{"gene":"FBXO4","run_date":"2026-06-09T23:54:43","timeline":{"discoveries":[{"year":2006,"finding":"FBX4 (FBXO4) and alphaB-crystallin form an SCF (Skp1-Cul1-F-box) E3 ubiquitin ligase complex that recognizes Thr286-phosphorylated cyclin D1 and catalyzes its polyubiquitination, leading to proteasomal degradation. Purified SCF(FBX4-alphaB crystallin) catalyzed polyubiquitination of cyclin D1 in vitro.","method":"In vitro ubiquitination reconstitution assay, Co-IP, overexpression and knockdown with cyclin D1 turnover readout","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro reconstitution of ubiquitination, Co-IP, and functional validation with overexpression/knockdown in multiple cell lines","pmids":["17081987"],"is_preprint":false},{"year":2002,"finding":"AlphaB-crystallin physically interacts with FBXO4 (FBX4), and this interaction is enhanced by pseudophosphorylation at Ser-19 and Ser-45 of alphaB-crystallin (S19D/S45D). Both pseudophosphorylated alphaB-crystallin and the R120G mutant translocate FBXO4 to the detergent-insoluble fraction and stimulate ubiquitination of target proteins.","method":"Co-IP, phosphomimetic mutagenesis, fractionation assays, ubiquitination assay in cells","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal Co-IP with mutagenesis, replicated by subsequent studies confirming the same complex","pmids":["12468532"],"is_preprint":false},{"year":2005,"finding":"FBXO4 (FBX4/PinX3) interacts with both Pin2 and TRF1 isoforms of Pin2/TRF1, promotes their ubiquitination in vitro and in vivo, and regulates telomere length. Overexpression of Fbx4 reduces endogenous Pin2/TRF1 and causes telomere elongation; RNAi-mediated inhibition stabilizes Pin2/TRF1 and promotes telomere shortening.","method":"Two-hybrid, Co-IP, in vitro and in vivo ubiquitination assay, RNAi knockdown with telomere length measurement","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — in vitro and in vivo ubiquitination, reciprocal interaction validation, RNAi with defined phenotype, multiple orthogonal methods","pmids":["16275645"],"is_preprint":false},{"year":2008,"finding":"GSK3beta phosphorylates FBXO4 in a cell-cycle-dependent manner, triggering FBXO4 dimerization, which is required for SCF(Fbx4) E3 ligase activity toward cyclin D1. Inactivating mutations in Fbx4 found in human cancer impair this dimerization, leading to nuclear cyclin D1 accumulation and oncogenic transformation. The Ras-Akt-GSK3beta pathway regulates this temporal phosphorylation-dimerization event.","method":"Phosphorylation assays, dimerization assays, mutagenesis, cell transformation assays, sequencing of human cancer samples","journal":"Cancer cell","confidence":"High","confidence_rationale":"Tier 2 / Strong — pathway epistasis (Ras-Akt-GSK3beta), mutagenesis, dimerization assays, functional transformation readout, human cancer mutations validated","pmids":["18598945"],"is_preprint":false},{"year":2010,"finding":"Crystal structure of FBXO4 in complex with Skp1 reveals an antiparallel dimer configuration where the linker domain of one FBXO4 protomer interacts with the C-terminal substrate-binding domain of the other. Biochemical studies confirmed that both the N-terminal domain and a loop connecting the linker and C-terminal domain are critical for dimerization and activation of SCF(Fbx4) ubiquitination activity toward Pin2/TRF1.","method":"X-ray crystallography, in vitro ubiquitination assay, mutagenesis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — crystal structure with biochemical mutagenesis validation of dimerization requirement, single lab but multiple orthogonal methods","pmids":["20181953"],"is_preprint":false},{"year":2009,"finding":"Lysine 269 (K269) in cyclin D1 is specifically required for polyubiquitin-mediated proteasomal degradation by SCF(Fbx4/alphaB-crystallin). Mutation K269R stabilizes cyclin D1 and promotes nuclear accumulation and cell transformation, and renders cyclin D1 resistant to genotoxic stress-induced degradation, even though K269R does not prevent cyclin D1 ubiquitination in vivo.","method":"Site-directed mutagenesis, proteasomal degradation assay, in vivo ubiquitination assay, cell transformation assay","journal":"Oncogene","confidence":"High","confidence_rationale":"Tier 1 / Moderate — mutagenesis with in vivo and in vitro functional readouts, dissects ubiquitination from degradation","pmids":["19767775"],"is_preprint":false},{"year":2004,"finding":"Pseudophosphorylation of alphaB-crystallin at Ser-19 and Ser-45 (S19D/S45D) recruits FBXO4 to nuclear SC35 speckles. This co-localization is resistant to detergent, DNase I, and RNase A treatment, indicating stable protein-protein interaction at speckles independent of nucleic acids.","method":"Immunofluorescence, co-transfection, detergent fractionation, nuclease resistance assays","journal":"European journal of biochemistry","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — localization experiments with multiple treatments, but functional consequence of speckle recruitment not fully established","pmids":["15511225"],"is_preprint":false},{"year":2011,"finding":"Loss of Fbxo4 in mouse embryonic fibroblasts results in cyclin D1 stabilization and nuclear accumulation throughout cell division, increased proliferation, DNA damage checkpoint activation, and increased susceptibility to Ras-dependent transformation. Fbxo4 knockout and heterozygous mice develop lymphomas, histiocytic sarcomas, and other tumors with elevated cyclin D1.","method":"Fbxo4 knockout mouse model, MEF isolation and characterization, Ras transformation assay, protein stability assays","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic knockout with defined molecular and cellular phenotype, in vivo tumor model, multiple orthogonal readouts","pmids":["21911473"],"is_preprint":false},{"year":2013,"finding":"FBXO4 deficiency combined with Braf(V600E) induces melanoma in mice, and this phenotype depends on cyclin D1 accumulation. The substrate-binding mutation FBXO4 I377M selectively disrupts cyclin D1 degradation while preserving TRF1 proteolysis, demonstrating that distinct substrate interactions can be genetically separated.","method":"Mouse melanoma model (Braf/Fbxo4 compound mutant), site-directed mutagenesis (I377M), cyclin D1 and TRF1 degradation assays","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic epistasis in animal model, separation-of-function mutagenesis with two substrates, multiple orthogonal methods","pmids":["24019069"],"is_preprint":false},{"year":2017,"finding":"FBXO4 is an E3 ubiquitin ligase for Fxr1 (fragile X protein family member). Purification of SCFFbxo4 complexes identified FMRP, Fxr1, and Fxr2 as binding partners; biochemical