Beyond spectral tuning: human cone visual pigments adopt different transient conformations for chromophore regeneration

Abstract

Human red and green visual pigments are seven transmembrane receptors of cone photoreceptor cells of the retina that mediate color vision. These pigments share a very high degree of homology and have been assumed to feature analogous structural and functional properties. We report on a different regeneration mechanism among red and green cone opsins with retinal analogs using UV-Vis/fluorescence spectroscopic analyses, molecular modeling and site-directed mutagenesis. We find that photoactivated green cone opsin adopts a transient conformation which regenerates via an unprotonated Schiff base linkage with its natural chromophore, whereas red cone opsin forms a typical protonated Schiff base. The chromophore regeneration kinetics is consistent with a secondary retinal uptake by the cone pigments. Overall, our findings reveal, for the

Ligand Binding Mechanisms in Human Cone Visual Pigments

“…Of note, while the binding free energy required by 9CR is lower than that of 11CR [34], the photoisomerization of isorhodopsin (9CR-bound opsin) is three times slower than that of rhodopsin (11CR-bound opsin) [36]. These conformational differences are in line with the observed behavior between rhodopsin and red and green cone opsin regenerated with 9CR and 11CR [26,29].…”

“…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].…”

“…Intriguingly, these two highly homologous opsins displayed different regeneration modes likely due to different conformational dynamics prior to ligand binding. Another significant difference was that while photoactivated green cone opsin primarily formed an unprotonated SB linkage with endogenously added 11CR, red cone opsin showed the spectral features of a typical PSB linkage with its natural 11CR chromophore [26]. 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].…”

A G Protein-Coupled Receptor Dimerization Interface in Human Cone Opsins

“…As shown in Figure S12, the absorption maximum of 9- cis -retinal-bound green opsin was ~493 nm, and it was ~525 nm for 9- cis -retinal-bound red opsin, in agreement with previous observations. 69 A spectral shift of 6 nm toward shorter wavelengths was observed for red-TSV as compared to WT red cone opsin. However, no spectral shift was found for green-IAM compared to WT green cone opsin (Figure S12D).…”

Functional role of positively selected amino acid substitutions in mammalian rhodopsin evolution

“…In order to rule out that the retinal could be binding to other Lys residues 20 in the F13M mutant (other than the natural K296 at transmembrane helix 7) we constructed the quadruple mutant F13M/N2C/D282C/K296G where the site of retinal attachment was eliminated by the K296G mutation 21 . We could not get any chromophore formation for this mutant indicating that retinal was binding to the native K296 in the rescued triple mutant ( Supplementary Figure S2 ).…”