paper 2 cell evolution

8/6/2019 PAPER 2 Cell Evolution

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Glu

Asp

Asn Thr His

Aromatic

Trp

Gln

Ala

Ser

Pro

Val

Polar uncharged

Ile

Met

Leu

Non-polarhydrophobic

Lys

Phospholipids

Arg

Basic

PheTyr

Gly

Cys

Acidic

RNA

Mineralsurfacechemistry

Vesicles Self-synthesizingmembrane

RNA-DNA DNAEvolutionary time

Genomic analysis leaves no doubt that thethree main kingdoms of life — eubacteria,archaea and eukaryotes — evolved froma common ancestor1. The idea that alllife evolved from one source is perhapsthe most profound consequence of theDarwin–Wallace model of evolution. With

increasing amounts of DNA sequenceinformation available, the search is on tofind the minimum set of genes that existedin the last common ancestor (LCA) — theprecursor cell(s) from which all livingorganisms subsequently evolved. The LCAmust have been a rather sophisticated cellbecause it contained all the cellular machin-ery that is common to all present livingforms, including a DNA-based informationsystem2,3. From sequence analysis, the LCAis suggested to contain 250–600 genes4,5. Mycoplasma genitalium, the simplest knownprokaryote cell (which is neverthelessdependent on host-cell parasitism), requires270 of its 380 genes for normal function, asrevealed when all genes were individually knocked out6. So, the minimum conceivablecell needs a lot of genetic information.

The Earth is ~4.56 billion years (Gyr)old, and the best current estimates arguethat the first cells appeared by 3.0–3.3 Gyrat the latest7. How these first cells emergedis one of the biggest unsolved problems inbiology. It seems undeniable that the LCAmust have been preceded by a spontaneousgeneration of cells from abiotic precursor

molecules. Even Darwin in his theory of evolution was obliged to accept the necessity of some kind of spontaneous generation.He brilliantly speculated (in a letter toHooker) that in a “warm little pond” aparticular chemistry involving “all sorts of

ammonia and phosphoric salts, light, heat,electricity etc. was present so that a proteincompound was chemically formed ready to undergo more complex changes on thepath towards life”8. It seems likely that his(unpublished) hypothesis was not far fromthe mark. Modern theories were initiatedby Oparin in 1924 (REF. 9) and by Haldane10,who first discussed the origin of membranesand speculated that an ‘oily film’ on the sur-

face of sea water evolved into the lipid-richcell membrane.

Here, I attempt to build up a plausiblescenario for how cells may have evolved— with an emphasis on the more difficultquestion of the origin of the cell membrane.A brief discussion of the (more actively pur-sued) question of the evolution of cytoplasm,including the nucleic acids, sets the stage forthe main hypothesis.

The evolution of the cytoplasm

Cells are made up of nucleic acids, proteins,carbohydrates, lipids and many smallmolecules suspended in a particular ionic

O P IN IO N

Cell evolution and the problem of membrane topology

Gareth Griffiths

Abstract | Cells somehow evolved from primordial chemistry and their emergencedepended on the co-evolution of the cytoplasm, a genetic system and the cell

membrane. It is widely believed that the cytoplasm evolved inside a primordial lipid

vesicle, but here I argue that the earliest cytoplasm could have co-evolved to high

complexity outside a vesicle on the membrane surface. An invagination of the

membrane, aided by an early cytoskeletal system, may have formed the first cells

— initially within primordial vesicles.

Figure 1 | Biosynthetic pathways for amino acids, phospholipids and central metabolism.

It has been proposed by Davis22,26 and many others that primitive biochemical pathways existed before

a genetic-based system. These include the reductive carbon cycle (the equivalent of the tricarboxylic

acid (TCA) cycle working in reverse), the reductive pentose pathway and the central trunk (proposed

to be a remnant of the formose cycle), which together make up the central biochemical pathway (CBP).

Davis proposes that the emergence of genetically encoded amino acids correlates with the number

of chemical reactions from the CBP that are required to generate each amino acid (evolutionary steps1–14). Acidic amino acids (Glu and Asp) are close to the CBP (1–2 reactions), whereas aromatic residues

(such as Trp) require up to 14 steps to be synthesized and may have appeared later. The hydrophobic

amino acids that could associate with membranes required four steps. The synthesis of phospholipids

requires 10 steps and, therefore, self-synthesizing membranes might only have arisen after this point.

