{"gene":"FBXO21","run_date":"2026-06-09T23:54:43","timeline":{"discoveries":[{"year":2015,"finding":"SCF(FBXO21) ubiquitin ligase complex, with FBXO21 as the substrate-recognition subunit, polyubiquitylates EID1 both in vitro and in vivo, targeting it for proteasomal degradation. A peptidic degron in EID1 is necessary and sufficient for this polyubiquitylation. The degron partially overlaps with the retinoblastoma tumor suppressor protein-binding domain of EID1.","method":"In vitro ubiquitylation assay, co-immunoprecipitation, proteasomal degradation assay, degron mapping","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Strong — reconstituted in vitro ubiquitylation, replicated independently by two labs (PMID:26631746 and PMID:26085330) with orthogonal methods including CRISPR/Cas9 KO and in vitro assay","pmids":["26631746","26085330"],"is_preprint":false},{"year":2015,"finding":"FBXO21 interacts with EID1 via its central and C-terminal portions binding the C-terminal region of EID1; FBXO21 overexpression down-regulates EID1, and CRISPR/Cas9 disruption of FBXO21 stabilizes EID1 and causes its accumulation in both cytoplasm and nucleus.","method":"Co-immunoprecipitation, CRISPR/Cas9 knockout, in vitro ubiquitylation assay, DiPIUS proteomics","journal":"Genes to cells : devoted to molecular & cellular mechanisms","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro reconstitution, CRISPR KO, and interaction mapping; replicated across two independent labs","pmids":["26085330"],"is_preprint":false},{"year":2015,"finding":"FBXO21 targets P-glycoprotein (ABCB1/P-gp) for proteasomal degradation via ubiquitination. Ser291-phosphorylated CD44 inhibits FBXO21-directed degradation of P-gp, thereby promoting multidrug resistance.","method":"Ubiquitination assay, proteasomal degradation assay, co-immunoprecipitation","journal":"Oncotarget","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional ubiquitination and degradation demonstrated, single lab with multiple orthogonal assays","pmids":["26299618"],"is_preprint":false},{"year":2016,"finding":"FBXO21 functions as a component of the SCF (Skp1-Cul1-F-box) complex and promotes Lys29-linked (non-proteolytic) ubiquitination of ASK1 (apoptosis signal-regulating kinase 1), which activates ASK1 and downstream JNK and p38 signaling, leading to type I interferon production and antiviral innate immune response. FBXO21 deficiency in mouse cells impairs Lys29-linkage and activation of ASK1.","method":"Genetic knockout (Fbxo21-deficient mouse cells), ubiquitination linkage analysis, innate immune signaling assays, cytokine measurement","journal":"eLife","confidence":"High","confidence_rationale":"Tier 2 / Strong — Fbxo21-deficient mouse model, linkage-specific ubiquitination analysis, replicated signaling readouts across multiple assays in a single rigorous study","pmids":["27063938"],"is_preprint":false},{"year":2021,"finding":"FBXO21 interacts with ERK (by mass spectrometry and co-immunoprecipitation) and promotes ERK phosphorylation, thereby inhibiting autophagy in chondrocytes. JUNB transcription factor promotes FBXO21 expression by directly targeting the FBXO21 promoter, establishing a JUNB-FBXO21-ERK signaling axis that regulates cartilage matrix metabolism and apoptosis in osteoarthritis.","method":"Mass spectrometry, co-immunoprecipitation, in vivo and in vitro knockdown/overexpression, autophagy markers (LC3, p62, Beclin1), ChIP/promoter assay","journal":"Aging cell","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — MS-identified interaction confirmed by Co-IP, in vivo KD model, multiple functional readouts; single lab","pmids":["33450132"],"is_preprint":false},{"year":2021,"finding":"FBXO21 ubiquitinates Nr2f2 (COUP-TFII) and targets it for proteasomal degradation, thereby suppressing epithelial-to-mesenchymal transition (EMT) in gastric cancer cells. Nr2f2 protein abundance is negatively regulated by FBXO21 in gastric cancer tissues.","method":"Co-immunoprecipitation, ubiquitination assay, proteasomal degradation assay, rescue experiments with Nr2f2 re-expression/inhibition in stable FBXO21 overexpression/silenced lines","journal":"Journal of Cancer","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — ubiquitination and degradation demonstrated with rescue experiments; single lab","pmids":["33531987"],"is_preprint":false},{"year":2023,"finding":"FBXO21 ubiquitylates p85α, the regulatory subunit of