Structural transitions of transmembrane helix 6 in the formation of metarhodopsin I.

Absorption of light by the visual pigment rhodopsin triggers a rapid cis-trans photoisomerization of its retinal chromophore and a series of conformational changes in both the retinal and protein. The largest structural change is an outward tilt of transmembrane helix H6 that increases the separation of the intracellular ends of H6 and H3 and opens up the G-protein binding site. In the dark state of rhodopsin, Glu247 at the intracellular end of H6 forms a salt bridge with Arg135 on H3 to tether H6 in an inactive conformation. The Arg135-Glu247 interaction is broken in the active state of the receptor, and Arg135 is then stabilized by interactions with Tyr223, Met257, and Tyr306 on helices H5, H6, and H7, respectively. To address the mechanism of H6 motion, solid-state NMR measurements are undertaken of Metarhodopsin I (Meta I), the intermediate preceding the active Metarhodopsin II (Meta II) state of the receptor. (13)C NMR dipolar recoupling measurements reveal an interhelical contact of (13)Cζ-Arg135 with (13)Cε-Met257 in Meta I but not with (13)Cζ-Tyr223 or (13)Cζ-Tyr306. These observations suggest that helix H6 has rotated in the formation of Meta I but that structural changes involving helices H5 and H7 have not yet occurred. Together, our results provide insights into the sequence of events leading up to the outward motion of H6, a hallmark of G protein-coupled receptor activation.