Photoaffinity labeling of two rat liver plasma membrane proteins with [³²P]γ-azidoanilido GTP in response to vasopressin: Immunologic identification as α subunits of the G_q class of G proteins

Two proteins have been identified in rat liver plasma membranes that bind a photoreactive GTP analogue, [³²P]γ-azidoanilido GTP, in response to incubation with the Ca²⁺-mobilizing agonist, vasopressin. The labeled proteins possess apparent molecular masses of 42 and 43 kDa. Their labeling requires Mg²⁺ and can be inhibited by GTP, its analogues, and GDP but not by other nucleotides. Vasopressin-stimulated labeling is attenuated by a V₁ receptor-selective antagonist. The concentration of vasopressin required to stimulate labeling is in the same range (EC₅₀ = 4 nM) as that required for activation of GTPase and phosphoinositide-specific phospholipase C activities in liver plasma membranes. Immunodetection and immunoprecipitation of the [³²P]γ-azidoanilido GTP-labeled 42- and 43-kDa proteins with antisera raised against peptide sequences in α_q indicate that these proteins are members of the recently described G_q class of G proteins.