Characterization of a novel ZP3-independent sperm-binding ligand that facilitates sperm adhesion to the egg coat

During mammalian fertilization, sperm adhere to the extracellular coat of the egg, or zona pellucida, in a species-specific manner. In mouse, evidence suggests that sperm recognize and bind to specific oligosaccharide ligands within the zona pellucida glycoprotein, ZP3, via beta1,4-galactosyltransferase I (GalT I), a lectin-like receptor on the sperm surface. Although in vitro experiments using isolated gametes lend support to this model, recent in vivo studies of genetically altered mice question whether ZP3 and/or GalT I are solely responsible for sperm-egg binding. In this regard, sperm from GalT I-null mice bind poorly to ZP3 and fail to undergo a zona-induced acrosome reaction; however, they still bind to the ovulated egg coat in vitro. In this report, we characterize a novel ZP3- and GalT I-independent mechanism for sperm adhesion to the egg coat. Results show that the ovulated zona pellucida contains at least two distinct ligands for sperm binding: a ZP3-independent ligand that is peripherally associated with the egg coat and facilitates gamete adhesion; and a ZP3-dependent ligand that is present in the insoluble zona matrix and is recognized by sperm GalT I to facilitate acrosomal exocytosis. The ZP3-independent ligand is not a result of contamination by egg cortical granules, nor is it the mouse homolog of oviduct-specific glycoprotein. It behaves as a 250 kDa, WGA-reactive glycoprotein with a basic isoelectric point, distinguishing it from the acidic glycoproteins that form the insoluble matrix of the egg coat. When eluted from isoelectric focusing gels, the acidic matrix glycoproteins possess sperm-binding activity for wild-type sperm, but not for GalT I-null sperm, whereas the basic glycoprotein retains sperm-binding activity for both wild-type and GalT I-null sperm. Thus, GalT I-null sperm are able to resolve gamete recognition into at least two distinct binding events, leading to the characterization of a novel, peripherally associated, sperm-binding ligand on the ovulated zona pellucida.