Termination of Protease-activated Receptor-1 Signaling by β-Arrestins Is Independent of Receptor Phosphorylation*

Protease-activated receptor 1 (PAR1), a G protein-coupled receptor (GPCR) for thrombin, is the prototypic member of a family of protease-activated receptors. PAR1 is irreversibly proteolytically activated; thus, the magnitude and duration of thrombin cellular responses are determined primarily by mechanisms responsible for termination of receptor signaling. Both phosphorylation and β-arrestins contribute to rapid desensitization of PAR1 signaling. However, the relative contribution of each of these pathways to the termination of PAR1 signaling is not known. Co-expression of PAR1 with β-arrestin 1 (βarr1) in COS-7 cells resulted in a marked inhibition of PAR1 signaling, whereas β-arrestin 2 (βarr2) was essentially inactive. Strikingly, signaling by a PAR1 cytoplasmic tail mutant defective in agonist-induced phosphorylation was also attenuated more effectively by βarr1 compared with βarr2. In contrast, both β-arrestin isoforms were equally effective at desensitizing the substance P receptor, a classic reversibly activated GPCR. PAR1 coimmunoprecipitated βarr1 in an agonist-dependent manner, whereas βarr2 association was virtually undetectable. Remarkably, βarr1 also interacted with phosphorylation defective PAR1 mutant, whereas βarr2 did not. Moreover, constitutively active β-arrestin mutants, βarr1 R169E and βarr2 R170E, that bind to activated receptor independent of phosphorylation failed to enhance either wild type or mutant PAR1 desensitization compared with normal versions of these proteins. In contrast, β-arrestin mutants displayed enhanced activity at desensitizing the serotonin 5-hydroxytryptamine2A receptor. Taken together, these results suggest that, in addition to PAR1 cytoplasmic tail phosphorylation itself, β-arrestin binding independent of phosphorylation promotes desensitization of PAR1 signaling. These findings reveal a new level of complexity in the regulation of protease-activated GPCR signaling.