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

Component interactions, regulation and mechanisms of chloroplast signal recognition particle-dependent protein transport

The chloroplast proteome comprises nuclear- and plastome-encoded proteins. In order to function correctly these proteins must be transported, either cotranslationally or posttranslationally, to their final destination in the chloroplast. Here the chloroplast signal recognition particle (cpSRP) which is present in two different stromal pools plays an essential role. On the one hand, the conserved 54kDa subunit (cpSRP54) is associated with 70S ribosomes to function in the cotranslational transport of the plastid-encoded thylakoid membrane protein D1. On the other hand, the cpSRP consists of cpSRP54 and a unique 43kDa subunit (cpSRP43) and facilitates the transport of nuclear-encoded light-harvesting chlorophyll-binding proteins (LHCPs), the most abundant membrane proteins of the thylakoids. In addition to cpSRP, the cpSRP receptor cpFtsY and the thylakoid membrane protein Alb3 are required for posttranslational LHCP integration in a GTP-dependent manner. In contrast to the universally conserved cytosolic SRP, the chloroplast SRP of higher plants lacks an SRP-RNA component. Interestingly, cpSRP-RNA genes have been identified in the plastome of lower plants, indicating that their cpSRP structure resembles the cytosolic SRP.

Alb4 of Arabidopsis promotes assembly and stabilization of a non chlorophyll-binding photosynthetic complex, the CF1CF0-ATP synthase

All members of the YidC/Oxa1/Alb3 protein family are evolutionarily conserved and appear to function in membrane protein integration and protein complex stabilization. Here, we report on a second thylakoidal isoform of Alb3, named Alb4. Analysis of Arabidopsis knockout mutant lines shows that Alb4 is required in assembly and/or stability of the CF1CF0-ATP synthase (ATPase). alb4 mutant lines not only have reduced steady-state levels of ATPase subunits, but also their assembly into high-molecular-mass complexes is altered, leading to a reduction of ATP synthesis in the mutants. Moreover, we show that Alb4 but not Alb3 physically interacts with the subunits CF1beta and CF0II. Summarizing, the data indicate that Alb4 functions to stabilize or promote assembly of CF1 during its attachment to the membrane-embedded CF0 part.

The making of a chloroplast

Since its endosymbiotic beginning, the chloroplast has become fully integrated into the biology of the host eukaryotic cell. The exchange of genetic information from the chloroplast to the nucleus has resulted in considerable co-ordination in the activities of these two organelles during all stages of plant development. Here, we give an overview of the mechanisms of light perception and the subsequent regulation of nuclear gene expression in the model plant Arabidopsis thaliana, and we cover the main events that take place when proplastids differentiate into chloroplasts. We also consider recent findings regarding signalling networks between the chloroplast and the nucleus during seedling development, and how these signals are modulated by light. In addition, we discuss the mechanisms through which chloroplasts develop in different cell types, namely cotyledons and the dimorphic chloroplasts of the C(4) plant maize. Finally, we discuss recent data that suggest the specific regulation of the light-dependent phases of photosynthesis, providing a means to optimize photosynthesis to varying light regimes.

The evolution of YidC/Oxa/Alb3 family in the three domains of life: a phylogenomic analysis

BACKGROUND

YidC/Oxa/Alb3 family includes a group of conserved translocases that are essential for protein insertion into inner membranes of bacteria and mitochondria, and thylakoid membranes of chloroplasts. Because mitochondria and chloroplasts are of bacterial origin, Oxa and Alb3, like many other mitochondrial/chloroplastic proteins, are hypothetically derived from the pre-existing protein (YidC) of bacterial endosymbionts. Here, we test this hypothesis and investigate the evolutionary history of the whole YidC/Oxa/Alb3 family in the three domains of life.