PI3K, targeting it for proteasomal degradation. This results in decreased canonical PI3K signaling, dimerization of free p85α, and ERK activation in acute myeloid leukemia (AML). Silencing FBXO21 leads to AML cell differentiation, inhibition of tumor progression, and sensitization to chemotherapy.","method":"Mass spectrometry-based proteomic analysis, ubiquitination assay, shRNA silencing in AML cell lines and primary patient samples, PI3K/ERK signaling readouts","journal":"Leukemia","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — MS-identified substrate, functional ubiquitination demonstrated, silencing in primary AML; single lab, single study","pmids":["37689825"],"is_preprint":false},{"year":2024,"finding":"A small molecule designed to interfere with the FBXO21:p85α substrate-ligase interaction blocks p85α ubiquitination by FBXO21, leading to decreased PI3K pathway activation and AML cell death in vitro and in vivo, with selectivity for AML over healthy cells.","method":"Small molecule inhibitor of substrate:ligase interaction, ubiquitination assay, cell viability assays, in vivo AML model","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional small molecule validation of FBXO21-p85α interaction with in vivo confirmation; preprint, not yet peer-reviewed","pmids":["bio_10.1101_2024.12.13.628427"],"is_preprint":true},{"year":2022,"finding":"FBXO21 depletion in hematopoietic stem and progenitor cells (HSPCs) leads to loss of colony formation and increased differentiation in vitro, and Fbxo21 conditional knockout mice show decreased survival upon 5-fluorouracil stress. FBXO21 depletion alters ERK (not p38) signaling in cytokine-mediated pathways in HSPCs; steady-state hematopoiesis is minimally affected.","method":"shRNA silencing, conditional knockout mouse model, colony formation assay, ERK/p38 signaling analysis, 5-FU stress test","journal":"Experimental hematology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — conditional KO mouse plus in vitro silencing, multiple orthogonal readouts; single lab","pmids":["35987460"],"is_preprint":false}],"current_model":"FBXO21 is the substrate-recognition subunit of an SCF-type (Skp1-Cul1-F-box) E3 ubiquitin ligase complex that mediates both proteolytic (Lys48-linked, targeting EID1, P-glycoprotein, Nr2f2, and p85α for proteasomal degradation) and non-proteolytic (Lys29-linked, activating ASK1 to drive JNK/p38/type I interferon antiviral signaling) ubiquitination events; FBXO21 also interacts with ERK to modulate autophagy, and its expression is transcriptionally regulated by JUNB, placing it at the nexus of innate immunity, proteostasis, PI3K/ERK signaling, and autophagy control."},"narrative":{"mechanistic_narrative":"FBXO21 is the substrate-recognition subunit of an SCF (Skp1-Cul1-F-box) E3 ubiquitin ligase that controls diverse cellular programs by directing both proteolytic and non-proteolytic ubiquitination [PMID:26631746, PMID:26085330, PMID:27063938]. In its degradative mode it assembles Lys48-type polyubiquitin chains on substrates recognized through defined degrons, targeting EID1 for proteasomal destruction via a degron that overlaps the retinoblastoma protein-binding region [PMID:26631746, PMID:26085330], and similarly destabilizing P-glycoprotein (ABCB1) [PMID:26299618], the transcription factor Nr2f2/COUP-TFII to restrain epithelial-to-mesenchymal transition [PMID:33531987], and the PI3K regulatory subunit p85α to tune PI3K/ERK signaling in acute myeloid leukemia [PMID:37689825]. In a distinct non-degradative mode, FBXO21 catalyzes Lys29-linked ubiquitination of the kinase ASK1, activating downstream JNK and p38 signaling to drive type I interferon production and antiviral innate immunity [PMID:27063938]. Beyond its ligase activity, FBXO21 interacts with ERK and promotes ERK phosphorylation to inhibit autophagy, with its expression driven transcriptionally by JUNB [PMID:33450132]. Functionally, FBXO21 supports hematopoietic stem and progenitor cell maintenance and stress survival, acting through ERK rather than p38 signaling in this context [PMID:35987460], and disruption of the FBXO21:p85α interaction by a small molecule kills AML cells selectively [PMID:bio_10.1101_2024.12.13.628427].","teleology":[{"year":2015,"claim":"Established FBXO21 as the substrate-recognition subunit of an SCF E3 ligase by demonstrating it polyubiquitylates EID1 through a defined degron, answering what molecular