INHIBITION OF ALPHA-LYTIC PROTEASE BY PRO REGION C-TERMINAL STERIC OCCLUSION OF THE ACTIVE-SITE

alpha-Lytic protease, a chymotrypsin-like serine protease, is synthesi zed with an N-terminal 166 amino acid pro region which is absolutely r equired for folding of the protease. The pro region is also the most p otent inhibitor of the protease known with a K-i of similar to 10(-10) M. Compared to its role in the folding reaction, relatively little is known about the mechanism by which the pro region inhibits the mature protease. While proteinaceous protease inhibitors generally function by occluding the active sites of their respective targets [Bode, W., & Huber, R. (1992) Eur. J. Biochem. 204, 433-451], the pro region of al pha-lytic protease with its dual roles in folding and inhibition might be expected to show a novel mechanism of inhibition. However, experim ents that probe both the structural and enzymatic consequences of pro region binding indicate that the pro region does not measurably distor t the protease active site. Instead, the catalytic site is fully funct ional in the complex. Pro region inhibition of the protease is due to simple steric obstruction; the pro region C-terminus lies in the subst rate binding sites of the protease. The implications of these results are discussed with regard to alpha-lytic protease maturation and foldi ng. In addition, the proposed mechanism of alpha-lytic protease pro re gion inhibition is discussed with respect to data from other pro regio n families.