The presequence of a chimeric construct dictates which of two mechanisms are utilized for translocation across the thylakoid membrane: evidence for the existence of two distinct translocation systems

Tat transport of a Sec passenger leads to both completely translocated as well as membrane-arrested passenger proteins

We have studied the membrane transport of the chimeric precursor protein 16/33, which is composed of the Tat(1)-specific transport signal of OEC16 and the Sec passenger protein OEC33, both subunits of the oxygen-evolving system associated with photosystem II. Protein transport experiments performed with isolated pea thylakoids show that the 16/33 chimera is transported in a strictly Tat-dependent manner into the thylakoid vesicles yielding mature OEC33 (mOEC33) in two different topologies. One fraction accumulates in the thylakoid lumen and is thus resistant to externally added protease. A second fraction is arrested during transport in an N-in/C-out topology within the membrane. Chase experiments demonstrate that this membrane-arrested mOEC33 moiety does not represent a translocation intermediate but instead an alternative end product of the transport process. Transport arrest of mOEC33, which is embedded in the membrane with a mildly hydrophobic protein segment, requires more than 26 additional and predominantly hydrophilic residues C-terminal of the membrane-embedded segment. Furthermore, it is stimulated by mutations which potentially affect the conformation of mOEC33 suggesting that at least partial folding of the passenger protein is required for complete membrane translocation.

The Tat system of Gram-positive bacteria

The twin-arginine protein translocation (Tat) system has a unique ability to translocate folded and co-factor-containing proteins across lipid bilayers. The Tat pathway is present in bacteria, archaea and in the thylakoid membranes of chloroplasts and, depending on the organism and environmental conditions, it can be deemed important for cell survival, virulence or bioproduction. This review provides an overview of the current understanding of the Tat system with specific focus on Gram-positive bacteria. The 'universal minimal Tat system' is composed of a TatA and a TatC protein. However, this pathway is more commonly composed of two TatA-like proteins and one TatC protein. Often the TatA-like proteins have diverged to have two different functions and, in this case, the second TatA-like protein is usually referred to as TatB. The correct folding and/or incorporation of co-factors are requirements for translocation, and the known quality control mechanisms are examined in this review. A number of examples of crosstalk between the Tat system and other protein transport systems, such as the Sec-YidC translocon and signal peptidases or sheddases are also discussed. Further, an overview of specific Gram-positive bacterial Tat systems found in monoderm and diderm species is detailed. Altogether, this review highlights the unique features of Gram-positive bacterial Tat systems and pinpoints key questions that remain to be addressed in future research. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

Tat pathway-mediated translocation of the sec pathway substrate protein MexA, an inner membrane component of the MexAB-OprM xenobiotic extrusion pump in Pseudomonas aeruginosa

Pseudomonas aeruginosa is equipped with the Sec and Tat protein secretion systems, which translocate the xenobiotic transporter MexAB-OprM and the pathogenic factor phospholipase C (PlcH), respectively. When the signal sequence of MexA was replaced with that of PlcH, the hybrid protein was successfully expressed and recovered from the periplasmic fraction, suggesting that the hybrid protein had been translocated across the inner membrane. MexA-deficient cells harboring the plasmid carrying the plcH-mexA fusion gene showed antibiotic resistance comparable to that of the wild-type cells. This result suggested that MexA secreted via the Tat machinery was properly assembled and functioned as a subunit of the MexAB-OprM efflux pump. A mutation was introduced into the chromosomal tatC gene encoding an inner membrane component of the Tat protein secretion machinery in mexA-deficient cells, and they were transformed with the plasmid carrying the plcH-mexA fusion gene. The transformants showed antibiotic susceptibility comparable to that of mexA-deficient cells, indicating that the hybrid protein was not transported to the periplasm. Whole-cell lysate of the mexA-tatC double mutant harboring the plcH-mexA plasmid produced mainly unprocessed PlcH-MexA. The periplasmic fraction showed no detectable anti-MexA antibody-reactive material. On the basis of these results, we concluded that MexA could be translocated across the inner membrane through the Tat pathway and assembled with its cognate partners, MexB and OprM, and that this complex machinery was fully functional. This hybrid protein translocation system has the potential to be a powerful screening tool for antimicrobial agents targeting the Tat system, which is not present in mammalian cells.

Polyphenol oxidase can cross thylakoids by both the Tat and the Sec-dependent pathways: a putative role for two stromal processing sites

Polyphenol oxidase (PPO; EC 1.10.3.2 or EC 1.14.18.1), a thylakoid-lumen protein encoded by a nuclear gene, plays a role in the defense of plants against both herbivores and pathogens. Although previously reported to be a Tat (twin-arginine-dependent translocation) protein, the import of PPO by isolated chloroplasts was inhibited by azide, a diagnostic inhibitor of the Sec-dependent pathway. Import of PPO inhibited thylakoid translocation of a Tat protein and did not affect translocation of Sec-dependent proteins. In contrast, a pre-accumulated iPPO competed with Sec-dependent but not with Tat proteins. A previously reported second processing step in the stroma removes a twin-Arg that is part of a 'Sec-avoidance' motif in the thylakoid targeting domain of PPO. When the second processing site was mutated, the import of the resulting precursor showed Sec-dependent characteristics. The PPO transit peptide could drive thylakoid translocation of a Tat protein in the dark. Azide inhibited the secretion of a PPO intermediate that lacks a twin-Arg to the periplasm of Escherichia coli, but had no effect on the export of the intermediate containing the twin-Arg. PPO is synthesized in plants in response to wound and pathogen-related signals and it is possible that when the Tat pathway is unable to translocate adequate amounts of newly synthesized PPO, translocation is diverted to the Sec-dependent pathway by processing the intermediate at the second site and removing the twin-Arg.

