SECB BINDS ONLY TO A LATE NATIVE-LIKE INTERMEDIATE IN THE FOLDING PATHWAY OF BARSTAR AND NOT TO THE UNFOLDED STATE

SecB is a cytosolic, tetrameric chaperone of Escherichia coli which ma intains precursor proteins in a translocation competent state. We have investigated the effect of SecB on the refolding kinetics of the smal l protein barstar in I M guanidine hydrochloride at pH 7.0 and 25 degr ees C using fluorescence spectroscopy. We show that SecB does not bind either the native or the unfolded states of barstar but binds to a la te near-native intermediate along the folding pathway. For barstar, po lypeptide collapse and formation of a hydrophobic surface are required for binding to SecB. SecB does not change the apparent rate constant of barstar refolding. The kinetic data for SecB binding to barstar are not consistent with simple kinetic partitioning models.