The N-terminal portion of the preToc75 transit peptide interacts with membrane lipids and inhibits binding and import of precursor proteins into isolated chloroplasts

Understanding protein import in diverse non-green plastids

The spectacular diversity of plastids in non-green organs such as flowers, fruits, roots, tubers, and senescing leaves represents a Universe of metabolic processes in higher plants that remain to be completely characterized. The endosymbiosis of the plastid and the subsequent export of the ancestral cyanobacterial genome to the nuclear genome, and adaptation of the plants to all types of environments has resulted in the emergence of diverse and a highly orchestrated metabolism across the plant kingdom that is entirely reliant on a complex protein import and translocation system. The TOC and TIC translocons, critical for importing nuclear-encoded proteins into the plastid stroma, remain poorly resolved, especially in the case of TIC. From the stroma, three core pathways (cpTat, cpSec, and cpSRP) may localize imported proteins to the thylakoid. Non-canonical routes only utilizing TOC also exist for the insertion of many inner and outer membrane proteins, or in the case of some modified proteins, a vesicular import route. Understanding this complex protein import system is further compounded by the highly heterogeneous nature of transit peptides, and the varying transit peptide specificity of plastids depending on species and the developmental and trophic stage of the plant organs. Computational tools provide an increasingly sophisticated means of predicting protein import into highly diverse non-green plastids across higher plants, which need to be validated using proteomics and metabolic approaches. The myriad plastid functions enable higher plants to interact and respond to all kinds of environments. Unraveling the diversity of non-green plastid functions across the higher plants has the potential to provide knowledge that will help in developing climate resilient crops.

Polyglycine Acts as a Rejection Signal for Protein Transport at the Chloroplast Envelope

PolyGly is present in many proteins in various organisms. One example is found in a transmembrane β-barrel protein, translocon at the outer-envelope-membrane of chloroplasts 75 (Toc75). Toc75 requires its N-terminal extension (t75) for proper localization. t75 comprises signals for chloroplast import (n75) and envelope sorting (c75) in tandem. n75 and c75 are removed by stromal processing peptidase and plastidic type I signal peptidase 1, respectively. PolyGly is present within c75 and its deletion or substitution causes mistargeting of Toc75 to the stroma. Here we have examined the properties of polyGly-dependent protein targeting using two soluble passenger proteins, the mature portion of the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (mSS) and enhanced green fluorescent protein (EGFP). Both t75-mSS and t75-EGFP were imported into isolated chloroplasts and their n75 removed. Resultant c75-mSS was associated with the envelope at the intermembrane space, whereas c75-EGFP was partially exposed outside the envelope. Deletion of polyGly or substitution of tri-Ala for the critical tri-Gly segment within polyGly caused each passenger to be targeted to the stroma. Transient expression of t75-EGFP in Nicotiana benthamiana resulted in accumulation of c75-EGFP exposed at the surface of the chloroplast, but the majority of the EGFP passenger was found free in the cytosol with most of its c75 attachment removed. Results of circular dichroism analyses suggest that polyGly within c75 may form an extended conformation, which is disrupted by tri-Ala substitution. These data suggest that polyGly is distinct from a canonical stop-transfer sequence and acts as a rejection signal at the chloroplast inner envelope.

New Insights Into Roles of Ubiquitin Modification in Regulating Plastids and Other Endosymbiotic Organelles

Recent findings have revealed important and diverse roles for the ubiquitin modification of proteins in the regulation of endosymbiotic organelles, which include the primary plastids of plants as well as complex plastids: the secondary endosymbiotic organelles of cryptophytes, alveolates, stramenopiles, and haptophytes. Ubiquitin modifications have a variety of potential consequences, both to the modified protein itself and to cellular regulation. The ubiquitin-proteasome system (UPS) can target individual proteins for selective degradation by the cytosolic 26S proteasome. Ubiquitin modifications can also signal the removal of whole endosymbiotic organelles, for example, via autophagy as has been well characterized in mitochondria. As plastids must import over 90% of their proteins from the cytosol, the observation that the UPS selectively targets the plastid protein import machinery is particularly significant. In this way, the UPS may influence the development and interconversions of different plastid types, as well as plastid responses to stress, by reconfiguring the organellar proteome. In complex plastids, the Symbiont-derived ERAD-Like Machinery (SELMA) has coopted the protein transport capabilities of the ER-Associated Degradation (ERAD) system, whereby misfolded proteins are retrotranslocated from ER for proteasomal degradation, uncoupling them from proteolysis: SELMA components have been retargeted to the second outermost plastid membrane to mediate protein import. In spite of this wealth of new information, there still remain a large number of unanswered questions and a need to define the roles of ubiquitin modification further in the regulation of plastids.

