Affinage

FBXO21

F-box only protein 21 · UniProt O94952

Length
628 aa
Mass
72.3 kDa
Annotated
2026-04-28
12 papers in source corpus 9 papers cited in narrative 8 extracted findings

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

FBXO21 is the substrate-recognition subunit of an SCF (Skp1–Cul1–F-box) E3 ubiquitin ligase that controls innate immunity, hematopoiesis, and epithelial-mesenchymal transition by targeting distinct substrates for ubiquitination. SCF(FBXO21) catalyzes non-proteolytic Lys29-linked ubiquitination of ASK1 to activate JNK/p38 signaling and type I interferon production during antiviral responses (PMID:27063938), and mediates Lys48-linked polyubiquitination of EID1 (PMID:26631746, PMID:26085330), P-glycoprotein (PMID:26299618), Nr2f2 (PMID:33531987), and the PI3K regulatory subunit p85α (PMID:37689825) for proteasomal degradation. FBXO21-dependent degradation of p85α modulates PI3K/ERK signaling and is required for maintaining hematopoietic stem and progenitor cell self-renewal, while its loss promotes myeloid differentiation and sensitizes acute myeloid leukemia cells to chemotherapy (PMID:37689825, PMID:35987460). FBXO21 also inhibits autophagy in chondrocytes through a JUNB-driven transcriptional axis that promotes ERK phosphorylation (PMID:33450132).

Mechanistic history

Synthesis pass · year-by-year structured walk · 7 steps
  1. 2015 High

    Identification of EID1 as the first bona fide SCF(FBXO21) substrate established that FBXO21 functions as a canonical F-box protein directing Lys48-linked polyubiquitination and proteasomal degradation, with degron mapping revealing how the ligase recognizes its target.

    Evidence In vitro reconstituted ubiquitylation, CRISPR/Cas9 knockout stabilization of EID1, degron mapping in human cells, independently replicated

    PMID:26085330 PMID:26631746

    Open questions at the time
    • Structural basis of FBXO21–EID1 degron recognition is unresolved
    • Physiological consequence of EID1 stabilization in FBXO21-null organisms not characterized
  2. 2015 Medium

    Demonstration that FBXO21 ubiquitinates P-glycoprotein for degradation, with CD44 phosphorylation antagonizing this process, linked FBXO21 to multidrug resistance regulation.

    Evidence Co-immunoprecipitation, ubiquitination assays, phospho-CD44 mutant analysis in cancer cell lines

    PMID:26299618

    Open questions at the time
    • Direct ubiquitination of P-gp not reconstituted in vitro with purified components
    • Mechanism by which phospho-CD44 blocks FBXO21 access to P-gp is unknown
    • In vivo relevance to drug resistance not tested
  3. 2016 High

    Discovery that FBXO21 catalyzes Lys29-linked (non-degradative) ubiquitination of ASK1 revealed that SCF(FBXO21) can employ atypical ubiquitin linkages to activate, rather than degrade, a substrate, directly connecting FBXO21 to antiviral innate immunity.

    Evidence Fbxo21-deficient mouse cells, in vivo ubiquitination assays with linkage-type specificity, antiviral phenotyping

    PMID:27063938

    Open questions at the time
    • How FBXO21 specifies Lys29 linkage assembly rather than canonical Lys48 linkage is unknown
    • Identity of the E2 conjugating enzyme cooperating with SCF(FBXO21) for Lys29 chains not determined
  4. 2021 Medium

    Identification of Nr2f2 as an FBXO21 substrate whose degradation suppresses EMT extended FBXO21's substrate repertoire to transcription factor turnover and tumor-suppressive signaling in gastric cancer.

    Evidence Co-immunoprecipitation, ubiquitination assays, FBXO21 overexpression/knockdown with Nr2f2 rescue, EMT marker analysis in gastric cancer cells

    PMID:33531987

    Open questions at the time
    • Nr2f2 degron recognized by FBXO21 not mapped
    • Relevance beyond gastric cancer cell lines not established
    • No in vivo tumor model tested
  5. 2021 Medium

    Placing FBXO21 downstream of JUNB transcriptional control and upstream of ERK-dependent autophagy inhibition in chondrocytes established a tissue-specific signaling axis relevant to osteoarthritis.

