AFB2 (AUXIN SIGNALING F-BOX 2; At3g26810) is a leucine-rich-repeat F-box protein of the TIR1/AFB auxin receptor family. As the substrate-recognition subunit of an SCF(ASK-cullin-F-box) E3 ubiquitin ligase, it binds auxin in a pocket of its LRR domain and, with auxin acting as molecular glue, captures the degron (domain II) of Aux/IAA transcriptional repressors, forming a TIR1/AFB-Aux/IAA co-receptor. This targets Aux/IAAs for ubiquitination and proteasomal degradation, releasing AUXIN RESPONSE FACTOR transcription factors to activate auxin-responsive genes. AFB2 belongs to the AFB2/AFB3 clade and acts redundantly with TIR1 and AFB3; together with TIR1 it is one of the dominant auxin receptors in the seedling root, and quadruple tir1 afb1 afb2 afb3 loss causes auxin insensitivity and embryonic patterning defects resembling monopteros/bodenlos. AFB2 is found in the nucleus and the cytoplasm, and its transcript is targeted by miR393. The InsP6 cofactor-binding assignment is inferred from the conserved TIR1 receptor fold; direct adenylate cyclase activity has not been established for AFB2 by the available receptor assays.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000325 plant-type vacuole | HDA PMID:17151019 A proteomics dissection of Arabidopsis thaliana vacuoles iso... | UNDECIDED | Summary: Vacuolar proteomics reports AFB2 localization, but the target-level supplemental identification remains unverified. Reason: The source purified cultured-cell vacuoles and tested organelle contamination; generic contamination is not evidence against this AFB2 assignment. Native root imaging of nuclear/cytoplasmic AFB2 does not exclude a vacuolar pool or turnover route, and localization does not require a vacuolar signaling function. The accessible main text does not expose the AFB2 peptide row; the original supplemental endpoint returned HTTP 403 and PMC retrieval returned a challenge page. Retain uncertainty pending target-level supplemental evidence rather than treating nuclear residence as exclusion. Supporting Evidence: PMID:32067636 AFB2 through AFB5 are distributed between the nucleus and the cytoplasm |
| GO:0000822 inositol hexakisphosphate binding | ISS GO_REF:0000024 | ACCEPT | Summary: Conserved receptor-fold InsP6 binding is inferred from the TIR1 structure. Reason: Retain the curatorβs ISS assignment as an integral receptor cofactor property. The direct structural experiment is TIR1, not AFB2; this is explicitly a paralog-based structural inference, not an AFB2 occupancy measurement. Cofactor binding is not peripheral simply because it is not the signaling reaction. Supporting Evidence: PMID:17410169 the leucine-rich repeat domain of TIR1 contains an unexpected inositol hexakisphosphate co-factor |
| GO:0005515 protein binding | IPI PMID:15992545 Plant development is regulated by a family of auxin receptor... | MODIFY | Summary: IPI evidence that AFB2 binds the Aux/IAA protein IAA7/AXR2 in an auxin-dependent manner (pull-down from plant extracts). This is the substrate-recognition step of the TIR1/AFB-Aux/IAA co-receptor, i.e. the F-box protein recruiting its Aux/IAA substrate to SCF. Reason: Generic protein binding is uninformative. The auxin-dependent recruitment of an Aux/IAA substrate by an SCF F-box subunit is captured by ubiquitin-like ligase-substrate adaptor activity, which also has IBA support for this gene. Proposed replacements: ubiquitin-like ligase-substrate adaptor activity Supporting Evidence: PMID:15992545 Like TIR1, these proteins interact with the Aux/IAA proteins in an auxin-dependent manner PMID:15992545 all TIR1/AFB proteins interact with BDL, and BDL is stabilized in triple mutant plants |
| GO:0005515 protein binding | IPI PMID:20018756 Complex regulation of the TIR1/AFB family of auxin receptors... | MODIFY | Summary: IPI for AFB2-IAA7 interaction from in vitro binding assays between individual TIR1/AFB and Aux/IAA proteins. Interaction is auxin dependent and reflects co-receptor assembly with its Aux/IAA substrate. Reason: Replace the uninformative protein binding term with the substrate-adaptor activity of the F-box receptor. Proposed replacements: ubiquitin-like ligase-substrate adaptor activity Supporting Evidence: PMID:22466420 IAA12 interacted specifically with TIR1 and AFB2 at 100 ΞΌM IAA |
