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

New insights into the Tat protein transport cycle from characterizing the assembled Tat translocon

The twin-arginine protein translocation (Tat) system transports folded proteins across the bacterial cytoplasmic membrane and the thylakoid membrane of chloroplasts. The Tat translocation site is transiently assembled by the recruitment of multiple TatA proteins to a substrate-activated TatBC receptor complex in a process requiring the protonmotive force. The ephemeral nature of the Tat translocation site has so far precluded its isolation. We now report that detergent solubilization of membranes during active transport allows the recovery of receptor complexes that are associated with elevated levels of TatA. We apply this biochemical analysis in combination with live cell fluorescence imaging to Tat systems trapped in the assembled state. We resolve sub-steps in the Tat translocation cycle and infer that TatA assembly precedes the functional interaction of TatA with a polar cluster site on TatC. We observe that dissipation of the protonmotive force releases TatA oligomers from the assembled translocation site demonstrating that the stability of the TatA oligomer does not depend on binding to the receptor complex and implying that the TatA oligomer is assembled at the periphery of the receptor complex. This work provides new insight into the Tat transport cycle and advances efforts to isolate the active Tat translocon.

Liquid-Liquid Phase Transition Drives Intra-chloroplast Cargo Sorting

In eukaryotic cells, organelle biogenesis is pivotal for cellular function and cell survival. Chloroplasts are unique organelles with a complex internal membrane network. The mechanisms of the migration of imported nuclear-encoded chloroplast proteins across the crowded stroma to thylakoid membranes are less understood. Here, we identified two Arabidopsis ankyrin-repeat proteins, STT1 and STT2, that specifically mediate sorting of chloroplast twin arginine translocation (cpTat) pathway proteins to thylakoid membranes. STT1 and STT2 form a unique hetero-dimer through interaction of their C-terminal ankyrin domains. Binding of cpTat substrate by N-terminal intrinsically disordered regions of STT complex induces liquid-liquid phase separation. The multivalent nature of STT oligomer is critical for phase separation. STT-Hcf106 interactions reverse phase separation and facilitate cargo targeting and translocation across thylakoid membranes. Thus, the formation of phase-separated droplets emerges as a novel mechanism of intra-chloroplast cargo sorting. Our findings highlight a conserved mechanism of phase separation in regulating organelle biogenesis.

Surface-exposed domains of TatB involved in the structural and functional assembly of the Tat translocase in Escherichia coli

Twin-arginine-dependent translocases transport folded proteins across bacterial, archaeal, and chloroplast membranes. Upon substrate binding, they assemble from hexahelical TatC and single-spanning TatA and TatB membrane proteins. Although structural and functional details of individual Tat subunits have been reported previously, the sequence and dynamics of Tat translocase assembly remain to be determined. Employing the zero-space cross-linker N,N'-dicyclohexylcarbodiimide (DCCD) in combination with LC-MS/MS, we identified as yet unknown intra- and intermolecular contact sites of TatB and TatC. In addition to their established intramembrane binding sites, both proteins were thus found to contact each other through the soluble N terminus of TatC and the interhelical linker region around the conserved glutamyl residue Glu⁴⁹ of TatB from Escherichia coli Functional analyses suggested that by interacting with the TatC N terminus, TatB improves the formation of a proficient substrate recognition site of TatC. The Glu⁴⁹ region of TatB was found also to contact distinct downstream sites of a neighboring TatB molecule and to thereby mediate oligomerization of TatB within the TatBC receptor complex. Finally, we show that global DCCD-mediated cross-linking of TatB and TatC in membrane vesicles or, alternatively, creating covalently linked TatC oligomers prevents TatA from occupying a position close to the TatBC-bound substrate. Collectively, our results are consistent with a circular arrangement of the TatB and TatC units within the TatBC receptor complex and with TatA entering the interior TatBC-binding cavity through lateral gates between TatBC protomers.

