Molecular bases of color vision in vertebrates
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概要
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Visual pigments initiate vision and are characterized by their wavelengths of maximal absorption (λmax). Modifications of the λmax values of visual pigments have allowed organisms to adapt to diverse light environments. The availability of the functional assays of these visual pigments using cultured cells makes vision an ideal genetic system to study the molecular bases of adaptation and genetics of color vision in vertebrates. The visual pigments in vertebrate retinas are distinguished into five evolutionarily distinct groups RH1 (λmax = 490-500 nm), RH2 (470-510 nm), SWS1 (360-420 nm), SWS2 (440-455), and LWS/MWS (510-570 nm). Here, we review amino acid replacements that are associated with the shifts in the λmax values of these visual pigments. The λmax-shifts in several RH1 pigments seem to reflect adaptive changes to blue environments of the organisms and are explained mostly by amino acid replacements D83N (D → N at residue 83), E122Q, and A292S. Similarly, the blue-shifts in the λmax values of the RH2 pigments can be explained by D83N, E122Q, A164S, and M207L. For the SWS1 pigments of birds, only one amino acid replacement S84C seems to be responsible for the transformation of ultraviolet pigments from the violet pigment. For the LWS/MWS pigments, the additive effects of amino acid differences at 180, 197, 277, 285, and 308 fully explain the red-green color vision in a wide range of vertebrates. All of these observations suggest that the evolution of the extant visual pigments can be explained by amino acid replacements at only a small number of sites. Visual pigments, a group of G-protein-coupled receptors, initiate visual excitation (Wald 1968). Each visual pigment consists of a transmembrane protein, opsin, and the chromophore, 11-cis-retinal, and can be characterized by its wavelength of maximal absorption (λmax). Human color vision is mediated by `blue, `green, and `red visual pigments. The `blue pigments absorb wavelengths ranging from about 370 nm to 570 nm with a λmax at 420 nm, while both `green and `red pigments are sensitive to wavelength about 450-620 nm with λmax values at 530 nm and 560 nm, respectively (Nathans 1989). The molecular bases of the spectral tuning of these and other visual pigments in vertebrates are still not well understood. Recently, however, some significant progress has been made on this subject. To evaluate the mechanisms of the functional properties of visual pigments, a large number of amino acid changes have been introduced into the bovine rod-specific visual pigment (rhodopsin) by several groups of vision scientists (for a review, see Yokoyama 1997). In most of these analyses, charged amino acids have been considered. For example, an amino acid change from glutamic acid at residue 113 (E113) to glutamine (E113Q) shifts the λmax of the pigment from 500 nm to 380 nm (Sakmar et al. 1989; Zhukovsky and Oprian 1989; Nathans 1990a, b). E113 is the negatively charged counterion to the positively charged protonated Schiff base and the modification of this opsin structure causes a drastic shift in the λmax value of the pigment. Unfortunately, most of these amino acid changes, including E113Q, have not been found in nature. Thus, it is not immediately clear how these mutagenesis results are helpful in elucidating the molecular basis for the divergence of λmax values of visual pigments in nature (Yokoyama 1995, 1997). Since this abstract exceeds the limit of the space allocated to abstract, the last paragraph has to be deleted. Please refer to PDF for the rest of the abstract.
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