FBXO24

FBXO24 is an F-box protein that functions as the substrate-recognition subunit of SCF (Skp1-Cullin1-F-box) E3 ubiquitin ligase complexes, directing polyubiquitination and proteasomal degradation of a broad substrate repertoire across diverse cellular processes. SCF-FBXO24 ubiquitinates substrates including NDPK-A (K48-linked, at K85), PRMT6 (at K369), LSD1, FOXK2, DARS2, MYC, RAD51, IPO5, SLC25A26 (K6-linked, at K31), and MIWI (K48-linked), with substrate recognition regulated by post-translational modifications: acetylation of NDPK-A at K56 by GCN5 blocks FBXO24 binding, and VRK2-mediated phosphorylation of MYC at S281/S293 competes with FBXO24 for MYC binding (PMID:25582197, PMID:41073389, PMID:38735474, PMID:40657752). In the testis, FBXO24 has a dual role — it interacts with splicing factors SRSF2, SRSF3, and SRSF9 to regulate alternative splicing in round spermatids and ubiquitinates MIWI and IPO5 to ensure proper chromatoid body remodeling, axoneme assembly, and mitochondrial sheath formation during spermiogenesis; Fbxo24 knockout mice are male-sterile with defective sperm flagella, mitochondrial fragmentation, and aberrant piRNA production (PMID:38470475, PMID:39163107, PMID:40657752). FBXO24-mediated degradation of LSD1 suppresses breast cancer tumorigenesis, and ATF6-driven transcriptional upregulation of FBXO24 induces RAD51 degradation, creating a BRCAness state that sensitizes triple-negative breast cancer to PARP inhibitors (PMID:37540490, PMID:40764992).

1. 2015 High

   Established that FBXO24 functions as a bona fide SCF E3 ligase substrate adaptor, demonstrating its first identified substrate (NDPK-A) and showing that substrate acetylation directly blocks FBXO24 recognition — founding the paradigm that post-translational modifications gate FBXO24 substrate selection.

   Evidence In vitro ubiquitination, site-directed mutagenesis (K56 acetylation mimic, K85R), Co-IP, and wound-healing assays in cultured cells

   PMID:25582197

   Open questions at the time

   • Whether acetylation-dependent gating generalizes to other FBXO24 substrates
   • Structural basis of FBXO24 substrate recognition undetermined
   • In vivo physiological role of FBXO24 not yet addressed
2. 2020 Medium

   Extended the substrate repertoire to PRMT6, mapping the FBXO24 binding region (aa 270–275 of PRMT6) and showing that FBXO24-mediated PRMT6 degradation restrains cell proliferation, migration, and invasion.

   Evidence Co-IP, K369R mutagenesis, ubiquitination assay, and functional cell assays

   PMID:32828318

   Open questions at the time

   • Single-lab finding without independent replication
   • Cancer-type specificity of the FBXO24-PRMT6 axis not explored
   • Degron motif on PRMT6 not fully resolved
3. 2023 Medium

   An unbiased siRNA screen identified FBXO24 as the SCF subunit responsible for LSD1 turnover, linking FBXO24 to epigenetic regulation and breast cancer suppression.

   Evidence Systematic siRNA screen of F-box proteins, ubiquitination assay, Co-IP, and tumorigenesis assays in breast cancer cells

   PMID:37540490

   Open questions at the time

   • LSD1 ubiquitination site(s) not mapped
   • Whether FBXO24 regulation of LSD1 occurs in normal tissue contexts unknown
   • No in vivo tumor model validation
4. 2024 High

   Fbxo24 knockout mice revealed an essential in vivo role in spermiogenesis: FBXO24 mediates K48-linked ubiquitination of MIWI and interacts with splicing factors (SRSF2/3/9) to regulate alternative splicing, with loss causing piRNA dysregulation, histone retention, chromatoid body defects, and male sterility — establishing a dual ubiquitin ligase/splicing regulatory function.

