Structural Dynamics of Retinal in Rhodopsin Activation Viewed by Solid-State 2H NMR Spectroscopy
Rhodopsin is a member of the family of G-protein-coupled receptors (GPCRs) that are implicated in cellular signaling and constitute the majority of human pharmaceutical targets. Solid-state 2H NMR spectroscopy can be used to obtain structural and dynamical information unavailable from X-ray crystallography and other biophysical methods. In this approach, site-specific 2H labels are introduced into the methyl groups of retinal that play an important role in rhodopsin function. Analysis of angular-dependent 2H NMR lineshapes for rhodopsin in aligned membranes enables determination of the average ligand conformation within the binding pocket. Solid-state 2H NMR relaxation methods further allow the investigation of light-induced changes in local ps–ns timescale motions of retinal bound to rhodopsin. In terms of a multi-scale activation mechanism, changes in retinal structure and dynamics upon photon absorption activate fluctuations of transmembrane helices H5 and H6 in rhodopsin. Allosteric interactions due to light absorption of rhodopsin are propagated from the retinal-binding pocket to the binding site of the G-protein transducin. An ensemble of substates in the Meta I–Meta II equilibrium yields the first amplification step in the visual response and may be typical for the activation mechanisms of other GPCRs in a membrane environment.