Specific binding of spermatozoa to the zona pellucida that surrounds mammalian eggs is a key step in the fertilization process. However, the
sperm proteins that recognise zona pellucida receptors remain contentious despite longstanding research efforts to identify them. Here we present evidence that
proacrosin, a tissue-specific
protein found within the acrosomal vesicle of all mammalian spermatozoa, is a multifunctional
protein that mediates binding of acrosome-reacted spermatozoa to
zona glycoproteins via a stereospecific polysulfate recognition mechanism. Using sulfated versus non-sulfated forms of chemically defined compounds in binding assays employing native
proteins in their normal cellular location or conjugated to FluoSpheres, we have attempted to identify the sulfation "code" required for recognition. Results show that protein conformation is important for specificity and that at least 2
sulfate groups are required to cross-link spatially separated docking sites on
proacrosin. The consistently most effective inhibitory compounds were
suramin and quercetin-3beta-d-glucoside
sulfate. The results support our hypothesis that
proacrosin is one of several
proteins in the acrosomal matrix that retain acrosome reacted spermatozoa on the
zona surface prior to penetration. They also establish, as a proof-of-principle, the feasibility of synthesising sulfated compounds of high specificity as antifertility agents for human or animal use.