Essential cytoplasmic domains in the Escherichia coli TatC protein

A Potential Late Stage Intermediate of Twin-Arginine Dependent Protein Translocation in Escherichia coli

The twin-arginine translocation (Tat) system transports folded proteins across membranes of prokaryotes, plant plastids, and some mitochondria. According to blue-native polyacrylamide gel electrophoresis after solubilization with digitonin, distinct interactions between the components TatA, TatB, and TatC result in two major TatBC-containing complexes in Escherichia coli that can bind protein substrates. We now report the first detection of a TatABC complex that likely represents the state at which transport occurs. This complex was initially found when the photo cross-linking amino acid p-benzoyl-l-phenylalanine (Bpa) was introduced at position I50 on the periplasmic side of the first trans-membrane domain of TatC. Cross-linking of TatC_I50Bpa resulted in TatC-TatC-cross-links, indicating a close proximity to neighboring TatC in the complex. However, the new complex was not caused by cross-links but rather by non-covalent side chain interactions, as it was also detectable without UV-cross-linking or with an I50Y exchange. The new complex did not contain any detectable substrate. It was slightly upshifted relative to previously reported substrate-containing TatABC complexes. In the absence of TatA, an inactive TatBC_I50Bpa complex was formed of the size of wild-type substrate-containing TatABC complexes, suggesting that TatB occupies TatA-binding sites at TatC_I50Bpa. When substrate binding was abolished by point mutations, this TatBC_I50Bpa complex shifted analogously to active TatABC_I50Bpa complexes, indicating that a defect substrate-binding site further enhances TatB association to TatA-binding sites. Only TatA could shift the complex with an intact substrate-binding site, which explains the TatA requirement for substrate transport by TatABC systems.

The TatBC complex of the Tat protein translocase in Escherichia coli and its transition to the substrate-bound TatABC complex

The twin-arginine translocation (Tat) system serves to transport folded proteins across membranes of prokaryotes and plant plastids. In Escherichia coli, a complex consisting of multiple copies of TatB and TatC initiates the transport by binding the signal peptides of the Tat substrates. Using blue-native polyacrylamide gel electrophoresis, bands of TatBC-containing complexes can be detected at molecular masses of 440 and 580 kDa. We systematically analyzed the formation of Tat complexes with TatB or TatC variants that carried point mutations at selected positions. Several mutations resulted in specific disassembly patterns and alterations in the 440 kDa:580 kDa complex ratios. The 440 kDa complex contains only TatBC, whereas the 580 kDa complex consists of TatABC. Substrate binding results in a TatBC-Tat substrate complex at ~500 kDa and a TatABC-Tat substrate complex at ~600 kDa. Only the ~600 kDa complex was detected with nonrecombinant substrate levels and thus could be the physiologically most relevant species. The results suggest that some TatA is usually associated with TatBC, regardless of substrate binding.

Diversity and evolution of bacterial twin arginine translocase protein, TatC, reveals a protein secretion system that is evolving to fit its environmental niche

Background

The twin-arginine translocation (Tat) protein export system enables the transport of fully folded proteins across a membrane. This system is composed of two integral membrane proteins belonging to TatA and TatC protein families and in some systems a third component, TatB, a homolog of TatA. TatC participates in substrate protein recognition through its interaction with a twin arginine leader peptide sequence.

