|UVB-resistance protein UVR8|
|Crystal structure of UVB-resistance protein UVR8.|
|PDB||4DNW More structures|
|Chromosome||5: 25.55 - 25.56 Mb|
UV-B resistance 8 (UVR8) also known as ultraviolet-B receptor UVR8 is an UV-B – sensing protein found in plants and possibly other sources. It is responsible for sensing ultraviolet light in the range 280-315 nm and initiating the plant stress response. UVR8 was first identified as a crucial mediator of a plant's repsonse to UV-B in Arabidopsis thaliana containing a mutation in this protein. This plant was found to have a hypersensitivity to UV-B which damages DNA. UVR8 is thought to be a unique photoreceptor as it doesn't contain a prosthetic chromophore but its light-sensing ability is intrinsic to the molecule. Tryptophan (Trp) residue 285 has been suggested to act the UV-B sensor, while other Trp residues have been also seen to be involved (Trp233 > Trp337 > Trp94).
UVR8 is a β-propeller protein with 7 blade-shaped β-sheets. It shares sequence homology with mammalian proteins involved in regulating chromatin condensation, for example the human RCC1 gene product. In the dark state, UVR8 forms a homodimer that is localized in the cytosol, but UV-B illumination induces the dissociation of UVR8 dimer to its respective monomers and translocation to the nucleus occurs. The dimer is held together via a complex salt bridge network.
Upon UV-B irradiation, light is absorbed by one or more Trp residues which are situated adjacent to Arg residues which form salt bridges across the dimer interface. It is thought that this light absorption induces the disruption of the salt-bridges and thus leads to the molecule's monomerization. Following monomerization, UVR8 accumulates in the nucleus where it interacts with a protein called constitutively photomorphogenic 1 (COP1). COP1 is known to act as an E3 Ubiquitin ligase that targets key transcription factors for ubiquitination and proteasome-mediated degradation. However, in the case of UVR8, it has been shown to act as a positive regulator of UVR8-mediated UV-B signalling. Upon UV-B illumination, UVR8 interacts via a C-terminal 27 amino acid region with the WD40 domain of COP1 in the nucleus, which triggers the induction of a elongated hypocotyl5 (HY5)- a key transcription factor for several UV-B responsive genes- and overall results in UV-B acclimation.
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