Escherichia coli Cotranslational Targeting and Translocation

Abstract Targeting of proteins to their proper location inside the cell, in the membrane or in the extracellular space is crucial to the cell. The targeting information is provided in form of a signal sequence by the protein itself. In Escherichia coli , mostly membrane proteins are targeted cotranslationally via the signal recognition particle (SRP) to the membrane. The SRP and its receptor are both guanosine triphosphatases (GTPases) which handover the ribosome‐nascent chain complex to the protein‐conducting channel. Subsequently, the nascent polypeptide is translocated into or across the membrane. To date, a host of biochemical data and a number of structures of important complexes along the pathway are available which shed light on the molecular mechanism of the targeting and translocation process. Key Concepts: Nascent polypeptides with signal sequences are recognised by the SRP. The hydrophobicity of the signal sequence determines whether a protein is targeted co‐ or posttranslationally. Cotranslational protein targeting is mediated by the SRP and the SRP receptor. They deliver the translating ribosome to the translocation machinery in the membrane. The SecYEG translocon is a channel that allows hydrophilic polypeptides to cross the membrane. In addition, the channel can open laterally and release transmembrane helices into the lipid bilayer. YidC acts as a membrane protein insertase for small membrane proteins in a Sec‐independent manner. In a Sec‐associated form, YidC is suggested to assist in membrane protein folding. YidC, SecD, SecF and YajC can associate with the SecYEG protein‐conducting channel. The resulting complex is called the ‘holotranslocon’, which is suggested to be responsible for membrane protein integration, folding and complex assembly.