The Tat protein transport system: intriguing questions and conundrums

The Tat machinery catalyzes the transport of folded proteins across the cytoplasmic membrane in bacteria and the thylakoid membrane in plants. Transport occurs only in the presence of an electric field (Δψ) and/or a pH (ΔpH) gradient, and thus, Tat transport is considered to be dependent on the proton motive force (pmf). This presents a fundamental and major challenge, namely, that the Tat system catalyzes the movement of large folded protein cargos across a membrane without collapse of ion gradients. Current models argue that the active translocon assembles de novo for each cargo transported, thus providing an effective gating mechanism to minimize ion leakage. A limited structural understanding of the intermediates occurring during transport and the role of the pmf in stabilizing and/or driving this process have hindered the development of more detailed models. A fundamental question that remains unanswered is whether the pmf is actually 'consumed', providing an energetic driving force for transport, or alternatively, whether its presence is instead necessary to provide the appropriate environment for the translocon components to become active. Including addressing this issue in greater detail, we explore a series of additional questions that challenge current models, and, hopefully, motivate future work.

Analysis of haloarchaeal twin-arginine translocase pathway reveals the diversity of the machineries

The twin-arginine translocase (Tat) pathway transports folded proteins across the plasma membrane and plays a critical role in protein transport in haloarchaea. Computational analysis and previous experimental evidence suggested that the Tat pathway transports almost the entire secretome in haloarchaea. The TatC, receptor component of this pathway shows greater variation in membrane topology in haloarchaea than in other organisms. The presence of a unique fourteen-transmembrane TatC homolog (TatC_t) in haloarchaea, over and above the expected TatC topological variants, indicates a strong correlation between the additional homologs and the large number of substrates transported via the haloarchaeal Tat pathway. Various combinations of TatC homologs with different topologies—TatC_o, TatC_t, TatC_n, and TatC_x have been observed in haloarchaea. In this report, on the basis of these combinations we have segregated all haloarchaeal Tat substrates into two groups. The first group consists of substrates that are transported by TatC_t alone, whereas the second group consists of substrates that are transported by the other TatC homologs (TatC_o, TatC_n, and TatC_x). The various haloarchaea TatA components also shows the possible segregation towards the substrates. We have also identified the possible homologs for Tat substrate chaperones, which act as a quality-control mechanism for proper protein folding. Further sequence analysis implies that the two TatC domains of TatC_t complement each other's functionally. Substrate analysis also revealed subtle differences between the substrates being transported by various homologs: further experimental analysis is therefore required for better understanding of the complexities of the haloarchaeal Tat pathway.

Routing of thylakoid lumen proteins by the chloroplast twin arginine transport pathway

Thylakoids are complex sub-organellar membrane systems whose role in photosynthesis makes them critical to life. Thylakoids require the coordinated expression of both nuclear- and plastid-encoded proteins to allow rapid response to changing environmental conditions. Transport of cytoplasmically synthesized proteins to thylakoids or the thylakoid lumen is complex; the process involves transport across up to three membrane systems with routing through three aqueous compartments. Protein transport in thylakoids is accomplished by conserved ancestral prokaryotic plasma membrane translocases containing novel adaptations for the sub-organellar location. This review focuses on the evolutionarily conserved chloroplast twin arginine transport (cpTat) pathway. An overview is provided of known aspects of the cpTat components, energy requirements, and mechanisms with a focus on recent discoveries. Some of the most exciting new studies have been in determining the structural architecture of the membrane complex involved in forming the point of passage for the precursor and binding features of the translocase components. The cpTat system is of particular interest because it transports folded protein domains using only the proton motive force for energy. The implications for mechanism of translocation by recent studies focusing on interactions between membrane Tat components and with the translocating precursor will be discussed.

Thylakoid-integrated recombinant Hcf106 participates in the chloroplast twin arginine transport system

