A Second Thylakoid Membrane-localized Alb3/OxaI/YidC Homologue Is Involved in Proper Chloroplast Biogenesis in Arabidopsis thaliana

The Sec pathway gene yidC regulates the virulence of mesophilic Aeromonassalmonicida

Aeromonas salmonicida is a common pathogenic bacterial species found in both freshwater and marine fish, leading to significant economic losses in the aquaculture industry. YidC is an accessory to SecYEG and is essential for the SecYEG transporter to insert into the bacterial membrane. However, the roles of the yidC gene on the host immune response remain unclear. Here, we compared the pathogenicity of yidC gene-deleted (ΔyidC) strain and wild-type (SRW-OG1) strain of mesophilic A. salmonicida to Orange-spotted grouper (Epinephelus coioides), and explored the impacts of yidC gene on the immune response of E. coioides to mesophilic A. salmonicida infection by using Red/ET recombineering. In this study, the E. coioides in the Secondary infected group had a 53.9 % higher survival rate than those in the Primary infected group. In addition, the adhesion ability of ΔyidC strain decreased by about 83.36 % compared with that of the wild-type (SRW-OG1) strain. Further comparison of the biological phenotype of SRW-OG1 and ΔyidC revealed that this yidC gene could regulate the expression of genes related to iron metabolism and have no effect on bacterial growth under the limited iron concentration. In the low concentration of Fe3+ and Fe2+ environment, SRW-OG1 can obtain iron ions by regulating yidC. Based on the above results, yidC gene contributed to the pathogenicity of mesophilic A. salmonicida to E. coioides, deletion of yidC gene promoted the inflammation and immune response of E. coioides to mesophilic A. salmonicida infection.

STIC2 selectively binds ribosome-nascent chain complexes in the cotranslational sorting of Arabidopsis thylakoid proteins

Chloroplast-encoded multi-span thylakoid membrane proteins are crucial for photosynthetic complexes, yet the coordination of their biogenesis remains poorly understood. To identify factors that specifically support the cotranslational biogenesis of the reaction center protein D1 of photosystem (PS) II, we generated and affinity-purified stalled ribosome-nascent chain complexes (RNCs) bearing D1 nascent chains. Stalled RNCs translating the soluble ribosomal subunit uS2c were used for comparison. Quantitative tandem-mass spectrometry of the purified RNCs identified around 140 proteins specifically associated with D1 RNCs, mainly involved in protein and cofactor biogenesis, including chlorophyll biosynthesis, and other metabolic pathways. Functional analysis of STIC2, a newly identified D1 RNC interactor, revealed its cooperation with chloroplast protein SRP54 in the de novo biogenesis and repair of D1, and potentially other cotranslationally-targeted reaction center subunits of PSII and PSI. The primary binding interface between STIC2 and the thylakoid insertase Alb3 and its homolog Alb4 was mapped to STIC2's β-sheet region, and the conserved Motif III in the C-terminal regions of Alb3/4.

Transcriptomic analysis of hub genes regulating albinism in light- and temperature-sensitive albino tea cultivars 'Zhonghuang 1' and 'Zhonghuang 2'

Albino tea cultivars have high economic value because their young leaves contain enhanced free amino acids that improve the quality and properties of tea. Zhonghuang 1 (ZH1) and Zhonghuang 2 (ZH2) are two such cultivars widely planted in China; however, the environmental factors and molecular mechanisms regulating their yellow-leaf phenotype remain unclear. In this study, we demonstrated that both ZH1 and ZH2 are light- and temperature-sensitive. Under natural sunlight and low-temperature conditions, their young shoots were yellow with decreased chlorophyll and an abnormal chloroplast ultrastructure. Conversely, young shoots were green with increased chlorophyll and a normal chloroplast ultrastructure under shading and high-temperature conditions. RNA-seq analysis was performed for high light and low light conditions, and pairwise comparisons identified genes exhibiting different light responses between albino and green-leaf cultivars, including transcription factors, cytochrome P450 genes, and heat shock proteins. Weighted gene coexpression network analyses of RNA-seq data identified the modules related to chlorophyll differences between cultivars. Genes involved in chloroplast biogenesis and development, light signaling, and JA biosynthesis and signaling were typically downregulated in albino cultivars, accompanied by a decrease in JA-ILE content in ZH2 during the albino period. Furthermore, we identified the hub genes that may regulate the yellow-leaf phenotype of ZH1 and ZH2, including CsGDC1, CsALB4, CsGUN4, and a TPR gene (TEA010575.1), which were related to chloroplast biogenesis. This study provides new insights into the molecular mechanisms underlying leaf color formation in albino tea cultivars.

Thylakoid protein FPB1 synergistically cooperates with PAM68 to promote CP47 biogenesis and Photosystem II assembly

In chloroplasts, insertion of proteins with multiple transmembrane domains (TMDs) into thylakoid membranes usually occurs in a co-translational manner. Here, we have characterized a thylakoid protein designated FPB1 (Facilitator of PsbB biogenesis1) which together with a previously reported factor PAM68 (Photosynthesis Affected Mutant68) is involved in assisting the biogenesis of CP47, a subunit of the Photosystem II (PSII) core. Analysis by ribosome profiling reveals increased ribosome stalling when the last TMD segment of CP47 emerges from the ribosomal tunnel in fpb1 and pam68. FPB1 interacts with PAM68 and both proteins coimmunoprecipitate with SecY/E and Alb3 as well as with some ribosomal components. Thus, our data indicate that, in coordination with the SecY/E translocon and the Alb3 integrase, FPB1 synergistically cooperates with PAM68 to facilitate the co-translational integration of the last two CP47 TMDs and the large loop between them into thylakoids and the PSII core complex.

The thylakoid proton antiporter KEA3 regulates photosynthesis in response to the chloroplast energy status

Plant photosynthesis contains two functional modules, the light-driven reactions in the thylakoid membrane and the carbon-fixing reactions in the chloroplast stroma. In nature, light availability for photosynthesis often undergoes massive and rapid fluctuations. Efficient and productive use of such variable light supply requires an instant crosstalk and rapid synchronization of both functional modules. Here, we show that this communication involves the stromal exposed C-terminus of the thylakoid K+-exchange antiporter KEA3, which regulates the ΔpH across the thylakoid membrane and therefore pH-dependent photoprotection. By combining in silico, in vitro, and in vivo approaches, we demonstrate that the KEA3 C-terminus senses the energy state of the chloroplast in a pH-dependent manner and regulates transport activity in response. Together our data pinpoint a regulatory feedback loop by which the stromal energy state orchestrates light capture and photoprotection via multi-level regulation of KEA3.

