Mechanism of arrestin 2 function in rhabdomeric photoreceptors

Arrestins have emerged as one family of proteins that mediate the inactivation of G-protein-coupled receptors. We have isolated cDNA clones encoding two arrestin isoforms of the dipteran visual system, Calliphora arrestin 1 (Arr1) and arrestin 2 (Arr2). Microsequencing established that the arr2 gene encodes the Calliphora 49-kDa protein characterized previously as a photoreceptor-specific protein that undergoes reversible binding to light-activated rhodopsin and thereby activates the phosphorylation of metarhodopsin. Ultrastructural localization of Arr2 to the rhabdomeral part of the photoreceptor cell and quantitation of the amount of Arr2 bound suggest that Arr2 directly interacts with light-activated rhodopsin. In a reconstituted system containing affinity purified Arr2 and isolated rhabdomeric membranes, Arr2 binds to non-phosphorylated and phosphorylated metarhodopsin with comparable affinity. Reaction time courses reveal that Arr2 binding precedes phosphorylation of metarhodopsin, contrary to what has been reported so far for vertebrate photoreceptors. The phosphorylation-independent binding of Arr2 to metarhodopsin provides a mechanism for the rapid inactivation of the long-lived activated rhodopsin state which is characteristic for invertebrate photoreceptors. The dephosphorylation of rhodopsin is catalyzed by a Ca(2+)-dependent protein phosphatase which is shown here for the first time to exist in a membrane-associated form. Only metarhodopsin molecules with bound Arr2 are resistant to dephosphorylation. Thus, in fly photoreceptors, Arr2 acts as a regulatory protein that controls the phosphorylation as well as the dephosphorylation of the light-activated visual pigment.