Identification of the Cl⁻-Binding Site in the Human Red and Green Color Vision Pigments

Chloride ions are known to bind and alter the absorption spectra of some but not all visual pigments. In this report, the human red and green color vision pigments are shown to bind Cl⁻ and to undergo a large red shift in their absorption maxima. Mutation of 18 different positively charged amino acids in these pigments identified two residues, His¹⁹⁷ and Lys²⁰⁰, in the Cl⁻-binding site. His¹⁹⁷ and Lys²⁰⁰ are strictly conserved in all long-wavelength cone pigments but are absent in all rhodopsins and short-wavelength cone pigments. This fact suggests that the evolutionary branch of the long-wavelength pigments was established when an ancestral pigment acquired the ability to bind Cl⁻ and, as a result, shift the absorption maximum to longer wavelengths.