activity FBXO21 carries.","evidence":"In vitro ubiquitylation, co-IP, proteasomal degradation assays, and degron mapping; independently confirmed by CRISPR/Cas9 knockout and interaction mapping across two labs","pmids":["26631746","26085330"],"confidence":"High","gaps":["Physiological consequences of EID1 stabilization not defined","Whether the EID1 degron overlap with the Rb-binding region affects Rb function not tested"]},{"year":2015,"claim":"Extended the degradative repertoire of FBXO21 to P-glycoprotein and linked it to multidrug resistance regulation, showing its substrate scope reaches drug-efflux machinery.","evidence":"Ubiquitination, degradation, and co-IP assays, with phospho-CD44 shown to inhibit FBXO21-directed P-gp degradation","pmids":["26299618"],"confidence":"Medium","gaps":["Single lab","Direct vs indirect ubiquitination of P-gp not fully resolved","In vivo relevance to drug resistance not established"]},{"year":2016,"claim":"Revealed a non-proteolytic function of FBXO21, showing it builds Lys29-linked chains on ASK1 to activate JNK/p38 and antiviral interferon responses, distinguishing it from a purely degradative ligase.","evidence":"Fbxo21-deficient mouse cells, ubiquitin linkage analysis, and innate immune signaling and cytokine readouts","pmids":["27063938"],"confidence":"High","gaps":["Structural basis for Lys29 linkage specificity unknown","How substrate choice between degradative and non-degradative modes is determined not defined"]},{"year":2021,"claim":"Placed FBXO21 in an autophagy-regulating axis by showing it interacts with ERK to promote ERK phosphorylation and is transcriptionally driven by JUNB, connecting its expression control to downstream signaling.","evidence":"Mass spectrometry, co-IP, ChIP/promoter assay, and autophagy marker analysis in chondrocytes and osteoarthritis models","pmids":["33450132"],"confidence":"Medium","gaps":["Single lab","Whether ERK is a ubiquitination substrate or non-catalytic partner unclear","Mechanism by which FBXO21 promotes ERK phosphorylation not defined"]},{"year":2021,"claim":"Identified Nr2f2/COUP-TFII as a degradative substrate, linking FBXO21 to suppression of epithelial-to-mesenchymal transition in gastric cancer.","evidence":"Co-IP, ubiquitination and degradation assays, and rescue experiments in FBXO21 overexpression/silenced gastric cancer lines","pmids":["33531987"],"confidence":"Medium","gaps":["Single lab","Degron on Nr2f2 not mapped","In vivo tumor relevance limited"]},{"year":2023,"claim":"Defined p85α as a FBXO21 substrate whose degradation reshapes PI3K and ERK signaling, establishing FBXO21 as a driver of the undifferentiated state in AML.","evidence":"Mass spectrometry, ubiquitination assays, shRNA silencing in AML cell lines and primary patient samples with PI3K/ERK readouts","pmids":["37689825"],"confidence":"Medium","gaps":["Single lab","Mechanism linking p85α dimerization to ERK activation not fully resolved"]},{"year":2022,"claim":"Demonstrated a physiological requirement for FBXO21 in hematopoietic stem and progenitor cell function under stress, acting through ERK rather than p38 signaling.","evidence":"shRNA silencing, conditional knockout mice, colony formation, ERK/p38 analysis, and 5-FU stress test","pmids":["35987460"],"confidence":"Medium","gaps":["Single lab","Substrate responsible for HSPC phenotype not identified","Steady-state hematopoiesis minimally affected, narrowing the functional window"]},{"year":2024,"claim":"Provided proof-of-concept that pharmacological disruption of the FBXO21:p85α substrate-ligase interface is selectively lethal to AML, validating the interaction as a therapeutic target.","evidence":"Small molecule inhibitor of substrate:ligase interaction, ubiquitination and viability assays, in vivo AML model (preprint)","pmids":["bio_10.1101_2024.12.13.628427"],"confidence":"Medium","gaps":["Preprint, not yet peer-reviewed","Specificity across other FBXO21 substrates not characterized","Pharmacokinetics and off-target profile undefined"]},{"year":null,"claim":"How FBXO21 selects between Lys48-degradative and Lys29-non-degradative ubiquitination on a given substrate, and what governs its substrate repertoire across tissues, remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No structural model of the SCF(FBXO21)-substrate complex","Determinants of linkage specificity unknown","Comprehensive substrate map across cell types