Efficient assembly of photosystem II in Chlamydomonas reinhardtii requires Alb3.1p, a homolog of Arabidopsis ALBINO3

Alb3 homologs Oxa1 and YidC have been shown to be required for the integration of newly synthesized proteins into membranes. Here, we show that although Alb3.1p is not required for integration of the plastid-encoded photosystem II core subunit D1 into the thylakoid membrane of Chlamydomonas reinhardtii, the insertion of D1 into functional photosystem II complexes is retarded in the Alb3.1 deletion mutant ac29. Alb3.1p is associated with D1 upon its insertion into the membrane, indicating that Alb3.1p is essential for the efficient assembly of photosystem II. Furthermore, levels of nucleus-encoded light-harvesting proteins are vastly reduced in ac29; however, the remaining antenna systems are still connected to photosystem II reaction centers. Thus, Alb3.1p has a dual function and is required for the accumulation of both nucleus- and plastid-encoded protein subunits in photosynthetic complexes of C. reinhardtii.     

Evidence for a role of VIPP1 in the structural organization of the photosynthetic apparatus in Chlamydomonas

The vesicle-inducing protein in plastids (VIPP1) was suggested to play a role in thylakoid membrane formation via membrane vesicles. As this functional assignment is under debate, we investigated the function of VIPP1 in Chlamydomonas reinhardtii. Using immunofluorescence, we localized VIPP1 to distinct spots within the chloroplast. In VIPP1-RNA interference/artificial microRNA cells, we consistently observed aberrant, prolamellar body-like structures at the origin of multiple thylakoid membrane layers, which appear to coincide with the immunofluorescent VIPP1 spots and suggest a defect in thylakoid membrane biogenesis. Accordingly, using quantitative shotgun proteomics, we found that unstressed vipp1 mutant cells accumulate 14 to 20% less photosystems, cytochrome b(6)f complex, and ATP synthase but 30% more light-harvesting complex II than control cells, while complex assembly, thylakoid membrane ultrastructure, and bulk lipid composition appeared unaltered. Photosystems in vipp1 mutants are sensitive to high light, which coincides with a lowered midpoint potential of the Q(A)/Q(A)(-) redox couple and increased thermosensitivity of photosystem II (PSII), suggesting structural defects in PSII. Moreover, swollen thylakoids, despite reduced membrane energization, in vipp1 mutants grown on ammonium suggest defects in the supermolecular organization of thylakoid membrane complexes. Overall, our data suggest a role of VIPP1 in the biogenesis/assembly of thylakoid membrane core complexes, most likely by supplying structural lipids.     

Loss of Albino3 leads to the specific depletion of the light-harvesting system

The chloroplast Albino3 (Alb3) protein is a chloroplast homolog of the mitochondrial Oxa1p and YidC proteins of Escherichia coli, which are essential components for integrating membrane proteins. In vitro studies in vascular plants have revealed that Alb3 is required for the integration of the light-harvesting complex protein into the thylakoid membrane. Here, we show that the gene affected in the ac29 mutant of Chlamydomonas reinhardtii is Alb3.1. The availability of the ac29 mutant has allowed us to examine the function of Alb3.1 in vivo. The loss of Alb3.1 has two major effects. First, the amount of light-harvesting complex from photosystem II (LHCII) and photosystem I (LHCI) is reduced >10-fold, and total chlorophyll represents only 30% of wild-type levels. Second, the amount of photosystem II is diminished 2-fold in light-grown cells and nearly 10-fold in dark-grown cells. The accumulation of photosystem I, the cytochrome b(6)f complex, and ATP synthase is not affected in the ac29 mutant. Mild solubilization of thylakoid membranes reveals that Alb3 forms two distinct complexes, a lower molecular mass complex of a size similar to LHC and a high molecular mass complex. A homolog of Alb3.1, Alb3.2, is present in Chlamydomonas, with 37% sequence identity and 57% sequence similarity. Based on the phenotype of ac29, these two genes appear to have mostly nonredundant functions.     