The chloroplast twin arginine transport (cpTat) system distinguishes itself as a protein transport pathway by translocating fully folded proteins, using the proton-motive force (PMF) as the sole source of energy. The cpTat pathway is evolutionarily conserved with the Tat pathway found in the plasma membrane of many prokaryotes. The cpTat (Escherichia coli) system uses three proteins, Tha4 (TatA), Hcf106 (TatB), and cpTatC (TatC), to form a transient translocase allowing the passage of precursor proteins. Briefly, cpTatC and Hcf106, with Tha4, form the initial receptor complex responsible for precursor protein recognition and binding in an energy-independent manner, while a separate pool of Tha4 assembles with the precursor-bound receptor complex in the presence the PMF. Analysis by blue-native polyacrylamide gel electrophoresis (BN-PAGE) shows that the receptor complex, in the absence of precursor, migrates near 700 kDa and contains cpTatC and Hcf106 with little Tha4 remaining after detergent solubilization. To investigate the role that Hcf106 may play in receptor complex oligomerization and/or stability, systematic cysteine substitutions were made in positions from the N-terminal transmembrane domain to the end of the predicted amphipathic helix of the protein. BN-PAGE analysis allowed us to identify the locations of amino acids in Hcf106 that were critical for interacting with cpTatC. Oxidative cross-linking allowed us to map interactions of the transmembrane domain and amphipathic helix region of Hcf106. In addition, we showed that in vitro expressed, integrated Hcf106 can interact with the precursor signal peptide domain and imported cpTatC, strongly suggesting that a subpopulation of the integrated Hcf106 is participating in competent cpTat complexes.

Precursor-Receptor Interactions in the Twin Arginine Protein Transport Pathway Probed with a New Receptor Complex Preparation

The twin arginine translocation (Tat) system moves folded proteins across the cytoplasmic membrane of bacteria and the thylakoid membrane of plant chloroplasts. Signal peptide-bearing substrates of the Tat pathway (precursor proteins) are recognized at the membrane by the TatBC receptor complex. The only established preparation of the TatBC complex uses the detergent digitonin, rendering it unsuitable for biophysical analysis. Here we show that the detergent glyco-diosgenin (GDN) can be used in place of digitonin to isolate homogeneous TatBC complexes that bind precursor proteins with physiological specificity. We use this new preparation to quantitatively characterize TatBC-precursor interactions in a fully defined system. Additionally, we show that the GDN-solubilized TatBC complex co-purifies with substantial quantities of phospholipids.

Unanticipated functional diversity among the TatA-type components of the Tat protein translocase

Twin-arginine translocation (Tat) systems transport folded proteins that harbor a conserved arginine pair in their signal peptides. They assemble from hexahelical TatC-type and single-spanning TatA-type proteins. Many Tat systems comprise two functionally diverse, TatA-type proteins, denominated TatA and TatB. Some bacteria in addition express TatE, which thus far has been characterized as a functional surrogate of TatA. For the Tat system of Escherichia coli we demonstrate here that different from TatA but rather like TatB, TatE contacts a Tat signal peptide independently of the proton-motive force and restricts the premature processing of a Tat signal peptide. Furthermore, TatE embarks at the transmembrane helix five of TatC where it becomes so closely spaced to TatB that both proteins can be covalently linked by a zero-space cross-linker. Our results suggest that in addition to TatB and TatC, TatE is a further component of the Tat substrate receptor complex. Consistent with TatE being an autonomous TatAB-type protein, a bioinformatics analysis revealed a relatively broad distribution of the tatE gene in bacterial phyla and highlighted unique protein sequence features of TatE orthologs.

A Hinged Signal Peptide Hairpin Enables Tat-Dependent Protein Translocation

The Tat machinery catalyzes the transport of folded proteins across the bacterial cytoplasmic membrane and the thylakoid membrane in plants. Using fluorescence quenching and cross-linking approaches, we demonstrate that the Escherichia coli TatBC complex catalyzes insertion of a pre-SufI signal peptide hairpin that penetrates about halfway across the membrane bilayer. Analysis of 512 bacterial Tat signal peptides using secondary structure prediction and docking algorithms suggest that this hairpin interaction mode is generally conserved. An internal cross-link in the signal peptide that blocks transport but does not affect binding indicates that a signal peptide conformational change is required during translocation. These results suggest, to our knowledge, a novel hairpin-hinge model in which the signal peptide hairpin unhinges during movement of the mature domain across the membrane. Thus, in addition to enabling the necessary recognition, the interaction of Tat signal peptides with the receptor complex plays a critical role in the transport process itself.