Heterologous Expression of Jatropha curcas Fatty Acyl-ACP Thioesterase A (JcFATA) and B (JcFATB) Affects Fatty Acid Accumulation and Promotes Plant Growth and Development in Arabidopsis

Plant fatty acyl-acyl carrier protein (ACP) thioesterases terminate the process of de novo fatty acid biosynthesis in plastids by hydrolyzing the acyl-ACP intermediates, and determine the chain length and levels of free fatty acids. They are of interest due to their roles in fatty acid synthesis and their potential to modify plant seed oils through biotechnology. Fatty acyl-ACP thioesterases (FAT) are divided into two families, i.e., FATA and FATB, according to their amino acid sequence and substrate specificity. The high oil content in Jatropha curcas L. seed has attracted global attention due to its potential for the production of biodiesel. However, the detailed effects of JcFATA and JcFATB on fatty acid biosynthesis and plant growth and development are still unclear. In this study, we found that JcFATB transcripts were detected in all tissues and organs examined, with especially high accumulation in the roots, leaves, flowers, and some stages of developing seeds, and JcFATA showed a very similar expression pattern. Subcellular localization of the JcFATA-GFP and JcFATB-GFP fusion protein in Arabidopsis leaf protoplasts showed that both JcFATA and JcFATB localized in chloroplasts. Heterologous expression of JcFATA and JcFATB in Arabidopsis thaliana individually generated transgenic plants with longer roots, stems and siliques, larger rosette leaves, and bigger seeds compared with those of the wild type, indicating the overall promotion effects of JcFATA and JcFATB on plant growth and development while JcFATB had a larger impact. Compositional analysis of seed oil revealed that all fatty acids except 22:0 were significantly increased in the mature seeds of JcFATA-transgenic Arabidopsis lines, especially unsaturated fatty acids, such as the predominant fatty acids of seed oil, 18:1, 18:2, and 18:3. In the mature seeds of the JcFATB-transgenic Arabidopsis lines, most fatty acids were increased compared with those in wild type too, especially saturated fatty acids, such as 16:0, 18:0, 20:0, and 22:0. Our results demonstrated the promotion effect of JcFATA and JcFATB on plant growth and development, and their possible utilization to modify the seed oil composition and content in higher plants.

Chloroplast Ribosomes Interact With the Insertase Alb3 in the Thylakoid Membrane

Members of the Oxa1/YidC/Alb3 protein family are involved in the insertion, folding, and assembly of membrane proteins in mitochondria, bacteria, and chloroplasts. The thylakoid membrane protein Alb3 mediates the chloroplast signal recognition particle (cpSRP)-dependent posttranslational insertion of nuclear-encoded light harvesting chlorophyll a/b-binding proteins and participates in the biogenesis of plastid-encoded subunits of the photosynthetic complexes. These subunits are cotranslationally inserted into the thylakoid membrane, yet very little is known about the molecular mechanisms underlying docking of the ribosome-nascent chain complexes to the chloroplast SecY/Alb3 insertion machinery. Here, we show that nanodisc-embedded Alb3 interacts with ribosomes, while the homolog Alb4, also located in the thylakoid membrane, shows no ribosome binding. Alb3 contacts the ribosome with its C-terminal region and at least one additional binding site within its hydrophobic core region. Within the C-terminal region, two conserved motifs (motifs III and IV) are cooperatively required to enable the ribosome contact. Furthermore, our data suggest that the negatively charged C-terminus of the ribosomal subunit uL4c is involved in Alb3 binding. Phylogenetic analyses of uL4 demonstrate that this region newly evolved in the green lineage during the transition from aquatic to terrestrial life.

Fates of Sec, Tat, and YidC Translocases in Mitochondria and Other Eukaryotic Compartments

Formation of mitochondria by the conversion of a bacterial endosymbiont was a key moment in the evolution of eukaryotes. It was made possible by outsourcing the endosymbiont's genetic control to the host nucleus, while developing the import machinery for proteins synthesized on cytosolic ribosomes. The original protein export machines of the nascent organelle remained to be repurposed or were completely abandoned. This review follows the evolutionary fates of three prokaryotic inner membrane translocases Sec, Tat, and YidC. Homologs of all three translocases can still be found in current mitochondria, but with different importance for mitochondrial function. Although the mitochondrial YidC homolog, Oxa1, became an omnipresent independent insertase, the other two remained only sporadically present in mitochondria. Only a single substrate is known for the mitochondrial Tat and no function has yet been assigned for the mitochondrial Sec. Finally, this review compares these ancestral mitochondrial proteins with their paralogs operating in the plastids and the endomembrane system.

The Tomato SlVIPP1 Gene Is Required for Plant Survival Through the Proper Development of Chloroplast Thylakoid Membrane

Since membranes play essential roles in all living beings, all cells have developed mechanisms for efficient and fast repair of membrane damage. In Escherichia coli, the Phage shock stress A (PspA) protein is involved in the maintenance of the integrity of its inner membrane in response to the damage produced by exposure to stress conditions. A role in thylakoid membrane maintenance and reorganization has been proposed for Vesicle Inducing Protein in Plastid 1 (VIPP1), the putative PspA ortholog in Arabidopsis thaliana. While some membranes of plant cells have been extensively studied, the biosynthesis and maintenance of chloroplast thylakoid membrane remains poorly known. Here, we report the cloning and functional characterization of the tomato (Solanum lycopersicum L.) ortholog of Escherichia coli PspA and Arabidopsis thaliana VIPP1, which we dubbed SlVIPP1. Our genetic and molecular characterization of slvipp1, an insertional mutant, allowed us to conclude that the tomato SlVIPP1 gene is needed for development, as Arabidopsis VIPP1, but not Escherichia coli PspA. Homozygous slvipp1 tomato plants are albino and exhibit early lethality and highly aberrant chloroplast development with almost complete absence of thylakoids. The phenotype of tomato RNAi lines and that of additional slvipp1 alleles generated by CRISPR/Cas9 gene editing technology confirmed that the morphological and histological aberrations shown by slvipp1 homozygotes are caused by VIPP1 lack of function. We also found that tomato SlVIPP1 overexpression does not cause any visible effect on plant morphology and viability. Our work with slvipp1 plants evidences that SlVIPP1 is an essential gene required for tomato survival, since its function is crucial for the proper formation and/or maintenance of thylakoid membranes.

