Envelope membrane proteins that interact with chloroplastic precursor proteins

Identification of a signal that distinguishes between the chloroplast outer envelope membrane and the endomembrane system in vivo

Certain small outer envelope membrane proteins of chloroplasts are encoded by the nuclear genome without a cleavable N-terminal transit peptide. We investigated in vivo the targeting mechanism of AtOEP7, an Arabidopsis homolog of the small outer envelope membrane protein. AtOEP7 was expressed as a fusion protein with the green fluorescent protein (GFP) either transiently in protoplasts or stably in transgenic plants. In either case, fluorescence microscopy of transformed cells and protein gel blot analysis of fractionated proteins confirmed that the AtOEP7:GFP fusion protein was targeted to the chloroplast outer envelope membrane. In vivo targeting experiments revealed that two regions, the transmembrane domain (TMD) and its C-terminal neighboring seven-amino acid region, were necessary and sufficient for targeting to the chloroplast outer membrane. Substitution of aspartic acid or lysine residues with glycine residues or scrambling of the amino acid sequence of the seven-amino acid region caused mistargeting to the plasma membrane. Although the amino acid sequence of the TMD is not important for targeting, amino acid residues with large side chains inhibited targeting to the chloroplasts and resulted in the formation of large aggregates in the protoplasts. In addition, introduction of a proline residue within the TMD resulted in inhibition of targeting. Finally, a fusion protein, AtOEP7:NLS:GFP, was targeted efficiently to the chloroplast envelope membranes despite the presence of a nuclear localization signal. On the basis of these results, we conclude that the seven-amino acid region and the TMD are determinants for targeting to the chloroplast outer envelope membrane. The seven-amino acid region plays a critical role in AtOEP7 evading the endomembrane system and entering the chloroplast pathway, and the TMD plays critical roles in migration to the chloroplasts and/or subsequent insertion into the membrane.

Leaf-specific upregulation of chloroplast translocon genes by a CCT motif-containing protein, CIA 2

Chloroplasts are a major destination of protein traffic within leaf cells. Protein import into chloroplasts is mediated by a set of translocon complexes at the chloroplast envelope. Current data indicate that the expression of translocon genes is regulated in a tissue-specific manner, possibly to accommodate the higher import demand of chloroplasts in leaves and the lower demand of plastids in other tissues. We have designed a transgene-based positive screen to isolate mutants disrupted in protein import into plastids. The first locus we isolated, CIA2, encodes a protein containing a motif conserved within the CCT family of transcription factors. Biochemical analysis indicates that CIA2 is responsible for specific upregulation of the translocon genes atToc33 and atToc75 in leaves. Identification of CIA2 provides new insights into the tissue-specific regulation of translocon gene expression.

Leaf-specific upregulation of chloroplast translocon genes by a CCT motif-containing protein, CIA 2

Chloroplasts are a major destination of protein traffic within leaf cells. Protein import into chloroplasts is mediated by a set of translocon complexes at the chloroplast envelope. Current data indicate that the expression of translocon genes is regulated in a tissue-specific manner, possibly to accommodate the higher import demand of chloroplasts in leaves and the lower demand of plastids in other tissues. We have designed a transgene-based positive screen to isolate mutants disrupted in protein import into plastids. The first locus we isolated, CIA2, encodes a protein containing a motif conserved within the CCT family of transcription factors. Biochemical analysis indicates that CIA2 is responsible for specific upregulation of the translocon genes atToc33 and atToc75 in leaves. Identification of CIA2 provides new insights into the tissue-specific regulation of translocon gene expression.

Chloroplast protein translocon components atToc159 and atToc33 are not essential for chloroplast biogenesis in guard cells and root cells

Protein import into chloroplasts is mediated by a protein import apparatus located in the chloroplast envelope. Previous results indicate that there may be multiple import complexes in Arabidopsis. To gain further insight into the nature of this multiplicity, we analyzed the Arabidopsis ppi1 and ppi2 mutants, which are null mutants of the atToc33 and atToc159 translocon proteins, respectively. In the ppi2 mutant, in contrast to the extremely defective plastids in mesophyll cells, chloroplasts in guard cells still contained starch granules and thylakoid membranes. The morphology of root plastids in both mutants was similar to that in wild type. After prolonged light treatments, root plastids of both mutants and the wild type differentiated into chloroplasts. Enzymatic assays indicated that the activity of a plastid enzyme was reduced only in leaves but not in roots. These results indicated that both the ppi1 and ppi2 mutants had functional root and guard cell plastids. Therefore, we propose that import complexes are cell type specific rather than substrate or plastid specific.

Targeting of an abundant cytosolic form of the protein import receptor at Toc159 to the outer chloroplast membrane

Chloroplast biogenesis requires the large-scale import of cytosolically synthesized precursor proteins. A trimeric translocon (Toc complex) containing two homologous GTP-binding proteins (atToc33 and atToc159) and a channel protein (atToc75) facilitates protein translocation across the outer envelope membrane. The mechanisms governing function and assembly of the Toc complex are not yet understood. This study demonstrates that atToc159 and its pea orthologue exist in an abundant, previously unrecognized soluble form, and partition between cytosol-containing soluble fractions and the chloroplast outer membrane. We show that soluble atToc159 binds directly to the cytosolic domain of atToc33 in a homotypic interaction, contributing to the integration of atToc159 into the chloroplast outer membrane. The data suggest that the function of the Toc complex involves switching of atToc159 between a soluble and an integral membrane form.