Methodology/Principal Findings

The aim of this study was to explore TatC diversity, evolution and sequence conservation in bacteria to identify how TatC is evolving and diversifying in various bacterial phyla. Surveying bacterial genomes revealed that 77% of all species possess one or more tatC loci and half of these classes possessed only tatC and tatA genes. Phylogenetic analysis of diverse TatC homologues showed that they were primarily inherited but identified a small subset of taxonomically unrelated bacteria that exhibited evidence supporting lateral gene transfer within an ecological niche. Examination of bacilli tatCd/tatCy isoform operons identified a number of known and potentially new Tat substrate genes based on their frequent association to tatC loci. Evolutionary analysis of these Bacilli isoforms determined that TatCy was the progenitor of TatCd. A bacterial TatC consensus sequence was determined and highlighted conserved and variable regions within a three dimensional model of the Escherichia coli TatC protein. Comparative analysis between the TatC consensus sequence and Bacilli TatCd/y isoform consensus sequences revealed unique sites that may contribute to isoform substrate specificity or make TatA specific contacts. Synonymous to non-synonymous nucleotide substitution analyses of bacterial tatC homologues determined that tatC sequence variation differs dramatically between various classes and suggests TatC specialization in these species.

Conclusions/Significance

TatC proteins appear to be diversifying within particular bacterial classes and its specialization may be driven by the substrates it transports and the environment of its host.

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.

The glove-like structure of the conserved membrane protein TatC provides insight into signal sequence recognition in twin-arginine translocation

In bacteria, two signal-sequence-dependent secretion pathways translocate proteins across the cytoplasmic membrane. Although the mechanism of the ubiquitous general secretory pathway is becoming well understood, that of the twin-arginine translocation pathway, responsible for translocation of folded proteins across the bilayer, is more mysterious. TatC, the largest and most conserved of three integral membrane components, provides the initial binding site of the signal sequence prior to pore assembly. Here, we present two crystal structures of TatC from the thermophilic bacteria Aquifex aeolicus at 4.0 Å and 6.8 Å resolution. The membrane architecture of TatC includes a glove-shaped structure with a lipid-exposed pocket predicted by molecular dynamics to distort the membrane. Correlating the biochemical literature to these results suggests that the signal sequence binds in this pocket, leading to structural changes that facilitate higher order assemblies.

Mapping the signal peptide binding and oligomer contact sites of the core subunit of the pea twin arginine protein translocase

Twin arginine translocation (Tat) systems of thylakoid and bacterial membranes transport folded proteins using the proton gradient as the sole energy source. Tat substrates have hydrophobic signal peptides with an essential twin arginine (RR) recognition motif. The multispanning cpTatC plays a central role in Tat operation: It binds the signal peptide, directs translocase assembly, and may facilitate translocation. An in vitro assay with pea (Pisum sativum) chloroplasts was developed to conduct mutagenesis and analysis of cpTatC functions. Ala scanning mutagenesis identified mutants defective in substrate binding and receptor complex assembly. Mutations in the N terminus (S1) and first stromal loop (S2) caused specific defects in signal peptide recognition. Cys matching between substrate and imported cpTatC confirmed that S1 and S2 directly and specifically bind the RR proximal region of the signal peptide. Mutations in four lumen-proximal regions of cpTatC were defective in receptor complex assembly. Copurification and Cys matching analyses suggest that several of the lumen proximal regions may be important for cpTatC-cpTatC interactions. Surprisingly, RR binding domains of adjacent cpTatCs directed strong cpTatC-cpTatC cross-linking. This suggests clustering of binding sites on the multivalent receptor complex and explains the ability of Tat to transport cross-linked multimers. Transport of substrate proteins cross-linked to the signal peptide binding site tentatively identified mutants impaired in the translocation step.

Molecular dissection of TatC defines critical regions essential for protein transport and a TatB-TatC contact site

The twin arginine transport (Tat) system transports folded proteins across the prokaryotic cytoplasmic membrane and the plant thylakoid membrane. TatC is the largest and most conserved component of the Tat machinery. It forms a multisubunit complex with TatB and binds the signal peptides of Tat substrates. Here we have taken a random mutagenesis approach to identify substitutions in Escherichia coli TatC that inactivate protein transport. We identify 32 individual amino acid substitutions that abolish or severely compromise TatC activity. The majority of the inactivating substitutions fall within the first two periplasmic loops of TatC. These regions are predicted to have conserved secondary structure and results of extensive amino acid insertion and deletion mutagenesis are consistent with these conserved elements being essential for TatC function. Three inactivating substitutions were identified in the fifth transmembrane helix of TatC. The inactive M205R variant could be suppressed by mutations affecting amino acids in the transmembrane helix of TatB. A physical interaction between TatC helix 5 and the TatB transmembrane helix was confirmed by the formation of a site-specific disulphide bond between TatC M205C and TatB L9C variants. This is the first molecular contact site mapped to single amino acid level between these two proteins.

