Role of carbohydrates in receptor-mediated fertilization in mammals

The mouse sperm receptor, called ZP3, is a glycoprotein (83,000 Mr) that consists of a 44,000 Mr polypeptide chain (402 amino acids), three or four N-linked oligosaccharides, and an undetermined number of O-linked oligosaccharides. There are more than 10(9) copies of ZP3 present throughout the mouse egg extracellular coat, or zona pellucida. As a prelude to fertilization, each acrosome-intact sperm binds in a relatively species-specific manner to tens-of-thousands of copies of ZP3 at the surface of the zona pellucida. Binding to ZP3 induces sperm to undergo the acrosome reaction (membrane fusion) and, consequently, enables them to penetrate through the zona pellucida and to reach, and then fuse with, egg plasma membrane (fertilization). Purified ZP3, as well as a specific class of ZP3-derived O-linked oligosaccharides (3900 Mr), exhibit sperm receptor activity in vitro. The oligosaccharides, which represent a relatively low percentage of total ZP3 O-linked oligosaccharides, account for the glycoprotein's sperm receptor activity in vitro (i.e., recognition and binding). Furthermore, either enzymic removal or modification of certain sugars that constitute these oligosaccharides results in destruction of sperm receptor activity. These and other findings strongly suggest that during mammalian fertilization carbohydrates play a fundamental role in species-specific sperm-egg interactions.