Arabidopsis genes encoding components of the chloroplastic protein import apparatus

The process of protein import into plastids has been studied extensively using isolated pea (Pisum sativum) chloroplasts. As a consequence, virtually all of the known components of the proteinaceous apparatus that mediates import were originally cloned from pea. With the recent completion of the Arabidopsis genome sequencing project, it is now possible to identify putative homologs of the import components in this species. Our analysis has revealed that Arabidopsis homologs with high sequence similarity exist for all of the pea import complex subunits, making Arabidopsis a valid model for further study of this system. Multiple homologs can be identified for over one-half of the components. In all but one case it is known that more than one of the putative isoforms for a particular subunit are expressed. Thus, it is possible that multiple types of import complexes are present within the same cell, each having a unique affinity for different chloroplastic precursor proteins, depending upon the exact mix of isoforms it contains. Sequence analysis of the putative Arabidopsis homologs for the chloroplast protein import apparatus has revealed many questions concerning subunit function and evolution. It should now be possible to use the genetic tools available in Arabidopsis, including the generation of knockout mutants and antisense technology, to address these questions and learn more about the molecular functions of each of the components during the import process.

Cytometric analysis of an epitope-tagged transit peptide bound to the chloroplast translocation apparatus

Chloroplast transit peptides are necessary and sufficient for the targeting and translocation of precursor proteins across the chloroplast envelope. However, the mechanism by which transit peptides engage the translocation apparatus has not been investigated. To analyse this interaction, we have developed a novel epitope-tagged transit peptide derived from the precursor of the small subunit of pea Rubisco. The recombinant transit peptide, His-S-SStp, contains a removable dual-epitope tag, His-S, at its N-terminus that permits both rapid purification via immobilized metal affinity chromatography and detection by blotting, flow cytometry and laser-scanning confocal microscopy. Unlike other chimeric precursors, which place the passenger protein C-terminal to the transit peptide, His-S-SStp bound to the translocation apparatus yet did not translocate across the chloroplast envelope. This early translocation intermediate allowed non-radioactive detection using fluorescent and chemiluminescent reporters. The physiological relevance of this interaction was confirmed by protein import competitions, sensitivity to pre- and post-import thermolysin treatment, photochemical cross-linking and organelle fractionation. The interaction was specific for the transit peptide since His-S alone did not engage the chloroplast translocation apparatus. Quantitation of the bound transit peptide was determined by flow cytometry, showing saturation of binding yet only slight ATP-dependence. The addition of GTP showed inhibition of the binding of His-S-SStp to the chloroplasts indicating an involvement of GTP in the formation of this early translocation intermediate. In addition, direct visualization of His-S-SStp and Toc75 by confocal microscopy revealed a patch-like labeling, suggesting a co-ordinate localization to discrete regions on the chloroplast envelope. These findings represent the first direct visualization of a transit peptide interacting with the chloroplast translocation apparatus. Furthermore, identification of a chloroplast-binding intermediate may provide a novel tool to dissect interactions between a transit peptide and the chloroplast translocation apparatus.

Tomato allene oxide synthase and fatty acid hydroperoxide lyase, two cytochrome P450s involved in oxylipin metabolism, are targeted to different membranes of chloroplast envelope

Allene oxide synthase (AOS) and hydroperoxide lyase (HPL) are related cytochrome P450s that metabolize a common fatty acid hydroperoxide substrate to different classes of bioactive oxylipins within chloroplasts. Here, we report the use of in vitro import assays to investigate the targeting of tomato (Lycopersicon esculentum) AOS (LeAOS) and HPL (LeHPL) to isolated chloroplasts. LeAOS, which contains a typical N-terminal transit peptide, was targeted to the inner envelope membrane by a route that requires both ATP and proteinase-sensitive components on the surface of chloroplasts. Imported LeAOS was peripherally associated with the inner envelope; the bulk of the protein facing the stroma. LeHPL, which lacks a typical chloroplast-targeting sequence, was targeted to the outer envelope by an ATP-independent and protease-insensitive pathway. Imported LeHPL was integrated into the outer envelope with most of the protein exposed to the inter-membrane space. We conclude that LeAOS and LeHPL are routed to different envelope membranes by distinct targeting pathways. Partitioning of AOS and HPL to different envelope membranes suggests differences in the spatial organization of these two branches of oxylipin metabolism.

Chloroplast precursor proteins compete to form early import intermediates in isolated pea chloroplasts

In order to ascertain whether there is one site for the import of precursor proteins into chloroplasts or whether different precursor proteins are imported via different import machineries, chloroplasts were incubated with large quantities of the precursor of the 33 kDa subunit of the oxygen-evolving complex (pOE33) or the precursor of the light-harvesting chlorophyll a/b-binding protein (pLHCP) and tested for their ability to import a wide range of other chloroplast precursor proteins. Both pOE33 and pLHCP competed for import into chloroplasts with precursors of the stromally-targeted small subunit of Rubisco (pSSu), ferredoxin NADP(+) reductase (pFNR) and porphobilinogen deaminase; the thylakoid membrane proteins LHCP and the Rieske iron-sulphur protein (pRieske protein); ferrochelatase and the gamma subunit of the ATP synthase (which are both associated with the thylakoid membrane); the thylakoid lumenal protein plastocyanin and the phosphate translocator, an integral membrane protein of the inner envelope. The concentrations of pOE33 or pLHCP required to cause half-maximal inhibition of import ranged between 0.2 and 4.9 microM. These results indicate that all of these proteins are imported into the chloroplast by a common import machinery. Incubation of chloroplasts with pOE33 inhibited the formation of early import intermediates of pSSu, pFNR and pRieske protein.

NADPH:Protochlorophyllide oxidoreductase uses the general import route into chloroplasts

Chloroplast differentiation in angiosperm plants depends on the light-dependent conversion of protochlorophyllide to chlorophyllide by NADPH:protochlorophyllide oxidoreductase (PORA; EC 1.6.99.1), a nuclearly encoded protein. The protein import of the precursor form of PORA into plastids was shown previously to strictly depend on the presence of its substrate protochlorophyllide. PORA seemed to follow a novel, posttranslationally regulated import route. Here we demonstrate that the precursor of PORA from barley is imported into isolated barley plastids independently of protochlorophyllide. PORA as well as PORB import is competed for by the precursor of the small subunit of Rubisco. The data demonstrate that the PORA precursor uses the general import pathway into plastids. Furthermore, en route into chloroplasts the pea POR precursor can be cross-linked to the protein import channel in the outer envelope Toc75 from pea.