Phenotype and TMT-based quantitative proteomics analysis of Brassica napus reveals new insight into chlorophyll synthesis and chloroplast structure

The conversion of light energy into chemical energy in leaves is very important for plant growth and development. During this process, chlorophylls and their derivatives are indispensable as their fundamental role in the energy absorption and transduction activities. Chlorophyll variation mutants are important materials for studying chlorophyll metabolism, chloroplast biogenesis, photosynthesis and related physiological processes. Here, a chlorophyll-reduced mutant (crm1) was isolated from ethyl methanesulfonate (EMS) mutagenized Brassica napus. Compared to wild type, crm1 showed yellow leaves, reduced chlorophyll content, fewer thylakoid stacks and retarded growth. Quantitative mass spectrometry analysis with Tandem Mass Tag (TMT) isobaric labeling showed that totally 4575 proteins were identified from the chloroplast of Brassica napus leaves, and 466 of which displayed differential accumulations between wild type and crm1. The differential abundance proteins were found to be involved in chlorophyll metabolism, photosynthesis, phagosome and proteasome. Our results suggest that the decreased abundance of chlorophyll biosynthetic enzymes, proteins involved in photosynthesis might account for the reduced chlorophyll content, impaired thylakoid structure, and reduction of plant productivity. The increased abundance of proteins involved in phagosome and proteasome pathways might allow plants to adapt the proteome to environmental conditions to ensure growth and survival due to chlorophyll reduction. BIOLOGICAL SIGNIFICANCE: Photosynthesis, which consists of light and dark reactions, is fundamental to biomass production. Chloroplast is regarded as the main site for photosynthesis. During photosynthesis, the pigment chlorophyll is essential for light harvesting and energy transfer. This work provides new insights into protein expression patterns, and enables the identification of many attractive candidates for investigation of chlorophyll biosynthesis, chloroplast structure and photosynthesis in Brassica napus. These findings may be applied to improve the photosynthetic efficiency by genetic engineering in crops.

JASSY, a chloroplast outer membrane protein required for jasmonate biosynthesis

Jasmonates are vital plant hormones that not only act in the stress response to biotic and abiotic influences, such as wounding, pathogen attack, and cold acclimation, but also drive developmental processes in cooperation with other plant hormones. The biogenesis of jasmonates starts in the chloroplast, where several enzymatic steps produce the jasmonate precursor 12-oxophytodienoic acid (OPDA) from α-linolenic acid. OPDA in turn is exported into the cytosol for further conversion into active jasmonates, which subsequently induces the expression of multiple genes in the nucleus. Despite its obvious importance, the export of OPDA across the chloroplast membranes has remained elusive. In this study, we characterized a protein residing in the chloroplast outer membrane, JASSY, which has proven indispensable for the export of OPDA from the chloroplast. We provide evidence that JASSY has channel-like properties and propose that it thereby facilitates OPDA transport. Consequently, a lack of JASSY in Arabidopsis leads to a deficiency in accumulation of jasmonic acids, which results in impaired expression of jasmonate target genes on exposure to various stresses. This results in plants that are more susceptible to pathogen attack and also exhibit defects in cold acclimation.

The Conserved Role of YidC in Membrane Protein Biogenesis

YidC insertase plays a pivotal role in the membrane integration, folding, and assembly of a number of proteins, including energy-transducing respiratory complexes, both autonomously and in concert with the SecYEG channel in bacteria. The YidC family of proteins is widely conserved in all domains of life, with new members recently identified in the eukaryotic endoplasmic reticulum membrane. Bacterial and organellar members share the conserved 5-transmembrane core, which forms a unique hydrophilic cavity in the inner leaflet of the bilayer accessible from the cytoplasm and the lipid phase. In this chapter, we discuss the YidC family of proteins, focusing on its mechanism of substrate insertion independently and in association with the Sec translocon.

Molecular mechanism of SRP-dependent light-harvesting protein transport to the thylakoid membrane in plants

The light-harvesting chlorophyll a/b binding proteins (LHCP) belong to a large family of membrane proteins. They form the antenna complexes of photosystem I and II and function in light absorption and transfer of the excitation energy to the photosystems. As nuclear-encoded proteins, the LHCPs are imported into the chloroplast and further targeted to their final destination-the thylakoid membrane. Due to their hydrophobicity, the formation of the so-called 'transit complex' in the stroma is important to prevent their aggregation in this aqueous environment. The posttranslational LHCP targeting mechanism is well regulated through the interaction of various soluble and membrane-associated protein components and includes several steps: the binding of the LHCP to the heterodimeric cpSRP43/cpSRP54 complex to form the soluble transit complex; the docking of the transit complex to the SRP receptor cpFtsY and the Alb3 translocase at the membrane followed by the release and integration of the LHCP into the thylakoid membrane in a GTP-dependent manner. This review summarizes the molecular mechanisms and dynamics behind the posttranslational LHCP targeting to the thylakoid membrane of Arabidopsis thaliana.

En route into chloroplasts: preproteins' way home

Chloroplasts are the characteristic endosymbiotic organelles of plant cells which during the course of evolution lost most of their genetic information to the nucleus. Thus, they critically depend on the host cell for allocation of nearly their complete protein supply. This includes gene expression, translation, protein targeting, and transport-all of which need to be tightly regulated and perfectly coordinated to accommodate the cells' needs. To this end, multiple signaling pathways have been implemented that interchange information between the different cellular compartments. One of the most complex and energy consuming processes is the translocation of chloroplast-destined proteins into their target organelle. It is a concerted effort from chaperones, receptor proteins, channels, and regulatory elements to ensure correct targeting, efficient transport, and subsequent folding. Although we have discovered and learned a lot about protein import into chloroplasts in the last decades, there are still many open questions and debates about the roles of individual proteins as well as the mechanistic details. In this review, I will summarize and discuss the published data with a focus on the translocation complex in the chloroplast inner envelope membrane.

