The polysulphate binding domain of human proacrosin/acrosin is involved in both the enzyme activation and spermatozoa zona pellucida interaction

Mammalian acrosin is a protease present as a zymogen in the acrosome of a non-reacted mammalian sperm, and in vitro is able to carry out limited hydrolysis of homologous and heterologous zonae pellucidae. On the other hand, sulphated polymers and zona pellucida glycoproteins bind to acrosin on a domain different from the active site, named the polysulphate binding domain (PSBD). Thus it is believed that acrosome-reacted spermatozoa bind to glycan chains bf the zona pellucida through PSBD participating as secondary binding receptor. The aim of the present work was to study the role of PSBD during both human gamete interaction and acrosin activation. In this work we present evidence that the anti-human acrosin monoclonal antibody C5F10 is directed to an epitope located on or near the PSBD on human proacrosin/acrosin. Moreover, we show that this antibody is able to inhibit both proacrosin activation induced by fucoidan and the sperm binding to the zona pellucida. Our results suggest that the same PSBD is involved in both sperm secondary binding, during zona pellucida penetration, and proacrosin activation.