A mammalian cytochrome fused to a chloroplast transit peptide is a functional haemoprotein and is imported into isolated chloroplasts

The small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is a major chloroplast stromal protein that is cytosolically synthesized as a precursor with an N-terminal extension, known as the transit sequence or transit peptide (Tp). The Tp is essential for the post-translational uptake of the precursor by the chloroplast. The Tp is thought to influence the conformation of the precursor protein and to facilitate polypeptide translocation across the chloroplast envelope barrier via a Tp-selective translocon. To address these issues we have devised a novel strategy to generate substrate amounts of a chloroplast targeting sequence as a fusion with the chromogenic globular domain of cytochrome b(5) (Cyt). The chimaeric protein is an ideal probe for investigating the conformation of a preprotein and events surrounding protein import into isolated chloroplasts. The Cyt of liver endoplasmic reticulum was fused at its N-terminus with the Tp of the small subunit of Rubisco of Pisum sativum (pea). To enhance its production by clearance from the cytoplasm of Escherichia coli, the chimaera was engineered by further N-terminal linkage of a prokaryotic secretory signal. Expression of this tripartite fusion resulted in mg quantities of the signal sequence-processed Tp-Cyt protein, which was eventually targeted to the membranes. The chromogenic nature of the chimaera and its localization to the bacterial membrane facilitated the biochemical isolation of the precursor in a soluble and functional form. The purified preprotein displayed spectral and enzymic properties that were indistinguishable from the native parental Cyt, implying an absence of observable influence of the Tp on the conformation of the haemoprotein. The chimaeric precursor was imported into the stroma of the isolated chloroplasts in a dose-dependent manner. Import was also strongly dependent upon exogenously supplied ATP. The stromally imported chimaeric precursor protein was processed to a size characteristic of Cyt.

Insertion of OEP14 into the outer envelope membrane is mediated by proteinaceous components of chloroplasts

Most chloroplastic outer envelope membrane proteins are synthesized in the cytosol at their mature size without a cleavable targeting signal. Their insertion into the outer membrane is insensitive to thermolysin pretreatment of chloroplasts and does not require ATP. The insertion has been assumed to be mediated by a spontaneous mechanism or by interaction solely with the lipid components of the outer membrane. However, we show here that insertion of an outer membrane protein requires some trypsin-sensitive and some N-ethylmaleimide-sensitive components of chloroplasts. Association and insertion of the outer membrane protein are saturable and compete with the import of another outer membrane protein. These data suggest that import of chloroplastic outer membrane proteins occurs at specific proteinaceous sites on chloroplasts.

Chloroplast transit peptides: structure, function and evolution

It is thought that two to three thousand different proteins are targeted to the chloroplast, and the 'transit peptides' that act as chloroplast targeting sequences are probably the largest class of targeting sequences in plants. At a primary structural level, transit peptide sequences are highly divergent in length, composition and organization. An emerging concept suggests that transit peptides contain multiple domains that provide either distinct or overlapping functions. These functions include direct interaction with envelope lipids, chloroplast receptors and the stromal processing peptidase. The genomic organization of transit peptides suggests that these domains might have originated from distinct exons, which were shuffled and streamlined throughout evolution to yield a modern, multifunctional transit peptide. Although still poorly characterized, this evolutionary process could yield transit peptides with different domain organizations. The plasticity of transit peptide design is consistent with the diverse biological functions of chloroplast proteins.

Protein import: the hitchhikers guide into chloroplasts

Plastids originated from an endosymbiotic event between an early eukaryotic host cell and an ancestor of today's cyanobacteria. During the events by which the engulfed endosymbiont was transformed into a permanent organelle, many genes were transferred from the plastidal genome to the nucleus of the host cell. Proteins encoded by these genes are synthesised in the cytosol and subsequently translocated into the plastid. Therefore they contain an N-terminal cleavable transit sequence that is necessary for translocation. The sequence is plastid-specific, thus preventing mistargeting into other organelles. Receptors embedded into the outer envelope of the plastid recognise the transit sequences, and precursor proteins are translocated into the chloroplast by a proteinaceous import machinery located in both the outer and inner envelopes. Inside the stroma the transit sequences are cleaved off and the proteins are further routed to their final locations within the plastid.

A second, substrate-dependent site of protein import into chloroplasts

Chloroplasts must import a large number of proteins from the cytosol. It generally is assumed that this import proceeds for all stromal and thylakoid proteins in an identical manner and is caused by the operation of two distinctive protein import machineries in the outer and inner plastid envelope, which form the general import site. Here we show that there is a second site of protein translocation into chloroplasts of barley, tobacco, Arabidopsis thaliana, and five other tested monocotyledonous and dicotyledonous plant species. This import site is specific for the cytosolic precursor of the NADPH:protochlorophyllide (Pchlide) oxidoreductase A, pPORA. It couples Pchlide synthesis to pPORA import and thereby reduces the actual level of free Pchlide, which, because of its photodynamic properties, would be destructive to the plastids. Consequently, photoprotection is conferred onto the plant.

Membrane proteins and proteomics: un amour impossible?

Proteome analysis implies the ability to separate proteins as a first step prior to characterization. Thus, the overall performance of the analysis strongly depends on the performance of the separation tool, usually two-dimensional electrophoresis. This review shows how two-dimensional electrophoresis performs with membrane proteins from bacteria or animal or vegetable cells and tissues, the recent progress in this field, and it examines future prospects in this area.