RESULTS

Our comprehensive analyses of the phylogenetic distribution and phylogeny of the YidC/Oxa/Alb3 family lead to the following findings: 1) In archaea, YidC homologs are only sporadically distributed in Euryarchaeota; 2) Most bacteria contain only one YidC gene copy; some species in a few taxa (Bacillus, Lactobacillales, Actinobacteria and Clostridia) have two gene copies; 3) Eukaryotic Oxa and Alb3 have two separate prokaryotic origins, but they might not arise directly from the YidC of proteobacteria and cyanobacteria through the endosymbiosis origins of mitochondrium and chloroplast, respectively; 4) An ancient duplication occurred on both Oxa and Alb3 immediately after their origins, and thus most eukaryotes generally bear two Oxa and two Alb3. However, secondary loss, duplication or acquisition of new domain also occurred on the two genes in some lineages, especially in protists, resulting in a rich diversity or adaptive differentiation of the two translocases in these lineages.

CONCLUSION

YidC is distributed in bacteria and some Euryarchaeota. Although mitochondrial Oxa and chloroplastic Alb3 are derived from the prokaryotic YidC, their origin might be not related to the endosymbiosis events of the two organelles. In some eukaryotic lineages, especially in protists, Oxa and Alb3 have diverse evolutionary histories. Finally, a model for the evolutionary history of the entire YidC/Oxa/Alb3 family in the three domains of life is proposed.

Plastid protein import and sorting: different paths to the same compartments

Chloroplasts contain several thousand different proteins, of which more than 95% are encoded on nuclear genes, synthesized in the cytosol as precursor proteins, and imported into the organelle. The major pathways for import and routing have been described; a general import apparatus in the chloroplast envelope and several ancestral translocases in the thylakoid membranes. In this update we focus on some interesting and emerging areas: the Tat translocase, which operates in parallel with the Sec system but transports folded proteins; different routes to the envelope membranes, which promises an understanding of the ways the Tic apparatus sorts transmembrane domains (TMDs) and may also uncover developmental relationships between envelope and thylakoids; and novel routes for proteins into chloroplasts including delivery from the secretory system.

Auxiliary proteins involved in the assembly and sustenance of photosystem II

Chloroplast proteins that regulate the biogenesis, performance and acclimation of the photosynthetic protein complexes are currently under intense research. Dozens, possibly even hundreds, of such proteins in the stroma, thylakoid membrane and the lumen assist the biogenesis and constant repair of the water splitting photosystem (PS) II complex. During the repair cycle, assistance is required at several levels including the degradation of photodamaged D1 protein, de novo synthesis, membrane insertion, folding of the nascent protein chains and the reassembly of released protein subunits and different co-factors into PSII in order to guarantee the maintenance of the PSII function. Here we review the present knowledge of the auxiliary proteins, which have been reported to be involved in the biogenesis and maintenance of PSII.

Identification of the minus-dominance gene ortholog in the mating-type locus of Gonium pectorale

The evolution of anisogamy/oogamy in the colonial Volvocales might have occurred in an ancestral isogamous colonial organism like Gonium pectorale. The unicellular, close relative Chlamydomonas reinhardtii has a mating-type (MT) locus harboring several mating-type-specific genes, including one involved in mating-type determination and another involved in the function of the tubular mating structure in only one of the two isogametes. In this study, as the first step in identifying the G. pectorale MT locus, we isolated from G. pectorale the ortholog of the C. reinhardtii mating-type-determining minus-dominance (CrMID) gene, which is localized only in the MT- locus. 3'- and 5'-RACE RT-PCR using degenerate primers identified a CrMID-orthologous 164-amino-acid coding gene (GpMID) containing a leucine-zipper RWP-RK domain near the C-terminal, as is the case with CrMID. Genomic Southern blot analysis showed that GpMID was coded only in the minus strain of G. pectorale. RT-PCR revealed that GpMID expression increased during nitrogen starvation. Analysis of F1 progeny suggested that GpMID and isopropylmalate dehydratase LEU1S are tightly linked, suggesting that they are harbored in a chromosomal region under recombinational suppression that is comparable to the C. reinhardtii MT locus. However, two other genes present in the C. reinhardtii MT locus are not linked to the G. pectorale LEU1S/MID, suggesting that the gene content of the volvocalean MT loci is not static over time. Inheritance of chloroplast and mitochondria genomes in G. pectorale is uniparental from the plus and minus parents, respectively, as is also the case in C. reinhardtii.