Davis provides detailed analysis to argue that the evolutionary appearance of the different amino acids

correlates well with the emergence of their corresponding triplet codes; this implies co-evolution of

biochemistry and the genetic code, an idea extensively championed by Wong60. Davis also identified

an 11-amino acid sequence in the FtsZ–tubulin family that he mapped to his evolutionary stage 7.5,

which could support a role for this protein in cellularization. Gly, Cys and Pro cannot easily be placed

into any of the five categories shown.

1018 | DECEMBER 2007 | VOLUME 8 www.nature.com/reviews/molcellbio

PERSPECTIVES

© 2007 Nature Publishing Group



Mineral surface

PP

P P P PP

–

–– – – –

–

+ + + + + + + + +Mineralsurface

Gel

ProtocytoplasmCytoplasm insidevesicle

Cytoplasm outsidevesicle

Lipid vesicle

a

c

b d

Liposome

Unstable permeablebilayer Less permeable bilayer ‘Stable’ bilayer

Single acyl chain lipids• Monoglycerides• Fatty acids• Carboxylic acids

Impermeable membrane• Electric potential• Proton gradient

Two acyl chainphospholipids

Autonomouscell divides

Transient breaksin the membrane

Obcell

Protocell

RNARibosome

Protein

Pore/channel

environment. Before cells emerged, howwere sufficient amounts of the necessary starting materials generated? Following thepioneering experiments of Miller9, extensivestudies described many chemical reactionsthat plausibly occurred under the presumedconditions of the early Earth. These reac-tions could synthesize many (but not all)amino acids and other key precursors, suchas purines, and precursors of liposomessuch as carboxylic acids11–13.

The key precursors of life could havebeen made on Earth or in outer space andcarried to Earth via meteorites, such asthe Murchison meteorite that landed inAustralia in 1969 (REF. 14). Meteorites, aswell as interplanetary dust particles, bringan extraordinary selection and amountof chemicals and biochemical precursors of life from space, including >90 differentamino acids (of which 19 are found inliving organisms) and, as discussed below,bilayer-forming lipids15,16.

The cytoplasms of most modern cellshave a similar chemical composition with areducing environment, neutral pH and anionic composition that is rich in K+, Cl– andMg2+ but low in Ca2+ and Na+. Perhaps thisuniversal composition reflects the environ-ment where the first cells evolved; if so, theenvironment where the earliest chemistry that preceded life occurred is likely tohave been in fresh water17,18 rather thanthe high-salt ocean environment proposedby others10,19. The latter is also difficult toreconcile with the need for primordial lipid vesicles, which are unstable at high saltconcentrations 20.

For a protocytoplasm to emerge, many schemes have been postulated that involvethe occurrence of chemical catalysis andautocatalytic reactions on the surface of positively charged minerals, such as clay or iron pyrites21–25. A useful ‘yardstick’for timing the main events leading tocellularization is a scheme proposed by

Davis22,26 (FIG. 1; see below). The evolutionof the cytoplasm is often proposed to haveoccurred in parallel with the emergence of aliposome system, which eventually becamethe delimiting membrane of the cell.

The emergence of membranes

In a water-based system, many lipids canself-assemble into bilayer-containing vesicles. Phospholipids are the main bilayer-forming lipids in bacteria and eukaryotes(assuming that the ether-lipid-basedmembranes in archaea are a later adapta-tion to extreme environments22). However,the simplest bilayer-forming lipids arelong-chain (>C9) fatty acids, carboxylicacids and monoglycerides18,20. When suchlipids were extracted with solvents fromthe Murchison meteorite, they formedbilayered vesicles in aqueous solution16.Deamer and colleagues proposed that vesicles containing lipids made up of singleacyl chains formed the first template for the

