Distinct loops in arrestin differentially regulate ligand binding within the GPCR opsin

Abstract

G-protein-coupled receptors are universally regulated by arrestin binding. Here we show that rod arrestin induces uptake of the agonist all-trans-retinol in only half the population of phosphorylated opsin in the native membrane. Agonist uptake blocks subsequent entry of the inverse agonist 11-cis-retinal (that is, regeneration of rhodopsin), but regeneration is not blocked in the other half of aporeceptors. Environmentally sensitive fluorophores attached to arrestin reported that conformational changes in loop Vĝ̂'VI (N-domain) are coupled to the entry of agonist, while loop XVIIIĝ̂ 'XIX (C-domain) engages the aporeceptor even before agonist is added. The data are most consistent with a model in which each domain of arrestin engages its own aporeceptor, and the different binding preferences of the domains lead to asymmetric ligand binding by the aporeceptors. Such a mechanism would protect the rod cell in bright light by concurrently sequestering toxic all-trans-retinol and allowing regeneration with 11-cis-retinal. © 2012 Macmillan Publishers Limited. All rights reserved.