Sec- and Tat-mediated protein secretion across the bacterial cytoplasmic membrane--distinct translocases and mechanisms

In bacteria, two major pathways exist to secrete proteins across the cytoplasmic membrane. The general Secretion route, termed Sec-pathway, catalyzes the transmembrane translocation of proteins in their unfolded conformation, whereupon they fold into their native structure at the trans-side of the membrane. The Twin-arginine translocation pathway, termed Tat-pathway, catalyses the translocation of secretory proteins in their folded state. Although the targeting signals that direct secretory proteins to these pathways show a high degree of similarity, the translocation mechanisms and translocases involved are vastly different.

Unassisted membrane insertion as the initial step in DeltapH/Tat-dependent protein transport

In the thylakoid membrane of chloroplasts as well as in the cytoplasmic membrane of bacteria, the DeltapH/Tat-dependent protein transport pathway is responsible for the translocation of folded proteins. Using the chimeric 16/23 protein as model substrate in thylakoid transport experiments, we dissected the transport process into several distinct steps that are characterized by specific integral translocation intermediates. Formation of the early translocation intermediate Ti-1, which still exposes the N and the C terminus to the stroma, is observed with thylakoids pretreated with (i) solutions of chaotropic salts or alkaline pH, (ii) protease, or (iii) antibodies raised against TatA, TatB, or TatC. Membrane insertion takes place even into liposomes, demonstrating that proteinaceous components are not required. This suggests that Tat-dependent transport may be initiated by the unassisted insertion of the substrate into the lipid bilayer, and that interaction with the Tat translocase takes place only in later stages of the process.

Toc, Tic, Tat et al.: structure and function of protein transport machineries in chloroplasts

The chloroplast is an organelle of prokaryotic origin that is situated in an eukaryotic cellular environment. As a result of this formerly endosymbiotic situation, the chloroplast houses a unique set of protein transport machineries. Among those are evolutionarily young transport pathways which are responsible for the import of the nuclear-encoded proteins into the organelle as well as ancient pathways operating in the 'export' of proteins from the stroma (the former cyanobacterial cytosol) across the thylakoid membrane into the thylakoid lumen. In this review, we have tried to address the main features of these various transport pathways.

Rapid binding of copper(I) to folded aporusticyanin

Kinetics of copper uptake in both oxidation states by the folded and unfolded forms of the type 1 copper protein rusticyanin have been studied. The speed of the binding of copper(I) to the folded rusticyanin is fast, and of the same order of magnitude as copper(I) uptake by the unfolded form. Thus, the binding of copper can be subsequent to the protein folding, contrary to previous proposals. Implications for the mechanism of the formation of the active holoprotein in vivo are discussed.

Increase in xylanase production by Streptomyces lividans through simultaneous use of the Sec- and Tat-dependent protein export systems

Xylanase B1 (XlnB1) from Streptomyces lividans is a protein consisting of two discrete structural and functional units, an N-terminal catalytic domain and a C-terminal substrate binding domain. In the culture medium, two forms of xylanase B are present, namely, XlnB1 and XlnB2, the latter of which corresponds to the catalytic domain of XlnB1 deprived of the substrate binding domain. Both forms of the xylanase have the same activity on xylan. The enzyme is secreted through the Sec-dependent pathway with a better yield of XlnB1 than XlnB2. Interestingly, XlnB2 exhibits 80% identity with XlnC which is secreted exclusively through the Tat-dependent pathway. To demonstrate whether XlnB1 and XlnB2 could also be secreted through the Tat-dependent pathway, the Tat-targeting xlnC signal sequence was fused to the structural genes of xlnB1 and xlnB2. Both XlnB1 and XlnB2 were secreted through the Tat-dependent pathway, but XlnB2 was better produced than XlnB1. As XlnB1 and XlnB2 could be better secreted through the Sec- and Tat-dependent systems, respectively, a copy of the structural gene of xlnB1 fused to a Sec signal sequence and a copy of the structural gene of xlnB2 fused to a Tat signal sequence were inserted into the same plasmid under the control of the xlnA promoter. The transformant produced xylanase activity which corresponded approximately to the sum of activities of the individual strain producing xylanase by either the Sec- or Tat-dependent secretion system. This indicated that both secretion systems are functional and independent of each other in the recombinant strain. This is the first report on the efficient secretion of a protein using two different secretion systems at the same time. Assuming that the protein to be secreted could be properly folded prior to and after translocation via the Tat- and Sec-dependent pathways, respectively, the simultaneous use of the Sec- and Tat-dependent pathways provides an efficient means to increase the production of a given protein.

Mechanisms of protein import and routing in chloroplasts

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

Targeting of EGFP chimeras within chloroplasts

We have tested the potential of EGFP, a derivative of the green fluorescent protein (GFP), as a passenger protein for the analysis of protein transport processes across the thylakoid membranes in chloroplasts. In contrast to the majority of fusion proteins commonly used in such studies, EGFP is not of plant origin and can therefore be assumed to behave like a "neutral" passenger protein that is unaffected by any internal plant regulatory circuits. Our in vitro transport experiments clearly demonstrate that EGFP is a suitable passenger protein that can be correctly targeted either to the stroma or to the thylakoid lumen if fused to the appropriate transit peptide. The transport of EGFP across the thylakoid membrane shows, however, a clear pathway preference. While the protein is efficiently targeted by the deltapH/TAT pathway, transport by the Sec pathway is barely detectable, either with isolated thylakoids or with intact chloroplasts. This pathway specificity suggests that EGFP is folded immediately after import into the chloroplast stroma, thus preventing further translocation across the thylakoid membrane by the Sec translocase. The data obtained provide a good basis for the development of molecular tools for transport studies using EGFP as a passenger protein. Furthermore, plant lines expressing corresponding EGFP chimeras are expected to allow in vivo studies on the transport and sorting mechanisms involved in the biogenesis of the chloroplast.

