Graduate School of Frontier Biosciences, Osaka University, Suita, Osaka 565-0871, Japan.
J Biol Chem. 2012 Nov 30;287(49):41186-94. doi: 10.1074/jbc.M112.403717. Epub 2012 Oct 8.
Cone photoreceptors show lower light sensitivity and briefer light responses than rod photoreceptors. The light detection signal in these cells is amplified through a phototransduction cascade. The first step of amplification in the cascade is the activation of a GTP-binding protein, transducin (Tr), by light-activated visual pigment (R*). We quantified transducin activation by measuring the binding of GTPγS in purified carp rod and cone membrane preparations with the use of a rapid quench apparatus and found that transducin activation by an R* molecule is ∼5 times less efficient in cones than in rods. Transducin activation terminated in less than 1 s in cones, more quickly than in rods. The rate of GTP hydrolysis in Tr*, and thus the rate of Tr* inactivation, was ∼25 times higher in cones than in rods. This faster inactivation of Tr* ensures briefer light responses in cones. The expression level of RGS9 was found to be ∼20 times higher in cones than in rods, which explains higher GTP hydrolytic activity and, thus, faster Tr* inactivation in cones than in rods. Although carp rods and cones express rod- or cone-versions of visual pigment and transducin, these molecules themselves do not seem to induce the differences significantly in the transducin activation and Tr* inactivation in rods and cones. Instead, the differences seem to be brought about in a rod or cone cell-type specific manner.
视锥细胞的光敏感性比视杆细胞低，光反应时间也更短。这些细胞的光检测信号通过光转导级联放大。级联中的第一步放大是光激活视色素（R*）激活 GTP 结合蛋白转导蛋白（Tr）。我们通过使用快速淬火装置测量纯化的鲤鱼视杆和视锥膜制剂中 GTPγS 的结合，定量了转导蛋白的激活，发现 R分子在视锥细胞中的转导蛋白激活效率比视杆细胞低约 5 倍。在视锥细胞中，转导蛋白的激活在不到 1 秒内终止，比在视杆细胞中更快。Tr中的 GTP 水解速率，因此 Tr失活的速率，在视锥细胞中比在视杆细胞中高约 25 倍。这种更快的 Tr失活确保了视锥细胞的短光反应。发现 RGS9 的表达水平在视锥细胞中比在视杆细胞中高约 20 倍，这解释了视锥细胞中 GTP 水解活性更高，因此 Tr失活更快。尽管鲤鱼的视杆和视锥都表达视杆或视锥视色素和转导蛋白，但这些分子本身似乎并没有在视杆和视锥细胞中转导蛋白的激活和 Tr失活方面产生显著差异。相反，这些差异似乎是以一种视杆或视锥细胞类型特异性的方式产生的。