analysis confirmed Fxr1 as a direct substrate. Fxr1 overexpression in turn attenuates FBXO4 translation, creating a feedback loop that contributes to Fxr1 overexpression and loss of FBXO4 in head and neck squamous cell carcinoma.","method":"SCFFbxo4 complex purification/MS, Co-IP, in vitro ubiquitination, Fbxo4 knockout cells/tissues, translation assay","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 / Strong — complex purification by MS, biochemical substrate validation, KO phenotype, feedback loop confirmed by translation assay, multiple orthogonal methods","pmids":["29142209"],"is_preprint":false},{"year":2017,"finding":"FBXO4 interacts with and promotes ubiquitination and proteasomal degradation of Mcl-1 in lung cancer cells. Knockdown of FBXO4 elevates Mcl-1 protein levels and increases cell survival and chemotherapy resistance, while ectopic FBXO4 expression promotes Mcl-1 degradation and reduces survival.","method":"Co-IP, ubiquitination assay, knockdown and overexpression with Mcl-1 protein stability and apoptosis readouts","journal":"Cancer gene therapy","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — Co-IP and ubiquitination assay with functional phenotype, single lab, limited mechanistic depth","pmids":["28776569"],"is_preprint":false},{"year":2018,"finding":"FBXO4 interacts with Hsp20 and regulates ubiquitin-dependent degradation of PPARγ in adipocytes. Hsp20 acts as a negative regulator by interacting with FBXO4 and controlling PPARγ stability, linking β-adrenergic signaling to PPARγ activity via FBXO4-mediated ubiquitination.","method":"Co-IP, ubiquitination assay, Hsp20 knockout mice with adipocyte phenotype, PPARγ stability assay","journal":"Cell reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP, ubiquitination assay, and KO mouse model with defined phenotype; single lab","pmids":["29925002"],"is_preprint":false},{"year":2017,"finding":"FBXO4 directly interacts with ICAM-1 (intercellular adhesion molecule-1) via Co-IP and regulates its protein stability through ubiquitin-dependent degradation, influencing tumor progression and metastasis in breast cancer cells.","method":"Co-IP, ICAM-1 stability assay, knockdown/overexpression with tumor invasion phenotype","journal":"Oncotarget","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single Co-IP with stability assay, single lab, limited mechanistic follow-up","pmids":["29137327"],"is_preprint":false},{"year":2014,"finding":"Alternative splicing variants of Fbx4 (Fbx4β, Fbx4γ, Fbx4δ) are more abundant in human cancer compared to normal tissues. Unlike full-length Fbx4α (cytoplasmic), these splice variants localize to both cytoplasm and nucleus and disrupt cyclin D1 degradation, promoting cell proliferation and migration.","method":"RT-PCR, cloning/sequencing, subcellular fractionation/immunofluorescence, cyclin D1 stability assay, proliferation/migration assay","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — localization experiments and functional assays with multiple splice variants, single lab","pmids":["24704453"],"is_preprint":false},{"year":2021,"finding":"FBX4 mediates rapid cyclin D1 proteasomal degradation in response to DNA damage (γ-irradiation) in immortalized esophageal epithelial cells, in an ATM-dependent and p53-independent manner. FBX4 knockdown reversed cyclin D1 turnover and increased DNA breaks (γ-H2AX foci).","method":"FBX4 siRNA knockdown, MG132 proteasome inhibition, ATM inhibition, cycloheximide chase, γ-H2AX foci assay","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — genetic epistasis (ATM-FBX4) with defined cyclin D1 degradation and DNA damage phenotype, single lab","pmids":["33784509"],"is_preprint":false},{"year":2024,"finding":"Cryo-EM structure of the CUL1-RBX1-SKP1-FBXO4 complex (CRL1FBXO4) reveals that FBXO4 adopts a homodimer (domain-swapped dimer) architecture within the complex. FBXO4 interacts with SKP1 via hydrophobic and electrostatic interactions and also contacts CUL1 directly.","method":"Cryo-EM structural determination","journal":"Biochemical and biophysical research communications","confidence":"High","confidence_rationale":"Tier 1 / Moderate — cryo-EM structure with architectural analysis of the full complex, single lab but high-resolution structural data","pmids":["39406020"],"is_preprint":false},{"year":2024,"finding":"FBXO4 targets HPV-18 E6 oncoprotein for degradation in the absence of E6AP. Knockdown of FBXO4 (combined with E6AP knockdown) dramatically increases endogenous HPV-18 E6 levels, and the combined knockdown induces p53-dependent cell death in HPV-positive cervical cancer cells.","method":"siRNA library screen, knockdown validation, Western blot for E6 protein stability, p53-dependent cell death assay","journal":"mBio","confidence":"Medium","confidence_rationale":"Tier 3 / Weak — siRNA screen with follow-up knockdown and protein stability readout, single lab, no direct ubiquitination assay reported in abstract","pmids":["39688415"],"is_preprint":false},{"year":2025,"finding":"FBXO4 ubiquitinates and degrades β-catenin in muscle cells, thereby inhibiting the Wnt/β-catenin signaling pathway and suppressing muscle cell differentiation. FBXO4 expression is induced by IFNα/JAK/STAT1 signaling, with FBXO4 identified as a direct transcriptional target of STAT1.","method":"Co-IP, ubiquitination assay, FBXO4 knockout in muscle cells with RNA-seq, dual-luciferase reporter assay for STAT1-FBXO4 promoter regulation","journal":"Journal of inflammation research","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — Co-IP and ubiquitination assay with KO phenotype and transcription reporter, single lab, limited replication","pmids":["40599691"],"is_preprint":false}],"current_model":"FBXO4 (FBX4) is the substrate-recognition F-box subunit of an SCF (SKP1-CUL1-RBX1-FBXO4) E3 ubiquitin ligase that forms a GSK3β-regulated, cell-cycle-dependent homodimer (visualized by crystal structure and cryo-EM) and, in cooperation with alphaB-crystallin, binds Thr286-phosphorylated cyclin D1 at Lys269 to catalyze its polyubiquitination and proteasomal degradation; it also targets additional substrates including Pin2/TRF1 (regulating telomere length), Fxr1, Mcl-1, PPARγ, ICAM-1, β-catenin, and HPV E6, placing FBXO4 as a broad-spectrum tumor-suppressive E3 ligase whose dimerization, phosphorylation by GSK3β via the Ras-Akt axis, and interaction with co-adaptors like alphaB-crystallin govern its activity and substrate specificity."