The chicken retina contains rhodopsin (a rod visual pigment) and four kinds of cone visual pigments. The

A variety of animal species utilize the ultraviolet (UV) component of sunlight as their environmental cues, whereas physiological roles of UV photoreception in mammals, especially in human beings, remain open questions. Here we report that mouse neuropsin (OPN5) encoded by the Opn5 gene exhibited an absorption maximum (λmax) at 380 nm when reconstituted with 11-cis-retinal. Upon UV-light illumination, OPN5 was converted to a blue-absorbing photoproduct (λmax 470 nm), which was stable in the dark and reverted to the UV-absorbing state by the subsequent orange light illumination, indicating its bistable nature. Human OPN5 also had an absorption maximum at 380 nm with spectral properties similar to mouse OPN5, revealing that OPN5 is the first and hitherto unknown human opsin with peak sensitivity in the UV region. OPN5 was capable of activating heterotrimeric G protein Gi in a UV-dependent manner. Immuno-blotting analyses of mouse tissue extracts identified the retina, the brain and, unexpectedly, the outer ears as the major sites of OPN5 expression. In the tissue sections of mice, OPN5 immuno-reactivities were detected in a subset of non-rod/non-cone retinal neurons as well as in the epidermal and muscle cells of the outer ears. Most of these OPN5-immuno-reactivities in mice were co-localized with positive signals for the alpha-subunit of Gi. These results demonstrate the first example of UV photoreceptor in human beings and strongly suggest that OPN5 triggers a UV-sensitive Gi-mediated signaling pathway in the mammalian tissues.

The visual transduction processes in rod and cone photoreceptor cells begin with photon absorption by the different types of visual pigments. Cone visual pigments exhibit faster regeneration from 11-cis-retinal and opsin and faster decay of physiologically active intermediate (meta II) than does the rod visual pigment, rhodopsin, as expected, due to the functional difference between rod and cone photoreceptor cells. To identify the amino acid residue(s) responsible for the difference in molecular properties between rod and cone visual pigments, we selected three amino acid positions (64, 122, and 150), where cone visual pigments have amino acid residues electrically different from those of rhodopsin, and prepared mutants of rhodopsin and chicken greensensitive cone visual pigment. The results showed that the replacement of Glu-122 of rhodopsin by the residue containing green-or red-sensitive cone pigment converted rhodopsin's rates of regeneration and meta II decay into those of the respective cone pigments, whereas the introduction of Glu-122 into green-sensitive cone visual pigment changed the rates of these processes into rates similar to those of rhodopsin. Furthermore, exchange of the residue at position 122 between rhodopsin and chicken green-sensitive cone pigment interchanges their efficiencies in activating retinal G protein transducin. Thus, the amino acid residue at position 122 is a functional determinant of rod and cone visual pigments.

Scallop retinas contain ciliary photoreceptor cells that respond to light by hyperpolarization like vertebrate rods and cones, but the response is generated by a different phototransduction cascade from those of rods and cones. To elucidate the cascade, we investigated a visual pigment and a G-protein functioning in the hyperpolarizing cell. Sequencing of cDNAs and in situ hybridization experiments showed that the hyperpolarizing cells express a novel subtype of visual pigment, which showed significant differences in amino acid sequence from other visual pigments. Cloning cDNA genes of G-protein and immunohistochemical analysis revealed the presence of an alpha subunit of a G o type G-protein, 83% identical in amino acid sequence to mammalian G o(␣) in the nervous system, in the photoreceptive region of the cells. The results demonstrate that a novel, G o -mediated, phototransduction cascade is present in the hyperpolarizing cells. The phototransduction cascade in the scallop hyperpolarizing cell provides an alternative system to investigate G o -mediated transduction pathways in the nervous system. Molecular phylogenetic analysis strongly suggests that the G o -mediated phototransduction system emerged before the divergence of animals into vertebrate and invertebrate in the course of evolution.In the photoreceptor cells of animals' eyes, visual pigments trigger a G-protein-mediated phototransduction cascade, which eventually generates an electrical response of the cells.Two kinds of the phototransduction systems have been reported. One is the G t 1 -mediated system of vertebrate hyperpolarizing photoreceptor cells in which the visual pigment activates a cGMP-specific phosphodiesterase via a heterotrimeric G-protein, transducin (G t ) (1-3). The other is the G q -mediated system of invertebrate depolarizing cells, such as cephalopod's and arthropod's, where phospholipase C is activated via a G qtype G-protein (4 -7). The visual pigments of these two systems show sequence homology, but clearly split into two subtypes (G t -and G q -coupled ones) in a molecular phylogenetic tree (8).In addition to the depolarizing rhabdomeric photoreceptor cells present in invertebrates, scallop retinas contain ciliary photoreceptor cells that hyperpolarize in response to light (9). After the first electrophysiological recordings of Hartline (10), the mechanism of the hyperpolarizing response as well as its evolution have been discussed in comparison with vertebrate hyperpolarizing ciliary photoreceptor cells, rods and cones (11)(12)(13)(14). It has been reported, however, that the hyperpolarizing response in the scallop cell is due to opening of a cGMPsensitive potassium channel (11)(12)(13)(14), which is different from that of the vertebrate cells (closing of a cGMP-sensitive cationic channel) (15). Our immunohistochemical experiments showed that an antibody to frog G t did not cross-react to the scallop hyperpolarizing cell.2 Therefore we expected the presence of an unknown G-protein-mediated phototransduction cascade other tha...

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