Protein transport into secondary plastids and the evolution of primary and secondary plastids

Chloroplasts are key organelles in algae and plants due to their photosynthetic abilities. They are thought to have evolved from prokaryotic cyanobacteria taken up by a eukaryotic host cell in a process termed primary endocytobiosis. In addition, a variety of organisms have evolved by subsequent secondary endocytobioses, in which a heterotrophic host cell engulfed a eukaryotic alga. Both processes dramatically enhanced the complexity of the resulting cells. Since the first version of the endosymbiotic theory was proposed more than 100 years ago, morphological, physiological, biochemical, and molecular data have been collected substantiating the emerging picture about the origin and the relationship of individual organisms with different primary or secondary chloroplast types. Depending on their origin, plastids in different lineages may have two, three, or four envelope membranes. The evolutionary success of endocytobioses depends, among other factors, on the specific exchange of molecules between the host and endosymbiont. This raises questions concerning how targeting of nucleus-encoded proteins into the different plastid types occurs and how these processes may have developed. Most studies of protein translocation into plastids have been performed on primary plastids, but in recent years more complex protein-translocation systems of secondary plastids have been investigated. Analyses of transport systems in different algal lineages with secondary plastids reveal that during evolution existing translocation machineries were recycled or recombined rather than being developed de novo. This review deals with current knowledge about the evolution and function of primary and secondary plastids and the respective protein-targeting systems.

Post-translational protein translocation into thylakoids by the Sec and DeltapH-dependent pathways

Two distinct protein translocation pathways that employ hydrophobic signal peptides function in the plant thylakoid membrane. These two systems are precursor specific and distinguished by their energy and component requirements. Recent studies have shown that one pathway is homologous to the bacterial general export system called Sec. The other one, called the DeltapH-dependent pathway, was originally considered to be unique to plant thylakoids. However, it is now known that homologous transport systems are widely present in prokaryotes and even present in archaea. Here we review these protein transport pathways and discuss their capabilities and mechanisms of operation.

Twin-arginine translocation pathway in Streptomyces lividans

The recently discovered bacterial twin-arginine translocation (Tat) pathway was investigated in Streptomyces lividans, a gram-positive organism with a high secretion capacity. The presence of one tatC and two hcf106 homologs in the S. lividans genome together with the several precursor proteins with a twin-arginine motif in their signal peptide suggested the presence of the twin-arginine translocation pathway in the S. lividans secretome. To demonstrate its functionality, a tatC deletion mutant was constructed. This mutation impaired the translocation of the Streptomyces antibioticus tyrosinase, a protein that forms a complex with its transactivator protein before export. Also the chimeric construct pre-TorA-23K, known to be exclusively secreted via the Tat pathway in Escherichia coli, could be translocated in wild-type S. lividans but not in the tatC mutant. In contrast, the secretion of the Sec-dependent S. lividans subtilisin inhibitor was not affected. This study therefore demonstrates that also in general in Streptomyces spp. the Tat pathway is functional. Moreover, this Tat pathway can translocate folded proteins, and the E. coli TorA signal peptide can direct Tat-dependent transport in S. lividans.

Protein targeting by the twin-arginine translocation pathway

The twin-arginine translocation pathway operates in the thylakoid membrane of chloroplasts and in the plasma membrane of most free-living bacteria. Its main function is to transport fully folded proteins across the membrane. Three important tat genes have been identified and the sequences of the encoded proteins, together with the unusual properties of the pathway, indicate that the Tat system is completely different from other protein translocases.

Multiple pathways used for the targeting of thylakoid proteins in chloroplasts

The assembly of the chloroplast thylakoid membrane requires the import of numerous proteins from the cytosol and their targeting into or across the thylakoid membrane. It is now clear that multiple pathways are involved in the thylakoid-targeting stages, depending on the type of protein substrate. Two very different pathways are used by thylakoid lumen proteins; one is the Sec pathway which has been well-characterised in bacteria, and which involves the threading of the substrate through a narrow channel. In contrast, the more recently characterised twin-arginine translocation (Tat) system is able to translocate fully folded proteins across this membrane. Recent advances on bacterial Tat systems shed further light on the structure and function of this system. Membrane proteins, on the other hand, use two further pathways. One is the signal recognition particle-dependent pathway, involving a complex interplay between many different factors, whereas other proteins insert without the assistance of any known apparatus. This article reviews advances in the study of these pathways and considers the rationale behind the surprising complexity.

Diatom fucoxanthin chlorophyll a/c-binding protein (FCP) and land plant light-harvesting proteins use a similar pathway for thylakoid membrane Insertion

The light-harvesting proteins in plastids of different lineages including algae and land plants represent a superfamily of chlorophyll-binding proteins that seem to be phylogenetically related, although some of the light-harvesting complex (LHC) proteins bind different carotenoids. LHCs can be divided into chlorophyll a/b-binding proteins found in green algae, euglenoids, and higher plants and into chlorophyll a/c-binding proteins of various algal taxa. LHC proteins from diatoms are named fucoxanthin-chlorophyll a/c-binding proteins (FCP). In contrast to chlorophyll a/b-binding proteins, there is no information so far about the way FCPs integrate into thylakoid membranes. The diatom FCP preproteins have a bipartite presequence that is necessary to enable transport into the four membrane-bound diatom plastids, but similar to chlorophyll a/b-binding proteins there is apparently no presequence present for targeting to the thylakoid membrane. By establishing an in vitro import assay for diatom thylakoids, we demonstrated that thylakoid integration of diatom FCP depends on the presence of stromal factors and GTP. This indicates that a pathway involving signal recognition particles (SRP) is involved in membrane integration just as shown for LHCs in higher plants. We also demonstrate integration of diatom FCP into thylakoids of higher plants and vice versa SRP-dependent targeting of LHCs from pea and Arabidopsis into diatom thylakoids. The similar SRP-dependent modes of thylakoid integration of land plant LHCs and FCPs support recent analyses indicating a common origin of chlorophyll a/b- and a/c-binding proteins.

