cDNA cloning and characterization of a novel squid rhodopsin kinase encoding multiple modular domains

Rhodopsin phosphorylation is one of the key mechanisms of inactivation in vertebrate and invertebrate visual signal transduction. Here we report the cDNA cloning and protein characterization of a 70-kDa squid rhodopsin kinase, SQRK. The cDNA encoding the 70-kDa protein demonstrates high sequence identity with octopus rhodopsin kinase (92%) and mammalian beta-adrenergic receptor kinases (63-65%), but only 33% similarity with bovine rhodopsin kinase, suggesting that invertebrate rhodopsin kinases may be structurally similar to beta-adrenergic receptor kinases. This cDNA encodes three distinct modular domains: RGS, S/TKc, and PH domains. The native SQRK is an eye-specific protein that is only expressed in photoreceptor cells and the optic ganglion as determined by immunoblotting. Purified SQRK is able to phosphorylate both squid and bovine rhodopsin. Squid rhodopsin phosphorylation by purified SQRK was sensitive to both Mg2+ and GTPgammaS but was insensitive to Ca2+/CaM regulation. The ability of SQRK to phosphorylate rhodopsin was totally lost in the presence of SQRK-specific antibodies. Our results suggest that SQRK plays an important role in squid visual signal termination.