Affinity of TatCd for TatAd Elucidates Its Receptor Function in the Bacillus subtilis Twin Arginine Translocation (Tat) Translocase System

Length matters: Functional flip of the short TatA transmembrane helix

The twin arginine translocase (Tat) exports folded proteins across bacterial membranes. The putative pore-forming or membrane-weakening component (TatA_d in B. subtilis) is anchored to the lipid bilayer via an unusually short transmembrane α-helix (TMH), with less than 16 residues. Its tilt angle in different membranes was analyzed under hydrophobic mismatch conditions, using synchrotron radiation circular dichroism and solid-state NMR. Positive mismatch (introduced either by reconstitution in short-chain lipids or by extending the hydrophobic TMH length) increased the helix tilt of the TMH as expected. Negative mismatch (introduced either by reconstitution in long-chain lipids or by shortening the TMH), on the other hand, led to protein aggregation. These data suggest that the TMH of TatA is just about long enough for stable membrane insertion. At the same time, its short length is a crucial factor for successful translocation, as demonstrated here in native membrane vesicles using an in vitro translocation assay. Furthermore, when reconstituted in model membranes with negative spontaneous curvature, the TMH was found to be aligned parallel to the membrane surface. This intrinsic ability of TatA to flip out of the membrane core thus seems to play a key role in its membrane-destabilizing effect during Tat-dependent translocation.

The Twin-Arginine Pathway for Protein Secretion

The Tat pathway for protein translocation across bacterial membranes stands out for its selective handling of fully folded cargo proteins. In this review, we provide a comprehensive summary of our current understanding of the different known Tat components, their assembly into different complexes, and their specific roles in the protein translocation process. In particular, this overview focuses on the Gram-negative bacterium Escherichia coli and the Gram-positive bacterium Bacillus subtilis. Using these organisms as examples, we discuss structural features of Tat complexes alongside mechanistic models that allow for the Tat pathway's unique protein proofreading and transport capabilities. Finally, we highlight recent advances in exploiting the Tat pathway for biotechnological benefit, the production of high-value pharmaceutical proteins.

Transport of Folded Proteins by the Tat System

The twin-arginine protein translocation (Tat) system has been characterized in bacteria, archaea and the chloroplast thylakoidal membrane. This system is distinct from other protein transport systems with respect to two key features. Firstly, it accepts cargo proteins with an N-terminal signal peptide that carries the canonical twin-arginine motif, which is essential for transport. Second, the Tat system only accepts and translocates fully folded cargo proteins across the respective membrane. Here, we review the core essential features of folded protein transport via the bacterial Tat system, using the three-component TatABC system of Escherichia coli and the two-component TatAC systems of Bacillus subtilis as the main examples. In particular, we address features of twin-arginine signal peptides, the essential Tat components and how they assemble into different complexes, mechanistic features and energetics of Tat-dependent protein translocation, cytoplasmic chaperoning of Tat cargo proteins, and the remarkable proofreading capabilities of the Tat system. In doing so, we present the current state of our understanding of Tat-dependent protein translocation across biological membranes, which may serve as a lead for future investigations.

Twin-Arginine Protein Translocation

Twin-arginine protein translocation systems (Tat) translocate fully folded and co-factor-containing proteins across biological membranes. In this review, we focus on the Tat pathway of Gram-positive bacteria. The minimal Tat pathway is composed of two components, namely a TatA and TatC pair, which are often complemented with additional TatA-like proteins. We provide overviews of our current understanding of Tat pathway composition and mechanistic aspects related to Tat-dependent cargo protein translocation. This includes Tat pathway flexibility, requirements for the correct folding and incorporation of co-factors in cargo proteins and the functions of known cargo proteins. Tat pathways of several Gram-positive bacteria are discussed in detail, with emphasis on the Tat pathway of Bacillus subtilis. We discuss both shared and unique features of the different Gram-positive bacterial Tat pathways. Lastly, we highlight topics for future research on Tat, including the development of this protein transport pathway for the biotechnological secretion of high-value proteins and its potential applicability as an antimicrobial drug target in pathogens.

TatBC-independent TatA/Tat substrate interactions contribute to transport efficiency

The Tat system can transport folded, signal peptide-containing proteins (Tat substrates) across energized membranes of prokaryotes and plant plastids. A twin-arginine motif in the signal peptide of Tat substrates is recognized by TatC-containing complexes, and TatA permits the membrane passage. Often, as in the model Tat systems of Escherichia coli and plant plastids, a third component - TatB - is involved that resembles TatA but has a higher affinity to TatC. It is not known why most TatA dissociates from TatBC complexes in vivo and distributes more evenly in the membrane. Here we show a TatBC-independent substrate-binding to TatA from Escherichia coli, and we provide evidence that this binding enhances Tat transport. First hints came from in vivo cross-linking data, which could be confirmed by affinity co-purification of TatA with the natural Tat substrates HiPIP and NrfC. Two positions on the surface of HiPIP could be identified that are important for the TatA interaction and transport efficiency, indicating physiological relevance of the interaction. Distributed TatA thus may serve to accompany membrane-interacting Tat substrates to the few TatBC spots in the cells.