The chloroplast HSP70B-CDJ2-CGE1 chaperones catalyse assembly and disassembly of VIPP1 oligomers in Chlamydomonas

The vesicle-inducing protein in plastids (VIPP1) is essential for the biogenesis of thylakoid membranes in cyanobacteria and plants. VIPP1 and its bacterial ancestor PspA form large homo-oligomeric rings of >1 MDa. We recently demonstrated that VIPP1 interacts with the chloroplast J-domain co-chaperone CDJ2 and its chaperone partner HSP70B, and hypothesized that the chaperones might be involved in the assembly and/or disassembly of VIPP1 oligomers. To test this hypothesis, we analysed the composition of VIPP1/chaperone complexes in Chlamydomonas reinhardtii cell extracts and monitored effects of the chaperones on VIPP1 assembly states in vitro. We found that CGE1, the chloroplast GrpE homologue, is also part of complexes with HSP70B, CDJ2 and VIPP1. We observed that CDJ2-VIPP1 accumulated as low- and high-molecular-weight complexes in ATP-depleted cell extracts, but as intermediate-size complexes in extracts supplemented with ATP. This was consistent with a role for the chaperones in VIPP1 assembly and disassembly. Using purified proteins, we could demonstrate that the chaperones indeed facilitated both the assembly and disassembly of VIPP1 oligomers. Electron microscopy studies revealed that, in contrast to PspA, VIPP1 rings assembled into rod-shaped supercomplexes that were morphologically similar to microtubule-like structures observed earlier in various plastid types. VIPP1 rods, too, were disassembled by the chaperones, and chaperone-mediated rod disassembly also occurred when VIPP1 lacked an approximately 30-aa C-terminal extension present in VIPP1 homologues but absent in PspA. By regulating the assembly state of VIPP1, the chloroplast HSP70 chaperone system may play an important role in the maintenance/biogenesis of thylakoid membranes.     

Phenotypes of Alb3p and carotenoid synthesis mutants show similarities regarding light sensitivity, thylakoid structure and protein stability

Chloroplast proteins of the Alb3/YidC/Oxa1p family are necessary for assembly of photosynthetic complexes in the thylakoid membranes. Alb3p in Arabidopsis thaliana is essential for posttranslational LHCII-integration into thylakoid membranes and participates in cotranslational assembly of D1. However, the pleiotropic defects of an Alb3p mutant, albino3, suggest additional functions for Alb3p. To obtain an impression of such potential further Alb3p activities from phenotypic manifestations, properties of mutants disturbed in thylakoid membrane protein transport or carotenoid biosynthesis were compared with the albino3 mutant. Specific defects observed in albino3 were similar to those in a carotenoid synthesis mutant. While this correlation did not provide tangible evidence for Alb3p being involved in the integration of carotenoids in photosynthetic complexes, it suggests a possible avenue for future investigations.     

The yeast split-ubiquitin system to study chloroplast membrane protein interactions

Each photosynthetic complex within the thylakoid membrane consists of several different subunits. During formation of these complexes, numerous regulatory factors are required for the coordinated transport and assembly of the subunits. Interactions between transport/assembly factors and their specific polypeptides occur in a membraneous environment and are usually transient and short-lived. Thus, a detailed analysis of the underlying molecular mechanisms by biochemical techniques is often difficult to perform. Here, we report on the suitability of a genetic system, i.e. the yeast split-ubiquitin system, to investigate protein–protein interactions of thylakoid membrane proteins. The data confirm the previously established binding of the cpSec-translocase subunits, cpSecY and cpSecE, and the interaction of the cpSec-translocase from Arabidopsis thaliana with Alb3, a factor required for the insertion of the light-harvesting chlorophyll-binding proteins into the thylakoid membrane. In addition, the proposed interaction between D1, the reaction center protein of photosystem II and the soluble periplasmic PratA factor from Synechocystis sp. PCC 6803 was verified. A more comprehensive analysis of Alb3-interacting proteins revealed that Alb3 is able to form dimers or oligomers. Interestingly, Alb3 was also shown to bind to the PSII proteins D1, D2 and CP43, to the PSI reaction center protein PSI-A and the ATP synthase subunit CF₀III, suggesting an important role of Alb3 in the assembly of photosynthetic thylakoid membrane complexes.     