Projection structure of yidC: a conserved mediator of membrane protein assembly

Bacteria, mitochondria and chloroplasts harbour factors that facilitate the insertion, folding and assembly of membrane proteins. In Escherichia coli, yidC is required for membrane insertion, acting in both a Sec-dependent and a Sec-independent manner. There is an expanding volume of biochemical work on its role in this process, but none so far on its structure. We present the first of this class of membrane proteins determined by electron cryomicroscopy in the near-nativelike state of the membrane. yidC forms dimers in the membrane and each monomer has an area of low density that may be part of the path transmembrane segments follow during their insertion. Upon consideration of the structures of yidC and SecYEG, we speculate on the nature of the interfaces that facilitate the alternative pathways (Sec-dependent and -independent) of membrane protein insertion.

Chlorophylls, ligands and assembly of light-harvesting complexes in chloroplasts

Chlorophyll (Chl) b serves an essential function in accumulation of light-harvesting complexes (LHCs) in plants. In this article, this role of Chl b is explored by considering the properties of Chls and the ligands with which they interact in the complexes. The overall properties of the Chls, not only their spectral features, are altered as consequences of chemical modifications on the periphery of the molecules. Important modifications are introduction of oxygen atoms at specific locations and reduction or desaturation of sidechains. These modifications influence formation of coordination bonds by which the central Mg atom, the Lewis acid, of Chl molecules interacts with amino acid sidechains, as the Lewis base, in proteins. Chl a is a versatile Lewis acid and interacts principally with imidazole groups but also with sidechain amides and water. The 7-formyl group on Chl b withdraws electron density toward the periphery of the molecule and consequently the positive Mg is less shielded by the molecular electron cloud than in Chl a. Chl b thus tends to form electrostatic bonds with Lewis bases with a fixed dipole, such as water and, in particular, peptide backbone carbonyl groups. The coordination bonds are enhanced by H-bonds between the protein and the 7-formyl group. These additional strong interactions with Chl b are necessary to achieve assembly of stable LHCs.

Photosystem II assembly and repair are differentially localized in Chlamydomonas

Many proteins of the photosynthesis complexes are encoded by the genome of the chloroplast and synthesized by bacterium-like ribosomes within this organelle. To determine where proteins are synthesized for the de novo assembly and repair of photosystem II (PSII) in the chloroplast of Chlamydomonas reinhardtii, we used fluorescence in situ hybridization, immunofluorescence staining, and confocal microscopy. These locations were defined as having colocalized chloroplast mRNAs encoding PSII subunits and proteins of the chloroplast translation machinery specifically under conditions of PSII subunit synthesis. The results revealed that the synthesis of the D1 subunit for the repair of photodamaged PSII complexes occurs in regions of the chloroplast with thylakoids, consistent with the current model. However, for de novo PSII assembly, PSII subunit synthesis was detected in discrete regions near the pyrenoid, termed T zones (for translation zones). In two PSII assembly mutants, unassembled D1 subunits and incompletely assembled PSII complexes localized around the pyrenoid, where we propose that they mark an intermediate compartment of PSII assembly. These results reveal a novel chloroplast compartment that houses de novo PSII biogenesis and the regulated transport of newly assembled PSII complexes to thylakoid membranes throughout the chloroplast.