Multi-functional roles for the polypeptide transport associated domains of Toc75 in chloroplast protein import

Toc75 plays a central role in chloroplast biogenesis in plants as the membrane channel of the protein import translocon at the outer envelope of chloroplasts (TOC). Toc75 is a member of the Omp85 family of bacterial and organellar membrane insertases, characterized by N-terminal POTRA (polypeptide-transport associated) domains and C-terminal membrane-integrated β-barrels. We demonstrate that the Toc75 POTRA domains are essential for protein import and contribute to interactions with TOC receptors, thereby coupling preprotein recognition at the chloroplast surface with membrane translocation. The POTRA domains also interact with preproteins and mediate the recruitment of molecular chaperones in the intermembrane space to facilitate membrane transport. Our studies are consistent with the multi-functional roles of POTRA domains observed in other Omp85 family members and demonstrate that the domains of Toc75 have evolved unique properties specific to the acquisition of protein import during endosymbiotic evolution of the TOC system in plastids.

DOI:http://dx.doi.org/10.7554/eLife.12631.001

Chloroplasts are a hallmark feature of plant cells and the sites of photosynthesis – the process in which plants harness the energy in sunlight for their own needs. The first chloroplasts arose when a photosynthetic bacterium was engulfed by another host cell, and most of the original bacterial genes have been transferred to the host cell’s nucleus during the evolution of land plants. As a result, modern chloroplasts need to import the thousands of proteins encoded by these genes from the rest of the cell.

The chloroplast protein import system relies on a protein transporter in the chloroplast membrane that evolved from a family of bacterial transporters. However, the bacterial transporters were initially involved in protein export, and it was not known how the activity of these transporters adapted to move proteins in the opposite direction.

Paila et al. set out to better understand the chloroplast protein import system and produced mutated forms of the transporter in the model plant Arabidopsis thaliana. These experiments revealed that a part of the transporter that is conserved in many other organisms, the “protein transport associated domains”, has been adapted for three key roles in protein import. First, this part of the transporter interacts with the other components of the import system that make the transporter more selective and control which direction the proteins are transported. Second, the domains interact with proteins during transport to help move them across the chloroplast membrane. Finally, the domains recruit other molecules called chaperones, which stop the protein from aggregating or misfolding during the transport process. These activities are similar to those for the bacterial export transporters, but clearly evolved to allow transport in the opposite direction – that is, to import proteins into chloroplasts.

The next challenges are to explain how proteins destined for chloroplasts are recognized and transported through the chloroplast’s membrane.

DOI:http://dx.doi.org/10.7554/eLife.12631.002

Plastidic type I signal peptidase 1 is a redox-dependent thylakoidal processing peptidase

Thylakoids are the photosynthetic membranes in chloroplasts and cyanobacteria. The aqueous phase inside the thylakoid known as the thylakoid lumen plays an essential role in the photosynthetic electron transport. The presence and significance of thiol-disulfide exchange in this compartment have been recognized but remain poorly understood. All proteins found free in the thylakoid lumen and some proteins associated to the thylakoid membrane require an N-terminal targeting signal, which is removed in the lumen by a membrane-bound serine protease called thylakoidal processing peptidase (TPP). TPP is homologous to Escherichia coli type I signal peptidase (SPI) called LepB. Genetic data indicate that plastidic SPI 1 (Plsp1) is the main TPP in Arabidopsis thaliana (Arabidopsis) although biochemical evidence had been lacking. Here we demonstrate catalytic activity of bacterially produced Arabidopsis Plsp1. Recombinant Plsp1 showed processing activity against various TPP substrates at a level comparable to that of LepB. Plsp1 and LepB were also similar in the pH optima, sensitivity to arylomycin variants and a preference for the residue at -3 to the cleavage site within a substrate. Plsp1 orthologs found in angiosperms contain two unique Cys residues located in the lumen. Results of processing assays suggested that these residues were redox active and formation of a disulfide bond between them was necessary for the activity of recombinant Arabidopsis Plsp1. Furthermore, Plsp1 in Arabidopsis and pea thylakoids migrated faster under non-reducing conditions than under reducing conditions on SDS-PAGE. These results underpin the notion that Plsp1 is a redox-dependent signal peptidase in the thylakoid lumen.