    Evidence Mass spectrometry, Co-IP, JUNB promoter binding, in vivo knockdown/overexpression in OA rat model, autophagy marker analysis

    PMID:33450132

    Open questions at the time
    • Direct substrate through which FBXO21 promotes ERK phosphorylation in chondrocytes not identified
    • Whether the ERK effect is ubiquitin-ligase-dependent or scaffolding-dependent is unclear
  6. 2022 Medium

    Conditional knockout of Fbxo21 in murine hematopoiesis demonstrated that FBXO21 is required for hematopoietic stem and progenitor cell maintenance and colony-forming capacity, linking its E3 ligase activity to normal blood development.

    Evidence shRNA knockdown, hematopoiesis-specific conditional knockout mouse, colony formation assays, 5-FU stress, ERK signaling readout

    PMID:35987460

    Open questions at the time
    • Critical substrate(s) mediating the HSPC phenotype not definitively assigned at this stage
    • Long-term hematopoietic reconstitution capacity of Fbxo21-null HSPCs not fully assessed
  7. 2023 Medium

    Identification of p85α as the key FBXO21 substrate in AML unified the hematopoietic phenotype with a defined molecular target: FBXO21-mediated p85α degradation controls PI3K/ERK signaling balance, and its silencing promotes AML differentiation and chemosensitization.

    Evidence Mass spectrometry-based proteomics, ubiquitylation assays, FBXO21 knockdown in AML cell lines and primary patient samples, PI3K/ERK pathway analysis

    PMID:37689825

    Open questions at the time
    • Degron on p85α recognized by FBXO21 not mapped
    • Whether p85α is the sole substrate driving the hematopoietic phenotype is not resolved
    • Therapeutic window for targeting FBXO21 in AML versus normal hematopoiesis not defined

Open questions

Synthesis pass · forward-looking unresolved questions
  • The structural basis for FBXO21 substrate selectivity and linkage-type determination (Lys29 vs. Lys48) remains unknown, and no atomic-resolution structure of the SCF(FBXO21) complex or any substrate–ligase interface has been reported.
  • No crystal or cryo-EM structure of FBXO21 or its SCF complex
  • Mechanism selecting Lys29 vs. Lys48 ubiquitin chain assembly unresolved
  • Full catalog of physiological substrates likely incomplete

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0016874 ligase activity 5 GO:0140096 catalytic activity, acting on a protein 5
Localization
GO:0005829 cytosol 2
Pathway
R-HSA-162582 Signal Transduction 3 R-HSA-168256 Immune System 1
Partners
ABCB1ASK1CUL1EID1NR2F2PIK3R1SKP1
Complex memberships
SCF(FBXO21)

