Probing Human Red Cone Opsin Activity with Retinal Analogues

Retinal analogues have been used to probe the chromophore binding pocket and function of the rod visual pigment rhodopsin. Despite the high homology between rod and cone visual pigment proteins, conclusions drawn from rhodopsin studies should not necessarily be extrapolated to cone visual pigment proteins. In this study, the effects of full-length and truncated retinal analogues on the human red cone opsin's ability to activate transducin, the G protein in visual transduction, were assessed. The result with beta-ionone (6) confirms that a covalent bond is not necessary to deactivate the red cone opsin. In addition, several small compounds were found able to deactivate this opsin. However, as the polyene chain is extended in a trans configuration beyond the 9-carbon position, the analogues became agonists up to all-trans-retinal (3). The 22-carbon analogue (2) appeared to be neither an agonist nor an inverse agonist. Although the all-trans-C17 (5) analogue was an agonist, the 9-cis-C17 (11) compound was an inverse agonist, a result that differs from that with rhodopsin. These results suggest that the red cone opsin has a more open structure in the chromophore binding region than rhodopsin and its activation or deactivation as a G-protein receptor may be less selective than rhodopsin.