Towards the molecular basis of sperm and egg interaction during mammalian fertilization

During the past 2 decades, a number of genes have been cloned from mammals which encode polypeptides that participate in the process of fertilization. Among these are glycoproteins ZP1-3 that constitute the zona pellucida of eggs from mice to human beings. In mice, one of these glycoproteins, mZP3, acts as a primary sperm receptor and acrosome reaction-inducer. The evidence suggests that acrosome-intact sperm recognize and bind to a specific class of mZP3 oligosaccharides present on two serine residues (O-linked) located near the carboxy-terminus of the polypeptide. Mutagenesis of either of these residues results in the synthesis of an inactive form of the receptor. Therefore, mammalian fertilization is a carbohydrate-mediated event. It is possible that changes in the structure of these oligosaccharides (e.g., composition, sequence, linkages, modifications, etc.) could account for species-specific binding of sperm to eggs. Stably transfected somatic cells, null mutant animals, and DNA constructs are now available to test this possibility both in vivo and in vitro.