review article oxidative protein-folding systems in plant...

Hindawi Publishing CorporationInternational Journal of Cell BiologyVolume 2013 Article ID 585431 15 pageshttpdxdoiorg1011552013585431

Review ArticleOxidative Protein-Folding Systems in Plant Cells

Yayoi Onda

Department of Food and Applied Life Sciences Yamagata University Yamagata 997-8555 Japan

Correspondence should be addressed to Yayoi Onda yondatds1tryamagata-uacjp

Received 28 May 2013 Accepted 1 August 2013

Academic Editor Kenji Inaba

Copyright copy 2013 Yayoi Onda This is an open access article distributed under the Creative Commons Attribution License whichpermits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Plants are unique among eukaryotes in having evolved organelles the protein storage vacuole protein body and chloroplastDisulfide transfer pathways that function in the endoplasmic reticulum (ER) and chloroplasts of plants play critical roles in thedevelopment of protein storage organelles and the biogenesis of chloroplasts respectively Disulfide bond formation requires thecooperative function of disulfide-generating enzymes (eg ER oxidoreductase 1) which generate disulfide bonds de novo anddisulfide carrier proteins (eg protein disulfide isomerase) which transfer disulfides to substrates by means of thiol-disulfideexchange reactions Selective molecular communication between disulfide-generating enzymes and disulfide carrier proteinswhich reflects the molecular and structural diversity of disulfide carrier proteins is key to the efficient transfer of disulfides tospecific sets of substrates This review focuses on recent advances in our understanding of the mechanisms and functions of thevarious disulfide transfer pathways involved in oxidative protein folding in the ER chloroplasts and mitochondria of plants

1 Introduction

The endoplasmic reticulum (ER) is the first organelle in thesecretory pathway and this dynamic and highly specializedorganelle contains enzymes and chaperones that mediate thefolding and assembly of newly synthesized proteins [1] Akey step in oxidative protein folding is the formation ofdisulfide bonds which covalently link the side chains ofpairs of Cys residues impart thermodynamic andmechanicalstability to proteins and control protein folding and activity[2] The introduction of disulfide bonds into polypeptidesrequires the cooperative function of a pair of enzymes a denovo disulfide-generating enzyme (eg ER oxidoreductase 1[ERO1]) and a disulfide carrier protein (eg protein disulfideisomerase [PDI]) [3] PDIs which are ubiquitous thiol-disulfide oxidoreductases in all eukaryotic cells directlydonate disulfides to substrate proteins by means of thiol-disulfide exchange reactions The oxidized form of PDI isregenerated by ERO1 which relays the oxidizing power frommolecular oxygen to the reduced form of PDI (Figure 1)The genomes of higher plants includingArabidopsis thalianaGlycine max (soybean) Oryza sativa (rice) and Zea mays(maize) encode approximately 10 to 20 members of the PDIfamily which show wide variation in the organization of

their thioredoxin (TRX)-fold domains [4 5] Although thenetwork of enzymes involved in disulfide bond formation hasnot yet been fully elucidated emerging evidence suggests thatthe molecular and structural diversity of the PDIs and thespecific combinations of disulfide-generating enzymes andPDIs are key to determining the functions of these enzymesand their substrate specificity

Plant cells have two distinct types of vacuoles theprototypical lytic vacuole (LV) which has high hydrolyticactivity and shares functions with the yeast vacuole andmammalian lysosome and the protein storage vacuole (PSV)a plant-specific organelle that is specialized in accumulatingreserve proteins and is prevalent in seeds [6] The developingendosperm cells of seeds actively synthesize large amountsof storage proteins that acquire disulfide bonds in theER Among the physicochemically and structurally diversedisulfide-rich seed storage proteins soluble storage proteinsare transported through the endomembrane system from theER to PSVs where they accumulate [7] Insoluble proteinshowever form large oligomeric accretions within the ERand are deposited in ER-derived protein bodies (PBs) whichbud and disconnect from the ER but remain surrounded byER membranes [8] The development of the PSVs and PBsenables massive structurally stable accumulations of storage

2 International Journal of Cell Biology

SubstrateDisulfide-

generatingenzyme

Disulfidecarrier

Electron transfer

Disulfide transfer

-SH HS- PDIox

PDIred ERO1ox

ERO1red

-S-S-

O2

H2O2

2eminus

2eminus

2eminus

Figure 1 Disulfide relay from ERO1 to PDI in the ER Disulfidebond formation involves electron transfer from the substrate to adisulfide carrier protein (eg PDI) and then to a de novo disulfide-generating enzyme (eg ERO1) PDI directly transfers a disulfide totwo Cys residues of a substrate protein by means of a thiol-disulfideexchange reaction The reduced form of PDI is reoxidized by ERO1which relays the oxidizing power from molecular oxygen via theFAD cofactor to the reduced form of PDI

Endosperm cellsPhotosynthetic cells

VKOR-TRXLQY1CYO1

PDI Chloroplast

PSV

ERV1MIA40

Mitochondrion

ERO1ERO1PDIPDI

ERPB

Golgi

Nucleus

Figure 2 Overview of the sites of oxidative protein folding inphotosynthetic and endosperm cells of plants Disulfide-generatingenzymes and disulfide carrier proteins characterized to date andtheir subcellular localizations are shown in photosynthetic (left) andendosperm (right) cells of plants ER endoplasmic reticulum PBprotein body PSV protein storage vacuole

proteins which serve as nutritional reserves of nitrogensulfur and carbon for the germinating seedlings (reviewedin [9 10]) The redox state of storage proteins containingdisulfide groups dramatically changes during seed develop-mentmaturation and germination [11] seed storage proteinsare synthesized in reduced form on the rough ER and areconverted to disulfide-bonded forms duringmaturation thenthey are converted back to the reduced form to facilitaterapid mobilization during germination The h-type TRXfunctions in the reduction of seed storage proteins in theendosperm increasing their proteolytic susceptibility andmaking nitrogen and sulfur available during germination[12] In contrast to the TRX-dependent system that actsmainly in the reduction of disulfides the disulfide transfersystems involving ERO1 and PDIs function primarily to formthe correct patterns of disulfide bonds thereby facilitatingthe stable accumulation of seed storage proteins preventingtheir denaturation and decreasing their susceptibility toproteolysis

In chloroplasts the redox state of disulfide bonds inredox-regulated proteins is linked to the control of metabolic

pathways and gene expression [11 13 14] The most-studiedredox regulatory system is the ferredoxin-TRX system thatreversibly reduces and oxidizes thiol groups reducing equiv-alents are transferred from photosystem I (PSI) to ferredoxinand then to TRX via ferredoxin-dependent TRX reductase[11 13] Inside chloroplasts the thylakoid membranes whichenclose the lumen that originated as the bacterial periplasmperform the primary events of oxygenic photosynthesisUnder strong illumination however photosystem II (PSII)in the thylakoid membranes is subjected to photodamageTo maintain photosynthetic activity PSII requires the rapidreassembly of a thylakoid membrane protein complex com-posed of dozens of proteins [15] which is dependent ondisulfide bond formation catalyzed by enzyme catalystsincluding a zinc finger protein LQY1 and a vitamin K epoxidereductase (VKOR) homolog

Emerging evidence has shown that disulfide transferpathways that function in the ER and chloroplasts of plantsplay critical roles in the development of PSVs and PBsand the biogenesis of chloroplasts respectively This reviewfocuses on recent advances in our understanding of themechanisms and functions of the various disulfide transferpathways involved in oxidative protein folding in the ERand chloroplasts of plants (Figure 2) The disulfide transferpathways in plant mitochondria which involve the disulfide-generating enzyme ERV1 and the disulfide carrier proteinMIA40 (Figure 2) are also discussed in comparison to therespective pathways in yeast and mammalian cells

2 Disulfide Bond Formation in the Plant ER

21 Seed Storage Proteins Seed storage proteins are con-ventionally classified on the basis of their solubility inwater (eg albumins) saline (eg 120572-globulin) alcohol (egprolamins) and acidic or basic solutions (eg glutelins)[16] The globulins the most widely distributed group ofstorage proteins in both dicots and monocots are dividedinto two subgroups on the basis of their sedimentationcoefficients the vicilin-like 7S globulins and the legumin-like11S globulins Rice glutelins (60 of total seed protein) [17]and soybean glycinins (40of total seed protein) [18] both ofwhich are 11S globulin homologs are synthesized from theirprecursors proglutelins and preproglycinins respectivelythe precursors are processed at a conserved Asp-Gly site byan asparaginyl endopeptidase [19 20] to produce acidic andbasic polypeptide subunits which remain linked by disulfidebonds [21]

The prolamin family which includes maize prolamins(referred to as zeins) and rice prolamins is encoded bymultiple genes and is composed of Cys-rich and Cys-poormembers [16 22] Rice prolamins (20 of total seed protein)[17] are clustered into three subgroups 13-kD prolamins con-taining 0ndash8 Cys residues 10-kD prolamins containing 9ndash11Cys residues and 16-kD prolamin containing 13 Cys residues[22] Maize prolamins include four structurally distinct typesof proteins 120572- 120573- 120574- and 120575-zeins The 120573- 120574- and 120575-zeinsare rich in Cys residues whereas the 120572-zeins are Cys poor[16] Prolamins and 2S albumins contain conserved motifs

International Journal of Cell Biology 3

SFA-8 (Ha)

Region B

Q Lx x x xx

Region A Region C

P C

58 M L N H C

C C CL

G M 86 K Q L C C M Q L K N L 126 H N L P I E C N L82 F L G Q C V E 110 Q Q Q C C H Q I R Q V 156 Q Q L T A M C G L

124 I L G Q C V E 152 R Q Q C C Q Q L R Q V 199 Q Q L T A M C G L57 R Q P Q C S P 84 R Q Q C C Q Q Q M R M 162 S Y R T N P C G V44 P L D A C R Q 75 R M Q C C Q Q L Q D V 165 A Q L P S M C R V39 T M D P C R Q 69 Q Q Q C C M Q L Q G M 113 M Q M P Y M C N M43 V L S P Y N E 77 - - - - - Q Q L A L V 124 F N V P S R Y G I

120572-Globulin (Os)

16-kD 120574-zein (Zm)27-kD 120574-zein (Zm)15-kD 120573-zein (Zm)

13-kD Cys-depleted prolamin (Os)10-kD Cys-rich prolamin (Os)

Figure 3 Conserved Cys residues in members of the prolamin superfamily Amino acid sequences in regions A B and C of Zea mays (Zm)and Oryza sativa (Os) storage proteins are compared to Helianthus annuus (Ha) SFA-8 Amino acid sequences of SFA-8 (CAA40015) 16-kD120574-zein (NP 001105337) 27-kD 120574-zein (NP 001105354) 15-kD 120573-zein (NP 001106004) 120572-globulin (BAA09308) and Cys-rich 10-kD prolamin(crP10 NP 001051380) were aligned by means of CLUSTALW For comparison the corresponding sequence of Cys-depleted 13-kD prolamin(cpP13 NP 001055213) is shown Two disulfide bonds in SFA-8 Cys62ndashCys89 and Cys90ndashCys132 are indicated with solid lines Red-on-blackletters indicate Cys residues white-on-black letters indicate amino acid residues conserved in at least four of the sequences analyzed

in three separate regions [16] LxxC in region A CCxQLin region B and PxxC in region C Cys-rich prolamins ofrice and the 120573- and 120574-zeins of maize but not the 120575-zeinscontain the conserved Cys residues found in regions A Band C (Figure 3) [16 22] Structural analysis shows that thesunflower (Helianthus annuus) 2S albumin SFA-8 forms adisulfide bond between regions A and B (between Cys62 ofLxxC62 and Cys89 of C89CxQL) and one between regions Band C (between Cys90 of CC90xQL and Cys132 of PxxC132)[23] These conserved Cys residues are also found in othermembers of the prolamin family including a 10-kD Cys-richprolamin from rice and a 16-kD 120574-zein frommaize (Figure 3)[22]

22 Protein Storage Organelles To enable the stable accu-mulation of massive amounts of seed storage proteins andprevent their degradation plants have evolved specializedmembrane-bound storage organelles PSVs that containmatrix and crystalloid components and PBs derived from theER [10]Themultisubunit 7S and 11S globulins are synthesizedin precursor form and then transported from the lumen of therough ER to PSVs where these proteins undergo proteolyticcleavage and assemble to form dense accretions [24] Incontrast prolamins generally form PBs directly within thelumen of the rough ER PBs in maize and rice remain in theER lumen whereas those in wheat and barley are transportedfrom the ER to PSVs by an autophagic process [25]

