The G protein βγ subunit transduces the muscarinic receptor signal for Ca²⁺ release in Xenopus oocytes

At least 30 G protein-linked receptors stimulate phosphatidylinositol 4,5-bisphosphate phosphodiesterase (phospholipase Cβ, PLCβ) through G protein subunits to release intracellular calcium from the endoplasmic reticulum (Clapham, D. E. (1995) Cell 80, 259-268). Although both Gα and Gβγ G protein subunits have been shown to activate purified PLCβ in vitro, Gαq has been presumed to mediate the pertussis toxin-insensitive response in vivo. In this study, we show that Gβγ plays a dominant role in muscarinic-mediated activation of PLCβ by employing the Xenopus oocyte expression system. Antisense nucleotides and antibodies to Gαq/11 blocked the m3-mediated signal transduction by inhibiting interaction of the muscarinic receptor with the G protein. Agents that specifically bound free Gβγ subunits (Gα-GDP and a β-adrenergic receptor kinase fragment) inhibited acetylcholine-induced signal transduction to PLCβ, and injection of Gβγ subunits into oocytes directly induced release of intracellular Ca²⁺. We conclude that receptor coupling specificity of the Gαq/Gβγ heterotrimer is determined by Gαq; Gβγ is the predominant signaling molecule activating oocyte PLCβ.