Many G protein coupled receptors, visual opsins included, weakly activate G protein in the absence of ligand. Opsins are unique in their binding of an inverse agonist at rest, 11-cis retinal, which is transformed into an agonist by light. The interactions between rod opsin and its 11-cis retinal chromophore have been well studied for their roles in pigment stability, activation and spectral tuning. Recently, it has been proposed that opsin possesses two additional retinoid binding sites that enable retinoids to approach and exit the pigment-forming chromophore binding pocket. In this study, we will provide further evidence for multiple ligand binding sites on opsin. More importantly, we hypothesize that opsins bind other ligands besides retinoids and that the functions of the binding sites extend beyond visual cycle. Using single cell physiology, single cell microspectrophotometry and spectroscopy on rhodopsin in solution, we propose experiments to test three functions. The light-regulated channel is susceptible to potent inhibition by retinoids. One function of the alternate binding sites may be to allow rhodopsin and opsin to serve as buffers that protect the cGMP-gated channel from retinoid inhibition after exposure to bright light, when vast amounts of retinoids are exchanged with the pigment epithelium. A second function of retinoid binding to alternate sites is to modulate the catalytic activity of rhodopsin and opsin. This function may play roles in conferring reproducibility to the single photon response, in setting the absolute sensitivity and response kinetics of the receptor. A third function may be to enable compounds to absorb light and transfer the energy to rhodopsin, causing the latter to photoisomerize and give rise to an electrical response as if by direct absorption. Thus spectral sensitivity may be altered without a change in opsin protein expression. Each of these functions may depend upon opsin type. A strong link has been established between phototransduction cascade activity and retinal disease raising the possibility that mutations affecting the alternate ligand binding sites or that affect ligand processing may be responsible for or contribute to certain retinal pathologies. Then pharmaceuticals that target the alternate site may be used for the treatment of a retinal disease or for the enhancement of vision. Therefore, this proposal will be highly significant to our understanding of the structure and function of visual opsins and opens new directions for vision research.
