UVR8 (UV RESISTANCE LOCUS 8) is the plant UV-B photoreceptor, 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, and UV-B absorption disrupts the cross-dimer salt-bridge/cation-pi network, causing instantaneous monomerization. The UVR8 monomer accumulates rapidly in the nucleus, where it binds the E3 ubiquitin ligase COP1 through its C-terminal C27 region. This interaction stabilizes the bZIP transcription factor HY5 and triggers UV-B photomorphogenesis, acclimation, and UV-protective gene expression (including flavonoid and sinapate biosynthesis, DNA photorepair, and antioxidant genes). UVR8 also associates with chromatin in the HY5 promoter region via histone H2B. The negative regulators RUP1 and RUP2, themselves UV-B-induced, bind UVR8 and mediate its redimerization to reset the photoreceptor. Although UVR8 is structurally homologous to human RCC1, a guanine-nucleotide exchange factor for the GTPase Ran, it has negligible Ran GEF activity and does not function as a RanGEF.
Definition: Detecting and responding to ultraviolet-B (UV-B, 280-315 nm) radiation as an informational light signal, via a chromophore-bearing protein that undergoes a UV-B-triggered conformational change to initiate downstream signaling.
Justification: GO currently lacks a wavelength-specific molecular-function term for UV-B photoreception. UVR8 is the defining plant UV-B photoreceptor, using intrinsic tryptophans rather than an external chromophore. A dedicated 'UV-B photoreceptor activity' MF child of GO:0009881 would more precisely capture this function.
Parent term: photoreceptor activity
Supporting Evidence:
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005634 nucleus | IEA GO_REF:0000044 | ACCEPT | Summary: Electronic subcellular-location mapping placing UVR8 in the nucleus. UVR8 nuclear localization is well established experimentally, so this electronic annotation is correct and corresponds to a core location. Reason: Nuclear localization is experimentally confirmed and mechanistically central. Supporting Evidence: PMID:17720867 UV-B stimulates the nuclear accumulation of both a green fluorescent protein (GFP)-UVR8 fusion and native UVR8 |
| GO:0005829 cytosol | IEA GO_REF:0000044 | KEEP AS NON CORE | Summary: Electronic subcellular-location mapping placing UVR8 in the cytosol. UVR8 is indeed present in the cytosol in the dark/ground state and redistributes to the nucleus upon UV-B. Reason: Cytosolic localization is experimentally supported but represents the resting pool; the functionally decisive location is the nucleus. Supporting Evidence: PMID:16330762 GFP-UVR8 fluorescence was present in the nucleus but also detectable in the cytosol |
| GO:0005515 protein binding | IPI PMID:19165148 Interaction of COP1 and UVR8 regulates UV-B-induced photomor... | MARK AS OVER ANNOTATED | Summary: IPI annotation capturing the UV-B-dependent UVR8-COP1 interaction (with COP1, P43254). The interaction is real and biologically important, but the bare 'protein binding' term is uninformative per curation guidelines. Reason: GO:0005515 'protein binding' conveys no specific function; the COP1 interaction it records is captured more informatively by the photomorphogenesis core functions. Supporting Evidence: PMID:19165148 the wild type but not the mutant UVR8 and COP1 proteins directly interact in a UV-B-dependent, rapid manner in planta |
| GO:0005515 protein binding | IPI PMID:21041653 Negative feedback regulation of UV-B-induced photomorphogene... | MARK AS OVER ANNOTATED | Summary: IPI annotation capturing the UVR8 interaction with the negative regulators RUP1 and RUP2 (RUP2, Q9FFA7). A genuine and functionally relevant interaction, but the bare 'protein binding' term is uninformative. Reason: GO:0005515 'protein binding' is uninformative; the RUP1/RUP2 interaction is better represented through the negative-feedback regulation of UVR8 signaling. Supporting Evidence: PMID:21041653 REPRESSOR OF UV-B PHOTOMORPHOGENESIS 1 (RUP1) and RUP2, that interact directly with UVR8 as potent repressors of UV-B signaling |
| GO:0042802 identical protein binding | IPI PMID:22388820 Structural basis of ultraviolet-B perception by UVR8. | ACCEPT | Summary: Self-interaction (homodimerization) of UVR8, confirmed by the crystal structure showing a symmetric homodimer. This is mechanistically central. Reason: The UVR8 homodimer is the functional ground state of the photoreceptor and its UV-B-triggered dissociation initiates signaling. Supporting Evidence: PMID:22388820 a symmetric homodimer of seven-bladed Ξ²-propeller that is devoid of any external cofactor as the chromophore |
| GO:0005634 nucleus | EXP PMID:17720867 UV-B promotes rapid nuclear translocation of the Arabidopsis... | ACCEPT | Summary: Experimental demonstration of nuclear localization and UV-B-induced nuclear accumulation of UVR8. Core location for signaling output. Reason: Direct experimental evidence; nuclear accumulation is a defining step of UVR8 function. Supporting Evidence: PMID:17720867 Nuclear accumulation of UVR8 is specific to UV-B, occurs at low fluence rates, and is observed within 5 min of UV-B exposure file:ARATH/UVR8/UVR8-deep-research-falcon.md Notably, nuclear localization alone is insufficient to activate signaling; UV-B/monomerization remains required. |
