Pro Region C-Terminus:Protease Active Site Interactions Are Critical in Catalyzing the Folding of α-Lytic Protease^†

α-Lytic protease is encoded with a large (166 amino acid) N-terminal pro region that is required transiently both in vivo and in vitro for the correct folding of the protease domain [Silen, J. L., and Agard, D. A. (1989) Nature 341, 462−464; Baker, D., et al. (1992) Nature 356, 263−265]. The pro region also acts as a potent inhibitor of the mature enzyme [Baker, D., et al. (1992) Proteins:  Struct., Funct., Genet. 12, 339−344]. This inhibition is mediated through direct steric occlusion of the active site by the C-terminal residues of the pro region [Sohl, J. L., et al. (1997) Biochemistry 36, 3894−3904]. Through mutagenesis and structure−function analyses we have begun to investigate the mechanism by which the pro region acts as a single turnover catalyst to facilitate folding of the mature protease. Of central interest has been mapping the interface between the pro region and the protease and identifying interactions critical for stabilizing the rate-limiting folding transition state. Progressive C-terminal deletions of the pro region were found to have drastic effects on the rate at which the pro region folds the protease but surprisingly little effect on inhibition of protease activity. The observed kinetic data strongly support a model in which the detailed interactions between the pro region C-terminus and the protease are remarkably similar to those of known substrate/inhibitor complexes. Further, mutation of two protease residues near the active site have significant effects on stabilization of the folding transition state (k_cat) or in binding to the folding intermediate (K_M). Our results suggest a model for the α-lytic protease pro region-mediated folding reaction that may be generally applicable to other pro region-dependent folding reactions.