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.

Abrogation of the twin arginine transport system in Salmonella enterica serovar Typhimurium leads to colonization defects during infection

TatC (STM3975) is a highly conserved component of the Twin Arginine Transport (Tat) systems that is required for transport of folded proteins across the inner membrane in gram-negative bacteria. We previously identified a ΔtatC mutant as defective in competitive infections with wild type ATCC14028 during systemic infection of Salmonella-susceptible BALB/c mice. Here we confirm these results and show that the ΔtatC mutant is internalized poorly by cultured J774-A.1 mouse macrophages a phenotype that may be related to the systemic infection defect. This mutant is also defective for short-term intestinal and systemic colonization after oral infection of BALB/c mice and is shed in reduced numbers in feces from orally infected Salmonella-resistant (CBA/J) mice. We show that the ΔtatC mutant is highly sensitive to bile acids perhaps resulting in the defect in intestinal infection that we observe. Finally, the ΔtatC mutant has an unusual combination of motility phenotypes in Salmonella; it is severely defective for swimming motility but is able to swarm well. The ΔtatC mutant has a lower amount of flagellin on the bacterial surface during swimming motility but normal levels under swarming conditions.

The Tat Protein Export Pathway

Proteins that reside partially or completely outside the bacterial cytoplasm require specialized pathways to facilitate their localization. Globular proteins that function in the periplasm must be translocated across the hydrophobic barrier of the inner membrane. While the Sec pathway transports proteins in a predominantly unfolded conformation, the Tat pathway exports folded protein substrates. Protein transport by the Tat machinery is powered solely by the transmembrane proton gradient, and there is no requirement for nucleotide triphosphate hydrolysis. Proteins are targeted to the Tat machinery by N-terminal signal peptides that contain a consensus twin arginine motif. In Escherichia coli and Salmonella there are approximately thirty proteins with twin arginine signal peptides that are transported by the Tat pathway. The majority of these bind complex redox cofactors such as iron sulfur clusters or the molybdopterin cofactor. Here we describe what is known about Tat substrates in E. coli and Salmonella, the function and mechanism of Tat protein export, and how the cofactor insertion step is coordinated to ensure that only correctly assembled substrates are targeted to the Tat machinery.

Effects of altered TatC proteins on protein secretion efficiency via the twin-arginine translocation pathway of Bacillus subtilis

Protein translocation via the Tat machinery in thylakoids and bacteria occurs through a cooperation between the TatA, TatB and TatC subunits, of which the TatC protein forms the initial Tat substrate-binding site. The Bacillus subtilis Tat machinery lacks TatB and comprises two separate TatAC complexes with distinct substrate specificities: PhoD is secreted by the TatAdCd complex, whereas YwbN is secreted by the TatAyCy complex. To study the role of the Gram-positive TatC proteins in Tat-dependent protein secretion efficiency, we applied several genetic engineering approaches to modify and analyse the B. subtilis TatCd and TatCy proteins. Cytoplasmic and transmembrane domain exchange between TatCd and TatCy resulted in stable chimeric proteins that were unable to secrete both known substrates of the B. subtilis Tat system. Site-directed mutagenesis of conserved residues in the N-terminal part of both TatC proteins revealed significant differences in the degree of importance of these residues between TatCd, TatCy and Escherichia coli TatC. In addition, two small C-terminal deletions in TatCy completely abolished YwbN translocation, indicating that this terminus is essential for Tat translocation activity. Important differences from previous observations for E. coli TatC and implications for substrate binding and translocation are discussed.

