ACTIVATION OF A G-PROTEIN COMPLEX BY AGGREGATION OF BETA-1,4-GALACTOSYLTRANSFERASE ON THE SURFACE OF SPERM

Fertilization is initialed by the species-specific binding of sperm to the extracellular coat of the egg. One sperm receptor for the mouse e gg is beta-1,4-galactosyltransferase (GalTase), which binds O-linked o ligosaccharides on the egg coat glycoprotein ZP3. ZP3 binding induces acrosomal exocytosis through the activation of a pertussis toxin-sensi tive heterotrimeric guanine nucleotide-binding protein (G protein). Th e cytoplasmic domain of sperm surface GalTase bound to and activated a heterotrimeric G protein complex that contained the G(i alpha) subuni t. Aggregation of GalTase by multivalent ligands elicited G protein ac tivation. Sperm from transgenic mice that overexpressed GalTase had hi gher rates of G protein activation than did wild-type sperm, which ren dered transgenic sperm hypersensitive to their ZP3 ligand. Thus, the c ytoplasmic domain of cell surface GalTase appears to enable it to func tion as a signal-transducing receptor for extracellular oligosaccharid e ligands.