Highly selective binding of nascent polypeptides by an Escherichia coli chaperone protein in

Chaperone proteins bind to newly synthesized polypeptides and assist in various assembly reactions. The Escherichia coli chaperone protein SecB binds precursors of exported proteins and assists in export. In vitro, SecB can bind to many unfolded proteins. In this report, we demonstrate that SecB binding in vivo is highly selective; the major polypeptides that are bound by SecB are nascent precursors of the exported proteins maltose-binding protein (MBP), LamB, OmpF, and OmpA. These results support the hypothesis that the primary physiological function of SecB is to stimulate protein export. By interacting with nascent polypeptides, SecB probably stimulates their cotranslational association with the membrane-bound protein translocation apparatus. Chaperone proteins are a class of proteins that bind to newly synthesized polypeptides and assist in assembly reac-tions, including folding of monomers, oligomerization of multimers, and interaction of precursors with the protein translocation apparatus (5). The Eschenchia coli chaperone protein SecB, a cytoplasmic oligomeric protein (11), is one of several components of the E. coli protein translocatio