Chloride-dependent spectral tuning mechanism of L-group cone visual pigments.

Most vertebrates have one type of rhodopsin and multiple types of cone visual pigments with different absorption maxima in their retinas. The spectral sensitivities of multiple cone visual pigments contribute to color discrimination in these animals. Vertebrate cone visual pigments are classified into four groups based on their amino acid sequences. Among these groups, many pigments in the longer wavelength-sensitive group (L-group) have a unique spectral tuning mechanism, that is, the red-shift of absorption maximum induced by the binding of chloride to His181 of the protein moiety (chloride effect). However, a few pigments such as mouse green and guinea pig green pigments in L-group have a tyrosine residue instead of a histidine at position 181. Interestingly, mouse green shows no chloride effect, whereas guinea pig green shows a significant chloride effect. In the present site-directed mutational analysis, we revealed that this difference in the chloride effect in rodent pigments is completely explained by the replacements of two residues at positions 289 and 292. In addition, mutations at positions 181, 289, and 292 abolished 80% of the chloride effect in monkey red and green. Further analysis with chimeras showed that the residual 20% of the chloride effect could be attributed to helical interactions within the pigments. Thus, we concluded that these three amino acid residues are the main determinants of the chloride-dependent spectral shift in L-group pigments.