Iron deficiency in rice shoots: identification of novel induced genes using RDA and possible relation to leaf senescence

Rice plants are highly susceptible to Fe-deficiency. Under nutrient deprivation, plant cells undergo extensive metabolic changes for their continued survival. To provide further insight into the pathways induced during Fe-deficiency, rice seedlings were grown for 3, 6 and 9 days in the presence or absence of Fe. Using RDA (Representational Difference Analysis), sequences of 32 induced genes in rice shoots under Fe-deficiency were identified. About 30% of the sequences found have been previously reported as responsive to other abiotic and even biotic stresses. However, this is the first report that indicates their relation to Fe deprivation. Differential expression of selected genes was confirmed by semi-quantitative RT-PCR analysis. The identification of classical senescence-related sequences, such as lipase EC 3.1.1.-, ubiquitin-conjugating enzyme EC 6.3.2.19, beta-Glucosidase EC 3.2.1.21 and cysteine synthase EC 2.5.1.47, besides the higher accumulation of total soluble sugars prior to the decrease of total chlorophyll content in Fe-deficient leaves, indicate that sugar accumulation may be one of the factors leading to premature leaf senescence induced by Fe-deficiency.

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.

Structure, function and regulation of plant photosystem I

Photosystem I (PSI) is a multisubunit protein complex located in the thylakoid membranes of green plants and algae, where it initiates one of the first steps of solar energy conversion by light-driven electron transport. In this review, we discuss recent progress on several topics related to the functioning of the PSI complex, like the protein composition of the complex in the plant Arabidopsis thaliana, the function of these subunits and the mechanism by which nuclear-encoded subunits can be inserted into or transported through the thylakoid membrane. Furthermore, the structure of the native PSI complex in several oxygenic photosynthetic organisms and the role of the chlorophylls and carotenoids in the antenna complexes in light harvesting and photoprotection are reviewed. The special role of the 'red' chlorophylls (chlorophyll molecules that absorb at longer wavelength than the primary electron donor P700) is assessed. The physiology and mechanism of the association of the major light-harvesting complex of photosystem II (LHCII) with PSI during short term adaptation to changes in light quality and quantity is discussed in functional and structural terms. The mechanism of excitation energy transfer between the chlorophylls and the mechanism of primary charge separation is outlined and discussed. Finally, a number of regulatory processes like acclimatory responses and retrograde signalling is reviewed with respect to function of the thylakoid membrane. We finish this review by shortly discussing the perspectives for future research on PSI.

Canonical signal recognition particle components can be bypassed for posttranslational protein targeting in chloroplasts

The chloroplast signal recognition particle (cpSRP) and its receptor (cpFtsY) target proteins both cotranslationally and posttranslationally to the thylakoids. This dual function enables cpSRP to utilize its posttranslational activities for targeting a family of nucleus-encoded light-harvesting chlorophyll binding proteins (LHCPs), the most abundant membrane proteins in plants. Previous in vitro experiments indicated an absolute requirement for all cpSRP pathway soluble components. In agreement, a cpFtsY mutant in Arabidopsis thaliana exhibits a severe chlorotic phenotype resulting from a massive loss of LHCPs. Surprisingly, a double mutant, cpftsy cpsrp54, recovers to a great extent from the chlorotic cpftsy phenotype. This establishes that in plants, a new alternative pathway exists that can bypass cpSRP posttranslational targeting activities. Using a mutant form of cpSRP43 that is unable to assemble with cpSRP54, we complemented the cpSRP43-deficient mutant and found that this subunit is required for the alternative pathway. Along with the ability of cpSRP43 alone to bind the ALBINO3 translocase required for LHCP integration, our results indicate that cpSRP43 has developed features to function independently of cpSRP54/cpFtsY in targeting LHCPs to the thylakoid membranes.

