Despite its importance, the molecular basis of mammalian gamete recognition has remained unclear. The enzyme beta-1,4-galactosyltransferase (Gal-transferase) has been viewed traditionally as a biosynthetic component of the Golgi complex, but is also found on the surface of many cells where it can bind its specific glycoside substrate on adjacent cell surfaces or in the extracellular matrix. In mouse it has been suggested that Gal-transferase on the sperm head mediates fertilization by binding oligosaccharide residues in the egg coat, or zona pellucida, and that the ability of the zona pellucida to bind sperm is conferred by oligosaccharides of the ZP3 glycoprotein. However, it has not been confirmed that Gal-transferase and ZP3 are in fact complementary gamete receptors whose interaction mediates sperm-egg binding. Here we show that mouse sperm Gal-transferase specifically recognizes those oligosaccharides on ZP3 that have sperm-binding activity, but does not interact with other zona pellucida glycoproteins. In contrast, all zona pellucida glycoproteins are recognized by non-sperm Gal-transferase, demonstrating a more stringent substrate specificity for the sperm enzyme. This interaction is required for sperm-egg binding because blocking or removing the binding site for Gal-transferase on ZP3 inhibits its ability to bind sperm. After the release of the sperm acrosome, the transferase relocalizes to a new membrane domain where it can no longer bind to ZP3, which is consistent with the inability of acrosome-reacted sperm to bind ZP3 or to initiate binding to the zona pellucida. Following fertilization, ZP3 is modified by egg cortical granule secretions so that it loses sperm receptor activity, which can be accounted for by a selective loss of its binding site for sperm Gal-transferase. These results show that sperm surface beta-1,4-galactosyltransferase and the egg-coat glycoprotein ZP3 are complementary adhesion molecules that mediate primary gamete binding in the mouse.

Recent studies suggest that gamete recognition in a number of species is mediated by complementary proteins and carbohydrates on opposing gamete surfaces. Studies in invertebrates and vertebrates have shown that carbohydrate-binding proteins on the sperm surface recognize and bind to complementary glycoconjugates on the egg's extracellular coat. This chapter reviews our current knowledge of gamete recognition in the mouse. The complementary receptors for both mouse sperm and egg have been identified, purified, and characterized. Their synthesis during gametogenesis has been defined, as have the effects of sperm capacitation and of the acrosome reaction on their expression and distribution. Their relationship to gamete receptors that function in other species is discussed. Finally, evidence is presented that suggests that one of the receptors that mediate mouse gamete recognition belongs to a family of cell surface receptors that function during multiple cellular interactions in development.Key words: acrosome reaction, capacitation, galactosyltransferase, gamete receptors, glycoconjugates, lectins, spermatogenesis, zona pellucida INTRODUCTIONThe binding and union of two separate gametes to form a new organism has intrigued investigators for the last century. In 1913, F.R. Lillie proposed that complementary cell surface receptors mediate gamete recognition and binding [Lillie, 19131, but not until the last 15 years has progress been made toward identifying these receptors.Although the molecular mechanisms underlying gamete recognition and fusion are yet to be clearly resolved, a recurring theme extends throughout several orders of the animal kingdom. It appears that sperm surface carbohydrate-binding proteins mediate gamete recognition by binding with high affinity and specificity to complex glycoconjugates of the egg coat (Fig. 1) species, sperm interactions with the egg coat are species specific. This specificity is attributed to complementarity between interacting molecules on opposing gamete surfaces. It is not surprising that interactions between complementary cell surface proteins and glycoconjugates participate in fertilization, since a number of somatic cells appear to utilize a similar paradigm for intercellular recognition [Damsky et al., 1984; Ivatt, 19841. This chapter will focus primarily on the manner in which glycoconjugate recognition mediates the initial phases of gamete interaction during fertilization. A brief overview is presented to indicate that gamete recognition is mediated in a number of systems by protein-carbohydrate complementarity. To illustrate this concept more fully, the discussion focuses on one of the more thoroughly characterized systems-the mouse. The cell surface molecules that mediate gamete recognition in the mouse have been identified and characterized, as have their expression during gametogenesis and their fate following initial gamete interaction. A brief discussion follows of analogous mechanisms in other species, and in other cellular interactions. PROTEIN: CARBOHYDR...

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