. Implications
for structure–function relationships, J. Biol. Chem. 269 (1994)
10370–10377.
[142] R. Allikmets, L.M. Schriml, A. Hutchinson, V. Romano-Spica, M.
Dean, A human placenta-specific ATP-binding cassette gene (ABCP)
on chromosome 4q22 that is involved in multidrug resistance,
Cancer Res. 58 (1998) 5337–5339.
[143] M. Maliepaard, G.L. Scheffer, I.F. Faneyte, M.A. van Gastelen, A.C.
Pijnenborg, A.H. Schinkel, M.J. van De Vijver, R.J. Scheper, J.H.
Schellens, Subcellular localization and distribution of the breast
cancer resistance protein transporter in normal human tissues, Cancer
Res. 61 (2001) 3458–3464.
[144] J. Xu, Y. Liu, Y. Yang, S. Bates, J.T. Zhang, Characterization of
oligomeric human half-ABC transporter ATP-binding cassette G2, J.
Biol. Chem. 279 (2004) 19781–19789.
[145] S.E. Bates, R. Robey, K. Miyake, K. Rao, D.D. Ross, T. Litman, The
role of half-transporters in multidrug resistance, J. Bioenerg.
Biomembranes 33 (2001) 503–511.
[146] T. Litman, M. Brangi, E. Hudson, P. Fetsch, A. Abati, D.D. Ross, K.
Miyake, J.H. Resau, S.E. Bates, The multidrug-resistant phenotype
associated with overexpression of the new ABC half-transporter,
MXR (ABCG2), J. Cell. Sci. 113 (2000) 2011–2021.
[147] L.A. Doyle, W. Yang, L.V. Abruzzo, T. Krogmann, Y. Gao, A.K.
Rishi, D.D. Ross, A multidrug resistance transporter from human
MCF-7 breast cancer cells, Proc. Natl. Acad. Sci. 95 (1998)
15665–15670.
[148] H. Woehlecke, A. Pohl, N. Alder-Baerens, H. Lage, A. Herrmann,
Enhanced exposure of phosphatidylserine in human gastric carci-
noma cells overexpressing the half-size ABC transporter BCRP
(ABCG2), Biochem. J. 376 (2003) 489–495.
[149] T. Janvilisri, H. Venter, S. Shahi, G. Reuter, L. Balakrishnan, H.W.
van Veen, Sterol transport by the human breast cancer resistance
protein (ABCG2) expressed in Lactococcus lactis, J. Biol. Chem.
278 (2003) 20645–20651.
[150] K.E. Berge, H. Tian, G.A. Graf, L. Yu, N.V. Grishin, J. Schultz, P.
Kwiterovich, B. Shan, R. Barnes, H.H. Hobbs, Accumulation of
dietary cholesterol in sitosterolemia caused by mutations in adjacent
ABC transporters, Science 290 (2000) 1771–1775.
[151] J.J. Repa, K.E. Berge, C. Pomajzl, J.A. Richardson, H. Hobbs, D.J.
Mangelsdorf, Regulation of ATP-binding cassette sterol transporters
ABCG5 and ABCG8 by the liver X receptors alpha and beta, J. Biol.
Chem. 277 (2002) 18793–18800.
[152] G.A. Graf, W.P. Li, R.D. Gerard, I. Gelissen, A. White, J.C. Cohen,
H.H. Hobbs, Coexpression of ATP-binding cassette proteins ABCG5
and ABCG8 permits their transport to the apical surface, J. Clin.
Invest. 110 (2002) 659–669.
[153] G.A. Graf, L. Yu, W.P. Li, R. Gerard, P.L. Tuma, J.C. Cohen, H.H.
Hobbs, ABCG5 and ABCG8 are obligate heterodimers for protein
trafficking and biliary cholesterol excretion, J. Biol. Chem. 278
(2003) 48275–48282.
[154] L. Yu, R.E. Hammer, J. Li-Hawkins, K. von Bergmann, D.
Lutjohann, J.C. Cohen, H.H. Hobbs, Disruption of Abcg5 and
Abcg8 in mice reveals their crucial role in biliary cholesterol
secretion, Proc. Natl. Acad. Sci. 99 (2002) 16237–16242.
[155] M.H. Lee, K. Lu, S. Hazard, H. Yu, H.S. Shulenin, H. Hidaka, H.
Kojima, R. Allikmets, N. Sakuma, R. Pegoraro, A.K. Srivastava, G.
Salen, M. Dean, S. Patel, Identification of a gene, ABCG5, important
in the regulation of dietary cholesterol absorption, Nat. Genet. 27
(2001) 79–83.