Mechanisms of protein import into thylakoids of chloroplasts

The thylakoid membrane of chloroplasts contains the major photosynthetic complexes, which consist of several either nuclear or chloroplast encoded subunits. The biogenesis of these thylakoid membrane complexes requires coordinated transport and subsequent assembly of the subunits into functional complexes. Nuclear-encoded thylakoid proteins are first imported into the chloroplast and then directed to the thylakoid using different sorting mechanisms. The cpSec pathway and the cpTat pathway are mainly involved in the transport of lumenal proteins, whereas the spontaneous pathway and the cpSRP pathway are used for the insertion of integral membrane proteins into the thylakoid membrane. While cpSec-, cpTat- and cpSRP-mediated targeting can be classified as 'assisted' mechanisms involving numerous components, 'unassisted' spontaneous insertion does not require additional targeting factors. However, even the assisted pathways differ fundamentally with respect to stromal targeting factors, the composition of the translocase and energy requirements.

YidC as an essential and multifunctional component in membrane protein assembly

Membrane proteins fulfill a number of vital functions in prokaryotic and eukaryotic cells. They are often organized in multicomponent complexes, folded within the membrane bilayer and interacting with the cytoplasmic and periplasmic or external soluble compartments. For the biogenesis of integral membrane proteins, the essential biochemical steps are (1) the insertion and topogenesis of the transmembrane protein segments into the lipid bilayer, (2) the three-dimensional folding of the translocated hydrophilic domains, and (3) the assembly into multimeric complexes. Intensive research has elucidated the basic mechanisms of membrane protein insertion in the homologous translocation machineries of different cellular systems. Whereas the Sec translocation system is found in the endoplasmic reticulum of eukaryotic cells and in the prokaryotic plasma membrane, the YidC-Oxa1 membrane insertase is present in prokaryotic and organellar membranes. This review focuses on the discoveries of the YidC system in bacterial as well as the Oxa1/Alb3 protein family of eukaryotic cells and will particularly emphasize evolutionary aspects.

The synechocystis sp PCC 6803 oxa1 homolog is essential for membrane integration of reaction center precursor protein pD1

Synechocystis sp PCC 6803 Slr1471p, an Oxa1p/Alb3/YidC homolog, is an essential protein for cell viability for which functions in thylakoid membrane biogenesis and cell division have been proposed. Using a fusion of green fluorescent protein to the C terminus of Slr1471p, we found that the mutant slr1471-gfp is photochemically inhibited when light intensities increase to 80 micromol x m(-2) x s(-1). We show that photoinhibition correlates with an increased redox potential of the reaction center quinone Q(A)(-) and a decreased redox potential of Q(B)(-). Analysis reveals that membrane integration of the D1 precursor protein is affected, leading to the accumulation of pD1 in the membrane phase. We show that Slr1471p interacts directly with the D1 protein and discuss why the accumulation of pD1 in two reaction center assembly intermediates is dependent on Slr1471p.

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.

One of two alb3 proteins is essential for the assembly of the photosystems and for cell survival in Chlamydomonas

Proteins of the YidC/Oxa1p/ALB3 family play an important role in inserting proteins into membranes of mitochondria, bacteria, and chloroplasts. In Chlamydomonas reinhardtii, one member of this family, Albino3.1 (Alb3.1), was previously shown to be mainly involved in the assembly of the light-harvesting complex. Here, we show that a second member, Alb3.2, is located in the thylakoid membrane, where it is associated with large molecular weight complexes. Coimmunoprecipitation experiments indicate that Alb3.2 interacts with Alb3.1 and the reaction center polypeptides of photosystem I and II as well as with VIPP1, which is involved in thylakoid formation. Moreover, depletion of Alb3.2 by RNA interference to 25 to 40% of wild-type levels leads to a reduction in photosystems I and II, indicating that the level of Alb3.2 is limiting for the assembly and/or maintenance of these complexes in the thylakoid membrane. Although the levels of several photosynthetic proteins are reduced under these conditions, other proteins are overproduced, such as VIPP1 and the chloroplast chaperone pair Hsp70/Cdj2. These changes are accompanied by a large increase in vacuolar size and, after a prolonged period, by cell death. We conclude that Alb3.2 is required directly or indirectly, through its impact on thylakoid protein biogenesis, for cell survival.

