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

Assembly pathway of a bacterial complex iron sulfur molybdoenzyme

Protein folding and assembly into macromolecule complexes within the living cell are complex processes requiring intimate coordination. The biogenesis of complex iron sulfur molybdoenzymes (CISM) requires use of a system specific chaperone - a redox enzyme maturation protein (REMP) - to help mediate final folding and assembly. The CISM dimethyl sulfoxide (DMSO) reductase is a bacterial oxidoreductase that utilizes DMSO as a final electron acceptor for anaerobic respiration. The REMP DmsD strongly interacts with DMSO reductase to facilitate folding, cofactor-insertion, subunit assembly and targeting of the multi-subunit enzyme prior to membrane translocation and final assembly and maturation into a bioenergetic catalytic unit. In this article, we discuss the biogenesis of DMSO reductase as an example of the participant network for bacterial CISM maturation pathways.

How to achieve Tat transport with alien TatA

TatA is an essential and structurally conserved component of all known Twin-arginine transport (Tat) machineries which are able to catalyse membrane transport of fully folded proteins. Here we have investigated if bacterial TatA, or chimeric pea/E. coli TatA derivatives, are capable of replacing thylakoidal TatA in function. While authentic E. coli TatA does not show any transport activity in thylakoid transport experiments, TatA chimeras comprising the transmembrane helix (TMH) of pea TatA are fully active. For minimal catalytic activity it is even sufficient to replace three residues within TMH of E. coli TatA by the corresponding pea residues. Almost any further substitution within TMH gradually raises transport activity in the thylakoid system, while functional characterization of the same set of TatA derivatives in E. coli yields essentially inverse catalytic activities. Closer inspection of the substituted residues suggests that the two transport systems have deviating demands with regard to the hydrophobicity of the transmembrane helix.

In silico analysis and experimental validation of lipoprotein and novel Tat signal peptides processing in Anabaena sp. PCC7120

Signal peptide (SP) plays a pivotal role in protein translocation. Lipoprotein- and twin arginine translocase (Tat) dependent signal peptides were studied in All3087, a homolog of competence protein of Synechocystis PCC6803 and in two putative alkaline phosphatases (ALPs, Alr2234 and Alr4976), respectively. In silico analysis of All3087 is shown to possess the characteristics feature of competence proteins such as helix-hairpin-helix, N and C-terminal HKD endonuclease domain, calcium binding domain and N-terminal lipoprotein signal peptide. The SP recognition-cleavage site in All3087 was predicted (AIA-AC) using SignalP while further in-depth analysis using Pred-Lipo and WebLogo analysis for consensus sequence showed it as IAA-C. Activities of putative ALPs were confirmed by heterologous overexpression, activity assessment and zymogram analysis. ALP activity in Anabaena remains cell bound in log-phase, but during late log/stationary phase, an enhanced ALP activity was detected in extracellular milieu. The enhancement of ALP activity during stationary phase was not only due to inorganic phosphate limitation but also contributed by the presence of novel bipartite Tat-SP. The Tat signal transported the folded active ALPs to the membrane, followed by anchoring into the membrane and successive cleavage enabling transportation of the ALPs to the extracellular milieu, because of bipartite architecture and processing of transit Tat-SP.

Targeting of lumenal proteins across the thylakoid membrane

The biogenesis of the plant thylakoid network is an enormously complex process in terms of protein targeting. The membrane system contains a large number of proteins, some of which are synthesized within the organelle, while many others are imported from the cytosol. Studies in recent years have shown that the targeting of imported proteins into and across the thylakoid membrane is particularly complex, with four different targeting pathways identified to date. Two of these are used to target membrane proteins: a signal recognition particle (SRP)-dependent pathway and a highly unusual pathway that appears to require none of the known targeting apparatus. Two further pathways are used to translocate lumenal proteins across the thylakoid membrane from the stroma and, again, the two pathways differ dramatically from each other. One is a Sec-type pathway, in which ATP hydrolysis by SecA drives the transport of the substrate protein through the membrane in an unfolded conformation. The other is the twin-arginine translocation (Tat) pathway, where substrate proteins are transported in a folded state using a unique mechanism that harnesses the proton motive force across the thylakoid membrane. This article reviews progress in studies on the targeting of lumenal proteins, with reference to the mechanisms involved, their evolution from endosymbiotic progenitors of the chloroplast, and possible elements of regulation.

The Tat-dependent protein translocation pathway

The twin-arginine translocation (Tat) pathway is found in bacteria, archaea, and plant chloroplasts, where it is dedicated to the transmembrane transport of fully folded proteins. These proteins contain N-terminal signal peptides with a specific Tat-system binding motif that is recognized by the transport machinery. In contrast to other protein transport systems, the Tat system consists of multiple copies of only two or three usually small (∼8–30 kDa) membrane proteins that oligomerize to two large complexes that transiently interact during translocation. Only one of these complexes includes a polytopic membrane protein, TatC. The other complex consists of TatA. Tat systems of plants, proteobacteria, and several other phyla contain a third component, TatB. TatB is evolutionarily and structurally related to TatA and usually forms tight complexes with TatC. Minimal two-component Tat systems lacking TatB are found in many bacterial and archaeal phyla. They consist of a ‘bifunctional’ TatA that also covers TatB functionalities, and a TatC. Recent insights into the structure and interactions of the Tat proteins have various important implications.