[156] S. Shulenin, L.M. Schriml, A.T. Remaley, S. Fojo, B. Brewer, R.
Allikmets, M. Dean, A liver-specific ATP-binding cassette gene
(ABCG5) from the ABCG (White) gene subfamily maps to human
A. Pohl et al. / Biochimica et Biophysica Acta 1733 (2005) 29–52 51
chromosome 2p21 in the region of the Sitosterolemia locus,
Cytogenet. Cell Genet. 92 (2001) 204–208.
[157] L. Yu, J. Li-Hawkins, R.E. Hammer, K.E. Berge, J.D. Horton, J.C.
Cohen, H.H. Hobbs, Overexpression of ABCG5 and ABCG8
promotes biliary cholesterol secretion and reduces fractional
absorption of dietary cholesterol, J. Clin. Invest. 110 (2002)
671–680.
[158] C. Albrecht, J.I. Elliott, A. Sardini, T. Litman, B. Stieger, P.J. Meier,
C.F. Higgins, Functional analysis of candidate ABC transporter
proteins for sitosterol transport, Biochim. Biophys. Acta 1567 (2003)
133–142.
[159] L. Yu, J. York, K. von Bergmann, D. Lutjohann, J.C. Cohen, H.H.
Hobbs, Stimulation of cholesterol excretion by the liver X receptor
agonist requires ATP-binding cassette transporters G5 and G8, J.
Biol. Chem. 278 (2003) 15565–15570.
[160] K.J. Mengerink, V.D. Vacquier, An ATP-binding cassette transporter
is a major glycoprotein of sea urchin sperm membranes, J. Biol.
Chem. 277 (2002) 40729–40734.
[161] J.M. Perez-Victoria, A. Parodi-Talice, C. Torres, F. Gamarro, S.
Castany, ABC transporters in the protozoan parasite Leishmania, Int.
Microbiol. 4 (2001) 159–166.
[162] D. Legare, S. Cayer, A.K. Singh, D. Richard, B. Papadopoulou, M.
Ouellette, ABC proteins of Leishmania, J. Bioenerg. Biomembranes
33 (2001) 469–474.
[163] A. Parodi-Talice, J.M. Araujo, C. Torres, J.M. Perez-Victoria, F.
Gamarro, S. Castanys, The overexpression of a new ABC transporter
in Leishmania is related to phospholipid trafficking and reduced
infectivity, Biochim. Biophys. Acta 1612 (2003) 195–207.
[164] D.M. Henderson, C.D. Sifri, M. Rodgers, D.F. Wirth, N. Hendrick-
son, B. Ullman, Multidrug resistance in Leishmania donovani is
conferred by amplification of a gene homologous to the mammalian
mdr1 gene, Mol. Cell. Biol. 12 (1992) 2855–2865.
[165] P.W. Denny, D. Goulding, M.A. Ferguson, D.F. Smith, Sphingolipid-
free Leishmania are defective in membrane trafficking, differ-
entiation and infectivity, Mol. Microbiol. 52 (2004) 313–327.
[166] J.M. Perez-Victoria, F.J. Perez-Victoria, A. Parodi-Talice, I.A.
Jimenez, A.G. Ravelo, S. Castany, F. Gamarro, Alkyl-lysophospho-
lipid resistance in multidrug-resistant Leishmania tropica and chemo-
sensitization by a novel P-glycoprotein-like transporter modulator,
Antimicrob. Agents Chemother. 45 (2001) 2468–2474.
[167] R. Sanchez-Fernandez, T.G. Davies, J.O. Coleman, P.A. Rea, The
Arabidopsis thaliana ABC protein superfamily, a complete inventory,
J. Biol. Chem. 276 (2001) 30231–30244.
[168] M. Hayashi, K. Nito, R. Takei-Hoshi, M. Yagi, M. Kondo, A.
Suenaga, T. Yamaya, M. Nishimura, Ped3p is a peroxisomal ATP-
binding cassette transporter that might supply substrates for fatty acid
beta-oxidation, Plant Cell Physiol. 43 (2002) 1–11.
[169] S. Footitt, S.P. Slocombe, V. Larner, S. Kurup, Y. Wu, T. Larson, I.
Graham, A. Baker, M. Holdsworth, Control of germination and lipid
mobilization by COMATOSE, the Arabidopsis homologue of human
ALDP, EMBO J. 21 (2002) 2912–2922.
[170] B.K. Zolman, I.D. Silva, B. Bartel, The Arabidopsis pxa1 mutant is
defective in an ATP-binding cassette transporter-like protein required
for peroxisomal fatty acid beta-oxidation, Plant Physiol. 127 (2001)
1266–1278.