lacking"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[0,2,3,5,6]},{"term_id":"GO:0016874","term_label":"ligase activity","supporting_discovery_ids":[0,3]},{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[4]}],"localization":[{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[1]},{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[1]}],"pathway":[{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[0,5,6]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[3]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[4,6]},{"term_id":"R-HSA-9612973","term_label":"Autophagy","supporting_discovery_ids":[4]}],"complexes":["SCF (Skp1-Cul1-F-box) E3 ubiquitin ligase"],"partners":["EID1","ASK1","ERK","NR2F2","P85Α","ABCB1","SKP1","CUL1"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"O94952","full_name":"F-box only protein 21","aliases":[],"length_aa":628,"mass_kda":72.3,"function":"Substrate-recognition component of the SCF (SKP1-CUL1-F-box protein)-type E3 ubiquitin ligase complex","subcellular_location":"","url":"https://www.uniprot.org/uniprotkb/O94952/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/FBXO21","classification":"Not Classified","n_dependent_lines":5,"n_total_lines":1208,"dependency_fraction":0.0041390728476821195},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/FBXO21","total_profiled":1310},"omim":[{"mim_id":"609095","title":"F-BOX ONLY PROTEIN 21; FBXO21","url":"https://www.omim.org/entry/609095"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Centrosome","reliability":"Approved"},{"location":"Cytosol","reliability":"Approved"},{"location":"Primary cilium","reliability":"Additional"},{"location":"Primary cilium tip","reliability":"Additional"},{"location":"Basal body","reliability":"Additional"}],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in all","driving_tissues":[{"tissue":"fallopian tube","ntpm":108.4}],"url":"https://www.proteinatlas.org/search/FBXO21"},"hgnc":{"alias_symbol":["FBX21","KIAA0875"],"prev_symbol":[]},"alphafold":{"accession":"O94952","domains":[{"cath_id":"-","chopping":"97-186","consensus_level":"medium","plddt":87.2501,"start":97,"end":186},{"cath_id":"-","chopping":"194-381","consensus_level":"high","plddt":93.3529,"start":194,"end":381},{"cath_id":"-","chopping":"433-487","consensus_level":"medium","plddt":66.8147,"start":433,"end":487},{"cath_id":"2.30.30.390","chopping":"495-620","consensus_level":"high","plddt":92.3217,"start":495,"end":620}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/O94952","model_url":"https://alphafold.ebi.ac.uk/files/AF-O94952-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-O94952-F1-predicted_aligned_error_v6.png","plddt_mean":85.25},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=FBXO21","jax_strain_url":"https://www.jax.org/strain/search?query=FBXO21"},"sequence":{"accession":"O94952","fasta_url":"https://rest.uniprot.org/uniprotkb/O94952.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/O94952/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/O94952"}},"corpus_meta":[{"pmid":"33450132","id":"PMC_33450132","title":"JUNB-FBXO21-ERK axis promotes cartilage degeneration in osteoarthritis by inhibiting autophagy.","date":"2021","source":"Aging cell","url":"https://pubmed.ncbi.nlm.nih.gov/33450132","citation_count":84,"is_preprint":false},{"pmid":"27063938","id":"PMC_27063938","title":"Lys29-linkage of ASK1 by Skp1-Cullin 1-Fbxo21 ubiquitin ligase complex is required for antiviral innate response.","date":"2016","source":"eLife","url":"https://pubmed.ncbi.nlm.nih.gov/27063938","citation_count":55,"is_preprint":false},{"pmid":"26299618","id":"PMC_26299618","title":"CD44 promotes multi-drug resistance by protecting P-glycoprotein from FBXO21-mediated ubiquitination.","date":"2015","source":"Oncotarget","url":"https://pubmed.ncbi.nlm.nih.gov/26299618","citation_count":46,"is_preprint":false},{"pmid":"26631746","id":"PMC_26631746","title":"Peptidic degron in EID1 is recognized by an SCF E3 ligase complex containing the orphan F-box protein FBXO21.","date":"2015","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/26631746","citation_count":24,"is_preprint":false},{"pmid":"33531987","id":"PMC_33531987","title":"Fbxo21 regulates the epithelial-to-mesenchymal transition through ubiquitination of Nr2f2 in gastric cancer.","date":"2021","source":"Journal of Cancer","url":"https://pubmed.ncbi.nlm.nih.gov/33531987","citation_count":21,"is_preprint":false},{"pmid":"26085330","id":"PMC_26085330","title":"FBXO21 