Salt sensitivity of minimal twin arginine translocases

Bacterial twin arginine translocation (Tat) pathways have evolved to facilitate transport of folded proteins across membranes. Gram-negative bacteria contain a TatABC translocase composed of three subunits named TatA, TatB, and TatC. In contrast, the Tat translocases of most Gram-positive bacteria consist of only TatA and TatC subunits. In these minimal TatAC translocases, a bifunctional TatA subunit fulfils the roles of both TatA and TatB. Here we have probed the importance of conserved residues in the bifunctional TatAy subunit of Bacillus subtilis by site-specific mutagenesis. A set of engineered TatAy proteins with mutations in the cytoplasmic hinge and amphipathic helix regions were found to be inactive in protein translocation under standard growth conditions for B. subtilis or when heterologously expressed in Escherichia coli. Nevertheless, these mutated TatAy proteins did assemble into TatAy and TatAyCy complexes, and they facilitated membrane association of twin arginine precursor proteins in E. coli. Interestingly, most of the mutated TatAyCy translocases were salt-sensitive in B. subtilis. Similarly, the TatAC translocases of Bacillus cereus and Staphylococcus aureus were salt-sensitive when expressed in B. subtilis. Taken together, our present observations imply that salt-sensitive electrostatic interactions have critical roles in the preprotein translocation activity of certain TatAC type translocases from Gram-positive bacteria.

Environmental salinity determines the specificity and need for Tat-dependent secretion of the YwbN protein in Bacillus subtilis

Twin-arginine protein translocation (Tat) pathways are required for transport of folded proteins across bacterial, archaeal and chloroplast membranes. Recent studies indicate that Tat has evolved into a mainstream pathway for protein secretion in certain halophilic archaea, which thrive in highly saline environments. Here, we investigated the effects of environmental salinity on Tat-dependent protein secretion by the Gram-positive soil bacterium Bacillus subtilis, which encounters widely differing salt concentrations in its natural habitats. The results show that environmental salinity determines the specificity and need for Tat-dependent secretion of the Dyp-type peroxidase YwbN in B. subtilis. Under high salinity growth conditions, at least three Tat translocase subunits, namely TatAd, TatAy and TatCy, are involved in the secretion of YwbN. Yet, a significant level of Tat-independent YwbN secretion is also observed under these conditions. When B. subtilis is grown in medium with 1% NaCl or without NaCl, the secretion of YwbN depends strictly on the previously described “minimal Tat translocase” consisting of the TatAy and TatCy subunits. Notably, in medium without NaCl, both tatAyCy and ywbN mutants display significantly reduced exponential growth rates and severe cell lysis. This is due to a critical role of secreted YwbN in the acquisition of iron under these conditions. Taken together, our findings show that environmental conditions, such as salinity, can determine the specificity and need for the secretion of a bacterial Tat substrate.

The twin-arginine translocation system and its capability for protein secretion in biotechnological protein production

The biotechnological production of recombinant proteins is challenged by processes that decrease the yield, such as protease action, aggregation, or misfolding. Today, the variation of strains and vector systems or the modulation of inducible promoter activities is commonly used to optimize expression systems. Alternatively, aggregation to inclusion bodies may be a desired starting point for protein isolation and refolding. The discovery of the twin-arginine translocation (Tat) system for folded proteins now opens new perspectives because in most cases, the Tat machinery does not allow the passage of unfolded proteins. This feature of the Tat system can be exploited for biotechnological purposes, as expression systems may be developed that ensure a virtually complete folding of a recombinant protein before purification. This review focuses on the characteristics that make recombinant Tat systems attractive for biotechnology and discusses problems and possible solutions for an efficient translocation of folded proteins.

The thylakoid proton gradient promotes an advanced stage of signal peptide binding deep within the Tat pathway receptor complex

Tat (twin arginine translocation) systems transport folded proteins across the thylakoid membrane of chloroplasts and the plasma membrane of most bacteria. Tat precursors are targeted by hydrophobic cleavable signal peptides with twin arginine (RR) motifs. Bacterial precursors possess an extended consensus, (S/T)RRXFLK, of which the two arginines and the phenylalanine are essential for efficient transport. Thylakoid Tat precursors possess twin arginines but lack the consensus phenylalanine. Here, we have characterized two stages of precursor binding to the thylakoid Tat signal peptide receptor, the 700-kDa cpTatC-Hcf106 complex. The OE17 precursor tOE17 binds to the receptor by RR-dependant electrostatic interactions and partially dissociates during blue native gel electrophoresis. In addition, the signal peptide of thylakoid-bound tOE17 is highly exposed to the membrane surface, as judged by accessibility to factor Xa of cleavage sites engineered into signal peptide flanking regions. By contrast, tOE17 containing a consensus phenylalanine in place of Val(-20) (V - 20F) binds the receptor more strongly and is completely stable during blue native gel electrophoresis. Thylakoid bound V - 20F is also completely protected from factor Xa at the identical sites. This suggests that the signal peptide is buried deeply in the cpTatC-Hcf106 binding site. We further provide evidence that the proton gradient, which is required for translocation, induces a tighter interaction between tOE17 and the cpTat machinery, similar to that exhibited by V - 20F. This implies that translocation involves a very intimate association of the signal peptide with the receptor complex binding site.

The Tat pathway in bacteria and chloroplasts (review)

Both in prokaryotic organisms and in chloroplasts, a specialized protein transport pathway exists which is capable of translocating proteins in a fully folded conformation. Transport is mediated in both instances by signal peptides harbouring a twin-arginine consensus motif (twin-arginine translocation (Tat) pathway). The Tat translocase comprises the three functionally different membrane proteins TatA, TatB, and TatC. While TatB and TatC are involved in the specific recognition of the substrate, TatA might be the major pore-forming component. Current evidence suggests that a functional Tat translocase is assembled from separate TatBC and TatA assemblies only on demand, i.e., in the presence of transport substrate and a transmembrane H+-motive force.