Toc64, a new component of the protein translocon of chloroplasts

A subunit of the preprotein translocon of the outer envelope of chloroplasts (Toc complex) of 64 kD is described, Toc64. Toc64 copurifies on sucrose density gradients with the isolated Toc complex. Furthermore, it can be cross-linked in intact chloroplasts to a high molecular weight complex containing both Toc and Tic subunits and a precursor protein. The 0 A cross-linker CuCl(2) yields the reversible formation of disulfide bridge(s) between Toc64 and the established Toc complex subunits in purified outer envelope membranes. Toc64 contains three tetratricopeptide repeat motifs that are exposed at the chloroplast cytosol interface. We propose that Toc64 functions early in preprotein translocation, maybe as a docking protein for cytosolic cofactors of the protein import into chloroplasts.

A chloroplast envelope-transfer sequence functions as an export signal in Escherichia coli

The small subunit precursor of pea ribulose-1,5-bisphosphate carboxylase/oxygenase engineered with prokaryotic elements was expressed in Escherichia coli. This resulted in a dependable level of synthesis of the precursor protein in E. coli. The bacterially synthesised plant precursor protein was translocated from the cytoplasm and targeted to the outer membrane of the envelope zone. During the translocation step, a significant proportion of the precursor was processed to a soluble, mature SSU and found localised in the periplasm. The determined amino acid sequence of the isolated precursor showed that it had a deletion of an arginine residue at position -15 in the transit peptide. Expression of this transit peptide-appended mammalian cytochrome b(5) in E. coli displayed a targeting profile of the chromogenic chimera that was similar to that observed with the plant precursor protein.

Initial binding of preproteins involving the Toc159 receptor can be bypassed during protein import into chloroplasts

Two integral outer envelope GTPases, Toc34 and Toc86, are proposed to regulate the recognition and translocation of nuclear-encoded preproteins during the early stages of protein import into chloroplasts. Defining the precise roles of Toc86 and Toc34 has been complicated by the inability to distinguish their GTPase activities. Furthermore, the assignment of Toc86 function is rendered equivocal by recent reports suggesting that the standard protocol for the isolation of chloroplasts results in significant proteolysis of Toc86 (B. Bolter, T. May, J. Soll [1998] FEBS Lett 441: 59-62; G. Schatz [1998] Nature 395: 439-440). We demonstrate that Toc86 corresponds to a native protein of 159 kD in pea (Pisum sativum), designated Toc159. We take advantage of the proteolytic sensitivity of Toc159 to selectively remove its 100-kD cytoplasmic GTPase domain and thereby distinguish its activities from other import components. Proteolysis eliminates detectable binding of preproteins at the chloroplast surface, which is consistent with the proposed role of Toc159 as a receptor component. Remarkably, preprotein translocation across the outer membrane can occur in the absence of the Toc159 cytoplasmic domain, suggesting that binding can be bypassed. Translocation remains sensitive to GTP analogs in the absence of the Toc159 GTP-binding domain, providing evidence that Toc34 plays a key role in the regulation of translocation by GTP.

The major protein import receptor of plastids is essential for chloroplast biogenesis

Light triggers the developmental programme in plants that leads to the production of photosynthetically active chloroplasts from non-photosynthetic proplastids. During this chloroplast biogenesis, the photosynthetic apparatus is rapidly assembled, mostly from nuclear-encoded imported proteins, which are synthesized in the cytosol as precursors with cleavable amino-terminal targeting sequences called transit sequences. Protein translocon complexes at the outer (Toc complex) and inner (Tic complex) envelope membranes recognize these transit sequences, leading to the precursors being imported. The Toc complex in the pea consists of three major components, Toc75, Toc34 and Toc159 (formerly termed Toc86). Toc159, which is an integral membrane GTPase, functions as a transit-sequence receptor. Here we show that Arabidopsis thaliana Toc159 (atToc159) is essential for the biogenesis of chloroplasts. In an Arabidopsis mutant (ppi2) that lacks atToc159, photosynthetic proteins that are normally abundant are transcriptionally repressed, and are found in much smaller amounts in the plastids, although ppi2 does not affect either the expression or the import of less abundant non-photosynthetic plastid proteins. These findings indicate that atToc159 is required for the quantitative import of photosynthetic proteins. Two proteins that are related to atToc159 (atToc120 and atToc132) probably help to maintain basal protein import in ppi2, and so constitute components of alternative, atToc159-independent import pathways.

14-3-3 proteins form a guidance complex with chloroplast precursor proteins in plants

Transit sequences of chloroplast-destined precursor proteins are phosphorylated on a serine or threonine residue. The amino acid motif around the phosphorylation site is related to the phosphopeptide binding motif for 14-3-3 proteins. Plant 14-3-3 proteins interact specifically with wheat germ lysate-synthesized chloroplast precursor proteins and require an intact phosphorylation motif within the transit sequence. Chloroplast precursor proteins do not interact with 14-3-3 when synthesized in the heterologous reticulocyte lysate. In contrast, a precursor protein destined for plant mitochondria was found to be associated with 14-3-3 proteins present in the reticulocyte lysate but not with 14-3-3 from wheat germ lysate. This indicates an unrecognized selectivity of 14-3-3 proteins for precursors from mitochondria and plastids in plants in comparison to fungi and animals. The heterooligomeric complex has an apparent size of 200 kD. In addition to the precursor protein, it contains 14-3-3 (probably as a dimer) and a heat shock protein Hsp70 isoform. Dissociation of the precursor complex requires ATP. Protein import experiments of precursor from the oligomeric complex into intact pea chloroplasts reveal three- to fourfold higher translocation rates compared with the free precursor, which is not complexed. We conclude that the 14-3-3-Hsp70-precursor protein complex is a bona fide intermediate in the in vivo protein import pathway in plants.

