Novel G(o)-mediated phototransduction cascade in scallop visual cells

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(alpha)) 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.