| GO:0005634 nucleus | IEA GO_REF:0000044 | ACCEPT | Summary: Nuclear localization from UniProt subcellular location mapping. AFB2 is detected in the nucleus, where it acts on nuclear Aux/IAA repressors. Reason: Consistent with UniProt (nucleus, experimentally supported) and with imaging showing AFB2 distributed between nucleus and cytoplasm. Supporting Evidence: PMID:32067636 We find that AFB2 through AFB5 are distributed between the nucleus and the cytoplasm, at least in epidermal cells of the root |
| GO:0005737 cytoplasm | ISM GO_REF:0000122 | ACCEPT | Summary: Cytoplasm predicted by AtSubP. Live imaging of AFB2-mCitrine shows a substantial cytoplasmic pool in addition to the nuclear pool, unlike the predominantly nuclear TIR1. Reason: Prediction is corroborated by experimental imaging (Prigge et al. 2020). Supporting Evidence: PMID:32067636 We find that AFB2 through AFB5 are distributed between the nucleus and the cytoplasm, at least in epidermal cells of the root |
| GO:0009733 response to auxin | IEA GO_REF:0000117 | MODIFY | Summary: AFB2 participates directly in auxin signaling. Reason: Refine the true broad response term to auxin-activated signaling, independently supported by native AFB2 genetics and auxin-dependent Aux/IAA interaction, rather than relying on the ARBA rule as evidence. Proposed replacements: auxin-activated signaling pathway Supporting Evidence: PMID:15992545 Like TIR1, these proteins interact with the Aux/IAA proteins in an auxin-dependent manner |
| GO:0009734 auxin-activated signaling pathway | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic inference that TIR1/AFB family F-box proteins function in the auxin-activated signaling pathway. Reason: AFB2 is an auxin co-receptor; this is its core biological process, consistent with genetic data (tir1 afb2 afb3 mutants) and auxin-dependent Aux/IAA binding. The actual PTHR16134 PAINT IBD table places this assertion at PTN008357904; this receptor/signaling node is scoped to Embryophyta. Supporting Evidence: PMID:15992545 Like TIR1, these proteins interact with the Aux/IAA proteins in an auxin-dependent manner PMID:20018756 TIR1 and AFB2 are the dominant auxin receptors in the seedling root file:interpro/panther/PTHR16134/PTHR16134-paint.tsv |
| GO:0009734 auxin-activated signaling pathway | IMP PMID:20018756 Complex regulation of the TIR1/AFB family of auxin receptors... | ACCEPT | Summary: Mutant analysis (Parry et al. 2009) showing that loss of AFB2 reduces auxin response; TIR1 and AFB2 are the dominant auxin receptors in the seedling root. Reason: Direct genetic evidence for AFB2 participation in auxin signaling. Supporting Evidence: PMID:20018756 TIR1 and AFB2 are the dominant auxin receptors in the seedling root |
| GO:0009734 auxin-activated signaling pathway | ISS GO_REF:0000024 | ACCEPT | Summary: Sequence-similarity transfer of auxin-activated signaling pathway from TIR1. Reason: Consistent with experimental data for AFB2 itself (Dharmasiri et al. 2005; Parry et al. 2009). Supporting Evidence: PMID:15992545 Like TIR1, these proteins interact with the Aux/IAA proteins in an auxin-dependent manner |
| GO:0010011 auxin binding | ISS GO_REF:0000024 | ACCEPT | Summary: Auxin binding transferred from TIR1. Auxin binds in the TIR1/AFB LRR pocket and is enclosed by the Aux/IAA degron; the F-box protein and Aux/IAA together form the high-affinity co-receptor. Reason: Retain the curatorβs TIR1-based inference, reinforced by auxin-dependent AFB2βAux/IAA interactions. PMID22466420 directly assays AFB2 interaction combinations, but its quoted TIR1/IAA7 radioligand affinity is not an AFB2-specific binding constant. Supporting Evidence: PMID:22466420 TIR1 and the Aux/IAA are both necessary and sufficient for auxin binding and act as auxin co-receptors PMID:17410169 auxin enhances the TIR1-substrate interactions by acting as a 'molecular glue' |
| GO:0016567 protein ubiquitination | IEA GO_REF:0000041 | ACCEPT | Summary: Protein ubiquitination via UniPathway mapping. As the substrate receptor of SCF(AFB2), AFB2 participates in the ubiquitination of Aux/IAA proteins. Reason: Correct for an F-box subunit of an SCF E3 ligase; the more specific SCF-dependent proteasomal catabolic process is also annotated. Supporting Evidence: file:ARATH/AFB2/AFB2-uniprot.txt Component of SCF(ASK-cullin-F-box) E3 ubiquitin ligase PMID:15992545 all TIR1/AFB proteins interact with BDL, and BDL is stabilized in triple mutant plants |