},"narrative":{"mechanistic_narrative":"FBXO4 (FBX4) is the substrate-recognition F-box subunit of an SCF (SKP1-CUL1-RBX1-FBXO4) E3 ubiquitin ligase that functions as a tumor suppressor by targeting growth-promoting proteins for proteasomal degradation [PMID:17081987, PMID:21911473]. Its best-defined substrate is Thr286-phosphorylated cyclin D1, which FBXO4 binds in cooperation with the co-adaptor alphaB-crystallin and polyubiquitinates at Lys269 to drive proteasomal turnover [PMID:17081987, PMID:19767775]; loss of this activity stabilizes nuclear cyclin D1 and, in knockout mice, produces lymphomas, sarcomas, and Ras- or Braf-driven tumors [PMID:21911473, PMID:24019069]. Catalytic activity requires cell-cycle-dependent FBXO4 dimerization, which is triggered by GSK3β phosphorylation downstream of the Ras-Akt axis and is structurally organized as an antiparallel domain-swapped homodimer in which the linker of one protomer engages the C-terminal substrate-binding domain of the other within the CRL1 complex [PMID:18598945, PMID:20181953, PMID:39406020]. Beyond cyclin D1, FBXO4 ubiquitinates a range of additional substrates—Pin2/TRF1 to control telomere length, Fxr1, Mcl-1, PPARγ, and β-catenin—and separation-of-function mutations such as I377M genetically uncouple cyclin D1 from TRF1 targeting, indicating substrate-specific recognition surfaces [PMID:16275645, PMID:24019069, PMID:29142209, PMID:28776569, PMID:29925002, PMID:40599691]. FBXO4-mediated cyclin D1 degradation is also engaged as a genotoxic-stress response in an ATM-dependent, p53-independent manner [PMID:33784509].","teleology":[{"year":2002,"claim":"Established that FBXO4 does not act alone but partners with alphaB-crystallin, whose phosphorylation state controls FBXO4 localization and ligase output—defining a co-adaptor mechanism.","evidence":"Co-IP with phosphomimetic mutagenesis and cellular ubiquitination/fractionation assays","pmids":["12468532"],"confidence":"High","gaps":["Did not identify the physiological substrate of the complex","Mechanism by which phosphorylated alphaB-crystallin redirects FBXO4 not resolved"]},{"year":2004,"claim":"Mapped the alphaB-crystallin-driven recruitment of FBXO4 to nuclear SC35 speckles, localizing the complex to a defined subnuclear compartment.","evidence":"Immunofluorescence co-transfection with detergent and nuclease resistance assays","pmids":["15511225"],"confidence":"Medium","gaps":["Functional consequence of speckle recruitment not established","Whether substrate degradation occurs at speckles unknown"]},{"year":2005,"claim":"Identified the first FBXO4 substrate, Pin2/TRF1, linking the ligase to telomere length homeostasis.","evidence":"Two-hybrid, Co-IP, in vitro/in vivo ubiquitination, and RNAi with telomere length measurement","pmids":["16275645"],"confidence":"High","gaps":["Did not address dimerization or regulatory inputs","Relationship to other substrates unknown at the time"]},{"year":2006,"claim":"Reconstituted SCF(FBX4-alphaB crystallin) as a bona fide E3 ligase for Thr286-phosphorylated cyclin D1, establishing the central tumor-suppressive substrate.","evidence":"In vitro ubiquitination reconstitution, Co-IP, and overexpression/knockdown with cyclin D1 turnover","pmids":["17081987"],"confidence":"High","gaps":["How ligase activity is temporally regulated not addressed","Acceptor lysine on cyclin D1 not yet defined"]},{"year":2008,"claim":"Revealed that GSK3β phosphorylation drives cell-cycle-dependent FBXO4 dimerization required for ligase activity, and that cancer-associated mutations abolishing dimerization cause oncogenic cyclin D1 accumulation.","evidence":"Phosphorylation/dimerization assays, mutagenesis, transformation assays, and human cancer sequencing","pmids":["18598945"],"confidence":"High","gaps":["Structural basis of dimerization not yet visualized","Direct GSK3β phosphosite occupancy not fully mapped"]},{"year":2009,"claim":"Identified Lys269 of cyclin D1 as the residue required for degradation, dissecting ubiquitination from proteolytic targeting.","evidence":"Site-directed mutagenesis with in vivo ubiquitination and degradation/transformation assays","pmids":["19767775"],"confidence":"High","gaps":["K269R still permits ubiquitination, so the degradation-specific signal is unexplained","How K269-linked chains differ from non-degradative chains unknown"]},{"year":2010,"claim":"Provided the crystal structure of FBXO4-SKP1 showing an antiparallel dimer and defined the domains essential for dimerization-dependent activity.","evidence":"X-ray crystallography with in vitro ubiquitination and mutagenesis","pmids":["20181953"],"confidence":"High","gaps":["Structure lacked CUL1/RBX1 and substrate","Did not capture the GSK3β-phosphorylated state"]},{"year":2011,"claim":"Demonstrated in vivo that Fbxo4 loss stabilizes nuclear cyclin D1, drives proliferation and transformation, and produces tumors, validating FBXO4 as a tumor suppressor.","evidence":"Fbxo4 knockout mouse, MEF characterization, Ras transformation, and stability assays","pmids":["21911473"],"confidence":"High","gaps":["Contribution of non-cyclin-D1 substrates to tumorigenesis not isolated","Tissue-specific requirements not dissected"]},{"year":2013,"claim":"Showed substrate targeting is genetically separable—I377M disrupts cyclin D1 but not TRF1 degradation—and that cyclin D1 accumulation mediates FBXO4-deficient melanoma.","evidence":"Braf/Fbxo4 compound mouse model and separation-of-function mutagenesis with dual-substrate assays","pmids":["24019069"],"confidence":"High","gaps":["Structural determinants distinguishing substrate surfaces not mapped","Generalizability of separation-of-function to other substrates unknown"]},{"year":2017,"claim":"Expanded the FBXO4 substrate repertoire to Fxr1 and Mcl-1, linking the ligase to a translational feedback loop and to apoptotic/chemoresistance control.","evidence":"SCFFbxo4 complex purification/MS, Co-IP, in vitro ubiquitination, KO cells, and translation/apoptosis readouts","pmids":["29142209","28776569"],"confidence":"High","gaps":["Mcl-1 targeting rests on a single lab without structural validation","Whether dimerization/GSK3β regulation applies to these substrates not tested"]},{"year":2018,"claim":"Connected FBXO4 to metabolic control by showing it degrades PPARγ under Hsp20 negative regulation, extending the ligase beyond proliferation.","evidence":"Co-IP, ubiquitination assay, and Hsp20 knockout mice with adipocyte phenotype","pmids":["29925002"],"confidence":"Medium","gaps":["Single-lab evidence","Direct PPARγ acceptor lysine and chain topology not defined"]},{"year":2021,"claim":"Placed FBXO4-mediated cyclin D1 degradation within the DNA damage response as an ATM-dependent, p53-independent event.","evidence":"siRNA knockdown, proteasome/ATM inhibition, cycloheximide chase, and γ-H2AX foci assay","pmids":["33784509"],"confidence":"Medium","gaps":["How ATM signals to FBXO4 mechanistically