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.

Escherichia coli tatC mutations that suppress defective twin-arginine transporter signal peptides

In vitro studies have suggested that the TatBC complex serves as the receptor for signal peptides targeted for export via the twin-arginine translocation (Tat) pathway. Substitution of the hallmark twin-arginine dipeptide with two lysines abrogates export of physiological substrates in all organisms. We report the isolation and characterization of suppressor mutations that allow export of an ssTor(KK)-GFP-SsrA tripartite fusion. We identified two amino acid suppressor mutations in the first cytoplasmic loop of TatC. In addition, two other amino acids in the first cytoplasmic loop exhibit epistatic suppression. Surprisingly, we also identified a suppressor mutation predicted to lie within the second periplasmic loop of TatC, a region that is not expected to interact directly with the signal peptide. The suppressor mutations allowed export of the native Esherichia coli Tat substrate trimethylamine N-oxide reductase with a twin-lysine substitution in its signal sequence. The cytoplasmic suppressor mutations conferred SDS sensitivity and partial filamentation, indicating that Tat export of authentic substrates was impaired.

Expression of mitochondrial tatC in Nicotiana tabacum is responsive to benzothiadiazole and salicylic acid

A cDNA, up-regulated upon treatment of tobacco cells with salicylic acid and benzo (1,2,3) thiadiazole-7-carbothioic acid S-methyl ester, was identified by differential RNA display and the full sequence obtained. This mitochondrial gene, twin arginine translocation (tatC), resembles orthologues across different species, including the gene that codes for a sec-independent membrane translocating protein in bacteria. Hypothetical tatC proteins have also been identified in the mitochondria of Arabidopsis thaliana, Oenothera berteriana, Beta vulgaris, Oryza sativa and Marchantia polymorpha. Comparative protein analysis indicates a similar function for the tatC gene. The up-regulation of the tatC gene in a 3kbp transcript was confirmed by RNA gel blot analysis.

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.

Analysis of Twin-Arginine Translocation Pathway Homologue in Staphylococcus aureus

Staphylococcus aureus releases a large number of exoproteins, including membrane-active proteins and toxins with superantigenic activity involved in pathogenicity. However, the export pathways of exoproteins in S. aureus have not been reported. We analyzed the function of the staphylococcal twin-arginine translocation (Tat) pathway homologue, the presence of which was recently discovered according to the genome database. The amino-acid sequences of the Tat homologues of S. aureus do not have a high similarity with those of Escherichia coli and other bacteria. Constructed tatC-deficient mutants from distinct parent strains showed the same patterns of exoproteins compared with those of parent strains on two-dementional gel electrophoresis, and the amounts of secreted staphylococcal enterotoxins and toxic shock syndrome toxin-1, of which signal peptides have some features often seen in signal sequences of Tat-dependent proteins, did not change with Western blotting analyses. Therefore, it seems that the Tat pathway does not play a major role in the secretion system of S. aureus, but other export pathways may play an important role in toxin secretion. This is the first experimental report showing the influence of the Tat pathway on the secretion of S. aureus.

Cysteine scanning mutagenesis and topological mapping of the Escherichia coli twin-arginine translocase TatC Component

The TatC protein is an essential component of the Escherichia coli twin-arginine (Tat) protein translocation pathway. It is a polytopic membrane protein that forms a complex with TatB, together acting as the receptor for Tat substrates. In this study we have constructed 57 individual cysteine substitutions throughout the protein. Each of the substitutions resulted in a TatC protein that was competent to support Tat-dependent protein translocation. Accessibility studies with membrane-permeant and -impermeant thiol-reactive reagents demonstrated that TatC has six transmembrane helices, rather than the four suggested by a previous study (K. Gouffi, C.-L. Santini, and L.-F. Wu, FEBS Lett. 525:65-70, 2002). Disulfide cross-linking experiments with TatC proteins containing single cysteine residues showed that each transmembrane domain of TatC was able to interact with the same domain from a neighboring TatC protein. Surprisingly, only three of these cysteine variants retained the ability to cross-link at low temperatures. These results are consistent with the likelihood that most of the disulfide cross-links are between TatC proteins in separate TatBC complexes, suggesting that TatC is located on the periphery of the complex.