TatC is a specificity determinant for protein secretion via the twin-arginine translocation pathway

The recent discovery of a ubiquitous translocation pathway, specifically required for proteins with a twin-arginine motif in their signal peptide, has focused interest on its membrane-bound components, one of which is known as TatC. Unlike most organisms of which the genome has been sequenced completely, the Gram-positive eubacterium Bacillus subtilis contains two tatC-like genes denoted tatCd and tatCy. The corresponding TatCd and TatCy proteins have the potential to be involved in the translocation of 27 proteins with putative twin-arginine signal peptides of which approximately 6-14 are likely to be secreted into the growth medium. Using a proteomic approach, we show that PhoD of B. subtilis, a phosphodiesterase belonging to a novel protein family of which all known members are synthesized with typical twin-arginine signal peptides, is secreted via the twin-arginine translocation pathway. Strikingly, TatCd is of major importance for the secretion of PhoD, whereas TatCy is not required for this process. Thus, TatC appears to be a specificity determinant for protein secretion via the Tat pathway. Based on our observations, we hypothesize that the TatC-determined pathway specificity is based on specific interactions between TatC-like proteins and other pathway components, such as TatA, of which three paralogues are present in B. subtilis.

Signal peptide-dependent protein transport in Bacillus subtilis: a genome-based survey of the secretome

One of the most salient features of Bacillus subtilis and related bacilli is their natural capacity to secrete a variety of proteins into their environment, frequently to high concentrations. This has led to the commercial exploitation of bacilli as major "cell factories" for secreted enzymes. The recent sequencing of the genome of B. subtilis has provided major new impulse for analysis of the molecular mechanisms underlying protein secretion by this organism. Most importantly, the genome sequence has allowed predictions about the composition of the secretome, which includes both the pathways for protein transport and the secreted proteins. The present survey of the secretome describes four distinct pathways for protein export from the cytoplasm and approximately 300 proteins with the potential to be exported. By far the largest number of exported proteins are predicted to follow the major "Sec" pathway for protein secretion. In contrast, the twin-arginine translocation "Tat" pathway, a type IV prepilin-like export pathway for competence development, and ATP-binding cassette transporters can be regarded as "special-purpose" pathways, through which only a few proteins are transported. The properties of distinct classes of amino-terminal signal peptides, directing proteins into the various protein transport pathways, as well as the major components of each pathway are discussed. The predictions and comparisons in this review pinpoint important differences as well as similarities between protein transport systems in B. subtilis and other well-studied organisms, such as Escherichia coli and the yeast Saccharomyces cerevisiae. Thus, they may serve as a lead for future research and applications.

The twin arginine consensus motif of Tat signal peptides is involved in Sec-independent protein targeting in Escherichia coli

In Escherichia coli a subset of periplasmic proteins is exported through the Tat pathway to which substrates are directed by an NH(2)-terminal signal peptide containing a consensus SRRXFLK "twin arginine" motif. The importance of the individual amino acids of the consensus motif for in vivo Tat transport has been assessed by site-directed mutagenesis of the signal peptide of the Tat substrate pre-SufI. Although the invariant arginine residues are crucial for efficient export, we find that slow transport of SufI is still possible if a single arginine is conservatively substituted by a lysine residue. Thus, in at least one signal peptide context there is no absolute dependence of Tat transport on the arginine pair. The consensus phenylalanine residue was found to be a critical determinant for efficient export but could be functionally substituted by leucine, another amino acid with a highly hydrophobic side chain. Unexpectedly, the consensus lysine residue was found to retard Tat transport. These observations and others suggest that the sequence conservation of the Tat consensus motif is a reflection of the functional importance of the consensus residues. Tat signal peptides characteristically have positively charged carboxyl-terminal regions. However, changing the sign of this charge does not affect export of SufI.

Different lumen-targeting pathways for nuclear-encoded versus cyanobacterial/plastid-encoded Hcf136 proteins

Lumenal proteins are transported across the thylakoid membrane by two very different pathways: Sec-dependent or twin-arginine translocase (Tat)-dependent, where the substrate protein can be transported in a folded state. We present the first evidence that a given protein can be targeted by different pathways in different organisms. Arabidopsis Hcf136 is targeted exclusively by the Tat pathway in pea chloroplasts and no Sec-dependent transport is evident even when the twin-arginine is replaced by twin-lysine. However, twin-arginine motifs are absent from the presequences of Hcf136 proteins encoded by plastid or cyanobacterial genomes, strongly implying translocation by another pathway (presumably Sec). We suggest that the Hcf136 protein was transferred to the Tat pathway when the gene became incorporated into the nuclear genome, possibly due to the tighter folding associated with the more involved, post-translational targeting pathway.