[171] A. Decottignies, A. Goffeau, Complete inventory of the yeast ABC
proteins, Nat. Genet. 15 (1997) 137–145.
[172] A. Decottignies, M. Kolaczkowski, E. Balzi, A. Goffeau, Solubili-
zation and characterization of the overexpressed PDR5 multidrug
resistance nucleotide triphosphatase of yeast, J. Biol. Chem. 269
(1994) 12797–12803.
[173] M. Kolaczkowski, M. van der Rest, A. Cybularz-Kolaczkowska, J.P.
Soumillion, W.N. Konings, A. Goffeau, Anticancer drugs, iono-
phoric peptides, and steroids as substrates of the yeast multidrug
transporter Pdr5p, J. Biol. Chem. 271 (1996) 31543–31548.
[174] A. Decottignies, A.M. Grant, J.W. Nichols, H. de Wet, D.B.
McIntosh, A. Goffeau, ATPase and multidrug transport activities
of the overexpressed yeast ABC protein Yor1p, J. Biol. Chem. 273
(1998) 12612–12622.
[175] D. Sanglard, F. Ischer, M. Monod, J. Bille, Cloning of Candida
albicans genes conferring resistance to azole antifungal agents:
characterization of CDR2, a new multidrug ABC transporter gene,
Microbiology 143 (1997) 405–416.
[176] I. Balan, A.M. Alarco, M. Raymond, The bZip transcription
factor Cap1p is involved in multidrug resistance and oxidative
stress response in Candida albicans, J. Bacteriol. 179 (1997)
7210–7218.
[177] S. Dogra, S. Krishnamurthy, V. Gupta, B.L. Dixit, C.M. Gupta, D.
Sanglard, R. Prasad, Asymmetric distribution of phosphatidyletha-
nolamine in C. albicans: possible mediation by CDR1, a multidrug
transporter belonging to ATP binding cassette (ABC) superfamily,
Yeast 15 (1999) 111–121.
[178] Smriti, S. Krishnamurthy, B.L. Dixit, C.M. Gupta, S. Milewski, R.
Prasad, ABC transporters Cdr1p, Cdr2p and Cdr3p of a human
pathogen Candida albicans are general phospholipid translocators,
Yeast 19 (2002) 303–318.
[179] B.L. Browne, V. McClendon, D.M. Bedwell, Mutations within the
first LSGGQ motif of Ste6p cause defects in a-factor transport and
mating in Saccharomyces cerevisiae, J. Bacteriol. 178 (1996)
1712–1719.
[180] S. Ruetz, M. Brault, W. Dalton, P. Gros, Functional interactions
between synthetic alkyl phospholipids and the ABC transporters P-
glycoprotein, Ste-6, MRP, and Pgh 1, Biochemistry 36 (1997)
8180–8188.
[181] S. Ruetz, M. Brault, W. Dalton, P. Gros, Functional interactions
between synthetic alkyl phospholipids and the ABC transporters P-
glycoprotein, ste-6, MRP, and pgh-1, Biochemistry 38 (1999) 2860.
[182] D.J. Katzmann, E.A. Epping, W.S. Moye-Rowley, Mutational
disruption of plasma membrane trafficking of Saccharomyces
cerevisiae Yor1p, a homologue of mammalian multidrug resistance
protein, Mol. Cell. Biol. 19 (1999) 2998–3009.
[183] Z. Cui, D. Hirata, E. Tsuchiya, H. Osada, T. Miyakawa, The
multidrug resistance-associated protein (MRP) subfamily (Yrs1/
Yor1) of Saccharomyces cerevisiae is important for the tolerance
to a broad range of organic anions, J. Biol. Chem. 271 (1996)
14712–14716.
[184] E.H. Hettema, C.W. van Roermund, B. Distel, M. van den Berg, C.
Vilela, C. Rodrigues-Pousada, R.J. Wanders, H.F. Tabak, The ABC
transporter proteins Pat1 and Pat2 are required for import of long-
chain fatty acids into peroxisomes of Saccharomyces cerevisiae,
EMBO J. 15 (1996) 3813–3822.
[185] N. Shani, D. Valle, A Saccharomyces cerevisiae homolog of the
human adrenoleukodystrophy transporter is a heterodimer of two
half ATP-binding cassette transporters, Proc. Natl. Acad. Sci. 93
(1996) 11901–11906.
[186] M. Karow, C. Georgopoulos, The essential Escherichia coli msbA
gene, a multicopy suppressor of null mutations in the htrB gene, is
related to the universally conserved family of ATP-dependent
translocators, Mol. Microbiol. 7 (1993) 69–79.
