DEFINING OLIGOSACCHARIDE SPECIFICITY FOR INITIAL SPERM-ZONA PELLUCIDAADHESION IN THE MOUSE

The identity of the sperm surface protein(s) responsible for sperm-zon a pellucida binding in the mouse, as well as the characteristics of th e oligosaccharide groups on zona pellucida glycoprotein 3 (ZP3) having ligand activity toward this receptor, remain controversial. Conflicti ng results from several groups have made interpretation of the current data difficult. By developing a quantitative binding assay to evaluat e the molecular interactions between mammalian sperm and the zona pell ucida during initial gamete interactions, we directly quantified sperm -ZP binding interactions at the molecular level for the first time. Th e ZP binding assay demonstrated that live, capacitated mouse sperm bin d solubilized I-125-labeled ZP glycoproteins in a concentration-depend ent manner characterized by a rapid forward rate constant of 3.0 x 10( 7) M(-1) min(-1). Following the initial characterization, the binding assay was used to examine the roles of the sperm surface enzymes galac tosyltransferase (GalTase) and fucosyltransferase (FucTase) in sperm-z one pellucida binding in the mouse. These data indicate that substrate s for FucTase, but not for GalTase, inhibit sperm-ZP binding, in contr ast to earlier reports in which GalTase substrates significantly inhib ited sperm binding to intact ZPs. A model is presented which resolves conflicting results between assays using intact ZPs and the results ob tained here using soluble I-125-Zps. Assuming a complex binding/recogn ition site, monosaccharides that could occupy part of the binding site would have a dramatic effect on sperm-ZP binding to the intact ZP, si nce they need only occupy the binding sites for a short time (similar to 100 msec) to disrupt binding. The current results suggest that the sperm ZP3 receptor binding site minimally recognizes the gal beta 1,3G lcNAc moiety also recognized by FucTases. The current data do not excl ude the possibility that additional sugar residues form part of the li gand oligosaccharide group and are recognized by a yet-to-be-identifie d sperm surface protein which serves as the ZP3 receptor. (C) 1996 Wil ey-Liss, Inc.