mediates the ubiquitylation and proteasomal degradation of EID1.","date":"2015","source":"Genes to cells : devoted to molecular & cellular mechanisms","url":"https://pubmed.ncbi.nlm.nih.gov/26085330","citation_count":14,"is_preprint":false},{"pmid":"36278814","id":"PMC_36278814","title":"circRNA-MSR regulates the expression of FBXO21 to inhibit chondrocyte autophagy by targeting miR-761 in osteoarthritis.","date":"2022","source":"The Kaohsiung journal of medical sciences","url":"https://pubmed.ncbi.nlm.nih.gov/36278814","citation_count":12,"is_preprint":false},{"pmid":"37689825","id":"PMC_37689825","title":"FBXO21 mediated degradation of p85α regulates proliferation and survival of acute myeloid leukemia.","date":"2023","source":"Leukemia","url":"https://pubmed.ncbi.nlm.nih.gov/37689825","citation_count":7,"is_preprint":false},{"pmid":"36443856","id":"PMC_36443856","title":"Epigenetic modification of miR-217 promotes intervertebral disc degeneration by targeting the FBXO21-ERK signalling pathway.","date":"2022","source":"Arthritis research & therapy","url":"https://pubmed.ncbi.nlm.nih.gov/36443856","citation_count":2,"is_preprint":false},{"pmid":"35987460","id":"PMC_35987460","title":"Ubiquitin E3 ligase FBXO21 regulates cytokine-mediated signaling pathways, but is dispensable for steady-state hematopoiesis.","date":"2022","source":"Experimental hematology","url":"https://pubmed.ncbi.nlm.nih.gov/35987460","citation_count":2,"is_preprint":false},{"pmid":"40089779","id":"PMC_40089779","title":"F-box protein FBXO21 overexpression inhibits the proliferation and metastasis of clear cell renal cell carcinoma and is closely related to the CREB pathway and tumor immune cell infiltration.","date":"2025","source":"Journal of translational medicine","url":"https://pubmed.ncbi.nlm.nih.gov/40089779","citation_count":1,"is_preprint":false},{"pmid":"41660081","id":"PMC_41660081","title":"Comprehensive machine learning and experimental verification reveal the mechanism of action of autophagy-related genes FIZ1 and FBXO21 in acute kidney injury.","date":"2026","source":"PeerJ","url":"https://pubmed.ncbi.nlm.nih.gov/41660081","citation_count":0,"is_preprint":false},{"pmid":null,"id":"bio_10.1101_2024.12.13.628427","title":"Small molecule targeting of FBXO21 mediated p85α ubiquitylation in acute  myeloid leukemia","date":"2024-12-17","source":"bioRxiv","url":"https://doi.org/10.1101/2024.12.13.628427","citation_count":0,"is_preprint":true}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":8720,"output_tokens":2483,"usd":0.031703,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":9573,"output_tokens":3169,"usd":0.063545,"stage2_stop_reason":"end_turn"},"total_usd":0.095248,"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\": 2015,\n      \"finding\": \"SCF(FBXO21) ubiquitin ligase complex, with FBXO21 as the substrate-recognition subunit, polyubiquitylates EID1 both in vitro and in vivo, targeting it for proteasomal degradation. A peptidic degron in EID1 is necessary and sufficient for this polyubiquitylation. The degron partially overlaps with the retinoblastoma tumor suppressor protein-binding domain of EID1.\",\n      \"method\": \"In vitro ubiquitylation assay, co-immunoprecipitation, proteasomal degradation assay, degron mapping\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — reconstituted in vitro ubiquitylation, replicated independently by two labs (PMID:26631746 and PMID:26085330) with orthogonal methods including CRISPR/Cas9 KO and in vitro assay\",\n      \"pmids\": [\"26631746\", \"26085330\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"FBXO21 interacts with EID1 via its central and C-terminal portions binding the C-terminal region of EID1; FBXO21 overexpression down-regulates EID1, and CRISPR/Cas9 disruption of FBXO21 stabilizes EID1 and causes its accumulation in both cytoplasm and nucleus.\",\n      \"method\": \"Co-immunoprecipitation, CRISPR/Cas9 knockout, in vitro ubiquitylation assay, DiPIUS proteomics\",\n      \"journal\": \"Genes to cells : devoted to molecular & cellular mechanisms\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro reconstitution, CRISPR KO, and interaction mapping; replicated across two independent labs\",\n      \"pmids\": [\"26085330\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"FBXO21 targets P-glycoprotein (ABCB1/P-gp) for proteasomal degradation via ubiquitination. Ser291-phosphorylated CD44 inhibits FBXO21-directed degradation of P-gp, thereby promoting multidrug resistance.