Export of periplasmic galactose-binding protein in Escherichia coli depends on the chaperone SecB

The efficient export of galactose-binding protein to the periplasm of Escherichia coli is shown to be dependent on the presence of the cytosolic chaperone SecB. SecB is a molecular chaperone that facilitates the export of proteins to the periplasmic space and to the outer membrane of Escherichia coli. SecB binds precursor polypeptides in the cytosol and delivers them to SecA, a component of the mem-brane-associated translocation apparatus. In addition, SecB maintains the precursors in a state compatible with the transfer across the cytoplasmic membrane by preventing their folding and/or aggregation (2). Not all exported proteins require the function of SecB, and even for those that do depend on SecB, the requirement is not absolute. In the complete absence of the chaperone, some proportion of the protein is properly local-ized, but the process is slow, and often a substantial amount of the precursors accumulates in the cytosol in a folded or aggre