Selective activation of Gi1 and Gi2 in mouse sperm by the zona pellucida, the egg's extracellular matrix

Mammalian sperm acrosomal exocytosis as induced by the egg's zona pellucida (ZP) appears to be a G protein-mediated event. Previously, we demonstrated that ZP3, the acrosome reaction-inducing component of mouse ZP, activates one or more Gi subtypes in isolated sperm membranes (Ward, C. R., Storey, B. T., and Kopf, G. S. (1992) J. Biol. Chem. 267, 14061-14067). To determine the identity of the Gi subtype(s) activated in the sperm, we examined ligand-dependent decreases in pertussis toxin-catalyzed in vitro [32P]ADP-ribosylation of the different Gi alpha subtypes that were identified by immunoprecipitation using Gi alpha-specific antisera. Membranes treated with solubilized ZP or mastoparan and subsequently [32P]ADP-ribosylated using pertussis toxin displayed 35 and 56% decreases, respectively, in [32P]ADP-ribosylation when compared with controls. These changes were quantitatively similar to the percentage increases in 35S-labeled guanosine 5'-O-(thiotriphosphate) binding in membranes treated with equivalent concentrations of ZP or mastoparan (Ward et al., 1992). Immunoprecipitation with Gi alpha subtype-specific antisera and subsequent autoradiography revealed that mastoparan activated Gi1, Gi2, and Gi3 in the membranes. ZP, in contrast, selectively activated Gi1 and Gi2. These data demonstrate that the egg's extracellular matrix has the ability to activate selectively sperm membrane Gi subtypes.