The bacterial twin-arginine translocation pathway

The twin-arginine translocation (Tat) pathway is responsible for the export of folded proteins across the cytoplasmic membrane of bacteria. Substrates for the Tat pathway include redox enzymes requiring cofactor insertion in the cytoplasm, multimeric proteins that have to assemble into a complex prior to export, certain membrane proteins, and proteins whose folding is incompatible with Sec export. These proteins are involved in a diverse range of cellular activities including anaerobic metabolism, cell envelope biogenesis, metal acquisition and detoxification, and virulence. The Escherichia coli translocase consists of the TatA, TatB, and TatC proteins, but little is known about the precise sequence of events that leads to protein translocation, the energetic requirements, or the mechanism that prevents the export of misfolded proteins. Owing to the unique characteristics of the pathway, it holds promise for biotechnological applications.

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

Twin arginine translocation (Tat) systems catalyze the transport of folded proteins across the bacterial cytosolic membrane or the chloroplast thylakoid membrane. In the Tat systems of Escherichia coli and many other species TatA-, TatB-, and TatC-like proteins have been identified as essential translocase components. In contrast, the Bacillus subtilis phosphodiesterase PhoD-specific system consists only of a pair of TatA(d)/TatC(d) proteins and involves a TatA(d) protein engaged in a cytosolic and a membrane-embedded localization. Because soluble TatA(d) was able to bind the twin arginine signal peptide of prePhoD prior to membrane integration it could serve to recruit its substrate to the membrane via the interaction with TatC(d). By analyzing the distribution of TatA(d) and studying the mutual affinity with TatC(d) we have shown here that TatC(d) assists the membrane localization of TatA(d). Besides detergent-solubilized TatC(d), membrane-integrated TatC(d) showed affinity for soluble TatA(d). By using a peptide library-specific binding of TatA(d) to cytosolic loops of membrane protein TatC(d) was demonstrated. Depletion of TatC(d) in B. subtilis resulted in a drastic reduction of TatA(d), indicating a stabilizing effect of TatC(d) for TatA(d). In addition, the presence of the substrate prePhoD was the prerequisite for appropriate localization in the cytosolic membrane of B. subtilis as demonstrated by freeze-fracture experiments.

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.

Recombinant protein secretion in Escherichia coli

The secretory production of recombinant proteins by the Gram-negative bacterium Escherichia coli has several advantages over intracellular production as inclusion bodies. In most cases, targeting protein to the periplasmic space or to the culture medium facilitates downstream processing, folding, and in vivo stability, enabling the production of soluble and biologically active proteins at a reduced process cost. This review presents several strategies that can be used for recombinant protein secretion in E. coli and discusses their advantages and limitations depending on the characteristics of the target protein to be produced.

Evidence for interactions between domains of TatA and TatB from mutagenesis of the TatABC subunits of the twin-arginine translocase

