PROOMPA CONTAINS SECONDARY AND TERTIARY STRUCTURE PRIOR TO TRANSLOCATION AND IS SHIELDED FROM AGGREGATION BY ASSOCIATION WITH SECB PROTEIN

Escherichia coli protein export involves cytosolic components termed molecular chaperones which function to stabilize precursors for membrane translocation. It has been suggested that chaperones maintain precursor proteins in a loosely folded state. We now demonstrate that purified proOmpA in its translocation component conformation contains both secondary and tertiary structure as analyzed by circular dichroism and intrinsic tryptophan fluorescence. Association with one molecular chaperone, SecB, subtly modulates the conformation of proOmpA and stabilizes it by inhibiting aggregation, permitting its translocation across inverted E. coli inner membrane vesicles. These results suggest that translocation competence does not simply result from the maintenance of an unfolded state and that molecular chaperones can stabilize precursor proteins by inhibiting their oligomerization.