Retinal orientation and interactions in rhodopsin reveal a two-stage trigger mechanism for activation

• Published in: Nature communications Vol. 7; no. 1; p. 12683
• Main Authors: Kimata, Naoki, Pope, Andreyah, Eilers, Markus, Opefi, Chikwado A, Ziliox, Martine, Hirshfeld, Amiram, Zaitseva, Ekaterina, Vogel, Reiner, Sheves, Mordechai, Reeves, Philip J, Smith, Steven O
• Format: Journal Article
• Language: English
• Published: England Nature Publishing Group 02.09.2016; Nature Portfolio
• Subjects: Binding sites; Fourier transforms; Ligands; Light; NMR; Nuclear magnetic resonance; Proteins
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/ncomms12683

The 11-cis retinal chromophore is tightly packed within the interior of the visual receptor rhodopsin and isomerizes to the all-trans configuration following absorption of light. The mechanism by which this isomerization event drives the outward rotation of transmembrane helix H6, a hallmark of activated G protein-coupled receptors, is not well established. To address this question, we use solid-state NMR and FTIR spectroscopy to define the orientation and interactions of the retinal chromophore in the active metarhodopsin II intermediate. Here we show that isomerization of the 11-cis retinal chromophore generates strong steric interactions between its β-ionone ring and transmembrane helices H5 and H6, while deprotonation of its protonated Schiff's base triggers the rearrangement of the hydrogen-bonding network involving residues on H6 and within the second extracellular loop. We integrate these observations with previous structural and functional studies to propose a two-stage mechanism for rhodopsin activation.