unknown","Single-lab, single cell system"]},{"year":2024,"claim":"Resolved the full CRL1FBXO4 architecture by cryo-EM, confirming a domain-swapped homodimer engaging SKP1 and contacting CUL1, and added HPV-18 E6 as an E6AP-independent target.","evidence":"Cryo-EM structural determination; and siRNA screen with E6 stability and p53-dependent death assays","pmids":["39406020","39688415"],"confidence":"High","gaps":["Cryo-EM lacked bound substrate","HPV E6 targeting lacked a direct ubiquitination assay"]},{"year":2025,"claim":"Identified FBXO4 as a STAT1-induced regulator that degrades β-catenin to suppress muscle differentiation, embedding the ligase in IFNα/JAK/STAT-Wnt signaling.","evidence":"Co-IP, ubiquitination assay, KO muscle cells with RNA-seq, and STAT1-FBXO4 promoter luciferase reporter","pmids":["40599691"],"confidence":"Medium","gaps":["Single-lab evidence","Whether β-catenin targeting requires alphaB-crystallin or dimerization not tested"]},{"year":null,"claim":"How distinct co-adaptors and substrate surfaces select among FBXO4's many substrates, and whether GSK3β-dependent dimerization regulates all of them, remains unresolved.","evidence":"No substrate-bound CRL1FBXO4 structure or unified regulatory model across substrates in the corpus","pmids":[],"confidence":"Low","gaps":["No structure of FBXO4 with any substrate bound","Co-adaptor requirement for non-cyclin-D1 substrates unknown","Whether phosphorylation-dependent dimerization gates all targets untested"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[0,2,9]},{"term_id":"GO:0016874","term_label":"ligase activity","supporting_discovery_ids":[0,7]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[0,8]}],"localization":[{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[13]},{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[6,13]}],"pathway":[{"term_id":"R-HSA-1640170","term_label":"Cell Cycle","supporting_discovery_ids":[0,3,7]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[0,9]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[7,8]}],"complexes":["SCF (SKP1-CUL1-RBX1-FBXO4) / CRL1FBXO4"],"partners":["SKP1","CUL1","RBX1","CRYAB","FXR1","TRF1"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q9UKT5","full_name":"F-box only protein 4","aliases":[],"length_aa":387,"mass_kda":44.1,"function":"Substrate recognition component of a SCF (SKP1-CUL1-F-box protein) E3 ubiquitin-protein ligase complex that mediates the ubiquitination and subsequent proteasomal degradation of target proteins (PubMed:10531035, PubMed:18598945, PubMed:20181953, PubMed:29142209). Promotes ubiquitination of cyclin-D1 (CCND1) and its subsequent proteasomal degradation (PubMed:18598945). However, it does not act as a major regulator of CCND1 stability during the G1/S transition (By similarity). Recognizes TERF1 and promotes its ubiquitination together with UBE2D1 (PubMed:16275645, PubMed:20159592). Promotes ubiquitination of FXR1 following phosphorylation of FXR1 by GSK3B, leading to FXR1 degradation by the proteasome (PubMed:29142209)","subcellular_location":"Cytoplasm","url":"https://www.uniprot.org/uniprotkb/Q9UKT5/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/FBXO4","classification":"Not Classified","n_dependent_lines":1,"n_total_lines":1208,"dependency_fraction":0.0008278145695364238},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/FBXO4","total_profiled":1310},"omim":[{"mim_id":"609090","title":"F-BOX ONLY PROTEIN 4; FBXO4","url":"https://www.omim.org/entry/609090"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Nucleoplasm","reliability":"Approved"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/FBXO4"},"hgnc":{"alias_symbol":["FBX4"],"prev_symbol":[]},"alphafold":{"accession":"Q9UKT5","domains":[{"cath_id":"1.20.1280.50","chopping":"59-154","consensus_level":"medium","plddt":82.1116,"start":59,"end":154},{"cath_id":"3.40.50.300","chopping":"179-262_272-387","consensus_level":"high","plddt":88.1275,"start":179,"end":387}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9UKT5","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9UKT5-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9UKT5-F1-predicted_aligned_error_v6.png","plddt_mean":78.38},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=FBXO4","jax_strain_url":"https://www.jax.org/strain/search?query=FBXO4"},"sequence":{"accession":"Q9UKT5","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9UKT5.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9UKT5/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9UKT5"}},"corpus_meta":[{"pmid":"17081987","id":"PMC_17081987","title":"Phosphorylation-dependent ubiquitination of cyclin D1 by the SCF(FBX4-alphaB crystallin) complex.","date":"2006","source":"Molecular cell","url":"https://pubmed.ncbi.nlm.nih.gov/17081987","citation_count":315,"is_preprint":false},{"pmid":"18598945","id":"PMC_18598945","title":"Mutations in Fbx4 inhibit dimerization of the SCF(Fbx4) ligase and contribute to cyclin D1 overexpression in human cancer.","date":"2008","source":"Cancer cell","url":"https://pubmed.ncbi.nlm.nih.gov/18598945","citation_count":131,"is_preprint":false},{"pmid":"12468532","id":"PMC_12468532","title":"The small heat-shock protein alpha B-crystallin promotes FBX4-dependent ubiquitination.","date":"2002","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/12468532","citation_count":110,"is_preprint":false},{"pmid":"16275645","id":"PMC_16275645","title":"The F-box protein FBX4 targets PIN2/TRF1 for ubiquitin-mediated degradation and regulates telomere maintenance.","date":"2005","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/16275645","citation_count":89,"is_preprint":false},{"pmid":"15511225","id":"PMC_15511225","title":"Mimicking phosphorylation of the small heat-shock protein alphaB-crystallin recruits the F-box protein FBX4 to nuclear SC35 speckles.","date":"2004","source":"European journal of biochemistry","url":"https://pubmed.ncbi.nlm.nih.gov/15511225","citation_count":64,"is_preprint":false},{"pmid":"29142209","id":"PMC_29142209","title":"Fbxo4-mediated degradation of Fxr1 suppresses tumorigenesis in head and neck squamous cell carcinoma.","date":"2017","source":"Nature communications","url":"https://pubmed.ncbi.nlm.nih.gov/29142209","citation_count":58,"is_preprint":false},{"pmid":"21911473","id":"PMC_21911473","title":"The Fbx4 tumor suppressor regulates cyclin D1 accumulation and prevents neoplastic transformation.","date":"2011","source":"Molecular and cellular biology","url":"https://pubmed.ncbi.nlm.nih.gov/21911473","citation_count":43,"is_preprint":false},{"pmid":"20181953","id":"PMC_20181953","title":"Structural basis of dimerization-dependent ubiquitination by the SCF(Fbx4) ubiquitin