Reconstitution of muscarinic receptor-mediated inhibition of adenylyl cyclase.

Inhibition of bovine brain calmodulin-sensitive adenylyl cyclase was examined in a system consisting of the reconstituted purified porcine atrial muscarinic acetylcholine receptor, the purified inhibitory guanine nucleotide-binding protein (Gi), and the partially purified stimulatory guanine nucleotide-binding protein.adenylyl cyclase complex. Under conditions where Gi existed mainly as the Gi.GDP complex, adenylyl cyclase was selectively preactivated with guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S). Addition of carbachol formed the receptor.carbachol complex, which catalyzed the exchange of GDP bound to Gi for GTP gamma S, initiating Gi-mediated inhibition of adenylyl cyclase. Adenylyl cyclase activated by calcium plus calmodulin was more sensitive to inhibition by carbachol than either unstimulated adenylyl cyclase or adenylyl cyclase activated by GTP gamma S or forskolin. Studies using the resolved subunits of Gi showed that the beta gamma subunit could inhibit adenylyl cyclase activated by GTP gamma S or calcium plus calmodulin, as well as the unactivated enzyme. The alpha subunit of Gi inhibited adenylyl cyclase only when adenylyl cyclase was activated by calcium plus calmodulin. Possible explanations for these results are discussed.