| GO:0042803 protein homodimerization activity | IPI PMID:23277547 Reversion of the Arabidopsis UV-B photoreceptor UVR8 to the ... | ACCEPT | Summary: Homodimerization activity of UVR8. The homodimer is the UV-B-absorbing ground state; RUP1/RUP2-mediated redimerization regenerates the reactivatable receptor. Reason: Homodimerization defines the photoreceptor ground state and its reversible monomerization/redimerization cycle is central to UVR8 function. Supporting Evidence: PMID:23277547 UVR8 monomerization is reversible in vivo, restoring the homodimeric ground state file:ARATH/UVR8/UVR8-deep-research-falcon.md Constitutive monomer and constitutive dimer mutants (e.g., W285A versus W285F) support that monomerization is the signaling-competent state. |
| GO:0005829 cytosol | HDA PMID:25293756 A proteomic strategy for global analysis of plant protein co... | KEEP AS NON CORE | Summary: Cytosolic localization from a global high-throughput cytosolic protein-complex survey of Arabidopsis leaves. Consistent with the cytosolic resting pool. Reason: Corroborates the cytosolic resting pool already supported by direct evidence; the core functional location is the nucleus. Supporting Evidence: PMID:16330762 GFP-UVR8 fluorescence was present in the nucleus but also detectable in the cytosol |
| GO:0009536 plastid | HDA PMID:28887381 Global Analysis of Membrane-associated Protein Oligomerizati... | MARK AS OVER ANNOTATED | Summary: Plastid localization derived from a global membrane-associated protein correlation profiling survey of more than 1350 proteins. UVR8 is consistently characterized as a nucleocytoplasmic photoreceptor with no functional evidence for a plastid role; most likely an artifact. Reason: Inferred from a single high-throughput proteomic localization survey with known cross-compartment partitioning; contradicts well-established localization and lacks functional support for a plastid role. Supporting Evidence: PMID:28887381 Over 150 proteins had a complicated localization pattern, and were clearly partitioned between cytosolic and membrane-associated pools file:ARATH/UVR8/UVR8-notes.md comes from a global membrane-associated protein-oligomerization proteomics survey |
| GO:0005737 cytoplasm | ISM GO_REF:0000122 | KEEP AS NON CORE | Summary: Sequence-based (ISM, AtSubP) prediction of cytoplasmic localization. UVR8 does have a cytosolic pool, so the general 'cytoplasm' term is not wrong. Reason: Computational prediction consistent with the experimentally supported cytosolic resting pool, but general and non-core. Supporting Evidence: PMID:16330762 GFP-UVR8 fluorescence was present in the nucleus but also detectable in the cytosol |
| GO:0005515 protein binding | IPI PMID:22988111 C-terminal region of the UV-B photoreceptor UVR8 initiates s... | MARK AS OVER ANNOTATED | Summary: IPI annotation capturing interactions of the UVR8 C-terminal C27 region with COP1 (P43254), RUP2 (Q9FFA7), and RUP1 (Q9LTJ6). Pivotal interactions, but the bare 'protein binding' term is uninformative. Reason: GO:0005515 'protein binding' is uninformative; the C27-mediated COP1/RUP interactions are better represented by the photomorphogenesis functions. Supporting Evidence: PMID:22988111 C27 is both necessary and sufficient for the interaction of UVR8 with the WD40 domain of COP1 |
| GO:0000785 chromatin | IDA PMID:20031919 Interaction of the Arabidopsis UV-B-specific signaling compo... | ACCEPT | Summary: UVR8 associates with chromatin in vivo, preferentially via histone H2B, including at the HY5 locus. Direct experimental evidence. Reason: Direct experimental demonstration that native UVR8 binds chromatin in vivo. Note: the Falcon deep-research synthesis flags UVR8 chromatin/histone-H2B association as disputed, citing later rigorous ChIP/nucleosome-binding work (Binkert et al. 2016, BMC Plant Biology) that failed to confirm specific chromatin association; this verdict is retained pending direct reassessment of that conflicting primary evidence. Supporting Evidence: PMID:20031919 UVR8 associates with histones in vivo and competition experiments indicate that the interaction is preferentially with histone H2B |
| GO:0009649 entrainment of circadian clock | IMP PMID:21395889 Functional interaction of the circadian clock and UV RESISTA... | KEEP AS NON CORE | Summary: Genetic evidence that UVR8 (with COP1) is required for entrainment of the circadian clock by photomorphogenic UV-B. A downstream physiological role. Reason: A bona fide UVR8-dependent process, but one of several downstream outputs of UVR8 UV-B signaling rather than its core photoreceptor function. Supporting Evidence: PMID:21395889 UV RESISTANCE LOCUS 8 (UVR8) and CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1) are required, but ELONGATED HYPOCOTYL 5 (HY5) and HY5 HOMOLOG (HYH) are dispensable for this process |
