Protein-protein interactions controlling acrosin release and solubilization during the boar sperm acrosome reaction

In this study we used previously characterized monoclonal antibodies to acrosin (ACR.2) and to an acrosomal matrix antigen (ACR.4) to analyze the acrosin-binding activity of a 28-kDa putative acrosin-binding protein from the acrosomal matrix. The 28-kDa protein bound proacrosin and 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 the 28-kDa protein was stimulated by Ca2+, inhibited by Mg2+, and removed by disulphide bond reduction. Induction of the acrosome reaction by a calcium ionophore resulted in proteolytic cleavage of the 28-kDa protein, giving rise to a 12-kDa degradation product that was the only form of ACR.4 antigen released to incubation media; the release of the ACR.4 antigen was closely correlated with that of acrosin. The release of alpha-acrosin to incubation media was accelerated in the presence of ACR.4 antibody. In a cell-free system, a limited cleavage of the purified 28-kDa protein into immunoreactive degradation products was catalyzed by acrosin but not by trypsin or chymotrypsin. The data suggest that the 28-kDa acrosomal protein helps to maintain acrosomal matrix integrity and controls the acrosin release from acrosome-reacted cells.