Proacrosin-acrosomal matrix binding interactions in ejaculated bovine spermatozoa

The mechanisms regulating hydrolase release during the mammalian sperm acrosome reaction are poorly understood. The present study demonstrates that specific domains of the acrosomal matrix of bovine spermatozoa function to maintain a particulate proacrosin pool and to regulate proacrosin/acrosin release. In sonicated sperm suspensions, 50-60% of the total proacrosin activity was sedimentable, and the amount of sedimentable proacrosin activity remained unchanged over time. Serial centrifugation and resuspension experiments demonstrated that the particulate proacrosin fraction resulted from an equilibrium binding of proacrosin to a stable sperm structure. To identify the proacrosin-binding structure of the acrosome, a purified sperm head fraction was isolated on sucrose density gradients. The sperm heads were extracted with Triton X-100, and a homogeneous acrosomal subfraction, the matrix complex associated with the outer acrosomal membrane (OMC), was isolated on Percoll density gradients. A centrifugation assay was then used to demonstrate that the OMC specifically binds proacrosin in a dose-dependent manner. These data demonstrate that the OMC represents a stable structural component of the acrosome that maintains a particulate proacrosin pool. We propose that the OMC regulates proacrosin release during the acrosome reaction and maintains elevated acrosin concentrations at the site of sperm-egg interaction.