PROTEIN-PROTEIN INTERACTION CONTROLLING ACROSIN RELEASE AND SOLUBILIZATION DURING THE BOAR SPERM ACROSOME REACTION

In this study we used previously characterized monoclonal antibodies t o acrosin (ACR.2) and to an acrosomal matrix antigen (ACR.4) to analyz e the acrosin-binding activity of a 28-kDa putative acrosin-binding pr otein from the acrosomal matrix. The 28-kDa protein bound proacrosin a nd the 49-kDa form of acrosin (alpha-acrosin) but it did not bind the 36-kDa acrosin form (beta-acrosin). The acrosin-binding activity of th e 28-kDa protein was stimulated by Ca2+, inhibited by Mg2+, and remove d by disulphide bond reduction. Induction of the acrosome reaction by a calcium ionophore resulted in proteolytic cleavage of the 28-kDa pro tein, giving rise to a 12-kDa degradation product that was the only fo rm of ACR.4 antigen released to incubation media; the release of the A CR.4 antigen was closely correlated with that of acrosin. The release of alpha-acrosin to incubation media was accelerated in the presence o f ACR.4 antibody. In a cell-free system, a limited cleavage of the pur ified 28-kDa protein into immunoreactive degradation products was cata lyzed by acrosin but not by trypsin or chymotrypsin. The data suggest that the 28-kDa acrosomal protein helps to maintain acrosomal matrix i ntegrity and controls the acrosin release from acrosome-reacted cells.