Molecular basis on the functional difference between rods and cones

锥形感光器比杆状感光器表现出更短的光反应。我们之前的研究表明，视色素磷酸化是光激活视觉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(Tchibanaki等，Proc.Nati.Acad Science USA，98,14044-14049,2001)。在这个项目中，我们用新建造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进程，并直接将其与锥体中的光响应时间进程进行了比较。在光响应恢复时，发现几乎两种磷酸盐被结合到漂白的锥形颜料分子中。这一结果首次表明，视觉色素的磷酸化与光响应的恢复是一致的。视锥细胞的快速磷酸化归因于视锥细胞中的视色素激酶(GRK7)具有很高的活性。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被磷酸化，这可能解释了为什么即使在非常明亮的光线下，锥体的光反应也会迅速恢复，并且在强烈的背景光下不会饱和。另一方面，杆状视觉色素的磷酸化在低漂白剂水平下已经饱和，这合理地解释了为什么杆状视觉色素很容易被更弱的光饱和。GRK7的高活性是由一种高效酶的高含量所带来的。GRK7的表达水平是杆状蛋白激酶(GRK1)的10倍，单分子GRK7的比活性约为GRK1的10倍。GRK7是目前已知的比活性最高的GRKs。我们的结果似乎从多个方面解释了视锥感光细胞的反应特征，包括白天视觉中视锥反应的非饱和性。

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.