Location of Trp265 in Metarhodopsin II: Implications for the Activation Mechanism of the Visual Receptor Rhodopsin

Isomerization of the 11-cis retinal chromophore in the visual pigment rhodopsin is coupled to motion of transmembrane helix H6 and receptor activation. We present solid-state magic angle spinning NMR measurements of rhodopsin and the metarhodopsin II intermediate that support the proposal that interaction of Trp2656.48 with the retinal chromophore is responsible for stabilizing an inactive conformation in the dark, and that motion of the β-ionone ring allows Trp2656.48 and transmembrane helix H6 to adopt active conformations in the light. Two-dimensional dipolar-assisted rotational resonance NMR measurements are made between the C19 and C20-methyl groups of the retinal and uniformly 13C-labeled Trp2656.48. The retinal C20–Trp2656.48 contact present in the dark-state of rhodopsin is lost in metarhodopsin II, and a new contact is formed with the C19 methyl group. We have previously shown that the retinal translates 4–5 Å toward H5 in metarhodopsin II. This motion, in conjunction with the Trp–C19 contact, implies that the Trp2656.48 side-chain moves significantly upon rhodopsin activation. NMR measurements also show that a packing interaction in rhodopsin between Trp2656.48 and Gly1213.36 is lost in metarhodopsin II, consistent with H6 motion away from H3. However, a close contact between Gly1203.35 on H3 and Met862.53 on H2 is observed in both rhodopsin and metarhodopsin II, suggesting that H3 does not change orientation significantly upon receptor activation.