Plant and algal chlorophyll synthases function in Synechocystis and interact with the YidC/Alb3 membrane insertase

In the model cyanobacterium Synechocystis sp. PCC 6803, the terminal enzyme of chlorophyll biosynthesis, chlorophyll synthase (ChlG), forms a complex with high light‐inducible proteins, the photosystem II assembly factor Ycf39 and the YidC/Alb3/OxaI membrane insertase, co‐ordinating chlorophyll delivery with cotranslational insertion of nascent photosystem polypeptides into the membrane. To gain insight into the ubiquity of this assembly complex in higher photosynthetic organisms, we produced functional foreign chlorophyll synthases in a cyanobacterial host. Synthesis of algal and plant chlorophyll synthases allowed deletion of the otherwise essential native cyanobacterial gene. Analysis of purified protein complexes shows that the interaction with YidC is maintained for both eukaryotic enzymes, indicating that a ChlG‐YidC/Alb3 complex may be evolutionarily conserved in algae and plants.

Plant Mitochondrial Inner Membrane Protein Insertion

During the biogenesis of the mitochondrial inner membrane, most nuclear-encoded inner membrane proteins are laterally released into the membrane by the TIM23 and the TIM22 machinery during their import into mitochondria. A subset of nuclear-encoded mitochondrial inner membrane proteins and all the mitochondrial-encoded inner membrane proteins use the Oxa machinery-which is evolutionarily conserved from the endosymbiotic bacterial ancestor of mitochondria-for membrane insertion. Compared to the mitochondria from other eukaryotes, plant mitochondria have several unique features, such as a larger genome and a branched electron transport pathway, and are also involved in additional cellular functions such as photorespiration and stress perception. This review focuses on the unique aspects of plant mitochondrial inner membrane protein insertion machinery, which differs from that in yeast and humans, and includes a case study on the biogenesis of Cox2 in yeast, humans, two plant species, and an algal species to highlight lineage-specific similarities and differences. Interestingly, unlike mitochondria of other eukaryotes but similar to bacteria and chloroplasts, plant mitochondria appear to use the Tat machinery for membrane insertion of the Rieske Fe/S protein.

The integration of chloroplast protein targeting with plant developmental and stress responses

The plastids, including chloroplasts, are a group of interrelated organelles that confer photoautotrophic growth and the unique metabolic capabilities that are characteristic of plant systems. Plastid biogenesis relies on the expression, import, and assembly of thousands of nuclear encoded preproteins. Plastid proteomes undergo rapid remodeling in response to developmental and environmental signals to generate functionally distinct plastid types in specific cells and tissues. In this review, we will highlight the central role of the plastid protein import system in regulating and coordinating the import of functionally related sets of preproteins that are required for plastid-type transitions and maintenance.

Identification and Fine Mapping of a White Husk Gene in Barley (Hordeum vulgare L.)

Barley is the only crop in the Poaceae family with adhering husks at maturity. The color of husk at barely development stage could influence the agronomic traits and malting qualities of grains. A barley mutant with a white husk was discovered from the malting barley cultivar Supi 3 and designated wh (white husk). Morphological changes and the genetics of white husk barley were investigated. Husks of the mutant were white at the heading and flowering stages but yellowed at maturity. The diastatic power and α-amino nitrogen contents also significantly increased in wh mutant. Transmission electron microscopy examination showed abnormal chloroplast development in the mutant. Genetic analysis of F₂ and BC₁F₁ populations developed from a cross of wh and Yangnongpi 5 (green husk) showed that the white husk was controlled by a single recessive gene (wh). The wh gene was initially mapped between 49.64 and 51.77 cM on chromosome 3H, which is syntenic with rice chromosome 1 where a white husk gene wlp1 has been isolated. The barley orthologous gene of wlp1 was sequenced from both parents and a 688 bp deletion identified in the wh mutant. We further fine-mapped the wh gene between SSR markers Bmac0067 and Bmag0508a with distances of 0.36 cM and 0.27 cM in an F₂ population with 1115 individuals of white husk. However, the wlp1 orthologous gene was mapped outside the interval. New candidate genes were identified based on the barley genome sequence.

Functional Update of the Auxiliary Proteins PsbW, PsbY, HCF136, PsbN, TerC and ALB3 in Maintenance and Assembly of PSII

Assembly of Photosystem (PS) II in plants has turned out to be a highly complex process which, at least in part, occurs in a sequential order and requires many more auxiliary proteins than subunits present in the complex. Owing to the high evolutionary conservation of the subunit composition and the three-dimensional structure of the PSII complex, most plant factors involved in the biogenesis of PSII originated from cyanobacteria and only rarely evolved de novo. Furthermore, in chloroplasts the initial assembly steps occur in the non-appressed stroma lamellae, whereas the final assembly including the attachment of the major LHCII antenna proteins takes place in the grana regions. The stroma lamellae are also the place where part of PSII repair occurs, which very likely also involves assembly factors. In cyanobacteria initial PSII assembly also occurs in the thylakoid membrane, in so-called thylakoid centers, which are in contact with the plasma membrane. Here, we provide an update on the structures, localisations, topologies, functions, expression and interactions of the low molecular mass PSII subunits PsbY, PsbW and the auxiliary factors HCF136, PsbN, TerC and ALB3, assisting in PSII complex assembly and protein insertion into the thylakoid membrane.

