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

大鼠垂体细胞膜与生长抑素孵育可刺激GI2和GOα亚基的释放，但在低浓度GTPGammaS(20 NM)存在下，GI2和GOα亚基不能释放GI3。百日咳毒素可抑制生长抑素的作用。此外，在完整的GH4C1细胞中加入生长抑素可降低粗膜中GI2α亚基的水平，而胞浆部分的免疫印迹分析明显显示GI2α亚基的存在。这些数据表明，当生长抑素激活时，GH4C1细胞中对百日咳毒素敏感的G蛋白解离为α亚基和β-γ复合体，α亚基从膜上释放出来。另一方面，VIP和PACAP也刺激Gsα亚基的释放。在与Gs缺失的S49细胞膜重组时，培养上清液中的Gsα亚基刺激腺苷环化酶的活性。此外，在完整的GH4C1细胞中加入VIP后，粗膜部分中Gsα亚基的水平降低，而胞浆部分的免疫印迹分析显示Gsalpha亚基增加。细胞长时间暴露于VIP 2-24小时后，细胞膜Gsα亚基水平下降21-54%，胞浆部分细胞总水平上升6%。这些数据表明，VIP受体激活Gsα亚基，并诱导膜上α亚基的释放，同时伴随着降解速率的增加。

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.