Protein translocation across the membrane is a key reaction in the biogenesis of noncytoplasmic proteins. Topogenic signals carried on various portions of the polypeptide, as well as proteinaceous factors play essential roles in the asymmetric protein-membrane interaction. Factors mediating co-translational translocation as well as factors modulating the folding pathway of polypeptides for post-translational translocation have been identified. The SecY protein of E. coli is an integral membrane component that facilitates the translocation step, possibly by forming a channel for polypeptide passage.