   Evidence Fbxo24 KO mice, Co-IP, K48-linkage ubiquitination assay, RNA-seq splicing analysis, electron microscopy, phenotypic characterization

   PMID:38470475

   Open questions at the time

   • Whether the splicing regulatory role is direct or mediated through ubiquitination of splicing factors
   • How MIWI degradation timing is coordinated with spermatid differentiation stages
   • Whether FBXO24 has roles in female reproduction
5. 2024 High

   Parallel KO mouse studies broadened the spermiogenesis mechanism: FBXO24 ubiquitinates IPO5 (importin-5), and its loss causes abnormal accumulation of membraneless RNP-like granules in sperm flagella and impaired motility, explaining the flagellar defects underlying FBXO24-dependent infertility.

   Evidence Fbxo24 KO mice, electron microscopy, ubiquitination assay, sperm motility analysis, immunofluorescence

   PMID:39163107

   Open questions at the time

   • How IPO5 accumulation mechanistically leads to RNP granule formation in flagella
   • Whether additional nuclear transport factors are FBXO24 substrates
   • Relevance to human male infertility not yet demonstrated
6. 2024 High

   FBXO24 was shown to ubiquitinate transcription factor FOXK2 (via C-terminal binding, aa 428–478) and mitochondrial tRNA synthetase DARS2, with substrate acetylation again blocking DARS2 degradation — extending the acetylation-gating paradigm beyond NDPK-A and linking FBXO24 to mitochondrial function and innate immune responses during pneumonia.

   Evidence Co-IP, domain mapping, ubiquitination assays, Fbxo24 heterozygous and KO mouse models with bacterial pneumonia, mitochondrial function and cytokine measurements

   PMID:38735474 PMID:39039092

   Open questions at the time

   • Identity of the acetyltransferase modifying DARS2 to block FBXO24 binding not determined
   • Whether FBXO24-FOXK2 axis operates outside lung/bacterial infection contexts
   • Structural basis of FBXO24 C-terminal substrate engagement unknown
7. 2025 High

   Quantitative proteomics in KO mice identified SLC25A26 (mitochondrial SAM transporter) as a substrate of atypical K6-linked polyubiquitination by FBXO24, demonstrating that FBXO24 uses non-canonical ubiquitin chain types and directly controls mitochondrial integrity and ATP production in spermatids.

   Evidence Fbxo24 KO mice, quantitative proteomics, K6-linkage-specific ubiquitination assay, mitochondrial function and sperm motility analysis

   PMID:40657752

   Open questions at the time

   • Whether K6-linked chains serve as a degradation signal or have signaling functions
   • How FBXO24 specifies chain-type selection across different substrates
   • E2 enzyme partner for K6-linked ubiquitination not identified
8. 2025 Medium

   Two studies expanded FBXO24 into oncology-relevant signaling: VRK2-mediated phosphorylation of MYC competes with FBXO24 binding to stabilize MYC, and ER stress-induced ATF6 transcriptionally upregulates FBXO24 to degrade RAD51, creating a druggable BRCAness state in triple-negative breast cancer.

   Evidence Co-IP, mutagenesis (MYC S281/S293), ChIP (ATF6 on FBXO24 promoter), ubiquitination assays, xenograft and PDTX models

   PMID:40764992 PMID:41073389

   Open questions at the time

   • Whether FBXO24 is a general mediator of MYC turnover or context-specific
   • Transcriptional regulation of FBXO24 beyond ATF6 and FoxP1 is poorly characterized
   • Clinical relevance of ATF6-FBXO24-RAD51 axis in patient tumors not validated

• Key unresolved questions include the structural basis for FBXO24's broad substrate recognition, how ubiquitin chain-type specificity (K48, K63, K6) is determined for different substrates, whether FBXO24 mutations cause human male infertility, and the full scope of its splicing regulatory function independent of its ligase activity.
• No crystal structure or cryo-EM model of FBXO24 or its SCF complex
• No human genetic studies linking FBXO24 variants to infertility or disease
• Mechanism of chain-type selectivity across substrates unknown