14-3-3 proteins form a guidance complex with chloroplast precursor proteins in plants

Transit sequences of chloroplast-destined precursor proteins are phosphorylated on a serine or threonine residue. The amino acid motif around the phosphorylation site is related to the phosphopeptide binding motif for 14-3-3 proteins. Plant 14-3-3 proteins interact specifically with wheat germ lysate-synthesized chloroplast precursor proteins and require an intact phosphorylation motif within the transit sequence. Chloroplast precursor proteins do not interact with 14-3-3 when synthesized in the heterologous reticulocyte lysate. In contrast, a precursor protein destined for plant mitochondria was found to be associated with 14-3-3 proteins present in the reticulocyte lysate but not with 14-3-3 from wheat germ lysate. This indicates an unrecognized selectivity of 14-3-3 proteins for precursors from mitochondria and plastids in plants in comparison to fungi and animals. The heterooligomeric complex has an apparent size of 200 kD. In addition to the precursor protein, it contains 14-3-3 (probably as a dimer) and a heat shock protein Hsp70 isoform. Dissociation of the precursor complex requires ATP. Protein import experiments of precursor from the oligomeric complex into intact pea chloroplasts reveal three- to fourfold higher translocation rates compared with the free precursor, which is not complexed. We conclude that the 14-3-3-Hsp70-precursor protein complex is a bona fide intermediate in the in vivo protein import pathway in plants.

A processing intermediate of a stromal chloroplast import protein in Chlamydomonas

Proteins synthesized in the cytoplasm and destined for importation into the chloroplast across the double envelope membrane contain an N-terminal transit sequence which upon import is cleaved off by a stromal-processing peptidase. Since for stromal-residing proteins no intermediates have ever been found in vivo, it is assumed that precursor proteins are cleaved to the mature size by one proteolytic event which occurs immediately after translocation across both envelope membranes. During import of the precursor of the small subunit of ribulose-1,5-bisphosphate carboxylase (pSS) into isolated chloroplasts of Chlamydomonas we identified an intermediate-sized product, called iSS. It might be identical to a previously described iSS obtained in vitro by a partially purified soluble chloroplast protease [Su and Boschetti (1993) Eur. J. Biochem. 217, 1039-1047]. The kinetics of the formation of iSS in chloroplasts suggest that pSS is processed to the mature small subunit (SS) not by one, but by two steps via this intermediate product. Since, after an induction period, the ratio of iSS/SS was constant under various experimental conditions of import, the formation of iSS was considered not to be a side-reaction. The location of iSS in the intermembrane space of the envelope, as suggested by protease treatment of chloroplasts, questions the one-step translocation mechanism of precursor import into chloroplasts.

Protein import into chloroplasts

Three proteins from the chloroplastic outer envelope membrane and four proteins from the inner envelope membrane have been identified as components of the chloroplastic protein import apparatus. Multiple molecular chaperones and a stromal processing peptidase are also important components of the import machinery. The interactions of these proteins with each other and with the precursors destined for transport into chloroplasts are gradually being described using both biochemical and genetic strategies. Homologs of some transport components have been identified in cyanobacteria suggesting that at least some of import machinery was inherited from the cyanobacterial ancestors that gave rise to chloroplasts.

GTP promotes the formation of early-import intermediates but is not required during the translocation step of protein import into chloroplasts

Protein import into chloroplasts is an energy-requiring process mediated by a proteinaceous import apparatus. Although previous work has shown that low levels of ATP or GTP can support precursor binding, the role of GTP during the import process remains unclear. Specifically, it is unknown whether GTP plays a separate role from ATP during the early stages of protein import and whether GTP has any role in the later stages of transport. We investigated the role of GTP during the various stages of protein import into chloroplasts by using purified GTP analogs and an in vitro import assay. GTP, GDP, the nonhydrolyzable analog GMP-PNP, and the slowly hydrolyzable analogs guanosine 5'-O-(2-thiodiphosphate) and guanosine 5'-O-(3-thiotriphosphate) were used in this study. Chromatographically purified 5'-guanylyl-imido-diphosphate and guanosine 5'-O-(3-thiotriphosphate) were found to inhibit the formation of early-import intermediates, even in the presence of ATP. We also observed that GTP does not play a role during the translocation of precursors from the intermediate state. We conclude that GTP hydrolysis influences events leading to the formation of early-import intermediates, but not subsequent steps such as precursor translocation.

Chloroplast precursor protein translocon

Chloroplasts are believed to have originated from a photosynthetic, prokaryotic ancestor. As the result of endosymbiotic evolution, most of the genes of the endocytobiont were displaced to the host nucleus. Today's chloroplasts must import most of their proteins from the cytosol as precursors. Oligomeric protein complexes in the chloroplast outer and inner envelope membranes are responsible for the specific recognition and membrane translocation of precursor proteins. The translocon at the outer membrane of chloroplasts and the inner membrane of chloroplasts act jointly during the import process. Several translocon subunits have been partially characterized in their molecular structure and function. Initial evidence indicates the prokaryotic origin of some chloroplast translocon components.

Protein import into chloroplasts

Chloroplasts have evolved an elaborate system of membrane and soluble subcompartments to organize and regulate photosynthesis and essential aspects of amino acid and lipid metabolism. The biogenesis and maintenance of organellar architecture rely on protein subunits encoded by both nuclear and plastid genomes. Import of nuclear-encoded proteins is mediated by interactions between the intrinsic N-terminal transit sequence of the nuclear-encoded preprotein and a common import machinery at the chloroplast envelope. Recent investigations have shown that there are two unique membrane-bound translocation systems, in the outer and inner envelope membranes, which physically associate during import to transport preproteins from the cytoplasm to the internal stromal compartment. This review discusses current understanding of these translocation systems and models for the way in which they might function.

