Reviews of Physiology, Biochemistry and Pharmacology : Springer : 9783540002284 : 3540002286 : 25 Mar 2003 : Protein transport events occurring at the endoplasmic reticulum (ER) of eukaryotic cells and the cytoplasmic membrane of prokaryotic organisms share many similarities. Resident proteins of both membranes span the lipid bilayer once or several times by a-helical stretches and their integration is usually mediated by uncleaved signal?anchor sequences. Proteins that are translocated across either membrane, collectively also termed secretory proteins, harbour cleavable N-terminal signal sequences. Prokaryotic and eukaryotic signal sequences have the same modular structure and are functionally exchangeable. Integration of membrane proteins and translocation of secretory proteins basically occur at the same sites (pores) within each membrane. In both types of membranes, these pores are c- posed of homologous components forming the Sec translocons. Parts of the Sec trans- cons are foun