Targeting and assembly of components of the TOC protein import complex at the chloroplast outer envelope membrane

The translocon at the outer envelope membrane of chloroplasts (TOC) initiates the import of thousands of nuclear encoded preproteins required for chloroplast biogenesis and function. The multimeric TOC complex contains two GTP-regulated receptors, Toc34 and Toc159, which recognize the transit peptides of preproteins and initiate protein import through a β-barrel membrane channel, Toc75. Different isoforms of Toc34 and Toc159 assemble with Toc75 to form structurally and functionally diverse translocons, and the composition and levels of TOC translocons is required for the import of specific subsets of coordinately expressed proteins during plant growth and development. Consequently, the proper assembly of the TOC complexes is key to ensuring organelle homeostasis. This review will focus on our current knowledge of the targeting and assembly of TOC components to form functional translocons at the outer membrane. Our analyses reveal that the targeting of TOC components involves elements common to the targeting of other outer membrane proteins, but also include unique features that appear to have evolved to specifically facilitate assembly of the import apparatus.

Evolution and targeting of Omp85 homologs in the chloroplast outer envelope membrane

Translocon at the outer-envelope-membrane of chloroplasts 75 (Toc75) is the core component of the chloroplast protein import machinery. It belongs to the Omp85 family whose members exist in various Gram-negative bacteria, mitochondria, and chloroplasts of eukaryotes. Chloroplasts of Viridiplantae contain another Omp85 homolog called outer envelope protein 80 (OEP80), whose exact function is unknown. In addition, the Arabidopsis thaliana genome encodes truncated forms of Toc75 and OEP80. Multiple studies have shown a common origin of the Omp85 homologs of cyanobacteria and chloroplasts but their results about evolutionary relationships among cyanobacterial Omp85 (cyanoOmp85), Toc75, and OEP80 are inconsistent. The bipartite targeting sequence-dependent sorting of Toc75 has been demonstrated but the targeting mechanisms of other chloroplast Omp85 homologs remain largely unexplored. This study was aimed to address these unresolved issues in order to further our understanding of chloroplast evolution. Sequence alignments and recently determined structures of bacterial Omp85 homologs were used to predict structures of chloroplast Omp85 homologs. The results enabled us to identify amino acid residues that may indicate functional divergence of Toc75 from cyanoOmp85 and OEP80. Phylogenetic analyses using Omp85 homologs from various cyanobacteria and chloroplasts provided strong support for the grouping of Toc75 and OEP80 sister to cyanoOmp85. However, this support was diminished when the analysis included Omp85 homologs from other bacteria and mitochondria. Finally, results of import assays using isolated chloroplasts support outer membrane localization of OEP80tr and indicate that OEP80 may carry a cleavable targeting sequence.

Probing Arabidopsis chloroplast diacylglycerol pools by selectively targeting bacterial diacylglycerol kinase to suborganellar membranes

