FBXO21

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].

1. 2015 High

   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

   PMID:26085330 PMID:26631746

   Open questions at the time

   • Physiological consequences of EID1 stabilization not defined
   • Whether the EID1 degron overlap with the Rb-binding region affects Rb function not tested
2. 2015 Medium

   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

   PMID:26299618

   Open questions at the time

   • Single lab
   • Direct vs indirect ubiquitination of P-gp not fully resolved
   • In vivo relevance to drug resistance not established
3. 2016 High

   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

   PMID:27063938

   Open questions at the time

   • Structural basis for Lys29 linkage specificity unknown
   • How substrate choice between degradative and non-degradative modes is determined not defined
4. 2021 Medium

   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

   PMID:33450132

   Open questions at the time

   • Single lab
   • Whether ERK is a ubiquitination substrate or non-catalytic partner unclear
   • Mechanism by which FBXO21 promotes ERK phosphorylation not defined
5. 2021 Medium

   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

   PMID:33531987

   Open questions at the time

   • Single lab
   • Degron on Nr2f2 not mapped
   • In vivo tumor relevance limited
6. 2023 Medium

   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

   PMID:37689825

   Open questions at the time

   • Single lab
   • Mechanism linking p85α dimerization to ERK activation not fully resolved
7. 2022 Medium

   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

   PMID:35987460

   Open questions at the time

   • Single lab
   • Substrate responsible for HSPC phenotype not identified
   • Steady-state hematopoiesis minimally affected, narrowing the functional window
8. 2024 Medium

   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)

   PMID:bio_10.1101_2024.12.13.628427

   Open questions at the time

   • Preprint, not yet peer-reviewed
   • Specificity across other FBXO21 substrates not characterized
   • Pharmacokinetics and off-target profile undefined

• 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.
• No structural model of the SCF(FBXO21)-substrate complex
• Determinants of linkage specificity unknown
• Comprehensive substrate map across cell types lacking