Dual regulation by G proteins of agonist-dependent phosphorylation of muscarinic acetylcholine receptors

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Muscarinic acetylcholine receptors purified from porcine atrium were phosphorylated, depending on the presence of agonists, by a protein kinase partially purified from porcine brain, which had similar properties to the β-adrenergic receptor kinase. GTP-binding regulatory proteins (Go) had dual effects on the phosphorylation of muscarinic receptors, i.e. stimulation at lower concentrations and inhibition at higher concentrations. The stimulatory effect was reproduced with the βγ subunit of Go and the inhibitory effect with the combination of the α and βγ subunits.