Diacylglycerol (DAG) is an intermediate in metabolism of both triacylglycerols and membrane lipids. Probing the steady-state pools of DAG and understanding how they contribute to the synthesis of different lipids is important when designing plants with altered lipid metabolism. However, traditional methods of assaying DAG pools are difficult, because its abundance is low and because fractionation of subcellular membranes affects DAG pools. To manipulate and probe DAG pools in an in vivo context, we generated multiple stable transgenic lines of Arabidopsis (Arabidopsis thaliana) that target an Escherichia coli DAG kinase (DAGK) to each leaflet of each chloroplast envelope membrane. E. coli DAGK is small, self inserts into membranes, and has catalytic activity on only one side of each membrane. By comparing whole-tissue lipid profiles between our lines, we show that each line has an individual pattern of DAG, phosphatidic acid, phosphatidylcholine, and triacylglycerol steady-state levels, which supports an individual function of DAG in each membrane leaflet. Furthermore, conversion of DAG in the leaflets facing the chloroplast intermembrane space by DAGK impairs plant growth. As a result of DAGK presence in the outer leaflet of the outer envelope membrane, phosphatidic acid accumulation is not observed, likely because it is either converted into other lipids or removed to other membranes. Finally, we use the outer envelope-targeted DAGK line as a tool to probe the accessibility of DAG generated in response to osmotic stress.

The chloroplast protein import system: from algae to trees

Chloroplasts are essential organelles in the cells of plants and algae. The functions of these specialized plastids are largely dependent on the ~3000 proteins residing in the organelle. Although chloroplasts are capable of a limited amount of semiautonomous protein synthesis - their genomes encode ~100 proteins - they must import more than 95% of their proteins after synthesis in the cytosol. Imported proteins generally possess an N-terminal extension termed a transit peptide. The importing translocons are made up of two complexes in the outer and inner envelope membranes, the so-called Toc and Tic machineries, respectively. The Toc complex contains two precursor receptors, Toc159 and Toc34, a protein channel, Toc75, and a peripheral component, Toc64/OEP64. The Tic complex consists of as many as eight components, namely Tic22, Tic110, Tic40, Tic20, Tic21 Tic62, Tic55 and Tic32. This general Toc/Tic import pathway, worked out largely in pea chloroplasts, appears to operate in chloroplasts in all green plants, albeit with significant modifications. Sub-complexes of the Toc and Tic machineries are proposed to exist to satisfy different substrate-, tissue-, cell- and developmental requirements. In this review, we summarize our understanding of the functions of Toc and Tic components, comparing these components of the import machinery in green algae through trees. We emphasize recent findings that point to growing complexities of chloroplast protein import process, and use the evolutionary relationships between proteins of different species in an attempt to define the essential core translocon components and those more likely to be responsible for regulation. This article is part of a Special Issue entitled: Protein Import and Quality Control in Mitochondria and Plastids.

OEP80, an essential protein paralogous to the chloroplast protein translocation channel Toc75, exists as a 70-kD protein in the Arabidopsis thaliana chloroplast outer envelope

Toc75 and OEP80 are paralogous proteins found in the Viridiplantae lineages, and appear to have evolved from a protein in the outer membrane of an ancient cyanobacterium. Toc75 is known to act as a protein translocation channel at the outer membrane of the chloroplast envelope, whereas the exact function of OEP80 is not understood. In Arabidopsis thaliana, each protein is encoded by a single gene, and both are essential for plant viability from embryonic stages onward. Sequence annotation and immunoblotting data with an antibody against its internal sequence (αOEP80(325-337)) indicated that the molecular weight of OEP80 is ca. 80 kD. Here we present multiple data to show that the size of A. thaliana OEP80 is smaller than previously estimated. First, we prepared the antibody against a recombinant protein consisting of annotated full-length A. thaliana OEP80 with an N-terminal hexahistidine tag (αOEP80(1-732)). This antibody recognized a 70-kD protein in the A. thaliana chloroplast membrane fraction which migrated faster than the His-tagged antigen and the protein recognized by the αOEP80(325-337) antibody on SDS-PAGE. Immunoprecipitation followed by LC-MS/MS analysis confirmed that the 70-kD protein was encoded by the OEP80 cDNA. Next, we performed a genetic complementation assay using embryo-lethal oep80-null plants and constructs encoding OEP80 and its variants. The results revealed that the nucleotide sequence encoding the 52 N-terminal amino acids was not required for functional expression of OEP80 and accumulation of the 70-kD protein. The data also indicated that an additional C-terminal T7 tag remained intact without disrupting the functionality of OEP80, and was not exposed to the cytoplasmic surface of the chloroplast envelope. Finally, OEP80-T7 and Toc75 showed distinct migration patterns on blue native-PAGE. This study provides molecular tools to investigate the function of OEP80, and also calls for caution in using an anti-peptide antibody.

