PDF(Pubmed)
Abstract：
Rhodopsin (Rho) and cone opsins are essential for detection of light. They respond via photoisomerization, converting their Schiff-base-adducted 11-cis-retinylidene chromophores to the all-trans configuration, eliciting conformational changes to activate opsin signaling. Subsequent Schiff-base hydrolysis releases all-trans-retinal, initiating two important cycles that maintain continuous vision-the Rho photocycle and visual cycle pathway. Schiff-base hydrolysis has been thoroughly studied with photoactivated Rho but not with cone opsins. Using established methodology, we directly measured the formation of Schiff-base between retinal chromophores with mammalian visual and nonvisual opsins of the eye. Next, we determined the rate of light-induced chromophore hydrolysis. We found that retinal hydrolysis from photoactivated cone opsins was markedly faster than from photoactivated Rho. Bovine retinal G protein-coupled receptor (bRGR) displayed rapid hydrolysis of its 11-cis-retinylidene photoproduct to quickly supply 11-cis-retinal and re-bind all-trans-retinal. Hydrolysis within bRGR in native retinal pigment epithelium microsomal membranes was >6-times faster than that of bRGR purified in detergent micelles. N-terminal-targeted antibodies significantly slowed bRGR hydrolysis, while C-terminal antibodies had no effect. Our study highlights the much faster photocycle of cone opsins relative to Rho and the crucial role of RGR in chromophore recycling in daylight. By contrast, in our experimental conditions, bovine peropsin did not form pigment in the presence of all-trans-retinal nor with any mono-cis retinal isomers, leaving uncertain the role of this opsin as a light sensor.
摘要：
视紫红质（Rho）和视锥视蛋白对于检测光至关重要。它们通过光异构化反应，将它们的席夫碱加成的11-顺式-视黄基发色团转化为全反式构型，引发构象变化以激活视蛋白信号。随后的希夫碱水解释放全反式视网膜，启动保持连续视力的两个重要周期-Rho光循环和视觉周期通路。希夫碱水解已使用光活化视紫红质进行了彻底研究，但未使用视锥细胞。使用既定的方法，我们直接测量了视网膜发色团与哺乳动物视觉和非视觉视蛋白之间席夫碱的形成。接下来,我们确定了光诱导发色团水解的速率。我们发现，光活化视锥视蛋白的视网膜水解明显快于光活化视紫红质。牛视网膜G蛋白偶联受体RGR（bRGR）显示其11-顺式-视黄叉光产物的快速水解，以快速提供11-顺式-视黄醛，重新结合全反式视网膜。天然视网膜色素上皮（RPE）微粒体膜中bRGR内的水解比洗涤剂胶束中纯化的bRGR快6倍。N-末端靶向抗体显著减缓bRGR水解，而C端抗体没有影响。我们的研究强调了锥形视蛋白相对于Rho的光循环要快得多，以及RGR在白天发色团回收中的关键作用。相比之下,在我们的实验条件下，牛过视蛋白（bRRH）在全反式视网膜的存在下不形成色素，也没有任何单顺式视网膜异构体，不确定这种视蛋白作为光传感器的作用。