Structural features of the TatC membrane protein that determine docking and insertion of a twin-arginine signal peptide

Twin-arginine translocation (Tat) systems transport folded proteins across cellular membranes with the concerted action of mostly three membrane proteins: TatA, TatB, and TatC. Hetero-oligomers of TatB and TatC form circular substrate-receptor complexes with a central binding cavity for twin-arginine-containing signal peptides. After binding of the substrate, energy from an electro-chemical proton gradient is transduced into the recruitment of TatA oligomers and into the actual translocation event. We previously reported that Tat-dependent protein translocation into membrane vesicles of Escherichia coli is blocked by the compound N,N'-dicyclohexylcarbodiimide (DCCD, DCC). We have now identified a highly conserved glutamate residue in the transmembrane region of E. coli TatC, which when modified by DCCD interferes with the deep insertion of a Tat signal peptide into the TatBC receptor complex. Our findings are consistent with a hydrophobic binding cavity formed by TatB and TatC inside the lipid bilayer. Moreover, we found that DCCD mediates discrete intramolecular cross-links of E. coli TatC involving both its N- and C-tails. These results confirm the close proximity of two distant sequence sections of TatC proposed to concertedly function as the primary docking site for twin-arginine signal peptides.

Signal Peptide Hydrophobicity Modulates Interaction with the Twin-Arginine Translocase

The general secretory pathway (Sec) and twin-arginine translocase (Tat) operate in parallel to export proteins across the cytoplasmic membrane of prokaryotes and the thylakoid membrane of plant chloroplasts. Substrates are targeted to their respective machineries by N-terminal signal peptides that share a tripartite organization; however, Tat signal peptides harbor a conserved and almost invariant arginine pair that is critical for efficient targeting to the Tat machinery. Tat signal peptides interact with a membrane-bound receptor complex comprised of TatB and TatC components, with TatC containing the twin-arginine recognition site. Here, we isolated suppressors in the signal peptide of the Tat substrate, SufI, that restored Tat transport in the presence of inactivating substitutions in the TatC twin-arginine binding site. These suppressors increased signal peptide hydrophobicity, and copurification experiments indicated that they restored binding to the variant TatBC complex. The hydrophobic suppressors could also act in cis to suppress substitutions at the signal peptide twin-arginine motif that normally prevent targeting to the Tat pathway. Highly hydrophobic variants of the SufI signal peptide containing four leucine substitutions retained the ability to interact with the Tat system. The hydrophobic signal peptides of two Sec substrates, DsbA and OmpA, containing twin lysine residues, were shown to mediate export by the Tat pathway and to copurify with TatBC. These findings indicate that there is unprecedented overlap between Sec and Tat signal peptides and that neither the signal peptide twin-arginine motif nor the TatC twin-arginine recognition site is an essential mechanistic feature for operation of the Tat pathway.IMPORTANCE Protein export is an essential process in all prokaryotes. The Sec and Tat export pathways operate in parallel, with the Sec machinery transporting unstructured precursors and the Tat pathway transporting folded proteins. Proteins are targeted to the Tat pathway by N-terminal signal peptides that contain an almost invariant twin-arginine motif. Here, we make the surprising discovery that the twin arginines are not essential for recognition of substrates by the Tat machinery and that this requirement can be bypassed by increasing the signal peptide hydrophobicity. We further show that signal peptides of bona fide Sec substrates can also mediate transport by the Tat pathway. Our findings suggest that key features of the Tat targeting mechanism have evolved to prevent mistargeting of substrates to the Sec pathway rather than being a critical requirement for function of the Tat pathway.

TatA complexes exhibit a marked change in organisation in response to expression of the TatBC complex

The twin-arginine translocation (Tat) system is an integral membrane protein complex that accomplishes the remarkable feat of transporting large, fully folded polypeptides across the inner membrane of bacteria, into the periplasm. In Escherichia coli, Tat comprises three membrane proteins: TatA, TatB and TatC. How these proteins arrange themselves in the inner membrane to permit passage of Tat substrates, whilst maintaining membrane integrity, is still poorly understood. TatA is the most abundant component of this complex and facilitates assembly of the transport mechanism. We have utilised immunogold labelling in combination with array tomography to gain insight into the localisation and distribution of the TatA protein in E. coli cells. We show that TatA exhibits a uniform distribution throughout the inner membrane of E. coli and that altering the expression of TatBC shows a previously uncharacterised distribution of TatA in the inner membrane. Array tomography was used to provide our first insight into this altered distribution of TatA in three-dimensional space, revealing that this protein forms linear clusters in the inner membrane of E. coli upon increased expression of TatBC. This is the first indication that TatA organisation in the inner membrane alters in response to changes in Tat subunit stoichiometry.