Figure 2 | Surface interactions and the membrane problem. a | A pure

lipid vesicle in contact with a protocytoplasmic milieu. The system provides

two catalytic surfaces, the positively charged mineral surface and the

negatively charged, hydrophobic lipid vesicle. b | Two proposed membrane

scenarios for cellularization during evolution. In the ‘inside-out’ or ‘leaky

liposome’ model (left) the cytoplasm evolved within the vesicle and the

topology of the membrane (outer leaflet outside) remains in place during

the evolutionary process. In the ‘cytoplasm outside’ model, the cytoplasm

co-evolves with the membrane by associating with the outer membrane

surface (which will later become the cytoplasmic surface). A gel-like fila-

mentous material may prevent the diffusion of protocytoplasmic compo-

nents36. c | The ‘cytoplasm inside the vesicle’ hypothesis requires that the

molecular precursors of life must have found a way to pass selectively

through the liposome barrier. Deamer proposed that the earliest vesicles

contained bilayer membranes made up of single acyl chain lipids that are

more permeable to many molecules, including ions. As the system evolved

to use the more complex two acyl chain phospholipids, the membrane

became more impermeable. Additional proposed mechanisms that over-

come the permeability barrier include osmotic forces, transient breaks in

the membrane caused by polymerized amino acids such as polyleucine,

and transient openings in the bilayer caused by temperature changes or

freeze-thaw cycles20. d | The obcell model of the ‘outside-in’ hypothesis of

cellularization, as proposed by Blobel38 and Cavalier-Smith39. The early

liposome system is postulated to induce cisternae to fuse with themselves

to form double-membrane ‘obcells’ (inside-out cells)39. The model necessi-

tates the loss of the outer of the two membranes to release a protocell that

has the correct topology (with the luminal domains of the membrane

proteins facing outwards).

P E R S P E C T I V E S

NATURE REVIEWS | MOLECULAR CELL BIOLOGY VOLUME 8 | DECEMBER 2007 | 1019




Golgi

Spore

Nucleus

Mitochondrion

pindleole body

b Budding yeast

a Vaccinia virus

c Autophagy

Fusionwithlysosome

(Microtubule) (Actin)IMV EEV

TGN

Golgi complex

Plasma membrane

cell membrane and that more complex (twoacyl chain) phospholipids emerged later20 (FIG. 2). Phospholipids can be synthesizedwithout enzymes under plausible abioticconditions12,13,27–29, but it seems likely thatthese lipids became more important whenRNA ribozymes or ribosomes and proteinenzymes emerged that could synthesizethem (FIG. 1).

The evolution of cells

If we assume that an increasingly complexcytoplasm with protein-synthesizing ribo-somes, nucleotides, RNA and even DNA

emerged in contact with a system of lipo-somes, we face a crucial unresolved issue inunderstanding the origin of cells and the cellmembrane: on which side of the liposomesdid the first key reactions occur?

The scenario of life within the vesicle. Mostspecialists think that such a system couldonly have evolved within the primordial vesicle20,30–34 (FIG. 2b). How then could theprecursors of the key molecules of life havecrossed a bilayer that presents a significantbarrier to charged molecules? Although vesicles comprised of single acyl chain

lipid membranes are more permeable touncharged and charged molecules, theiroverall permeability is several orders of magnitude lower than phospholipid bilayers.Significantly, all bilayers are highly imper-meable to ions such as K+ (REFS 18,20).As membrane complexity increases, thebilayer becomes more impermeable. In this‘cytoplasm within the vesicle’ scenario, theemerging ribosomes inside the vesicle wouldeventually evolve the capacity to insertmembrane proteins from the inside withtheir extracellular domains facing outwards,as in modern cells (FIG. 2b,c).

Many elegant experiments have been con-ducted to try and reconstitute some aspectsof this hypothesis. For example, investigatorstrapped enzymes and whole transcription–

translation systems within vesicles and foundthat these became functionally active 20,33–35 (FIG. 2c). Such analyses are still a long way from reconstituting life in such vesicles, asconceded even by some of the strongestproponents of this hypothesis18.

