CORRELATION BETWEEN REQUIREMENT FOR SECA DURING EXPORT AND FOLDING PROPERTIES OF PRECURSOR POLYPEPTIDES

The structural complexity of a ligand in association with the molecula r chaperones SecB and SecA was investigated using three species of pre cursor maltose-binding protein, which differ in their stability as a r esult of an amino acid substitution in each that affects the rate of f olding of the polypeptide. In the presence of high concentrations of b oth SecB and SecA, the precursors were translocated in vitro with indi stinguishable kinetics. However, when SecA was limiting, the transloca tion was more rapid for precursor species, which had lower stability i n the native state relative to the stability of the wild-type precurso r. We propose that, when in complex with SecB, precursors can form an element of tertiary structure and that these tertiary contacts are blo cked when SecA is bound.