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Physiological role during export for the retardation of folding by the leader peptide of maltose-binding protein.

It has been shown that folding of precursor maltose-binding protein of Escherichia coli in vitro is retarded by the leader peptide. We now present evidence that this modulation of folding plays a role during the export of maltose-binding protein in vivo. Maltose-binding protein synthesized in vivo without a leader sequence did not engage the cellular export apparatus. However, the requirement for the leader in at least one step, that of binding the export factor SecB, could be overcome by an amino acid substitution in the mature portion of maltose-binding protein. This substitution retarded the folding of the polypeptide even in the absence of a leader. Investigations using purified proteins in vitro demonstrated that SecB would stably bind to species of maltose-binding protein devoid of a leader when the folding of the binding proteins was sufficiently slow. Thus, we conclude that one of the roles of the leader is to retard folding and expose the binding site for SecB.