Inactivation of a predicted leader peptidase prevents photoautotrophic growth of Synechocystis sp. strain PCC 6803

To establish the role of the two putative type I leader peptidases (LepB1 and LepB2) encoded in the genome of the cyanobacterium Synechocystis sp. strain PCC 6803, we generated independent knockout mutants for both genes by introducing kanamycin resistance cassettes into the two open reading frames (sll0716 [lepB1] and slr1377 [lepB2], respectively). Although the insertion was successful in both instances, it was not possible to select homozygous mutant cells for lepB2, suggesting that the function of this gene is essential for cell viability. In contrast, LepB1 is apparently essential only for photoautotrophic growth, because homozygous lepB1::Km(r) cells could be propagated under heterotrophic conditions. They were even capable to some extent of photosynthetic oxygen evolution. However, the photosynthetic activity decreased gradually with extended incubation in the light and was particularly affected by high light intensities. Both features were indicative of photooxidative damage, which was probably caused by inefficient replacement of damaged components of the photosynthetic machinery due to the lack of a leader peptidase removing the signal peptides from photosynthetic precursor proteins. Indeed, processing of the PsbO precursor polypeptide to the corresponding mature protein was significantly affected in the mutant, and reduced amounts of other proteins that are synthesized as precursors with signal peptides accumulated in the cells. These results strongly suggest that LepB1 is important for removal of the signal peptides after membrane transport of the components of the photosynthetic machinery, which in turn is a prerequisite for the biogenesis of a functional photosynthetic electron transport chain.

Tat-dependent protein targeting in prokaryotes and chloroplasts

The twin-arginine translocation (Tat) system operates in the chloroplast thylakoid and the plasma membranes of a wide range of bacteria. It recognizes substrates bearing cleavable signal peptides in which a twin-arginine motif almost invariably plays a key role in recognition by the translocation machinery. These signal peptides are surprisingly similar to those used to specify transport by Sec-type systems, but the Tat pathway differs in fundamental respects from Sec-type and other protein translocases. Its key attribute is its ability to translocate large, fully folded (even oligomeric) proteins across tightly sealed membranes. To date, three key tat genes have been characterised and the first details of the Tat system are beginning to emerge. In this article we review the salient features of Tat systems, with an emphasis on the targeting signals involved, the substrate specificities of Tat systems, our current knowledge of Tat complex structures and the known mechanistic features. Although the article is focused primarily on bacterial systems, we incorporate relevant aspects of plant thylakoid Tat work and we discuss how the plant and bacterial systems may differ in some respects.

Dissecting the twin-arginine translocation pathway using genome-wide analysis

A recently discovered route for protein export, known as the twin-arginine translocation (Tat) pathway, has received much attention owing to several atypical characteristics that distinguish it from other transport mechanisms. For instance, recent evidence has clearly established that this pathway only transports folded polypeptides. Moreover, several studies have demonstrated a vital role for the Tat pathway in bacterial pathogenesis. In this review, we discuss genomic approaches that have been employed to determine the prevalence and capacity of the Tat system and how the information generated from these approaches is helping to connect Tat transport to bacterial physiology and virulence.

An Escherichia coli twin-arginine signal peptide switches between helical and unstructured conformations depending on the hydrophobicity of the environment

The Tat system catalyzes the transport of folded globular proteins across the bacterial plasma membrane and the chloroplast thylakoid. It recognizes cleavable signal peptides containing a critical twin-arginine motif but little is known of the overall structure of these peptides. In this report, we have analyzed the secondary structure of the SufI signal peptide, together with those of two nonfunctional variants in which the region around the twin-arginine, RRQFI, is replaced by KKQFI or RRQAA. Circular dichroism studies show that the SufI peptide exists as an unstructured peptide in aqueous solvent with essentially no stable secondary structure. In membrane-mimetic environments such as SDS micelles or water/trifluoroethanol, however, the peptide adopts a structure containing up to about 40% alpha-helical content. Secondary structure predictions and molecular modelling programs strongly suggest that the helical region begins at, or close to, the twin-arginine motif. Studies on the thermal stability of the helix demonstrate a sharp transition between the unstructured and helical states, suggesting that the peptide exists in one of two distinct states. The two nonfunctional peptides exhibit almost identical spectra and properties to the wild-type SufI peptide, indicating that it is the arginine sidechains, and not their contribution to the helical structure, that are critical in this class of peptide.

Membrane-specific targeting of green fluorescent protein by the Tat pathway in the cyanobacterium Synechocystis PCC6803

The transport and sorting of extracytoplasmic proteins in cyanobacteria is made complex by the presence of a highly differentiated membrane system. Proteins destined for the periplasm and thylakoid lumen are initially transported by Sec- and Tat-type pathways but little is known of the mechanisms that ultimately direct them to the correct destinations. We have generated a Synechocystis PCC6803 transformant that expresses a fusion protein comprising the Tat-specific targeting signal of Escherichia coli TorA linked to green fluorescent protein (GFP). Immunoblotting indicates the presence of mature-size GFP but no precursor form, demonstrating that efficient translocation has taken place. Confocal microscopy and immunogold electron microscopy reveal GFP to be almost exclusively located in the periplasm, with almost no protein evident in the thylakoid network. These data point to the operation of highly effective sorting pathways for soluble proteins in this cyanobacterium. The observed sorting of the GFP suggests that either (a) the Tat apparatus is located only in the plasma membrane or (b) the TorA-GFP is targeted across either membrane but the GFP is subsequently directed to the periplasm, perhaps by a default sorting pathway to this compartment.

