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

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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.