Protein Translocation Across the Bacterial Cytoplasmic Membrane

About 25% to 30% of the bacterial proteins function in the cell envelope or outside of the cell. These proteins are synthesized in the cytosol, and the vast majority is recognized as a ribosome-bound nascent chain by the signal recognition particle (SRP) or by the secretion-dedicated chaperone SecB. Subsequently, they are targeted to the Sec translocase in the cytoplasmic membrane, a multimeric membrane protein complex composed of a highly conserved protein-conducting channel, SecYEG, and a peripherally bound ribosome or ATP-dependent motor protein SecA. The Sec translocase mediates the translocation of proteins across the membrane and the insertion of membrane proteins into the cytoplasmic membrane. Translocation requires the energy sources of ATP and the proton motive force (PMF) while the membrane protein insertion is coupled to polypeptide chain elongation at the ribosome. This review summarizes the present knowledge of the mechanism and structure of the Sec translocase, with a special emphasis on unresolved questions and topics of current research.