Chloroplast Omp85 proteins change orientation during evolution

The majority of outer membrane proteins (OMPs) from gram-negative bacteria and many of mitochondria and chloroplasts are β-barrels. Insertion and assembly of these proteins are catalyzed by the Omp85 protein family in a seemingly conserved process. All members of this family exhibit a characteristic N-terminal polypeptide-transport-associated (POTRA) and a C-terminal 16-stranded β-barrel domain. In plants, two phylogenetically distinct and essential Omp85's exist in the chloroplast outer membrane, namely Toc75-III and Toc75-V. Whereas Toc75-V, similar to the mitochondrial Sam50, is thought to possess the original bacterial function, its homolog, Toc75-III, evolved to the pore-forming unit of the TOC translocon for preprotein import. In all current models of OMP biogenesis and preprotein translocation, a topology of Omp85 with the POTRA domain in the periplasm or intermembrane space is assumed. Using self-assembly GFP-based in vivo experiments and in situ topology studies by electron cryotomography, we show that the POTRA domains of both Toc75-III and Toc75-V are exposed to the cytoplasm. This unexpected finding explains many experimental observations and requires a reevaluation of current models of OMP biogenesis and TOC complex function.

Biosynthesis and functions of the plant sulfolipid

Higher-plant chloroplast membranes are composed primarily of four characteristic lipids, namely monogalactosyldiacylglycerol, digalactosyldiacylglycerol, sulfoquinovosyldiacylglycerol (SQDG), and phosphatidylglycerol. Among them, SQDG is the only sulfur-containing anionic glycerolipid and is the least prevalent component of photosynthetic membrane lipids. SQDG biosynthesis is mostly mediated by UDP-sulfoquinovose synthase (SQD1) and SQDG synthase (SQD2). Recently, another essential gene for SQDG synthesis, UGP3, was identified using transcriptome coexpression analysis and reverse genetics. UGP3 is a novel plastid UDP-glucose pyrophosphorylase that supplies UDP-glucose to SQD1 in plastids. In Arabidopsis, SQDG is dispensable under normal growth conditions but important in certain environments, particularly phosphate-depleted conditions. The function of SQDG under phosphate-limited growth conditions is highly correlated with the regulation of other plant glycerolipid biosyntheses. This review summarizes recent research defining the mechanism for SQDG biosynthesis and its biological function in higher plants, particularly under phosphate-starved conditions.

The galactolipid monogalactosyldiacylglycerol (MGDG) contributes to photosynthesis-related processes in Arabidopsis thaliana

Processes putatively dependent on the galactolipid monogalactosyldiacylglycerol (MGDG) were recently studied using the knockdown monogalactosyldiacylglycerol synthase 1 (mgd1-1) mutant ( approximately 40% reduction in MGDG). Surprisingly, targeting of chloroplast proteins was not affected in mgd1-1 mutants, suggesting they retain sufficient MGDG to maintain efficient targeting. However, in dark-grown mgd1-1 plants the photoactive to photoinactive protochlorophyllide (Pchlide) ratio was increased, suggesting that photoprotective responses are induced in them. Nevertheless, mgd1-1 could not withstand high light intensities, apparently due to impairment of another photoprotective mechanism, the xanthophyll cycle (and hence thermal dissipation). This was mediated by increased conductivity of the thylakoid membrane leading to a higher pH in the thylakoid interior, which impaired the pH-dependent activation of violaxanthin de-epoxidase (VDE) and PsbS. These findings suggest that MGDG contribute directly to the regulation of photosynthesis-related processes.

