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.
Tsukamoto
Hisao
H
Department of Biophysics, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan.
Terakita
Akihisa
A
Shichida
Yoshinori
Y
D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D002417:Cattle; D004252:DNA Mutational Analysis; D019204:GTP-Binding Proteins / Q000378:metabolism; D006801:Humans; D008958:Models, Molecular; D008969:Molecular Sequence Data; D015394:Molecular Structure; D016297:Mutagenesis, Site-Directed; D011485:Protein Binding; D017433:Protein Structure, Secondary; D043562:Receptors, G-Protein-Coupled / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D012172:Retinaldehyde / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D012243:Rhodopsin / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D017299:Rod Opsins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism
D043562:Receptors, G-Protein-Coupled; D017299:Rod Opsins; D012243:Rhodopsin; D019204:GTP-Binding Proteins; D012172:Retinaldehyde