The twin-arginine translocation system: a novel means of transporting folded proteins in chloroplasts and bacteria

Protein translocases have been characterised in several membrane systems and the translocation mechanisms have been shown to differ in critical respects. Nevertheless, the majority were believed to transport proteins only in a largely unfolded state, and this widespread characteristic was viewed as a likely evolutionary effort to minimise the diameter of translocation pore required. Within the last few years, however, studies on the chloroplast thylakoid membrane have revealed a novel class of protein translocase which possesses the apparently unique ability to transport fully-folded proteins across a tightly sealed energy-transducing membrane. A related system, (the twin-arginine translocation, or Tat system) has now been characterised in the Escherichia coli plasma membrane and considerations of its substrate specificity again point to its involvement in the transport of folded proteins. The emerging data suggest a critical involvement in many membranes for the biogenesis of two types of globular protein: those that are obliged to fold prior to translocation, and those that fold too tightly or rapidly for other types of protein translocase to handle.

Transport of proteins into and across the thylakoid membrane

The biogenesis of thylakoid proteins is a complex issue that requires the operation of at least four pathways within the chloroplast. Two of the pathways are used for soluble lumenal proteins, where the proteins bear cleavable targeting signals that are recognized by one of two distinct translocases. These pathways differ in fundamental respects. A subset of lumenal proteins are transported in an unfolded state by a typical Sec system, whereas others are transported by a novel class of translocase that appears to function primarily in the transport of fully-folded proteins. Related protein translocases have now been shown to operate in a wide variety of bacterial species, suggesting a widespread requirement for the translocation of folded proteins across biological membranes. Numerous integral membrane proteins are also targeted into the thylakoid membrane, and these too follow at least two distinct routes. Some proteins use a signal recognition particle-dependent pathway that requires GTP and unidentified apparatus in the thylakoid membrane. Others, however, require none of the known targeting factors and may insert spontaneously into the membrane. In this article, the rationale behind this pathway complexity is discussed in relation to the properties of the substrate proteins and the evolutionary origins of the chloroplast.

The Tat protein export pathway

The Tat (twin-arginine translocation) system is a bacterial protein export pathway with the remarkable ability to transport folded proteins across the cytoplasmic membrane. Preproteins are directed to the Tat pathway by signal peptides that bear a characteristic sequence motif, which includes consecutive arginine residues. Here, we review recent progress on the characterization of the Tat system and critically discuss the structure and operation of this major new bacterial protein export pathway.

Chloroplast SecY is complexed to SecE and involved in the translocation of the 33-kDa but not the 23-kDa subunit of the oxygen-evolving complex

SecY is a component of the protein-conducting channel for protein transport across the cytoplasmic membrane of prokaryotes. It is intimately associated with a second integral membrane protein, SecE, and together with SecA forms the minimal core of the preprotein translocase. A chloroplast homologue of SecY (cpSecY) has previously been identified and determined to be localized to the thylakoid membrane. In the present work, we demonstrate that a SecE homologue is localized to the thylakoid membrane, where it forms a complex with cpSecY. Digitonin solubilization of thylakoid membranes releases the SecY/E complex in a 180-kDa form, indicating that other components are present and/or the complex is a higher order oligomer of the cpSecY/E dimer. To test whether cpSecY forms the protein-conducting channel of the thylakoid membrane, translocation assays were conducted with the SecA-dependent substrate OE33 and the SecA-independent substrate OE23, in the presence and absence of antibodies raised against cpSecY. The antibodies inhibited translocation of OE33 but not OE23, indicating that cpSecY comprises the protein-conducting channel used in the SecA-dependent pathway, whereas a distinct protein conducting channel is used to translocate OE23.

Protein translocation into and across the bacterial plasma membrane and the plant thylakoid membrane

Over the past decade, some familiar themes have emerged on how proteins are inserted into or translocated across the plant chloroplast thylakoid membrane and bacterial inner membranes. In the SecA and signal recognition particle (SRP) pathways, nucleotides and soluble factors are used to translocate proteins across the membrane bilayer in the unfolded state. However, the delta pH-dependent pathway in thylakoids uses a radically different mechanism: transport of proteins across the membrane is driven by the transmembrane pH gradient, and neither stromal factors nor nucleotide triphosphates are needed. In addition, this pathway, which requires the membrane-bound protein Hcf106, appears to translocate proteins in a tightly folded form. Recently, a similar pathway has been shown to operate in eubacteria, and several of its components have been identified.

The sec-independent twin-arginine translocation system can transport both tightly folded and malfolded proteins across the thylakoid membrane

A subset of lumen proteins is transported across the thylakoid membrane by a Sec-independent translocase that recognizes a twin-arginine motif in the targeting signal. A related system operates in bacteria, apparently for the export of redox cofactor-containing proteins. In this report we describe a key feature of this system, the ability to transport folded proteins. The thylakoidal system is able to transport dihydrofolate reductase (DHFR) when an appropriate signal is attached, and the transport efficiency is almost undiminished by the binding of folate analogs such as methotrexate that cause the protein to fold very tightly. The system is moreover able to transport DHFR into the lumen with methotrexate bound in the active site, demonstrating that the DeltapH-driven transport of large, native structures is possible by this pathway. However, correct folding is not a prerequisite for transport. Truncated, malfolded DHFR can be translocated by this system, as can physiological substrates that are severely malfolded by the incorporation of amino acid analogs.