Genetic and Physical Interaction Studies Reveal Functional Similarities between ALBINO3 and ALBINO4 in Arabidopsis

ALBINO3 (ALB3) is a well-known component of a thylakoid protein-targeting complex that interacts with the chloroplast signal recognition particle (cpSRP) and the cpSRP receptor, chloroplast filamentous temperature-sensitive Y (cpFtsY). Its protein-inserting function has been established mainly for light-harvesting complex proteins, which first interact with the unique chloroplast cpSRP43 component and then are delivered to the ALB3 integrase by a GTP-dependent cpSRP-cpFtsY interaction. In Arabidopsis (Arabidopsis thaliana), a subsequently discovered ALB3 homolog, ALB4, has been proposed to be involved not in light-harvesting complex protein targeting, but instead in the stabilization of the ATP synthase complex. Here, however, we show that ALB3 and ALB4 share significant functional overlap, and that both proteins are required for the efficient insertion of cytochrome f and potentially other subunits of pigment-bearing protein complexes. Genetic and physical interactions between ALB4 and ALB3, and physical interactions between ALB4 and cpSRP, suggest that the two ALB proteins may engage similar sets of interactors for their specific functions. We propose that ALB4 optimizes the insertion of thylakoid proteins by participating in the ALB3-cpSRP pathway for certain substrates (e.g. cytochrome f and the Rieske protein). Although ALB4 has clearly diverged from ALB3 in relation to the partner-recruiting C-terminal domain, our analysis suggests that one putative cpSRP-binding motif has not been entirely lost.

YidC/Alb3/Oxa1 Family of Insertases

The YidC/Alb3/Oxa1 family functions in the insertion and folding of proteins in the bacterial cytoplasmic membrane, the chloroplast thylakoid membrane, and the mitochondrial inner membrane. All members share a conserved region composed of five transmembrane regions. These proteins mediate membrane insertion of an assorted group of proteins, ranging from respiratory subunits in the mitochondria and light-harvesting chlorophyll-binding proteins in chloroplasts to ATP synthase subunits in bacteria. This review discusses the YidC/Alb3/Oxa1 protein family as well as their function in membrane insertion and two new structures of the bacterial YidC, which suggest a mechanism for membrane insertion by this family of insertases.

Biogenesis of light harvesting proteins

The LHC family includes nuclear-encoded, integral thylakoid membrane proteins, most of which coordinate chlorophyll and xanthophyll chromophores. By assembling with the core complexes of both photosystems, LHCs form a flexible peripheral moiety for enhancing light-harvesting cross-section, regulating its efficiency and providing protection against photo-oxidative stress. Upon its first appearance, LHC proteins underwent evolutionary diversification into a large protein family with a complex genetic redundancy. Such differentiation appears as a crucial event in the adaptation of photosynthetic organisms to changing environmental conditions and land colonization. The structure of photosystems, including nuclear- and chloroplast-encoded subunits, presented the cell with a number of challenges for the control of the light harvesting function. Indeed, LHC-encoding messages are translated in the cytosol, and pre-proteins imported into the chloroplast, processed to their mature size and targeted to the thylakoids where are assembled with chromophores. Thus, a tight coordination between nuclear and plastid gene expression, in response to environmental stimuli, is required to adjust LHC composition during photoacclimation. In recent years, remarkable progress has been achieved in elucidating structure, function and regulatory pathways involving LHCs; however, a number of molecular details still await elucidation. In this review, we will provide an overview on the current knowledge on LHC biogenesis, ranging from organization of pigment-protein complexes to the modulation of gene expression, import and targeting to the photosynthetic membranes, and regulation of LHC assembly and turnover. Genes controlling these events are potential candidate for biotechnological applications aimed at optimizing light use efficiency of photosynthetic organisms. This article is part of a Special Issue entitled: Chloroplast biogenesis.

Assembly of F1F0-ATP synthases

F1F0-ATP synthases are multimeric protein complexes and common prerequisites for their correct assembly are (i) provision of subunits in appropriate relative amounts, (ii) coordination of membrane insertion and (iii) avoidance of assembly intermediates that uncouple the proton gradient or wastefully hydrolyse ATP. Accessory factors facilitate these goals and assembly occurs in a modular fashion. Subcomplexes common to bacteria and mitochondria, but in part still elusive in chloroplasts, include a soluble F1 intermediate, a membrane-intrinsic, oligomeric c-ring, and a membrane-embedded subcomplex composed of stator subunits and subunit a. The final assembly step is thought to involve association of the preformed F1-c10-14 with the ab2 module (or the ab8-stator module in mitochondria)--mediated by binding of subunit δ in bacteria or OSCP in mitochondria, respectively. Despite the common evolutionary origin of F1F0-ATP synthases, the set of auxiliary factors required for their assembly in bacteria, mitochondria and chloroplasts shows clear signs of evolutionary divergence. This article is part of a Special Issue entitled: Chloroplast Biogenesis.

Structure and dynamics of thylakoids in land plants

Thylakoids of land plants have a bipartite structure, consisting of cylindrical grana stacks, made of membranous discs piled one on top of the other, and stroma lamellae which are helically wound around the cylinders. Protein complexes predominantly located in the stroma lamellae and grana end membranes are either bulky [photosystem I (PSI) and the chloroplast ATP synthase (cpATPase)] or are involved in cyclic electron flow [the NAD(P)H dehydrogenase (NDH) and PGRL1-PGR5 heterodimers], whereas photosystem II (PSII) and its light-harvesting complex (LHCII) are found in the appressed membranes of the granum. Stacking of grana is thought to be due to adhesion between Lhcb proteins (LHCII or CP26) located in opposed thylakoid membranes. The grana margins contain oligomers of CURT1 proteins, which appear to control the size and number of grana discs in a dosage- and phosphorylation-dependent manner. Depending on light conditions, thylakoid membranes undergo dynamic structural changes that involve alterations in granum diameter and height, vertical unstacking of grana, and swelling of the thylakoid lumen. This plasticity is realized predominantly by reorganization of the supramolecular structure of protein complexes within grana stacks and by changes in multiprotein complex composition between appressed and non-appressed membrane domains. Reversible phosphorylation of LHC proteins (LHCPs) and PSII components appears to initiate most of the underlying regulatory mechanisms. An update on the roles of lipids, proteins, and protein complexes, as well as possible trafficking mechanisms, during thylakoid biogenesis and the de-etiolation process complements this review.

