. Rev. Biochem. 60:101-124, 1991 With the aim of eventual elucidation of the role of ATP binding and hydrolysis in the export cycle, we compared the effects of treating a growing culture with sodium azide and the expression of an allele of SecA that has an altered ATPbinding site.

SecB, a molecular chaperone involved in protein export in Escherichia coli, displays the remarkable ability to selectively bind many different polypeptide ligands whose only common feature is that of being nonnative. The selectivity is explained in part by a kinetic partitioning between the folding of a polypeptide and its association with SecB. SecB has no affinity for native, stably folded polypeptides but interacts tightly with polypeptides that are nonnative. In order to better understand the nature of the binding, we have examined the interaction of SecB with intermediates along the folding pathway of maltose-binding protein. Taking advantage of forms of maltosebinding protein that are altered in their folding properties, we show that the first intermediate in folding, represented by the collapsed state, binds to SecB, and that the polypeptide remains active as a ligand until it crosses the final energy barrier to attain the native state.

No abstract