Regulation of adenylyl cyclase by GTP-binding Proteins

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GTP-binding protein (G proteins) consisting of alpha, beta, gamma subunits, carry signals from membrane receptors to effectors such as enzymes or ion channels. G proteins dissociate into the GTP-bound alpha and betagamma subunits upon their interaction with agonist-receptor complex. In the present studies, we investigated mechanisms by which the activities of adenylyl cyclase and phosphatidylinositol (PI) 3-kinase are regulated by G proteins, especially the betagamma subunits.In guinea pig neutrophils, the cellular cAMP formation by prostagladin (PG) E_1 was markedly potentiated by the chemoattractant formyl-Met-Leu-Phe (fMLP). This potentiation was abolished by prior treatment of the cells with pertussis toxin, but not by the prevention of fMLP-induced intracellular Ca^<2+> increase in the cells, indicating the direct involvement of the inhibitory G protein (G_i) in the fMLP-induced potentiation of cAMP formation. Adenylyl cyclase responsible for the G_i-induced potentiation was parti … More ally purified from the GTPgammaS-treated cell membranes with a forskolin-agarose column. The cyclase appeared to be a complex form with the GTPgammaS-bound alpha subunit of the stimulatory G_S protein (G_Salpha), but not with the betagamma subunits. The GTPgammaS-bound G_Salpha-stimulated cyclase activity was further enhanced by betagamma subunits. These results indicate that GTP-bound G_Salpha and betagamma subunits released from G_i synergistically stimulate adenylyl cyclase activity in neutrophils. In relation to this, we also found that the activity of PI 3-kinase is inhibited by wortmannin and that several types of PI 3-kinase are stimulated by bg subunits. Interestingly, a peptide containing phosphorylated Tyr and betagamma subunits synergistically activated a certain type of PI 3-kinase. Such synergistic activation was also observed in intact neutrophil-like cells upon their stimulation with insulin and fMLP.Thus, G_i activation appeared to have a cross-talk with Tyr-phosphorylation signaling pathway via betagamma subunits in PI 3-kinase. Less