Sites and roles of arrestin phosphorylation in regulating interactions between arrestin and rhodopsin of squid eyes

In squid (Loligo pealei), activation of phototransduction is mediated by rhodopsin isomerization to metarhodopsin and the activation of a phospholipase C pathway, resulting in depolarization of the retinal membrane. Inactivation is mediated by squid rhodopsin kinase (SQRK) and arrestin. Arrestin binding to metarhodopsin is the key inactivation step and SQRK-mediated phosphorylation of rhodopsin is not required for arrestin binding. SQRK can also phosphorylate arrestin, a function that is not shared by other receptor kinases.;To determine the site(s) and function of arrestin phosphorylation, I expressed recombinant arrestins in E. coli and used site-directed mutagenesis. Squid arrestin was phosphorylated at Ser392 and/or Ser 397 by SQRK. Arrestin phosphorylation did not affect arrestin binding to rhodopsin in the light but inhibited its binding to dark-adapted membranes. Furthermore, the phosphorylation significantly increased arrestin dissociation from dark-adapted membranes at high salt concentrations in vitro. This work suggested that the role of arrestin phosphorylation was to increase dissociation of the protein from rhodopsin once it has been photoconverted back to inactive state.