\",\n      \"method\": \"Ubiquitination assay, proteasomal degradation assay, co-immunoprecipitation\",\n      \"journal\": \"Oncotarget\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional ubiquitination and degradation demonstrated, single lab with multiple orthogonal assays\",\n      \"pmids\": [\"26299618\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"FBXO21 functions as a component of the SCF (Skp1-Cul1-F-box) complex and promotes Lys29-linked (non-proteolytic) ubiquitination of ASK1 (apoptosis signal-regulating kinase 1), which activates ASK1 and downstream JNK and p38 signaling, leading to type I interferon production and antiviral innate immune response. FBXO21 deficiency in mouse cells impairs Lys29-linkage and activation of ASK1.\",\n      \"method\": \"Genetic knockout (Fbxo21-deficient mouse cells), ubiquitination linkage analysis, innate immune signaling assays, cytokine measurement\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — Fbxo21-deficient mouse model, linkage-specific ubiquitination analysis, replicated signaling readouts across multiple assays in a single rigorous study\",\n      \"pmids\": [\"27063938\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"FBXO21 interacts with ERK (by mass spectrometry and co-immunoprecipitation) and promotes ERK phosphorylation, thereby inhibiting autophagy in chondrocytes. JUNB transcription factor promotes FBXO21 expression by directly targeting the FBXO21 promoter, establishing a JUNB-FBXO21-ERK signaling axis that regulates cartilage matrix metabolism and apoptosis in osteoarthritis.\",\n      \"method\": \"Mass spectrometry, co-immunoprecipitation, in vivo and in vitro knockdown/overexpression, autophagy markers (LC3, p62, Beclin1), ChIP/promoter assay\",\n      \"journal\": \"Aging cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — MS-identified interaction confirmed by Co-IP, in vivo KD model, multiple functional readouts; single lab\",\n      \"pmids\": [\"33450132\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"FBXO21 ubiquitinates Nr2f2 (COUP-TFII) and targets it for proteasomal degradation, thereby suppressing epithelial-to-mesenchymal transition (EMT) in gastric cancer cells. Nr2f2 protein abundance is negatively regulated by FBXO21 in gastric cancer tissues.\",\n      \"method\": \"Co-immunoprecipitation, ubiquitination assay, proteasomal degradation assay, rescue experiments with Nr2f2 re-expression/inhibition in stable FBXO21 overexpression/silenced lines\",\n      \"journal\": \"Journal of Cancer\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — ubiquitination and degradation demonstrated with rescue experiments; single lab\",\n      \"pmids\": [\"33531987\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"FBXO21 ubiquitylates p85α, the regulatory subunit of PI3K, targeting it for proteasomal degradation. This results in decreased canonical PI3K signaling, dimerization of free p85α, and ERK activation in acute myeloid leukemia (AML). Silencing FBXO21 leads to AML cell differentiation, inhibition of tumor progression, and sensitization to chemotherapy.\",\n      \"method\": \"Mass spectrometry-based proteomic analysis, ubiquitination assay, shRNA silencing in AML cell lines and primary patient samples, PI3K/ERK signaling readouts\",\n      \"journal\": \"Leukemia\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — MS-identified substrate, functional ubiquitination demonstrated, silencing in primary AML; single lab, single study\",\n      \"pmids\": [\"37689825\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"A small molecule designed to interfere with the FBXO21:p85α substrate-ligase interaction blocks p85α ubiquitination by FBXO21, leading to decreased PI3K pathway activation and AML cell death in vitro and in vivo, with selectivity for AML over healthy cells.\",\n      \"method\": \"Small molecule inhibitor of substrate:ligase interaction, ubiquitination assay, cell viability assays, in vivo AML model\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional small molecule validation of FBXO21-p85α interaction with in vivo confirmation; preprint, not yet peer-reviewed\",\n      \"pmids\": [\"bio_10.1101_2024.12.13.628427\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"FBXO21 depletion in hematopoietic stem and progenitor cells (HSPCs) leads to loss of colony formation and increased differentiation in vitro, and Fbxo21 conditional knockout mice show decreased survival upon 5-fluorouracil stress. FBXO21 depletion alters ERK (not p38) signaling in cytokine-mediated pathways in HSPCs; steady-state hematopoiesis is minimally affected.