A PROTEIN-FOLDING REACTION UNDER KINETIC CONTROL

SYNTHESIS of alpha-lytic protease is as a precursor containing a 166 amino-acid pro region 1 transiently required for the correct folding of the protease domain 2-4. By omitting the pro region in an in vitro refolding reaction we trapped an inactive, but folding competent state (I) having an expanded radius yet native-like secondary structure. The I state is stable for weeks at physiological pH in the absence of denaturant, but rapidly folds to the active, native state on addition of the pro region as a separate polypeptide chain. The mechanism of action of the pro region is distinct from that of the chaperonins 5,6: rather than reducing the rate of off-pathway reactions, the pro region accelerates the rate-limiting step on the folding pathway by more than 10 7. Because both the I and native states are stable under identical conditions with no detectable interconversion, the folding of alpha-lytic protease must be under kinetic and not thermodynamic control.