ligase.","date":"2010","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/20181953","citation_count":36,"is_preprint":false},{"pmid":"30746571","id":"PMC_30746571","title":"The RNA-binding protein FXR1 modulates prostate cancer progression by regulating FBXO4.","date":"2019","source":"Functional & integrative genomics","url":"https://pubmed.ncbi.nlm.nih.gov/30746571","citation_count":34,"is_preprint":false},{"pmid":"18818515","id":"PMC_18818515","title":"SCF(Fbx4/alphaB-crystallin) E3 ligase: when one is not enough.","date":"2008","source":"Cell cycle (Georgetown, Tex.)","url":"https://pubmed.ncbi.nlm.nih.gov/18818515","citation_count":33,"is_preprint":false},{"pmid":"28776569","id":"PMC_28776569","title":"FBXO4 inhibits lung cancer cell survival by targeting Mcl-1 for degradation.","date":"2017","source":"Cancer gene therapy","url":"https://pubmed.ncbi.nlm.nih.gov/28776569","citation_count":33,"is_preprint":false},{"pmid":"29925002","id":"PMC_29925002","title":"An Hsp20-FBXO4 Axis Regulates Adipocyte Function through Modulating PPARγ Ubiquitination.","date":"2018","source":"Cell reports","url":"https://pubmed.ncbi.nlm.nih.gov/29925002","citation_count":31,"is_preprint":false},{"pmid":"24019069","id":"PMC_24019069","title":"The FBXO4 tumor suppressor functions as a barrier to BRAFV600E-dependent metastatic melanoma.","date":"2013","source":"Molecular and cellular biology","url":"https://pubmed.ncbi.nlm.nih.gov/24019069","citation_count":31,"is_preprint":false},{"pmid":"19767775","id":"PMC_19767775","title":"Lysine 269 is essential for cyclin D1 ubiquitylation by the SCF(Fbx4/alphaB-crystallin) ligase and subsequent proteasome-dependent degradation.","date":"2009","source":"Oncogene","url":"https://pubmed.ncbi.nlm.nih.gov/19767775","citation_count":30,"is_preprint":false},{"pmid":"17224055","id":"PMC_17224055","title":"SCF Fbx4/alphaB-crystallin cyclin D1 ubiquitin ligase: a license to destroy.","date":"2007","source":"Cell division","url":"https://pubmed.ncbi.nlm.nih.gov/17224055","citation_count":25,"is_preprint":false},{"pmid":"25801820","id":"PMC_25801820","title":"FBXO4 loss facilitates carcinogen induced papilloma development in mice.","date":"2015","source":"Cancer biology & therapy","url":"https://pubmed.ncbi.nlm.nih.gov/25801820","citation_count":24,"is_preprint":false},{"pmid":"35272674","id":"PMC_35272674","title":"RUNX3-mediated circDYRK1A inhibits glutamine metabolism in gastric cancer by up-regulating microRNA-889-3p-dependent FBXO4.","date":"2022","source":"Journal of translational 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connections","url":"https://pubmed.ncbi.nlm.nih.gov/33617468","citation_count":0,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":14264,"output_tokens":4667,"usd":0.056398,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":12718,"output_tokens":4272,"usd":0.085195,"stage2_stop_reason":"end_turn"},"total_usd":0.141593,"stage1_batch_id":"msgbatch_01AsR7g8dGTEiEw9U83bJeWn","stage2_batch_id":"msgbatch_01YV5ZyxtFNUZEjxRcpd9jzb","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2006,\n      \"finding\": \"FBX4 (FBXO4) and alphaB-crystallin form an SCF (Skp1-Cul1-F-box) E3 ubiquitin ligase complex that recognizes Thr286-phosphorylated cyclin D1 and catalyzes its polyubiquitination, leading to proteasomal degradation. Purified SCF(FBX4-alphaB crystallin) catalyzed polyubiquitination of cyclin D1 in vitro.\",\n      \"method\": \"In vitro ubiquitination reconstitution assay, Co-IP, overexpression and knockdown with cyclin D1 turnover readout\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro reconstitution of ubiquitination, Co-IP, and functional validation with overexpression/knockdown in multiple cell lines\",\n      \"pmids\": [\"17081987\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"AlphaB-crystallin physically interacts with FBXO4 (FBX4), and this interaction is enhanced by pseudophosphorylation at Ser-19 and Ser-45 of alphaB-crystallin (S19D/S45D). Both pseudophosphorylated alphaB-crystallin and the R120G mutant translocate FBXO4 to the detergent-insoluble fraction and stimulate ubiquitination of target proteins.\",\n      \"method\": \"Co-IP, phosphomimetic mutagenesis, fractionation assays, ubiquitination assay in cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal Co-IP with mutagenesis, replicated by subsequent studies confirming the same complex\",\n      \"pmids\": [\"12468532\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"FBXO4 (FBX4/PinX3) interacts with both Pin2 and TRF1 isoforms of Pin2/TRF1, promotes their ubiquitination in vitro and in vivo, and regulates telomere length. Overexpression of Fbx4 reduces endogenous Pin2/TRF1 and causes telomere elongation; RNAi-mediated inhibition stabilizes Pin2/TRF1 and promotes telomere shortening.\",\n      \"method\": \"Two-hybrid, Co-IP, in vitro and in vivo ubiquitination assay, RNAi knockdown with telomere length measurement\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — in vitro and in vivo ubiquitination, reciprocal interaction validation, RNAi with defined phenotype, multiple orthogonal methods\",\n      \"pmids\": [\"16275645\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"GSK3beta phosphorylates FBXO4 in a cell-cycle-dependent manner, triggering FBXO4 dimerization, which is required for SCF(Fbx4) E3 ligase activity toward cyclin D1. Inactivating mutations in Fbx4 found in human cancer impair this dimerization, leading to nuclear cyclin D1 accumulation and oncogenic transformation. The Ras-Akt-GSK3beta pathway regulates this temporal phosphorylation-dimerization event.\",\n      \"method\": \"Phosphorylation assays, dimerization assays, mutagenesis, cell transformation assays, sequencing of human cancer samples\",\n      \"journal\": \"Cancer cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — pathway epistasis (Ras-Akt-GSK3beta), mutagenesis, dimerization assays, functional transformation readout, human cancer mutations validated\",\n      \"pmids\": [\"18598945\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Crystal structure of FBXO4 in complex with Skp1 reveals an antiparallel dimer configuration where the linker domain of one FBXO4 protomer interacts with the C-terminal substrate-binding domain of the other. Biochemical studies confirmed that both the N-terminal domain and a loop connecting the linker and C-terminal domain are critical for dimerization and activation of SCF(Fbx4) ubiquitination activity toward Pin2/TRF1.