Targeting of thylakoid proteins by the delta pH-driven twin-arginine translocation pathway requires a specific signal in the hydrophobic domain in conjunction with the twin-arginine motif

Superficially similar cleavable targeting signals specify whether lumenal proteins are transported across the thylakoid membrane by a Sec- or delta pH-dependent pathway. A twin-arginine motif is essential but not sufficient to direct delta pH-dependent targeting; here we show that a second determinant is located in the hydrophobic region. A highly hydrophobic amino acid is found either two or three residues C-terminal to the twin-arginine in all known transfer peptides for the delta pH-dependent system, and substitution of this residue in the 23-kDa (23K) peptide markedly inhibits translocation. Further, whereas the insertion of twin-arginine in a Sec-dependent precursor does not permit efficient delta pH-dependent targeting, the simultaneous presence of a leucine at the +3 position (relative to the RR) enables the peptide to function as efficiently as an authentic transfer peptide. RRNVL, RRAAL and RRALA within a Sec targeting signal all support efficient delta pH-dependent targeting, RRNVA is less effective and RRNAA/RRNAG are totally ineffective. We conclude that the core signal for this pathway is a twin-arginine together with an adjacent hydrophobic determinant.

Multiple pathways for the targeting of thylakoid proteins in chloroplasts

The assembly of the photosynthetic apparatus requires the import of numerous cytosolically synthesised proteins and their correct targeting into or across the thylakoid membrane. Biochemical and genetic studies have revealed the operation of several targeting pathways for these proteins, some of which are used for thylakoid lumen proteins whereas others are utilised by membrane proteins. Some pathways can be traced back to the prokaryotic ancestors of chloroplasts but at least one pathway appears to have arisen in response to the transfer of genes from the organelle to the nucleus. In this article we review recent findings in this field that point to the operation of a mechanistically unique protein translocase in both plastids and bacteria, and we discuss emerging data that reconcile the remarkable variety of targeting pathways with the natures of the substrate precursor proteins.

Sec-independent insertion of thylakoid membrane proteins. Analysis of insertion forces and identification of a loop intermediate involving the signal peptide

A group of membrane proteins are synthesized with cleavable signal sequences but inserted into the thylakoid membrane by an unusual Sec/SRP-independent mechanism. In this report we describe a key intermediate in the insertion of one such protein, photosystem II subunit W (PSII-W). A single mutation in the terminal cleavage site partially blocks processing and leads to the formation of an intermediate-size protein in the thylakoid membrane during chloroplast import assays. This protein is in the form of a loop structure: the N and C termini are exposed on the stromal face, whereas the cleavage site has been translocated into the lumen. In this respect the insertion of this protein resembles that of M13 procoat, which also adopts a loop structure during insertion, and we present preliminary evidence that a similar mechanism is used by another thylakoid protein, PSII-X. However, whereas the negatively charged region of procoat is translocated by an apparently electrophoretic mechanism using the DeltamuH+, the corresponding region of PSII-W is equally acidic but insertion is DeltamuH+ independent. We furthermore show that neutralization of this region has no apparent effect on the insertion process. We propose that a central element in this insertion mechanism is a loop structure whose formation is driven by hydrophobic interactions.

Targeting signals for a bacterial Sec-independent export system direct plant thylakoid import by the delta pH pathway

Preproteins targeted to the Sec-independent protein transport systems of plant thylakoids and of bacteria both have unusual transfer peptides bearing a consensus twin-arginine motif. Possible mechanistic similarity between the two Sec-independent transport pathways was investigated by assessing the ability of bacterial twin-arginine transfer peptides to direct thylakoid import. High efficiency import was observed. This process was demonstrated to occur specifically via the Sec-independent deltapH pathway and to depend on an intact twin-arginine motif on the transfer peptide. These results provide strong evidence for the operation of mechanistically related Sec-independent protein transport pathways in chloroplasts and bacteria.

An essential component of a novel bacterial protein export system with homologues in plastids and mitochondria

Proteins are transported across the bacterial plasma membrane and the chloroplast thylakoid membrane by means of protein translocases that recognize N-terminal targeting signals in their cognate substrates. Transport of many of these proteins involves the well defined Sec apparatus that operates in both membranes. We describe here the identification of a novel component of a bacterial Sec-independent translocase. The system probably functions in a similar manner to a Sec-independent translocase in the thylakoid membrane, and substrates for both systems bear a characteristic twin-arginine motif in the targeting peptide. The translocase component is encoded in Escherichia coli by an unassigned reading frame, yigU, disruption of which blocks the export of at least five twin-Arg-containing precursor proteins that are predicted to bind redox cofactors, and hence fold, prior to translocation. The Sec pathway remains unaffected in the deletion strain. The gene has been designated tatC (for twin-arginine translocation), and we show that homologous genes are present in a range of bacteria, plastids, and mitochondria. These findings suggest a central role for TatC-type proteins in the translocation of tightly folded proteins across a spectrum of biological membranes.

Tip loci: six Chlamydomonas nuclear suppressors that permit the translocation of proteins with mutant thylakoid signal sequences

Mutations within the signal sequence of cytochrome f (cytf) in Chlamydomonas inhibit thylakoid membrane protein translocation and render cells nonphotosynthetic. Twenty-seven suppressors of the mutant signal sequences were selected for their ability to restore photoautotrophic growth and these describe six nuclear loci named tip1 through 6 for thylakoid insertion protein. The tip mutations restore the translocation of cytf and are not allele specific, as they suppress a number of different cytf signal sequence mutations. Tip5 and 2 may act early in cytf translocation, while Tip1, 3, 4, and 6 are engaged later. The tip mutations have no phenotype in the absence of a signal sequence mutation and there is genetic interaction between tip4, and tip5 suggesting an interaction of their encoded proteins. As there is overlap in the energetic, biochemical and genetic requirements for the translocation of nuclear and chloroplast-encoded thylakoid proteins, the tip mutations likely identify components of a general thylakoid protein translocation apparatus.

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.

