Abstract
In vertebrate retinas, cone cells exhibit photoresponses with a more restrained sensitivity and rapid shutoff kinetics than those of rods. For these characteristics, we have paid special attention to termination of the phototransduction through opsin phosphorylation by G protein-coupled receptor kinase (GRK) 1 and 7 in zebrafish. We identified GRK isoform, GRK1A expressed in the rods whereas GRK1B and GRK7-1 in the cones. Kinetics analysis revealed that Vmax of a major subtype of the cone kinase, GRK7-1, was considerably higher than that of the rod kinase, GRK1A. The results suggest the activity of GRK being an important factor for photoresponse properties and also for the difference in photosensitivity. In the activation process, G protein transducin plays a central role in both cells. The γ-subunits of rod and cone transducin (Gγ1 = Tγ, Gγ8, respectively) are farnesylated, whereas the other subtypes of Gγ are geranylgeranylated. We investigated physiological significance of selective farnesylation of rod Tγ by using knock-in mice, in which farnesylation of Tγ was switched to geranylgeranylation. The isoprenyl replacement markedly impaired light-adaptation without affecting the phototransduction properties. The reduced light-adaptation was associated with slowdown of light-dependent translocation of Tβγ in rod cells, implying that the regulation of visual sensitivity requires the selective farnesylation of Tγ in order to facilitate its intracellular movement. The proper localization of transducin also required isoprenylation of Tγ, suggesting a more critical role of farnesylation of transducin for global organization of the visual cell. [J Physiol Sci. 2007;57 Suppl:S17]
