An Essential Component of a Novel Bacterial Protein Export System with Homologues in Plastids and Mitochondria

TLDR

Proteins are transported across bacterial and chloroplast membranes by translocases that recognize N‑terminal targeting signals, often via the Sec apparatus. The authors identify a novel component of a Sec‑independent bacterial translocase. They show that the component is encoded by the unassigned E. coli reading frame yigU (later named tatC) and that its disruption blocks export of multiple twin‑arginine‑containing precursor proteins while leaving the Sec pathway intact.

Abstract

Proteins are transported across the bacterial plasma membrane and the chloroplast thylakoid membrane by means of protein translocases that recognize N-terminal targeting signals in their cognate substrates. Transport of many of these proteins involves the well defined Sec apparatus that operates in both membranes. We describe here the identification of a novel component of a bacterial Sec-independent translocase. The system probably functions in a similar manner to a Sec-independent translocase in the thylakoid membrane, and substrates for both systems bear a characteristic twin-arginine motif in the targeting peptide. The translocase component is encoded in<i>Escherichia coli</i> by an unassigned reading frame,<i>yigU</i>, disruption of which blocks the export of at least five twin-Arg-containing precursor proteins that are predicted to bind redox cofactors, and hence fold, prior to translocation. The Sec pathway remains unaffected in the deletion strain. The gene has been designated <i>tatC</i> (for twin-argininetranslocation), and we show that homologous genes are present in a range of bacteria, plastids, and mitochondria. These findings suggest a central role for TatC-type proteins in the translocation of tightly folded proteins across a spectrum of biological membranes.