LPA2 is required for efficient assembly of photosystem II in Arabidopsis thaliana

To elucidate the molecular mechanism of photosystem II (PSII) assembly, we characterized the low psii accumulation2 (lpa2) mutant of Arabidopsis thaliana, which is defective in the accumulation of PSII supercomplexes. The levels and processing patterns of the RNAs encoding the PSII subunits are unaltered in the mutant. In vivo protein-labeling experiments showed that the synthesis of CP43 (for chlorophyll a binding protein) was greatly reduced, but CP47, D1, and D2 were synthesized at normal rates in the lpa2-1 mutant. The newly synthesized CP43 was rapidly degraded in lpa2-1, and the turnover rates of D1 and D2 were higher in lpa2-1 than in wild-type plants. The newly synthesized PSII proteins were assembled into PSII complexes, but the assembly of PSII was less efficient in the mutant than in wild-type plants. LPA2 encodes an intrinsic thylakoid membrane protein, which is not an integral subunit of PSII. Yeast two-hybrid assays indicated that LPA2 interacts with the PSII core protein CP43 but not with the PSII reaction center proteins D1 and D2. Moreover, direct interactions of LPA2 with Albino3 (Alb3), which is involved in thylakoid membrane biogenesis and cell division, were also detected. Thus, the results suggest that LPA2, which appears to form a complex with Alb3, is involved in assisting CP43 assembly within PSII.     

YidC--an evolutionary conserved device for the assembly of energy-transducing membrane protein complexes

Members of the YidC/Oxa1/Alb3 membrane protein family are multifunctional mediators of membrane protein integration, folding and assembly into large complexes. Their evolutionary conserved and physiologically important role appears to relate to the assembly of major energy-transducing membrane protein complexes.     

The C Terminus of the Alb3 Membrane Insertase Recruits cpSRP43 to the Thylakoid Membrane

The YidC/Oxa1/Alb3 family of membrane proteins controls the insertion and assembly of membrane proteins in bacteria, mitochondria, and chloroplasts. Here we describe the molecular mechanisms underlying the interaction of Alb3 with the chloroplast signal recognition particle (cpSRP). The Alb3 C-terminal domain (A3CT) is intrinsically disordered and recruits cpSRP to the thylakoid membrane by a coupled binding and folding mechanism. Two conserved, positively charged motifs reminiscent of chromodomain interaction motifs in histone tails are identified in A3CT that are essential for the Alb3-cpSRP43 interaction. They are absent in the C-terminal domain of Alb4, which therefore does not interact with cpSRP43. Chromodomain 2 in cpSRP43 appears as a central binding platform that can interact simultaneously with A3CT and cpSRP54. The observed negative cooperativity of the two binding events provides the first insights into cargo release at the thylakoid membrane. Taken together, our data show how Alb3 participates in cpSRP-dependent membrane targeting, and our data provide a molecular explanation why Alb4 cannot compensate for the loss of Alb3. Oxa1 and YidC utilize their positively charged, C-terminal domains for ribosome interaction in co-translational targeting. Alb3 is adapted for the chloroplast-specific Alb3-cpSRP43 interaction in post-translational targeting by extending the spectrum of chromodomain interactions.     

A second thylakoid membrane-localized Alb3/OxaI/YidC homologue is involved in proper chloroplast biogenesis in Arabidopsis thaliana

The integral membrane proteins Alb3, OxaI, and YidC belong to an evolutionary conserved protein family mediating protein insertion into the thylakoid membrane of chloroplasts, the inner membrane of mitochondria, and bacteria, respectively. Whereas OxaI and YidC are involved in the insertion of a wide range of membrane proteins, the function of Alb3 seems to be limited to the insertion of a subset of the light-harvesting chlorophyll-binding proteins. In this study, we identified a second chloroplast homologue of the Alb3/OxaI/YidC family, named Alb4. Alb4 is almost identical to the Alb3/OxaI/YidC domain of the previously described 110-kDa inner envelope protein Artemis. We show that Alb4 is expressed as a separate 55-kDa protein and that Artemis was identified mistakenly. Alb4 is located in the thylakoid membrane of Arabidopsis thaliana chloroplasts. Analysis of an Arabidopsis mutant (Salk_136199) and RNA interference lines with a reduced level of Alb4 revealed chloroplasts with an altered ultrastructure. Mutant plastids are larger and more spherical in appearance, and the grana stacks within the mutant lines are less appressed than in the wild-type chloroplasts. These data indicate that Alb4 is required for proper chloroplast biogenesis.     

