GAMETE INTERACTIONS DURING MAMMALIAN FERTILIZATION

Complementary adhesion molecules are located on the surface of mouse e ggs and sperm. These molecules support species-specific interactions b etween sperm and eggs that lead to gamete fusion (fertilization). Modi fication of these molecules shortly after gamete fusion assists in pre vention of polyspermic fertilization. mZP3, an 83,000 MW glycoprotein located in the egg extracellular coat, or zona pellucida, serves as pr imary sperm receptor. Gamete adhesion in mice is carbohydrate-mediated , since sperm recognize and bind to certain mZP3 serine/threonine- (O- ) linked oligosaccharides. As a consequence of binding to mZP3, sperm undergo the acrosome reaction (exocytosis), which enables them to pene trate the zona pellucida and fertilize the egg. Initiation of the acro some reaction involves components of a signal transduction pathway ana logous to that found in somatic cells. sp56, a 56,000 MW protein locat ed in plasma membrane surrounding acrosome-intact mouse sperm heads, i s a putative primary egg-binding protein. It is suggested that sp56 re cognizes and binds to certain mZP3 O-linked oligosaccharides. Acrosome -reacted sperm remain bound to eggs by interacting with mZP2, a 120,00 0 MW zona pellucida glycoprotein. Thus, mZP2 serves as secondary sperm receptor. Perhaps a sperm protease associated with inner acrosomal me mbrane, possibly (pro)acrosin, serves as secondary egg-binding protein . mZP3 and mZP2 are inactivated as sperm receptors following sperm-egg fusion. In this manner, these egg and sperm surface molecules regulat e fertilization in mice. Homologous molecules apparently regulate fert ilization in other mammals, including human beings.