Interaction of zona pellucida glycoproteins, sulphated carbohydrates and synthetic polymers with proacrosin, the putative egg-binding protein from mammalian spermatozoa

Abstract

Fertilization in mammals is a unique cell-cell recognition event that involves specific receptors on the surface of each gamete. Previous work has shown that proacrosin, a protein found within the acrosome of mammalian spermatozoa, binds non-enzymatically to zona pellucida glycoproteins (ZPGPs) that surround the egg and that this binding can be inhibited by sulphated polysaccharides such as fucoidan. The mechanism of this interaction has been investigated using 125I-ZPGPs and 125I-fucoidan as probes. Results show that it involves poly(sulphate) groups on zona glycoproteins that bind with high affinity (Kd = 1.2 to 5.0 × 10(−8)M) to complementary ‘docking’ sites on proacrosin. The spatial orientation of these sulphates, together with the tertiary structure of the target protein, determines the selectivity of polymer binding. Thus, dextran sulphate and poly(vinyl sulphate) are strong inhibitors of the above probes whereas dextran, chondroitin sulphates A and C and poly(vinyl phosphate) are ineffective. Proacrosin, therefore, has properties analogous to those described for ‘bindin’, the egg adhesion protein found within the acrosomal vesicle of sea urchin spermatozoa.