The evolutionary origin of the protein-translocating channel of chloroplastic envelope membranes: identification of a cyanobacterial homolog

The known envelope membrane proteins of the chloroplastic protein import apparatus lack sequence similarity to proteins of other eukaryotic or prokaryotic protein transport systems. However, we detected a putative homolog of the gene encoding Toc75, the protein-translocating channel from the outer envelope membrane of pea chloroplasts, in the genome of the cyanobacterium Synechocystis sp. PCC 6803. We investigated whether the low sequence identity of 21% reflects a structural and functional relationship between the two proteins. We provide evidence that the cyanobacterial protein is also localized in the outer membrane. From this information and the similarity of the predicted secondary structures, we conclude that Toc75 and the cyanobacterial protein, referred to as SynToc75, are structural homologs. synToc75 is essential, as homozygous null mutants were not recovered after directed mutagenesis. Sequence analysis indicates that SynToc75 belongs to a family of outer membrane proteins from Gram-negative bacteria whose function is not yet known. However, we demonstrate that these proteins are related to a specific group of prokaryotic secretion channels that transfer virulence factors, such as hemolysins and adhesins, across the outer membrane.

Origin of a chloroplast protein importer

During evolution, chloroplasts have relinquished the majority of their genes to the nucleus. The products of transferred genes are imported into the organelle with the help of an import machinery that is distributed across the inner and outer plastid membranes. The evolutionary origin of this machinery is puzzling because, in the putative predecessors, the cyanobacteria, the outer two membranes, the plasma membrane, and the lipopolysaccharide layer lack a functionally similar protein import system. A 75-kDa protein-conducting channel in the outer envelope of pea chloroplasts, Toc75, shares approximately 22% amino acid identity to a similarly sized protein, designated SynToc75, encoded in the Synechocystis PCC6803 genome. Here we show that SynToc75 is located in the outer membrane (lipopolysaccharide layer) of Synechocystis PCC6803 and that SynToc75 forms a voltage-gated, high conductance channel with a high affinity for polyamines and peptides in reconstituted liposomes. These findings suggest that a component of the chloroplast protein import system, Toc75, was recruited from a preexisting channel-forming protein of the cyanobacterial outer membrane. Furthermore, the presence of a protein in the chloroplastic outer envelope homologous to a cyanobacterial protein provides support for the prokaryotic nature of this chloroplastic membrane.

A protein import receptor in pea chloroplasts, Toc86, is only a proteolytic fragment of a larger polypeptide

The protein import complex of the chloroplastic outer envelope (Toc-complex) contains a prominent subunit of 86 kDa molecular weight (Toc86). Toc86 was identified as a putative precursor receptor. The Arabidopsis genome sequencing project indicates that Toc86 represents only a proteolytic fragment of a larger polypeptide of 160 kDa. The 160-kDa protein, which we name Toc160, is only present in significant amounts in pea chloroplasts isolated under stringent conditions. The capacity of chloroplasts to import an in vitro translated precursor protein correlates well with the integrity of Toc160. We conclude that Toc160 is still a bonafide subunit of the protein import machinery of chloroplasts.

Tic20 and Tic22 are new components of the protein import apparatus at the chloroplast inner envelope membrane

Two components of the chloroplast envelope, Tic20 and Tic22, were previously identified as candidates for components of the general protein import machinery by their ability to covalently cross-link to nuclear-encoded preproteins trapped at an intermediate stage in import across the envelope (Kouranov, A., and D.J. Schnell. 1997. J. Cell Biol. 139:1677-1685). We have determined the primary structures of Tic20 and Tic22 and investigated their localization and association within the chloroplast envelope. Tic20 is a 20-kD integral membrane component of the inner envelope membrane. In contrast, Tic22 is a 22-kD protein that is located in the intermembrane space between the outer and inner envelope membranes and is peripherally associated with the outer face of the inner membrane. Tic20, Tic22, and a third inner membrane import component, Tic110, associate with import components of the outer envelope membrane. Preprotein import intermediates quantitatively associate with this outer/inner membrane supercomplex, providing evidence that the complex corresponds to envelope contact sites that mediate direct transport of preproteins from the cytoplasm to the stromal compartment. On the basis of these results, we propose that Tic20 and Tic22 are core components of the protein translocon of the inner envelope membrane of chloroplasts.

Domains of a transit sequence required for in vivo import in Arabidopsis chloroplasts

Nuclear-encoded precursors of chloroplast proteins are synthesized with an amino-terminal cleavable transit sequence, which contains the information for chloroplastic targeting. To determine which regions of the transit sequence are most important for its function, the chloroplast uptake and processing of a full-length ferredoxin precursor and four mutants with deletions in adjacent regions of the transit sequence were analyzed. Arabidopsis was used as an experimental system for both in vitro and in vivo import. The full-length wild-type precursor translocated efficiently into isolated Arabidopsis chloroplasts, and upon expression in transgenic Arabidopsis plants only mature-sized protein was detected, which was localized inside the chloroplast. None of the deletion mutants was imported in vitro. By analyzing transgenic plants, more subtle effects on import were observed. The most N-terminal deletion resulted in a fully defective transit sequence. Two deletions in the middle region of the transit sequence allowed translocation into the chloroplast, although with reduced efficiencies. One deletion in this region strongly reduced mature protein accumulation in older plants. The most C-terminal deletion was translocated but resulted in defective processing. These results allow the dissection of the transit sequence into separate functional regions and give an in vivo basis for a domain-like structure of the ferredoxin transit sequence.

