Abstract
Photoactivation of the visual rhodopsin, a prototypical G protein-coupled receptor (GPCR), involves efficient conversion of the intrinsic inverse-agonist 11-cis-retinal to the all-trans agonist. This event leads to the rearrangement of the heptahelical transmembrane bundle, which is thought to be shared by hundreds of GPCRs. To examine this activation mechanism, we determined the x-ray crystallographic model of the photoreaction intermediate of rhodopsin, lumirhodopsin, which represents the conformational state having the nearly complete all-trans agonist form of the retinal. A difference electron density map clearly indicated that the distorted all-trans-retinal in the precedent intermediate bathorhodopsin relaxes by dislocation of the beta-ionone ring in lumirhodopsin, along with significant peptide displacement in the middle of helix III, including approximately two helical turns. This local movement results in the breaking of the electrostatic interhelical restraints mediated by many of the conserved residues among rhodopsin-like GPCRs, with consequent acquisition of full activity.