A signal sequence suppressor mutant that stabilizes an assembled state of the twin arginine translocase

The twin-arginine protein translocation (Tat) system mediates transport of folded proteins across the cytoplasmic membrane of bacteria and the thylakoid membrane of chloroplasts. The Tat system of Escherichia coli is made up of TatA, TatB, and TatC components. TatBC comprise the substrate receptor complex, and active Tat translocases are formed by the substrate-induced association of TatA oligomers with this receptor. Proteins are targeted to TatBC by signal peptides containing an essential pair of arginine residues. We isolated substitutions, locating to the transmembrane helix of TatB that restored transport activity to Tat signal peptides with inactivating twin arginine substitutions. A subset of these variants also suppressed inactivating substitutions in the signal peptide binding site on TatC. The suppressors did not function by restoring detectable signal peptide binding to the TatBC complex. Instead, site-specific cross-linking experiments indicate that the suppressor substitutions induce conformational change in the complex and movement of the TatB subunit. The TatB F13Y substitution was associated with the strongest suppressing activity, even allowing transport of a Tat substrate lacking a signal peptide. In vivo analysis using a TatA-YFP fusion showed that the TatB F13Y substitution resulted in signal peptide-independent assembly of the Tat translocase. We conclude that Tat signal peptides play roles in substrate targeting and in triggering assembly of the active translocase.

Assembling the Tat protein translocase

The twin-arginine protein translocation system (Tat) transports folded proteins across the bacterial cytoplasmic membrane and the thylakoid membranes of plant chloroplasts. The Tat transporter is assembled from multiple copies of the membrane proteins TatA, TatB, and TatC. We combine sequence co-evolution analysis, molecular simulations, and experimentation to define the interactions between the Tat proteins of Escherichia coli at molecular-level resolution. In the TatBC receptor complex the transmembrane helix of each TatB molecule is sandwiched between two TatC molecules, with one of the inter-subunit interfaces incorporating a functionally important cluster of interacting polar residues. Unexpectedly, we find that TatA also associates with TatC at the polar cluster site. Our data provide a structural model for assembly of the active Tat translocase in which substrate binding triggers replacement of TatB by TatA at the polar cluster site. Our work demonstrates the power of co-evolution analysis to predict protein interfaces in multi-subunit complexes.

DOI:http://dx.doi.org/10.7554/eLife.20718.001

High Throughput Screen for Escherichia coli Twin Arginine Translocation (Tat) Inhibitors

The twin arginine translocation (Tat) pathway transports fully-folded and assembled proteins in bacteria, archaea and plant thylakoids. The Tat pathway contributes to the virulence of numerous bacterial pathogens that cause disease in humans, cattle and poultry. Thus, the Tat pathway has the potential to be a novel therapeutic target. Deciphering the Tat protein transport mechanism has been challenging since the active translocon only assembles transiently in the presence of substrate and a proton motive force. To identify inhibitors of Tat transport that could be used as biochemical tools and possibly as drug development leads, we developed a high throughput screen (HTS) to assay the effects of compounds in chemical libraries against protein export by the Escherichia coli Tat pathway. The primary screen is a live cell assay based on a fluorescent Tat substrate that becomes degraded in the cytoplasm when Tat transport is inhibited. Consequently, low fluorescence in the presence of a putative Tat inhibitor was scored as a hit. Two diverse chemical libraries were screened, yielding average Z'-factors of 0.74 and 0.44, and hit rates of ~0.5% and 0.04%, respectively. Hits were evaluated by a series of secondary screens. Electric field gradient (Δψ) measurements were particularly important since the bacterial Tat transport requires a Δψ. Seven low IC₅₀ hits were eliminated by Δψ assays, suggesting ionophore activity. As Δψ collapse is generally toxic to animal cells and efficient membrane permeability is generally favored during the selection of library compounds, these results suggest that secondary screening of hits against electrochemical effects should be done early during hit validation. Though none of the short-listed compounds inhibited Tat transport directly, the screening and follow-up assays developed provide a roadmap to pursue Tat transport inhibitors.

