A subset of bacterial inner membrane proteins integrated by the twin‐arginine translocase

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Abstract
Summary: A group of bacterial exported proteins are synthesized with N‐terminal signal peptides containing a SRRxFLK ‘twin‐arginine’ amino acid motif. Proteins bearing twin‐arginine signal peptides are targeted post‐translationally to the twin‐arginine translocation (Tat) system which transports folded substrates across the inner membrane. In Escherichia coli, most integral inner membrane proteins are assembled by a co‐translational process directed by SRP/FtsY, the SecYEG translocase, and YidC. In this work we define a novel class of integral membrane proteins assembled by a Tat‐dependent mechanism. We show that at least five E. coli Tat substrate proteins contain hydrophobic C‐terminal transmembrane helices (or ‘C‐tails’). Fusions between the identified transmembrane C‐tails and the exclusively Tat‐dependent reporter proteins TorA and SufI render the resultant chimeras membrane‐bound. Export‐linked signal peptide processing and membrane integration of the chimeras is shown to be both Tat‐dependent and YidC‐independent. It is proposed that the mechanism of membrane integration of proteins by the Tat system is fundamentally distinct from that employed for other bacterial inner membrane proteins.