Biogenesis of thylakoid membranes with emphasis on the process in Chlamydomonas

Recent results obtained by electron microscopic and biochemical analyses of greening Chlamydomonas reinhardtii y1 suggest that localized expansion of the plastid envelope is involved in thylakoid biogenesis. Kinetic analyses of the assembly of light-harvesting complexes and development of photosynthetic function when degreened cells of the alga are exposed to light suggest that proteins integrate into membrane at the level of the envelope. Current information, therefore, supports the earlier conclussion that the chloroplast envelope is a major biogenic structure, from which thylakoid membranes emerge. Chloroplast development in Chlamydomonas provides unique opportunities to examine in detail the biogenesis of thylakoids.Abbreviations Rubisco ribulose bisphosphate carboxylase/oxygenase - CAB Chl a/b-binding (proteins) - Chlide chlorophyllide - LHC I light-harvesting complex of PS I - LHC II light-harvesting complex of PS II - Pchlide protochlorophyllide     

Role of vesicle-inducing protein in plastids 1 in cpTat transport at the thylakoid

VIPP1 has been shown to be required for the proper formation of thylakoid membranes. However, studies on VIPP1 itself, as well as on PspA, its bacterial homolog, suggests that this protein may be involved in a number of additional functions, including protein translocation. The role of VIPP1 in protein translocation in the chloroplast has not been investigated. To this end, we conducted in vitro thylakoid protein transport assays to look at the effect of VIPP1 on the cpTat pathway, which is one of three translocation pathways found in both the chloroplast and its bacterial progenitor. We found that VIPP1 does indeed enhance protein transport through the cpTat pathway by up to 100%. The VIPP1 effect on cpTat activity occurs without interacting with the substrates or components of the translocon, and does not alter the energy potentials driving this translocation pathway. Instead, VIPP1 greatly enhances the amount of substrate bound productively to the thylakoids. Moreover, the presence of increasing VIPP1 concentrations in the reactions resulted in greater interactions between thylakoid membranes. Taken together, these results demonstrate a stimulatory role for VIPP1 in cpTat transport by enhancement of substrate binding, probably to the membrane lipid regions of the thylakoid. We propose a model in which VIPP1 facilitates reorganization of the thylakoid structure to increase substrate access to productive binding regions of the membrane as an early step in the cpTat pathway.     

Functional and mutational analysis of the light-harvesting chlorophyll a/b protein of thylakoid membranes

The precursor for a Lemna light-harvesting chlorophyll a/b protein (pLHCP) has been synthesized in vitro from a single member of the nuclear LHCP multigene family. We report the sequence of this gene. When incubated with Lemna chloroplasts, the pLHCP is imported and processed into several polypeptides, and the mature form is assembled into the light-harvesting complex of photosystem II (LHC II). The accumulation of the processed LHCP is enhanced by the addition to the chloroplasts of a precursor and a co-factor for chlorophyll biosynthesis. Using a model for the arrangement of the mature polypeptide in the thylakoid membrane as a guide, we have created mutations that lie within the mature coding region. We have studied the processing, the integration into thylakoid membranes, and the assembly into light-harvesting complexes of six of these deletions. Four different mutant LHCPs are found as processed proteins in the thylakoid membrane, but only one appears to have an orientation in the membrane that is similar to that of the wild type. No mutant LHCP appears in LHC II. The other two mutant LHCPs cannot be detected within the chloroplasts. We conclude that stable complex formation is not required for the processing and insertion of altered LHCPs into the thylakoid membrane. We discuss the results in light of our model.     

Evolution of YidC/Oxa1/Alb3 insertases: three independent gene duplications followed by functional specialization in bacteria, mitochondria and chloroplasts

Members of the YidC/Oxa1/Alb3 protein family facilitate the insertion, folding and assembly of proteins of the inner membranes of bacteria and mitochondria and the thylakoid membrane of plastids. All homologs share a conserved hydrophobic core region comprising five transmembrane domains. On the basis of phylogenetic analyses, six subgroups of the family can be distinguished which presumably arose from three independent gene duplications followed by functional specialization. During evolution of bacteria, mitochondria and chloroplasts, subgroup-specific regions were added to the core domain to facilitate the association with ribosomes or other components contributing to the substrate spectrum of YidC/Oxa1/Alb3 proteins.     