Protein transport by the bacterial Tat pathway

The twin-arginine translocation (Tat) system accomplishes the remarkable feat of translocating large - even dimeric - proteins across tightly sealed energy-transducing membranes. All of the available evidence indicates that it is unique in terms of both structure and mechanism; however its very nature has hindered efforts to probe the core translocation events. At the heart of the problem is the fact that two large sub-complexes are believed to coalesce to form the active translocon, and 'capturing' this translocation event has been too difficult. Nevertheless, studies on the individual components have come a long way in recent years, and structural studies have reached the point where educated guesses can be made concerning the most interesting aspects of Tat. In this article we review these studies and the emerging ideas in this field. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

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.

Mapping the twin-arginine protein translocation network of Bacillus subtilis

Bacteria employ twin-arginine translocation (Tat) pathways for the transport of folded proteins to extracytoplasmic destinations. In recent years, most studies on bacterial Tat pathways addressed the membrane-bound TatA(B)C subunits of the Tat translocase, and the specific interactions between this translocase and its substrate proteins. In contrast, relatively few studies investigated possible coactors in the TatA(B)C-dependent protein translocation process. The present studies were aimed at identifying interaction partners of the Tat pathway of Bacillus subtilis, which is a paradigm for studies on protein secretion by Gram-positive bacteria. Specifically, 36 interaction partners of the TatA and TatC subunits were identified by rigorous application of the yeast two-hybrid (Y2H) approach. Our Y2H analyses revealed that the three TatA isoforms of B. subtilis can form homo- and heterodimers. Subsequently, the secretion of the Tat substrates YwbN and PhoD was tested in mutant strains lacking genes for the TatAC interaction partners identified in our genome-wide Y2H screens. Our results show that the cell wall-bound protease WprA is important for YwbN secretion, and that the HemAT and CsbC proteins are required for PhoD secretion under phosphate starvation conditions. Taken together, our findings imply that the Bacillus Tat pathway is embedded in an intricate protein-protein interaction network.

Twin-arginine-dependent translocation of folded proteins

Twin-arginine translocation (Tat) denotes a protein transport pathway in bacteria, archaea and plant chloroplasts, which is specific for precursor proteins harbouring a characteristic twin-arginine pair in their signal sequences. Many Tat substrates receive cofactors and fold prior to translocation. For a subset of them, proofreading chaperones coordinate maturation and membrane-targeting. Tat translocases comprise two kinds of membrane proteins, a hexahelical TatC-type protein and one or two members of the single-spanning TatA protein family, called TatA and TatB. TatC- and TatA-type proteins form homo- and hetero-oligomeric complexes. The subunits of TatABC translocases are predominantly recovered from two separate complexes, a TatBC complex that might contain some TatA, and a homomeric TatA complex. TatB and TatC coordinately recognize twin-arginine signal peptides and accommodate them in membrane-embedded binding pockets. Advanced binding of the signal sequence to the Tat translocase requires the proton-motive force (PMF) across the membranes and might involve a first recruitment of TatA. When targeted in this manner, folded twin-arginine precursors induce homo-oligomerization of TatB and TatA. Ultimately, this leads to the formation of a transmembrane protein conduit that possibly consists of a pore-like TatA structure. The translocation step again is dependent on the PMF.

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.

Towards understanding the Tat translocation mechanism through structural and biophysical studies of the amphipathic region of TatA from Escherichia coli

The twin-arginine translocase (Tat) system is used by many bacteria and plants to move folded proteins across the cytoplasmic or thylakoid membrane. In most bacteria, the TatA protein is believed to form a defined pore in the membrane through homo-oligomerization with other TatA protomers. The predicted secondary structure of TatA includes a transmembrane helix, an amphipathic helix, and an unstructured C-terminal region. Here biophysical and structural investigations were performed on a synthetic peptide representing the amphipathic region of TatA (residues 22 to 44, abbreviated TatAH2). The C-terminal region of TatA (residues 44-89) was previously shown to be accessible from both the cytoplasmic and periplasmic sides of the membrane only when the membrane potential was intact, suggesting dependence of its topology on an energized membrane (Chan et al. 2007 Biochemistry 46: 7396-404). Such observation suggests that the TatAH2 region would have unique lipid interactions that may be related to the function of TatA during translocation and thus warranted further investigations. NMR and CD spectroscopy of TatAH2 show that it adopts a predominantly helical structure in a membrane environment while remaining unstructured in aqueous solution. Differential scanning calorimetry studies also reveal that TatAH2 interacts with DPPG lipids but not with DPPC, suggesting that negatively charged phospholipid head groups contribute to the membrane interactions with TatA.

