Molecular aspects of mammalian fertilization

The initial communication between the gametes is a molecular, receptor-mediated process that takes place at the surface of the egg coat. The zona pellucida plays a central role in this process such that, on the one hand, spermatozoa may bind to it and, on the other hand, it prevents polyspermy. In the mouse, ZP3, a glycoprotein of the zona pellucida with a mol. wt of 84 kd, serves as a sperm receptor. Only a relatively small part of ZP3, namely certain O-linked carbohydrate side chains, is involved in the process of binding. These oligosaccharides probably become bound to enzymes associated with the plasma membrane of the sperm head and thus form an enzyme-substrate complex. A terminal alpha-galactose has been found to be one of the decisive sugar molecules and, moreover, the critical chemical group. After sperm binding to the zona pellucida has taken place, the polypeptide chain of ZP3 initiates the acrosome reaction in the sperm head. In the mouse, numerous binding proteins have been detected in the sperm plasma membrane: these are enzymes such as glycosyl transferases, proteinases, and glycosidases. A galactosyl transferase has been found on the surface of the mouse sperm that binds specifically to N-acetylglucosamine in the mouse zona pellucida. It is therefore apparent that carbohydrate-binding proteins on the sperm surface mediate gamete recognition through their high affinity and specificity for complex glycoconjugates in the egg coat. In fact, it is not at all surprising that complementary cell-surface protein and glycoconjugates are involved in fertilization, since many somatic cells exhibit a similar mechanism of cell recognition.