Chlorophyll a/b-binding proteins: an extended family

A large proportion of the chlorophyll in a plant is engaged in harvesting light energy and transferring it to the photochemical reaction centres. These 'antenna' chlorophylls are non-covalently bound to specific proteins to form chlorophyll-protein complexes. The chlorophyll a/b-binding (CAB) polypeptides are encoded by an extended family of nuclear genes. It has recently been discovered that other proteins not known to bind chlorophyll, the early light-inducible proteins (ELIPs), are also related and could be considered part of this family. We suggest that the latter proteins may be involved in pigment biosynthesis or in assembly of the thylakoid membrane.     

Insertion of proteins into the inner membrane of mitochondria: the role of the Oxa1 complex

The inner mitochondrial membrane harbors a large number of proteins that display a wide range of topological arrangements. The majority of these proteins are encoded in the cell's nucleus, but a few polytopic proteins, all subunits of respiratory chain complexes are encoded by the mitochondrial genome. A number of distinct sorting mechanisms exist to direct these proteins into the mitochondrial inner membrane. One of these pathways involves the export of proteins from the matrix into the inner membrane and is used by both proteins synthesized within the mitochondria, as well as by a subset of nuclear encoded proteins. Prior to embarking on the export pathway, nuclear encoded proteins using this sorting route are initially imported into the mitochondrial matrix from the cytosol, their site of synthesis. Protein export from the matrix into the inner membrane bears similarities to Sec-independent protein export in bacteria and requires the function of the Oxa1 protein. Oxa1 is a component of a general protein insertion site in yeast mitochondrial inner membrane used by both nuclear and mitochondrial DNA encoded proteins. Oxa1 is a member of the conserved Oxa1/YidC/Alb3 protein family found throughout prokaryotes throughout eukaryotes (where it is found in mitochondria and chloroplasts). The evidence to demonstrate that the Oxa1/YidC/Alb3 protein family represents a novel evolutionarily conserved membrane insertion machinery is reviewed here.     

Saccharomyces cerevisiae Cox18 complements the essential Sec-independent function of Escherichia coli YidC

Members of the YidC/Oxa1/Alb3 protein family function in the biogenesis of membrane proteins in bacteria, mitochondria and chloroplasts. In Escherichia coli, YidC plays a key role in the integration and assembly of many inner membrane proteins. Interestingly, YidC functions both in concert with the Sec-translocon and as a separate insertase independent of the translocon. Mitochondria of higher eukaryotes contain two distant homologues of YidC: Oxa1 and Cox18/Oxa2. Oxa1 is required for the insertion of membrane proteins into the mitochondrial inner membrane. Cox18/Oxa2 plays a poorly defined role in the biogenesis of the cytochrome c oxidase complex. Employing a genetic complementation approach by expressing the conserved region of yeast Cox18 in E. coli, we show here that Cox18 is able to complement the essential Sec-independent function of YidC. This identifies Cox18 as a bona fide member of the YidC/Oxa1/Alb3 family.     

Interaction studies between the chloroplast signal recognition particle subunit cpSRP43 and the full-length translocase Alb3 reveal a membrane-embedded binding region in Alb3 protein

Posttranslational targeting of the light-harvesting chlorophyll a,b-binding proteins depends on the function of the chloroplast signal recognition particle, its receptor cpFtsY, and the translocase Alb3. The thylakoid membrane protein Alb3 of Arabidopsis chloroplasts belongs to the evolutionarily conserved YidC/Oxa1/Alb3 protein family; the members of this family facilitate the insertion, folding, and assembly of membrane proteins in bacteria, mitochondria, and chloroplasts. Here, we analyzed the interaction sites of full-length Alb3 with the cpSRP pathway component cpSRP43 by using in vitro and in vivo studies. Bimolecular fluorescence complementation and Alb3 proteoliposome studies showed that the interaction of cpSRP43 is dependent on a binding domain in the C terminus of Alb3 as well as an additional membrane-embedded binding site in the fifth transmembrane domain (TMD5) of Alb3. The C-terminal binding domain was mapped to residues 374-388, and the binding domain within TMD5 was mapped to residues 314-318 located close to the luminal end of TMD5. A direct binding between cpSRP43 and these binding motifs was shown by pepspot analysis. Further studies using blue-native gel electrophoresis revealed that full-length Alb3 is able to form dimers. This finding and the identification of a membrane-embedded cpSRP43 binding site in Alb3 support a model in which cpSRP43 inserts into a dimeric Alb3 translocation pore during cpSRP-dependent delivery of light-harvesting chlorophyll a,b-binding proteins.