Residency time of agonists does not affect the stability of GPCR–arrestin complexes

Abstract

Background and Purpose: The interaction of arrestins with G-protein coupled receptors (GPCRs) desensitizes agonist-dependent receptor responses and often leads to receptor internalization. GPCRs that internalize without arrestin have been classified as “class A” GPCRs whereas “class B” GPCRs co-internalize with arrestin into endosomes. The interaction of arrestins with GPCRs requires both agonist activation and receptor phosphorylation. Here, we ask the question whether agonists with very slow off-rates can cause the formation of particularly stable receptor–arrestin complexes. Experimental Approach: The stability of GPCR–arrestin-3 complexes at two class A GPCRs, the β2-adrenoceptor and the μ opioid receptor, was assessed using two different techniques, fluorescence resonance energy transfer (FRET) and fluorescence recovery after photobleaching (FRAP) employing several ligands with very different off-rates. Arrestin trafficking was determined by confocal microscopy. Key Results: Upon agonist washout, GPCR–arrestin-3 complexes showed markedly different dissociation rates in single-cell FRET experiments. In FRAP experiments, however, all full agonists led to the formation of receptor–arrestin complexes of identical stability whereas the complex between the μ receptor and arrestin-3 induced by the partial agonist morphine was less stable. Agonists with very slow off-rates could not mediate the co-internalization of arrestin-3 with class A GPCRs into endosomes. Conclusions and Implications: Agonist off-rates do not affect the stability of GPCR–arrestin complexes but phosphorylation patterns do. Our results imply that orthosteric agonists are not able to pharmacologically convert class A into class B GPCRs.