Bacterial protein translocase: a unique molecular machine with an army of substrates

Secretion of most polypeptides across the bacterial plasma membrane is cata lyzed by the Sec protein translocase. This complex molecular machine compri ses a flexible transmembrane conduit coupled to a motor-like component and displays four activities: (a) it is a specific receptor at its cytoplasmic side for all secretory polypeptides, (b) it converts metabolic energy from ATP and proton gradients into mechanical motion, (c) it prevents substrates from folding in statu translocanti and (d) it binds and releases short seg ments of the polymeric substrate sequentially. Combination of these activit ies allows translocase to move processively along the length of the substra te. Substrates are thus gradually expelled from the membrane and are releas ed for subsequent extracytoplasmic folding. (C) 2000 Federation of European Biochemical Societies. Published by Elsevier Science B.V. All rights reser ved.