Folding quality control in the export of proteins by the bacterial twin-arginine translocation pathway

To examine the relationship between folding and export competence by the twin-arginine translocation (Tat) pathway we analyzed the subcellular localization of fusions between a set of eight putative Tat leader peptides and alkaline phosphatase in isogenic Escherichia coli strains that either allow or disfavor the formation of protein disulfide bonds in the cytoplasm. We show that export by the Tat translocator is observed only in strains that enable oxidative protein folding in the cytoplasm. Further, we show that other disulfide-containing proteins, namely single-chain Fv and heterodimeric F(AB) antibody fragments, are export-competent only in strains having an oxidizing cytoplasm. Functional, heterodimeric F(AB) protein was exported from the cytoplasm by means of a Tat leader peptide fused to the heavy chain alone, indicating that the formation of a disulfide-bonded dimer preceeds export. These results demonstrate that in vivo only proteins that have attained the native conformation are exported by the Tat translocator, indicating that a folding quality-control mechanism is intrinsic to the export process. The ability to export proteins with disulfide bonds and the folding proofing feature of the Tat pathway are of interest for biotechnology applications.

An alternative model of the twin arginine translocation system

The twin arginine translocation (Tat) system is a machinery which can translocate folded proteins across energy transducing membranes. Currently it is supposed that Tat substrates bind directly to Tat translocon components before a ApH-driven translocation occurs. In this review, an alternative model is presented which proposes that membrane integration could precede Tat-dependent translocation. This idea is mainly supported by the recent observations of Tat-independent membrane insertion of Tat substrates in vivo and in vitro. Membrane insertion may allow i) a quality control of the folded state by membrane bound proteases like FtsH, ii) the recognition of the membrane spanning signal peptide by Tat system components, and iii) a pulling mechanism of translocation. In some cases of folded Tat substrates, the membrane targeting process may require ATP-dependent N-terminal unfolding-steps.

Specific inhibition of the translocation of a subset of Escherichia coli TAT substrates by the TorA signal peptide

The SufI protein and the trimethylamine N-oxide reductase (TorA) are the two best-characterized prototype proteins exported by the Escherichia coli TAT system. Whereas SufI does not contain cofactors, TorA is a molybdo-enzyme and the acquisition of the molybdo-cofactor is a prerequisite for its translocation. The overproduction of each protein leads to the saturation of its translocation, but it was unknown if the overproduction of one substrate could saturate the TAT apparatus and block thus the translocation of other TAT substrates. Here, we showed that the overproduction of SufI saturated only its own translocation, but had no effect of the translocation of TorA and other TAT substrate analyzed. To dissect the saturation mechanism of TorA translocation, we shortened by about one-third of the TorA protein and removed nine consensus molybdo-cofactor-binding ligands. Like SufI, the truncated TorA (TorA502) did not contain cofactor and would not compete with the full length TorA for molybdo-cofactor acquisition. The overproduction of TorA502 completely inhibited the export of the full length TorA and dimethyl sulfoxide (DMSO) reductase, but had no effect on the translocation of SufI, nitrate-induced formate dehydrogenase and hydrogenase-2. Importantly, deletion of the twin-arginine signal peptide of TorA502 abolished the inhibitory effect. Moreover, the overproduction of the TorA signal peptide fused to the green fluorescence protein (GFP) was sufficient to block the TorA translocation. These results demonstrated that the twin-arginine signal peptide of the TorA protein specifically inhibits the translocation of a subset of TAT substrates, probably at the step of their targeting to the TAT apparatus.

Moving folded proteins across the bacterial cell membrane

The Tat protein export system is located in the bacterial cytoplasmic membrane and operates in parallel to the well-known Sec pathway. While the Sec system only transports unstructured substrates, the function of the Tat pathway is to translocate folded proteins. The Tat translocase thus faces the formidable challenge of moving structured macromolecular substrates across the bacterial cytoplasmic membrane without rendering the membrane freely permeable to protons and other ions. The substrates of the Tat pathway are often proteins that bind cofactor molecules in the cytoplasm, and are thus folded, prior to export. Such periplasmic cofactor-containing proteins are essential for most types of bacterial respiratory and photosynthetic energy metabolism. In addition, the Tat pathway is involved in outer membrane biosynthesis and in bacterial pathogenesis. Substrates are targeted to the Tat pathway by amino-terminal signal sequences harbouring consecutive, essentially invariant, arginine residues, and movement of proteins through the Tat system is energized by the transmembrane proton electrochemical gradient. The TatA protein probably forms the transport channel while the TatBC proteins act as a receptor complex that recognizes the signal peptide of the substrate protein.

Localization and function of the IdiA homologue Slr1295 in the cyanobacterium Synechocystis sp. strain PCC 6803

Slr1295 (and Slr0513) in the cyanobacterium Synechocystis sp. PCC 6803 has amino acid similarity to the bacterial FbpA protein family and also to IdiA of Synechococcus PCC 6301/PCC 7942. To determine whether Slr1295 is the periplasm-located component of an iron transporter, or has a function in protecting photosystem (PS) II, subcellular localization and Deltaslr1295 mutant characterization studies were performed. Localization of Slr1295 provided evidence that it has an intracellular function, since virtually no Slr1295 was detected in the soluble protein fraction of the periplasm or in the cytoplasmic membrane. Characterization of a Deltaslr1295 Synechocystis PCC 6803 mutant indicated that PS II is more susceptible to inactivation in the mutant than in the wild-type (WT). Under mild iron limitation, modification of PS I to the PS I-IsiA complex is more advanced in the Deltaslr1295 mutant, indicating that iron deficiency leads more rapidly to changes in the photosynthetic apparatus in the mutant than in the WT. Biochemical fractionation procedures provide evidence that Slr1295 co-purifies with PS II. These results suggest a function of Slr1295 that is comparable to the function of IdiA in Synechococcus PCC 6301/PCC 7942 being a protein that protects PS II under iron limitation in an as yet unknown way.