An Rh1-GFP fusion protein is in the cytoplasmic membrane of a white mutant strain of Chlamydomonas reinhardtii

The major Rhesus (Rh) protein of the green alga Chlamydomonas reinhardtii, Rh1, is homologous to Rh proteins of humans. It is an integral membrane protein involved in transport of carbon dioxide. To localize a fusion of intact Rh1 to the green fluorescent protein (GFP), we used as host a white (lts1) mutant strain of C. reinhardtii, which is blocked at the first step of carotenoid biosynthesis. The lts1 mutant strain accumulated normal amounts of Rh1 heterotrophically in the dark and Rh1-GFP was at the periphery of the cell co-localized with the cytoplasmic membrane dye FM4-64. Although Rh1 carries a potential chloroplast targeting sequence at its N-terminus, Rh1-GFP was clearly not associated with the chloroplast envelope membrane. Moreover, the N-terminal half of the protein was not imported into chloroplasts in vitro and N-terminal regions of Rh1 did not direct import of the small subunit of ribulose bisphosphate carboxylase (SSU). Despite caveats to this interpretation, which we discuss, current evidence indicates that Rh1 is a cytoplasmic membrane protein and that Rh1-GFP is among the first cytoplasmic membrane protein fusions to be obtained in C. reinhardtii. Although lts1 (white) mutant strains cannot be used to localize proteins within sub-compartments of the chloroplast because they lack thylakoid membranes, they should nonetheless be valuable for localizing many GFP fusions in Chlamydomonas.

Monogalactosyldiacylglycerol deficiency in Arabidopsis affects pigment composition in the prolamellar body and impairs thylakoid membrane energization and photoprotection in leaves

Monogalactosyldiacylglycerol (MGDG) is the major lipid constituent of chloroplast membranes and has been proposed to act directly in several important plastidic processes, particularly during photosynthesis. In this study, the effect of MGDG deficiency, as observed in the monogalactosyldiacylglycerol synthase1-1 (mgd1-1) mutant, on chloroplast protein targeting, phototransformation of pigments, and photosynthetic light reactions was analyzed. The targeting of plastid proteins into or across the envelope, or into the thylakoid membrane, was not different from wild-type in the mgd1 mutant, suggesting that the residual amount of MGDG in mgd1 was sufficient to maintain functional targeting mechanisms. In dark-grown plants, the ratio of bound protochlorophyllide (Pchlide, F656) to free Pchlide (F631) was increased in mgd1 compared to the wild type. Increased levels of the photoconvertible pigment-protein complex (F656), which is photoprotective and suppresses photooxidative damage caused by an excess of free Pchlide, may be an adaptive response to the mgd1 mutation. Leaves of mgd1 suffered from a massively impaired capacity for thermal dissipation of excess light due to an inefficient operation of the xanthophyll cycle; the mutant contained less zeaxanthin and more violaxanthin than wild type after 60 min of high-light exposure and suffered from increased photosystem II photoinhibition. This is attributable to an increased conductivity of the thylakoid membrane at high light intensities, so that the proton motive force is reduced and the thylakoid lumen is less acidic than in wild type. Thus, the pH-dependent activation of the violaxanthin de-epoxidase and of the PsbS protein is impaired.

The Omp85-related chloroplast outer envelope protein OEP80 is essential for viability in Arabidopsis

beta-Barrel proteins of the Omp85 (Outer membrane protein, 85 kD) superfamily exist in the outer membranes of Gram-negative bacteria, mitochondria, and chloroplasts. Prominent Omp85 proteins in bacteria and mitochondria mediate biogenesis of other beta-barrel proteins and are indispensable for viability. In Arabidopsis (Arabidopsis thaliana) chloroplasts, there are two distinct types of Omp85-related protein: Toc75 (Translocon at the outer envelope membrane of chloroplasts, 75 kD) and OEP80 (Outer Envelope Protein, 80 kD). Toc75 functions as a preprotein translocation channel during chloroplast import, but the role of OEP80 remains elusive. We characterized three T-DNA mutants of the Arabidopsis OEP80 (AtOEP80) gene. Selectable markers associated with the oep80-1 and oep80-2 insertions segregated abnormally, suggesting embryo lethality of the homozygous genotypes. Indeed, no homozygotes were identified among >100 individuals, and heterozygotes of both mutants produced approximately 25% aborted seeds upon self-pollination. Embryo arrest occurred at a relatively late stage (globular embryo proper) as revealed by analysis using Nomarski optics microscopy. This is substantially later than arrest caused by loss of the principal Toc75 isoform, atToc75-III (two-cell stage), suggesting a more specialized role for AtOEP80. Surprisingly, the oep80-3 T-DNA (located in exon 1 between the first and second ATG codons of the open reading frame) did not cause any detectable developmental defects or affect the size of the AtOEP80 protein in chloroplasts. This indicates that the N-terminal region of AtOEP80 is not essential for the targeting, biogenesis, or functionality of the protein, in contrast with atToc75-III, which requires a bipartite targeting sequence.