Disulfide bonds play a critical role in the accumula-tion of seed storage proteins in PSVs and PBs [26 27]In the endosperm cells of rice seeds proglutelins acquireintramolecular disulfide bonds in the ER before they aretargeted via the Golgi to a PSV (designated type-II PB[PB-II] a crystalloid structure with a diameter of 2ndash4120583m)They are processed into acidic (37ndash39 kD) and basic (22ndash23 kD) subunits by a vacuolar processing enzyme (VPE)and accumulate as higher-order complexes held togetherby intermolecular disulfide bonds and hydrophobic inter-actions in the crystalloids of PSV [28ndash31] The formationof intramolecular disulfide bonds is also required for thesorting of 120572-globulins to the matrix of PSV [27] The 10-kDCys-rich prolamins (Figure 3) however directly accumulate

within the ER of rice endosperm cells to form the centercore region of an ER-derived PB (designated type-I PB[PB-I] a spherical structure with a diameter of 1ndash2120583m)at early stages of PB development before the 13-kD Cys-depleted prolamins (Figure 3) are localized to the outerlayer of ER-derived PB [32 33] The assembly of prolaminsinto the ER-derived PB is stabilized by the formation ofintermolecular disulfide bonds [34 35] In the endospermcells of maize seeds however the 120573- and 120574-zeins formspherical accretions (with a diameter of 02120583m) at earlystages of PB development Subsequently the 120572-zeins pene-trate the network of the 120573- and 120574-zeins and fill the centralregion of the PB (which has a diameter of 1ndash2120583m) [3637]

23 Roles of PDIs in the Development of Protein StorageOrganelles Different Subcellular Localizations and SubstrateSpecificities PDIs are multifunctional enzymes that catalyzea wide range of thiol-disulfide exchange reactions includ-ing oxidation reduction and isomerization reactions andalso display chaperone activity [38] PDIs are multidomainproteins which have at least one redox-active TRX domaindesignated domain a The oxidized form of the redox-activeCxxCmotif in the a domain transfers a disulfide to a substrateand is converted to the reduced form Members of the PDIfamily vary widely in the number and arrangement of theira domains and of another type of TRX domain designateddomain b which has a similar TRX-like structure but lacksthe redox-active motif For example Pdi1p one of the fivePDIs encoded by the Saccharomyces cerevisiae genome (andthe only one that is essential for viability [39]) has beendescribed as a U-shaped molecule composed of four TRX-fold domains (a-b-b1015840-a1015840) and an acidic C-terminal tail tworedox-active CGHC motifs are in the N-terminal and C-terminal TRX domains (a and a1015840 resp) and face eachother on either side of the U shape [40] The subsequentlydetermined crystal structure indicates that the shape the yeastPdi1p adopts is more that of a boat with the b and b1015840 domainsat the bottom and the a and a1015840 domains at the bow and stern[41] The flexible arms composed of the a and a1015840 domainsare connected to the relatively rigid core composed of the


Table 1 Components of the system for disulfide bond formation in higher plants

Species Name Domain Active sites LocationDisulfide-generating enzymes

Rice ERO1 Cx4C CxxC ERm

Arabidopsis ERV1 Erv1ALR CxxC Cx4C MArabidopsis LTO1 VKOR TRX Cx6C CxxC CxxC Tm

Arabidopsis QSOX1 TRX Erv1ALR CxxC CxxC CxxC WThiol-disulfide oxidoreductases

PDI family proteinsRice OsPDIL11 a-b-b1015840-a1015840 CGHC CGHC ERL

Rice OsPDIL23 a-a1015840-b CGHC CGHC PBER

Arabidopsis AtPDIL11 a-b-b1015840-a1015840 CGHC CGHC ER G LV PSVArabidopsis AtPDIL13 c-a-b-b1015840-a1015840 CGAC CGHC C ERArabidopsis AtPDIL14 c-a-b-b1015840-a1015840 CGHC CGHC ER G N V WArabidopsis AtPDIL21 a-a1015840-D CGHC CGHC ERL

Soybean GmPDIL-1 a-b-b1015840-a1015840 CGHC CGHC ERL

Soybean GmPDIL-2 c-a-b-b1015840-a1015840 CGHC CGHC ERL

Soybean GmPDIS-1 a-a1015840-D CGHC CGHC ERL


Soybean GmPDIM a-a1015840-b CGHC CGHC ERL

Zn finger proteinsArabidopsis LQY1 DnaJ Zn finger (CxxCxGxG)4 Tm

Arabidopsis CYO1 DnaJ Zn finger (CxxCxGxG)2 Tm

Disulfide carrier proteinArabidopsis MIA40 CPC M

Disulfide-generating enzymes and disulfide carrier proteins from rice (Oryza sativa) Arabidopsis (Arabidopsis thaliana) and soybean (Glycine max)characterized to date C chloroplasts ER endoplasmic reticulum ERL ER lumen ERm ER membranes G Golgi LV lytic vacuole N nucleus Mmitochondria PBER ER-derived protein body PSV protein storage vacuole Tm thylakoid membranes V vacuole W cell wall

b and b1015840 domains resulting in a highly flexible structure[41] The primary substrate-binding sites of yeast Pdi1pas well as Homo sapiens (human Hs) HsPDI have beenmapped to the hydrophobic pocket in the b1015840 domain whosesubstrate-binding ability seems likely to be influenced by theneighboring domains (b and a1015840) and by the x-linker regionbetween b1015840 and a1015840 [40 42ndash44] The domain organization ofPDIs likely contributes strongly to their substrate specificity

Whole-genome sequencing has identified 13 genes encod-ing PDI proteins in Arabidopsis (At) 22 in soybean (Gm)and 12 each in rice (Os) and maize (Zm) [4 5] ArabidopsisAtPDIL11 and AtPDIL12 rice OsPDIL11 OsPDIL12 andOsPDIL13 maize ZmPDIL11 and ZmPDIL12 and soybeanGmPDIL-1 show the a-b-b1015840-a1015840 domain structure (Table 1Figure 4(a)) [4 5 32 45] similar to human HsPDI andHsERp57 [46] Arabidopsis AtPDIL13 and AtPDIL14 riceOsPDIL14 maize ZmPDIL13 and ZmPDIL14 and soybeanGmPDIL-2 show a c-a-b-b1015840-a1015840 domain structure with anacidic N-terminal domain (c domain Table 1 Figure 4(a))[4 5 45] In contrast PDIL21ndash23 members show a three-domain structure in which two redox-active TRX domainsrepeated tandemly in the N-terminal region are followed bya redox-inactive domain either a TRX-like b domain (a-a1015840-b domain structure Arabidopsis AtPDIL22 and AtPDIL23rice OsPDIL23 maize ZmPDIL23 and soybean GmPDIM

[4 5 32 47] similar to human HsP5 [46]) or an 120572-helical Ddomain as found in human HsERp29 (a-a1015840-D domain struc-ture Arabidopsis AtPDIL21 rice OsPDIL21 and OsPDIL22maize ZmPDIL21 and ZmPDIL22 and soybean GmPDIS-1and GmPDIS-2 [4 5 48]) (Table 1 Figure 4(a))

Specific members of the PDI family have been suggestedto be involved in oxidative folding of storage proteins Forexample soybean GmPDIS-1 and GmPDIM (orthologs ofAtPDIL21 and OsPDIL23 resp) are localized in the ERand associate with proglycinin in the cotyledon cells [4748] GmPDIL-1 and GmPDIL-2 (orthologs of AtPDIL11 andAtPDIL13 resp) which show higher activity for refoldingof reduced denatured RNase than do GmPDIS-1 GmPDIS-2 and GmPDIM [45 47 48] are localized in the ER andassociate with proglycinin and 120573-conglycinin [45] Quanti-tative comparison of the transcript levels of maize ZmPDILgenes by massively parallel signature sequencing showedthat ZmPDIL11 is highly expressed in all organs and tissuessurveyed except for pollen [4] In the maize endospermZmPDIL11 is by far the most highly expressed PDI followedby ZmPDIL23 [4] The level of ZmPDIL11 is upregulated inthe seeds of floury-2 mutant which accumulates an abnor-mally processed 120572-zein [49] In OsERO1-knockdown mutantseeds of rice (ero1) which exhibit the abnormal accumulationof storage proteins and unfolded protein response-relatedinduction of the protein-folding chaperone BiP OsPDIL11


D

b

FADCxxC

CxxC CxxC

CxxCCxxC

CxxC

CxxCCxxC

CxxC

ERO1

PDIL11PDIs

PDIL14

PDIL21

PDIL23

TMD

a

a

a

ac

b

b

Cx4C

a998400

a998400

a998400

a998400

b998400

b998400

(a) ER

VKOR TRXTMD

a b

ZnfCYO1

TMDLQY1

CxxC

CxxCCxxC

CxxCCxxC

CxxC CxxCCxxC

CxxC

CxxCLTO1

PDIL13

VKOR-TRX

PDI

Zn finger proteins

c

Znf

a998400b998400

Cx6C

(b) Chloroplasts

CxxCERV1 FAD Erv1ALR

MIA40CPC

Cx4C

Cx16C

Cx9CCx9C

(c) Mitochondria

Figure 4 Main characterized components of the disulfide bond formation pathway in the ER chloroplasts and mitochondria of plant cells(a) Schematic illustration of ERO1 and PDIs in the plant ER (b) Schematic illustration of a VKOR homolog (LTO1) and thiol-disulfideoxidoreductases (PDIL13 LQY1 and CYO1) in chloroplasts (c) Schematic illustration of ERV1 and MIA40 in mitochondria In disulfide-generating enzymes (ERO1 LTO1 and ERV1) the predicted active-site Cys pairs shuttle Cys pairs and regulatory Cys pairs are indicated withred blue and green circles respectively The FAD cofactor is shown in yellow No cofactor of LTO1 has been identified yet (question markcircled in orange) In disulfide carrier proteins and TRX domain of LTO1 the predicted redox-active Cys pairs (CxxC in TRX domains andZn finger domains CPC in MIA40) are indicated with white circles The predicted structural Cys pairs in MIA40 (Cx

9C) and ERV1 (Cx

16C)

are indicated with brown lines Redox-active TRX (a and a1015840 domains blue boxes) redox-inactive TRX (b and b1015840 white boxes) 120572-helical D(white box) VKOR (green box) DnaJ Zn finger (Znf orange boxes) and Erv1ALR (cyan box) domains are predicted by NCBI ConservedDomain searches (httpwwwncbinlmnihgovStructurecddcddshtml) Transmembrane helices (TMD purple boxes) are predicted byTMHMM v 20 (httpwwwcbsdtudkservicesTMHMM) Acidic N-terminal c domains are also indicated (white boxes) Amino acidsequence data can be found in the GenBankEMBL databases under the following accession numbers OsERO1 OSJNEc05N03 OsPDIL11NP 001067436 AtPDIL14 NP 001190581 AtPDIL21 NP 973708 OsPDIL23 NP 001063331 AtPDIL13 NP 191056 AtLTO1 NP 567988AtLQY1 NP 177698 AtCYO1 NP 566627 AtERV1 NP 564557 AtMIA40 NP 680211

(a ZmPDIL11 ortholog a-b-b1015840-a1015840 Figure 4(a)) accumulatesto lower levels than in the wild type whereas OsPDIL23(a ZmPDIL23 ortholog a-a1015840-b Figure 4(a)) accumulates tohigher levels than in the wild type [50]

In a recent study we demonstrated that rice OsPDIL11and OsPDIL23 when expressed in the same cell show

distinct subcellular localizations substrate specificities andfunctions [32] OsPDIL11 whose redox activity for oxidativeprotein folding depends on the redox state of the catalyticactive sites (Cys69ndashCys72 and Cys414ndashCys417) is distributedin the ER lumen of endosperm cells [32 50] and plays animportant role in the formation of disulfide bonds in the


PSV-targeted storage proteins proglutelins and 120572-globulin[32 50 51] OsPDIL23 which does not complement thefunction of OsPDIL11 in the formation of disulfide bonds invivo exhibits lower activity for oxidative folding of reduceddenatured 120572-globulin and RNase than does OsPDIL11whereas OsPDIL23 exhibits higher activity for the formationof nonnative intermolecular disulfide bonds between 120572-globulin Cys79Phe mutant proteins [32] Interestingly theredox-active form of OsPDIL23 is predominantly targetedto the surface of the ER-derived PB and this localizationof OsPDIL23 is highly regulated by the redox state of theredox-active sites (Cys59ndashCys62 and Cys195ndashCys198) [32]The knockdown of OsPDIL23 inhibits the accumulation ofa Cys-rich 10-kD prolamin (Figure 3) in the core of the ER-derived PB indicating that OsPDIL23 has a specific functionin the intermolecular disulfide bond-assisted assembly ofpolypeptides into the ER-derived PB [32 52] Note that BiPis localized on the surface of the ER-derived PB in an ATP-sensitive manner [53] In cultured human cells HsP5 formsa noncovalent complex with BiP and BiP client proteins [5455] The high sequence similarity between human HsP5 andrice OsPDIL23 suggests that OsPDIL23 may also interactwith BiP If so OsPDIL23 and BiPmay be involved in recruit-ing a specific group of proteins to the ER-derived PB In addi-tion because the knockout of OsPDIL11 causes aggregationof proglutelins through nonnative intermolecular disulfidebonds and inhibits the development of the ER-derived PBand PSV [50 51] the distinct but coordinated function ofOsPDIL11 and OsPDIL23 likely supports the developmentof the ER-derived PB and PSV in rice endosperm

