Abstract:Electrophysiological studies of photoreceptors from the horseshoe crabLimulus polyphemuscontinue to provide fundamental new knowledge of the photoresponse in invertebrates. Therefore, it is of particular interest to characterize the molecular components of the photoresponse in this system. Here we describe an arrestin cloned from a cDNA library constructed using poly(A)+RNA isolated fromLimuluslateral eyes. The protein, deduced from the arrestin cDNA, is most similar to arrestin from locust antennae (56% identity) andDrosophilaphosrestin I (53% identity).Limulusarrestin was expressed in a heterologous system, and its properties were compared with those of a 46‐kDa light‐regulated phosprotein (pp46A) inLimulusphotoreceptors described in previous studies from this laboratory. Arrestin and pp46A (a) have the same apparent molecular weight on sodium dodecyl sulfate‐polyacrylamide gel electrophoresis, (b) have an isoelectric point in the basic pH range, (c) require calmodulin and elevated Ca2+levels for phosphorylation, (d) are immunoreactive with monoclonal antibody C10C10 directed against a sequence in bovine arrestin (S‐antigen) that is perfectly conserved in the deduced arrestin protein, and (e) are associated with photoreceptors. We conclude that the arrestin described here and pp46A are the same protein. The results of this and previous studies show that inLimulusphotoreceptors, light regulates the phosphorylation of arrestin in compl