\",\n      \"method\": \"X-ray crystallography, in vitro ubiquitination assay, mutagenesis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — crystal structure with biochemical mutagenesis validation of dimerization requirement, single lab but multiple orthogonal methods\",\n      \"pmids\": [\"20181953\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Lysine 269 (K269) in cyclin D1 is specifically required for polyubiquitin-mediated proteasomal degradation by SCF(Fbx4/alphaB-crystallin). Mutation K269R stabilizes cyclin D1 and promotes nuclear accumulation and cell transformation, and renders cyclin D1 resistant to genotoxic stress-induced degradation, even though K269R does not prevent cyclin D1 ubiquitination in vivo.\",\n      \"method\": \"Site-directed mutagenesis, proteasomal degradation assay, in vivo ubiquitination assay, cell transformation assay\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — mutagenesis with in vivo and in vitro functional readouts, dissects ubiquitination from degradation\",\n      \"pmids\": [\"19767775\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Pseudophosphorylation of alphaB-crystallin at Ser-19 and Ser-45 (S19D/S45D) recruits FBXO4 to nuclear SC35 speckles. This co-localization is resistant to detergent, DNase I, and RNase A treatment, indicating stable protein-protein interaction at speckles independent of nucleic acids.\",\n      \"method\": \"Immunofluorescence, co-transfection, detergent fractionation, nuclease resistance assays\",\n      \"journal\": \"European journal of biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — localization experiments with multiple treatments, but functional consequence of speckle recruitment not fully established\",\n      \"pmids\": [\"15511225\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Loss of Fbxo4 in mouse embryonic fibroblasts results in cyclin D1 stabilization and nuclear accumulation throughout cell division, increased proliferation, DNA damage checkpoint activation, and increased susceptibility to Ras-dependent transformation. Fbxo4 knockout and heterozygous mice develop lymphomas, histiocytic sarcomas, and other tumors with elevated cyclin D1.\",\n      \"method\": \"Fbxo4 knockout mouse model, MEF isolation and characterization, Ras transformation assay, protein stability assays\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic knockout with defined molecular and cellular phenotype, in vivo tumor model, multiple orthogonal readouts\",\n      \"pmids\": [\"21911473\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"FBXO4 deficiency combined with Braf(V600E) induces melanoma in mice, and this phenotype depends on cyclin D1 accumulation. The substrate-binding mutation FBXO4 I377M selectively disrupts cyclin D1 degradation while preserving TRF1 proteolysis, demonstrating that distinct substrate interactions can be genetically separated.\",\n      \"method\": \"Mouse melanoma model (Braf/Fbxo4 compound mutant), site-directed mutagenesis (I377M), cyclin D1 and TRF1 degradation assays\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic epistasis in animal model, separation-of-function mutagenesis with two substrates, multiple orthogonal methods\",\n      \"pmids\": [\"24019069\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"FBXO4 is an E3 ubiquitin ligase for Fxr1 (fragile X protein family member). Purification of SCFFbxo4 complexes identified FMRP, Fxr1, and Fxr2 as binding partners; biochemical analysis confirmed Fxr1 as a direct substrate. Fxr1 overexpression in turn attenuates FBXO4 translation, creating a feedback loop that contributes to Fxr1 overexpression and loss of FBXO4 in head and neck squamous cell carcinoma.\",\n      \"method\": \"SCFFbxo4 complex purification/MS, Co-IP, in vitro ubiquitination, Fbxo4 knockout cells/tissues, translation assay\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — complex purification by MS, biochemical substrate validation, KO phenotype, feedback loop confirmed by translation assay, multiple orthogonal methods\",\n      \"pmids\": [\"29142209\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"FBXO4 interacts with and promotes ubiquitination and proteasomal degradation of Mcl-1 in lung cancer cells. Knockdown of FBXO4 elevates Mcl-1 protein levels and increases cell survival and chemotherapy resistance, while ectopic FBXO4 expression promotes Mcl-1 degradation and reduces survival.\",\n      \"method\": \"Co-IP, ubiquitination assay, knockdown and overexpression with Mcl-1 protein stability and apoptosis readouts\",\n      \"journal\": \"Cancer gene therapy\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — Co-IP and ubiquitination assay with functional phenotype, single lab, limited mechanistic depth\",\n      \"pmids\": [\"28776569\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"FBXO4 interacts with Hsp20 and regulates ubiquitin-dependent degradation of PPARγ in adipocytes. Hsp20 acts as a negative regulator by interacting with FBXO4 and controlling PPARγ stability, linking β-adrenergic signaling to PPARγ activity via FBXO4-mediated ubiquitination.\",\n      \"method\": \"Co-IP, ubiquitination assay, Hsp20 knockout mice with adipocyte phenotype, PPARγ stability assay\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP, ubiquitination assay, and KO mouse model with defined phenotype; single lab\",\n      \"pmids\": [\"29925002\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"FBXO4 directly interacts with ICAM-1 (intercellular adhesion molecule-1) via Co-IP and regulates its protein stability through ubiquitin-dependent degradation, influencing tumor progression and metastasis in breast cancer cells.\",\n      \"method\": \"Co-IP, ICAM-1 stability assay, knockdown/overexpression with tumor invasion phenotype\",\n      \"journal\": \"Oncotarget\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single Co-IP with stability assay, single lab, limited mechanistic follow-up\",\n      \"pmids\": [\"29137327\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Alternative splicing variants of Fbx4 (Fbx4β, Fbx4γ, Fbx4δ) are more abundant in human cancer compared to normal tissues. Unlike full-length Fbx4α (cytoplasmic), these splice variants localize to both cytoplasm and nucleus and disrupt cyclin D1 degradation, promoting cell proliferation and migration.\",\n      \"method\": \"RT-PCR, cloning/sequencing, subcellular fractionation/immunofluorescence, cyclin D1 stability assay, proliferation/migration assay\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — localization experiments and functional assays with multiple splice variants, single lab\",\n      \"pmids\": [\"24704453\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"FBX4 mediates rapid cyclin D1 proteasomal degradation in response to DNA damage (γ-irradiation) in immortalized esophageal epithelial cells, in an ATM-dependent and p53-independent manner. FBX4 knockdown reversed cyclin D1 turnover and increased DNA breaks (γ-H2AX foci).