FBXO44

FBXO44 is an F-box protein that functions as the substrate-recognition subunit of both canonical SCF (Skp1/CUL1/RBX1) and non-canonical CUL4B/DDB1 E3 ubiquitin ligase complexes to target multiple substrates—including BRCA1, RGS2, PXR, and FOXP1—for K48-linked ubiquitination and proteasomal degradation (PMID:23086937, PMID:25970626, PMID:37969738, PMID:41051444). Substrate selection is regulated by degron-specific mechanisms: FOXP1 degradation requires prior AURKA-mediated phosphorylation at Ser440, whereas RGS2 degradation is inhibited by phosphorylation of its N-terminal Ser3 (PMID:41051444, PMID:33008920). Structurally, FBXO44 belongs to the FBA subfamily but has a remodeled substrate-binding β-sandwich that lacks the carbohydrate-binding pocket of glycan-recognizing paralogs, instead engaging protein substrates through its F-box-associated domain (PMID:26460611, PMID:18203720). Independent of its role in targeted proteolysis, FBXO44 binds H3K9me3-marked nucleosomes at replication forks and recruits SUV39H1, CRL4, and Mi-2/NuRD to maintain epigenetic silencing of repetitive elements in cancer cells; loss of this function reactivates repetitive elements, triggers MAVS/STING-dependent viral mimicry, and sensitizes tumors to immunotherapy (PMID:33357448).

1. 2002 Low

   Identification of FBXO44 as a novel FBA-subfamily F-box protein established it as a potential SCF E3 ligase component with a conserved ~180 aa C-terminal domain, setting the stage for functional characterization.

   Evidence cDNA cloning, genomic mapping, and tissue expression profiling

   PMID:12383498

   Open questions at the time

   • No functional or substrate data; gene identification only
   • Tissue expression limited to Northern analysis without protein-level validation
2. 2008 High

   Demonstrating that FBXO44 assembles into a canonical SCF complex yet uniquely fails to bind glycans among FBA family members raised the question of what its true substrates are.

   Evidence Glycan array screening across FBA family, Co-IP of Skp1/CUL1/RBX1, site-directed mutagenesis of glycan-binding residues

   PMID:18203720

   Open questions at the time

   • No endogenous substrate identified
   • Basis for loss of glycan binding was inferred but not structurally resolved
3. 2011 Medium

   Showing that Skp1 co-expression is required for FBXO44 folding and prevents aggregation revealed a chaperone-like dependency that explained why isolated FBXO44 appeared non-functional in earlier assays.

   Evidence Co-expression with Skp1, ConA-binding assay, Co-IP

   PMID:21640084

   Open questions at the time

   • Single lab study
   • Whether Skp1 dependency extends to all FBXO44 complexes (e.g., CUL4B-based) was not tested
4. 2012 High

   Identification of BRCA1 as the first bona fide SCF(FBXO44) substrate demonstrated that this atypical FBA member functions as a protein-targeting E3 ligase rather than a glycoprotein-targeting one.

   Evidence Mass spectrometry, siRNA screen of F-box proteins, in vitro ubiquitination assay, Co-IP with domain mapping

   PMID:23086937

   Open questions at the time

   • Physiological contexts triggering BRCA1 degradation not defined
   • Whether FBXO44-mediated BRCA1 turnover affects DNA damage responses not tested
5. 2015 High

   Discovery that FBXO44 degrades RGS2 through a non-canonical CUL4B/DDB1 complex—not the SCF complex—revealed that FBXO44 operates in at least two distinct E3 ligase architectures with different substrate specificities.

   Evidence Genome-wide siRNA screen, reciprocal Co-IP distinguishing CUL1- and CUL4B-containing complexes, proteasomal inhibition

   PMID:25970626

   Open questions at the time

   • How FBXO44 is partitioned between SCF and CUL4B complexes is unknown
   • Whether additional cullins are involved was not explored
6. 2015 High

   The 2.6 Å crystal structure of Skp1–FBXO44 resolved the structural basis for loss of glycan binding: altered hydrogen-bond networks in four loops collapse the carbohydrate pocket, confirming FBXO44 engages non-glycan substrates.