The membrane insertase YidC

The membrane insertases YidC-Oxa1-Alb3 provide a simple cellular system that catalyzes the transmembrane topology of newly synthesized membrane proteins. The insertases are composed of a single protein with 5 to 6 transmembrane (TM) helices that contact hydrophobic segments of the substrate proteins. Since YidC also cooperates with the Sec translocase it is widely involved in the assembly of many different membrane proteins including proteins that obtain complex membrane topologies. Homologues found in mitochondria (Oxa1) and thylakoids (Alb3) point to a common evolutionary origin and also demonstrate the general importance of this cellular process. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

Analysis of essential Arabidopsis nuclear genes encoding plastid-targeted proteins

The Chloroplast 2010 Project (http://www.plastid.msu.edu/) identified and phenotypically characterized homozygous mutants in over three thousand genes, the majority of which encode plastid-targeted proteins. Despite extensive screening by the community, no homozygous mutant alleles were available for several hundred genes, suggesting that these might be enriched for genes of essential function. Attempts were made to generate homozygotes in ~1200 of these lines and 521 of the homozygous viable lines obtained were deposited in the Arabidopsis Biological Resource Center (http://abrc.osu.edu/). Lines that did not yield a homozygote in soil were tested as potentially homozygous lethal due to defects either in seed or seedling development. Mutants were characterized at four stages of development: developing seed, mature seed, at germination, and developing seedlings. To distinguish seed development or seed pigment-defective mutants from seedling development mutants, development of seeds was assayed in siliques from heterozygous plants. Segregating seeds from heterozygous parents were sown on supplemented media in an attempt to rescue homozygous seedlings that could not germinate or survive in soil. Growth of segregating seeds in air and air enriched to 0.3% carbon dioxide was compared to discover mutants potentially impaired in photorespiration or otherwise responsive to CO2 supplementation. Chlorophyll fluorescence measurements identified CO2-responsive mutants with altered photosynthetic parameters. Examples of genes with a viable mutant allele and one or more putative homozygous-lethal alleles were documented. RT-PCR of homozygotes for potentially weak alleles revealed that essential genes may remain undiscovered because of the lack of a true null mutant allele. This work revealed 33 genes with two or more lethal alleles and 73 genes whose essentiality was not confirmed with an independent lethal mutation, although in some cases second leaky alleles were identified.

Arabidopsis thaliana Oxa proteins locate to mitochondria and fulfill essential roles during embryo development

Members of the Alb3/Oxa1/YidC protein family function as insertases in chloroplasts, mitochondria, and bacteria. Due to independent gene duplications, all organisms possess two isoforms, Oxa1 and Oxa2 except gram-negative bacteria, which encode only for one YidC-like protein. The genome of Arabidopsis thaliana however, encodes for eight different isoforms. The localization of three of these isoforms has been identified earlier: Alb3 and Alb4 located in thylakoid membranes of chloroplasts while AtOxa1 was found in the inner membrane of mitochondria. Here, we show that the second Oxa1 protein, Oxa1b as well as two Oxa2 proteins are also localized in mitochondria. The last two isoforms most likely encode truncated versions of Oxa-like proteins, which might be inoperable pseudogenes. Homozygous mutant lines were only obtained for Oxa1b, which did not reveal any significant phenotypes, while T-DNA insertion lines of Oxa1a, Oxa2a and Oxa2b resulted only in heterozygous plants indicating that these genes are indispensable for plant development. Phenotyping heterozygous lines showed that embryos are either retarded in growth, display an albino phenotype or embryo formation was entirely abolished suggesting that Oxa1a and both Oxa2 proteins function in embryo formation although at different developmental stages as indicated by the various phenotypes observed.

Intra-plastid protein trafficking: how plant cells adapted prokaryotic mechanisms to the eukaryotic condition

Protein trafficking and localization in plastids involve a complex interplay between ancient (prokaryotic) and novel (eukaryotic) translocases and targeting machineries. During evolution, ancient systems acquired new functions and novel translocation machineries were developed to facilitate the correct localization of nuclear encoded proteins targeted to the chloroplast. Because of its post-translational nature, targeting and integration of membrane proteins posed the biggest challenge to the organelle to avoid aggregation in the aqueous compartments. Soluble proteins faced a different kind of problem since some had to be transported across three membranes to reach their destination. Early studies suggested that chloroplasts addressed these issues by adapting ancient-prokaryotic machineries and integrating them with novel-eukaryotic systems, a process called 'conservative sorting'. In the last decade, detailed biochemical, genetic, and structural studies have unraveled the mechanisms of protein targeting and localization in chloroplasts, suggesting a highly integrated scheme where ancient and novel systems collaborate at different stages of the process. In this review we focus on the differences and similarities between chloroplast ancestral translocases and their prokaryotic relatives to highlight known modifications that adapted them to the eukaryotic situation. This article is part of a Special Issue entitled: Protein Import and Quality Control in Mitochondria and Plastids.

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.

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.

Loss of the SPHF homologue Slr1768 leads to a catastrophic failure in the maintenance of thylakoid membranes in Synechocystis sp. PCC 6803

Background

In cyanobacteria the photosystems are localised to, and maintained in, specialist membranes called the thylakoids. The mechanism driving the biogenesis of the thylakoid membranes is still an open question, with only two potential biogenesis factors, Vipp1 and Alb3 currently identified.

Methodology/Principal Findings

We generated a slr1768 knockout using the pGEM T-easy vector and REDIRECT. By comparing growth and pigment content (chlorophyll a fluoresence) of the Δslr1768 mutant with the wild-type, we found that Δslr1768 has a conditional phenotype; specifically under high light conditions (130 µmol m⁻² s⁻¹) thylakoid biogenesis is disrupted leading to cell death on a scale of days. The thylakoids show considerable disruption, with loss of both structure and density, while chlorophyll a density decreases with the loss of thylakoids, although photosynthetic efficiency is unaffected. Under low light (30 µmol m⁻² s⁻¹) the phenotype is significantly reduced, with a growth rate similar to the wild-type and only a low frequency of cells with evident thylakoid disruption.

Conclusions/Significance

This is the first example of a gene that affects the maintenance of the thylakoid membranes specifically under high light, and which displays a phenotype dependent on light intensity. Our results demonstrate that Slr1768 has a leading role in acclimatisation, linking light damage with maintenance of the thylakoids.

Multiple FtsZ2 isoforms involved in chloroplast division and biogenesis are developmentally associated with thylakoid membranes in Arabidopsis

Seed plants and algae have two distinct FtsZ protein families, FtsZ1 and FtsZ2, involved in plastid division. Distinctively, seed plants and mosses contain two FtsZ2 family members (FtsZ2-1 and FtsZ2-2) thus raising the question of the role of these FtsZ2 paralogs in plants. We show that both FtsZ2 paralogs, in addition to being present in the stroma, are associated with the thylakoid membranes and that association is developmentally regulated. We also show that several FtsZ2-1 isoforms are present with distinct intra-plastidial localization. Mutant analyses show that FtsZ2-1 is essential for chloroplast division and that FtsZ2-2 plays a specific role in chloroplast morphology and internal organisation in addition to participating in chloroplast partition.

