A Role for Mouse Sperm Binding to Sperm Surface Galactosyltransferase the Egg Zona Pellucida in

Past studies have suggested that mouse sperm surface galactosyltransferase may participate during fertilization by binding N-acetylglucosamine (GIcNAc) residues in the zona pellucida. In this paper, we examined further the role of sperm surface galactosyltransferase in mouse fertilization. Two reagents that specifically perturb sperm surface galactosyltransferase activity both inhibit sperm-zona binding. The presence of the milk protein a-lactalbumin specifically modifies the substrate specificity of sperm galactosyltransferase away from GIcNAc and towards glucose and simultaneously inhibits sperm binding to the zona pellucida. Similarly, UDP-dialdehyde inhibits sperm binding to the zona pellucida and sperm surface galactosyltransferase activity to identical degrees. Of five other sperm enzymes assayed, four are unaffected by UDP-dialdehyde, and one is affected only slightly. Covalent linkage of UDPdialdehyde to sperm dramatically inhibits binding to eggs, while treatment of eggs with UDPdialdehyde has no effect on sperm binding. Heat-solubilized or pronase-digested zona pellucida inhibit sperm-zona binding, and they can be glycosylated by sperm with UDP-galactose. Sperm are also able to glycosylate intact zona pellucida with UDP-galactose. Thus, solubitized and intact zona pellucida act as substrates for sperm surface GIcNAc:galactosyltransferases. Finally, pretreatment of eggs with fi-N-acetylglucosaminidase inhibits sperm binding by up to 86%, while under identical conditions, pretreatment with fl-galactosidase increases sperm binding by 55%. These studies, in conjunction with those of the preceding paper dealing with surface galactosyltransferase changes during capacitation, directly suggest that galactosyltransferase is at least one of the components necessary for sperm binding to the zona pellucida. This paper addresses the molecular mechanism of mouse sperm binding to the zona pellucida. It has been 70 years since Lillie first proposed that gamete recognition may involve interactions between complementary cell surface receptors (4). Only in the last 10 years have any specific molecular hypotheses been proposed to account for species-specific gamete recognition. Evidence suggests that sperm receptor proteins may bind specific carbohydrate residues on the egg surface (5-7). For example, sea urchin sperm acrosome vesicles contain "bindin", a lectinlike protein that is capable of agglutinating eggs of the homologous species (8), and a glycopeptide receptor has been isolated from sea urchin eggs by bindin-specific affinity chromatography (9). Mammalian fertilization also likely involves specific complementary receptors on interacting gamete surfaces (10). A particular zona pellucida glycoprotein has been implicated as the receptor for mouse sperm (11). Recently, evidence has sug574 gested that mouse sperm surface galactosyltransferase participates during fertilization by binding GIcNAc residues in the zona pellucida (1-3). Two types of results support this possibility. First, in the preceding paper (3), results showed that sperm capacitation is associated with the release of specific sperm bound galactosyltransferase substrates, thereby exposing the surface galactosyltransferase for binding to the zona pellucida. The released galactosyl acceptors serve as "decapacitation factors" (i.e., inhibit capacitated sperm binding to the zona pellucida) when added back to in vitro fertilization assays. Glycosides not recognized by sperm surface galactosyltransferases do not inhibit sperm binding to the zona pellucida. Second, sperm bearing mutant alleles of the T/t-complex, which show increased fertilizing ability, have a specific fourfold increase in surface galactosyltransferase activity. Eight other tsperm enzyme activities are indistinguishable from normal (1). Sperm bearing recombinant t-chromosomes, or one of four THE JOURNAL OF CELL BIOLOGY. VOLUM[ 95 NOVEMB[r 1982 574-579 © The Rockefeller University Press 0021-9525/82/11/0574/06 $1.00 on A ril 6, 2017 D ow nladed fom Published November 1, 1982