\",\n      \"method\": \"shRNA silencing, conditional knockout mouse model, colony formation assay, ERK/p38 signaling analysis, 5-FU stress test\",\n      \"journal\": \"Experimental hematology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — conditional KO mouse plus in vitro silencing, multiple orthogonal readouts; single lab\",\n      \"pmids\": [\"35987460\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"FBXO21 is the substrate-recognition subunit of an SCF-type (Skp1-Cul1-F-box) E3 ubiquitin ligase complex that mediates both proteolytic (Lys48-linked, targeting EID1, P-glycoprotein, Nr2f2, and p85α for proteasomal degradation) and non-proteolytic (Lys29-linked, activating ASK1 to drive JNK/p38/type I interferon antiviral signaling) ubiquitination events; FBXO21 also interacts with ERK to modulate autophagy, and its expression is transcriptionally regulated by JUNB, placing it at the nexus of innate immunity, proteostasis, PI3K/ERK signaling, and autophagy control.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"FBXO21 is the substrate-recognition subunit of an SCF (Skp1-Cul1-F-box) E3 ubiquitin ligase that controls diverse cellular programs by directing both proteolytic and non-proteolytic ubiquitination [#0, #3]. In its degradative mode it assembles Lys48-type polyubiquitin chains on substrates recognized through defined degrons, targeting EID1 for proteasomal destruction via a degron that overlaps the retinoblastoma protein-binding region [#0, #1], and similarly destabilizing P-glycoprotein (ABCB1) [#2], the transcription factor Nr2f2/COUP-TFII to restrain epithelial-to-mesenchymal transition [#5], and the PI3K regulatory subunit p85\\u03b1 to tune PI3K/ERK signaling in acute myeloid leukemia [#6]. In a distinct non-degradative mode, FBXO21 catalyzes Lys29-linked ubiquitination of the kinase ASK1, activating downstream JNK and p38 signaling to drive type I interferon production and antiviral innate immunity [#3]. Beyond its ligase activity, FBXO21 interacts with ERK and promotes ERK phosphorylation to inhibit autophagy, with its expression driven transcriptionally by JUNB [#4]. Functionally, FBXO21 supports hematopoietic stem and progenitor cell maintenance and stress survival, acting through ERK rather than p38 signaling in this context [#8], and disruption of the FBXO21:p85\\u03b1 interaction by a small molecule kills AML cells selectively [#7].\",\n  \"teleology\": [\n    {\n      \"year\": 2015,\n      \"claim\": \"Established FBXO21 as the substrate-recognition subunit of an SCF E3 ligase by demonstrating it polyubiquitylates EID1 through a defined degron, answering what molecular activity FBXO21 carries.\",\n      \"evidence\": \"In vitro ubiquitylation, co-IP, proteasomal degradation assays, and degron mapping; independently confirmed by CRISPR/Cas9 knockout and interaction mapping across two labs\",\n      \"pmids\": [\"26631746\", \"26085330\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological consequences of EID1 stabilization not defined\", \"Whether the EID1 degron overlap with the Rb-binding region affects Rb function not tested\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Extended the degradative repertoire of FBXO21 to P-glycoprotein and linked it to multidrug resistance regulation, showing its substrate scope reaches drug-efflux machinery.\",\n      \"evidence\": \"Ubiquitination, degradation, and co-IP assays, with phospho-CD44 shown to inhibit FBXO21-directed P-gp degradation\",\n      \"pmids\": [\"26299618\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab\", \"Direct vs indirect ubiquitination of P-gp not fully resolved\", \"In vivo relevance to drug resistance not established\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Revealed a non-proteolytic function of FBXO21, showing it builds Lys29-linked chains on ASK1 to activate JNK/p38 and antiviral interferon responses, distinguishing it from a purely degradative ligase.