Structural basis of ultraviolet-B perception by UVR8

The Arabidopsis thaliana protein UVR8 is a photoreceptor for ultraviolet-B. Upon ultraviolet-B irradiation, UVR8 undergoes an immediate switch from homodimer to monomer, which triggers a signalling pathway for ultraviolet protection. The mechanism by which UVR8 senses ultraviolet-B remains largely unknown. Here we report the crystal structure of UVR8 at 1.8 Å resolution, revealing a symmetric homodimer of seven-bladed β-propeller that is devoid of any external cofactor as the chromophore. Arginine residues that stabilize the homodimeric interface, principally Arg 286 and Arg 338, make elaborate intramolecular cation–π interactions with surrounding tryptophan amino acids. Two of these tryptophans, Trp 285 and Trp 233, collectively serve as the ultraviolet-B chromophore. Our structural and biochemical analyses identify the molecular mechanism for UVR8-mediated ultraviolet-B perception, in which ultraviolet-B radiation results in destabilization of the intramolecular cation–π interactions, causing disruption of the critical intermolecular hydrogen bonds mediated by Arg 286 and Arg 338 and subsequent dissociation of the UVR8 homodimer. The X-ray crystal structure of ultraviolet-B-sensing protein UVR8 is determined, revealing that, unlike other known photoreceptors, the chromophore is not an external cofactor but rather two amino acids. How plants 'see' ultraviolet-B Plants have evolved elaborate systems of photoreceptors to perceive light of different wavelengths. Photoreceptors were known for far-red, red, blue and ultraviolet-A light, but the photoreceptor for ultraviolet-B light was identified only last year. The ultraviolet-B receptor in Arabidopsis is the protein UVR8, and now the mechanism by which it senses ultraviolet-B has been determined. The X-ray crystal structure reveals that — in contrast to that of phytochrome and other known photoreceptors — the chromophore of UVR8 is not an external cofactor. Instead, the ultraviolet-B chromophore consists of two tryptophans. Structural and biochemical experiments suggest that the presence of ultraviolet-B destabilizes key intramolecular cation–π interactions, leading to the disassembly of the UVR8 homodimer and triggering a signalling cascade.