\",\n      \"method\": \"FBX4 siRNA knockdown, MG132 proteasome inhibition, ATM inhibition, cycloheximide chase, γ-H2AX foci assay\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — genetic epistasis (ATM-FBX4) with defined cyclin D1 degradation and DNA damage phenotype, single lab\",\n      \"pmids\": [\"33784509\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Cryo-EM structure of the CUL1-RBX1-SKP1-FBXO4 complex (CRL1FBXO4) reveals that FBXO4 adopts a homodimer (domain-swapped dimer) architecture within the complex. FBXO4 interacts with SKP1 via hydrophobic and electrostatic interactions and also contacts CUL1 directly.\",\n      \"method\": \"Cryo-EM structural determination\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — cryo-EM structure with architectural analysis of the full complex, single lab but high-resolution structural data\",\n      \"pmids\": [\"39406020\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"FBXO4 targets HPV-18 E6 oncoprotein for degradation in the absence of E6AP. Knockdown of FBXO4 (combined with E6AP knockdown) dramatically increases endogenous HPV-18 E6 levels, and the combined knockdown induces p53-dependent cell death in HPV-positive cervical cancer cells.\",\n      \"method\": \"siRNA library screen, knockdown validation, Western blot for E6 protein stability, p53-dependent cell death assay\",\n      \"journal\": \"mBio\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Weak — siRNA screen with follow-up knockdown and protein stability readout, single lab, no direct ubiquitination assay reported in abstract\",\n      \"pmids\": [\"39688415\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"FBXO4 ubiquitinates and degrades β-catenin in muscle cells, thereby inhibiting the Wnt/β-catenin signaling pathway and suppressing muscle cell differentiation. FBXO4 expression is induced by IFNα/JAK/STAT1 signaling, with FBXO4 identified as a direct transcriptional target of STAT1.\",\n      \"method\": \"Co-IP, ubiquitination assay, FBXO4 knockout in muscle cells with RNA-seq, dual-luciferase reporter assay for STAT1-FBXO4 promoter regulation\",\n      \"journal\": \"Journal of inflammation research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — Co-IP and ubiquitination assay with KO phenotype and transcription reporter, single lab, limited replication\",\n      \"pmids\": [\"40599691\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"FBXO4 (FBX4) is the substrate-recognition F-box subunit of an SCF (SKP1-CUL1-RBX1-FBXO4) E3 ubiquitin ligase that forms a GSK3β-regulated, cell-cycle-dependent homodimer (visualized by crystal structure and cryo-EM) and, in cooperation with alphaB-crystallin, binds Thr286-phosphorylated cyclin D1 at Lys269 to catalyze its polyubiquitination and proteasomal degradation; it also targets additional substrates including Pin2/TRF1 (regulating telomere length), Fxr1, Mcl-1, PPARγ, ICAM-1, β-catenin, and HPV E6, placing FBXO4 as a broad-spectrum tumor-suppressive E3 ligase whose dimerization, phosphorylation by GSK3β via the Ras-Akt axis, and interaction with co-adaptors like alphaB-crystallin govern its activity and substrate specificity.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"FBXO4 (FBX4) is the substrate-recognition F-box subunit of an SCF (SKP1-CUL1-RBX1-FBXO4) E3 ubiquitin ligase that functions as a tumor suppressor by targeting growth-promoting proteins for proteasomal degradation [#0, #7]. Its best-defined substrate is Thr286-phosphorylated cyclin D1, which FBXO4 binds in cooperation with the co-adaptor alphaB-crystallin and polyubiquitinates at Lys269 to drive proteasomal turnover [#0, #5]; loss of this activity stabilizes nuclear cyclin D1 and, in knockout mice, produces lymphomas, sarcomas, and Ras- or Braf-driven tumors [#7, #8]. Catalytic activity requires cell-cycle-dependent FBXO4 dimerization, which is triggered by GSK3β phosphorylation downstream of the Ras-Akt axis and is structurally organized as an antiparallel domain-swapped homodimer in which the linker of one protomer engages the C-terminal substrate-binding domain of the other within the CRL1 complex [#3, #4, #15]. Beyond cyclin D1, FBXO4 ubiquitinates a range of additional substrates—Pin2/TRF1 to control telomere length, Fxr1, Mcl-1, PPARγ, and β-catenin—and separation-of-function mutations such as I377M genetically uncouple cyclin D1 from TRF1 targeting, indicating substrate-specific recognition surfaces [#2, #8, #9, #10, #11, #17]. FBXO4-mediated cyclin D1 degradation is also engaged as a genotoxic-stress response in an ATM-dependent, p53-independent manner [#14].\",\n  \"teleology\": [\n    {\n      \"year\": 2002,\n      \"claim\": \"Established that FBXO4 does not act alone but partners with alphaB-crystallin, whose phosphorylation state controls FBXO4 localization and ligase output—defining a co-adaptor mechanism.\",\n      \"evidence\": \"Co-IP with phosphomimetic mutagenesis and cellular ubiquitination/fractionation assays\",\n      \"pmids\": [\"12468532\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not identify the physiological substrate of the complex\", \"Mechanism by which phosphorylated alphaB-crystallin redirects FBXO4 not resolved\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Mapped the alphaB-crystallin-driven recruitment of FBXO4 to nuclear SC35 speckles, localizing the complex to a defined subnuclear compartment.\",\n      \"evidence\": \"Immunofluorescence co-transfection with detergent and nuclease resistance assays\",\n      \"pmids\": [\"15511225\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional consequence of speckle recruitment not established\", \"Whether substrate degradation occurs at speckles unknown\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Identified the first FBXO4 substrate, Pin2/TRF1, linking the ligase to telomere length homeostasis.\",\n      \"evidence\": \"Two-hybrid, Co-IP, in vitro/in vivo ubiquitination, and RNAi with telomere length measurement\",\n      \"pmids\": [\"16275645\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not address dimerization or regulatory inputs\", \"Relationship to other substrates unknown at the time\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Reconstituted SCF(FBX4-alphaB crystallin) as a bona fide E3 ligase for Thr286-phosphorylated cyclin D1, establishing the central tumor-suppressive substrate.