Involvement of an inhibitory G-protein in the signal transduction pathway of maturation-inducing hormone (17α,20β-dihydroxy-4-pregnen-3-one) action in rainbow trout (oncorhynchus mykiss) oocytes

To investigate the mechanism by which the hormone 17α,20β-dihydroxy-4-pregnen-3-one(17α,20β-DP) acts on a receptor on the external surface of rainbow trout oocytes to induce maturation, the interaction between 17α,20β-DP receptors and G-proteins was examined. Pertussis toxin (PT) catalyzed the ADP ribosylation of a 40-kDa protein in crude membranes from rainbow trout postvitellogenic oocytes, and cholera toxin (CT) labeled several proteins, including a major protein with an apparent molecular weight of 43 kDa. The 40-kDa protein was recognized by an antibody against the α subunit of inhibitory G-proteins (G_i), whereas the 43-kDa protein was recognized by an antibody against the α subunit of stimulatory G-proteins (G_s). Treating the membrane fraction with 17α,20β-DP decreased the PT-catalyzed ADP ribosylation of the 40-kDa protein. In contrast, there was no significant change in the CT-catalyzed ribosylation of the 43-kDa protein after exposure to 17α,20β-DP. The specific binding of 17α,20β-DP to membrane fractions was decreased by PT. 17α,20β-DP binding was also inhibited by nonhydrolyzable GTP analogs such as guanosine 5′-O-(3-thiotriphosphate) (GTP-γS) and guanylylimidodiphosphate (GppNHp), but not by either ATP or guanosine 5′-O-(2-thiodiphosphate) (GDPβS). Scatchard analysis revealed that GppNHp induced a, 3.8-fold increase inthe dissociation constant without a significant change in the number of binding sites, suggesting that the GppNHp-induced decrease in 17α,20β-DP binding is due to the decrease in binding affinity between 17α,20β-DP and its receptors. We conclude that the PT-sensitive G_i is involved in the signal transduction pathway of 17α,20β-DP in rainbow trout oocytes.