Transport and proofreading of proteins by the twin-arginine translocation (Tat) system in bacteria

The twin-arginine translocation (Tat) system operates in plant thylakoid membranes and the plasma membranes of most free-living bacteria. In bacteria, it is responsible for the export of a number of proteins to the periplasm, outer membrane or growth medium, selecting substrates by virtue of cleavable N-terminal signal peptides that contain a key twin-arginine motif together with other determinants. Its most notable attribute is its ability to transport large folded proteins (even oligomeric proteins) across the tightly sealed plasma membrane. In Gram-negative bacteria, TatABC subunits appear to carry out all of the essential translocation functions in the form of two distinct complexes at steady state: a TatABC substrate-binding complex and separate TatA complex. Several studies favour a model in which these complexes transiently coalesce to generate the full translocase. Most Gram-positive organisms possess an even simpler "minimalist" Tat system which lacks a TatB component and contains, instead, a bifunctional TatA component. These Tat systems may involve the operation of a TatAC complex together with a separate TatA complex, although a radically different model for TatAC-type systems has also been proposed. While bacterial Tat systems appear to require the presence of only a few proteins for the actual translocation event, there is increasing evidence for the operation of ancillary components that carry out sophisticated "proofreading" activities. These activities ensure that redox proteins are only exported after full assembly of the cofactor, thereby avoiding the futile export of apo-forms. This article is part of a Special Issue entitled Protein translocation across or insertion into membranes.

TatB functions as an oligomeric binding site for folded Tat precursor proteins

The TatABC subunits of the twin-arginine translocation machinery allow transport of folded proteins by an unknown mechanism. Here we show that the entire surfaces of folded Tat substrates contact TatB via both of its predicted helices. Our data suggest that TatB forms an oligomeric binding site that transiently accommodates folded Tat precursors.

Twin-arginine-containing signal sequences mediate the transmembrane transport of folded proteins. The cognate twin-arginine translocation (Tat) machinery of Escherichia coli consists of the membrane proteins TatA, TatB, and TatC. Whereas Tat signal peptides are recognized by TatB and TatC, little is known about molecular contacts of the mature, folded part of Tat precursor proteins. We have placed a photo-cross-linker into Tat substrates at sites predicted to be either surface-exposed or hidden in the core of the folded proteins. On targeting of these variants to the Tat machinery of membrane vesicles, all surface-exposed sites were found in close proximity to TatB. Correspondingly, incorporation of the cross-linker into TatB revealed multiple precursor-binding sites in the predicted transmembrane and amphipathic helices of TatB. Large adducts indicative of TatB oligomers contacting one precursor molecule were also obtained. Cross-linking of Tat substrates to TatB required an intact twin-arginine signal peptide and disappeared upon transmembrane translocation. Our collective data are consistent with TatB forming an oligomeric binding site that transiently accommodates folded Tat precursors.

Subcellular localization of TatAd of Bacillus subtilis depends on the presence of TatCd or TatCy

The gram-positive bacterium Bacillus subtilis contains two minimal Tat translocases, TatAdCd and TatAyCy, which are each involved in the secretion of one or more specific protein substrates. We have investigated the subcellular localization of the TatA components by employing C-terminal green fluorescent protein (GFP) fusions and fluorescence microscopy. When expressed from a xylose-inducible promoter, the TatA-GFP fusion proteins displayed a dual localization pattern, being localized peripherally and showing bright foci which are predominantly located at the division sites and/or poles of the cells. Importantly, the localization of TatAd-GFP was similar when the protein was expressed from its own promoter under phosphate starvation conditions, indicating that these foci are not the result of artificial overexpression. Moreover, the TatAd-GFP fusion protein was shown to be functional in the translocation of its substrate PhoD, provided that TatCd is also present. Furthermore, we demonstrate that the localization of TatAd-GFP in foci depends on the presence of the TatCd component. Remarkably, however, the TatAd-GFP foci can also be observed in the presence of TatCy, indicating that TatAd can interact not only with TatCd but also with TatCy. These results suggest that the formation of TatAd complexes in B. subtilis is controlled by TatC.

