All-trans-retinal forms a visible-absorbing pigment with human rod opsin.

Rhodopsin activation elicits transmembrane currents due to electrostatic events associated with conformational changes. We employed the sensitive rhodopsin early receptor current approach to reevaluate whether all-trans-retinal can form a visual pigment with rod opsin apoprotein. An opsin shift above 440 nm is induced in the action spectrum of charge motions caused by visible flashes in cells expressing human rod opsin and regenerated with all-trans-retinal, compared to cells without opsin. Near-ultraviolet stimulation of opsin regenerated with all-trans-retinal promotes charge motions similar to those arising from the meta-II signaling state while photochemically regenerating a pigment with ground state charge motion properties. These results indicate that all-trans-retinal can form a visual pigment with opsin, through both protonated and unprotonated Schiff base linkages and likely within the native ligand binding pocket at lysine-296. The agonist effects of all-trans-retinal may relate to its structural accommodation within the core of opsin, similar to other G-protein-coupled receptors.