Molecular basis on the functional difference between rods and cones

锥状光感受器表现出比杆状光感受器更短的光反应。我们之前的研究表明，视色素磷酸化（光激活的视色素的一种猝灭机制）在视锥细胞中比在视杆细胞中要快得多（Tachibanaki et al., acta . acad Sci.）。美国,98年,14044 - 14049,2001)。在本项目中，我们用新构建的计算机辅助快速猝灭装置测量了这种快速磷酸化的早期时间过程，并直接将其与锥细胞的光响应时间过程进行了比较。在光响应恢复时，发现几乎两种磷酸盐被纳入漂白的锥体色素分子中。这一结果首次表明视色素磷酸化与光响应恢复一致。视锥细胞的快速磷酸化归因于视锥细胞中视色素激酶（GRK7）的高活性。由于这种高活性，锥体色素在非常高的漂白剂水平下很容易磷酸化，这可能解释了为什么锥体光反应即使在非常明亮的光线下也能迅速恢复，并且在强烈的背景光下不会饱和。另一方面，杆状体视觉色素的磷酸化在低漂白剂水平下已经饱和，这就合理地解释了为什么杆状体在较弱的光线下很容易饱和。高GRK7活性是由高含量的高效酶带来的。GRK7的表达量比杆状激酶（GRK1）高10倍，单个GRK7分子的比活性比GRK1高10倍左右。GRK7的比活性是目前已知的grk中最高的。我们的结果似乎从多方面解释了锥状光感受器的响应特性，包括在日光视觉中锥状光感受器的不饱和响应。

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.