The twin-arginine translocation (Tat) system transports folded proteins across the bacterial plasma membrane. Three subunits, TatA, B and C, are known to be involved but their modes of action are poorly understood, as are the inter-subunit interactions occurring within Tat complexes. We have generated mutations in the single transmembrane (TM) spans of TatA and TatB, with the aim of generating structural distortions. We show that substitution in TatB of three residues by glycine, or a single residue by proline, has no detectable effect on translocation, whereas the presence of three glycines in the TatA TM span completely blocks Tat translocation activity. The results show that the integrity of the TatA TM span is vital for Tat activity, whereas that of TatB can accommodate large-scale distortions. Near-complete restoration of activity in TatA mutants is achieved by the simultaneous presence of a V12P mutation in the TatB TM span, strongly implying a direct functional interaction between the TatA/B TM spans. We also analyzed the predicted amphipathic regions in TatA and TatB and again find evidence of direct interaction; benign mutations in either subunit completely blocked translocation of two Tat substrates when present in combination. Finally, we have re-examined the effects of previously analyzed TatABC mutations under conditions of high translocation activity. Among numerous TatA or TatB mutations tested, TatA F39A alone blocked translocation, and only substitutions of P48 and F94 in TatC blocked translocation activity.

Mutations in Subunits of the Escherichia coli Twin-arginine Translocase Block Function via Differing Effects on Translocation Activity or Tat Complex Structure

We have used a combination of blue-native (BN) gel electrophoresis and protein purification to analyze the effects of TatA or TatC mutations on the structures of the primary TatABC and multimeric TatA complexes in Escherichia coli. Expression of wild-type TatABC leads to the production of a single major TatABC complex of 370 kDa and a heterogeneous set of TatA complexes of <100 kDa to approximately 500 kDa. Two TatC mutations that block translocation have different effects on complex structures. P48A causes massive defects in TatABC assembly, including a marked separation of the TatBC subunits and the production of TatB and TatC aggregates. In contrast, TatABC complexes from the inactive TatC F94A mutant are structurally intact, suggesting that this mutation affects translocation activity rather than assembly. Neither TatC mutation affects the separate TatA complexes, showing that assembly of the TatA complexes is independent of TatABC assembly or activity. In contrast, three TatA mutations affect both the TatA and TatABC complexes. F39A assembles into smaller, incorrectly organized TatA complexes and the TatABC complexes contain an incorrect TatB:TatC ratio and unusually large amounts of TatA. A triple mutant in the amphipathic region forms slightly larger TatA complexes that are likewise disorganized, and a mutant containing three glycine substitutions in the transmembrane (TM) span assembles as grossly affected TatA complexes that are much larger than wild-type complexes. These mutants lead to a partial failure of TatB to assemble correctly. The data show that the amphipathic and TM regions play critical roles in TatA complex assembly. All of the TatA mutations lead to partial or substantial defects in TatABC complex formation, demonstrating that the properties of TatA can have a marked influence on the TatABC complex.

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.

The Escherichia coli Twin-arginine Translocation Apparatus Incorporates a Distinct Form of TatABC Complex, Spectrum of Modular TatA Complexes and Minor TatAB Complex

The Tat system transports folded proteins across bacterial plasma and plant thylakoid membranes. To date, three key Tat subunits have been identified and mechanistic studies indicate the presence of two types of complex: a TatBC-containing substrate-binding unit and a separate TatA complex. Here, we used blue-native gel electrophoresis and affinity purification to study the nature of these complexes in Escherichia coli. Analysis of solubilized membrane shows that the bulk of TatB and essentially all of the TatC is found in a single 370kDa TatABC complex. TatABC was purified to homogeneity using an affinity tag on TatC and this complex runs apparently as an identical band. We conclude that this is the primary core complex, predicted to contain six or seven copies of TatBC together with a similar number of TatA subunits. However, the data indicate the presence of an additional form of Tat complex containing TatA and TatB, but not TatC; we speculate that this may be an assembly or disassembly intermediate of the translocator. The vast majority of TatA is found in separate complexes that migrate in blue-native gels as a striking ladder of bands with sizes ranging from under 100 kDa to over 500 kDa. Further analysis shows that the bands differ by an average of 34 kDa, indicating that TatA complexes are built largely, but possibly not exclusively, from modules of three or four TatA molecules. The range and nature of these complexes are similar in a TatC mutant that is totally inactive, indicating that the ladder of bands does not stem from ongoing translocation activity, and we show that purified TatA can self-assemble in vitro to form similar complexes. This spectrum of TatA complexes may provide the flexibility required to generate a translocon capable of transporting substrates of varying sizes across the plasma membrane in a folded state.

