Vertebrate cone photoreceptors are known to show lower light sensitivity and briefer photoresponses than rod photoreceptors. To understand the molecular mechanisms characterizing cone photoresponses, we compared some of the reactions in the phototransduction cascade between rods and cones. For this purpose, rods and cones were obtained in quantities large enough to do biochemical studies. The cells were purified from the retina of carp (Cyprinus carpio) with a stepwise Percoll gradient. The purified rod fraction contained almost no other kinds of cells besides rods, and the purified cone fraction contained a mixture of red-, green-, and blue-sensitive cones in the ratio 3:Ϸ1:Ϸ1. We prepared membrane preparations from the rod and the cone fraction, and in these membranes, we measured activation efficiencies of the reactions in the phototransduction cascade. The results showed that the signal amplification is lower in the cone membranes, which accounts for the lower light sensitivity in cones. Furthermore, we measured the time courses of visual pigment phosphorylation. The result showed that the phosphorylation is much faster in the cone membranes, which also explains the lower light sensitivity and, in addition, the briefer photoresponse in cones.

Cone photoreceptors show briefer photoresponses than rod photoreceptors. Our previous study showed that visual pigment phosphorylation, a quenching mechanism of light-activated visual pigment, is much more rapid in cones than in rods. Here, we measured the early time course of this rapid phosphorylation with good time resolution and directly compared it with the photoresponse time course in cones. At the time of photoresponse recovery, almost two phosphates were incorporated into a bleached cone pigment molecule, which indicated that the visual pigment phosphorylation coincides with the photoresponse recovery. The rapid phosphorylation in cones is attributed to very high activity of visual pigment kinase [G protein-coupled receptor kinase (GRK) 7] in cones. Because of this high activity, cone pigment is readily phosphorylated at very high bleach levels, which probably explains why cone photoresponses recover quickly even after a very bright light and do not saturate under intense background light. The high GRK7 activity is brought about by high content of a highly potent enzyme. The expression level of GRK7 was 10 times higher than that of rod kinase (GRK1), and the specific activity of a single GRK7 molecule was Ϸ10 times higher than that of GRK1. The specific activity of GRK7 is the highest among the GRKs so far known. Our result seems to explain the response characteristics of cone photoreceptors in many aspects, including the nonsaturation of the cone responses during daylight vision.rod ͉ photoreceptors ͉ retina ͉ phototransduction O ur visual system consists of two components: rods and cones (1, 2). These photoreceptors differ in their light sensitivity so that rods mediate twilight vision, and cones mediate daylight vision. Rods and cones are distinguished not only in their light sensitivity, but also in other response characteristics. The photoresponse time course is much briefer in cones, which improves the time resolution of our daylight vision greatly. Rods are saturated with bright background light and do not respond to more intense light (3). In contrast, cones are not saturated and respond to very bright light (4, 5). The molecular mechanisms underlying in the differences of these response characteristics are not yet known. In previous biochemical studies on the cone phototransduction mechanism, only slight quantitative differences were known in the transduction components between rods and cones (6-8).In our previous study, we obtained a large quantity of isolated cones (enough to perform biochemistry) and showed that transducin activation and cGMP phosphodiesterase activation, the reactions involved in the generation of a photoresponse, are less efficient in cones (9). These findings reasonably explained the lower light sensitivity in cones. Another remarkable difference was found in the phosphorylation of light-activated visual pigment. When light-activated, visual pigment is phosphorylated by a class of kinase known as rhodopsin kinase (rod kinase or G protein-coupled receptor kinase (GRK) 1) in...

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