24 Role of PDI in the Regulation of Programmed Cell DeathIn vivo and in vitro evidence strongly suggests that membersof the PDI family play different physiological roles in differenttypes of plant cells For example Arabidopsis AtPDIL14 (c-a-b-b1015840-a1015840 Figure 4(a)) which restores the wild-type phenotypein the E coli protein-foldingmutant dsbA is highly expressedin root tips and developing seeds and interacts with BiP inthe ER thereby establishing that it is involved in oxidativeprotein folding [56] whereas the ER-localized AtPDIL21 (a-a1015840-D Figure 4(a)) which can restore thewild-type phenotypein a yeast PDI1 null mutant is highly expressed in themicropylar region of the ovule and plays a role in embryo sacdevelopment [57]

In contrast Arabidopsis AtPDIL11 (PDI5 a-b-b1015840-a1015840Figure 4(a)) may play a role in the regulation of programmedcell death (PCD) a ubiquitous physiological process thatoccurs in both prokaryotes and eukaryotes during develop-ment and under various stresses [58] The PCD pathwaysin plants and animals share some components includingspecialized Cys proteases called caspases (Cys-containingAsn-specific proteases) that function as integration pointsfor life-or-death decisions by cells [59 60] Vacuoles havebeen proposed to play a central role in various plantPCD pathways including developmentally controlled andpathogen-induced PCD pathways the collapse of vacuolesreleases enzymes into the cytosol that attack organelles andDNA leading to cell death [61 62] Vacuole-localized VPE

which was discovered originally as a novel Cys proteaseresponsible for the maturation of seed storage proteins[63] shares enzymatic properties with caspase-1 and bothenzymes undergo self-catalytic conversionactivation fromtheir inactive precursors (reviewed in [64]) VPE cleavescaspase-specific substrates and regulates the death of cellsduring the hypersensitive response and the death of limitedcell layers during early embryogenesis [64 65] Andeme-Ondzighi et al have shown that Arabidopsis AtPDIL11 acts asa redox-sensitive regulator of the activity of the noncaspase-type PCD-related Cys proteases RD21 and CP42 whichcontain a redox-sensitive active site (GxCGSCW) composed oftwo Cys residues AtPDIL11 which travels from the ER viathe Golgi to vacuoles (LVs and PSVs Table 1) specificallyinteracts with RD21 and CP42 to prevent their prematureactivation during embryogenesis and to control the timing ofthe onset of PCD by protease activation in endothelial cells[66]

25 The ERO1 System in the Plant ER The introductionof disulfides into substrate proteins leads to concomitantreduction of PDIs the reduced form of which needs to bereoxidized for the next round of the reaction cycle (Figure 1)A key question is how the plant ER establishes a redoxenvironment that favors PDI regeneration and promotesoxidative folding of nascent polypeptides As described in theintroductory section disulfide bond formation involves thetransfer of electrons from a substrate protein to a disulfidecarrier protein (eg PDI) and then to a de novo disulfide-generating enzyme (Figure 1) [3] An example of disulfide-generating enzyme is the flavoenzyme Ero1p which was firstidentified in yeast by mutation studies of the temperature-sensitive conditional mutant ero1-1 [67 68]The ERO1 familymembers are highly conserved in eukaryotic organismsincluding yeast humans and plantsThe Saccharomyces cere-visiae Caenorhabditis elegans and Drosophila melanogastergenomes each encode only a single-copy ERO1 gene Genomeduplication is observed in humans HsERO1120572 and HsERO1120573[69ndash71] In most higher and lower plants including Ara-bidopsis [72] maize and moss (Physcomitrella patens) thereare two paralogues of ERO1 [73] Rice contains a singleortholog of ERO1 [50] as do Chlamydomonas reinhardtii andVolvox carteri [73]

ERO1 introduces disulfides in PDI by relaying oxidizingpower from molecular oxygen to the reduced form of PDIvia the ERO1 flavin cofactor [74ndash76] ERO1 proteins possesstwo conserved catalytic Cys pairs a shuttle Cys pair andan active-site Cys pair (Cx

4C and CxxC resp Figure 4(a))

The shuttle Cys pair of Ero1p (Cys100ndashCys105 yeast Ero1pnumbering) directly oxidizes the active site of Pdi1p bymeansof a thiol-disulfide exchange reaction and is subsequentlyreoxidized by the active-site Cys pair (Cys352ndashCys355 yeastEro1p numbering) [77ndash79] The active-site Cys pair of Ero1pis reoxidized by transferring their electrons to the flavincofactor and then to molecular oxygen [74ndash76] Plant ERO1proteins share greater homology with human HsERO1120572 thanwith yeast Ero1p [50 73] Modeling analysis of ArabidopsisAtERO1 shows that the locations of the conserved Cys


residues in AtERO1 are similar to the locations in HsERO1120572(PDB ID code 3AHQ [80]) [73] Rice OsERO1 possesses twopairs of Cys residues Cys134ndashCys139 and Cys391ndashCys394which correspond to Cys94-Cys99 (shuttle Cys pair) andCys394ndashCys397 (active-site Cys pair) of human HsERO1120572respectively (Figure 4(a)) [50]

Yeast Ero1p is tightly associated with the luminal faceof the ER membrane via the 127-aa C-terminal tail [6781] Human HsERO1120572 which lacks the C-terminal domainfound in yeast Ero1p contains a 23-aa signal peptide and isretained in the ER by covalent interactions with HsPDI andHsERp44 [70 81 82] Although the subcellular localizationof Arabidopsis AtERO1 proteins has not been characterizedAtERO1 proteins have been shown to be glycosylated [72]Unlike ERO1 proteins from yeast and human rice OsERO1 islocalized to the ER membrane and this localization dependson the N-terminal region (Met1-Ser55) which contains anAlaPro-rich sequence (Met1-Trp36) and a putative trans-membrane domain (TMD Ala37-Ser55 Figure 4(a)) [50]

We studied the functions of rice OsERO1 by means ofRNAi-induced knockdown of OsERO1 under the control ofan endosperm-specific promoter The OsERO1-knockdownseeds which produce OsERO1 at dramatically reduced lev-els fail to develop the typical PSV and ER-derived PBand abnormal aggregates form instead [50] Knockdown ofOsERO1 inhibits the formation of native disulfide bondsin proglutelins and promotes the formation of proglutelin-containing aggregates through nonnative intermoleculardisulfide bonds in the ER as does knockout of OsPDIL11[50]These results indicate that the ERO1-dependent pathwayplays an important role in the formation of native disulfidebonds in the ER of rice endosperm cells

ERO1 proteins possess a feedback system that regulatesERO1 activity in response to fluctuations in the ER redoxenvironment allowing the cell to maintain the redox condi-tions in the ER favorable for native disulfide bond formation[83 84] Yeast Ero1p contains two noncatalytic regulatoryCys pairs (Cys90ndashCys349 and Cys150ndashCys295) reduction ofthe disulfide bonds between these Cys pairs increases Ero1pactivity [83] Human HsERO1120572 possesses a different modeof regulation HsERO1120572 activity is regulated by an internaldisulfide rearrangement in which Cys94 forms a catalyticdisulfide bond with Cys99 (shuttle Cys pair Cys94ndashCys99) ora regulatory disulfide bond with Cys131 [80 84] A similarregulatory system may also exist in plant ERO1 proteinsincluding Arabidopsis AtERO1 and rice OsERO1 in whichthe Cys residues corresponding to Cys150ndashCys295 of yeastEro1p are missing but those corresponding to Cys94 Cys99and Cys131 of human HsERO1120572 are present as indicated bysequence similarity analysis [50]

Additionally the ERO1-mediated disulfide transfer sys-tem produces hydrogen peroxide a reactive oxygen species(ROS) as a by-product (Figure 5(a)) [76] Overexpression ofERO1 has been shown to cause a significant increase in ROSwhereas a partial reduction of ERO1 activity suppresses ROSaccumulation and increases resistance to the lethal effects ofER stress [85 86] Although hydrogen peroxide and otherROS are cytotoxic hydrogen peroxide sometimes plays a keyrole as a signal transduction messenger in eukaryotic cells

[87] High concentrations of hydrogen peroxide may causeperoxidation of membrane lipids and gradually destroy themembrane integrity in plant cells leading to the leakage ofsmall molecules including water [88] The production ofhydrogen peroxide in the ER of rice endosperm cells corre-lates with the oxidation of sulfhydryl groups in seed storageproteins [50]The homozygousmutant seeds of EM49 whichhave fewer sulfhydryl groups and generate less hydrogenperoxide than wild-type seeds contain a higher proportionof water than wild-type seeds during seed development andshow delayed seed desiccation and maturation [50] Unlikethe adjacent 2n embryo the 3n endosperm cells whichsynthesize vast amounts of disulfide-rich proteins duringearly seed development are destined for PCD during seeddesiccation and maturation [89] The hydrogen peroxidegenerated during the formation of disulfide bonds in the ERof endosperm cells may serve as a signal for inducing PCDand subsequent seed desiccation and maturation

Another key question is how ERO1 recognizes andoxidizes specific PDI proteins in the plant ER Yeast Ero1ppreferentially oxidizes the CxxC motif in the N-terminalTRX domain of Pdi1p [90] In mammalian cells specificcombinations of ERO1 and PDI members drive the oxidativefolding pathways For example human HsERO1120572 showsdifferent affinities for human HsPDI and HsERp57 [82 9192] Crystallographic and biochemical analyses have revealedthat electrostatic and hydrophobic interactions betweenHsERO1120572 and the PDI b1015840 domain allow for effective oxidationof specific PDI proteins by HsERO1120572 [80] Although humanHsERp57 shares the a-b-b1015840-a1015840 domain organization of humanHsPDI HsERp57 exhibits a different electrostatic potentialon the b-b1015840 domain and lacks the hydrophobic pocket thatis essential for substrate binding in the b1015840 domain [80 93]It remains unclear which members of the rice PDI familyare specific partners of OsERO1 in the ER (Figure 5(a))However given that OsPDIL11 and OsPDIL23 differ indomain organization (a-b-b1015840-a1015840 and a-a1015840-b resp) and thatsequence similarity between OsPDIL11 and OsPDIL14 ishigher in domains a and a1015840 than in domains b and b1015840 it isplausible that OsERO1 may show different affinities for thesePDI proteins

26 Alternative Pathways for Disulfide Bond Formation inthe Plant ER The yeast ERO1 gene is essential for viability[67 68] In human cells HsERO1120572 is expressed in mostcell types whereas HsERO1120573 is expressed only in selectedtissues [69ndash71] Interestingly disruption of HsERO1120572 andHsERO1120573 which have selective and nonredundant functionsin oxidative protein folding causes only a modest delay inIgM production and is not lethal suggesting that there areERO1-independent pathways for disulfide bond formationin mammalian cells [94] In fact a candidate enzyme for aparallel ERO1-independent pathway has recently been uncov-ered ER-localized peroxiredoxin (PRX) IV uses hydrogenperoxide to convert thiols into disulfides in the ER andtransfers these disulfides to PDI which in turn introducesdisulfide bonds into nascent polypeptides [95] In additionthe human glutathione peroxidase familymembersGPx7 and


-SH HS-

-S-S-

-SH HS-

-S-S-

O2

H2O2

2eminus2eminus

2eminus2eminus

2eminus

2eminus

Xred

Xox Yred

Yox

PDILxxox

PDILxxox

PDILxxred

PDILxxred

ERO1ox

ERO1red

(a) ER

-SH HS-

-S-S-

-SH HS-

-S-S-

-SH HS-

-S-S-

2eminus

2eminus2eminus

2eminus

2eminus2eminus

2eminus

Xred

Xred

Xox

Xox

Yred

Yred

Yox

Yox

2eminusYred

YoxPDILxxox

PDILxxred

VKOR-TRXox

VKOR-TRXred

ZnfPox

ZnfPred

(b) Chloroplasts

Respiratory chain

-SH HS-

-S-S-

H2O

O2

H2O22eminus2eminus

2eminus

2eminus

2eminus

2eminus

MIA40ox

MIA40red ERV1ox

ERV1red cyt cox

cyt cred

12O2

(c) Mitochondria

Figure 5 Multiple electron transfer pathways for oxidative protein folding in the ER chloroplasts and mitochondria of plant cells (a)ERO1-dependent and PDI-dependent pathways in the ER (b) VKOR-TRX-dependent Zn finger protein-dependent and PDI-dependentpathways in chloroplasts LTO1 is indicated as VKOR-TRX LQY1 and CYO1 are indicated as ZnfP (c) ERV1-MIA40-meditaed pathwayin mitochondria One-electron carrier cytochrome c is indicated as cyt c Xox and Yox represent unidentified de novo disulfide-generatingenzymes and electron acceptors respectively Whether de novo disulfide-generating enzymes (Xox) are involved in the LQY1-dependent andCYO1-dependent disulfide transfer pathways is not clear