Interplay between the cpSRP pathway components, the substrate LHCP and the translocase Alb3: an in vivo and in vitro study

The chloroplast signal recognition particle (cpSRP) and its receptor, cpFtsY, posttranslationally target the nuclear-encoded light-harvesting chlorophyll-binding proteins (LHCPs) to the translocase Alb3 in the thylakoid membrane. In this study, we analyzed the interplay between the cpSRP pathway components, the substrate protein LHCP and the translocase Alb3 by using in vivo and in vitro techniques. We propose that cpSRP43 is crucial for the binding of LHCP-loaded cpSRP and cpFtsY to Alb3. In addition, our data suggest that a direct interaction between Alb3 and LHCP contributes to the formation of this complex.

Inserting membrane proteins: the YidC/Oxa1/Alb3 machinery in bacteria, mitochondria, and chloroplasts

The evolutionarily conserved YidC/Oxa1p/Alb3 family of proteins plays important roles in the membrane biogenesis in bacteria, mitochondria, and chloroplasts. The members in this family function as novel membrane protein insertases, chaperones, and assembly factors for transmembrane proteins, including energy transduction complexes localized in the bacterial and mitochondrial inner membrane, and in the chloroplast thylakoid membrane. In this review, we will present recent progress with this class of proteins in membrane protein biogenesis and discuss the structure/function relationships. This article is part of a Special Issue entitled Protein translocation across or insertion into membranes.

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.

Biogenesis of bacterial inner-membrane proteins

All cells must traffic proteins into and across their membranes. In bacteria, several pathways have evolved to enable protein transfer across the inner membrane, the periplasm, and the outer membrane. The major route of protein translocation in and across the cytoplasmic membrane is the general secretion pathway (Sec-pathway). The biogenesis of membrane proteins not only requires protein translocation but also coordinated targeting to the membrane beforehand and folding and assembly into their protein complexes afterwards to function properly in the cell. All these processes are responsible for the biogenesis of membrane proteins that mediate essential functions of the cell such as selective transport, energy conversion, cell division, extracellular signal sensing, and motility. This review will highlight the most recent developments on the structure and function of bacterial membrane proteins, focusing on the journey that integral membrane proteins take to find their final destination in the inner membrane.

DONGLE and DEFECTIVE IN ANTHER DEHISCENCE1 lipases are not essential for wound- and pathogen-induced jasmonate biosynthesis: redundant lipases contribute to jasmonate formation

Lipases are involved in the generation of jasmonates, which regulate responses to biotic and abiotic stresses. Two sn-1-specific acyl hydrolases, DEFECTIVE IN ANTHER DEHISCENCE1 (DAD1) and DONGLE (DGL), have been reported to be localized in plastids and to be essential and sufficient for jasmonate biosynthesis in Arabidopsis (Arabidopsis thaliana) leaves. Here, we show that levels of 12-oxo-phytodienoic acid (OPDA) and jasmonic acid in three different DGL RNA interference lines and the dad1 mutant were similar to wild-type levels during the early wound response as well as after Pseudomonas infection. Due to the lack of sn-2 substrate specificity, synthesis of dinor OPDA was not expected and also not found to be affected in DGL knockdown and DGL-overexpressing lines. As reported, DAD1 participates in jasmonate formation only in the late wound response. In addition, DGL protein was found to be localized in lipid bodies and not in plastids. Furthermore, jasmonate levels in 16 additional mutants defective in the expression of lipases with predicted chloroplast localization did not show strong differences from wild-type levels after wounding, except for a phospholipase A (PLA) PLA-Igamma1 (At1g06800) mutant line that displayed diminished wound-induced dinor OPDA, OPDA, and jasmonic acid levels. A quadruple mutant defective in four DAD1-like lipases displayed similar jasmonate levels as the mutant line of PLA-Igamma1 after wounding. Hence, we identify PLA-Igamma1 as a novel target gene to manipulate jasmonate biosynthesis. Our results suggest that, in addition to DAD1 and PLA-Igamma1, still unidentified enzymes with sn-1 and sn-2 hydrolase activity are involved in wound- and pathogen-induced jasmonate formation, indicating functional redundancy within the lipase family.

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.

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.

From the Sec complex to the membrane insertase YidC

The key enzymes that catalyze the insertion of proteins into membranes are the Sec translocase and the YidC membrane insertase. Recent insights into the structure and functional intermediates of these enzymes have provided a first molecular glimpse of how they help the newly synthesized proteins to enter the membrane bilayer. In this process, the new proteins undergo a number of specific interactions in the cytoplasm and at the membrane surface before they insert into the bilayer and translocate their external domains across the membrane. The components involved in this pathway recognize each other at the molecular level, forming a route the membrane protein can move along.

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.

Localization and integration of thylakoid protein translocase subunit cpTatC

Thylakoid membranes have a unique complement of proteins, most of which are nuclear encoded synthesized in the cytosol, imported into the stroma and translocated into thylakoid membranes by specific thylakoid translocases. Known thylakoid translocases contain core multi-spanning, membrane-integrated subunits that are also nuclear-encoded and imported into chloroplasts before being integrated into thylakoid membranes. Thylakoid translocases play a central role in determining the composition of thylakoids, yet the manner by which the core translocase subunits are integrated into the membrane is not known. We used biochemical and genetic approaches to investigate the integration of the core subunit of the chloroplast Tat translocase, cpTatC, into thylakoid membranes. In vitro import assays show that cpTatC correctly localizes to thylakoids if imported into intact chloroplasts, but that it does not integrate into isolated thylakoids. In vitro transit peptide processing and chimeric precursor import experiments suggest that cpTatC possesses a stroma-targeting transit peptide. Import time-course and chase assays confirmed that cpTatC targets to thylakoids via a stromal intermediate, suggesting that it might integrate through one of the known thylakoid translocation pathways. However, chemical inhibitors to the cpSecA-cpSecY and cpTat pathways did not impede cpTatC localization to thylakoids when used in import assays. Analysis of membranes isolated from Arabidopsis thaliana mutants lacking cpSecY or Alb3 showed that neither is necessary for cpTatC membrane integration or assembly into the cpTat receptor complex. These data suggest the existence of another translocase, possibly one dedicated to the integration of chloroplast translocases.