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.

The most C-terminal tri-glycine segment within the polyglycine stretch of the pea Toc75 transit peptide plays a critical role for targeting the protein to the chloroplast outer envelope membrane

The protein translocation channel at the outer envelope membrane of chloroplasts (Toc75) is synthesized as a larger precursor with an N-terminal transit peptide. Within the transit peptide of the pea Toc75, a major portion of the 10 amino acid long stretch that contains nine glycine residues was shown to be necessary for directing the protein to the chloroplast outer membrane in vitro. In order to get insights into the mechanism by which the polyglycine stretch mediates correct targeting, we divided it into three tri-glycine segments and examined the importance of each domain in targeting specificity in vitro. Replacement of the most C-terminal segment with alanine residues resulted in mistargeting the protein to the stroma, while exchange of either of the other two tri-glycine regions had no effect on correct targeting. Furthermore, simultaneous replacement of the N-terminal and middle tri-glycine segments with alanine repeats did not cause mistargeting of the protein as much as those of the N- and C-terminal, or the middle and C-terminal segments. These results indicate that the most C-terminal tri-glycine segment is important for correct targeting. Exchanging this portion with a repeat of leucine or glutamic acid also caused missorting of Toc75 to the stroma. By contrast, its replacement with repeats of asparagine, aspartic acid, serine, and proline did not largely affect correct targeting. These data suggest that relatively compact and nonhydrophobic side chains in this particular region play a crucial role in correct sorting of Toc75.

Evolution of the general protein import pathway of plastids (review)

The evolutionary process that transformed a cyanobacterial endosymbiont into contemporary plastids involved not only inheritance but also invention. Because gram-negative bacteria lack a system for polypeptide import, the envelope translocon complex of the general protein import pathway was the most important invention of organelle evolution resulting in a pathway to import back into plastids those nuclear-encoded proteins supplemented with a transit peptide. Genome information of cyanobacteria, phylogenetically diverse plastids, and the nuclei of the first red alga, a diatom, and Arabidopsis thaliana allows us to trace back the evolutionary origin of the twelve currently known translocon components and to partly deduce their assembly sequence. Development of the envelope translocon was initiated by recruitment of a cyanobacterial homolog of the protein-import channel Toc75, which belongs to a ubiquitous and essential family of Omp85/D15 outer membrane proteins of gram-negative bacteria that mediate biogenesis of beta-barrel proteins. Likewise, three other translocon subunits (Tic20, Tic22, and Tic55) and several stromal chaperones have been inherited from the ancestral cyanobacterium and modified to take over the novel function of precursor import. Most of the remaining subunits seem to be of eukaryotic origin, recruited from pre-existing nuclear genes. The next subunits that joined the evolving protein import complex likely were Toc34 and Tic110, as indicated by the presence of homologous genes in the red alga Cyanidioschyzon merolae, followed by the stromal processing peptidase, members of the Toc159 receptor family, Toc64, Tic40, and finally some regulatory redox components (Tic62, Tic32), all of which were probably required to increase specificity and efficiency of precursor import.

Chloroplast outer membrane protein targeting and insertion

Proteins in the chloroplast outer envelope membrane are nuclear encoded and post-translationally targeted to the chloroplast. The targeting and membrane insertion of these proteins is not well understood. Although early work suggested otherwise, the best-studied outer membrane proteins (OMPs) use both proteins within the chloroplast and NTPs for insertion. There have been conflicting reports in the field regarding protein targeting and insertion, which have probably arisen because of differences in experimental methodology and different interpretations of reduction (versus abolition) of integration. This review summarizes what is known to date about the mechanism of chloroplast OMP targeting.