GPx8 are involved in the pathway that couples hydrogenperoxide production to disulfide bond formation GPx7 andGPx8 are localized in the ER and preferentially oxidize PDIin the presence of hydrogen peroxide enabling disulfide bondformation in a reduced unfolded protein [96]

Native and nonnative disulfide bonds form in OsERO1-knockdownmutant seeds of rice suggesting that unidentifiedde novo disulfide-generating enzymes are also involved inthe oxidative protein folding in the ER of endosperm cells(Figure 5(a)) [50] In plants PRXs play central roles in theantioxidant defense system including thiol-based reductionof hydrogen peroxide and in the dithiol-disulfide redox regu-latory network [97]Themembers of the plant PRX family arelocalized in chloroplasts mitochondria the cytosol and thenucleus but no ER-localized PRX has yet been identified inplants [97ndash99] Among the putative TRX-dependent peroxi-dases of plants glutathione-dependent peroxidase Gpx5 hasbeen predicted to showdual localization to the ER and cytosol[98]

3 Disulfide Bond Formation in Chloroplasts

31 Role of PDI in Chloroplasts Because it is derived from acyanobacterial ancestor the chloroplast combines prokary-otic and eukaryotic features of gene expression Thousands

of chloroplast proteins including photosynthesis-related pro-teins are encoded by nuclear genes and are synthesized in thecytoplasmCytoplasmically synthesized chloroplastic precur-sorsmust be unfolded for translocation across the chloroplas-tic double envelope membranes [100] Most of the unfoldingtakes place within the membranes themselves and proteinsare then refolded before localizing to their target destinationsIn addition to the nuclear-encoded chloroplast proteins thereare also approximately 100 chloroplast proteins encoded bythe organellersquos own genome [101]The transcription of chloro-plast genes is driven by plastid transcription systems nuclear-encoded andplastid-encoded plastidRNApolymerases [102]

Oxygen-evolving photosynthetic organisms are subjectto irreversible light-induced damage Photodamage occursprimarily in PSII in the thylakoid membranes and leadsto inactivation of photosynthetic electron transport andsubsequent oxidative damage of the reaction center proteins[103] To maintain PSII activity the damaged D1 and D2 pro-teins in the PSII reaction center are degraded and replacedand expression of plastid-encoded psbA and psbD (whichencode D1 and D2 resp) is rapidly activated at the level oftranscription in response to light (reviewed in [102 104]) Inchloroplasts reducing equivalents are generated from lightby PSII and PSI and are transferred to the ferredoxin-TRXsystem which in turn reduces regulatory redox-active sitesof target proteins including key proteins involved in carbon


dioxide assimilation and ATP synthesis [13 105] It has beensuggested that a reductive signal transduced by TRX activatesa 51015840 protein complex with high affinity for the 51015840 UTR of psbAmRNA [14]The genomes of the green algae Chlamydomonasreinhardtii andVolvox carteri contain 8 and 7 genes encodingPDI proteins respectively [5] Chlamydomonas reinhardtiiRB60 which contains two CGHCmotifs and the C-terminalKDEL signal functions as a redox-responsive translationalregulator in chloroplasts [106 107] The reduction-oxidationof RB60 located in a complex bound to the 51015840 UTR of psbAmRNA plays a key role in determining the rate of psbAtranslation [106 108]

Arabidopsis AtPDIL13 (Figure 4(b)) which shows highsequence similarity to RB60 and contains the KDEL signalbut not a cleavable chloroplast-targeting signal is localized tothe stromal-starch interface in chloroplasts in addition to theER [109ndash111] The expression of AtPDIL13 is upregulated bylight during chloroplast development and this upregulationis correlated with starch synthesis [109 112] AtPDIL13 likelyplays an important role in protein folding or in enzymeregulation of proteins associated with starch metabolism inchloroplasts [109]

32 VKORHomolog-Dependent Disulfide Transfer Pathway inChloroplasts Roles in Assembly of the Photosystem ComplexesAlthough plants are subject to irreversible photodamage asdescribed above they have evolved a highly specialized repairmechanism that restores the functional status of PSII PSIIrepair requires disassembly and reassembly of a thylakoidmembrane protein complex composed of dozens of proteins(including the reaction center proteins and oxygen-evolvingproteins of PSII) and hundreds of cofactors and the breakageand reformation of disulfide bonds among PSII proteins [15113 114] Recently a disulfide-generating VKOR homolog hasbeen demonstrated to play important roles in assembly of thephotosystem complexes in chloroplasts

In mammals VKOR catalyzes a step in the vitamin Kcycle which occurs in the ER membrane and is required forsustaining blood coagulation Human HsVKOR an integralmembrane protein of the ER consists of four TMDs with thesame membrane topology as the core of a bacterial homologof VKOR from Synechococcus sp [115 116] The physiologicalredox partner of HsVKOR is a membrane-anchoredmemberof the PDI family with which HsERO1120572 interacts only poorly[116] VKOR is a member of a large family of homologsthat are present in various organisms including vertebratesplants bacteria and archaea [117] All homologs containan active-site CxxC motif and an additional pair of loopCys residues [115 117] In some homologs from plants andbacteria includingArabidopsis cyanobacteria Synechococcussp and Synechocystis 6803 the domain homologous toHsVKOR is fused with the TRX-like domain typical of thePDI family (Figure 4(b)) [115 117ndash119] A crystal structureof the Synechococcus VKOR has revealed that electrons aretransferred from a substrate protein to the reduced Cys pair(Cys209ndashCys212) of the soluble C-terminal TRX-like domainand then to the two conserved loop Cys residues (Cys50and Cys56) of the VKOR domain Subsequently electrons

are transferred from the loop Cys pair (Cys50ndashCys56) tothe active-site CxxC pair (Cys130ndashCys133) [115] Like E coliDsbB VKOR accepts the oxidizing power from quinoneTheC-terminal Cys residue (Cys133) in the active-site Cys pair islocated on the side of the ring of quinone [115]

Arabidopsis VKOR homolog (designated LTO1) is athylakoid membrane-localized protein that is composed ofone VKOR domain one TRX domain and four TMDs(Figure 4(b)) as found in the Synechococcus VKOR andcatalyzes the formation of disulfide bonds (Figure 5(b)) [119ndash121] LTO1 is required for the assembly of PSII by means ofthe formation of a disulfide bond in PsbO a luminal subunitof the PSII oxygen-evolving complex [120] AdditionallyLTO1may regulate the redox state of Cys-containing proteinsresiding in and facing the thylakoid lumen In vitro enzymaticassays indicate that LTO1 is active in reducing phylloquinonea structural cofactor tightly bound to PSI [122] to phylloqui-non but does not reduce either phylloquinone epoxide orplastoquinone [119] Whether phylloquinone is involved inthe LTO1-mediated thiol-oxidation pathway in chloroplastsis not clear

33 Zinc Finger Protein-Dependent Disulfide Transfer Path-ways in Chloroplasts Roles in Photosystem Repair and Chloro-plast Biogenesis Other chloroplast proteins for which disul-fide bond-forming activity has been reported are zinc finger-like proteins LQY1 [123] and CYO1 [124] (Figures 4(b) and5(b)) The LQY1 protein which is localized in the thylakoidmembranes and binds to the PSII coremonomer is suggestedto be involved in maintaining PSII activity and regulatingthe repair and reassembly of PSII under high-light conditions[123] Arabidopsis mutants that lack the LQY1 protein showelevated accumulation of ROS and reduced levels of PSII-light-harvesting complex II supercomplex under high-lightconditions [123] Proteins homologous to Arabidopsis CYO1are widely found in plants including rice maize and soy-bean but not in moss and algae [124] The levels of the CYO1transcript and the CYO1 protein increase in response to lightand theCYO1 protein is localized to the thylakoidmembranesin Arabidopsis [124] The cyo1 mutation does not affect thebiogenesis of etioplasts under dark conditions but severelyimpairs the development of thylakoid membranes underlight conditions indicating that CYO1 plays an importantrole in chloroplast biogenesis [124] Taken together with theevidence that LQY1 andCYO1 each contain a zinc fingermotif(Figure 4(b)) similar to that in E coli DnaJ bind Zn2+ andexhibit PDI-like activity [123ndash125] the above data suggestthat the PDI-like activity (including disulfide isomerizationactivity and chaperone activity) of LQY1 and CYO1 maybe required for the repair of PSII and thylakoid biogenesisrespectively

4 Disulfide Bond Formation inPlant Mitochondria

In mitochondria which are also surrounded by a dou-ble membrane the disulfide transfer pathway for oxidativeprotein folding is linked to the redox-regulated import


of precursor proteins Mitochondria play critical roles innumerous cellular processes including energy metabolismand PCDMitochondria are derived from bacterial endosym-bionts and their genomes possess limited coding capacity(approximately 40ndash50 genes) [126] thus they largely dependon the energy-dependent import of nuclear-encoded pro-teins that are synthesized in the cytosol The translocase ofthe outer membrane designated the TOM complex is themain entry gate for importing mitochondrial preproteinsIn yeast N-terminal presequence-carrying and hydrophobicprecursor proteins are initially recognized by Tom20 andTom70 respectively subsequently transferred to the cen-tral receptor Tom22 and then inserted into the channelof the central component Tom40 [127] Among the foursubsequent sorting pathways leading to intramitochondrialdestinations the mitochondrial intermembrane space (IMS)import and assembly machinery (MIA) directs small Cys-rich precursors into the IMS [127] The MIA pathway is aredox-driven import pathway that requires the cooperativefunction of the essential IMS proteins Yeast Mia40 bindsto the precursors of IMS proteins with either twin Cx

3C or

Cx9C motifs including members of the inner mitochondrial

membrane chaperone complex (TIM) Tim8ndashTim13 In yeasta ternary complex composed of Mia40 (disulfide carrierprotein) the FAD-bound sulfhydryl oxidase Erv1 (disulfide-generating enzyme) and the substrate facilitates the relayof disulfides from Erv1 to the precursor protein via Mia40and the reduced form of Mia40 is reoxidized by Erv1 whichtransfers electrons via cytochrome c (one-electron carrier) tothe respiratory chain and then to molecular oxygen or canuse molecular oxygen directly resulting in the generation ofhydrogen peroxide (Figure 5(c)) [127ndash130]

Plant orthologs of yeast Mia40 and Erv1 have been foundArabidopsis AtMIA40 and AtERV1 each contain a highlyconserved redox-active Cys pair (CPC motif in AtMIA40CxxC in AtERV1 Figure 4(c)) and structural Cys pairs(two Cx

9C in AtMIA40 Cx

16C in AtERV1 Figure 4(c)) and

localize to mitochondria [131 132] Arabidopsis AtERV1 alsocontains noncovalently bound FAD (Figure 4(c)) and showssulfhydryl oxidase activity [132] Conservation of Mia40 andErv1 orthologs and the presence ofMIA import pathway sub-strates in yeast animals and plants suggest the common useof the MIA pathway [133] Both Mia40 and Erv1 are essentialin Saccharomyces cerevisiae [134 135] and deletion of Ara-bidopsis AtERV1 causes embryonic lethality [131] Howeverdeletion of AtMIA40 in Arabidopsis causes no clear phe-notype [131] Additionally unlike yeast Mia40 ArabidopsisAtMIA40 which is localized to both mitochondria and per-oxisomes does not seem to play an essential role in the importandor assembly of small Tim proteins in mitochondriaInstead the loss of AtMIA40 leads to the absence frommito-chondria of two IMS-localized proteins copperzinc super-oxide dismutase (CSD1 which has a dissimilar Cys pair fromthe Cys motif found in Mia40 substrates) and the chaperonethat inserts copper into CSD1 although both are still success-fully imported intomitochondria [131] Loss of AtMIA40 alsoresults in a decrease in the capacity of complex I and the effi-ciency of the assembly of subunits into complex I [131] Ara-bidopsis AtERV1 which is unable to substitute for the yeast

enzyme in vivo contains both the common features of theErv1ALR protein family and unique features AtERV1 lacksCxxC (shuttle Cys pair) in the N-terminal domain found inyeast Erv1 and human ALR but contains a C-terminal Cx

4C

motif conserved among plant ERV1 homologs which is likelyto function in the interdisulfide relay [132] MIA40 and ERV1may thus play both conserved and distinct roles in plants