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 CF(0)III, suggesting an important role of Alb3 in the assembly of photosynthetic thylakoid membrane complexes.

Mechanisms of protein import and routing in chloroplasts

The vast majority of the approximately 3000 different proteins required to build a fully functional chloroplast are encoded by the nuclear genome and translated on cytosolic ribosomes. As chloroplasts are each surrounded by a double-membrane system, or envelope, sophisticated mechanisms are necessary to mediate the import of these nucleus-encoded proteins into chloroplasts. Once inside the organelle, many chloroplast proteins engage one of four additional protein sorting mechanisms that direct targeting to the internal thylakoid membrane system.

Conservative sorting in a primitive plastid. The cyanelle of Cyanophora paradoxa

Higher plant chloroplasts possess at least four different pathways for protein translocation across and protein integration into the thylakoid membranes. It is of interest with respect to plastid evolution, which pathways have been retained as a relic from the cyanobacterial ancestor ('conservative sorting'), which ones have been kept but modified, and which ones were developed at the organelle stage, i.e. are eukaryotic achievements as (largely) the Toc and Tic translocons for envelope import of cytosolic precursor proteins. In the absence of data on cyanobacterial protein translocation, the cyanelles of the glaucocystophyte alga Cyanophora paradoxa for which in vitro systems for protein import and intraorganellar sorting were elaborated can serve as a model: the cyanelles are surrounded by a peptidoglycan wall, their thylakoids are covered with phycobilisomes and the composition of their oxygen-evolving complex is another feature shared with cyanobacteria. We demonstrate the operation of the Sec and Tat pathways in cyanelles and show for the first time in vitro protein import across cyanobacteria-like thylakoid membranes and protease protection of the mature protein.

YidC family members are involved in the membrane insertion, lateral integration, folding, and assembly of membrane proteins

Members of the YidC family exist in all three domains of life, where they control the assembly of a large variety of membrane protein complexes that function as transporters, energy devices, or sensor proteins. Recent studies in bacteria have shown that YidC functions on its own as a membrane protein insertase independent of the Sec protein–conducting channel. YidC can also assist in the lateral integration and folding of membrane proteins that insert into the membrane via the Sec pathway.

A homolog of Albino3/OxaI is essential for thylakoid biogenesis in the cyanobacterium Synechocystis sp. PCC6803

YidC/OxaI play essential roles in the insertion of a wide range of membrane proteins in Eschericha coli and mitochondria, respectively. In contrast, the chloroplast thylakoid homolog Albino3 (Alb3) facilitates the insertion of only a specialized subset of proteins, and the vast majority insert into thylakoids by a pathway that is so far unique to chloroplasts. In this study, we have analyzed the role of Alb3 in the cyanobacterium Synechocystis sp. PCC6803, which contains internal thylakoids that are similar in some respects to those of chloroplasts. The single alb3 gene (slr1471) was disrupted by the introduction of an antibiotic cassette, and photoautotrophic growth resulted in the generation of a merodiploid species (but not full segregation), indicating an essential role for Alb3 in maintaining the photosynthetic apparatus. Thylakoid organization is lost under these conditions, and the levels of photosynthetic pigments fall to approximately 40% of wild-type levels. Photosynthetic electron transport and oxygen evolution are reduced by a similar extent. Growth on glucose relieves the selective pressure to maintain photosynthetic competence, and under these conditions, the cells become completely bleached, again indicating that Alb3 is essential for thylakoid biogenesis. Full segregation could not be achieved under any growth regime, strongly suggesting that the slr1471 open reading frame is essential for cell viability.

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.

