11 The Basis of Recognition of Nonnative Structure by the Chaperone SecB

Abstract

I. INTRODUCTION
Chaperones selectively bind polypeptide ligands that have no feature in common except that of being nonnative. In this chapter, we discuss this amazing ability with respect to SecB, a chaperone facilitating protein export in Escherichia coli. Intensive study of this protein has led to a rudimentary understanding of the mechanism of ligand binding and of how that binding relates to function. The principles emerging are, we believe, pertinent to the function of chaperones in other pathways.

II. THE FUNCTION OF SECB DURING PROTEIN EXPORT
SecB is involved in the export of a subset of the proteins that are located in either the periplasmic space or the outer membrane of E. coli (Kumamoto and Beckwith 1983Kumamoto and Beckwith 1985). It interacts with them either while they are nascent growing polypeptide chains or after completion of synthesis but before they acquire their native structure (Kumamoto and Gannon 1988; Kumamoto 1989; Kumamoto and Francetic 1993). The primary functions of SecB are to maintain these polypeptide ligands in a state competent for translocation across the cytoplasmic membrane (Randall and Hardy 1986; Kumamoto and Gannon 1988; Kusters et al. 1989; Lecker et al. 1989; Weiss et al. 1988) and to deliver them to SecA, which in turn interacts with the integral membrane proteins SecY, SecE, and band 1 to form a translocase (see Fig. 1) (Hartl et al. 1990). The passage of the polypeptide through the cytoplasmic membrane mediated by the translocase complex requires both hydrolysis of ATP by SecA and proton motive force (Schiebel...