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.

Department of Biochemistry and Biology of Reproduction Institute of Molecular Genetics Prague Czechoslovakia

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