Agonist-Receptor-Arrestin, an Alternative Ternary Complex with High Agonist Affinity*

The rapid decrease of a response to a persistent stimulus, often termed desensitization, is a widespread biological phenomenon. Signal transduction by numerous G protein-coupled receptors appears to be terminated by a strikingly uniform two-step mechanism, most extensively characterized for the β2-adrenergic receptor (β2AR), m2 muscarinic cholinergic receptor (m2 mAChR), and rhodopsin. The model predicts that activated receptor is initially phosphorylated and then tightly binds an arrestin protein that effectively blocks further G protein interaction. Here we report that complexes of β2AR-arrestin and m2 mAChR-arrestin have a higher affinity for agonists (but not antagonists) than do receptors not complexed with arrestin. The percentage of phosphorylated β2AR in this high affinity state in the presence of full agonists varied with different arrestins and was enhanced by selective mutations in arrestins. The percentage of high affinity sites also was proportional to the intrinsic activity of an agonist, and the coefficient of proportionality varies for different arrestin proteins. Certain mutant arrestins can form these high affinity complexes with unphosphorylated receptors. Mutations that enhance formation of the agonist-receptor-arrestin complexes should provide useful tools for manipulating both the efficiency of signaling and rate and specificity of receptor internalization.