AGONIST-RECEPTOR-ARRESTIN, AN ALTERNATIVE TERNARY COMPLEX WITH HIGH AGONIST AFFINITY

The rapid decrease of a response to a persistent stimulus, often terme d desensitization, is a widespread biological phenomenon, Signal trans duction by numerous G protein-coupled receptors appears to be terminat ed by a strikingly uniform two-step mechanism, most extensively charac terized for the beta(2)-adrenergic receptor (beta(2)AR), m2 muscarinic cholinergic receptor (m2 mAChR), and rhodopsin, The model predicts th at activated receptor is initially phosphorylated and then tightly bin ds an arrestin protein that effectively blocks further G protein inter action, Here we report that complexes of beta(2)AR-arrestin and m2 mAC hR-arrestin have a higher affinity for agonists (but not antagonists) than do receptors not complexed with arrestin. The percentage of phosp horylated beta(2)AR in this high affinity state in the presence of ful l agonists varied with different arrestins and was enhanced by selecti ve mutations in arrestins, The percentage of high affinity sites also was proportional to the intrinsic activity of an agonist, and the coef ficient of proportionality varies for different arrestin proteins. Cer tain mutant arrestins can form these high affinity complexes with unph osphorylated receptors, Mutations that enhance formation of the agonis t-receptor-arrestin complexes should provide useful tools for manipula ting both the efficiency of signaling and rate and specificity of rece ptor internalization.