Twin-arginine-specific protein export in Escherichia coli

In many prokaryotic organisms, secretory proteins harboring a twin-arginine consensus motif are exported in a fully folded conformation via the twin-arginine translocation (Tat) pathway. In Escherichia coli, Tat involves the three structurally and functionally different membrane proteins TatA, TatB, and TatC. Whereas TatC proteins function in the specific recognition of substrate, TatA might be the major pore-forming subunit.

The Core TatABC Complex of the Twin-arginine Translocase in Escherichia coli: TatC Drives Assembly Whereas TatA is Essential for Stability

Current models for the action of the twin-arginine translocation (Tat) system propose that substrates bind initially to the TatBC subunits, after which a separate TatA complex is recruited to form an active translocon. Here, we have studied the roles of individual subunits in the assembly and stability of the core TatBC-containing substrate-binding complex. Previous studies have shown that TatB and TatC are active when fused together; we show here that deletion of the entire TatB transmembrane span from this Tat(BC) fusion inactivates the Tat system but does not affect assembly of the core complex. In this mutated complex, TatA is present but more loosely bound, indicating a role for TatB in the correct binding of TatA. In the absence of TatA, the truncated TatBC fusion protein still assembles into a complex of the correct magnitude, demonstrating that the transmembrane spans of TatC are the only determinants within the membrane bilayer that specify assembly of this complex. Further studies on both the Tat(BC) construct and the wild-type TatBC subunits show that the TatBC complex is unstable in the absence of TatA, and we show that TatA stabilises the TatB subunit specifically within this complex. The results demonstrate a dual role and location for TatA: in the functioning/maintenance of the core complex, and as a separate homo-oligomeric complex.

Dual topology of the Escherichia coli TatA protein

The Escherichia coli Tat system has unusual capacity of translocating folded proteins across the cytoplasmic membrane. The TatA protein is the most abundant known Tat component and consists of a transmembrane segment followed by an amphipathic helix and a hydrophilic C terminus. To study the operation mechanism of the Tat apparatus, we analyzed the topology of TatA. Intriguingly, alkaline phosphatase (PhoA)-positive fusions were obtained at positions Gly-38, Lys-40, Asp-51, and Thr-53, which are all located at the cytoplasmic C terminus of the TatA protein. Interestingly, replacing phoA with uidA at Thr-53 led to positive beta-glucuronidase fusion, implying cytoplasmic location of the TatA C terminus. To further determine cellular localization of the TatA C terminus, we deleted the phoA gene and left 46 exogenous residues, including the tobacco etch virus (Tev) protease cleavage site (Tcs) after Thr-53, yielding TatA(T53)::Tcs. Unlike the PhoA and UidA fusions, which abolished the TatA function, the TatA(T53)::Tcs construct was able to restore the growth of tatA mutants on the minimal trimethlyamine N-oxide media. In vitro and in vivo proteolysis assay showed that the Tcs site of TatA(T53)::Tcs was accessible from both the periplasm and cytoplasm, indicating a dual topology of the TatA C terminus. Importantly, growth conditions seemed to influence the protein level of TatA and the cytoplasmic accessibility of the Tcs site of TatA(T53)::Tcs. A function-linked change of the TatA topology is suggested, and its implication in protein transport is discussed.

Differential interactions between a twin-arginine signal peptide and its translocase in Escherichia coli

The twin-arginine translocation (Tat) machinery of the Escherichia coli inner membrane is dedicated to the export of proteins harboring a conserved SRRxFLK motif in their signal sequence. TatA, TatB, and TatC are the functionally essential constituents of the Tat machinery, but their precise function is unknown. Using site-specific crosslinking, we have analyzed interactions of the twin-arginine precursor preSufI with the Tat proteins upon targeting to inner membrane vesicles. TatA association is observed only in the presence of a transmembrane H(+) gradient. TatB is found in contact with the entire signal sequence and adjacent parts of mature SufI. Interaction of TatC with preSufI is, however, restricted to a discrete area around the consensus motif. The results reveal a hierarchy in targeting of a Tat substrate such that for the primary interaction, TatC is both necessary and sufficient while a subsequent association with TatB likely mediates transfer from TatC to the actual Tat pore.

