CHAPERONE SECB FROM ESCHERICHIA-COLI MEDIATES KINETIC PARTITIONING VIA A DYNAMIC EQUILIBRIUM WITH ITS LIGANDS

We have shown that the complexes between SecB, a chaperone from Escher ichia coli, and two physiological ligands, galactose-binding protein a nd maltose-binding protein, are in rapid, dynamic equilibrium between the bound and free states, Binding to SecB is readily reversible, and each time the ligand is released it undergoes a kinetic partitioning b etween folding to its native state and re-binding to SecB. Binding req uires that the polypeptide be devoid of tertiary structure; once the p rotein has folded, it is no longer a ligand. Conditions were establish ed in which folding of the polypeptides was sufficiently slow so that at each cycle of dissociation rebinding was favored over folding and a kinetically stable complex between SecB and each polypeptide ligand w as observed, Evidence that the ligand is continually released to the b ulk solution and rebound was obtained by altering the conditions to in crease the rate of folding of each ligand so that folding of the ligan d was faster than reassociation with SecB thereby allowing the system to partition to free SecB and folded polypeptide ligand. We conclude t hat complexes between the chaperone SecB and ligands are in dynamic, r apid equilibrium with the free states. This mode of binding is simpler than that documented for chaperones that function to facilitate foldi ng such as the Hsp70s and Hsp60s, where hydrolysis of ATP is coupled t o the binding and release of ligands, This difference may reflect the fact that SecB does not mediate folding but is specialized to facilita te protein export. Without a requirement for exogenous energy it effic iently performs its sole duty: to keep proteins in a nonnative conform ation and thus competent for export.