\",\n      \"evidence\": \"In vitro ubiquitination reconstitution, Co-IP, and overexpression/knockdown with cyclin D1 turnover\",\n      \"pmids\": [\"17081987\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How ligase activity is temporally regulated not addressed\", \"Acceptor lysine on cyclin D1 not yet defined\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Revealed that GSK3β phosphorylation drives cell-cycle-dependent FBXO4 dimerization required for ligase activity, and that cancer-associated mutations abolishing dimerization cause oncogenic cyclin D1 accumulation.\",\n      \"evidence\": \"Phosphorylation/dimerization assays, mutagenesis, transformation assays, and human cancer sequencing\",\n      \"pmids\": [\"18598945\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of dimerization not yet visualized\", \"Direct GSK3β phosphosite occupancy not fully mapped\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Identified Lys269 of cyclin D1 as the residue required for degradation, dissecting ubiquitination from proteolytic targeting.\",\n      \"evidence\": \"Site-directed mutagenesis with in vivo ubiquitination and degradation/transformation assays\",\n      \"pmids\": [\"19767775\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"K269R still permits ubiquitination, so the degradation-specific signal is unexplained\", \"How K269-linked chains differ from non-degradative chains unknown\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Provided the crystal structure of FBXO4-SKP1 showing an antiparallel dimer and defined the domains essential for dimerization-dependent activity.\",\n      \"evidence\": \"X-ray crystallography with in vitro ubiquitination and mutagenesis\",\n      \"pmids\": [\"20181953\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structure lacked CUL1/RBX1 and substrate\", \"Did not capture the GSK3β-phosphorylated state\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Demonstrated in vivo that Fbxo4 loss stabilizes nuclear cyclin D1, drives proliferation and transformation, and produces tumors, validating FBXO4 as a tumor suppressor.\",\n      \"evidence\": \"Fbxo4 knockout mouse, MEF characterization, Ras transformation, and stability assays\",\n      \"pmids\": [\"21911473\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Contribution of non-cyclin-D1 substrates to tumorigenesis not isolated\", \"Tissue-specific requirements not dissected\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Showed substrate targeting is genetically separable—I377M disrupts cyclin D1 but not TRF1 degradation—and that cyclin D1 accumulation mediates FBXO4-deficient melanoma.\",\n      \"evidence\": \"Braf/Fbxo4 compound mouse model and separation-of-function mutagenesis with dual-substrate assays\",\n      \"pmids\": [\"24019069\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural determinants distinguishing substrate surfaces not mapped\", \"Generalizability of separation-of-function to other substrates unknown\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Expanded the FBXO4 substrate repertoire to Fxr1 and Mcl-1, linking the ligase to a translational feedback loop and to apoptotic/chemoresistance control.\",\n      \"evidence\": \"SCFFbxo4 complex purification/MS, Co-IP, in vitro ubiquitination, KO cells, and translation/apoptosis readouts\",\n      \"pmids\": [\"29142209\", \"28776569\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mcl-1 targeting rests on a single lab without structural validation\", \"Whether dimerization/GSK3β regulation applies to these substrates not tested\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Connected FBXO4 to metabolic control by showing it degrades PPARγ under Hsp20 negative regulation, extending the ligase beyond proliferation.\",\n      \"evidence\": \"Co-IP, ubiquitination assay, and Hsp20 knockout mice with adipocyte phenotype\",\n      \"pmids\": [\"29925002\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab evidence\", \"Direct PPARγ acceptor lysine and chain topology not defined\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Placed FBXO4-mediated cyclin D1 degradation within the DNA damage response as an ATM-dependent, p53-independent event.\",\n      \"evidence\": \"siRNA knockdown, proteasome/ATM inhibition, cycloheximide chase, and γ-H2AX foci assay\",\n      \"pmids\": [\"33784509\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"How ATM signals to FBXO4 mechanistically unknown\", \"Single-lab, single cell system\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Resolved the full CRL1FBXO4 architecture by cryo-EM, confirming a domain-swapped homodimer engaging SKP1 and contacting CUL1, and added HPV-18 E6 as an E6AP-independent target.\",\n      \"evidence\": \"Cryo-EM structural determination; and siRNA screen with E6 stability and p53-dependent death assays\",\n      \"pmids\": [\"39406020\", \"39688415\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Cryo-EM lacked bound substrate\", \"HPV E6 targeting lacked a direct ubiquitination assay\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Identified FBXO4 as a STAT1-induced regulator that degrades β-catenin to suppress muscle differentiation, embedding the ligase in IFNα/JAK/STAT-Wnt signaling.\",\n      \"evidence\": \"Co-IP, ubiquitination assay, KO muscle cells with RNA-seq, and STAT1-FBXO4 promoter luciferase reporter\",\n      \"pmids\": [\"40599691\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab evidence\", \"Whether β-catenin targeting requires alphaB-crystallin or dimerization not tested\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How distinct co-adaptors and substrate surfaces select among FBXO4's many substrates, and whether GSK3β-dependent dimerization regulates all of them, remains unresolved.\",\n      \"evidence\": \"No substrate-bound CRL1FBXO4 structure or unified regulatory model across substrates in the corpus\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No structure of FBXO4 with any substrate bound\", \"Co-adaptor requirement for non-cyclin-D1 substrates unknown\", \"Whether phosphorylation-dependent dimerization gates all targets untested\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [0, 2, 9]},\n      {\"term_id\": \"GO:0016874\", \"supporting_discovery_ids\": [0, 7]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [0, 8]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [13]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [6, 13]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1640170\", \"supporting_discovery_ids\": [0, 3, 7]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [0, 9]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [7, 8]}\n    ],\n    \"complexes\": [\"SCF (SKP1-CUL1-RBX1-FBXO4) / CRL1FBXO4\"],\n    \"partners\": [\"SKP1\", \"CUL1\", \"RBX1\", \"CRYAB\", \"Fxr1\", \"TRF1\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":5,"faith_total":5,"faith_pct":100.0}}