Characterisation of an Arabidopsis cDNA encoding a thylakoid lumen protein related to a novel 'pentapeptide repeat' family of proteins

We have cloned an Arabidopsis cDNA encoding a novel thylakoid lumen protein, P17.4, that has been previously isolated from lumen extracts of spinach chloroplasts. The protein is synthesised with a bipartite presequence containing a Sec-type lumen-targeting signal peptide and the precursor protein is imported into the lumen of pea chloroplasts. The encoded protein is homologous to an Anabaena protein that is essential for correct glycolipid localisation, and is also related to at least 16 unassigned open reading frames in Synechocystis. This family of proteins is characterised by the presence of numerous pentapeptide repeats with the consensus structure AXLXX, and its members are predicted to be located in the cytosol, plasma membrane and periplasm/lumen. P17.4 is therefore the first higher plant member of an extended family of putative cyanobacterial proteins that may serve important roles in lipid transport or assembly.

Sec/SRP-independent insertion of two thylakoid membrane proteins bearing cleavable signal peptides

Two imported thylakoid membrane proteins, PSII-X and PSII-W, are synthesised with cleavable N-terminal signal peptides that closely resemble those of Sec-dependent lumenal proteins. In this report we have reconstituted the insertion of pre-PSII-X and pre-PSII-W into isolated thylakoids. We show that insertion does not require either nucleoside triphosphates or stromal extracts, both of which are required for Sec- and signal recognition particle (SRP)-dependent targeting mechanisms. Insertion is furthermore unaffected by protease treatments that destroy the known protein translocation apparatus in the thylakoid membrane. We conclude that these membrane proteins are inserted by an unusual Sec/SRP-independent mechanism that probably resembles that used by CFoII, and we discuss possible parallels with the biogenesis of phage M13 procoat.

An Arabidopsis cDNA encodes an apparent polyprotein of two non-identical thylakoid membrane proteins that are associated with photosystem II and homologous to algal ycf32 open reading frames

We have characterised an Arabidopsis thaliana cDNA homologous to the ycf32 open reading frames present in the Synechocystis genome and the plastid genomes of several eukaryotic algae. The predicted protein is also homologous to a novel protein reported to be associated with photosystem II. The protein is synthesised as a 23 kDa precursor with an N-terminal presequence that appears to be bipartite in structure, and the protein is targeted into the thylakoid membrane of pea chloroplasts. Although the Ycf32 presequence contains an apparent signal peptide, we find that this protein is not imported by either of the standard Sec- or deltapH-dependent pathways. The mature protein is also unusual in two respects. First, there are two distinct, non-identical copies of typical single-span Ycf32 sequences in the Arabidopsis sequence, separated by an additional hydrophobic region. Secondly, the imported protein runs as a doublet of 6 kDa and 7 kDa polypeptides whereas the mature protein is predicted to be 14 kDa. We speculate that the protein undergoes further maturation once inserted into the thylakoid membrane to yield two separate Ycf32-like polypeptides.

Identification of cDNAS encoding plastid-targeted glutathione peroxidase

A cDNA was isolated from pea leaf RNA which encodes a phospholipid hydroperoxide glutathione peroxidase (PHGPX; E.C. 1.1.1.1.9). The N-terminal section of this PHGPX encodes a recognisable chloroplast transit peptide. Efficient import in vitro of the pre-PHGPX protein into the stroma of isolated pea chloroplasts confirmed that the PHGPX is a chloroplast-located enzyme. The pea PHGPX has highly conserved homologues in Arabidopsis, citrus and Nicotiana sylvestris and the authors suggest that these proteins are also localised in the chloroplast and not in the cytosol as previously supposed.

Transport of CtpA protein from the cyanobacterium Synechocystis 6803 across the thylakoid membrane in chloroplasts

The CtpA protein in the cyanobacterium Synechocystis 6803 is a C-terminal processing protease that is essential for the assembly of the manganese cluster of the photosystem II complex. When fused to different chloroplast-targeting transit peptides, CtpA can be imported into isolated spinach chloroplasts and is subsequently translocated into the thylakoid lumen. Thylakoid transport is mediated by the cyanobacterial signal peptide which demonstrates that the protein transport machinery in thylakoid membranes is functionally conserved between chloroplasts and cyanobacteria. Transport of CtpA across spinach thylakoid membranes is affected by both nigericin and sodium azide indicating that the SecA protein and a transthylakoidal proton gradient are involved in this process. Saturation of the Sec-dependent thylakoid transport route by high concentrations of the precursor of the 33-kDa subunit of the oxygen-evolving system leads to a strongly reduced rate of thylakoid translocation of CtpA which demonstrates transport by the Sec pathway. However, thylakoid transport of CtpA is affected also by excess amounts of the 23-kDa subunit of the oxygen-evolving system, though to a lesser extent. This suggests that the cyanobacterial protein is capable of also interacing with components of the deltapH-dependent route and that transport of a protein across the thylakoid membrane may not always be restricted to a single pathway.

Alterations in the Chlamydomonas plastocyanin transit peptide have distinct effects on in vitro import and in vivo protein accumulation

Nucleus-encoded chloroplast proteins that reside in the thylakoid lumen are synthesized as precursors with bipartite transit peptides that contain information for uptake and intra-chloroplast localization. We have begun to apply the superb molecular and genetic attributes of Chlamydomonas to study chloroplast protein import by creating a series of deletions in the transit peptide of plastocyanin and determining their effects on translocation into isolated Chlamydomonas chloroplasts. Most N-terminal mutations dramatically inhibited in vitro import, whereas replacement with a transit peptide from the gamma-subunit of chloroplast ATPase restored uptake. Thus, the N-terminal region has stroma-targeting function. Deletions within the C-terminal portion of the transit peptide resulted in the appearance of import intermediates, suggesting that this region is required for lumen translocation and processing. Thus, despite its short length and predicted structural differences, the Chlamydomonas plastocyanin transit peptide has functional domains similar to those of vascular plants. Similar mutations have been analyzed in vivo by transforming altered genes into a mutant defective at the plastocyanin locus (K. L. Kindle, manuscript in preparation). Most mutations affected in vitro import more severely than plastocyanin accumulation in vivo. One exception was a deletion that removed residues 2-8, which nearly eliminated in vivo accumulation but had a modest effect in vitro. We suggest that this mutant precursor may not compete successfully with other proteins for the translocation pathway in vivo. Apparently, in vivo and in vitro analyses reveal different aspects of chloroplast protein biogenesis.