5 Disulfide Bond Formation Outside the Cell

Quiescin sulfhydryl oxidase (QSOX) is an atypical disulfidecatalyst localized at various subcellular locations or outsidethe cellMammalianQSOXproteins are localized to theGolgiin the post-ER secretory pathways or secreted fromcells [136ndash138] and Arabidopsis AtQSOX1 when expressed in the leafepidermis is found in the cell wall rather than the plasmamembrane (Table 1) [139] QSOX in which the Erv domainhas been linked with a redox-active TRX domain during evo-lution catalyzes both de novo disulfide generation and disul-fide transfer Generation and transfer of disulfide bonds byQSOX are mediated by two redox-active Cys pairs one in anErv domain and the other in a TRX-like domain Intramolec-ular disulfides are catalytically generated at the noncovalentlybound FAD-proximal Cys pair in the Erv domain of QSOXand are in turn transferred to the Cys pair in the TRX domainby means of interdomain electron transfer which is followedby intermolecular thiol-disulfide exchange between QSOXand a substrate [140ndash142] AtQSOX1 contains a CxxC motifin the TRXdomain and thismotif exhibits sulfhydryl oxidaseactivity on the small-molecule substrate dithiothreitol but notelectron transfer activity from the TRX domain to the Ervdomain [143]The expression ofAtQSOX1 in leaves is upregu-lated byK+ starvation andAtQSOX1 plays a role in regulatingcation homeostasis by activating root systems that load K+into the xylem [139] This K+ efflux system may be regulatedby AtQSOX1-mediated oxidation of sulfhydryl groups of thetransporter at the external side of the plasmamembrane [139]

In conclusion plant cells have evolved multiple anddistinct oxidative protein-folding systems in the ER chloro-plasts and mitochondria These systems which requirethe cooperative functions of disulfide-generating enzymesand disulfide carrier proteins support cell function anddevelopment and protect against environmental fluctuationsIndividual disulfide carrier proteins (eg PDIs) recognizespecific sets of substrate proteins Current interest lies inuncovering the broad and specific network for oxidativeprotein folding in plant cells Further study is needed to definethe functions of known disulfide-generating enzymes thatsupply oxidizing power to specific sets of disulfide carrierproteins (eg PDIs) to discover additional such enzymesand to identify the final electron acceptors in the oxidativeprotein-folding systems in plant cells

Abbreviations

ER Endoplasmic reticulumERO1 ER oxidoreductase 1PB Protein body


PCD Programmed cell deathPDI Protein disulfide isomerasePRX PeroxiredoxinPSV Protein storage vacuoleQSOX Quiescin sulfhydryl oxidaseROS Reactive oxygen speciesTMD Transmembrane domainTRX ThioredoxinVKOR Vitamin K epoxide reductaseVPE Vacuolar processing enzyme

Acknowledgments

This work was supported by a Grant-in-Aid for YoungScientists from the Japan Society for the Promotion of Science(23780101) by a Shiseido Female Researcher Science Grantand by the Ministry of Education Culture Sports Scienceand Technology of Japan

References

[1] M Molinari ldquoN-glycan structure dictates extension of proteinfolding or onset of disposalrdquoNature Chemical Biology vol 3 no6 pp 313ndash320 2007

[2] D Fass ldquoDisulfide bonding in protein biophysicsrdquo AnnualReview of Biophysics vol 41 pp 63ndash79 2012

[3] J Riemer N Bulleid and J M Herrmann ldquoDisulfide formationin the ER and mitochondria two solutions to a commonprocessrdquo Science vol 324 no 5932 pp 1284ndash1287 2009

[4] N L Houston C Z Fan Q-Y Xiang J-M Schulze R Jungand R S Boston ldquoPhylogenetic analyses identify 10 classesof the protein bisulfide isomerase family in plants includingsingle-domain protein disulfide isomerase-related proteinsrdquoPlant Physiology vol 137 no 2 pp 762ndash778 2005

[5] B Selles J-P Jacquot and N Rouhier ldquoComparative genomicstudy of protein disulfide isomerases from photosyntheticorganismsrdquo Genomics vol 97 no 1 pp 37ndash50 2011

[6] D C Bassham and N V Raikhel ldquoUnique features of the plantvacuolar sorting machineryrdquo Current Opinion in Cell Biologyvol 12 no 4 pp 491ndash495 2000

[7] E M Herman and B A Larkins ldquoProtein storage bodies andvacuolesrdquoThe Plant Cell vol 11 no 4 pp 601ndash613 1999

[8] E M Herman ldquoEndoplasmic reticulum bodies solving theinsolublerdquo Current Opinion in Plant Biology vol 11 no 6 pp672ndash679 2008

[9] K Muntz ldquoDeposition of storage proteinsrdquo Plant MolecularBiology vol 38 no 1-2 pp 77ndash99 1998

[10] V Ibl and E Stoger ldquoThe formation function and fate of proteinstorage compartments in seedsrdquo Protoplasma vol 249 no 2 pp379ndash392 2012

[11] B B Buchanan and Y Balmer ldquoRedox regulation a broadeninghorizonrdquo Annual Review of Plant Biology vol 56 pp 187ndash2202005

[12] K Kobrehel J H Wong A Balogh F Kiss B C Yee and BB Buchanan ldquoSpecific reduction of wheat storage proteins bythioredoxin hrdquoPlant Physiology vol 99 no 3 pp 919ndash924 1992

[13] P Schurmann and B B Buchanan ldquoThe ferredoxinthioredoxinsystem of oxygenic photosynthesisrdquo Antioxidants amp RedoxSignaling vol 10 no 7 pp 1235ndash1273 2008

[14] A Danon and S P Mayfield ldquoLight-regulated translation ofchloroplast messenger RNAs through redox potentialrdquo Sciencevol 266 no 5191 pp 1717ndash1719 1994

[15] E Baena-Gonzalez and E-M Aro ldquoBiogenesis assembly andturnover of photosystem II unitsrdquo Philosophical Transactions ofthe Royal Society B vol 357 no 1426 pp 1451ndash1460 2002

[16] P R Shewry J A Napier and A S Tatham ldquoSeed storageproteins structures and biosynthesisrdquoThe Plant Cell vol 7 no7 pp 945ndash956 1995

[17] X X Li and T W Okita ldquoAccumulation of prolamines andglutelins during rice seed development a quantitative evalua-tionrdquo Plant and Cell Physiology vol 34 no 3 pp 385ndash390 1993

[18] S Utsumi ldquoPlant food protein engineeringrdquo Advances in Foodand Nutrition Research vol 36 pp 89ndash208 1992

[19] M P Scott R Jung K Muntz and N C Nielsen ldquoA proteaseresponsible for post-translational cleavage of a conserved Asn-Gly linkage in glycinin the major seed storage protein ofsoybeanrdquo Proceedings of the National Academy of Sciences of theUnited States of America vol 89 no 2 pp 658ndash662 1992

[20] I Hara-Nishimura T Shimada N Hiraiwa andM NishimuraldquoVacuolar processing enzyme responsible for maturation ofseed proteinsrdquo Journal of Plant Physiol vol 145 pp 632ndash6401995

[21] N CNielsen CDDickinson T J Cho et al ldquoCharacterizationof the glycinin gene family in soybeanrdquoThePlant Cell vol 1 no3 pp 313ndash328 1989

[22] Y Onda and Y Kawagoe ldquoOxidative protein folding selectivepressure for prolamin evolution in ricerdquo Plant Signaling ampBehavior vol 6 no 12 pp 1966ndash1972 2011

[23] D Pantoja-Uceda P R Shewry M Bruix A S Tatham JSantoro and M Rico ldquoSolution structure of a methionine-rich2S albumin from sunflower seeds relationship to its allergenicand emulsifying propertiesrdquo Biochemistry vol 43 no 22 pp6976ndash6986 2004

[24] C D Dickinson E H Hussein and N C Nielsen ldquoRole ofposttranslational cleavage in glycinin assemblyrdquoThe Plant Cellvol 1 no 4 pp 459ndash469 1989

[25] H Levanony R Rubin Y Altschuler and G Galili ldquoEvidencefor a novel route of wheat storage proteins to vacuolesrdquo TheJournal of Cell Biology vol 119 no 5 pp 1117ndash1128 1992

[26] R Jung Y W Nam I Saalbach K Muntz and N C NielsenldquoRole of the sulfhydryl redox state and disulfide bonds inprocessing and assembly of 11S seed globulinsrdquo The Plant Cellvol 9 no 11 pp 2037ndash2050 1997

[27] Y Kawagoe K Suzuki M Tasaki et al ldquoThe critical role ofdisulfide bond formation in protein sorting in the endospermof ricerdquoThe Plant Cell vol 17 no 4 pp 1141ndash1153 2005

[28] H Yamagata T Sugimoto K Tanaka and Z Kasai ldquoBiosyn-thesis of storage proteins in developing rice seedsrdquo PlantPhysiology vol 70 no 4 pp 1094ndash1100 1982

[29] H B Krishnan and TW Okita ldquoStructural relationship amongthe rice glutelin polypeptidesrdquo Plant Physiology vol 81 no 3pp 748ndash753 1986

[30] Y H Wang S S Zhu S J Liu et al ldquoThe vacuolar processingenzyme OsVPE1 is required for efficient glutelin processing inricerdquoThe Plant Journal vol 58 no 4 pp 606ndash617 2009

[31] T Kumamaru Y Uemura Y Inoue et al ldquoVacuolar processingenzyme plays an essential role in the crystalline structure ofglutelin in rice seedrdquo Plant and Cell Physiology vol 51 no 1pp 38ndash46 2010


[32] Y Onda A Nagamine M Sakurai T Kumamaru M Ogawaand Y Kawagoe ldquoDistinct roles of protein disulfide isomeraseand P5 sulfhydryl oxidoreductases in multiple pathways foroxidation of structurally diverse storage proteins in ricerdquo ThePlant Cell vol 23 no 1 pp 210ndash223 2011

[33] A Nagamine H Matsusaka T Ushijima et al ldquoA role for thecysteine-rich 10 kDa prolamin in protein body I formation inricerdquoPlant andCell Physiology vol 52 no 6 pp 1003ndash1016 2011

[34] M Ogawa T Kumamaru H Satoh et al ldquoPurification ofprotein body-I of rice seed and its polypeptide compositionrdquoPlant and Cell Physiology vol 28 no 8 pp 1517ndash1527 1987

[35] N Mitsukawa R Konishi K Kidzu K Ohtsuki T Masumuraand K Tanaka ldquoAmino acid sequencing and cDNA cloning ofrice seed storage proteins the 13kDa prolamins extracted fromtype I protein bodiesrdquoPlant Biotechnology vol 16 no 2 pp 103ndash113 1999

[36] C R Lending and B A Larkins ldquoChanges in the zein composi-tion of protein bodies during maize endosperm developmentrdquoThe Plant Cell vol 1 no 10 pp 1011ndash1023 1989

[37] D R HoldingM S Otegui B Li et al ldquoThemaize Floury1 geneencodes a novel endoplasmic reticulumprotein involved in zeinprotein body formationrdquoThe Plant Cell vol 19 no 8 pp 2569ndash2582 2007

[38] F Hatahet and L W Ruddock ldquoProtein disulfide isomerase acritical evaluation of its function in disulfide bond formationrdquoAntioxidants amp Redox Signaling vol 11 no 11 pp 2807ndash28502009

[39] R Farquhar N Honey S J Murant et al ldquoProtein disulfideisomerase is essential for viability in Saccharomyces cerevisiaerdquoGene vol 108 no 1 pp 81ndash89 1991

[40] G Tian S Xiang R Noiva W J Lennarz and H SchindelinldquoThe crystal structure of yeast protein disulfide isomerasesuggests cooperativity between its active sitesrdquo Cell vol 124 no1 pp 61ndash73 2006

[41] G Tian F-X Kober U Lewandrowski A Sickmann WJ Lennarz and H Schindelin ldquoThe catalytic activity ofprotein-disulfide isomerase requires a conformationally flexiblemoleculerdquo The Journal of Biological Chemistry vol 283 no 48pp 33630ndash33640 2008

[42] N J Darby E Penka and R Vincentelli ldquoThe multi-domainstructure of protein disulfide isomerase is essential for highcatalytic efficiencyrdquo Journal of Molecular Biology vol 276 no1 pp 239ndash247 1998

[43] P Klappa LW Ruddock N J Darby and R B Freedman ldquoTheb1015840 domain provides the principal peptide-binding site of proteindisulfide isomerase but all domains contribute to binding ofmisfolded proteinsrdquo The EMBO Journal vol 17 no 4 pp 927ndash935 1998

[44] L J Byrne A Sidhu A K Wallis et al ldquoMapping of theligand-binding site on the b1015840 domain of human PDI interactionwith peptide ligands and the x-linker regionrdquo The BiochemicalJournal vol 423 no 2 pp 209ndash217 2009

