The study of the structure and the function of soluble GTP-binding protein which involved in the somatostatin receptor

在低浓度GTP γ S（20 nM）存在下，用生长抑素孵育GH4 C1大鼠垂体细胞膜刺激Gi 2和Go α亚基的释放，但不刺激Gi 3从膜中释放。生长抑素的作用被百日咳毒素预处理的GH4 C1细胞所抑制。此外，在完整的GH4 C1细胞中加入生长抑素降低了粗膜中Gi 2 α亚基的水平，而胞质组分的免疫印迹分析清楚地显示了Gi 2 α亚基的存在。这些数据表明，百日咳毒素敏感的G蛋白在GH4 C1细胞解离成α亚基和β γ复合物的α亚基从膜释放后生长抑素激活。VIP和PACAP也能促进Gs α亚基的释放。在用来自GS-缺陷S49细胞的膜重建时，上清液中的Gs α亚基刺激腺苷酸环化酶的活性。此外，添加VIP到完整的GH4 C1细胞中降低了粗膜组分中Gs α亚基的水平，而胞质组分的免疫印迹分析显示Gsalpha亚基增加。延长暴露的细胞VIP 2-24小时，导致21-54%的降低Gs α亚基水平的膜和6%的增加，在细胞溶质组分中的总细胞水平。这些数据表明，VIP受体激活Gs α亚单位，并诱导α亚单位从膜释放沿着降解速率的增加。

Incubation of GH4C1 rat pituitary cell membranes with somatostatin stimulte the release of Gi2 and Go alpha subunits but not Gi3 from membranes in the presence of a low concentration of GTPgammaS (20 nM) . The effects of somatostatin are inhibited by pretreatment of GH4C1 cells with pertussis toxin. Furthermore, the addition of somatostatin to intact GH4C1 cells decreases the level of Gi2 alpha subunits in the crude membrane whereas immunoblot analysis of the cytosolic fraction learly shows the presence of Gi2 alpha subunits. These data indicate that pertussis toxin-sensitive G proteins in GH4C1 cells dissociate into alpha subunits and beta gamma complex with the release of the alpha subunits from the membranes upon somatostatin activation. On the other hand, VIP and PACAP also stimulte the release of Gs alpha subunit. On reconstitution with membranes from Gs-deficint S49 cyc-cells, the Gs alpha subunit in the supernatant stimulate the activity of adenylatecyclase. Furthermore, the addition of VIP to intact GH4C1 cells decreases the levels of Gs alpha subunits in the crude membrane fraction whereas immunoblot analysis of cytosolic fraction shows an increase in Gsalpha subunits. Prolonged exposure of cells to VIP for 2-24 h, caused a 21-54% decrease in Gs alpha subunit levels in membranes and a 6% increase in total cellular levels in the cytosolic fractions. These data indicate that the VIP receptor activates Gs alpha subunits and induced the release of alpha subunit from membranes along with an increase in the degradation rate.