Activation of solubilized G-proteins by muscarinic acetylcholine receptors

Binding of GTP and its analogue, guanosine 5'-O-[gamma-thio]triphosphate (GTP[S]) to G-proteins, and release of GTP[S] from G-proteins are stimulated by muscarinic acetylcholine (mACh) receptors in intact cardiac membranes. Upon solubilization of receptors and G-proteins by membrane extraction with the detergent, 3-[(cholamidopropyl)dimethylammonio]-1-propanesulphonate, followed by sucrose density gradient centrifugation, agonist-liganded mACh receptors stimulated binding of GTP[S] and hydrolysis of GTP by G-proteins with similar requirements as in intact membranes. One soluble agonist-activated mACh receptor induced binding of GTP[S] to several (about seven) soluble G-proteins. In contrast to intact membranes, however, agonist activation of mACh receptors did not induce release of GTP[S] from solubilized G-proteins. The data presented indicate that mACh receptors can interact with and efficiently activate G-proteins even in solution, whereas the possible interaction of receptors with GTP[S]-liganded G-proteins observed in intact membranes is lost upon solubilization of these components.