ABSTRACT
There are one third of synthesized proteins must be secreted to the cell surface or to the surrounding environment to acquire their native functional state. Most of them are exported by Sec translocase (secretion pathway). Sec translocase consists of a membrane embedded protein-conducting channel, termed SecYEG and a peripherally associated motor domain, the ATPase SecA. The SecDFyajC heterotrimeric membrane protein complex can facilitates protein translocation. SecB is a molecular chaperone that functions in the protein translocation pathway. SecM (secretion monitor) encoded by the 5' region of the secM-secA mRNA, which elongation arrest is required for upregulated expression of SecA. The signal sequence in the N terminus of the nascent peptide is first recognized by the signal recognition particle (SRP). SecB, the Sec-system-specific chaperone, channels the preprotein to the Sec translocation pathway and, ad- ditionally, actively targets the bound precursor to the translocase by its ability to bind SecA. The preprotein-bearing SecA then binds to the membrane, at a high-affinity SecA-binding site, SecYEG, which constitutes a channel for polypeptide movement. Continued translocation requires cycles of ATP hydrolysis bySecA, which is thought to occur in a step-wise fashion with a step of 20~30 amino acid residues.