Separate analysis of twin-arginine translocation (Tat)-specific membrane binding and translocation in Escherichia coli

The twin-arginine translocation (Tat) pathway exports those precursor proteins to the periplasmic space of bacteria that harbor a twin-arginine (RR) consensus motif in their signal sequences. We have reproduced translocation of several Tat substrates into inside-out plasma membrane vesicles from Escherichia coli. Translocation proceeding at an efficiency of up to 20% occurs specifically via the Tat pathway as indicated by (i) its requirement for elevated levels of the TatABC proteins in the membrane vesicles, (ii) competition by an intact twin-arginine signal peptide, and (iii) susceptibility toward dissipation of the transmembrane H(+) gradient. The latter treatment, while blocking translocation, still allows for functional membrane association of Tat precursors. This is shown by the finding that translocation of isolated membrane-bound Tat precursor is restored upon re-energization of the vesicles.

In vivo dissection of the Tat translocation pathway in Escherichia coli

The bacterial Tat pathway is capable of exporting folded proteins carrying a special twin arginine (RR) signal peptide. By using two in vivo reporter proteins, we assessed factors that affect Tat pathway transport. We observed that, like the intact RR signal peptide, those with a KR or RK substitution were still capable of mediating the translocation of the folded green fluorescent protein (GFP). However, the translocation efficiency decreased in the order of RR>KR>RK. The KK motif was unable to mediate GFP translocation. The translocation of the RR-GFP fusion required TatA, TatB and TatC proteins. By exploiting the periplasmic bactericidal property of colicin V (ColV), we constructed a translocation-suicide probe, RR-ColV. The translocation of RR-ColV fully inhibited the growth of wild-type Escherichia coli and those of the DeltatatD and DeltatatE mutants. In contrast, the deletion of the tatC gene blocked RR-ColV in the cytoplasm and this strain exhibited a normal growth phenotype. Interestingly, the growth of DeltatatA and tatB mutants was inhibited partially by RR-ColV. Moreover, KR, RK and KK motifs were capable of mediating the ColV translocation with a decreasing RR=KR>RK>KK efficiency. In addition to TatE and TatC proteins, either TatA or TatB was sufficient for the translocation of RR-ColV or KR-ColV. In contrast, TatA plus the conserved N-terminal domain of TatB were required to mediate the killing effect of ColV fused to the less-efficient RK signal peptide. Taken together, these results suggest that a fully efficient Tat pathway transport is determined by the sequence of the signal peptide, the composition of the Tat apparatus, and the intrinsic characteristics of exported proteins.

Essential cytoplasmic domains in the Escherichia coli TatC protein

The twin-arginine translocation (Tat) system mediates the transport of proteins across the bacterial plasma membrane and chloroplast thylakoid membrane. Operating in parallel with Sec-type systems in these membranes, the Tat system is completely different in both structural and mechanistic terms, and is uniquely able to catalyze the translocation of fully folded proteins across coupled membranes. TatC is an essential, multispanning component that has been proposed to form part of the binding site for substrate precursor proteins. In this study we have tested the importance of conserved residues on the periplasmic and cytoplasmic face of the Escherichia coli protein. We find that many of the mutations on the cytoplasmic face have little or no effect. However, substitution at several positions in the extreme N-terminal cytoplasmic region or the predicted first cytoplasmic loop lead to a significant or complete loss of Tat-dependent export. The mutated strains are unable to grow anaerobically on trimethylamine N-oxide minimal media and are unable to export trimethylamine-N-oxide reductase (TorA). The same mutants are completely unable to export a chimeric protein, comprising the TorA signal peptide linked to green fluorescent protein, indicating that translocation is blocked rather than cofactor insertion into the TorA mature protein. The data point to two essential cytoplasmic domains on the TatC protein that are essential for export.

The twin-arginine signal peptide of PhoD and the TatAd/Cd proteins of Bacillus subtilis form an autonomous Tat translocation system

The bacterial twin-arginine translocation (Tat) pathway has been recently described for PhoD of Bacillus subtilis, a phosphodiesterase containing a twin-arginine signal peptide. The expression of phoD is co-regulated with the expression of tatA(d) and tatC(d) genes localized downstream of phoD. To characterize the specificity of PhoD transport further, translocation of PhoD was investigated in Escherichia coli. By using gene fusions, we analyzed the particular role of the signal peptide and the mature region of PhoD in canalizing the transport route. A hybrid protein consisting of the signal peptide of beta-lactamase and mature PhoD was transported in a Sec-dependent manner indicating that the mature part of PhoD does not contain information canalizing the selected translocation route. Pre-PhoD, as well as a fusion protein consisting of the signal peptide of PhoD (SP(PhoD)) and beta-galactosidase (LacZ), remained cytosolic in the E. coli. Thus, SP(PhoD) is not recognized by E. coli transport systems. Co-expression of B. subtilis tatA(d)/C(d) genes resulted in the processing of SP(PhoD)-LacZ and periplasmic localization of LacZ illustrating a close substrate specificity of the TatA(d)/C(d) transport system. While blockage of the Sec-dependent transport did not affect the localization of SP(PhoD)-LacZ, translocation and processing was dependent on the pH gradient of the cytosolic membrane. Thus, the minimal requirement of a functional Tat-dependent protein translocation system consists of a twin-arginine signal peptide-containing Tat substrate, its specific TatA/C proteins, and the pH gradient across the cytosolic membrane.