Amino-terminal and hydrophobic regions of the Chlamydomonas reinhardtii plastocyanin transit peptide are required for efficient protein accumulation in vivo

Nucleus-encoded chloroplast proteins of vascular plants are synthesized as precursors and targeted to the chloroplast by stroma-targeting domains in N-terminal transit peptides. Transit peptides in Chlamydomonas reinhardtii are considerably shorter than those in vascular plants, and their stroma-targeting domains have similarities to both mitochondrial and chloroplast targeting sequences. To examine Chlamydomonas transit peptide function in vivo, deletions were introduced into the transit peptide coding region of the petE gene, which encodes the thylakoid lumen protein plastocyanin (PC). The mutant petE genes were introduced into a plastocyanin-deficient Chlamydomonas strain, and transformants that accumulated petE mRNA were analyzed for PC accumulation. The most profound defects were observed with deletions at the N-terminus and those that extended into the hydrophobic region in the C-terminal half of the transit peptide. PC precursors were detected among pulse-labeled proteins in transformants with N-terminal deletions, suggesting that these precursors cannot be imported and are degraded in the cytosol. Intermediate PC species were observed in a transformant deleted for part of the hydrophobic region, suggesting that this protein is defective in lumen translocation and/or processing. Thus, despite its shorter length, the bipartite nature of the Chlamydomonas PC transit peptide appears similar to that of lumen-targeted proteins in vascular plants. Analysis of the synthesis, stability, and accumulation of PC species in transformants bearing deletions in the stroma-targeting domain suggests that specific regions probably have distinct roles in vivo.

A mutant deficient in the plastid lipid DGD is defective in protein import into chloroplasts

Most proteins in chloroplasts are encoded by the nuclear genome and synthesized in the cytosol with N-terminal extensions called transit peptides. Transit peptides function as the import signal to chloroplasts. The import process requires several protein components in the envelope and stroma and also requires the hydrolysis of ATP. Lipids have been implicated in the import process based on theories or experiments with in vitro model systems. We show here that chloroplasts isolated from an Arabidopsis mutant deficient in the plastid lipid digalactosyl diacylglycerol (DGD) were normal in importing a chloroplast outer membrane protein, but were defective in importing precursor proteins targeted to the interior of chloroplasts. The impairment includes the binding, or docking, step of the import process that is supported by 100 microM ATP.

The role of lipids in plastid protein transport

The elaborate compartmentalization of plant cells requires multiple mechanisms of protein targeting and trafficking. In addition to the organelles found in all eukaryotes, the plant cell contains a semi-autonomous organelle, the plastid. The plastid is not only the most active site of protein transport in the cell, but with its three membranes and three aqueous compartments, it also represents the most topologically complex organelle in the cell. The chloroplast contains both a protein import system in the envelope and multiple protein export systems in the thylakoid. Although significant advances have identified several proteinaceous components of the protein import and export apparatuses, the lipids found within plastid membranes are also emerging as important players in the targeting, insertion, and assembly of proteins in plastid membranes. The apparent affinity of chloroplast transit peptides for chloroplast lipids and the tendency for unsaturated MGDG to adopt a hexagonal II phase organization are discussed as possible mechanisms for initiating the binding and/or translocation of precursors to plastid membranes. Other important roles for lipids in plastid biogenesis are addressed, including the spontaneous insertion of proteins into the outer envelope and thylakoid, the role of cubic lipid structures in targeting and assembly of proteins to the prolamellar body, and the repair process of D1 after photoinhibition. The current progress in the identification of the genes and their associated mutations in galactolipid biosynthesis is discussed. Finally, the potential role of plastid-derived tubules in facilitating macromolecular transport between plastids and other cellular organelles is discussed.

Protein translocation into and across the chloroplastic envelope membranes

Post-translational protein import into chloroplasts follows a common route characterised by the need for nucleoside-triphosphates at various steps and two distinct protein import machineries at the outer and inner envelope membrane, respectively. Several subunits of these complexes have been elucidated. In contrast, protein translocation into the chloroplastic outer envelope uses distinct and various but poorly characterised insertion pathways. A topological framework for single-membrane spanning proteins of the chloroplastic outer envelope is presented.

PROTEIN TARGETING TO THE THYLAKOID MEMBRANE

▪ Abstract  The assembly of the photosynthetic apparatus at the thylakoid begins with the targeting of proteins from their site of synthesis in the cytoplasm or stroma to the thylakoid membrane. Plastid-encoded proteins are targeted directly to the thylakoid during or after synthesis on plastid ribosomes. Nuclear-encoded proteins undergo a two-step targeting process requiring posttranslational import into the organelle from the cytoplasm and subsequent targeting to the thylakoid membrane. Recent investigations have revealed a single general import machinery at the envelope that mediates the direct transport of preproteins from the cytoplasm to the stroma. In contrast, at least four distinct pathways exist for the targeting of proteins to the thylakoid membrane. At least two of these systems are homologous to translocation systems that operate in bacteria and at the endoplasmic reticulum, indicating that elements of the targeting mechanisms have been conserved from the original prokaryotic endosymbiont.

Chloroplast biogenesis: mixing the prokaryotic and the eukaryotic?

Chloroplast biogenesis requires the translocation of proteins across the outer and inner envelopes. The membrane components of this transport machinery completely differ from those of other organelles, but recently homologues of some of the components have been detected in prokaryotes.

Analysis of the interactions of preproteins with the import machinery over the course of protein import into chloroplasts

We have investigated the interactions of two nuclear-encoded preproteins with the chloroplast protein import machinery at three stages in import using a label-transfer crosslinking approach. During energy-independent binding at the outer envelope membrane, preproteins interact with three known components of the outer membrane translocon complex, Toc34, Toc75, and Toc86. Although Toc75 and Toc86 are known to associate with preproteins during import, a role for Toc34 in preprotein binding previously had not been observed. The interaction of Toc34 with preproteins is regulated by the binding, but not hydrolysis of GTP. These data provide the first evidence for a direct role for Toc34 in import, and provide insights into the function of GTP as a regulator of preprotein recognition. Toc75 and Toc86 are the major targets of cross-linking upon insertion of preproteins across the outer envelope membrane, supporting the proposal that both proteins function in translocation at the outer membrane as well as preprotein recognition. The inner membrane proteins, Tic(21) and Tic22, and a previously unidentified protein of 14 kD are the major targets of crosslinking during the late stages in import. These data provide additional support for the roles of these components during protein translocation across the inner membrane. Our results suggest a defined sequence of molecular interactions that result in the transport of nuclear-encoded preproteins from the cytoplasm into the stroma of chloroplasts.