The ‘outside the vesicle’ scenario. Giventhe difficulties with the above hypothesis,let us consider another scenario in whichthe cytoplasm evolved to a high degree of complexity outside the vesicles. A mineralsurface could provide an environmenton which lipid vesicles become attached,and these surfaces could provide theenvironment where some rudimentary

components of a complex protocytoplasm— for example, perhaps ATP, GTP, proteins,RNA and maybe even DNA — could evolve.Given the problem of these key precursorsdiffusing away from the site of action, it isattractive to consider the emergence of apolymer in the space around the vesiclesthat can form a hydrophilic gel (REF. 36). Thiscould initially be a simple polymer such aspolysugars or polyglutamic acid that mightlater be replaced by a cytoskeletal proteinpolymer such as actin (see below). Suchsystems of filaments could serve to attachthe vesicles to the mineral surface, captureprecursor molecules within their gel-likematrix or provide a scaffold for protectionas well as an additional catalytic surface foremerging biochemistry. Under these condi-tions, there would be three catalytic surfaces— the positively charged mineral surface24,25,the surface of the gel36 and the negatively charged lipid vesicle surface — that couldprovide a rich surface for many reactions37.The mineral surface could itself catalyse theassembly of lipid vesicles25. In this micro-environment, sandwiched between thesesurfaces, a sophisticated protocytoplasm

Box 1 | Double membrane compartments in modern cells

There are at least three examples in modern cells that are topologically similar to the ‘life outside the

vesicle’ model (see figure). a | During the cellular release of vaccinia virus, the transGolgi network

(TGN) cisternal domain engulfs the intracellular mature virus (IMV)56. This intracellular enveloped

virus is transported along microtubules and fuses with the plasma membrane. During fusion with the

plasma membrane, or shortly thereafter, the local polymerization of actin around the TGN-derived

membrane facilitates virus release into the extracellular space (extracellular enveloped virus (EEV))57.

As proposed in the evolutionary schemes presented in FIG. 3, actin and microtubules also interact

here with membranes. b | During sporulation in the budding yeast Saccharomyces cerevisiae, a

double-membrane prospore cisterna is formed that originates from post-Golgi vesicles that fuse

around the forming (haploid) spore (only one spore is shown but up to four may be made). The Golgi

vesicles aggregate on the surface of the nucleus (pink) at the spindle pole body to enclose the spore

within two membranes58. c | The formation of autophagic vacuoles is proposed to occur by the

formation of a double-membrane structure59. A cytoplasmic cisternal structure (of unknown origin

but suspected to originate from the endoplasmic reticulum) wraps around cytoplasmic components

such as a mitochondrion to form a double-membrane vesicle. The outer membrane fuses with a late

endocytic compartment, whereas the inner membrane is expected to be lysed by the hydrolytic

conditions of the lysosomal lumen. In parts a andc, the purple ball and stick structure represents a

membrane-spanning protein, with the ball representing the luminal domain.






LamellipodiaCell motility

Endocytosis

PhagocytosisMacropinocytosis

Fusion

Exocytosis

Cytokinesis

Filopodia

Microtubule

+ Motor + Motor

Membranenucleationof actin +

+

+

–

–

–

Comet

Cytokinesis

MreB (actin homologue)

FtsA (actin homologue)

FtsZ (tubulin homologue)

Myosin

might have emerged that interacted firstwith the outer surface of the vesicles. Smallmolecules may indeed have crossed spon-taneously into the vesicle lumen and I willrevisit this issue below.

If ribosomes could make hydrophobicpolypeptides, these would interact with the vesicle outer surface and, in time, the systemwould evolve the capacity to insert mem-brane proteins that spanned the membrane,

with their extracellular domains facing theinside of the vesicle38. To develop normalcell-membrane topology, the vesicle wouldhave to invert upon itself and fuse to form adouble-layered vesicle (FIG. 2d). The outer of the two vesicles would then need to lyse torelease the cell. This clever idea by Blobel38 is rarely cited by origin-of-life specialistsbut has been extensively championed by Cavalier-Smith39, who conceived the idea

of an ‘obcell’ or ‘inside-out cell’ (FIG. 2d).Whereas the ‘life within the vesicle’ model(FIG. 2c) has no obvious mechanistic prece-dents in modern cells, there are examplesfrom present-day cells in which a cisternalwrapping process occurs that is topologi-cally similar to the ‘cytoplasm outside the vesicle’ model (BOX 1). Below, I arguethat this (admittedly complex) inversionoccurred by numerous steps and dependedon the earliest cytoskeletal filaments.

