Molecular Mechanism of Photoreception in Cones.

本研究主要从视色素的分子特性和光化学反应两个方面来阐明视杆细胞和视锥细胞对光反应的差异是如何从视色素的角度来解释的.利用cDNA克隆技术，我们分离到了四种编码鸡视锥色素的cDNA克隆和两种编码壁虎视色素的cDNA克隆。根据不同视色素氨基酸序列的差异，推测了负责视色素光谱调谐的氨基酸残基。根据氨基酸同源性构建了脊椎动物视色素的系统发育树。这表明，一种祖先视色素首先进化成四组锥状视色素，而视杆细胞视色素的视紫红质组后来从四组中的一组分化出来。因此，这表明动物首先获得了辨别颜色的能力，然后获得了暗视觉.本文用低温光谱和激光光解研究了碘视蛋白的漂白过程。结果表明，碘视蛋白在漂白过程中有类似的中间体，它们分别对应于视紫红质的光-、浴-、BL-、光-、Meta I-和Meta II-中间体。此外，与变视紫红质II一样，偏碘视蛋白II具有与转导蛋白结合的能力，这表明它具有与变视紫红质II相似的生理作用。锥视色素、碘视蛋白、鸡绿色和鸡蓝色与视杆视色素视紫红质的比较研究清楚地表明，锥视色素从11-顺式-视网膜和视蛋白再生的速率比视紫红质快，以及生理活性中间体（Meta II-中间体）的形成和衰变。这些差异分别与视锥细胞比视杆细胞更快的暗适应、更快的光反应和更低的光敏性密切相关。

The present studies focused on the molecular properties and photochemical reactions of visual pigments in order to elucidate how the difference in light response between rods and cones can be explained on the basis of visual pigments.1. Using a cDNA cloning technique, we have isolated four kinds of cDNA clones encoding chicken cone visual pigments and two clones encoding nocturnal gecko visual pigments. The amino acid residues responsible for the spectral tuning of the visual pigments were speculated on the basis of the differences in amino acid sequence among the visual pigments. A phylogenetic tree of vertebrate visual pigments was then constructed on the basis of amino acid identity. It indicated that an ancestral visual pigment evolved first into four groups of cone visual pigments and that group of rhodopsins, the rod visual pigments, diverged later from one of the four groups. Thus, it is suggested that animals had acquired first the ability to distinguish color and then acquired scotopic vision.2. We have investigated the bleaching process of iodopsin by means of low temperature spectroscopy and laser photolyses. The results showed that iodopsin has similar intermediates corresponding to photo-, batho-, BL, lumi, meta I-, and meta II-intermediates of rhodopsin in its bleaching prosess. Furthemore, like metarhodopsin II, metaiodopsin II has a binding ability to transducin, suggesting that it has a physiological role similar to metarhodopsin II.Comparative studies of cone visual pigments, iodopsin, chicken green and chicken blue with the rod visual pigment rhodopsin clearly showed that cone visual pigments are faster than rhodopsin in rate of regeneration from 11-cis-retinal and opsin, and those of formation and decay of physiologically active intermediate (meta II-intermediate). These difference closely correlate to faster dark-adaptation, faster light response and less photosensitivity of cones than rods, respectively.