The Tat protein translocation pathway and its role in microbial physiology

The Tat (twin arginine translocation) protein transport system functions to export folded protein substrates across the bacterial cytoplasmic membrane and to insert certain integral membrane proteins into that membrane. It is entirely distinct from the Sec pathway. Here, we describe our current knowledge of the molecular features of the Tat transport system. In addition, we discuss the roles that the Tat pathway plays in the bacterial cell, paying particular attention to the involvement of the Tat pathway in the biogenesis of cofactor-containing proteins, in cell wall biosynthesis and in bacterial pathogenicity.

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.

Consensus structural features of purified bacterial TatABC complexes

The twin-arginine translocation (Tat) system transports folded proteins across bacterial plasma membranes and the chloroplast thylakoid membrane. Here, we investigate the composition and structural organization of three different purified Tat complexes from Escherichia coli, Salmonella typhimurium and Agrobacterium tumefaciens. First, we demonstrate the functional activity of these Tat systems in vivo, since expression of the tatABC operons from S.typhimurium or A.tumefaciens in an E.coli tat null mutant strain resulted in efficient Tat-dependent export of an E.coli cofactor-containing substrate, TMAO reductase. The three isolated, affinity-tagged Tat complexes comprised TatA, TatB and TatC in each case, demonstrating a strong interaction between these three subunits. Single-particle electron microscopy studies of all three complexes revealed approximately oval-shaped, asymmetric particles with maximal dimensions up to 13 nm. A common feature is a number of stain-excluding densities surrounding more or less central pools of stain, suggesting protein-lined pores or cavities. The characteristics of size variation among the particles suggest a modular form of assembly and/or the recruitment of varying numbers of TatBC/TatA units. Despite low levels of sequence homology, the combined data indicate structural and functional conservation in the Tat systems of these three bacterial species.

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.

Identification of key regions within the Escherichia coli TatAB subunits

The twin-arginine translocation (Tat) system catalyzes the transport of folded proteins across the bacterial plasma membrane or the chloroplast thylakoid membrane. In Escherichia coli and most other species, three important tat genes have been identified but the structure and mechanism of this system are poorly understood; the role and location of TatA are particularly unclear. In this report we have used site-specific mutagenesis to probe the significance of conserved features of the related TatA/B subunits. We find that an apparent 'hinge' region between the transmembrane (TM) span and an adjacent amphipathic region is important in both proteins, in that substitution of turn-inducing residues inhibits the export of a natural Tat substrate. Surprisingly, large-scale mutagenesis of the conserved amphipathic regions of TatA and TatB leads only to minor effects on Tat-dependent export suggesting that this particular feature is not central to the translocation mechanism. This domain is, however, critical for the translocation process and we identify Gly/Pro residues in these regions of TatA/B that are essential for efficient export.

Topology determination and functional analysis of the Escherichia coli TatC protein

The TatC protein is an essential component of the bacterial Tat system. By using alkaline phosphatase and beta-glucuronidase fusions we found that TatC contains four transmembrane helices. Three insertions of Ala-Ser dipeptide at the cytoplasmic N- and C-termini and in the cytoplasmic loop had no or only partial effect on the TatC function. In contrast, five of seven insertions in the two periplasmic loops abolished the Tat function. Four insertions analyzed had no effect on the stability of the altered TatC proteins or on membrane assembly of the TatA and TatB proteins. These data provide a novel base for more detailed studies of the mechanism of the Tat system.