| GO:0019005 SCF ubiquitin ligase complex | IBA GO_REF:0000033 | ACCEPT | Summary: AFB2 is the F-box subunit of an SCF(ASK-cullin-F-box) E3 ligase, phylogenetically inferred from the TIR1/AFB clade. Reason: Core complex membership; F-box domain binds ASK/SKP1. The actual PTHR16134 PAINT IBD table places this assertion at PTN009029653; its broader F-box/SCF scope is appropriate for the conserved complex/adaptor contribution, not evidence that every family member is an auxin receptor. Supporting Evidence: file:ARATH/AFB2/AFB2-uniprot.txt Component of SCF(ASK-cullin-F-box) E3 ubiquitin ligase file:interpro/panther/PTHR16134/PTHR16134-paint.tsv |
| GO:0019005 SCF ubiquitin ligase complex | ISS GO_REF:0000024 | ACCEPT | Summary: SCF complex membership transferred from TIR1. Reason: Consistent with the conserved F-box domain and family biochemistry. Supporting Evidence: file:ARATH/AFB2/AFB2-uniprot.txt Component of SCF(ASK-cullin-F-box) E3 ubiquitin ligase |
| GO:0031146 SCF-dependent proteasomal ubiquitin-dependent protein catabolic process | IBA GO_REF:0000033 | ACCEPT | Summary: SCF-dependent proteasomal ubiquitin-dependent protein catabolism, inferred for the TIR1/AFB clade. Loss of TIR1/AFBs stabilizes the Aux/IAA BDL/IAA12. Reason: AFB2 directs SCF-mediated degradation of Aux/IAA repressors; BDL is stabilized in tir1 afb mutants. The actual PTHR16134 PAINT IBD table places this assertion at PTN009029653; its broader F-box/SCF scope is appropriate for the conserved complex/adaptor contribution, not evidence that every family member is an auxin receptor. Supporting Evidence: PMID:15992545 all TIR1/AFB proteins interact with BDL, and BDL is stabilized in triple mutant plants file:interpro/panther/PTHR16134/PTHR16134-paint.tsv |
| GO:0038198 auxin receptor activity | IBA GO_REF:0000033 | ACCEPT | Summary: Auxin receptor activity inferred phylogenetically from TIR1/AFB family members. Reason: Core function: AFB2 is one of the two dominant auxin receptors in seedling roots and forms auxin-dependent co-receptor complexes with Aux/IAAs. The actual PTHR16134 PAINT IBD table places this assertion at PTN008357904; this receptor/signaling node is scoped to Embryophyta. Supporting Evidence: PMID:20018756 TIR1 and AFB2 are the dominant auxin receptors in the seedling root PMID:22466420 IAA12 interacted specifically with TIR1 and AFB2 at 100 ΞΌM IAA file:interpro/panther/PTHR16134/PTHR16134-paint.tsv |
| GO:1990756 ubiquitin-like ligase-substrate adaptor activity | IBA GO_REF:0000033 | ACCEPT | Summary: Ubiquitin-like ligase-substrate adaptor activity: the F-box protein recruits Aux/IAA substrates to SCF in an auxin-dependent manner. Reason: Core molecular function of TIR1/AFB F-box receptors. The actual PTHR16134 PAINT IBD table places this assertion at PTN009029653; its broader F-box/SCF scope is appropriate for the conserved complex/adaptor contribution, not evidence that every family member is an auxin receptor. Supporting Evidence: PMID:15992545 Like TIR1, these proteins interact with the Aux/IAA proteins in an auxin-dependent manner PMID:17410169 auxin enhances the TIR1-substrate interactions by acting as a 'molecular glue' file:interpro/panther/PTHR16134/PTHR16134-paint.tsv |
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Download this section (compressed HTML)Q: Does the substantial cytoplasmic pool of AFB2 (unlike nuclear TIR1) have a distinct function, e.g. in rapid non-transcriptional auxin responses, or is it a reservoir?
Q: Which Aux/IAA partners are preferentially degraded by SCF(AFB2) versus SCF(TIR1) in vivo, given their different auxin-dependent affinities in yeast and in vitro assays?
Q: Does purified, properly expressed AFB2 catalyze cAMP production, and can catalytic mutations preserve Aux/IAA recognition as in TIR1?
Q: Which AFB2 peptides and vacuolar fraction underlie the original PMID17151019 identification, and is the pool reproducible with targeted imaging or fractionation?
Experiment: Compare Aux/IAA degradation kinetics (e.g. ratiometric degron reporters for IAA12/BDL, IAA17, IAA7) in tir1 versus afb2 backgrounds to resolve receptor-specific substrate preferences in planta.
Hypothesis: AFB2 and TIR1 differ in substrate preference and auxin sensitivity, contributing to tissue-specific auxin responses.
Experiment: Express nucleus-restricted versus cytoplasm-restricted AFB2 in tir1 afb2 mutants and score complementation of root growth and rapid root growth inhibition.
Hypothesis: Cytoplasmic AFB2 contributes to fast, non-transcriptional auxin responses.
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