Formation and Decay of Active Structure in Photoactivated Rhodopsin Probed by Wide-Angle X-Ray Scattering

Among the photoproducts of vertebrate rhodopsin, only metarhodopsin II (Meta-II) preferentially adopts the active structure in which transmembrane helices are rearranged. Light-induced helical rearrangement of rhodopsin in membrane-embedded form was directly monitored by wide-angle X-ray scattering (WAXS) using nanodiscs. The change in WAXS curve for the formation of Meta-II was characterized by peak at 0.2 A-1 and valley at 0.6 A-1, which were not observed in metarhodopsin I and opsin. However, acid-induced active opsin (Opsin*) showed 0.2 A-1 peak but no 0.6 A-1 valley. Analyses using the model structures based on the crystal structures of dark state and Meta-II suggest that the outward movement of helix VI occurred in Opsin*. However, the displaced helices III and V in Meta-II resulting from the disruption of cytoplasmic ionic lock were restored in Opsin*, which is likely to destabilize the G-protein activating structure of opsin.