A nuclear-coded chloroplastic inner envelope membrane protein uses a soluble sorting intermediate upon import into the organelle

The chloroplastic inner envelope protein of 110 kD (IEP110) is part of the protein import machinery in the pea. Different hybrid proteins were constructed to assess the import and sorting pathway of IEP110. The IEP110 precursor (pIEP110) uses the general import pathway into chloroplasts, as shown by the mutual exchange of presequences with the precursor of the small subunit of ribulose-1,5-bisphosphate carboxylase (pSSU). Sorting information to the chloroplastic inner envelope is contained in an NH2-proximal part of mature IEP110 (110N). The NH2-terminus serves to anchor the protein into the membrane. Large COOH-terminal portions of this protein (80-90 kD) are exposed to the intermembrane space in situ. Successful sorting and integration of IEP110 and the derived constructs into the inner envelope are demonstrated by the inaccessability of processed mature protein to the protease thermolysin but accessibility to trypsin, i.e., the imported protein is exposed to the intermembrane space. A hybrid protein consisting of the transit sequence of SSU, the NH2-proximal part of mature IEP110, and mature SSU (tpSSU-110N-mSSU) is completely imported into the chloroplast stroma, from which it can be recovered as soluble, terminally processed 110NmSSU. The soluble 110N-mSSU then enters a reexport pathway, which results not only in the insertion of 110N-mSSU into the inner envelope membrane, but also in the extrusion of large portions of the protein into the intermembrane space. We conclude that chloroplasts possess a protein reexport machinery for IEPs in which soluble stromal components interact with a membrane-localized translocation machinery.

Chloroplast SRP54 interacts with a specific subset of thylakoid precursor proteins

Signal recognition particles (SRPs) have been identified in organisms as diverse as mycoplasma and mammals; in several cases these SRPs have been shown to play a key role in protein targeting. In each case the recognition of appropriate targeting signals is mediated by SRP subunits related to the 54-kDa protein of mammalian SRP (SRP54). In this study we have characterized the specificity of 54CP, a chloroplast homologue of SRP54 which is located in the chloroplast stroma. We have used a nascent chain cross-linking approach to detect the interactions of 54CP with heterologous endoplasmic reticulum-targeting signals. 54CP functions as a bona fide signal recognition factor which can discriminate between functional and non-functional targeting signals. Using a range of authentic thylakoid precursor proteins we found that 54CP discriminates between thylakoid-targeting signals, interacting with only a subset of protein precursors. Thus, the light-harvesting chlorophyll a/b-binding protein, cytochrome f, and the Rieske FeS protein all showed strong cross-linking products with 54CP. In contrast, no cross-linking to the 23- and 33-kDa proteins of the oxygen-evolving complex were detected. The selectivity of 54CP correlates with the hydrophobicity of the thylakoid-targeting signal and, in the case of light-harvesting chlorophyll a/b-binding protein, with previously determined transport/integration requirements. We propose that 54CP mediates the targeting of a specific subset of precursors to the thylakoid membrane, i.e. those with particularly hydrophobic signal sequences.

Unusual characteristics of amino-terminal and hydrophobic domains in nuclear-encoded thylakoid signal peptides

Thylakoid transfer signals carry information specifying translocation by either a Sec- or delta pH-dependent protein translocator in the chloroplast thylakoid membrane, yet all resemble classical signal peptides in overall structural terms. Comparison of known transfer signals reveals two differences: (a) signals for the delta pH-driven system invariably contain a critical twin-arginine (Arg-Arg) motif prior to the hydrophobic (H) domain, whereas known Sec-dependent signals contain lysine, and (b) the H-domains of Sec-dependent signals are generally longer. Previous work has shown that a twin-Arg motif before the H-domain is critical for targeting by the delta pH-dependent pathway; in this report we show that the charge characteristics of this region are not important for sorting by the Sec pathway. Twin-Lys, twin-Arg or single Arg are all acceptable to the Sec system, although single Lys/Arg is preferred. The single Lys in pre-plastocyanin can even be replaced by an uncharged residue without apparent effect. We have also generated a pre-plastocyanin mutant containing an H-domain which, in terms of hydropathy profile, is identical to that of a delta pH-dependent protein. This mutant is also transported efficiently by the Sec system, demonstrating that hydrophobicity per se is not a key sorting determinant. However, the characteristics of the H-domain may be important in avoiding a different form of mis-targeting: to the endoplasmic reticulum. Thylakoid signal peptides have undergone substantial structural changes during the evolution of the chloroplast from endosymbiotic cyanobacterium: plastid-encoded and cyanobacterial signals contain H-domains that are highly hydrophobic and enriched in Leu and aromatic residues, whereas nuclear-encoded counterparts are Ala-rich and far less hydrophobic. We speculate that this trend may reflect a need to avoid mistargeting through recognition by cytosolic signal recognition particle, which preferentially interacts with more hydrophobic signal peptides.