Human color vision is mediated by the red, green, and blue cone visual pigments. Cone opsins are G-protein-coupled receptors consisting of an opsin apoprotein covalently linked to the 11-cis-retinal chromophore. All visual pigments share a common evolutionary origin, and red and green cone opsins exhibit a higher homology, whereas blue cone opsin shows more resemblance to the dim light receptor rhodopsin. Here we show that chromophore regeneration in photoactivated blue cone opsin exhibits intermediate transient conformations and a secondary retinoid binding event with slower binding kinetics. We also detected a fine-tuning of the conformational change in the photoactivated blue cone opsin binding site that alters the retinal isomer binding specificity. Furthermore, the molecular models of active and inactive blue cone opsins show specific molecular interactions in the retinal binding site that are not present in other opsins. These findings highlight the differential conformational versatility of human cone opsin pigments in the chromophore regeneration process, particularly compared to rhodopsin, and point to relevant functional, unexpected roles other than spectral tuning for the cone visual pigments.

Deuteranopia is an X-linked congenital dichromatic condition in which single point mutations in green cone opsin lead to defective non-functional cone photoreceptor cells. Green cone opsin belongs to the G protein-coupled receptor superfamily and consists of a seven transmembrane helical apoprotein covalently bound to 11-cis-retinal, by means of a protonated Schiff base linkage, in its inactive dark state. Several point mutations in green cone opsin have been reported to cause deuteranopia, but the structural details underlying the molecular mechanisms behind the malfunction of mutated opsins have not been clearly established. Here, deutan N94K and R330Q mutants were studied by introducing these substitutions into the native green cone opsin gene by site-directed mutagenesis. The mutant proteins were purified and analyzed using UV-vis spectroscopy and transducin activation assay. We find that the N94K mutant binds the retinal chromophore by means of an unprotonated Schiff base linkage in contrast to previous studies that reported no chromophore regeneration. The other mutant studied, R330Q, showed impaired functionality as measured by its reduced transducin activation ability when compared to wild-type green cone opsin. A double Cys mutant that could form a stabilizing disulfide bond was used in an attempt to address the instability of the green opsin mutants. Our results suggest the presence of key intramolecular networks which may be disrupted in deuteranopia, and these findings could help in finding therapeutic solutions for treating color blindness. Furthermore, our results can also have implications for the study of other visual pigments and other rhodopsin-like G protein-coupled receptors.

Visual rhodopsins are membrane proteins that function as light photoreceptors in the vertebrate retina. Specific amino acids have been positively selected in visual pigments during mammal evolution, which, as products of adaptive selection, would be at the base of important functional innovations. We have analyzed the top candidates for positive selection at the specific amino acids and the corresponding reverse changes (F13M, Q225R and A346S) in order to unravel the structural and functional consequences of these important sites in rhodopsin evolution. We have constructed, expressed and immunopurified the corresponding mutated pigments and analyzed their molecular phenotypes. We find that position 13 is very important for the folding of the receptor and also for proper protein glycosylation. Position 225 appears to be important for the function of the protein affecting the G-protein activation process, and position 346 would also regulate functionality of the receptor by enhancing G-protein activation and presumably affecting protein phosphorylation by rhodopsin kinase. Our results represent a link between the evolutionary analysis, which pinpoints the specific amino acid positions in the adaptive process, and the structural and functional analysis, closer to the phenotype, making biochemical sense of specific selected genetic sequences in rhodopsin evolution.

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