The chloroplastic protein import machinery contains a Rieske-type iron-sulfur cluster and a mononuclear iron-binding protein

Transport of precursor proteins across the chloroplastic envelope membranes requires the interaction of protein translocons localized in both the outer and inner envelope membranes. Analysis by blue native gel electrophoresis revealed that the translocon of the inner envelope membranes consisted of at least six proteins with molecular weights of 36, 45, 52, 60, 100 and 110 kDa, respectively. Tic110 and ClpC, identified as components of the protein import apparatus of the inner envelope membrane, were prominent constituents of this complex. The amino acid sequence of the 52 kDa protein, deduced from the cDNA, contains a predicted Rieske-type iron-sulfur cluster and a mononuclear iron-binding site. Diethylpyrocarbonate, a Rieske-type protein-modifying reagent, inhibits the translocation of precursor protein across the inner envelope membrane, whereas binding of the precursor to the outer envelope membrane is still possible. In another independent experimental approach, the 52 kDa protein could be co-purified with a trapped precursor protein in association with the chloroplast protein translocon subunits Toc86, Toc75, Toc34 and Tic110. Together, these results strongly suggest that the 52 kDa protein, named Tic55 due to its calculated molecular weight, is a member of the chloroplastic inner envelope protein translocon.

Reconstitution of a chloroplast protein import channel

The chloroplastic outer envelope protein OEP75 with a molecular weight of 75 kDa probably forms the central pore of the protein import machinery of the outer chloroplastic membrane. Patch-clamp analysis shows that heterologously expressed, purified and reconstituted OEP75 constitutes a voltage-gated ion channel with a unit conductance of Lambda = 145pS. Activation of the OEP75 channel in vitro is completely dependent on the magnitude and direction of the voltage gradient. Therefore, movements of protein charges of parts of OEP75 in the membrane electric field are required either for pore formation or its opening. In the presence of precursor protein from only one side of the bilayer, strong flickering and partial closing of the channel was observed, indicating a specific interaction of the precursor with OEP75. The comparatively low ionic conductance of OEP75 is compatible with a rather narrow aqueous pore (dporeapproximately equal to 8-9 A). Provided that protein and ion translocation occur through the same pore, this implies that the environment of the polypeptide during the transit is mainly hydrophilic and that protein translocation requires almost complete unfolding of the precursor.

Identification of protein transport complexes in the chloroplastic envelope membranes via chemical cross-linking

Transport of cytoplasmically synthesized proteins into chloroplasts uses an import machinery present in the envelope membranes. To identify the components of this machinery and to begin to examine how these components interact during transport, chemical cross-linking was performed on intact chloroplasts containing precursor proteins trapped at a particular stage of transport by ATP limitation. Large cross-linked complexes were observed using three different reversible homobifunctional cross-linkers. Three outer envelope membrane proteins (OEP86, OEP75, and OEP34) and one inner envelope membrane protein (IEP110), previously reported to be involved in protein import, were identified as components of these complexes. In addition to these membrane proteins, a stromal member of the hsp100 family, ClpC, was also present in the complexes. We propose that ClpC functions as a molecular chaperone, cooperating with other components to accomplish the transport of precursor proteins into chloroplasts. We also propose that each envelope membrane contains distinct translocation complexes and that a portion of these interact to form contact sites even in the absence of precursor proteins.

Stable association of chloroplastic precursors with protein translocation complexes that contain proteins from both envelope membranes and a stromal Hsp100 molecular chaperone

Cytoplasmically synthesized precursors interact with translocation components in both the outer and inner envelope membranes during transport into chloroplasts. Using co-immunoprecipitation techniques, with antibodies specific to known translocation components, we identified stable interactions between precursor proteins and their associated membrane translocation components in detergent-solubilized chloroplastic membrane fractions. Antibodies specific to the outer envelope translocation components OEP75 and OEP34, the inner envelope translocation component IEP110 and the stromal Hsp100, ClpC, specifically co-immunoprecipitated precursor proteins under limiting ATP conditions, a stage we have called docking. A portion of these same translocation components was co-immunoprecipitated as a complex, and could also be detected by co-sedimentation through a sucrose density gradient. ClpC was observed only in complexes with those precursors utilizing the general import apparatus, and its interaction with precursor-containing translocation complexes was destabilized by ATP. Finally, ClpC was co-immunoprecipitated with a portion of the translocation components of both outer and inner envelope membranes, even in the absence of added precursors. We discuss possible roles for stromal Hsp100 in protein import and mechanisms of precursor binding in chloroplasts.

Import of proteins into mitochondria and chloroplasts

Although mitochondria and chloroplasts synthesize some of their own proteins, they must import most of them from the cytosol. Import is mediated by molecular chaperones in the cytosol, receptors and channels in the organelle membranes and ATP-driven 'import motors' inside the organelles. Many of these components are now known, allowing informed guesses on how they might work.

Import inhibition of poly(His) containing chloroplast precursor proteins by Ni2+ ions

The precursor of the small subunit of ribulose-1,5-bisphosphate carboxylase (pSS) and a modified pSS containing a C-terminal hexahistidyl tail (pSS(His)6) were imported into isolated Chlamydomonas chloroplasts with comparable efficiency. In the presence of Ni2+ ions the import of pSS(His)6 was inhibited and the precursor bound to the envelope remained protease sensitive, while import of pSS was not affected. Addition of an excess of L-histidine suppressed the inhibition demonstrating that the hexahistidyl-Ni2+ complex was responsible for import inhibition. Inhibition could be observed between about 0.5 and 10 mM Ni2+, depending on the total protein content in the assay. Import incompetent Ni2+-precursor complexes can be used to study early events in chloroplast protein import.