Molecular basis on the functional difference between rods and cones

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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 (Tachibanaki et al., Proc.Nati.Acad Sci.USA, 98,14044-14049,2001). In this project, we measured early time course of this rapid phosphorylation with a newly constructed, computer-assisted rapid quench apparatus and directly compared it with the photoresponse time course in cones. At the time of photoresponse recovery, almost two phosphates were found to be incorporated into a bleached cone pigment molecule. This result indicated for the first time that the visual pigment phosphorylation coincides with the photoresponse recovery. The rapid phosphorylation in cones is attributed to a very high activity of visual pigment kinase in cones (GRK7). 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. On the other hand, phosphorylation of rod visual pigment was already saturated at low bleach levels, which reasonably explains why rods are easily saturated with much weaker 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 about 10 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 non-saturation of the cone responses during daylight vision.