INVOLVEMENT OF AN INHIBITORY G-PROTEIN IN THE SIGNAL-TRANSDUCTION PATHWAY OF MATURATION-INDUCING HORMONE (17-ALPHA,20-BETA-DIHYDROXY-4-PREGNEN-3-ONE) ACTION IN RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) OOCYTES

To investigate the mechanism by which the hormone 17 alpha,20 beta-dih ydroxy-4-pregnen-3-one(17 alpha,20 beta-DP) acts on a receptor on the external surface of rainbow trout oocytes to induce maturation, the in teraction between 17 alpha,20 beta-DP receptors and G-proteins was exa mined. Pertussis toxin (PT) catalyzed the ADP ribosylation of a 40-kDa protein in crude membranes from rainbow trout postvitellogenic oocyte s, and cholera toxin (CT) labeled several proteins, including a major protein with an apparent molecular weight of 43 kDa. The 40-kDa protei n was recognized by an antibody against the alpha subunit of inhibitor y G-proteins (G(i)), whereas the 43-kDa protein was recognized by an a ntibody against the alpha subunit of stimulatory G-proteins (G(5)). Tr eating the membrane fraction with 17 alpha,20 beta-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-k Da protein after exposure to 17 alpha,20 beta-DP. The specific binding of 17 alpha,20 beta-DP to membrane fractions was decreased by PT. 17 alpha,20 beta-DP binding was also inhibited by nonhydrolyzable GTP ana logs such as guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) and gua nylylimidodiphosphate (GppNHp), but not by either ATP or guanosine 5'- O-(2-thiodiphosphate) (GDP beta S). Scatchard analysis revealed that G ppNHp induced a 3.8-fold increase in the dissociation constant without a significant change in the number of binding sites, suggesting that the GppNHp-induced decrease in 17 alpha,20 beta-DP binding is due to t he decrease in binding affinity between 17 alpha,20 beta-DP and its re ceptors. We conclude that the PT-sensitive G(i) is involved in the sig nal transduction pathway of 17 alpha,20 beta-DP in rainbow trout oocyt es. (C) 1994 Academic Press, Inc.