   Evidence X-ray crystallography at 2.6 Å, structure-based mutagenesis

   PMID:26460611

   Open questions at the time

   • No co-crystal with any protein substrate obtained
   • Structure represents only the SCF-associated form, not the CUL4B complex
7. 2020 High

   Mapping the RGS2 N-terminal degron and showing that Ser3 phosphorylation blocks FBXO44 binding established a phosphorylation-dependent switch that protects RGS2 from degradation, paralleling classical phosphodegron logic in reverse.

   Evidence Site-directed mutagenesis, Co-IP, ubiquitination assays with RGS2 variants

   PMID:33008920

   Open questions at the time

   • Kinase responsible for Ser3 phosphorylation not identified
   • In vivo relevance of phospho-protection not demonstrated
8. 2020 High

   Discovery that FBXO44 binds H3K9me3-marked nucleosomes at replication forks to recruit SUV39H1/CRL4/NuRD for post-replicative silencing of repetitive elements revealed a chromatin-regulatory function entirely distinct from its proteolytic roles, and linked its loss to viral mimicry and immunotherapy sensitization in cancer.

   Evidence RNAi screen, Co-IP, chromatin fractionation, loss-of-function across multiple cancer cell lines with MAVS/STING pathway readouts

   PMID:33357448

   Open questions at the time

   • Whether the chromatin function uses the CUL4B or SCF complex or neither is unclear
   • Mechanism of H3K9me3 recognition by FBXO44 not structurally defined
   • Whether this silencing role operates in normal (non-cancer) cells is unknown
9. 2023 High

   Identification of the nuclear receptor PXR as an FBXO44 substrate, with binding mediated through the F-box-associated domain to the PXR ligand-binding domain, extended the substrate repertoire to xenobiotic-sensing pathways.

   Evidence RNAi library screen, Co-IP, ubiquitination assay, domain-mapping truncations, bidirectional modulation of PXR levels

   PMID:37969738

   Open questions at the time

   • Which cullin complex mediates PXR ubiquitination not determined
   • Whether ligand occupancy of PXR modulates FBXO44-mediated turnover not tested
10. 2025 High

    Demonstration that AURKA phosphorylates FOXP1 at Ser440 to create a phosphodegron recognized by FBXO44, leading to K48-linked ubiquitination at K377 and derepression of Cyclin E2, established a kinase–E3 ligase axis driving colorectal cancer proliferation.

    Evidence Co-IP, K48-linkage-specific ubiquitination, phosphomimetic mutagenesis, AURKA kinase assay, in vivo xenograft

    PMID:41051444

    Open questions at the time

    • Whether other kinases can substitute for AURKA not explored
    • Generalizability beyond colorectal cancer models not tested
11. 2025 Medium

    Fine-mapping of the RGS2–FBXO44 interface by peptide arrays and AlphaFold modeling, together with identification of a small-molecule inhibitor of this interaction, provided proof-of-concept that FBXO44 substrate engagement is pharmacologically tractable.

    Evidence Systematic peptide array, AlphaFold/MD simulations, NanoBiT PPI assay, compound screening with siRNA validation

    PMID:40199141 PMID:40992661

    Open questions at the time

    • Inhibitor potency (19.6 µM) is modest; selectivity and in vivo efficacy untested
    • Structural model is computationally predicted and awaits experimental co-structure

• Key unresolved questions include how FBXO44 is partitioned between SCF and CUL4B complexes, the structural basis of H3K9me3 nucleosome recognition, whether the chromatin-silencing function is ubiquitin-dependent, and whether additional physiological substrates exist.
• No co-structure of FBXO44 with any protein substrate
• Mechanism linking FBXO44 to H3K9me3 not structurally resolved
• Regulation of FBXO44 expression and turnover largely unexplored