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.

TatE as a Regular Constituent of Bacterial Twin-arginine Protein Translocases

Twin-arginine translocation (Tat) systems mediate the transmembrane translocation of completely folded proteins that possess a conserved twin-arginine (RR) motif in their signal sequences. Many Tat systems consist of three essential membrane components named TatA, TatB, and TatC. It is not understood why some bacteria, in addition, constitutively express a functional paralog of TatA called TatE. Here we show, in live Escherichia coli cells, that, upon expression of a Tat substrate protein, fluorescently labeled TatE-GFP relocates from a rather uniform distribution in the plasma membrane into a number of discrete clusters. Clustering strictly required an intact RR signal peptide and the presence of the TatABC subunits, suggesting that TatE-GFP associates with functional Tat translocases. In support of this notion, site-specific photo cross-linking revealed interactions of TatE with TatA, TatB, and TatC. The same approach also disclosed a pronounced tendency of TatE and TatA to hetero-oligomerize. Under in vitro conditions, we found that TatE replaces TatA inefficiently. Our collective results are consistent with TatE being a regular constituent of the Tat translocase in E. coli.

The TatC component of the twin-arginine protein translocase functions as an obligate oligomer

The Tat protein export system translocates folded proteins across the bacterial cytoplasmic membrane and the plant thylakoid membrane. The Tat system in Escherichia coli is composed of TatA, TatB and TatC proteins. TatB and TatC form an oligomeric, multivalent receptor complex that binds Tat substrates, while multiple protomers of TatA assemble at substrate‐bound TatBC receptors to facilitate substrate transport. We have addressed whether oligomerisation of TatC is an absolute requirement for operation of the Tat pathway by screening for dominant negative alleles of tatC that inactivate Tat function in the presence of wild‐type tatC. Single substitutions that confer dominant negative TatC activity were localised to the periplasmic cap region. The variant TatC proteins retained the ability to interact with TatB and with a Tat substrate but were unable to support the in vivo assembly of TatA complexes. Blue‐native PAGE analysis showed that the variant TatC proteins produced smaller TatBC complexes than the wild‐type TatC protein. The substitutions did not alter disulphide crosslinking to neighbouring TatC molecules from positions in the periplasmic cap but abolished a substrate‐induced disulphide crosslink in transmembrane helix 5 of TatC. Our findings show that TatC functions as an obligate oligomer.

Initial assembly steps of a translocase for folded proteins

The so-called Tat (twin-arginine translocation) system transports completely folded proteins across cellular membranes of archaea, prokaryotes and plant chloroplasts. Tat-directed proteins are distinguished by a conserved twin-arginine (RR-) motif in their signal sequences. Many Tat systems are based on the membrane proteins TatA, TatB and TatC, of which TatB and TatC are known to cooperate in binding RR-signal peptides and to form higher-order oligomeric structures. We have now elucidated the fine architecture of TatBC oligomers assembled to form closed intramembrane substrate-binding cavities. The identification of distinct homonymous and heteronymous contacts between TatB and TatC suggest that TatB monomers coalesce into dome-like TatB structures that are surrounded by outer rings of TatC monomers. We also show that these TatBC complexes are approached by TatA protomers through their N-termini, which thereby establish contacts with TatB and membrane-inserted RR-precursors.

The twin-arginine translocation complex consists of TatA, TatB and TatC subunits and transports folded proteins across cellular membranes. Here, using photocrosslinking, the authors show that TatB monomers form dome-like structures that are surrounded by TatC monomers enabling lateral access of TatA.

Mechanistic Aspects of Folded Protein Transport by the Twin Arginine Translocase (Tat)

The twin arginine translocase (Tat) transports folded proteins of widely varying size across ionically tight membranes with only 2-3 components of machinery and the proton motive force. Tat operates by a cycle in which the receptor complex combines with the pore-forming component to assemble a new translocase for each substrate. Recent data on component and substrate organization in the receptor complex and on the structure of the pore complex inform models for translocase assembly and translocation. A translocation mechanism involving local transient bilayer rupture is discussed.

