Differential release of guinea pig sperm acrosomal components during exocytosis

The contents of the sperm acrosome are compartmentalized at the biochemical and morphological levels. Biochemically, the acrosome can be considered to be comprised of two compartments: one consisting of readily soluble protei ns and one containing a particulate acrosomal matrix. To test the hypothesi s that compartmentalization affects the release of acrosomal components dur ing the course of secretion in guinea pig sperm, we examined the relationsh ip between the presence of specific proteins and acrosomal status and monit ored the recovery of acrosomal constituents in the medium surrounding sperm induced to undergo exocytosis with the ionophore A23187. Cysteine-rich sec retory protein 2 (CRISP-2), a soluble component of the acrosome, was rapidl y lost from the acrosome soon after ionophore treatment. However, acrosomal matrix components remained associated with the sperm for longer periods. A M67, a matrix component and the guinea pig orthologue of the mouse sperm zo na pellucida-binding protein sp56, was released at a slower rate than was C RISP-2 but at a faster rate than were two other matrix proteins, AM50 and p roacrosin. Coincident with their release from the sperm, AM50 and proacrosi n were posttranslationally modified, probably by proteolysis. The release o f proacrosin from the matrix appears associated with the conversion of this protein to the enzymatically active acrosin protease. These results provid e strong support for the hypothesis that compartmentalization plays a signi ficant role in regulating the release of proteins during the course of acro somal exocytosis. Acrosomal matrix proteins remain associated with the sper m for prolonged periods of time following the induction of acrosomal exocyt osis, suggesting that transitional acrosomal intermediates may have signifi cant functions in the fertilization process.