Functions of the cytoskeleton

Not so long ago, textbooks dogmatically stated that only eukaryotes contain actin fila-ments and microtubules. Recently, this ideahas been overturned with the realization thatthe actin- and microtubule-family proteins

are universally expressed and interact withmembranes. The prokaryotic GTPase FtsZ,a homologue of tubulin, was shown to formfilaments and to interact with the membraneduring bacterial cell division40.

Actin homologues have been discoveredin eubacteria and archaea. It had been longappreciated that hexokinase and chaperonesof the heat shock protein-70 (HSP70) family were homologous to actin41. Recently, newmembers of this family, the MreB andMb1 subfamilies, were shown to be trueactin homologues that can form actin-likefilaments in vitro and in bacteria42,43. As ineukaryotic cells, these filaments polymerizewhile being attached to the membrane

surface, providing a force that contributes toprokaryote shape44,45. When these proteinsare knocked out in rod-shaped bacteria,for example, the cells are converted intospheres44. One actin homologue, FtsA, isalso involved in bacterial cell division andhas recently been shown to interact directly with FtsZ; thus, the actin- and tubulin-basedsystems work together to drive bacterialcytokinesis46 (BOX 2).

In eukaryotes, the actin family is involvedin many membrane-dependent processessuch as exocytosis, endocytosis, phago-cytosis, cell motility, cytokinesis and cellpolarity 47,48. In many fusion processes, actinpolymerizes on membranes and somehowprovides a force to pull membranes togetherand/or push them or keep them apart49–52 (BOX 2). Recent data have shown directinteractions between membrane nucleationmechanisms for actin assembly (N-WASP)and machinery for non-clathrin-mediatedendocytic vesicle formation and fordorsal cell ruffling. In these processes,sorting nexin-9 (SNX9) was identified as amembrane scaffold protein that stimulatesN-WASP and ARP2/3-dependent actin

Box 2 | Interactions of actin and microtubules with membranes in modern cells

In eukaryotes, the actin family is involved in many membrane-dependent processes such as

exocytosis, endocytosis, phagocytosis, cell motility (for example, using lamellipodia), cytokinesis and

cell polarity. Actin (red) and its prokaryotic homologues have many intimate connections with

membranes in eukaryotes (see figure; top panel) and prokaryotes (bottom panel). Membranes can

nucleate the assembly of actin and, in all known cases, the topology is such that the fast-growing end

(barbed end or plus end (+)) is localized adjacent to the membrane — this means that the insertion of

new monomers occurs at this site. Of the different mechanisms and processes shown, only two can be

considered to be well understood. First, myosin motors attached to membrane organelles can walk

along actin; most myosins move towards the plus ends. Second, actin comets, which were first

described in the intracellular transport of Listeria monocytogenes and other cytoplasmic pathogens,

transport vesicles such as endocytic vesicles and phagosomes. For the most part, however, the

mechanistic details of how actin or its homologues in prokaryotes interact with membranes is poorly

understood49,52. Even less is known about the interactions of microtubules with membranes, with the

exception of motor proteins that can be bound to membrane organelles; kinesin transports cargo

towards the plus end of the microtubule, whereas dynein transports cargo in the opposite direction.

In prokaryotes, actin homologues interact with the cell membrane, whereas the tubulin homologue

FtsZ, which also assembles into filaments (green), interacts with the membrane at the site of cell

division (it forms a ring structure at the septum known as the Z ring) and is essential to carry out

cytokinesis (bottom panel). There are no comprehensive models to predict how FtsZ–membrane

interactions operate mechanistically in this process but, recently, the actin homologue FtsA has

been shown to interact directly with FtsZ46. A similar lack of understanding pertains to the role of

actin in the much more complex process of cytokinesis in eukaryotic cells; microtubules and

microtubule-associated proteins are also implicated in this process.






+ + + + + + + + +

Pre-cytoplasmicenvironment

Microtubule/FtsZ

Actin

Lumen

RibosomeExtracellular space

External milieu

Protocell

a

de

cb

Luminalfusion

Cytoplasmicfusion

Inverted vesicle

Extracellular space

Cytokinesis

Autonomous cells

Channel

assembly, especially when it is allowed tooligomerize on phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P

2)-enriched

membrane domains in vitro. SNX9 containsa BAR domain that is known to facilitatemembrane bending53. Such studies startto resolve the long-elusive molecular linksbetween actin, its assembly and membranefunctions. Besides its role in cytoplasmicfusion processes (where the cytoplasmicleaflets of membranes first interact), arecent study shows that N-WASP-basedactin polymerization is also essential fortwo apposing plasma membrane luminaldomains to fuse completely duringDrosophila melanogaster myoblast fusion54.Thus, actin facilitates both cytoplasmic andluminal fusion events (FIG. 3).

