Abstract
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.
Highlights
Bacteria, archaea, and plant chloroplasts have the capability to transport precursor proteins in a folded state across membranes
We found TatE to interact with TatA, TatB, and TatC and to localize to active Tat translocases in vivo
We previously demonstrated that TatE displays the properties of a constitutive component of the E. coli Tat translocase, as it localizes to functional Tat translocases in living cells and interacts individually with TatA, TatB, and TatC28
Summary
TatE and TatB of E. coli share functional properties. We previously demonstrated that TatE displays the properties of a constitutive component of the E. coli Tat translocase, as it localizes to functional Tat translocases in living cells and interacts individually with TatA, TatB, and TatC28. The sequence alignment of B. subtilis TatAc and E. coli TatE, both exhibiting the ExxxxK motif, gave 45% identical amino acid residues within the aligned parts indicating that TatAc might more likely be a functional paralog of E. coli TatE than of E. coli TatA All these sequence compilations demonstrate a wide distribution of TatA-family members that possess a Lys at a position equivalent to the 8th position of enterobacaterial TatE (Fig. 3B). The Lys[8] residue, which obviously is a hallmark of TatE-type orthologs, is likely to mediate different contacts with TatC This is strongly suggested by the phenotype of an E. coli TatB variant, which due to a Glu[8] to Lys[8] substitution associates with TatC in a manner that allows proficient recognition of otherwise transport-defective signal peptides[38,39,40]. TatA-family members naturally displaying a positively charged residue at position 8, such as most
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