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

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 114; no. 10; pp. E1958 - E1967
• Main Authors: Huang, Qi, Alcock, Felicity, Kneuper, Holger, Deme, Justin C., Rollauer, Sarah E., Lea, Susan M., Berks, Ben C., Palmer, Tracy
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 07.03.2017
• Series: PNAS Plus
• Subjects: Amino Acid Sequence; Amino Acid Substitution; Arginine - chemistry; Arginine - metabolism; Binding Sites; Biological Sciences; Chloroplasts; Cytoplasm; E coli; Escherichia coli - genetics; Escherichia coli - metabolism; Escherichia coli Proteins - chemistry; Escherichia coli Proteins - genetics; Escherichia coli Proteins - metabolism; Gene Expression Regulation, Bacterial; Membrane Transport Proteins - chemistry; Membrane Transport Proteins - genetics; Membrane Transport Proteins - metabolism; Models, Molecular; Mutation; Peptides; PNAS Plus; Protein Binding; Protein Conformation, alpha-Helical; Protein Folding; Protein Interaction Domains and Motifs; Protein Sorting Signals; Protein Transport; Proteins; Substrate Specificity; protein transport; twin arginine signal peptide; Tat pathway; genetic suppressor
• DOI: 10.1073/pnas.1615056114
• ISSN: 0027-8424; 1091-6490

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.