Homologues of actin and tubulin havetherefore been identified in all kingdomsof life. The ATPase actin and the GTPasetubulin–FtsZ protein family may haveappeared before the cell became surrounded

by a membrane and, if so, could havefunctioned in cellularization. These sugges-tions are supported by an analysis by Davis,who argued that the order in which codedsynthesis of the different amino acids andlipids emerged during evolution correlateswith the number of reactions needed for theirsynthesis from an already evolved biochemi-cal system, which included the tricarboxylicacid (TCA) cycle and the pentose path-way 22,26 (FIG. 1). These reaction systems areuniversally involved in the synthesis of all 20common amino acids used by modern cells.Davis26 identified a conserved 11-residuesequence in the FtsZ–tubulin family that hemapped to his evolutionary stage 7.5, a stagehe classified as occurring before the systemcould self-synthesize membrane phospho-lipids (stage 10; FIG. 1). Because microtubulesand FtsZ, and especially actin, have intricateinteractions with modern membranes,I propose a speculative model for the role of their precursors in cellularization.

The cytoskeleton in cellularization?

My model of cellularization starts with inter-actions of the protocytoplasm with the outersurface of the initially pure lipid liposomes(FIG. 3a). In the model, I propose that proteins(made by ribosomes present in the proto-cytoplasm) evolved hydrophobic domainsthat allowed them to interact with liposomes.Later, membrane-spanning proteins, includ-ing channels and pumps, were inserted. Theproton and other pumps formed chemicaland electrical gradients and synthesized ATPon the cytoplasmic side of the membrane.The inside of the vesicle could then developa different composition to the outside andmight later have become the extracellularspace. Thus, in this model, even sophisticatedmembrane functions such as proton andelectrochemical gradients emerged beforecellularization.

Actin and/or tubulin ancestors theninteracted with the outer surface of the mem-brane and facilitated membrane bending.

Figure 3 | The outside-in model of cellularization. Shown is a possible evo-

lutionary mechanism by which the outside-in model (FIG. 2d) may have

occurred. a | An evolutionary stage exists in which a complex protocytoplasm

has a genetic code that is RNA- or even DNA-based, ribosomes for synthe-

sizing proteins and a relatively advanced biochemistry. The system has evolved

the capacity to insert membrane proteins such that the future extracellular or

luminal domains are inside the liposome. In parallel, an actin- and tubulin-

based cytoskeleton evolved the capacity to interact with the liposome surface.

b | The emergence of the cytoplasmic fusion machinery (orange) allows lipo-

somes to fuse together and bend membranes.c | The fusion machinery could

collaborate with the cytoskeleton to form an inwards-budding vesicle. This

budding process, which is similar to modern endocytosis but has the opposite

topology, allows the genetic material (not shown) and the protocytoplasm to

enter into a vesicle within a larger vesicle — the protocell. Channels and

transporters that allow ions to cross the membrane would be important for

maintaining the ionic homeostasis of the protocytoplasm inside the protocells

and in the extracellular space. d | The emergence of luminal fusion mecha-

nisms allows the process of fission out of the parental vesicle to occur.

This machinery also allows the protocells to fuse together within the extra-

cellular space. e | The cytoplasmic fusion machinery evolves into the process

of cytokinesis, which allows protocells to divide in a regulated fashion such

that each daughter cell contains everything it needs to metabolize and repli-

cate. The protocells are eventually released when the outer membrane lyses.

These are now independent living forms that are capable of self-replication.






A cytoplasmic machinery evolved that fusedthe vesicles together (FIG. 3b). A coordinatedeffort between the cytoskeleton and thecytoplasmic fusion machinery then allowedapposing membrane lipid coalescence andthe inwards pinching of vesicles to formprotocells (FIG. 3c). These ‘inverted’ vesicleswithin the larger vesicle enclose the proto-cytoplasm and the genetic material (DNA orRNA); it is implicitly assumed that this evolv-ing genetic material is intimately associatedwith the cytoplasmic surface of the vesicle(not shown in FIG. 3).