[45] S Kamauchi H Wadahama K Iwasaki et al ldquoMolecularcloning and characterization of two soybean protein disulfideisomerases as molecular chaperones for seed storage proteinsrdquoThe FEBS Journal vol 275 no 10 pp 2644ndash2658 2008

[46] C Appenzeller-Herzog and L Ellgaard ldquoThe human PDIfamily versatility packed into a single foldrdquo Biochimica etBiophysica Acta vol 1783 no 4 pp 535ndash548 2008

[47] H Wadahama S Kamauchi Y Nakamoto et al ldquoA novel plantprotein disulfide isomerase family homologous to animal P5mdashmolecular cloning and characterization as a functional protein

for folding of soybean seed-storage proteinsrdquoThe FEBS Journalvol 275 no 3 pp 399ndash410 2008

[48] H Wadahama S Kamauchi M Ishimoto T Kawada and RUrade ldquoProtein disulfide isomerase family proteins involved insoybean protein biogenesisrdquo The FEBS Journal vol 274 no 3pp 687ndash703 2007

[49] C P Li and B A Larkins ldquoExpression of protein disulfideisomerase is elevated in the endosperm of the maize floury-2mutantrdquo Plant Molecular Biology vol 30 no 5 pp 873ndash8821996

[50] Y Onda T Kumamaru and Y Kawagoe ldquoER membrane-localized oxidoreductase Ero1 is required for disulfide bondformation in the rice endospermrdquo Proceedings of the NationalAcademy of Sciences of the United States of America vol 106 no33 pp 14156ndash14161 2009

[51] Y Takemoto S J Coughlan T W Okita H Satoh M Ogawaand T Kumamaru ldquoThe rice mutant esp2 greatly accumulatesthe glutelin precursor and deletes the protein disulfide iso-meraserdquo Plant Physiology vol 128 no 4 pp 1212ndash1222 2002

[52] Y Onda and Y Kawagoe ldquoP5-type sulfhydryl oxidoreductasepromotes the sorting of proteins to protein body I in riceendosperm cellsrdquo Plant Signaling amp Behavior vol 8 no 2Article ID e23075 2013

[53] X Li Y Wu D-Z Zhang et al ldquoRice prolamine protein bodybiogenesis a BiP-mediated processrdquo Science vol 262 no 5136pp 1054ndash1056 1993

[54] L Meunier Y-K Usherwood K Tae Chung and L MHendershot ldquoA subset of chaperones and folding enzymesformmultiprotein complexes in endoplasmic reticulum to bindnascent proteinsrdquo Molecular Biology of the Cell vol 13 no 12pp 4456ndash4469 2002

[55] C E Jessop R H Watkins J J Simmons M Tasab andN J Bulleid ldquoProtein disulphide isomerase family membersshow distinct substrate specificity P5 is targeted to BiP clientproteinsrdquo Journal of Cell Science vol 122 part 23 pp 4287ndash42952009

[56] E J Cho C Y L Yuen B-H Kang C Andeme-Ondzighi L AStaehelin andD A Christopher ldquoProtein disulfide isomerase-2ofArabidopsismediates protein folding and localizes to both thesecretory pathway and nucleus where it interacts withmaternaleffect embryo arrest factorrdquo Molecules and Cells vol 32 no 5pp 459ndash475 2011

[57] H Wang L C Boavida M Ron and S McCormick ldquoTrun-cation of a protein disulfide isomerase PDIL2-1 delays embryosacmaturation anddisrupts pollen tube guidance inArabidopsisthalianardquoThe Plant Cell vol 20 no 12 pp 3300ndash3311 2008

[58] D L Vaux and S J Korsmeyer ldquoCell death in developmentrdquoCell vol 96 no 2 pp 245ndash254 1999

[59] E Lam and O del Pozo ldquoCaspase-like protease involvement inthe control of plant cell deathrdquo Plant Molecular Biology vol 44no 3 pp 417ndash428 2000

[60] E H Baehrecke ldquoHow death shapes life during developmentrdquoNature Reviews Molecular Cell Biology vol 3 no 10 pp 779ndash787 2002

[61] E Lam ldquoControlled cell death plant survival and developmentrdquoNature ReviewsMolecular Cell Biology vol 5 no 4 pp 305ndash3152004

[62] W G van Doorn and E J Woltering ldquoMany ways to exit Celldeath categories in plantsrdquo Trends in Plant Science vol 10 no3 pp 117ndash122 2005


[63] I Hara-Nishimura K Inoue and M Nishimura ldquoA uniquevacuolar processing enzyme responsible for conversion ofseveral proprotein precursors into the mature formsrdquo FEBSLetters vol 294 no 1-2 pp 89ndash93 1991

[64] I Hara-Nishimura N Hatsugai S Nakaune M Kuroyanagiand M Nishimura ldquoVacuolar processing enzyme an executorof plant cell deathrdquo Current Opinion in Plant Biology vol 8 no4 pp 404ndash408 2005

[65] N Hatsugai M Kuroyanagi K Yamada et al ldquoA plant vacuo-lar protease VPE mediates virus-induced hypersensitive celldeathrdquo Science vol 305 no 5685 pp 855ndash858 2004

[66] CAndeme-Ondzighi DAChristopher E J Cho S-C Changand L A Staehelin ldquoArabidopsis protein disulfide isomerase-5 inhibits cysteine proteases during trafficking to vacuolesbefore programmed cell death of the endothelium in developingseedsrdquoThe Plant Cell vol 20 no 8 pp 2205ndash2220 2008

[67] A R Frand andCAKaiser ldquoTheERO1 gene of yeast is requiredfor oxidation of protein dithiols in the endoplasmic reticulumrdquoMolecular Cell vol 1 no 2 pp 161ndash170 1998

[68] M G Pollard K J Travers and J S Weissman ldquoEro1p anovel and ubiquitous protein with an essential role in oxidativeprotein folding in the endoplasmic reticulumrdquo Molecular Cellvol 1 no 2 pp 171ndash182 1998

[69] A Cabibbo M Pagani M Fabbri et al ldquoERO1-L a humanprotein that favors disulfide bond formation in the endoplasmicreticulumrdquo The Journal of Biological Chemistry vol 275 no 7pp 4827ndash4833 2000

[70] M Pagani M Fabbri C Benedetti et al ldquoEndoplasmic retic-ulum oxidoreductin 1-L120573 (ERO1-L120573) a human gene inducedin the course of the unfolded protein responserdquo The Journal ofBiological Chemistry vol 275 no 31 pp 23685ndash23692 2000

[71] S Dias-Gunasekara J Gubbens M van Lith et al ldquoTissue-specific expression and dimerization of the endoplasmic reticu-lum oxidoreductase Ero1120573rdquoThe Journal of Biological Chemistryvol 280 no 38 pp 33066ndash33075 2005

[72] D P Dixon M van Lith R Edwards and A BenhamldquoCloning and initial characterization of theArabidopsis thalianaendoplasmic reticulum oxidoreductinsrdquo Antioxidants amp RedoxSignaling vol 5 no 4 pp 389ndash396 2003

[73] I Aller and A J Meyer ldquoThe oxidative protein folding machin-ery in plant cellsrdquoProtoplasma vol 250 no 4 pp 799ndash816 2013

[74] A R Frand and C A Kaiser ldquoEro1p oxidizes protein disulfideisomerase in a pathway for disulfide bond formation in theendoplasmic reticulumrdquo Molecular Cell vol 4 no 4 pp 469ndash477 1999

[75] B P Tu and J S Weissman ldquoThe FAD- and O2-dependent

reaction cycle of Ero1-mediated oxidative protein folding in theendoplasmic reticulumrdquoMolecular Cell vol 10 no 5 pp 983ndash994 2002

[76] E Gross C S Sevier N Heldman et al ldquoGenerating disulfidesenzymatically reaction products and electron acceptors of theendoplasmic reticulum thiol oxidase Ero1prdquo Proceedings of theNational Academy of Sciences of the United States of Americavol 103 no 2 pp 299ndash304 2006

[77] A R Frand and C A Kaiser ldquoTwo pairs of conserved cysteinesare required for the oxidative activity of Ero1p in protein disul-fide bond formation in the endoplasmic reticulumrdquo MolecularBiology of the Cell vol 11 no 9 pp 2833ndash2843 2000

[78] E Gross D B Kastner C A Kaiser and D Fass ldquoStructure ofEro1p source of disulfide bonds for oxidative protein folding inthe cellrdquo Cell vol 117 no 5 pp 601ndash610 2004

[79] C S Sevier and C A Kaiser ldquoDisulfide transfer between twoconserved cysteine pairs imparts selectivity to protein oxidationby Ero1rdquo Molecular Biology of the Cell vol 17 no 5 pp 2256ndash2266 2006

[80] K Inaba S Masui H Iida S Vavassori R Sitia andM SuzukildquoCrystal structures of human Ero1120572 reveal the mechanisms ofregulated and targeted oxidation of PDIrdquo The EMBO Journalvol 29 no 19 pp 3330ndash3343 2010

[81] M Pagani S Pilati G Bertoli B Valsasina and R SitialdquoThe C-terminal domain of yeast Ero1p mediates membranelocalization and is essential for functionrdquo FEBS Letters vol 508no 1 pp 117ndash120 2001

[82] M Otsu G Bertoli C Fagioli et al ldquoDynamic retention ofEro1120572 and Ero1120573 in the endoplasmic reticulum by interactionswith PDI and ERp44rdquo Antioxidants amp Redox Signaling vol 8no 3-4 pp 274ndash282 2006

[83] C S Sevier H Qu N Heldman E Gross D Fass and C AKaiser ldquoModulation of cellular disulfide-bond formation andthe ER redox environment by feedback regulation of Ero1rdquo Cellvol 129 no 2 pp 333ndash344 2007

[84] C Appenzeller-Herzog J Riemer B Christensen E SSoslashrensen and L Ellgaard ldquoA novel disulphide switch mech-anism in Ero1120572 balances ER oxidation in human cellsrdquo TheEMBO Journal vol 27 no 22 pp 2977ndash2987 2008

[85] H P Harding Y H Zhang H Q Zeng et al ldquoAn integratedstress response regulates amino acid metabolism and resistanceto oxidative stressrdquo Molecular Cell vol 11 no 3 pp 619ndash6332003

[86] S J Marciniak C Y Yun S Oyadomari et al ldquoCHOP inducesdeath by promoting protein synthesis and oxidation in thestressed endoplasmic reticulumrdquoGenes amp Development vol 18no 24 pp 3066ndash3077 2004

[87] E A Veal A M Day and B A Morgan ldquoHydrogen peroxidesensing and signalingrdquo Molecular Cell vol 26 no 1 pp 1ndash142007

[88] S E Sattler L Mene-Saffrane E E Farmer M Krischke M JMueller and D DellaPenna ldquoNonenzymatic lipid peroxidationreprograms gene expression and activates defense markers inArabidopsis tocopherol-deficient mutantsrdquo The Plant Cell vol18 no 12 pp 3706ndash3720 2006

[89] T E Young and D R Gallie ldquoProgrammed cell death duringendosperm developmentrdquo Plant Molecular Biology vol 44 no3 pp 283ndash301 2000

[90] E Vitu S Kim C S Sevier et al ldquoOxidative activity of yeastEro1p on protein disulfide isomerase and related oxidoreduc-tases of the endoplasmic reticulumrdquo The Journal of BiologicalChemistry vol 285 no 24 pp 18155ndash18165 2010

[91] A Mezghrani A Fassio A Benham T Simmen I Braakmanand R Sitia ldquoManipulation of oxidative protein folding and PDIredox state in mammalian cellsrdquoTheEMBO Journal vol 20 no22 pp 6288ndash6296 2001

[92] C Appenzeller-Herzog J Riemer E Zito et al ldquoDisulphideproduction by Ero1120572-PDI relay is rapid and effectively regu-latedrdquoThe EMBO Journal vol 29 no 19 pp 3318ndash3329 2010

[93] G Kozlov P Maattanen J D Schrag et al ldquoCrystal structureof the bb1015840 domains of the protein disulfide isomerase ERp57rdquoStructure vol 14 no 8 pp 1331ndash1339 2006

[94] E Zito K-T Chin J Blais H P Harding andD Ron ldquoERO1-120573a pancreas-specific disulfide oxidase promotes insulin biogen-esis and glucose homeostasisrdquo The Journal of Cell Biology vol188 no 6 pp 821ndash832 2010


[95] E Zito E P Melo Y Yang A Wahlander T A Neubert and DRon ldquoOxidative protein folding by an endoplasmic reticulum-localized peroxiredoxinrdquoMolecular Cell vol 40 no 5 pp 787ndash797 2010

[96] V D Nguyen M J Saaranen A-R Karala et al ldquoTwoendoplasmic reticulum PDI peroxidases increase the efficiencyof the use of peroxide during disulfide bond formationrdquo Journalof Molecular Biology vol 406 no 3 pp 503ndash515 2011