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.

The Rieske Fe/S protein of the cytochrome b6/f complex in chloroplasts: missing link in the evolution of protein transport pathways in chloroplasts?

The Rieske Fe/S protein, a nuclear-encoded subunit of the cytochrome b(6)/f complex in chloroplasts, is retarded in the stromal space after import into the chloroplast and only slowly translocated further into the thylakoid membrane system. As shown by the sensitivity to nigericin and to specific competitor proteins, thylakoid transport takes place by the DeltapH-dependent TAT pathway. The Rieske protein is an untypical TAT substrate, however. It is only the second integral membrane protein shown to utilize this pathway, and it is the first authentic substrate without a cleavable signal peptide. Transport is instead mediated by the NH(2)-terminal membrane anchor, which lacks, however, the twin-arginine motif indicative of DeltapH/TAT-dependent transport signals. Furthermore, transport is affected by sodium azide as well as by competitor proteins for the Sec pathway in chloroplasts, demonstrating for the first time some cross-talk of the two pathways. This might take place in the stroma where the Rieske protein accumulates after import in several complexes of high molecular mass, among which the cpn60 complex is the most prominent. These untypical features suggest that the Rieske protein represents an intermediate or early state in the evolution of the thylakoidal protein transport pathways.

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.

Evolutionarily related insertion pathways of bacterial, mitochondrial, and thylakoid membrane proteins

The inner membranes of eubacteria and mitochondria, as well as the chloroplast thylakoid membrane, contain essential proteins that function in oxidative phosphorylation and electron transport processes or in photosynthesis. Because most of the organellar proteins are nuclear encoded, they are synthesized in the cytoplasm and subsequently imported into the organelle before they are inserted into the membrane. This review focuses on the pathways of protein insertion into the inner membrane of eubacteria and mitochondria and into the chloroplast thylakoid membrane. In many respects, insertion of proteins into the inner membrane of bacteria is a process similar to that used by proteins of the thylakoid membrane. In both of these systems a signal recognition particle (SRP) and a SecYE-translocase are involved, as in translocation into the endoplasmic reticulum. The pathway of proteins into the mitochondrial membranes appears to be different in that it involves no SecYE-like components. A conservative pathway, recently identified in mitochondria, involves the Oxa1 protein for the insertion of proteins from the matrix. The presence of Oxa1 homologues in eubacteria and chloroplasts suggests that this pathway is evolutionarily conserved.

Protein traffic in bacteria: multiple routes from the ribosome to and across the membrane

Bacteria use several routes to target their exported proteins to the plasma membrane. The majority are exported through pores formed by SecY and SecE. Two different molecular machineries are used to target proteins to the SecYE translocon. Translocated proteins, synthesized as precursors with cleavable signal sequences, require cytoplasmic chaperones, such as SecB, to remain competent for posttranslational transport. In concert with SecB, SecA targets the precursors to SecY and energizes their translocation by its ATPase activity. The latter function involves a partial insertion of SecA itself into the SecYE translocon, a process that is strongly assisted by a couple of membrane proteins, SecG, SecD, SecF, YajC, and the proton gradient across the membrane. Integral membrane proteins, however, are specifically recognized by a direct interaction between their noncleaved signal anchor sequences and the bacterial signal recognition particle (SRP) consisting of Ffh and 4.5S RNA. Recognition occurs during synthesis at the ribosome and leads to a cotranslational targeting to SecYE that is mediated by FtsY and the hydrolysis of GTP. No other Sec protein is required for integration unless the membrane protein also contains long translocated domains that engage the SecA machinery. Discrimination between SecA/SecB- and SRP-dependent targeting involves the specificity of SRP for hydrophobic signal anchor sequences and the exclusion of SRP from nascent chains of translocated proteins by trigger factor, a ribosome-associated chaperone. The SecYE pore accepts only unfolded proteins. In contrast, a class of redox factor-containing proteins leaves the cell only as completely folded proteins. They are distinguished by a twin arginine motif of their signal sequences that by an unknown mechanism targets them to specific pores. A few membrane proteins insert spontaneously into the bacterial plasma membrane without the need for targeting factors and SecYE. Insertion depends only on hydrophobic interactions between their transmembrane segments and the lipid bilayer and on the transmembrane potential. Finally, outer membrane proteins of Gram-negative bacteria after having crossed the plasma membrane are released into the periplasm, where they undergo distinct folding events until they insert as trimers into the outer membrane. These folding processes require distinct molecular chaperones of the periplasm, such as Skp, SurA, and PpiD.

Specificity of signal peptide recognition in tat-dependent bacterial protein translocation

The bacterial twin arginine translocation (Tat) pathway translocates across the cytoplasmic membrane folded proteins which, in most cases, contain a tightly bound cofactor. Specific amino-terminal signal peptides that exhibit a conserved amino acid consensus motif, S/T-R-R-X-F-L-K, direct these proteins to the Tat translocon. The glucose-fructose oxidoreductase (GFOR) of Zymomonas mobilis is a periplasmic enzyme with tightly bound NADP as a cofactor. It is synthesized as a cytoplasmic precursor with an amino-terminal signal peptide that shows all of the characteristics of a typical twin arginine signal peptide. However, GFOR is not exported to the periplasm when expressed in the heterologous host Escherichia coli, and enzymatically active pre-GFOR is found in the cytoplasm. A precise replacement of the pre-GFOR signal peptide by an authentic E. coli Tat signal peptide, which is derived from pre-trimethylamine N-oxide (TMAO) reductase (TorA), allowed export of GFOR, together with its bound cofactor, to the E. coli periplasm. This export was inhibited by carbonyl cyanide m-chlorophenylhydrazone, but not by sodium azide, and was blocked in E. coli tatC and tatAE mutant strains, showing that membrane translocation of the TorA-GFOR fusion protein occurred via the Tat pathway and not via the Sec pathway. Furthermore, tight cofactor binding (and therefore correct folding) was found to be a prerequisite for proper translocation of the fusion protein. These results strongly suggest that Tat signal peptides are not universally recognized by different Tat translocases, implying that the signal peptides of Tat-dependent precursor proteins are optimally adapted only to their cognate export apparatus. Such a situation is in marked contrast to the situation that is known to exist for Sec-dependent protein translocation.

