The receptor function of galactosyltransferase during mammalian fertilization

The molecular mechanisms underlying mouse sperm binding to the egg zona pellucida are being examined. A variety of studies suggest that galactosyltransferase (GalTase) on the sperm surface at least partly mediates gamete adhesion by binding to its appropriate carbohydrate substrate in the egg zona pellucida. The first indication that GalTase serves as a sperm receptor came from a biochemical analysis of sperm bearing mutant alleles of the t-complex. t-bearing sperm, which are transmitted perferentially during fertilization relative to normal sperm, have four times the surface GalTase activity of wild-type, while eight other enzyme activities are indistinguishable between normal and t-sperm populations. Interestingly, mutant t-sperm that have lost their fertilizing superiority due to genetic recombination, no longer show elevated GalTase activity. Before sperm are capable of binding to the eggs, the GalTase active site becomes exposed on the sperm surface, due to the release of a competitive substrate. When added back to sperm/zona binding assays, this GalTase substrate inhibits binding by competing for the GalTase active site. Inhibition and/or modification of the sperm GalTase with either substrate analogues or modifier proteins, produces a parallel inhibition of sperm GalTase activity and sperm binding to the zona pellucida. The presence of the second GalTase substrate, UDPGal, causes the dissociation of sperm bound to the zona pellucida. UDPGal forces catalysis to occur, thus releasing the sperm GalTase from its zona pellucida substrate. Purified GalTase inhibits sperm/zona binding, as does galactosylation of the zona pellucida. Monospecific anti-GalTase IgG, and its Fab fragments, inhibit sperm/zona binding and GalTase catalytic activity. The sperm surface GalTase is localized by indirect immunofluorescence to the plasma membrane overlying the intact acrosome. Finally, the zona pellucida possess substrates for the sperm GalTase, and enzymatic modification of these substrates inhibits subsequent sperm binding. These studies, as well as others, all suggest that sperm GalTase serves as at least one, and possibly the principal, sperm surface receptor for binding to the zona pellucida during fertilization. Studies are in progress to identify the complementary GalTase substrate in the zona pellucida.