Human Blue Cone Opsin Regeneration Involves Secondary Retinal Binding with Analog Specificity

Abstract

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.

Ligand Binding Mechanisms in Human Cone Visual Pigments

“…Very recently, our regeneration experiments with photoactivated ligand-free cone opsins revealed the existence of such a site, which we suggested would be located between TMs 1 and 7, based on molecular docking (Fig. 3D) [26,27]. In fact, our predictions positioned the second retinal molecule in a very similar location to that of a detergent molecule recently resolved in rhodopsin (Fig.…”

“…QLT, a 9-cisretinoid analog has been tested as a potential therapeutic agent for the treatment of the LCA clinical phenotype [75]. Our recent findings, with the 9CR isomer, showed improved regeneration at various time-points after photoactivation in the case of red, green, and blue cone pigments as well as in the case of rhodopsin [25][26][27]. Overall, these results provide novel evidence for the great potential of specific retinal isomers (alone or in combination with other molecules [76] that can act as allosteric modulators), or chemically modified retinoid analogs, as therapeutic tools for the successful treatment of visual disorders associated with mutations in cone photoreceptor proteins.…”

“…Interestingly, a recent study shows how an 11-cis-6-membered ring locked retinal analog can regenerate rhodopsin but not green cone opsin, hence suggesting that green cone opsins have a tighter retinal binding site [29]. On the other hand, blue cone opsin appeared to behave in a similar way to red cone opsin in response to canonical 11CR binding [27]. Thus, further studies are required to unveil the physiological significance of the initial unprotonated SB linkage formation and its conversion to 11CR-PSB green opsin in the visual transduction process of green cone opsin.…”

“…In our recent studies on cone opsins, we showed that chromophore regeneration also involves dynamic conformations of the apoprotein [25][26][27]. Key differences with rhodopsin are that cone opsins: i) are more prone to spontaneous dissociation into free opsin and ATR, ii) exhibit a faster Meta II decay, and iii) have more serine and threonine residues in the C-terminus that can become phosphorylated [28].…”

“…This process may cause rapid chromophore regeneration by incidentally displacing ATR out of the orthosteric binding site; and (ii) the secondary binding site accommodates additional 11CR, this time with slower kinetics. This second slower step would be due to the conformational change induced in the regenerated protein and the reduced availability of bulk 11CR, which would synergistically slow-down 11CR binding to the secondary site [26,27]. While this secondary binding is not detected in the regeneration of red/green cone pigments by 9CR, this isomer can regenerate blue cone opsin via this mechanism.…”