Estevez Maureen E, Kolesnikov Alexander V, Ala-Laurila Petri, Crouch Rosalie K, Govardovskii Victor I, Cornwall M Carter
Department of Physiology and Biophysics, Boston University School of Medicine, Boston, MA 02118, USA.
J Gen Physiol. 2009 Aug;134(2):137-50. doi: 10.1085/jgp.200910232.
Cone photoreceptors of the vertebrate retina terminate their response to light much faster than rod photoreceptors. However, the molecular mechanisms underlying this rapid response termination in cones are poorly understood. The experiments presented here tested two related hypotheses: first, that the rapid decay rate of metarhodopsin (Meta) II in red-sensitive cones depends on interactions between the 9-methyl group of retinal and the opsin part of the pigment molecule, and second, that rapid Meta II decay is critical for rapid recovery from saturation of red-sensitive cones after exposure to bright light. Microspectrophotometric measurements of pigment photolysis, microfluorometric measurements of retinol production, and single-cell electrophysiological recordings of flash responses of salamander cones were performed to test these hypotheses. In all cases, cones were bleached and their visual pigment was regenerated with either 11-cis retinal or with 11-cis 9-demethyl retinal, an analogue of retinal lacking the 9-methyl group. Meta II decay was four to five times slower and subsequent retinol production was three to four times slower in red-sensitive cones lacking the 9-methyl group of retinal. This was accompanied by a significant slowing of the recovery from saturation in cones lacking the 9-methyl group after exposure to bright (>0.1% visual pigment photoactivated) but not dim light. A mathematical model of the turn-off process of phototransduction revealed that the slower recovery of photoresponse can be explained by slower Meta decay of 9-demethyl visual pigment. These results demonstrate that the 9-methyl group of retinal is required for steric chromophore-opsin interactions that favor both the rapid decay of Meta II and the rapid response recovery after exposure to bright light in red-sensitive cones.
脊椎动物视网膜中的视锥光感受器对光的反应终止速度比视杆光感受器快得多。然而，视锥细胞中这种快速反应终止背后的分子机制却知之甚少。本文所呈现的实验检验了两个相关假说：其一，红色敏感视锥细胞中视紫红质（Meta）II的快速衰减速率取决于视黄醛的9-甲基与色素分子的视蛋白部分之间的相互作用；其二，Meta II的快速衰减对于红色敏感视锥细胞在强光照射后从饱和状态快速恢复至关重要。为了检验这些假说，进行了色素光解的显微分光光度测量、视黄醇生成的显微荧光测量以及蝾螈视锥细胞闪光反应的单细胞电生理记录。在所有情况下，视锥细胞都被漂白，其视觉色素用11-顺式视黄醛或11-顺式9-去甲基视黄醛（一种缺乏9-甲基的视黄醛类似物）进行再生。在缺乏视黄醛9-甲基的红色敏感视锥细胞中，Meta II的衰减速度慢四至五倍，随后视黄醇的生成速度慢三至四倍。这伴随着缺乏9-甲基的视锥细胞在暴露于强光（>0.1%视觉色素光激活）而非弱光后从饱和状态恢复的显著减慢。光转导关闭过程的数学模型表明，光反应恢复较慢可以用9-去甲基视觉色素的Meta衰减较慢来解释。这些结果表明，视黄醛的9-甲基对于空间发色团-视蛋白相互作用是必需的，这种相互作用既有利于Meta II的快速衰减，也有利于红色敏感视锥细胞在暴露于强光后快速恢复反应。
