Folding pathway mediated by an intramolecular chaperone: characterization of the structural changes in pro-subtilisin E coincident with autoprocessing.

Mechanisms by which many N-terminal propeptides facilitate folding of proteins are unknown. The maturation of such proteins from their precursors involve three steps, namely: (1) folding of the precursor, (2) autoprocessing of the propeptide from the N terminus and (3) degradation of the cleaved propeptide. Using subtilisin E we have analyzed the mechanism of propeptide-mediated protein folding. Two active site mutations allow us to trap intermediates at stages of autoprocessing and degradation. An analysis of these intermediates has shown the existence of a molten-globule-like intermediate on the folding pathway. After autoprocessing of the propeptide, this intermediate undergoes a structural reorganization which reduces solvent-accessible hydrophobic surface area and increases the amount of its tertiary structure. Removal of the propeptide from the mature enzyme in this intermediate state occurs only by proteolytic degradation and contributes to the stability of the active enzyme.