Melanopsin is an opsin identified in photosensitive dermal melanophores of Xenopus laevis. It shares the highest homology with cephalopod opsins and has a long cytoplasmic tail with multiple phosphorylation sites, suggesting it is regulated. Melanopsin mRNA is expressed in hypothalamic regions thought to contain deep brain photoreceptors and in the iris, a structure known to be directly photosensitive in amphibians. It is also localized in retinal cells in the outermost lamina of the inner nuclear layer, where horizontal cells are typically found. Its expression in retinal and nonretinal tissues suggests a role in vision and nonvisual photoreceptive tasks, such as photic control of skin pigmentation, pupillary aperture, and circadian and photoperiodic physiology. Opsins are integral membrane proteins that bind retinaldehyde chromophores. Upon illumination, the chromophore is photoisomerized, causing a conformational change in the opsin and initiating an intracellular signaling cascade. Melanophores of Xenopus laevis, like retinal photoreceptors, are photosensitive. When maintained in vitro, melanosomes in dermal melanophores migrate to the cellular periphery in response to illumination or activation of G protein-coupled receptors. Melanophore photosensitivity is reversibly activated by retinaldehydes and has an action spectrum characteristic of opsin-like photopigments. To identify an opsin that may regulate melanosome migration, protein extracts of cultured melanophores were analyzed for opsin immunoreactivity and a melanophore cDNA library was screened for opsin-like nucleotide sequences. A 2.2-kb cDNA clone was isolated, encoding a 534-amino acid protein more homologous to known opsins. Melanopsin shares structural similarities with all known opsins, including an extracellular amino terminus and seven transmembrane domains. It has a lysine in the seventh transmembrane domain for chromophore attachment and a pair of cysteines in the second and third extracellular loops for disulfide bridge formation. It lacks N-glycosylation sites typically found in the extracellular amino terminus of most opsins. Melanopsin is more homologous to invertebrate opsins than vertebrate opsins. It shares 39% identity with octopus rhodopsin and approximately 30% identity with typical vertebrate opsins. Phylogenetic analysis indicates that melanopsin and invertebrate opsins diverged from a common ancestor. Melanopsin is localized in the iris, brain structures, and retinal cells, suggesting a role in nonvisual photoreception. Its long cytoplasmic tail contains 14 potential phosphorylation sites, suggesting it is regulated by kinases. Melanopsin may provide a mechanismMelanopsin is an opsin identified in photosensitive dermal melanophores of Xenopus laevis. It shares the highest homology with cephalopod opsins and has a long cytoplasmic tail with multiple phosphorylation sites, suggesting it is regulated. Melanopsin mRNA is expressed in hypothalamic regions thought to contain deep brain photoreceptors and in the iris, a structure known to be directly photosensitive in amphibians. It is also localized in retinal cells in the outermost lamina of the inner nuclear layer, where horizontal cells are typically found. Its expression in retinal and nonretinal tissues suggests a role in vision and nonvisual photoreceptive tasks, such as photic control of skin pigmentation, pupillary aperture, and circadian and photoperiodic physiology. Opsins are integral membrane proteins that bind retinaldehyde chromophores. Upon illumination, the chromophore is photoisomerized, causing a conformational change in the opsin and initiating an intracellular signaling cascade. Melanophores of Xenopus laevis, like retinal photoreceptors, are photosensitive. When maintained in vitro, melanosomes in dermal melanophores migrate to the cellular periphery in response to illumination or activation of G protein-coupled receptors. Melanophore photosensitivity is reversibly activated by retinaldehydes and has an action spectrum characteristic of opsin-like photopigments. To identify an opsin that may regulate melanosome migration, protein extracts of cultured melanophores were analyzed for opsin immunoreactivity and a melanophore cDNA library was screened for opsin-like nucleotide sequences. A 2.2-kb cDNA clone was isolated, encoding a 534-amino acid protein more homologous to known opsins. Melanopsin shares structural similarities with all known opsins, including an extracellular amino terminus and seven transmembrane domains. It has a lysine in the seventh transmembrane domain for chromophore attachment and a pair of cysteines in the second and third extracellular loops for disulfide bridge formation. It lacks N-glycosylation sites typically found in the extracellular amino terminus of most opsins. Melanopsin is more homologous to invertebrate opsins than vertebrate opsins. It shares 39% identity with octopus rhodopsin and approximately 30% identity with typical vertebrate opsins. Phylogenetic analysis indicates that melanopsin and invertebrate opsins diverged from a common ancestor. Melanopsin is localized in the iris, brain structures, and retinal cells, suggesting a role in nonvisual photoreception. Its long cytoplasmic tail contains 14 potential phosphorylation sites, suggesting it is regulated by kinases. Melanopsin may provide a mechanism