Identification of a zona-binding protein from boar spermatozoa as proacrosin

The initial stages of fertilization in vertebrates and invertebrates are thought to involve complementary recognition molecules on spermatozoa and eggs. In a previous work (C. R. Brown and R. Jones, 1987, Development) we described one such putative molecule (a protein of approximate molecular weight 53 kDa) in detergent extracts of boar spermatozoa that has affinity for glycoproteins from the zona pellucida of pig eggs. This molecule has now been identified as proacrosin, the zymogen form of the acrosomal protease acrosin, on the basis of its electrophoretic behavior, the ability of zona glycoproteins to recognize and bind to proacrosin on Western blots, and the cross-reactivity of specific antisera to the 53-kDa molecule and proacrosin. A role is proposed for this enzyme in binding the sperm head to the zona pellucida during the initial stages of sperm-egg interaction.