Evidence

Reading pass · 8 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2016 FBXO21, as a substrate-recognition subunit of the SCF (Skp1-Cul1-F-box) E3 ubiquitin ligase complex, catalyzes Lys29-linked ubiquitination of ASK1 (apoptosis signal-regulating kinase 1), which activates ASK1 and promotes downstream JNK and p38 signaling, type I interferon production, and antiviral innate immune response. FBXO21 deficiency impairs Lys29-linkage and activation of ASK1 in mouse cells, attenuates JNK/p38 signaling, decreases proinflammatory cytokines and type I interferon, and enhances virus replication. Genetic knockout (Fbxo21-deficient mouse cells), in vivo ubiquitination assays, co-immunoprecipitation, signaling pathway analysis eLife High 27063938
2015 SCF(FBXO21) recognizes a peptidic degron in EID1 (EP300-interacting inhibitor of differentiation 1) via the central and C-terminal portions of FBXO21, polyubiquitylates EID1 both in vitro and in vivo, and is required for efficient proteasomal degradation of EID1 in both cycling and quiescent cells. The EID1 degron partially overlaps with its retinoblastoma protein-binding domain. In vitro ubiquitylation assay, co-immunoprecipitation, CRISPR/Cas9 gene disruption, degron mapping, proteasomal degradation assays Proceedings of the National Academy of Sciences of the United States of America High 26085330 26631746
2015 FBXO21 targets P-glycoprotein (ABCB1/P-gp) for proteasomal degradation via ubiquitination, and Ser291-phosphorylated CD44 inhibits FBXO21-directed degradation of P-gp, thereby increasing P-gp-mediated multidrug resistance. Co-immunoprecipitation, ubiquitination assays, proteasomal degradation assays, phospho-CD44 mutant analysis Oncotarget Medium 26299618
2021 FBXO21 inhibits autophagy by interacting with and promoting ERK phosphorylation in chondrocytes; upstream, JUNB transcription factor directly targets the FBXO21 promoter to promote FBXO21 expression, establishing a JUNB-FBXO21-ERK axis that regulates cartilage degeneration in osteoarthritis. Mass spectrometry, co-immunoprecipitation, in vivo knockdown/overexpression in OA rat model, in vitro knockdown in chondrocytes, autophagy marker analysis Aging cell Medium 33450132
2021 FBXO21 ubiquitinates Nr2f2 (COUP-TFII) and promotes its proteasomal degradation, thereby inhibiting the epithelial-to-mesenchymal transition (EMT) in gastric cancer cells. Co-immunoprecipitation, ubiquitination assays, FBXO21 knockdown/overexpression, Nr2f2 rescue experiments, EMT marker analysis Journal of Cancer Medium 33531987
2023 FBXO21 ubiquitylates p85α (regulatory subunit of PI3K) for proteasomal degradation, resulting in decreased canonical PI3K signaling, dimerization of free p85α, and ERK activation; silencing FBXO21 in AML cells leads to differentiation, inhibition of tumor progression, and sensitization to chemotherapy. Mass spectrometry-based proteomic analysis, ubiquitylation assays, FBXO21 knockdown in AML cell lines and primary patient samples, PI3K/ERK signaling pathway analysis Leukemia Medium 37689825
2024 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 selective cell death in AML cells in vitro and in vivo. Small molecule inhibitor of substrate:ligase interaction, p85α ubiquitination assays, cell death assays, in vivo AML mouse model bioRxivpreprint Medium bio_10.1101_2024.12.13.628427
2022 Silencing FBXO21 in hematopoietic stem and progenitor cells (HSPCs) leads to loss of colony formation and increased differentiation in vitro, and depletion of FBXO21 alters ERK signaling in HSPCs; FBXO21 is highly expressed in HSPCs but not in mature myeloid populations. shRNA knockdown, conditional knockout mouse model (hematopoiesis-specific Fbxo21 cKO), colony formation assays, 5-fluorouracil stress model, ERK signaling analysis Experimental hematology Medium 35987460

Source papers

Stage 0 corpus · 12 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2021 JUNB-FBXO21-ERK axis promotes cartilage degeneration in osteoarthritis by inhibiting autophagy. Aging cell 83 33450132
2016 Lys29-linkage of ASK1 by Skp1-Cullin 1-Fbxo21 ubiquitin ligase complex is required for antiviral innate response. eLife 52 27063938
2015 CD44 promotes multi-drug resistance by protecting P-glycoprotein from FBXO21-mediated ubiquitination. Oncotarget 45 26299618
2015 Peptidic degron in EID1 is recognized by an SCF E3 ligase complex containing the orphan F-box protein FBXO21. Proceedings of the National Academy of Sciences of the United States of America 24 26631746
2021 Fbxo21 regulates the epithelial-to-mesenchymal transition through ubiquitination of Nr2f2 in gastric cancer. Journal of Cancer 20 33531987
2015 FBXO21 mediates the ubiquitylation and proteasomal degradation of EID1. Genes to cells : devoted to molecular & cellular mechanisms 14 26085330
2022 circRNA-MSR regulates the expression of FBXO21 to inhibit chondrocyte autophagy by targeting miR-761 in osteoarthritis. The Kaohsiung journal of medical sciences 11 36278814
2023 FBXO21 mediated degradation of p85α regulates proliferation and survival of acute myeloid leukemia. Leukemia 7 37689825
2022 Ubiquitin E3 ligase FBXO21 regulates cytokine-mediated signaling pathways, but is dispensable for steady-state hematopoiesis. Experimental hematology 2 35987460
2022 Epigenetic modification of miR-217 promotes intervertebral disc degeneration by targeting the FBXO21-ERK signalling pathway. Arthritis research & therapy 2 36443856
2025 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. Journal of translational medicine 1 40089779
2026 Comprehensive machine learning and experimental verification reveal the mechanism of action of autophagy-related genes FIZ1 and FBXO21 in acute kidney injury. PeerJ 0 41660081