Chromophore Structure in an Inactive State of a Novel Photosensor Protein Opn5L1: Resonance Raman Evidence for the Formation of a Deprotonated Adduct at the 11th Carbon Atom.

Opsins are photosensitive G protein-coupled receptor proteins and are classified into visual and nonvisual receptors. Opn5L1 is a nonvisual opsin that binds all- trans retinal as a chromophore. A unique feature of Opn5L1 is that the protein exhibits a photocyclic reaction upon photoexcitation. Determining the chromophore structures of intermediates in the photocycle is essential for understanding the functional mechanism of Opn5L1. A previous study revealed that a long-lived intermediate in the photocycle cannot activate the G protein and forms a covalent bond between the retinal chromophore and a nearby cysteine residue. However, the position of this covalent bond in the chromophore remains undetermined. Here, we report a resonance Raman study on isotopically labeled samples in combination with density functional theory calculations and reveal that the 11th carbon atom of the chromophore of the intermediate forms a covalent linkage to the cysteine residue. Furthermore, vibrational assignments based on the isotopic substitutions and density functional theory calculations suggested that the Schiff base of the intermediate is deprotonated. The chromophore structure determined in the present study well explains the mechanism of the photocyclic reaction, which is crucial to the photobiological function of Opn5L1.