Structure and dynamics of plant TatA in micelles and lipid bilayers studied by solution NMR

The polar amino acid in the TatA transmembrane helix is not strictly necessary for protein function

The twin-arginine translocation (Tat) pathway utilizes the proton-motive force (pmf) to transport folded proteins across cytoplasmic membranes in bacteria and archaea, as well as across the thylakoid membrane in plants and the inner membrane in mitochondria. In most species, the minimal components required for Tat activity consist of three subunits, TatA, TatB, and TatC. Previous studies have shown that a polar amino acid is present at the N-terminus of the TatA transmembrane helix (TMH) across many different species. In order to systematically assess the functional importance of this polar amino acid in the TatA TMH in Escherichia coli, we examined a complete set of 19-amino-acid substitutions. Unexpectedly, although being preferred overall, our experiments suggest that the polar amino acid is not necessary for a functional TatA. Hydrophilicity and helix-stabilizing properties of this polar amino acid were found to be highly correlated with the Tat activity. Specifically, change in charge status of the amino acid side chain due to pH resulted in a shift in hydrophilicity, which was demonstrated to impact the Tat transport activity. Furthermore, we identified a four-residue motif at the N-terminus of the TatA TMH by sequence alignment. Using a biochemical approach, we found that the N-terminal motif was functionally significant, with evidence indicating a potential role in the preference for utilizing different pmf components. Taken together, these findings yield new insights into the functionality of TatA and its potential role in the Tat transport mechanism.

Hydrophobic mismatch is a key factor in protein transport across lipid bilayer membranes via the Tat pathway

The twin-arginine translocation (Tat) pathway transports folded proteins across membranes in bacteria, thylakoids, plant mitochondria, and archaea. In most species, the active Tat machinery consists of three independent subunits: TatA, TatB, and TatC. TatA and TatB possess short transmembrane alpha helices (TMHs), both of which are only 15 residues long in Escherichia coli. Such short TMHs cause a hydrophobic mismatch between Tat subunits and the membrane bilayer, although the functional significance of this mismatch is unclear. Here, we sought to address the functional importance of the hydrophobic mismatch in the Tat transport mechanism in E. coli. We conducted three different assays to evaluate the effect of TMH length mutants on Tat activity and observed that the TMHs of TatA and TatB appear to be evolutionarily tuned to 15 amino acids, with activity dropping off following any modification of this length. Surprisingly, TatA and TatB with as few as 11 residues in their TMHs can still insert into the membrane bilayer, albeit with a decline in membrane integrity. These findings support a model of Tat transport utilizing localized toroidal pores that form when the membrane bilayer is thinned to a critical threshold. In this context, we conclude that the 15-residue length of the TatA and TatB TMHs can be seen as a compromise between the need for some hydrophobic mismatch to allow the membrane to reversibly reach the threshold thinness required for toroidal pore formation and the permanently destabilizing effect of placing even shorter helices into these energy-transducing membranes.

Soluble TatA forms oligomers that interact with membranes: Structure and insertion studies of a versatile protein transporter

The twin-arginine translocase (Tat) mediates the transport of already-folded proteins across membranes in bacteria, plants and archaea. TatA is a small, dynamic subunit of the Tat-system that is believed to be the active component during target protein translocation. TatA is foremost characterized as a bitopic membrane protein, but has also been found to partition into a soluble, oligomeric structure of yet unknown function. To elucidate the interplay between the membrane-bound and soluble forms we have investigated the oligomers formed by Arabidopsis thaliana TatA. We used several biophysical techniques to study the oligomeric structure in solution, the conversion that takes place upon interaction with membrane models of different compositions, and the effect on bilayer integrity upon insertion. Our results demonstrate that in solution TatA oligomerizes into large objects with a high degree of ordered structure. Upon interaction with lipids, conformational changes take place and TatA disintegrates into lower order oligomers. The insertion of TatA into lipid bilayers causes a temporary leakage of small molecules across the bilayer. The disruptive effect on the membrane is dependent on the liposome's negative surface charge density, with more leakage observed for purely zwitterionic bilayers. Overall, our findings indicate that A. thaliana TatA forms oligomers in solution that insert into bilayers, a process that involves reorganization of the protein oligomer.

Functional reconstitution of TatB into the thylakoidal Tat translocase

We have established an experimental system for the functional analysis of thylakoidal TatB, a component of the membrane-integral TatBC receptor complex of the thylakoidal Twin-arginine protein transport (Tat) machinery. For this purpose, the intrinsic TatB activity of isolated pea thylakoids was inhibited by affinity-purified antibodies and substituted by supplementing the assays with TatB protein either obtained by in vitro translation or purified after heterologous expression in E. coli. Tat transport activity of such reconstituted thylakoids, which was analysed with the authentic Tat substrate pOEC16, reached routinely 20-25% of the activity of mock-treated thylakoid vesicles analysed in parallel. In contrast, supplementation of the assays with the purified antigen comprising all but the N-terminal transmembrane helix of thylakoidal TatB did not result in Tat transport reconstitution which confirms that transport relies strictly on the activity of the TatB protein added and is not due to restoration of the intrinsic TatB activity by antibody release. Unexpectedly, even a mutated TatB protein (TatB,E10C) assumed to be incapable of assembling into the TatBC receptor complex showed low but considerable transport reconstitution underlining the sensitivity of the approach and its suitability for further functional analyses of protein variants. Finally, quantification of TatB demand suggests that TatA and TatB are required in approximately equimolar amounts to achieve Tat-dependent thylakoid transport.

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.

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.