The chloroplastic protein translocation channel Toc75 and its paralog OEP80 represent two distinct protein families and are targeted to the chloroplastic outer envelope by different mechanisms

Toc75 is postulated to form the protein translocation channel in the chloroplastic outer envelope membrane. Proteins homologous to Toc75 are present in a wide range of organisms, with the closest homologs occurring in cyanobacteria. Therefore, an endosymbiotic origin of Toc75 has been postulated. Recently, a gene encoding a paralog to Toc75 was identified in Arabidopsis and its product was named atToc75-V. In the present study, we characterized this new Toc75 paralog, and investigated extensively the relationships among Toc75 homologs from higher plants and bacteria in order to gain insights into the evolutionary origin of the chloroplastic protein translocation channel. First, we found that the native molecular weight of atToc75-V is 80 kDa and renamed it (AtOEP80) Arabidopsis thalianaouter envelope protein of 80 kDa. Second, we found that AtOEP80 and Toc75 utilize different mechanisms for their targeting to the chloroplastic envelope. Toc75 is directed with a cleavable bipartite transit peptide partly via the general import pathway, whereas AtOEP80 contains the targeting information within its mature sequence, and its targeting is independent of the general pathway. Third, we undertook phylogenetic analyses of Toc75 homologs from various organisms, and found that Toc75 and OEP80 represent two independent gene families that are most likely derived from cyanobacterial sequences. Our results suggest that Toc75 and OEP80 diverged early in the evolution of plastids from their common ancestor with modern cyanobacteria.

A polyglycine stretch is necessary for proper targeting of the protein translocation channel precursor to the outer envelope membrane of chloroplasts

Toc75 is a protein translocation channel in the outer envelope membrane of chloroplasts and its presence is essential for the biogenesis of the organelles. Toc75 is the only protein identified so far in the outer membrane of chloroplasts or mitochondria that is synthesized as a larger precursor, preToc75, with a bipartite transit peptide. Its N-terminus targets the protein to the stroma and is removed by the stromal processing peptidase, whereas its C-terminus mediates envelope targeting and is removed by a yet unknown peptidase. Several conserved domains have been identified in the C-terminal portion of the preToc75 transit peptide from six plant species. We evaluated their importance in the biogenesis of Toc75 by means of deletion or site-directed mutagenesis, followed by import experiments using isolated chlroplasts. Among the conserved domains, a polyglycine stretch was found to be necessary for envelope targeting. Substitution of this domain with other stretches of a single amino acid such as alanine caused mistargeting of the protein into the stroma, indicating an important role for this domain. Furthermore, a glutamate at +2 and two alanine residues at -3 and -1 to the second cleavage site were found to be important for processing. A potential mechanism for the biogenesis of Toc75 is discussed.

Protein import into chloroplasts

Protein import into chloroplasts is a highly regulated process. The activity of the major import receptors is regulated by protein phosphorylation, as well as by GTP binding and hydrolysis. Complete translocation into the organelle could depend on its redox status, as sensed by the Tic complex. A further possibility is that, upon phosphorylation, precursor proteins form a highly import-competent guidance complex in the cytosol. Hence, several levels of regulation seem to coexist.

Molecular chaperones involved in chloroplast protein import

Transport of cytoplasmically synthesized precursor proteins into chloroplasts, like the protein transport systems of mitochondria and the endoplasmic reticulum, appears to require the action of molecular chaperones. These molecules are likely to be the sites of the ATP hydrolysis required for precursor proteins to bind to and be translocated across the two membranes of the chloroplast envelope. Over the past decade, several different chaperones have been identified, based mainly on their association with precursor proteins and/or components of the chloroplast import complex, as putative factors mediating chloroplast protein import. These factors include cytoplasmic, chloroplast envelope-associated and stromal members of the Hsp70 family of chaperones, as well as stromal Hsp100 and Hsp60 chaperones and a cytoplasmic 14-3-3 protein. While many of the findings regarding the action of chaperones during chloroplast protein import parallel those seen for mitochondrial and endoplasmic reticulum protein transport, the chloroplast import system also has unique aspects, including its hypothesized use of an Hsp100 chaperone to drive translocation into the organelle interior. Many questions concerning the specific functions of chaperones during protein import into chloroplasts still remain that future studies, both biochemical and genetic, will need to address.