A novel protein transport system involved in the biogenesis of bacterial electron transfer chains

The Tat system is a recently discovered bacterial protein transport pathway that functions primarily in the biosynthesis of proteins containing redox active cofactors. Analogous transport systems are found in plant organelles. Remarkably and uniquely the Tat system functions to transported a diverse range of folded proteins across a biological membrane, a feat that must be achieved without rendering the membrane freely permeable to protons and other ions. Here we review the operation of the bacterial Tat system and propose a model for the structural organisation of the Tat preprotein translocase.

TatD is a cytoplasmic protein with DNase activity. No requirement for TatD family proteins in sec-independent protein export

The Escherichia coli Tat system mediates Sec-independent export of protein precursors bearing twin arginine signal peptides. Genes known to be involved in this process include tatA, tatB, and tatC that form an operon with a fourth gene, tatD. The tatD gene product has two homologues in E. coli coded by the unlinked ycfH and yjjV genes. An E. coli strain with in-frame chromosomal deletions in all three of tatD, ycfH, and yjjV exhibits no significant defect in the cellular location of five cofactor-containing enzymes that are synthesized with twin arginine signal peptides. Neither these mutations nor overproduction of the TatD protein cause any discernible effect on the export kinetics of an additional E. coli Tat pathway substrate. It is concluded that proteins of the TatD family have no obligate involvement in protein export by the Tat system. TatD is shown to be a cytoplasmic protein. TatD binds to immobilized Ni(2+) or Zn(2+) affinity columns and exhibits magnesium-dependent DNase activity. Features of the tatA operon that may control TatD expression are discussed.

Characterization of the early steps of OE17 precursor transport by the thylakoid DeltapH/Tat machinery

In order to probe the structure and protein translocation function of the thylakoid Tat machinery, a 25-residue C-terminal extension containing a 13-residue in vivo biotinylation tag and a 6x His tag was added to a mutant precursor of the 17-kDa subunit of the oxygen-evolving complex to form pOE17(C)-BioHis. When avidin was attached to biotinylated precursor in situ, the precursor-avidin complex was neither imported nor did it form a membrane-spanning translocation intermediate. It did, however, competitively inhibit the translocation of unbiotinylated precursor with an apparent KI unaffected by avidin. It is shown that the precursor protein achieves a stable folded structure upon dilution from urea, suggesting that the avidin-induced inhibition of transport results from a folding-induced proximity of N-terminal and C-terminal domains. It is further demonstrated that the majority of precursor rapidly binds to the thylakoid membrane, remaining import competent and yet undissociable by high salt or high pH treatment at ice temperature. The membrane binding event is unaffected by avidin. Import kinetics reveal that nonproton motive force-driven transport steps make up a major fraction of the transport time. These observations suggest that the N-terminal presequence on the avidin-bound precursor is available for membrane binding and initial recognition by the transport machinery, but the attached avidin signals the machinery that the precursor is an incorrectly configured substrate and thus import is aborted. Consequently, the DeltapH/Tat machinery's proofreading mechanism must operate after precursor recognition but before the committed step in transport.

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.

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.

Two distinct translocation intermediates can be distinguished during protein transport by the TAT (Deltaph) pathway across the thylakoid membrane

During thylakoid transport of the chimeric precursor protein 16/23 which takes place by the twin arginine translocation (TAT) (Deltaph)-dependent pathway, two distinct translocation intermediates can be identified which represent successive steps in the translocation process. Both intermediates are partially inserted into the thylakoid membrane and can be distinguished by specific degradation fragments occurring after thermolysin treatment of the thylakoids. While the formation of the early translocation intermediate does not depend on a functional TAT translocation machinery, the appearance of the late intermediate is strictly coupled to the Deltaph-dependent transport of the 16/23 chimera. Accordingly, this translocation intermediate is found associated with two distinct complexes in the thylakoid membrane having apparent molecular masses of approximately 560 and 620 kDa, respectively.

The maize tha4 gene functions in sec-independent protein transport in chloroplasts and is related to hcf106, tatA, and tatB

Proteins are translocated across the chloroplast thylakoid membrane by a variety of mechanisms. Some proteins engage a translocation machinery that is derived from the bacterial Sec export system and require an interaction with a chloroplast-localized SecA homologue. Other proteins engage a machinery that is SecA-independent, but requires a transmembrane pH gradient. Recently, a counterpart to this Delta pH mechanism was discovered in bacteria. Genetic studies revealed that one maize protein involved in this mechanism, HCF106, is related in both structure and function to the bacterial tatA and tatB gene products. We describe here the mutant phenotype and molecular cloning of a second maize gene that functions in the Delta pH mechanism. This gene, thylakoid assembly 4 (tha4), is required specifically for the translocation of proteins that engage the Delta pH pathway. The sequence of the tha4 gene product resembles those of the maize hcf106 gene and the bacterial tatA and tatB genes. Sequence comparisons suggest that tha4 more closely resembles tatA, and hcf106 more closely resembles tatB. These findings support the notion that this sec-independent translocation mechanism has been highly conserved during the evolution of eucaryotic organelles from bacterial endosymbionts.

