抄録
Upon receiving light, rod and cone photoreceptor cells in vertebrates exhibit characteristic photoresponses; i.e. high sensitivity in rods and rapid response in cones. To understand the molecular basis for this difference, we preformed in vitro studies focusing on G protein-coupled receptor kinase (GRK) 1 and 7, which phosphorylate light-activated opsins for terminating the phototransduction. In silico screening of both zebrafish whole genome shotgun and EST databases, and subsequent cloning using zebrafish ocular cDNAs revealed two homologs for each of GRK1 and 7. We classified them into three GRK subfamilies, GRK1A, 1B and 7 (termed GRK7-1 and 7-2) based on a phylogenetic analysis. In situ hybridization and immunohistochemical analyses localized GRK1A in rod outer segments, whereas both GRK1B and 7-1 were localized in cone outer segments. Estimated expression levels of GRK proteins in the rod and cone cells were similar to each other in the zebrafish. The kinase activities of these GRKs for the light-activated rhodopsin were measured by using recombinant proteins expressed in HEK293S cells, and we found that Vmax of a major subtype of the cone kinase, GRK7-1, was considerably higher than that of the rod kinase, GRK1A. These results demonstrate that the enzymatic activity responsible for termination of the light signaling should serve as an important factor that determines the photoresponse property of cone cells by affecting both amplification efficiency and shutoff kinetics of the light signaling. [Jpn J Physiol 55 Suppl:S48 (2005)]