[97] K-J Dietz ldquoPeroxiredoxins in plants and cyanobacteriardquoAntioxidants amp Redox Signaling vol 15 no 4 pp 1129ndash11592011

[98] N Rouhier and J-P Jacquot ldquoThe plant multigenic family ofthiol peroxidasesrdquo Free Radical Biology amp Medicine vol 38 no11 pp 1413ndash1421 2005

[99] B N Tripathi I Bhatt and K-J Dietz ldquoPeroxiredoxins aless studied component of hydrogen peroxide detoxification inphotosynthetic organismsrdquo Protoplasma vol 235 no 1ndash4 pp3ndash15 2009

[100] K Keegstra and K Cline ldquoProtein import and routing systemsof chloroplastsrdquoThe Plant Cell vol 11 no 4 pp 557ndash570 1999

[101] M Sugiura ldquoThe chloroplast chromosomes in land plantsrdquoAnnual Review of Cell Biology vol 5 pp 51ndash70 1989

[102] Y Toyoshima YOnda T Shiina andYNakahira ldquoPlastid tran-scription in higher plantsrdquo Critical Reviews in Plant Sciencesvol 24 no 1 pp 59ndash81 2005

[103] E-M Aro I Virgin and B Andersson ldquoPhotoinhibition ofphotosystem II Inactivation protein damage and turnoverrdquoBiochimica et Biophysica Acta vol 1143 no 2 pp 113ndash134 1993

[104] B Demmig-Adams andWW Adams III ldquoPhotoprotection andother responses of plants to high light stressrdquo Annual Review ofPlant Physiology and Plant Molecular Biology vol 43 no 1 pp599ndash626 1992

[105] B B Buchanan ldquoRegulation of CO2assimilation in oxygenic

photosynthesis the ferredoxinthioredoxin system Perspectiveon its discovery present status and future developmentrdquoArchives of Biochemistry and Biophysics vol 288 no 1 pp 1ndash91991

[106] J Kim and S P Mayfield ldquoProtein disulfide isomerase as aregulator of chloroplast translational activationrdquo Science vol278 no 5345 pp 1954ndash1957 1997

[107] A Levitan T Trebitsh V Kiss Y Pereg I Dangoor and ADanon ldquoDual targeting of the protein disulfide isomerase RB60to the chloroplast and the endoplasmic reticulumrdquo Proceedingsof the National Academy of Sciences of the United States ofAmerica vol 102 no 17 pp 6225ndash6230 2005

[108] T Trebitsh A Levitan A Sofer and A Danon ldquoTranslationof chloroplast psbA mRNA is modulated in the light bycounteracting oxidizing and reducing activitiesrdquoMolecular andCellular Biology vol 20 no 4 pp 1116ndash1123 2000

[109] D-P Lu and D A Christopher ldquoImmunolocalization of aprotein disulfide isomerase to Arabidopsis thaliana chloroplastsand its association with starch biogenesisrdquo International Journalof Plant Sciences vol 167 no 1 pp 1ndash9 2006

[110] U Armbruster A Hertle E Makarenko et al ldquoChloroplastproteins without cleavable transit peptides rare exceptions ora major constituent of the chloroplast proteomerdquo MolecularPlant vol 2 no 6 pp 1325ndash1335 2009

[111] N M Escobar S Haupt G Thow P Boevink S Chapmanand K Oparka ldquoHigh-throughput viral expression of cDNA-green fluorescent protein fusions reveals novel subcellularaddresses and identifies unique proteins that interact with

plasmodesmatardquo The Plant Cell vol 15 no 7 pp 1507ndash15232003

[112] S C Zeeman A Tiessen E Pilling K L Kato A M Donaldand A M Smith ldquoStarch synthesis in Arabidopsis Granulesynthesis composition and structurerdquo Plant Physiology vol129 no 2 pp 516ndash529 2002

[113] M Suorsa and E M Aro ldquoExpression assembly and auxiliaryfunctions of photosystem II oxygen-evolving proteins in higherplantsrdquo Photosynthesis Research vol 93 no 1ndash3 pp 89ndash1002007

[114] T Kieselbach ldquoOxidative folding in chloroplastsrdquo Antioxidantsamp Redox Signaling vol 19 no 1 pp 72ndash82 2013

[115] W K Li S Schulman R J Dutton D Boyd J Beckwith and TA Rapoport ldquoStructure of a bacterial homologue of vitamin Kepoxide reductaserdquoNature vol 463 no 7280 pp 507ndash512 2010

[116] S Schulman B Wang W K Li and T A Rapoport ldquoVita-min K epoxide reductase prefers ER membrane-anchoredthioredoxin-like redox partnersrdquo Proceedings of the NationalAcademy of Sciences of the United States of America vol 107 no34 pp 15027ndash15032 2010

[117] L Goodstadt and C P Ponting ldquoVitamin K epoxide reductasehomology active site and catalytic mechanismrdquo Trends inBiochemical Sciences vol 29 no 6 pp 289ndash292 2004

[118] A K Singh M Bhattacharyya-Pakrasi and H B PakrasildquoIdentification of an atypical membrane protein involved inthe formation of protein disulfide bonds in oxygenic photosyn-thetic organismsrdquoThe Journal of Biological Chemistry vol 283no 23 pp 15762ndash15770 2008

[119] F Furt C V Oostende J RWidhalmM A Dale J Wertz andG J C Basset ldquoA bimodular oxidoreductase mediates the spe-cific reduction of phylloquinone (vitamin K

1) in chloroplastsrdquo

The Plant Journal vol 64 no 1 pp 38ndash46 2010[120] M Karamoko S Cline K Redding N Ruiz and P P Hamel

ldquoLumen thiol oxidoreductase1 a disulfide bond-forming cata-lyst is required for the assembly of photosystem II inArabidop-sisrdquoThe Plant Cell vol 23 no 12 pp 4462ndash4475 2011

[121] W-K Feng L Wang Y Lu and X-Y Wang ldquoA proteinoxidase catalysing disulfide bond formation is localized to thechloroplast thylakoidsrdquo The FEBS Journal vol 278 no 18 pp3419ndash3430 2011

[122] K Brettel ldquoElectron transfer and arrangement of the redoxcofactors in photosystem Irdquo Biochimica et Biophysica Acta vol1318 no 3 pp 322ndash373 1997

[123] Y Lu D A Hall and R L Last ldquoA small zinc finger thylakoidprotein plays a role in maintenance of photosystem II inArabidopsis thalianardquoThePlant Cell vol 23 no 5 pp 1861ndash18752011

[124] H Shimada M Mochizuki K Ogura et al ldquoArabidopsiscotyledon-specific chloroplast biogenesis factor CYO1 is aprotein disulfide isomeraserdquo The Plant Cell vol 19 no 10 pp3157ndash3169 2007

[125] A Muranaka S Watanabe A Sakamoto and H ShimadaldquoArabidopsis cotyledon chloroplast biogenesis factor CYO1 usesglutathione as an electron donor and interacts with PSI (A1and A2) and PSII (CP43 and CP47) subunitsrdquo Journal of PlantPhysiology vol 169 no 12 pp 1212ndash1215 2012

[126] G BurgerMWGray and B F Lang ldquoMitochondrial genomesanything goesrdquo Trends in Genetics vol 19 no 12 pp 709ndash7162003

[127] A Chacinska C M Koehler D Milenkovic T Lithgow and NPfanner ldquoImporting mitochondrial proteins machineries andmechanismsrdquo Cell vol 138 no 4 pp 628ndash644 2009


[128] D Stojanovski D Milenkovic J M Muller et al ldquoMitochon-drial protein import precursor oxidation in a ternary complexwith disulfide carrier and sulfhydryl oxidaserdquoThe Journal of CellBiology vol 183 no 2 pp 195ndash202 2008

[129] K Bihlmaier N Mesecke N Terziyska M Bien K Hell andJ M Herrmann ldquoThe disulfide relay system of mitochondria isconnected to the respiratory chainrdquoThe Journal of Cell Biologyvol 179 no 3 pp 389ndash395 2007

[130] D V Dabir E P Leverich S-K Kim et al ldquoA role forcytochrome c and cytochrome c peroxidase in electron shuttlingfrom Erv1rdquo The EMBO Journal vol 26 no 23 pp 4801ndash48112007

[131] C Carrie E Giraud O Duncan et al ldquoConserved andnovel functions for Arabidopsis thaliana MIA40 in assemblyof proteins in mitochondria and peroxisomesrdquo The Journal ofBiological Chemistry vol 285 no 46 pp 36138ndash36148 2010

[132] A Levitan ADanon andT Lisowsky ldquoUnique features of plantmitochondrial sulfhydryl oxidaserdquo The Journal of BiologicalChemistry vol 279 no 19 pp 20002ndash20008 2004

[133] J W A Allen S J Ferguson and M L Ginger ldquoDistinctivebiochemistry in the trypanosome mitochondrial intermem-brane space suggests a model for stepwise evolution of the MIApathway for import of cysteine-rich proteinsrdquo FEBS Letters vol582 no 19 pp 2817ndash2825 2008

[134] A Chacinska S Pfannschmidt NWiedemann et al ldquoEssentialrole of Mia40 in import and assembly of mitochondrial inter-membrane space proteinsrdquo The EMBO Journal vol 23 no 19pp 3735ndash3746 2004

[135] H Lange T Lisowsky J Gerber U Muhlenhoff G Kispal andR Lill ldquoAn essential function of the mitochondrial sulfhydryloxidase Erv1pALR in the maturation of cytosolic FeS pro-teinsrdquo EMBO Reports vol 2 no 8 pp 715ndash720 2001

[136] D Coppock C Kopman J Gudas and D A Cina-PoppeldquoRegulation of the quiescence-induced genes quiescin Q6decorin and ribosomal protein S29rdquo Biochemical and Biophysi-cal Research Communications vol 269 no 2 pp 604ndash610 2000

[137] G Mairet-Coello A Tury D Fellmann P-Y Risold andB Griffond ldquoOntogenesis of the sulfhydryl oxidase QSOXexpression in rat brainrdquo Journal of Comparative Neurology vol484 no 4 pp 403ndash417 2005

[138] S Chakravarthi C E Jessop M Willer C J Stirling and NJ Bulleid ldquoIntracellular catalysis of disulfide bond formationby the human sulfhydryl oxidase QSOX1rdquo The BiochemicalJournal vol 404 no 3 pp 403ndash411 2007

[139] S Alejandro P L Rodrıguez J M Belles et al ldquoAn Arabidopsisquiescin-sulfhydryl oxidase regulates cation homeostasis at theroot symplast-xylem interfacerdquoThe EMBO Journal vol 26 no13 pp 3203ndash3215 2007

[140] C Thorpe K L Hoober S Raje et al ldquoSulfhydryl oxidasesemerging catalysts of protein disulfide bond formation ineukaryotesrdquo Archives of Biochemistry and Biophysics vol 405no 1 pp 1ndash12 2002

[141] S Raje and C Thorpe ldquoInter-domain redox communication inflavoenzymes of the quiescinsulfhydryl oxidase family role of athioredoxin domain in disulfide bond formationrdquo Biochemistryvol 42 no 15 pp 4560ndash4568 2003

[142] A Alon I Grossman Y Gat et al ldquoThe dynamic disulphiderelay of quiescin sulphydryl oxidaserdquo Nature vol 488 no 7411pp 414ndash418 2012

[143] K Limor-Waisberg A Alon T Mehlman and D Fass ldquoPhylo-genetics and enzymology of plant quiescin sulfhydryl oxidaserdquoFEBS Letters vol 586 no 23 pp 4119ndash4125 2012

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Anatomy Research International

PeptidesInternational Journal of


Hindawi Publishing Corporation httpwwwhindawicom

International Journal of

Volume 2014

Zoology


Molecular Biology International

GenomicsInternational Journal of


The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014


BioinformaticsAdvances in

Marine BiologyJournal of



Signal TransductionJournal of


BioMed Research International

Evolutionary BiologyInternational Journal of



Biochemistry Research International

ArchaeaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014


Genetics Research International


Advances in

Virolog y

Hindawi Publishing Corporationhttpwwwhindawicom

Nucleic AcidsJournal of

Volume 2014

Stem CellsInternational



Enzyme Research



Microbiology


SubstrateDisulfide-

generatingenzyme

Disulfidecarrier

Electron transfer

Disulfide transfer

-SH HS- PDIox

PDIred ERO1ox

ERO1red

-S-S-

O2

H2O2

2eminus

2eminus

2eminus

Figure 1 Disulfide relay from ERO1 to PDI in the ER Disulfidebond formation involves electron transfer from the substrate to adisulfide carrier protein (eg PDI) and then to a de novo disulfide-generating enzyme (eg ERO1) PDI directly transfers a disulfide totwo Cys residues of a substrate protein by means of a thiol-disulfideexchange reaction The reduced form of PDI is reoxidized by ERO1which relays the oxidizing power from molecular oxygen via theFAD cofactor to the reduced form of PDI

