Phosphorylation by Protein Kinase C of the Muscarinic Acetylcholine Receptor

Abstract

Muscarinic acetylcholine receptors purified from porcine cerebrum were phosphorylated by protein kinase C purified from the same tissue. More than 1 mol of phosphate was incorporated per mole of receptor, with both serine and threonine residues being phosphorylated. Neither the degree nor the rate of the phosphorylation was affected by the presence or absence of acetylcholine. GTP‐sensitive high‐affinity binding with acetylcholine was observed for muscarinic receptors reconstituted with GTP‐binding proteins (G_i or G_o), irrespective of whether muscarinic receptors or the GTP‐binding proteins had been phosphorylated by protein kinase C or not. This indicates that the interaction between purified muscarinic receptors and purified GTP‐binding proteins in vitro is not affected by their phosphorylation.