Larger inhibition of visual pigment kinase in cones than in rods

P>In the carp retina, visual pigment kinase, GRK1 (G-protein coupled receptor kinase 1) in rods and GRK7 in cones, is inhibited by a photoreceptor neuronal Ca2+-sensor protein, S-modulin (or recoverin) in rods and visinin (formerly named s26) in cones. Here, we compared Ca2+-dependent inhibition of GRK1 by S-modulin and that of GRK7 by visinin. First, the concentrations of S-modulin and visinin in the outer segment were estimated: the concentration of visinin (1.2 mM) was 20 times higher than that of S-modulin (53 mu M). Based on the determined concentrations of the Ca2+-sensor proteins and the known dark Ca2+ concentrations, we estimated that in situ Ca2+-dependent inhibition on GRK in cones would be 2.5 times higher than that in rods at the Ca2+ concentration in the dark. Because GRK activity is approximately 100 times higher in cones than in rods [Proc. Natl Acad. Sci. USA102 (2005) 21359], the range of Ca2+-dependent inhibition on GRK activity is more than 100 times wider in cones than in rods. The inhibitory effects of S-modulin and visinin on photoreceptor GRKs were indistinguishable, although these Ca2+-sensor proteins are expressed in a cell-type specific manner. The inhibition by these Ca2+-sensor proteins was slightly higher on GRK7 than GRK1 probably because of a characteristic specific to GRK7.