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

Rhodopsin activation elicits transmembrane currents due to electrostatic ev ents associated with conformational changes. We employed the sensitive rhod opsin early receptor current approach to reevaluate whether all-trans-retin al can form a visual pigment with rod opsin apoprotein. An opsin shift abov e 440 nm is induced in the action spectrum of charge motions caused by visi ble flashes in cells expressing human rod opsin and regenerated with all-tr ans-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 re generating a pigment with ground state charge motion properties. These resu lts indicate that all-trans-retinal can form a visual pigment with opsin, t hrough both protonated and unprotonated Schiff base linkages and likely wit hin the native ligand binding pocket at lysine-296. The agonist effects of all-trans-retinal may relate to its structural accommodation within the cor e of opsin, similar to other G-protein-coupled receptors.