[187] Z. Zhou, K.A. White, A. Polissi, C. Georgopoulos, C.R. Raetz,
Function of Escherichia coli MsbA, an essential ABC family
transporter, in lipid A and phospholipid biosynthesis, J. Biol. Chem.
273 (1998) 12466–12475.
[188] G. Chang, C.B. Roth, Structure of MsbA from E. coli: a homolog of
the multidrug resistance ATP binding cassette (ABC) transporters,
Science 293 (2001) 1793–1800.
[189] H.W. van Veen, R. Callaghan, L. Soceneantu, A. Sardini, W.N.
Konings, C.F. Higgins, A bacterial antibiotic-resistance gene that
complements the human multidrug-resistance P-glycoprotein gene,
Nature 391 (1998) 291–295.
[190] G. Reuter, T. Janvilisri, H. Venter, S. Shahi, L. Balakrishnan, H.W.
van Veen, The ATP binding cassette multidrug transporter LmrA and
lipid transporter MsbA have overlapping substrate specificities, J.
Biol. Chem. 278 (2003) 35193–35198.
A. Pohl et al. / Biochimica et Biophysica Acta 1733 (2005) 29–5252
[191] G. Chang, Multidrug resistance ABC transporters, FEBS Lett. 555
(2003) 102–105.
[192] W.T. Doerrler, M.C. Reedy, C.R. Raetz, An Escherichia coli
mutant defective in lipid export, J. Biol. Chem. 276 (2001)
11461–11464.
[193] W.T. Doerrler, C.R. Raetz, ATPase activity of the MsbA lipid flippase
of Escherichia coli, J. Biol. Chem. 277 (2002) 36697–36705.
[194] W.T. Doerrler, H.S. Gibbons, C.R. Raetz, MsbA-dependent trans-
location of lipids across the inner membrane of Escherichia coli, J.
Biol. Chem. 279 (2004) 45102–45109.
[195] G.J. Poelarends, P. Mazurkiewicz, W.N. Konings, Multidrug trans-
porters and antibiotic resistance in Lactococcus lactis, Biochim.
Biophys. Acta 1555 (2002) 1–7.
[196] H.W. van Veen, K. Venema, H. Bolhuis, I. Oussenko, J. Kok, B.
Poolman, A.J.M. Driessen, W.N. Konings, Multidrug resistance
mediated by a bacterial homolog of the human multidrug transporter
MDR1, Proc. Natl. Acad. Sci. 93 (1996) 10668–10672.
[197] A. Margolles, M. Putman, H.W. van Veen, W.N. Konings, The
purified and functionally reconstituted multidrug transporter LmrA
of Lactococcus lactis mediates the transbilayer movement of specific
fluorescent phospholipids, Biochemistry 38 (1999) 16298–16306.
[198] R.P.H. Huijbregts, A.I.P.M. de Kroon, B. de Kruijff, Rapid
transmembrane movement of C6-NBD-labeled phospholipids across
the inner membrane of Escherichia coli, Biochim. Biophys. Acta
1280 (1996) 41–50.
[199] R.P.H. Huijbregts, A.I.P.M. de Kroon, B. de Kruijff, Rapid
transmembrane movement of newly synthesized phosphatidyletha-
nolamine across the inner membrane of Escherichia coli, J. Biol.
Chem. 273 (1998) 18936–18942.
[200] J. Kubelt, A.K. Menon, P. Mqller, A. Herrmann, Transbilayer
movement of fluorescent phospholipid analogues in the cytoplasmic
membrane of Escherichia coli, Biochemistry 41 (2002) 5605–5612.
[201] M.K. McDonald, S.C. Cowley, F.E. Nano, Temperature-sensitive
lesions in the Francisella novicida valA gene cloned into an
Escherichia coli msbA lpxK mutant affecting deoxycholate resist-
ance and lipopolysaccharide assembly at the restrictive temperature,
J. Bacteriol. 179 (1997) 7638–7643.
[202] L.S. Kean, A.M. Grant, C. Angeletti, Y. Mahe, K. Kuchler, R.S.
Fuller, J.W. Nichols, Plasma membrane translocation of fluo-
rescent-labeled phosphatidylethanolamine is controlled by tran-
scription regulators, PDR1 and PDR3, J. Cell Biol. 138 (1997)
255–270.
[203] X. Zha, J. Genest Jr., R. McPherson, Endocytosis is enhanced in
Tangier fibroblasts: possible role of ATP-binding cassette protein A1
in endosomal vesicular transport, J. Biol. Chem. 276 (2001)
39476–39483.
