Abstract
Opsins are the photoreceptive proteins which trigger phototransduction cascades in visual and non-visual photoreceptor cells. I have conducted the following studies on the opsin genes to elucidate the function and evolution of the animal photoreceptor cells: 1) All the retinal photoreceptor cells are rod-shaped in a nocturnal gecko (Gekko gekko). Phylogenetic and biochemical analyses revealed that the gecko visual opsins are cone-type, suggesting that the gecko rods originate from their ancestral cones. 2) I identified a novel subtype of opsin and a Go-type G-protein in the ciliary photoreceptor cells of an invertebrate, scallop (Patinopecten yessoensis). The result suggests that the novel Go-mediated phototransduction cascade is responsible for the hyperpolarizing photoresponse in the scallop photoreceptor cells, and that multiple phototransduction systems emerged before the species divergence between vertebrates and arthropods. 3) Non-visual photosensitivity has been found in the brain of vertebrates. A search for brain opsin genes in zebrafish (Danio rerio) identified two kinds of novel opsins: exo-rhodopsin in the pineal gland and VAL-opsin in the deep brain. VAL-opsin was also found in the retinal horizontal cells, suggesting its contribution to retinal physiology. Transgenic experiments on the exo-rhodopsin promoter led to the discovery of a novel pineal-specific cis element, PIPE.
