A pivot between helices V and VI near the retinal-binding site is necessary for activation in rhodopsins.

Rhodopsins are photoreceptor proteins that have diverged from ligand-binding G protein-coupled receptors (GPCRs). Unlike other GPCRs, rhodopsins contain an intrinsic antagonist, 11-cis-retinal, which is converted to the agonist all-trans-retinal upon absorption of a photon. Through evolution, vertebrate rhodopsins have lost the ability of direct binding to the agonist, but some invertebrate and vertebrate non-visual rhodopsins have retained this ability. Here, we investigated the difference in the agonist-binding state between these rhodopsins to further our understanding of the structural and functional diversity of rhodopsins. Mutational analyses of agonist-binding rhodopsin showed that replacement of Ala-269, one of the residues constituting the antagonist-binding site, with bulky amino acids resulted in a large spectral shift in its active state and a great reduction in G protein activity, whereas these were rescued by subsequent replacement of Phe-208 with smaller amino acids. Although similar replacements in vertebrate rhodopsin did not cause a spectral shift in the active state, a similar reduction in and recovery of G protein activity was observed. Therefore, the agonist is located close to Ala-269 in the agonist-binding rhodopsin, but not in vertebrate rhodopsins, and Ala-269 with Phe-208 acts as a pivot for the formation of the G protein-activating state in both rhodopsins. The positions corresponding to Ala-269 and Phe-208 in other GPCRs have been reported to form part of an agonist-binding site. Therefore, an agonist-binding rhodopsin has the molecular architecture of the agonist-binding site similar to that of a general GPCR, whereas vertebrate rhodopsins changed the architecture, resulting in loss of agonist binding during molecular evolution.