Substrate-gated docking of pore subunit Tha4 in the TatC cavity initiates Tat translocase assembly

The twin-arginine translocase (Tat) transports folded proteins across tightly sealed membranes. cpTatC is the core component of the thylakoid translocase and coordinates transport through interactions with the substrate signal peptide and other Tat components, notably the Tha4 pore-forming component. Here, Cys-Cys matching mapped Tha4 contact sites on cpTatC and assessed the role of signal peptide binding on Tha4 assembly with the cpTatC-Hcf106 receptor complex. Tha4 made contact with a peripheral cpTatC site in nonstimulated membranes. In the translocase, Tha4 made an additional contact within the cup-shaped cavity of cpTatC that likely seeds Tha4 polymerization to form the pore. Substrate binding triggers assembly of Tha4 onto the interior site. We provide evidence that the substrate signal peptide inserts between cpTatC subunits arranged in a manner that conceivably forms an enclosed chamber. The location of the inserted signal peptide and the Tha4-cpTatC contact data suggest a model for signal peptide-gated Tha4 entry into the chamber to form the translocase.

Structural biology of Tat protein transport

The Tat protein transport system is found in the cytoplasmic membrane of prokaryotes and the thylakoid membrane of plant chloroplasts. Unusually, the Tat system translocates proteins only after they have folded. Proteins are targeted to the Tat system by specific N-terminal signal peptides. High resolution structures have recently been determined for the TatA and TatC proteins that form the Tat translocation site. These structures provide a molecular framework for understanding the mechanism of Tat transport. The interactions between TatC and the signal peptide of the substrate protein can be provisionally modelled. However, the way that TatA and TatC combine in the active translocation site remains to be definitively established.

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.

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.

Live cell imaging shows reversible assembly of the TatA component of the twin-arginine protein transport system

The twin-arginine translocation (Tat) machinery transports folded proteins across the cytoplasmic membrane of bacteria and the thylakoid membrane of chloroplasts. It has been inferred that the Tat translocation site is assembled on demand by substrate-induced association of the protein TatA. We tested this model by imaging YFP-tagged TatA expressed at native levels in living Escherichia coli cells in the presence of low levels of the TatA paralogue TatE. Under these conditions the TatA-YFP fusion supports full physiological Tat transport activity. In agreement with the TatA association model, raising the number of transport-competent substrate proteins within the cell leads to an increase in the number of large TatA complexes present. Formation of these complexes requires both a functional TatBC substrate receptor and the transmembrane proton motive force (PMF). Removing the PMF causes TatA complexes to dissociate, except in strains with impaired Tat transport activity. Based on these observations we propose that TatA assembly reaches a critical point at which oligomerization can be reversed only by substrate transport. In contrast to TatA-YFP, the oligomeric states of TatB-YFP and TatC-YFP fusions are not affected by substrate or the PMF, although TatB-YFP oligomerization does require TatC.

Substrate-dependent assembly of the Tat translocase as observed in live Escherichia coli cells

The twin-arginine translocation (Tat) pathway guides fully folded proteins across membranes of bacteria, archaea and plant chloroplasts. In Escherichia coli, Tat-specific transport is executed in a still largely unknown manner by three functionally diverse membrane proteins, termed TatA, TatB, and TatC. In order to follow the intracellular distribution of the TatABC proteins in live E. coli cells, we have individually expressed fluorophore-tagged versions of each Tat protein in addition to a set of chromosomally encoded TatABC proteins. In this way, a Tat translocase could form from the native TatABC proteins and be visualized via the association of a fluorescent Tat variant. A functionally active TatA-green fluorescent protein fusion was found to re-locate from a uniform distribution in the membrane into a few clusters preferentially located at the cell poles. Clustering was absolutely dependent on the co-expression of functional Tat substrates, the proton-motive force, and the cognate TatBC subunits. Likewise, polar cluster formation of a functional TatB-mCherry fusion required TatA and TatC and that of a functional TatC-mCherry fusion a functional Tat substrate. Furthermore we directly demonstrate the co-localization of TatA and TatB in the same fluorescent clusters. Our collective results are consistent with distinct Tat translocation sites dynamically forming in vivo in response to newly synthesized Tat substrates.