Rhodopsin is the light-sensitive G protein-coupled receptor of vertebrate rod photoreceptors. It consists of the protein opsin bound to the light-absorbing molecule 11-cis-retinal. Absorbing a photon changes the retinal configuration toward all-trans-retinal, initiating structural changes that activate the signaling partner transducin. The visual cycle supplies fresh 11-cis-retinal so the pigment can be regenerated.[ref]
The 2000 bovine rhodopsin structure, obtained by X-ray crystallography at angstrom resolution, showed seven membrane-spanning alpha helices surrounding the bound chromophore. The dark-state retinal helps stabilize an inactive receptor configuration. The structure made a molecular question visible: how can a small rearrangement of a bound light absorber alter the intracellular surface that engages another protein?[ref]
Crystal Structure of Rhodopsin: A G Protein-Coupled Receptor by Krzysztof Palczewski and others. Abstract, structure-determination section and Figures 2–3.
The distinction between opsin and functional pigment matters experimentally. In mice lacking RPE65, investigators detected rod opsin but no normal rhodopsin, together with loss of rod responses on electroretinography. Having the receptor chain is insufficient when the required retinal supply is disrupted.[ref]
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