Figure 1

The biogenesis of Sec-translocon dependent secretory and cytoplasmic membrane proteins in E. coli . In E. coli, most secretory and cytoplasmic membrane proteins require the Sec-translocon for their biogenesis. The Sec-translocon is a protein conducting channel in the cytoplasmic membrane (CM), which mediates the vectorial transfer of secretory proteins across and the biogenesis of membrane proteins in the cytoplasmic membrane [7]. Secretory proteins are equipped with a cleavable N-terminal signal sequence. The signal sequence determines whether a secretory protein is targeted to the Sec-translocon via the post-translational SecB-targeting pathway or the co-translational signal recognition particle (SRP)-targeting pathway, which is comprised of the SRP and its receptor FtsY. Upon translocation, the signal sequence is cleaved off by leader peptidase (Lep) and the secretory protein is released into the periplasm. In this compartment, the Dsb-system can catalyze the formation of disulfide bonds. The disulfide oxidoreductase DsbA catalyzes the de-novo formation of disulfide bonds in polypeptide chains. The disulfide bond formation protein B (DsbB) is essential to maintain DsbA in an oxidized state. Incorrectly formed disulfide bonds can be corrected by DsbC/D. For a more detailed description of disulfide bond formation in the periplasm of E. coli see [3, 5]. Cytoplasmic membrane proteins are targeted to the Sec-translocon via the SRP-targeting pathway. SecA = peripheral membrane ATPase associated with the Sec-translocon [18], OM = outer membrane, YidC = cytoplasmic membrane protein translocase/insertase [18].