[204] J.A. Hamilton, Fast flip-flop of cholesterol and fatty acids in
membranes: implications for membrane transport proteins, Curr.
Opin. Lipidol. 14 (2003) 263–271.
[205] D.M. Small, Role of ABC transporters in secretion of cholesterol
from liver into bile, Proc. Natl. Acad. Sci. 100 (2003) 4–6.
[206] K.P. Locher, A.T. Lee, D.C. Rees, The E. coli BtuCD structure: a
framework for ABC transporter architecture and mechanism, Science
296 (2002) 1091–1098.
[207] C.F. Higgins, K.J. Linton, The xyz of ABC transporters, Science 293
(2001) 1782–1784.
[208] G. Chang, Structure of MsbA from Vibrio cholera: a multidrug
resistance ABC transporter homolog in a closed conformation, J.
Mol. Biol. 330 (2003) 419–430.
[209] M.F. Rosenberg, A.B. Kamis, R. Callaghan, C.F. Higgins, R.C. Ford,
Three-dimensional structures of the mammalian multidrug resistance
P-glycoprotein demonstrate major conformational changes in the
transmembrane domains upon nucleotide binding, J. Biol. Chem.
278 (2003) 8294–8299.
[210] D.R. Stenham, J.D. Campbell, M.S.P. Sansom, C.F. Higgins, I.D.
Kerr, K.J. Linton, An atomic detail model for the human ATP
binding cassette transporter P-glycoprotein derived from disulfide
cross-linking and homology modeling, FASEB J. 17 (2003)
2287–2289.
[211] K.P. Hopfner, A. Karcher, D.S. Shin, L. Craig, L.M. Arthur, J.P.
Carney, J.A. Tainer, Structural biology of Rad50 ATPase: ATP-
driven conformational control in DNA double-strand break repair
and the ABC-ATPase superfamily, Cell 101 (2000) 789–800.
[212] K.P. Locher, E. Borths, ABC transporter architecture and mecha-
nism: implications from the crystal structures of BtuCD and BtuF,
FEBS Lett. 564 (2004) 264–268.
[213] M.F. Rosenberg, G.S. Velarde, R.C. Ford, C. Martin, G. Berridge, I.D.
Kerr, R. Callaghan, A. Schmidlin, C. Wooding, K.J. Linton, C.F.
Higgins, Repacking of the transmembrane domains of P-glycoprotein
during the transport ATPase cycle, EMBO J. 20 (2001) 5615–5625.
[214] C. Martin, G. Berridge, P. Mistry, C.F. Higgins, P. Charlton, R.
Callaghan, Drug binding sites on P-glycoprotein are altered by ATP
binding prior to nucleotide hydrolysis, Biochemistry 39 (2000)
11901–11906.
[215] Z.E. Sauna, M.M. Smith, M. Mqller, S.V. Ambudkar, Evidence for
the vectorial nature of drug (substrate)-stimulated ATP hydrolysis by
human P-glycoprotein, J. Biol. Chem. 276 (2001) 33301–33304.
[216] A.H. Schinkel, U. Mayer, E. Wagenaar, C.A. Mol, L. van Deemter, J.J
Smit, M.A. van der Valk, A.C. Voordouw, H. Spits, O. van Tellingen,
J.M. Zijlmans, W.E. Fibbe, P. Borst, Normal viability and altered
pharmacokinetics in mice lacking mdr1-type (drug-transporting) P-
glycoproteins, Proc. Natl. Acad. Sci. 94 (1997) 4028–4033.
[217] R.J. Raggers, I. Vogels, G. van Meer, Upregulation of the expression
of endogenous Mdr1 P-glycoprotein enhances lipid translocation in
MDCK cells transfected with human MRP2, Histochem Cell Biol.
117 (2002) 181–185.
[218] C.R. Sweet, A.H. Williams, M.J. Karbarz, C. Werts, S.R. Kalb,
R.J. Cotter, C.R. Raetz, Enzymatic synthesis of lipid A molecules
with four amide-linked acyl chains: LpxA acyltransferases selec-
tive for an analogue of UDP-N-acetylglucosamine in which an
amine replaces the 3V-hydroxyl group, J. Biol. Chem. 279 (2004)
25411–25419.
[219] C.F. Higgins, K.J. Linton, The ATP switch model for ABC
transporter, Nat. Struct. Mol. Biol. 11 (2004) 918–926.
[220] S. Beharry, M. Zhong, R.S. Molday, N-retinylidene-phosphatidyle-
thanolamine is the preferred retinoid substrate for the photoreceptor-
specific ABC transporter ABCA4 (ABCR), J. Biol. Chem. 52 (2004)
53972–53979.