Correlation between requirement for SecA during export and folding properties of precursor polypeptides

The structural complexity of a ligand in association with the molecular chaperones SecB and SecA was investigated using three species of precursor maltose-binding protein, which differ in their stability as a result of an amino acid substitution in each that affects the rate of folding of the polypeptide. In the presence of high concentrations of both SecB and SecA, the precursors were translocated in vitro with indistinguishable kinetics. However, when SecA was limiting, the translocation was more rapid for precursor species, which had lower stability in the native state relative to the stability of the wild-type precursor. We propose that, when in complex with SecB, precursors can form an element of tertiary structure and that these tertiary contacts are blocked when SecA is bound.