Endosperm cellsPhotosynthetic cells

VKOR-TRXLQY1CYO1

PDI Chloroplast

PSV

ERV1MIA40

Mitochondrion

ERO1ERO1PDIPDI

ERPB

Golgi

Nucleus

Figure 2 Overview of the sites of oxidative protein folding inphotosynthetic and endosperm cells of plants Disulfide-generatingenzymes and disulfide carrier proteins characterized to date andtheir subcellular localizations are shown in photosynthetic (left) andendosperm (right) cells of plants ER endoplasmic reticulum PBprotein body PSV protein storage vacuole

proteins which serve as nutritional reserves of nitrogensulfur and carbon for the germinating seedlings (reviewedin [9 10]) The redox state of storage proteins containingdisulfide groups dramatically changes during seed develop-mentmaturation and germination [11] seed storage proteinsare synthesized in reduced form on the rough ER and areconverted to disulfide-bonded forms duringmaturation thenthey are converted back to the reduced form to facilitaterapid mobilization during germination The h-type TRXfunctions in the reduction of seed storage proteins in theendosperm increasing their proteolytic susceptibility andmaking nitrogen and sulfur available during germination[12] In contrast to the TRX-dependent system that actsmainly in the reduction of disulfides the disulfide transfersystems involving ERO1 and PDIs function primarily to formthe correct patterns of disulfide bonds thereby facilitatingthe stable accumulation of seed storage proteins preventingtheir denaturation and decreasing their susceptibility toproteolysis

In chloroplasts the redox state of disulfide bonds inredox-regulated proteins is linked to the control of metabolic

pathways and gene expression [11 13 14] The most-studiedredox regulatory system is the ferredoxin-TRX system thatreversibly reduces and oxidizes thiol groups reducing equiv-alents are transferred from photosystem I (PSI) to ferredoxinand then to TRX via ferredoxin-dependent TRX reductase[11 13] Inside chloroplasts the thylakoid membranes whichenclose the lumen that originated as the bacterial periplasmperform the primary events of oxygenic photosynthesisUnder strong illumination however photosystem II (PSII)in the thylakoid membranes is subjected to photodamageTo maintain photosynthetic activity PSII requires the rapidreassembly of a thylakoid membrane protein complex com-posed of dozens of proteins [15] which is dependent ondisulfide bond formation catalyzed by enzyme catalystsincluding a zinc finger protein LQY1 and a vitamin K epoxidereductase (VKOR) homolog

Emerging evidence has shown that disulfide transferpathways that function in the ER and chloroplasts of plantsplay critical roles in the development of PSVs and PBsand the biogenesis of chloroplasts respectively This reviewfocuses on recent advances in our understanding of themechanisms and functions of the various disulfide transferpathways involved in oxidative protein folding in the ERand chloroplasts of plants (Figure 2) The disulfide transferpathways in plant mitochondria which involve the disulfide-generating enzyme ERV1 and the disulfide carrier proteinMIA40 (Figure 2) are also discussed in comparison to therespective pathways in yeast and mammalian cells

2 Disulfide Bond Formation in the Plant ER

21 Seed Storage Proteins Seed storage proteins are con-ventionally classified on the basis of their solubility inwater (eg albumins) saline (eg 120572-globulin) alcohol (egprolamins) and acidic or basic solutions (eg glutelins)[16] The globulins the most widely distributed group ofstorage proteins in both dicots and monocots are dividedinto two subgroups on the basis of their sedimentationcoefficients the vicilin-like 7S globulins and the legumin-like11S globulins Rice glutelins (60 of total seed protein) [17]and soybean glycinins (40of total seed protein) [18] both ofwhich are 11S globulin homologs are synthesized from theirprecursors proglutelins and preproglycinins respectivelythe precursors are processed at a conserved Asp-Gly site byan asparaginyl endopeptidase [19 20] to produce acidic andbasic polypeptide subunits which remain linked by disulfidebonds [21]

The prolamin family which includes maize prolamins(referred to as zeins) and rice prolamins is encoded bymultiple genes and is composed of Cys-rich and Cys-poormembers [16 22] Rice prolamins (20 of total seed protein)[17] are clustered into three subgroups 13-kD prolamins con-taining 0ndash8 Cys residues 10-kD prolamins containing 9ndash11Cys residues and 16-kD prolamin containing 13 Cys residues[22] Maize prolamins include four structurally distinct typesof proteins 120572- 120573- 120574- and 120575-zeins The 120573- 120574- and 120575-zeinsare rich in Cys residues whereas the 120572-zeins are Cys poor[16] Prolamins and 2S albumins contain conserved motifs


SFA-8 (Ha)

Region B

Q Lx x x xx

Region A Region C

P C

58 M L N H C

C C CL



































D

b

FADCxxC

CxxC CxxC

CxxCCxxC

CxxC

CxxCCxxC

CxxC

ERO1

PDIL11PDIs

PDIL14

PDIL21

PDIL23

TMD

a

a

a

ac

b

b

Cx4C

a998400

a998400

a998400

a998400

b998400

b998400

(a) ER

VKOR TRXTMD

a b

ZnfCYO1

TMDLQY1

CxxC

CxxCCxxC

CxxCCxxC

CxxC CxxCCxxC

CxxC

CxxCLTO1

PDIL13

VKOR-TRX

PDI

Zn finger proteins

c

Znf

a998400b998400

Cx6C

(b) Chloroplasts


MIA40CPC

Cx4C

Cx16C

Cx9CCx9C

(c) Mitochondria


9C) and ERV1 (Cx

16C)
























-SH HS-

-S-S-

-SH HS-

-S-S-

O2

H2O2

2eminus2eminus

2eminus2eminus

2eminus

2eminus

Xred

Xox Yred

Yox

PDILxxox

PDILxxox

PDILxxred

PDILxxred

ERO1ox

ERO1red

(a) ER

-SH HS-

-S-S-

-SH HS-

-S-S-

-SH HS-

-S-S-

2eminus

2eminus2eminus

2eminus

2eminus2eminus

2eminus

Xred

Xred

Xox

Xox

Yred

Yred

Yox

Yox

2eminusYred

YoxPDILxxox

PDILxxred

VKOR-TRXox

VKOR-TRXred

ZnfPox

ZnfPred

(b) Chloroplasts

Respiratory chain

-SH HS-

-S-S-

H2O

O2

H2O22eminus2eminus

2eminus

2eminus

2eminus

2eminus

MIA40ox

MIA40red ERV1ox

ERV1red cyt cox

cyt cred

12O2

(c) Mitochondria




















3C or




9C in AtMIA40 Cx




4C





Abbreviations




Acknowledgments


References

































































































































































Volume 2014

Zoology






















Advances in

Virolog y



Volume 2014




Enzyme Research



Microbiology


SFA-8 (Ha)

Region B

Q Lx x x xx

Region A Region C

P C

58 M L N H C

C C CL



































D

b

FADCxxC

CxxC CxxC

CxxCCxxC

CxxC

CxxCCxxC

CxxC

ERO1

PDIL11PDIs

PDIL14

PDIL21

PDIL23

TMD

a

a

a

ac

b

b

Cx4C

a998400

a998400

a998400

a998400

b998400

b998400

(a) ER

VKOR TRXTMD

a b

ZnfCYO1

TMDLQY1

CxxC

CxxCCxxC

CxxCCxxC

CxxC CxxCCxxC

CxxC

CxxCLTO1

PDIL13

VKOR-TRX

PDI

Zn finger proteins

c

Znf

a998400b998400

Cx6C

(b) Chloroplasts


MIA40CPC

Cx4C

Cx16C

Cx9CCx9C

(c) Mitochondria


9C) and ERV1 (Cx

16C)
























-SH HS-

-S-S-

-SH HS-

-S-S-

O2

H2O2

2eminus2eminus

2eminus2eminus

2eminus

2eminus

Xred

Xox Yred

Yox

PDILxxox

PDILxxox

PDILxxred

PDILxxred

ERO1ox

ERO1red

(a) ER

-SH HS-

-S-S-

-SH HS-

-S-S-

-SH HS-

-S-S-

2eminus

2eminus2eminus

2eminus

2eminus2eminus

2eminus

Xred

Xred

Xox

Xox

Yred

Yred

Yox

Yox

2eminusYred

YoxPDILxxox

PDILxxred

VKOR-TRXox

VKOR-TRXred

ZnfPox

ZnfPred

(b) Chloroplasts

Respiratory chain

-SH HS-

-S-S-

H2O

O2

H2O22eminus2eminus

2eminus

2eminus

2eminus

2eminus

MIA40ox

MIA40red ERV1ox

ERV1red cyt cox

cyt cred

12O2

(c) Mitochondria




















3C or




9C in AtMIA40 Cx




4C





Abbreviations




Acknowledgments


References

































































































































































Volume 2014

Zoology






















Advances in

Virolog y



Volume 2014




Enzyme Research



Microbiology
























D

b

FADCxxC

CxxC CxxC

CxxCCxxC

CxxC

CxxCCxxC

CxxC

ERO1

PDIL11PDIs

PDIL14

PDIL21

PDIL23

TMD

a

a

a

ac

b

b

Cx4C

a998400

a998400

a998400

a998400

b998400

b998400

(a) ER

VKOR TRXTMD

a b

ZnfCYO1

TMDLQY1

CxxC

CxxCCxxC

CxxCCxxC

CxxC CxxCCxxC

CxxC

CxxCLTO1

PDIL13

VKOR-TRX

PDI

Zn finger proteins

c

Znf

a998400b998400

Cx6C

(b) Chloroplasts


MIA40CPC

Cx4C

Cx16C

Cx9CCx9C

(c) Mitochondria


9C) and ERV1 (Cx

16C)
























-SH HS-

-S-S-

-SH HS-

-S-S-

O2

H2O2

2eminus2eminus

2eminus2eminus

2eminus

2eminus

Xred

Xox Yred

Yox

PDILxxox

PDILxxox

PDILxxred

PDILxxred

ERO1ox

ERO1red

(a) ER

-SH HS-

-S-S-

-SH HS-

-S-S-

-SH HS-

-S-S-

2eminus

2eminus2eminus

2eminus

2eminus2eminus

2eminus

Xred

Xred

Xox

Xox

Yred

Yred

Yox

Yox

2eminusYred

YoxPDILxxox

PDILxxred

VKOR-TRXox

VKOR-TRXred

ZnfPox

ZnfPred

(b) Chloroplasts

Respiratory chain

-SH HS-

-S-S-

H2O

O2

H2O22eminus2eminus

2eminus

2eminus

2eminus

2eminus

MIA40ox

MIA40red ERV1ox

ERV1red cyt cox

cyt cred

12O2

(c) Mitochondria




















3C or




9C in AtMIA40 Cx




4C





Abbreviations




Acknowledgments


References

































































































































































Volume 2014

Zoology






















Advances in

Virolog y



Volume 2014




Enzyme Research



Microbiology




















-SH HS-

-S-S-

-SH HS-

-S-S-

O2

H2O2

2eminus2eminus

2eminus2eminus

2eminus

2eminus

Xred

Xox Yred

Yox

PDILxxox

PDILxxox

PDILxxred

PDILxxred

ERO1ox

ERO1red

(a) ER

-SH HS-

-S-S-

-SH HS-

-S-S-

-SH HS-

-S-S-

2eminus

2eminus2eminus

2eminus

2eminus2eminus

2eminus

Xred

Xred

Xox

Xox

Yred

Yred

Yox

Yox

2eminusYred

YoxPDILxxox

PDILxxred

VKOR-TRXox

VKOR-TRXred

ZnfPox

ZnfPred

(b) Chloroplasts

Respiratory chain

-SH HS-

-S-S-

H2O

O2

H2O22eminus2eminus

2eminus

2eminus

2eminus

2eminus

MIA40ox

MIA40red ERV1ox

ERV1red cyt cox

cyt cred

12O2

(c) Mitochondria




















3C or




9C in AtMIA40 Cx




4C





Abbreviations




Acknowledgments


References

































































































































































Volume 2014

Zoology






















Advances in

Virolog y



Volume 2014




Enzyme Research



Microbiology













3C or




9C in AtMIA40 Cx




4C





Abbreviations




Acknowledgments


References

































































































































































Volume 2014

Zoology






















Advances in

Virolog y



Volume 2014




Enzyme Research



Microbiology



Acknowledgments


References

































































































































































Volume 2014

Zoology






















Advances in

Virolog y



Volume 2014




Enzyme Research



Microbiology


































































































































Volume 2014

Zoology






















Advances in

Virolog y



Volume 2014




Enzyme Research



Microbiology

review article oxidative protein-folding systems in plant...

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