The formation of these vesicles istopologically equivalent to the budding of vesicles into the lumen of specialized endo-cytic organelles — multivesicular bodies— in modern eukaryotic cells. Additional

machinery must have evolved that allowedthe luminal domains of the protocell mem-branes to fuse together. The same machinery could allow cells to bleb off a membrane vesicle by fission, a process equivalent toblebbing in modern cells. The protocellscould undergo selection by content mix-ing driven by fusion with themselves andwith newly made protocells bringing innew components from the external milieu(FIG. 3d).

At a later stage, the cytoplasmic fusionmachinery facilitates the separation of daughter protocells in a regulated fashion(cytokinesis) (FIG. 3e). The final stage is lysisof the first outer membrane to release inde-

pendent cells. These are expected to haveprobably several hundred DNA-encodedgenes in order for the minimal cell functionsto be permitted.

The extracellular compartment (FIG. 3c) would offer several advantages to theemerging cells bathing within this space.The evolution of mechanisms to transferglycoconjugates bound to lipids or proteinsonto and beyond the luminal surface of theboundary membrane of protocells couldprovide a reserve energy source for cellsif sugar hydrolases were also secreted intothis space. A high-viscosity environmenthere could also protect the enclosed cellsfrom extreme environmental changesin the external environment. Ion channels inthe boundary membrane and the protocellmembrane could concentrate some ions,such as Ca2+ and protons, which would bestored in the extracellular buffering com-partment and injected into the protocells asrequired. It is conceivable that many of themechanisms proposed for permeation of themembrane in the ‘inside the vesicle’ scenario(FIG. 2c) could be operational in the ‘outsidethe vesicle’ model (FIG. 3). However, instead

of molecules entering the future cytoplasm,in the latter model, the transport of mole-cules into the lumen of the vesicles wouldallow the future extracellular space to evolve.

The stage of protocells within a vesiclemight, perhaps, have existed for a relatively long time. The evolutionary split betweeneukaryotes and prokaryotes55 could also havedeveloped in this system, protected by twomembranes.

Conclusions and perspectives

I have outlined a plausible scenario forthe co-evolution of the cytoplasm and themembrane based on existing cell biologicalprinciples. I have discussed the two principaltheories with respect to the evolution of themembrane and suggested that the ‘cytoplasm

outside’ model (FIG. 3) is more plausible thanthe more favoured ‘cytoplasm inside the vesicle’ model. If the inside-out (or obcell)model is correct, it seems reasonable to sug-gest that it needed the active participation of a cytoskeleton to ‘invert’ the topology of themembrane system and initiate the cellular-ization process. The model predicts that theactin and tubulin family preceded the LCAand that they are probably as universal as the vacuolar ATPase family.

Experimentally, one could ask how aliposome system with a subset of recon-stituted membrane proteins would behavewith cytosolic extracts and factors such asglucose, GTP and ATP. If the cytoplasmic

domain of these membrane proteins all facedoutwards, some aspects of the ‘life outsidethe vesicle’ model presented here could betested. Alternatively, if they had the oppositeorientation, they could perhaps be usedto test some aspects of the ‘life within the vesicle’ scenario.

Gareth Griffiths is at the

European Molecular Biology Laboratory,

Heidelberg, Germany.

e-mail:

[email protected]

doi:10.1038/nrm2287

Published online 31 October 2007

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AcknowledgementsI thank T. Gibson, M. Knop, S. Trachtenberg, D. Lancet,

G. van Meer, L. Mayorga and J. Reich for their comments

and discussion, and C. Bleck for preparing the figures.

DATABASESEntrez Genome Project: http://www.ncbi.nlm.nih.gov/sites/

entrez?db=genomeprj

Listeria monocytogenes | Mycoplasma genitalium

UniProtKB: http://beta.uniprot.org/

FtsA | FtsZ | N-WASP| SNX9

FURTHER INFORMATIONGareth Griffiths’s homepage:

http://www-db.embl.de/jss/EmblGroupsOrg/g_79.html

ALL LINKS ARE ACTIVE IN THE ONLINE PDF




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