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Binding of pig sperm receptor in the zona pellucida to the boar sperm acrosome

Pig zona pellucida (ZP) contains three families of glycoproteins: PZP2, PZP3 and PZP3β. PZP3 mediates the binding of the ZP to spermatozoa. In this study, the binding site of pig ZP on boar spermatozoa and the zona-binding proteins of boar spermatozoa were studied using chemically modified zona glycoproteins or anti-pig ZP antiserum. Endo-β-galactosidase-digested PZP3 (EβG-PZP3), which is deficient in sulfated N-acetylpolylactosamine, as well as solubilized ZP, bound to the acrosomal region of acrosome-damaged or partially acrosome-reacted spermatozoa. However, they did not bind to acrosome-intact or fully acrosome-reacted spermatozoa. Solubilized ZP did bind to the acrosomal cap released upon acrosome reaction. In western blot analyses, EβG-PZP3 bound to the sperm proteins with molecular masses similar to proacrosin–acrosin and the binding was inhibited by fucoidan and anti-pig acrosin antiserum. These results suggest that the binding site of solubilized pig ZP and EβG-PZP3 on spermatozoa is located mainly in the acrosomal matrix and on the membranous compartments in the acrosome and suggest that EβG-PZP3 binds to proacrosin–acrosin. The binding of EβG-PZP3 to proacrosin–acrosin may be involved in the binding of the ZP to the acrosome of partially acrosome-reacted spermatozoa.