ABSTRACT
Binding of guinea-pig spermatozoa to the zona pelluclda of homologous eggs has been reported to involve ‘receptors’ on the inner acrosomal membrane (Huang et al. 1981). These receptors can be blocked by sulphated polysaccharides such as fucoidan (Huang and Yanagimachi, 1984). The aims of the present investigation were to identify these putative zona receptors using 1251-fucoidan as a probe and examine their mechanism of recognition. Results show that 1251-fucoidan binds to several proteins extracted from guinea-pig spermatozoa with molecular m of 95, 60, 48, 34, 30 and 18-20xloJ (K) on SOS-PAGE. The 48K, 34K and 30K components represent Jroacrosln and two forms of acrosin, respectively. 1 1-zona pelluclda glycoproteins also bound strongly to the 48K, 34K and 30K sperm proteins. The other high and low mass binding proteins were not positively identified but cytochemlcal experiments with fluorescelnamine-fucoidan and FITC-soybean trypsin inhibitor indicate that they are intraacrosomal. The mechanism of binding of 1251-fucoidan to proacrosin/acrosln (and also the 95K, 60K and 18K-20K components) involves multiple sulphate groups on the polysaccharlde in a specific orientation to allow them to interact with basic residues on the protein. It is suggested that guinea-pig spermatozoa retain sufficient proacrosin/acrosin bound to the inner acrosomal membrane after the acrosome reaction to mediate binding to the zona pellucida and that functionally proacrosin is analogous to sea urchin bindin.