The twin-arginine translocation (Tat) systems from Bacillus subtilis display a conserved mode of complex organization and similar substrate recognition requirements

The twin arginine translocation (Tat) system transports folded proteins across the bacterial plasma membrane. In Gram-negative bacteria, membrane-bound TatABC subunits are all essential for activity, whereas Gram-positive bacteria usually contain only TatAC subunits. In Bacillus subtilis, two TatAC-type systems, TatAdCd and TatAyCy, operate in parallel with different substrate specificities. Here, we show that they recognize similar signal peptide determinants. Both systems translocate green fluorescent protein fused to three distinct Escherichia coli Tat signal peptides, namely DmsA, AmiA and MdoD, and mutagenesis of the DmsA signal peptide confirmed that both Tat pathways recognize similar targeting determinants within Tat signals. Although another E. coli Tat substrate, trimethylamine N-oxide reductase, was translocated by TatAdCd but not by TatAyCy, we conclude that these systems are not predisposed to recognize only specific Tat signal peptides, as suggested by their narrow substrate specificities in B. subtilis. We also analysed complexes involved in the second Tat pathway in B. subtilis, TatAyCy. This revealed a discrete TatAyCy complex together with a separate, homogeneous, approximately 200 kDa TatAy complex. The latter complex differs significantly from the corresponding E. coli TatA complexes, pointing to major structural differences between Tat complexes from Gram-negative and Gram-positive organisms. Like TatAd, TatAy is also detectable in the form of massive cytosolic complexes.

Relaxed specificity of the Bacillus subtilis TatAdCd translocase in Tat-dependent protein secretion

Protein translocation via the twin arginine translocation (TAT) pathway is characterized by the translocation of prefolded proteins across the hydrophobic lipid bilayer of the membrane. In Bacillus subtilis, two different Tat translocases are involved in this process, and both display different substrate specificities: PhoD is secreted via TatAdCd, whereas YwbN is secreted via TatAyCy. It was previously assumed that both TatAy and TatCy are essential for the translocation of the YwbN precursor. Through complementation studies, we now show that TatAy can be functionally replaced by TatAd when the latter is offered to the cells in excess amounts. Moreover, under conditions of overproduction, TatAdCd, in contrast to TatAyCy, shows an increased tolerance toward the acceptance of various Tat-dependent proteins.

A minimal Tat system from a gram-positive organism: a bifunctional TatA subunit participates in discrete TatAC and TatA complexes

The Tat system transports folded proteins across bacterial and thylakoid membranes. In Gram-negative organisms, a TatABC substrate-binding complex and separate TatA complex are believed to coalesce to form an active translocon, with all three subunits essential for translocation. Most Gram-positive organisms lack a tatB gene, indicating major differences in organization and possible differences in mode of action. Here, we have studied Tat complexes encoded by the tatAdCd genes of Bacillus subtilis. Expression of tatAdCd in an Escherichia coli tat null mutant results in efficient export of a large, cofactor-containing E. coli Tat substrate, TorA. We show that the tatAd gene complements E. coli mutants lacking either tatAE or tatB, indicating a bifunctional role for this subunit in B. subtilis. Second, we have identified and characterized two distinct Tat complexes that are novel in key respects: a TatAdCd complex of approximately 230 kDa that is significantly smaller than the analogous E. coli TatABC complex (approximately 370 kDa on BN gels) and a separate TatAd complex. The latter is a discrete entity of approximately 270 kDa as judged by gel filtration chromatography, very different from the highly heterogeneous E. coli TatA complex that ranges in size from approximately 50 kDa to over 600 kDa. TatA heterogeneity has been linked to the varying size of Tat substrates being translocated, but the singular nature of the B. subtilis TatAd complex suggests that discrete TatAC and TatA complexes may form a single form of translocon.

Differential roles of individual domains in selection of secretion route of a Streptococcus parasanguinis serine-rich adhesin, Fap1

Fimbria-associated protein 1 (Fap1) is a high-molecular-mass glycosylated surface adhesin required for fimbria biogenesis and biofilm formation in Streptococcus parasanguinis. The secretion of mature Fap1 is dependent on the presence of SecA2, a protein with some homology to, but with a different role from, SecA. The signals that direct the secretion of Fap1 to the SecA2-dependent secretion pathway rather than the SecA-dependent secretion pathway have not yet been identified. In this study, Fap1 variants containing different domains were expressed in both secA2 wild-type and mutant backgrounds and were tested for their ability to be secreted by the SecA- or SecA2-dependent pathway. The presence or absence of the cell wall anchor domain (residues 2531 to 2570) at the C terminus did not alter the selection of the Fap1 secretion route. The Fap1 signal peptide (residues 1 to 68) was sufficient to support the secretion of a heterologous protein via the SecA-dependent pathway, suggesting that the signal peptide was sufficient for recognition by the SecA-dependent pathway. The minimal sequences of Fap1 required for the SecA2-dependent pathway included the N-terminal signal peptide, nonrepetitive region I (residues 69 to 102), and part of nonrepetitive region II (residues 169 to 342). The two serine-rich repeat regions (residues 103 to 168 and 505 to 2530) were not required for Fap1 secretion. However, they were both involved in the specific inhibition of Fap1 secretion via the SecA-dependent pathway.