\",\n      \"evidence\": \"Fbxo21-deficient mouse cells, ubiquitin linkage analysis, and innate immune signaling and cytokine readouts\",\n      \"pmids\": [\"27063938\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis for Lys29 linkage specificity unknown\", \"How substrate choice between degradative and non-degradative modes is determined not defined\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Placed FBXO21 in an autophagy-regulating axis by showing it interacts with ERK to promote ERK phosphorylation and is transcriptionally driven by JUNB, connecting its expression control to downstream signaling.\",\n      \"evidence\": \"Mass spectrometry, co-IP, ChIP/promoter assay, and autophagy marker analysis in chondrocytes and osteoarthritis models\",\n      \"pmids\": [\"33450132\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab\", \"Whether ERK is a ubiquitination substrate or non-catalytic partner unclear\", \"Mechanism by which FBXO21 promotes ERK phosphorylation not defined\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Identified Nr2f2/COUP-TFII as a degradative substrate, linking FBXO21 to suppression of epithelial-to-mesenchymal transition in gastric cancer.\",\n      \"evidence\": \"Co-IP, ubiquitination and degradation assays, and rescue experiments in FBXO21 overexpression/silenced gastric cancer lines\",\n      \"pmids\": [\"33531987\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab\", \"Degron on Nr2f2 not mapped\", \"In vivo tumor relevance limited\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Defined p85\\u03b1 as a FBXO21 substrate whose degradation reshapes PI3K and ERK signaling, establishing FBXO21 as a driver of the undifferentiated state in AML.\",\n      \"evidence\": \"Mass spectrometry, ubiquitination assays, shRNA silencing in AML cell lines and primary patient samples with PI3K/ERK readouts\",\n      \"pmids\": [\"37689825\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab\", \"Mechanism linking p85\\u03b1 dimerization to ERK activation not fully resolved\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Demonstrated a physiological requirement for FBXO21 in hematopoietic stem and progenitor cell function under stress, acting through ERK rather than p38 signaling.\",\n      \"evidence\": \"shRNA silencing, conditional knockout mice, colony formation, ERK/p38 analysis, and 5-FU stress test\",\n      \"pmids\": [\"35987460\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab\", \"Substrate responsible for HSPC phenotype not identified\", \"Steady-state hematopoiesis minimally affected, narrowing the functional window\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Provided proof-of-concept that pharmacological disruption of the FBXO21:p85\\u03b1 substrate-ligase interface is selectively lethal to AML, validating the interaction as a therapeutic target.\",\n      \"evidence\": \"Small molecule inhibitor of substrate:ligase interaction, ubiquitination and viability assays, in vivo AML model (preprint)\",\n      \"pmids\": [\"bio_10.1101_2024.12.13.628427\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Preprint, not yet peer-reviewed\", \"Specificity across other FBXO21 substrates not characterized\", \"Pharmacokinetics and off-target profile undefined\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How FBXO21 selects between Lys48-degradative and Lys29-non-degradative ubiquitination on a given substrate, and what governs its substrate repertoire across tissues, remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No structural model of the SCF(FBXO21)-substrate complex\", \"Determinants of linkage specificity unknown\", \"Comprehensive substrate map across cell types lacking\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [0, 2, 3, 5, 6]},\n      {\"term_id\": \"GO:0016874\", \"supporting_discovery_ids\": [0, 3]},\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [4]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [1]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [1]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [0, 5, 6]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [3]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [4, 6]},\n      {\"term_id\": \"R-HSA-9612973\", \"supporting_discovery_ids\": [4]}\n    ],\n    \"complexes\": [\"SCF (Skp1-Cul1-F-box) E3 ubiquitin ligase\"],\n    \"partners\": [\"EID1\", \"ASK1\", \"ERK\", \"Nr2f2\", \"p85\\u03b1\", \"ABCB1\", \"SKP1\", \"CUL1\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":4,"faith_total":5,"faith_pct":80.0}}