| GO:0010224 response to UV-B | IEP PMID:17720867 UV-B promotes rapid nuclear translocation of the Arabidopsis... | ACCEPT | Summary: UVR8 is the UV-B-specific signaling component that mediates plant responses to UV-B. Core biological process. Reason: UVR8 is the defining mediator of the UV-B response. Supporting Evidence: PMID:17720867 Nuclear accumulation of UVR8 is specific to UV-B, occurs at low fluence rates, and is observed within 5 min of UV-B exposure file:ARATH/UVR8/UVR8-deep-research-falcon.md These are core UVR8 pathway phenotypes and are strengthened by UVR8βCOP1 interaction evidence and genetic phenotypes |
| GO:0003682 chromatin binding | IDA PMID:16330762 A UV-B-specific signaling component orchestrates plant UV pr... | ACCEPT | Summary: Direct evidence that UVR8 binds chromatin (via histones), including the HY5 promoter region. Reason: Experimentally demonstrated histone/chromatin binding underpins UVR8's role in regulating HY5 and UV-protective gene expression. Note: the Falcon deep-research synthesis flags direct UVR8 chromatin/histone-H2B binding as disputed, with later ChIP/nucleosome-binding studies (Binkert et al. 2016) failing to confirm specific chromatin association; verdict retained pending direct reassessment of the conflicting evidence. Supporting Evidence: PMID:16330762 GFP-UVR8 associated with a chromatin fragment containing the HY5 promoter |
| GO:0005085 guanyl-nucleotide exchange factor activity | ISS PMID:12226503 Arabidopsis UVR8 regulates ultraviolet-B signal transduction... | REMOVE | Summary: Homology-based (ISS) inference from UVR8's sequence similarity to human RCC1, a Ran GEF. This is directly contradicted by experiment: UVR8 has negligible Ran GEF activity, does not interact with Arabidopsis Ran, and does not complement a yeast RCC1 mutant. Reason: Over-propagated electronic/homology (ISS) inference from RCC1 similarity that is experimentally refuted - UVR8 has only about 7 percent of RCC1's GEF activity, does not bind Ran, and does not complement yeast RCC1 loss. Supporting Evidence: PMID:16330762 UVR8 has sequence similarity to the eukaryotic guanine nucleotide exchange factor RCC1, but we found that it has little exchange activity PMID:16330762 UVR8 did not interact with Arabidopsis Ran1 and Ran2 in yeast two-hybrid assays and did not complement the yeast prp20 mutant lacking yeast RCC1 |
| GO:0005634 nucleus | IDA PMID:16330762 A UV-B-specific signaling component orchestrates plant UV pr... | ACCEPT | Summary: Direct evidence (GFP-UVR8) that UVR8 is localized principally in the nucleus. Reason: Direct experimental localization to the nucleus. Supporting Evidence: PMID:16330762 GFP-UVR8 fluorescence was present in the nucleus but also detectable in the cytosol |
| GO:0005829 cytosol | IDA PMID:16330762 A UV-B-specific signaling component orchestrates plant UV pr... | KEEP AS NON CORE | Summary: Direct evidence that UVR8 is also detectable in the cytosol. Resting/ground-state pool. Reason: Experimentally supported cytosolic pool, but the functionally decisive location is the nucleus. Supporting Evidence: PMID:16330762 GFP-UVR8 fluorescence was present in the nucleus but also detectable in the cytosol |
| GO:0009411 response to UV | IMP PMID:12226503 Arabidopsis UVR8 regulates ultraviolet-B signal transduction... | MODIFY | Summary: Mutant phenotype evidence (uvr8-1) that UVR8 is required for UV(-B) tolerance. More general than the specific 'response to UV-B' term. Reason: UVR8 acts specifically in the UV-B response; the more specific child term 'response to UV-B' (GO:0010224) is the appropriate annotation. Proposed replacements: response to UV-B Supporting Evidence: PMID:12226503 This mutation reduces the UV-B-mediated induction of flavonoids and blocks chalcone synthase mRNA and protein induction |
| GO:0009881 photoreceptor activity | IEA GO_REF:0000043 | NEW | Summary: Keyword-derived (UniProtKB-KW) photoreceptor activity, present in the UniProt GO cross-references but not in the current QuickGO GOA export. This is the appropriate molecular function for UVR8. GO does not currently provide a dedicated 'UV-B photoreceptor activity' MF term. Reason: UVR8 is an experimentally established UV-B photoreceptor; photoreceptor activity is its defining core molecular function. Added as a NEW annotation because it appears in the UniProt keyword-derived GO cross-references but is absent from the supplied GOA export. Supporting Evidence: PMID:22388820 Two of these tryptophans, Trp 285 and Trp 233, collectively serve as the ultraviolet-B chromophore file:ARATH/UVR8/UVR8-deep-research-falcon.md consistent with UVR8 functioning as a UV-B receptor rather than as an enzyme with a substrate-binding pocket |
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