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.

Structure analysis of the protein translocating channel TatA in membranes using a multi-construct approach

The twin-arginine-translocase (Tat) can transport proteins in their folded state across bacterial or thylakoid membranes. In Bacillus subtilis the Tat-machinery consists of only two integral (inner) membrane proteins, TatA and TatC. Multiple copies of TatA are supposed to form the transmembrane channel, but little structural data is available on this 70-residue component. We used a multi-construct approach for expressing several characteristic fragments of TatA(d), to determine their individual structures and to cross-validate them comprehensively within the architecture of the full-length protein. Here, we report the design, high-yield expression, detergent-aided purification and lipid-reconstitution of five constructs of TatA(d), overcoming difficulties associated with the very different hydrophobicities and sizes of these membrane protein fragments. Circular dichroism (CD) and oriented CD (OCD) were used to determine their respective conformations and alignments in suitable, negatively charged phospholipid bilayers. CD spectroscopy showed an N-terminal alpha-helix, a central helical stretch, and an unstructured C-terminus, thus proving the existence of these secondary structures in TatA(d) for the first time. The OCD spectra demonstrated a transmembrane orientation of the N-terminal alpha-helix and a surface alignment of the central amphiphilic helix in lipid bilayers, thus supporting the postulated topology model and function of TatA as a transmembrane channel.

The TatA Subunit ofEscherichia coliTwin-Arginine Translocase Has an N-in Topology

The twin-arginine translocase (Tat) system is used by many bacteria to translocate folded proteins across the cytoplasmic membrane. The TatA subunit is the predicted pore-forming subunit and has been shown to form a homo-oligomeric complex. Through accessibility experiments using the thiol-reactive reagents 4-acetamido-4'-maleimidylstilbene-2,2'-disulfonic acid and Nalpha-(3-maleimidylproprionyl)biocytin toward site-specific cysteine mutants in TatA, we show that the N-terminus of TatA is located in the cytoplasm rather than the previously assumed periplasm. We also confirm previous observations that the C-terminus has a dual topology. By treatment with the membrane uncoupler carbonyl cyanide-m-chlorophenyl hydrazone, we show that the topological state of the C-terminus is dependent on the membrane potential. These results suggest two architectures of TatA in the membrane: one with a single transmembrane helix and the other with two transmembrane helices. Molecular models of both topologies were used to develop and cartoon a homo-oligomeric complex as a channel with a diameter of approximately 50 A and suggest that the double transmembrane helix topology might be the building block for the translocation channel. Additionally, in vivo cross-linking experiments of Gly2Cys and Thr22Cys mutants showed that Gly2, at the beginning of transmembrane helix-1, is in close proximity with Gly2 of a neighboring TatA, as Cys2 cross-linked immediately upon the addition of copper phenanthroline. On the other hand, Cys22, at the other end of the transmembrane helix, took at least 10 min to cross-link, suggesting that a possible movement or reorientation is required to bring this residue into proximity with a neighboring TatA subunit.

The Tat pathway in Streptomyces lividans: interaction of Tat subunits and their role in translocation

The twin-arginine translocation (Tat) pathway transports folded proteins across bacterial cytoplasmic membranes. The Tat system of Streptomyces lividans consists of TatA, TatB and TatC, unlike most Gram-positive bacteria, which only have TatA and TatC subunits. Interestingly, in S. lividans TatA and TatB are localized in both the cytoplasm and the membrane. In the cytoplasm soluble TatA and TatB were found as monomers or as part of a hetero-oligomeric complex. Further analysis showed that specific information for recognition of the precursor by the soluble Tat components is mainly present in the twin-arginine signal peptide. Study of the role of the Tat subunits in complex assembly and stability in the membrane and cytoplasm showed that TatB stabilizes TatC whereas a key role in driving Tat complex assembly is suggested for TatC. Finally, by analysis of the oligomeric properties of TatA in the membrane of S. lividans and study of the affinity of membrane-embedded TatA for Tat/Sec precursors, a role for TatA as a translocator is postulated.