Component specificity for the thylakoidal Sec and Delta pH-dependent protein transport pathways

Prokaryotes and prokaryote-derived thylakoid membranes of chloroplasts share multiple, evolutionarily conserved pathways for protein export. These include the Sec, signal recognition particle (SRP), and Delta pH/Tat systems. Little is known regarding the thylakoid membrane components involved in these pathways. We isolated a cDNA clone to a novel component of the Delta pH pathway, Tha4, and prepared antibodies against pea Tha4, against maize Hcf106, a protein implicated in Delta pH pathway transport by genetic studies, and against cpSecY, the thylakoid homologue of the bacterial SecY translocon protein. These components were localized to the nonappressed thylakoid membranes. Tha4 and Hcf106 were present in approximately 10-fold excess over active translocation sites. Antibodies to either Tha4 or Hcf106 inhibited translocation of four known Delta pH pathway substrate proteins, but not of Sec pathway or SRP pathway substrates. This suggests that Tha4 and Hcf106 operate either in series or as subunits of a heteromultimeric complex. cpSecY antibodies inhibited translocation of Sec pathway substrates but not of Delta pH or SRP pathway substrates. These studies provide the first biochemical evidence that Tha4 and Hcf106 are specific components of the Delta pH pathway and provide one line of evidence that cpSecY is used specifically by the Sec pathway.

Expression of plastocyanin and cytochrome f of the cyanobacterium Phormidium laminosum in Escherichia coli and Paracoccus denitrificans and the role of leader peptides

The gene for plastocyanin from the cyanobacterium Phormidium laminosum was successfully expressed in Escherichia coli. Expression of the gene for cytochrome f resulted in the production of holocytochrome f in the periplasmic space of E. coli, but the yield was low. Expression in Paracoccus denitrificans yielded no holoprotein. When the region encoding the cytochrome f leader sequence was replaced with more typical bacterial leader sequences (those from the P. laminosum plastocyanin gene and the Paracoccus versutus cytochrome c-550 gene), much higher yields were consistently obtained in both species. Overexpressed proteins were compared to those isolated from P. laminosum and found to be identical in mass, isoelectric point, redox midpoint potential and (for plastocyanin) 1H-NMR spectrum.

Competition between Sec- and TAT-dependent protein translocation in Escherichia coli

Recently, a new protein translocation pathway, the twin-arginine translocation (TAT) pathway, has been identified in both bacteria and chloroplasts. To study the possible competition between the TAT- and the well-characterized Sec translocon-dependent pathways in Escherichia coli, we have fused the TorA TAT-targeting signal peptide to the Sec-dependent inner membrane protein leader peptidase (Lep). We find that the soluble, periplasmic P2 domain from Lep is re-routed by the TorA signal peptide into the TAT pathway. In contrast, the full-length TorA-Lep fusion protein is not re-routed into the TAT pathway, suggesting that Sec-targeting signals in Lep can override TAT-targeting information in the TorA signal peptide. We also show that the TorA signal peptide can be converted into a Sec-targeting signal peptide by increasing the hydrophobicity of its h-region. Thus, beyond the twin-arginine motif, the overall hydrophobicity of the signal peptide plays an important role in TAT versus Sec targeting. This is consistent with statistical data showing that TAT-targeting signal peptides in general have less hydrophobic h-regions than Sec-targeting signal peptides.

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.

Involvement of a chloroplast homologue of the signal recognition particle receptor protein, FtsY, in protein targeting to thylakoids

We isolated an Arabidopsis thaliana cDNA whose translated product shows sequence similarity to the FtsY, a bacterial homologue of SRP receptor protein. The Arabidopsis FtsY homologue contains a typical chloroplast transit peptide. The in vitro-synthesized 37 kDa FtsY homologue was imported into chloroplasts, and the processed 32 kDa polypeptide bound peripherally on the outer surface of thylakoids. Antibodies raised against the FtsY homologue also reacted with a thylakoid-bound 32 kDa protein. The antibodies inhibited the cpSRP-dependent insertion of the light-harvesting chlorophyll alb-binding protein into thylakoid membranes suggesting that the chloroplast FtsY homologue is involved in the cpSRP-dependent protein targeting to the thylakoid membranes.

Bacterial proteins carrying twin-R signal peptides are specifically targeted by the delta pH-dependent transport machinery of the thylakoid membrane system

Glucose-fructose oxidoreductase (GFOR), a periplasmic protein of Zymomonas mobilis, is synthesized as a precursor polypeptide with a twin-R signal peptide for Sec-independent protein export in bacteria. In higher plant chloroplasts, twin-R signal peptides are specific targeting signals for the Sec-independent delta pH pathway of the thylakoid membrane system. In agreement with the assumed common phylogenetic origin of the two protein transport mechanisms, GFOR can be efficiently translocated by the delta pH-dependent pathway when analyzed with isolated thylakoid membranes. Transport is sensitive to the ionophore nigericin and competes with specific substrates for the delta pH-dependent transport route. In contrast, neither sodium azide nor enzymatic destruction of the nucleoside triphosphates in the assays affects thylakoid transport of GFOR indicating that the Sec apparatus is not involved in this process. Mutagenesis of the twin-R motif in the GFOR signal peptide prevents membrane translocation of the protein emphasizing the importance of these residues for the transport process.

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.