Fine mapping and candidate gene analysis of a green-revertible albino gene gra(t) in rice

Green-revertible albino is a novel type of chlorophyll deficiency in rice (Oryza sativa L.), which is helpful for further research in chlorophyll synthesis and chloroplast development to illuminate their molecular mechanism. In the previous study, we had reported a single recessive gene, gra(t), controlling this trait on the long arm of chromosome 2. In this paper, we mapped the gra(t) gene using 1,936 recessive individuals with albino phenotype in the F(2) population derived from the cross between themo-photoperiod-sensitive genic male-sterile (T/PGMS) line Pei'ai 64S and the spontaneous mutant Qiufeng M. Eventually, it was located to a confined region of 42.4 kb flanked by two microsatellite markers RM2-97 and RM13553. Based on the annotation results of RiceGAAS system, 11 open reading frames (ORFs) were predicted in this region. Among them, ORF6 was the most possible gene related to chloroplast development, which encoded the chloroplast protein synthesis elongation factor Tu in rice. Therefore, we designated it as the candidate gene of gra(t). Sequence analysis indicated that only one base substitution C to T occurred in the coding region, which caused a missense mutation (Thr to Ile) in gra(t) mutant. These results are very valuable for further study on gra(t) gene.

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.

Non-identical contributions of two membrane-bound cpSRP components, cpFtsY and Alb3, to thylakoid biogenesis

The insertion of light-harvesting chlorophyll proteins (LHCPs) into the thylakoid membrane of the chloroplast is cpSRP-dependent, and requires the stromal components cpSRP54 and cpSRP43, the membrane-bound SRP receptor cpFtsY and the integral membrane protein Alb3. Previous studies demonstrated that the Arabidopsis mutant lacking both cpSRP54 and cpSRP43 had pale yellow leaves, but was viable, whereas the mutants lacking Alb3 exhibit an albino phenotype that is more severe and seedling lethality. We previously showed that a maize mutant lacking cpFtsY had a pale yellow-green phenotype and was seedling lethal. To compare the in vivo requirements of cpFtsY and Alb3 in thylakoid biogenesis in greater detail, we isolated Arabidopsis null mutants of cpftsY, and performed biochemical comparisons with the Arabidopsis alb3 mutant. Both cpftsY and alb3 null mutants were seedling lethal on a synthetic medium lacking sucrose, whereas on a medium supplemented with sucrose, they were able to grow to later developmental stages, but were mostly infertile. cpftsY mutant plants had yellow leaves in which the levels of LHCPs were reduced to 10-33% compared with wild type. In contrast, alb3 had yellowish white leaves, and the LHCP levels were less than or equal to 10% of those of wild type. Intriguingly, whereas accumulation of the Sec and Tat machineries were normal in both mutants, the Sec pathway substrate Cyt f was more severely decreased in the cpftsY mutant than in alb3, which may indicate a functional link between cpFtsY and Sec translocation machinery. These results suggest that cpFtsY and Alb3 have essentially similar, but slightly distinct, contributions to thylakoid biogenesis.

Protein trafficking to plastids: one theme, many variations

Plastids are a diverse group of essential organelles in plants that include chloroplasts. The biogenesis and maintenance of these organelles relies on the import of thousands of nucleus-encoded proteins. The complexity of plastid structure has resulted in the evolution of at least four general import pathways that target proteins into and across the double membrane of the plastid envelope. Several of these pathways can be further divided into specialty pathways that mediate and regulate the import of specific classes of proteins. The co-ordination of import by these specialized pathways with changes in gene expression is critical for plastid and plant development. Moreover, protein import is acutely regulated in response to physiological and metabolic changes within the cell. In the present review we summarize the current knowledge of the mechanism of import via these pathways and highlight the regulatory mechanisms that integrate the plastid protein-trafficking pathways with the developmental and metabolic state of the plant.

Phosphorylation regulates the assembly of chloroplast import machinery

Chloroplast function depends on the translocation of cytosolically synthesized precursor proteins into the organelle. The recognition and transfer of most precursor proteins across the outer membrane depend on a membrane inserted complex. Two receptor components of this complex, Toc34 and Toc159, are GTPases, which can be phosphorylated by kinases present in the hosting membrane. However, the physiological function of phosphorylation is not yet understood in detail. It is demonstrated that both receptors are phosphorylated within their G-domains. In vitro, the phosphorylation of Toc34 disrupts both homo- and heterodimerization of the G-domains as determined using a phospho-mimicking mutant. In endogenous membranes this mutation or phosphorylation of the wild-type receptor disturbs the association of Toc34, but not of Toc159 with the translocation pore. Therefore, phosphorylation serves as an inhibitor for the association of Toc34 with other components of the complex and phosphorylation can now be discussed as a mechanism to exchange different isoforms of Toc34 within this ensemble.

A novel serine/proline-rich domain in combination with a transmembrane domain is required for the insertion of AtTic40 into the inner envelope membrane of chloroplasts

AtTic40 is part of the chloroplastic protein import apparatus that is anchored in the inner envelope membrane by a single N-terminal transmembrane domain, and has a topology in which the bulk of the C-terminal domain is oriented toward the stroma. The targeting of AtTic40 to the inner envelope membrane involves two steps. Using an in vitro import assay, we showed that the sorting of AtTic40 requires a bipartite transit peptide, which was first cleaved by the stromal processing peptidase (SPP), thus generating a soluble AtTic40 stromal intermediate (iAtTic40). iAtTic40 was further processed by a second unknown peptidase, which generates its mature form (mAtTic40). Using deletion mutants, we identified a sequence motif N-terminal of the transmembrane domain that was essential for reinsertion of iAtTic40 into the inner envelope membrane. We have designated this region a serine/proline-rich (S/P-rich) domain and present a model describing its role in the targeting of AtTic40 to the inner envelope membrane.

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.