Regulation of adenylyl cyclase by GTP-binding Proteins

GTP结合蛋白（G蛋白）由α、β、γ亚基组成，将信号从膜受体传递到效应器，如酶或离子通道。G蛋白在与激动剂-受体复合物相互作用后解离成GTP结合的α和β γ亚基。在豚鼠中性粒细胞中，趋化因子甲酰-蛋氨酸-亮氨酸-苯丙氨酸（formyl-Met-Leu-Phe，fMLP）能显著增强辣根过氧化物酶（PG）E_1诱导的细胞内cAMP生成，并能抑制PG E_1诱导的细胞内cAMP生成。预先用百日咳毒素处理细胞可以消除这种增强作用，但不能阻止fMLP诱导的细胞内Ca^2+增加，这表明抑制性G蛋白（G_i）直接参与了fMLP诱导的cAMP形成增强作用。腺苷酸环化酶参与了G_i诱导的增强作用。 ...更多信息 用毛喉素-琼脂糖柱从GTP γ S处理的细胞膜中纯化。环化酶与刺激性G_S蛋白（G_Salpha）的GTP γ S结合α亚基形成复合物，而与β γ亚基不形成复合物。GTP γ S结合的G_α刺激的环化酶活性被β γ亚基进一步增强。这些结果表明，GTP结合的G_i释放的G_Salpha和β γ亚单位协同刺激中性粒细胞腺苷酸环化酶活性。与此相关，我们还发现，PI 3-激酶的活性被渥曼青霉素抑制，几种类型的PI 3-激酶被bg亚基刺激。有趣的是，含有磷酸化Tyr和β γ亚基的肽协同激活某种类型的PI 3-激酶。胰岛素和fMLP刺激后，在完整的类胰岛细胞中也观察到这种协同激活作用，因此，PI 3-激酶β γ亚基的Tyr磷酸化信号通路可能与Gi激活存在交叉作用。少

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