SELECTIVE ACTIVATION OF G(I1), AND G(I2), IN MOUSE SPERM BY THE ZONA-PELLUCIDA, THE EGGS EXTRACELLULAR-MATRIX

Mammalian sperm acrosomal exocytosis as induced by the egg's zona pell ucida (ZP) appears to be a G protein-mediated event. Previously, we de monstrated that ZP3, the acrosome reaction-inducing component of mouse ZP, activates one or more G(i) subtypes in isolated sperm membranes ( Ward, C. R., Storey, B. T., and Kopf, G. S. (1992) J. Biol. Chen. 267, 14061-14067). To determine the identity of the G(i) subtype(s) activa ted in the sperm, we examined ligand-dependent decreases in pertussis toxin-catalyzed in vitro [P-32]ADP-ribosylation of the different G(i a lpha) subtypes that were identified by immunoprecipitation using G(i a lpha)-specific antisera. Membranes treated with solubilized ZP or mast oparan and subsequently [P-32]ADP-ribosylated using pertussis toxin di splayed 35 and 56% decreases, respectively, in [P-32]ADP-ribosylation when compared with controls. These changes were quantitatively similar to the percentage increases in S-35-labeled guanosine 5'-O-(thiotriph osphate) binding in membranes treated with equivalent concentrations o f ZP or mastoparan (Ward et al., 1992). Immunoprecipitation with G(i a lpha) subtype-specific antisera and subsequent autoradiography reveale d that mastoparan activated G(i1), G(i2), and G(i3) in the membranes. ZP, in contrast, selectively activated G(i1) and G(i2). These data dem onstrate that the egg's extracellular matrix has the ability to activa te selectively sperm membrane G(i) subtypes.