TheArabidopsis thalianachloroplast inner envelope protein ARTEMIS is a functional member of the Alb3/Oxa1/YidC family of proteins

The Arabidopsis thaliana protein ARTEMIS is an integral component of the chloroplast inner envelope required for chloroplast division. It contains a domain of significant homology to members of the Alb3/Oxa1/YidC protein family. Here, we show that upon expression in yeast mitochondria, ARTEMIS can partially take over the function of yeast Oxa1 in the insertion and assembly of mitochondrial membrane proteins. This identifies ARTEMIS as a functional member of the Alb3/Oxa1/YidC protein family and suggests the existence of a novel protein sorting pathway in chloroplasts which integrates polypeptides from the stroma into the inner envelope by an evolutionary conserved process.

The Oxa2 protein of Neurospora crassa plays a critical role in the biogenesis of cytochrome oxidase and defines a ubiquitous subbranch of the Oxa1/YidC/Alb3 protein family

Proteins of the Oxa1/YidC/Alb3 family mediate the insertion of proteins into membranes of mitochondria, bacteria, and chloroplasts. Here we report the identification of a second gene of the Oxa1/YidC/Alb3 family in the genome of Neurospora crassa, which we have named oxa2. Its gene product, Oxa2, is located in the inner membrane of mitochondria. Deletion of the oxa2 gene caused a specific defect in the biogenesis of cytochrome oxidase and resulted in induction of the alternative oxidase (AOD), which bypasses the need for complex IV of the respiratory chain. The Oxa2 protein of N. crassa complements Cox18-deficient yeast mutants suggesting a common function for both proteins. The oxa2 sequence allowed the identification of a new subfamily of Oxa1/YidC/Alb3 proteins whose members appear to be ubiquitously present in mitochondria of fungi, plants, and animals including humans.

Structure, function and assembly of Photosystem II and its light-harvesting proteins

Photosystem II (PSII) is a multisubunit chlorophyll-protein complex that drives electron transfer from water to plastoquinone using energy derived from light. In green plants, the native form of PSII is surrounded by the light-harvesting complex (LHCII complex) and thus it is called the PSII-LHCII supercomplex. Over the past several years, understanding of the structure, function, and assembly of PSII and LHCII complexes has increased considerably. The unicellular green alga Chlamydomonas reinhardtii has been an excellent model organism to study PSII and LHCII complexes, because this organism grows heterotrophically and photoautotrophically and it is amenable to biochemical, genetic, molecular biological and recombinant DNA methodology. Here, the genes encoding and regulating components of the C. reinhardtii PSII-LHCII supercomplex have been thoroughly catalogued: they include 15 chloroplast and 20 nuclear structural genes as well as 13 nuclear genes coding for regulatory factors. This review discusses these molecular genetic data and presents an overview of the structure, function and assembly of PSII and LHCII complexes.

Functional interaction of chloroplast SRP/FtsY with the ALB3 translocase in thylakoids: substrate not required

Integration of thylakoid proteins by the chloroplast signal recognition particle (cpSRP) posttranslational transport pathway requires the cpSRP, an SRP receptor homologue (cpFtsY), and the membrane protein ALB3. Similarly, Escherichia coli uses an SRP and FtsY to cotranslationally target membrane proteins to the SecYEG translocase, which contains an ALB3 homologue, YidC. In neither system are the interactions between soluble and membrane components well understood. We show that complexes containing cpSRP, cpFtsY, and ALB3 can be precipitated using affinity tags on cpSRP or cpFtsY. Stabilization of this complex with GMP-PNP specifically blocks subsequent integration of substrate (light harvesting chl a/b-binding protein [LHCP]), indicating that the complex occupies functional ALB3 translocation sites. Surprisingly, neither substrate nor cpSRP43, a component of cpSRP, was necessary to form a complex with ALB3. Complexes also contained cpSecY, but its removal did not inhibit ALB3 function. Furthermore, antibody bound to ALB3 prevented ALB3 association with cpSRP and cpFtsY and inhibited LHCP integration suggesting that a complex containing cpSRP, cpFtsY, and ALB3 must form for proper LHCP integration.