α-lytic protease precursor:: Characterization of a structured folding intermediate

The bacterial alpha-lytic protease (alpha LP) is synthesized as a precursor containing a large N-terminal pro region (Pro) transiently required for correct folding of the protease [Silen, J. L., and Agard, D. A, (1989) Nature 341, 462-464]. Upon folding, the precursor is autocatalyticly cleaved to yield a tight-binding inhibitory complex of the pro region and the fully folded protease (Pro/alpha LP). An in vitro purification and refolding protocol has been developed for production of the disulfide-bonded precursor. A combination of spectroscopic approaches have been used to compare the structure and stability of the precursor with either the Pro/alpha LP complex or isolated Pro. The precursor and complex have significant similarities in secondary structure but some differences in tertiary structure, as well as a dramatic difference in stability. Correlations with isolated Pro suggest that the pro region part of the precursor is fully folded and acts to stabilize and structure the alpha LP region. Precursor folding is shown to be biphasic with the fast phase matching the rate of pro region folding. Further, the rate-limiting step in oxidative folding is formation of the disulfide bonds and autocatalytic processing occurs rapidly thereafter. These studies suggests a model in which the pro region folds first and catalyzes folding of the protease domain, forming the active site and finally causing autocatalytic cleavage of the bond separating pro region and protease. This last processing step is critical as it allows the protease N-terminus to rearrange, providing the majority of net stabilization of the product Pro/alpha LP complex.