Reconstitution of agonist-stimulated phosphatidylinositol 4,5-bisphosphate hydrolysis using purified m1 muscarinic receptor, G_q/11, and phospholipase C-β

We describe the reconstitution using purified proteins of the m1 muscarinic cholinergic pathway that activates phosphatidylinositol 4,5-bisphosphate-specific phospholipase C via the G protein G_q/11. Recombinant m1 muscarinic receptor was co-reconstituted in lipid vesicles with either hepatic G_q/11 or with cerebral α_q/11 and βγ subunits. The rate of [³⁵S]GTPγS binding to the reconstituted vesicles was stimulated 20-50-fold by agonist. Maximal receptor-catalyzed binding was 7 mol of GTPγS bound per mol of receptor. The m2 muscarinic receptor was a poor activator of G_q/11. The binding of [α-³²P]GTP and [γ-³²P]GTP to m1/G_q/11 vesicles indicated that the receptor could maintain up to 40% of the total coupled G_q/11 in the GTP bound state. The rate of hydrolysis of bound GTP, 0.8 mm^-1, is consistent with the rate predicted from the GTP binding data but is 3-5-fold lower than rates reported for other trimeric G proteins. Agonist-stimulated photoaffinity labeling with γ-(4-azidoanilido)-[α-³²P]GTP indicated that the receptor catalyzed binding to both α_q and α₁₁ with about equal efficiency. Receptor-catalyzed activation of G_q/11 by GTPγS, measured as the ability to activate purified phospholipase C-β1, paralleled receptor-catalyzed [³⁵S]GTPγS binding. Co-reconstitution of receptor, G_q/11, and phospholipase C-β1 restored GTPγS-dependent carbachol-stimulated hydrolysis of phosphatidylinositol 4,5-bis-phosphate. The m1 receptor, G_q/11, and phospholipase C-β1 are thus sufficient to initiate the hormonal inositol trisphosphate/diacylglycerol signaling pathway without additional proteins.