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

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 loopV−VI (N-domain) are coupled to the entry of agonist, while loopXVIII−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. Following retinalcis/trans isomerisation, the active form of the G-protein-coupled receptor rhodopsin decays to opsin and all-trans-retinal. In this study, arrestin, a regulator of G-protein-coupled receptor activity, is shown to facilitate the concurrent sequestering of toxic all-trans-retinal and regeneration of 11-cis-retinal within the opsin population.