Folding of subtilisin BPN': characterization of a folding intermediate

Subtilisin BPN', an extracellular serine protease from Bacillus amyloliquefaciens, requires a 77 amino acid pro-sequence for correct folding in vivo. We report the observation of a metastable folding intermediate during the refolding of wild-type and a proteolytically inactive mutant subtilisin BPN' that lack the pro-sequence. The addition of the pro-sequence as a separate polypeptide chain results in the folding of the intermediate to the native state. The intermediate state of subtilisin is stable at different temperatures, pH values, and salt concentrations for more than a week and retains its competence for folding. The intermediate state possesses a compactness between that of the native and unfolded states. Although it has native-like secondary structure, it shows no distinct near-UV CD spectrum and has a strongly reduced dispersion in the amide and methyl regions of the 1H NMR spectrum. These indicate considerably less tertiary structure than possessed by the native state. However, the intermediate conformation has regions of stable tertiary structure: it has a high-affinity calcium binding site and, after a first noncooperative transition, unfolds with guanidinium chloride in a cooperative process. These results support a folding mechanism for subtilisin BPN' that comprises a high energy transition state, which is lowered by the interaction with the pro-sequence. The similarity to the folding mechanism of alpha-lytic protease supports the hypothesis that a common folding mechanism has been developed through convergent evolution.