The Sec Protein Secretion System
Koreaki Ito and Hiroyuki Mori
from: Bacterial Secreted Proteins: Secretory Mechanisms and Role in Pathogenesis (Edited by: Karl Wooldridge). Caister Academic Press, U.K. (2009)
The majority of proteins destined for export across the cytoplasmic membrane or integration into the membrane are handled by the evolutionarily conserved Sec system. The Sec substrates have specific topogenic signals and are targeted to the membrane-embedded SecYEG translocon that serves as a polypeptide-conducting channel either co-translationally by SRP for lipid-phase integration or post-translationally by SecB for complete translocation. The plug helix of SecY that clogs the unused channel and the central constriction that seals around the translocating chain make the translocon function compatible with the permeability barrier of the membrane. The translocon also contains a lateral gate, through which it not only accepts a newly synthesized client protein but also allows its hydrophobic segment, if any, to partition into the lipid phase. The post-translational mode of translocation, characteristic of the bacterial systems, is driven by the SecA ATPase, which interacts with SecY and a preprotein and accordingly undergoes conformational transitions coupled with the ATPase cycles. While recent progress in structural analyses of these components is remarkable, real molecular understanding of their dynamic actions is left for future studies read more ...