UniProt: Q9FN03 | TAIR/Araport: AT5G63860 | NCBI Taxon: 3702
UVR8 is the plant UV-B photoreceptor. It is a seven-bladed beta-propeller protein
of the RCC1 structural family. In the absence of UV-B it exists as a homodimer.
Specific intrinsic tryptophan residues (principally Trp-285 and Trp-233) act as the
UV-B chromophore; UV-B absorption disrupts the cation-pi/salt-bridge network at the
dimer interface and triggers instantaneous monomerization. The monomer accumulates
in the nucleus, where it binds the E3 ubiquitin ligase COP1 (via the C-terminal C27
region), leading to stabilization of the bZIP transcription factor HY5 and induction
of UV-B photomorphogenesis, acclimation, and UV-protective gene expression
(flavonoid/sinapate biosynthesis, DNA repair, antioxidant genes). The negative
regulators RUP1 and RUP2 (UV-B-induced) bind UVR8 and drive its redimerization,
inactivating signaling. UVR8 also associates with chromatin via histone H2B in the
HY5 promoter region.
UVR8 has sequence similarity to human RCC1, a guanine-nucleotide exchange factor
(GEF) for the small GTPase Ran. However, experimentally UVR8 has essentially no Ran
GEF activity and does not function as a RanGEF:
- PMID:16330762.
- PMID:16330762.
- PMID:16330762.
- PMID:16330762.
- PMID:19165148.
=> The GOA annotation GO:0005085 "guanyl-nucleotide exchange factor activity" (ISS
from PMID:12226503, based purely on RCC1 sequence similarity) is an over-propagated
homology inference that is directly contradicted by experimental data in the SAME
gene's literature. This should be REMOVED.
GO:0009536 "plastid" (HDA, PMID:28887381) comes from a global membrane-associated
protein-oligomerization proteomics survey (protein correlation profiling of
1350 proteins). This is a high-throughput dataset with known cross-compartment
mixing; UVR8 is consistently characterized as a nucleocytoplasmic protein, and there
is no functional evidence for a plastid role. PMID:28887381 Treat as over-annotation (not a core/established location).
GO:0005829 "cytosol" (HDA, PMID:25293756) is from a global cytosolic SEC/MS protein
complex survey of Arabidopsis leaves. Cytosolic localization of UVR8 is independently
well supported (PMID:16330762, PMID:17720867), so this is consistent/corroborating.
Note on "protein binding" (GO:0005515): the IPI annotations from PMID:19165148 and
PMID:21041653 (with COP1 P43254, RUP2 Q9FFA7) and PMID:22988111 (COP1, RUP2, RUP1
Q9LTJ6) capture real, important interactions, but the bare GO:0005515 term is
uninformative per curation guidelines. These interactions are better represented by
the homodimerization MF term and the regulatory BP terms; the underlying COP1/RUP
interactions inform the photomorphogenesis/regulation functions rather than being
core MF in themselves.
Incorporated file:ARATH/UVR8/UVR8-deep-research-falcon.md as a supporting reference
(relevance HIGH, correctness VERIFIED with caveat that it is an AI-generated synthesis
and individual claims should be traced to primary literature). The report strongly
corroborates the core verdicts: UV-B photoreception via intrinsic tryptophans (not an
enzyme/RanGEF), homodimer->monomer photoconversion as the signaling-competent switch,
UV-B-dependent COP1 binding by the monomer, nuclear accumulation (with the caveat that
nuclear localization alone is insufficient), and the UV-B response/photomorphogenesis
BP. Falcon quotes added as supported_by to: photoreceptor activity (GO:0009881),
homodimerization activity (GO:0042803), response to UV-B (GO:0010224), nucleus EXP
(GO:0005634), and the COP1 core_function.
CONTRADICTION / refinement noted (verdicts NOT flipped, per augmentation rules):
The Falcon report flags direct UVR8 chromatin / histone-H2B binding as DISPUTED. It
reports that earlier histone-agarose/ChIP studies (PMID:16330762, PMID:20031919) were
followed by a careful re-examination (Binkert et al. 2016, BMC Plant Biology,
doi:10.1186/s12870-016-0732-5) that found no convincing UVR8 chromatin association at
HY5/MYB12 loci, no in vitro nucleosome binding, and that RCC1-like histone/DNA-binding
residues are not conserved in UVR8. The current review ACCEPTs both GO:0003682
(chromatin binding) and GO:0000785 (chromatin). I left these as ACCEPT but appended a
cautionary sentence to each reason flagging the dispute. A future full re-review
should weigh Binkert 2016 directly and may want to downgrade these to
MARK_AS_OVER_ANNOTATED / UNDECIDED. (Binkert PMID not asserted in the YAML because it
was not verified against PubMed here.)