FEM1B

FEM1B is a substrate-recognition subunit of the CRL2 (Cullin 2-RING) E3 ubiquitin ligase complex that targets diverse substrates for proteasomal degradation through recognition of C-terminal degrons, using a bipartite mechanism that engages both a ψ-Pro C-degron and an upstream aromatic residue, with NEDD8-dependent dimerization activating the ligase (PMID:38670995, PMID:33892462). Validated substrates include FNIP1 (a reductive stress sensor), Gli1 (a Hedgehog pathway transcription factor), SLBP, SMCR8, and Ankrd37, placing FEM1B at the nexus of stress sensing, developmental signaling, and cell cycle control (PMID:34994556, PMID:24076122, PMID:39140134, PMID:33892462, PMID:21723927). Beyond its E3 ligase function, FEM1B serves as a chromatin-associated adaptor that links Rad9 to CHK1 for ATR checkpoint signaling during replication stress and promotes extrinsic apoptosis through interaction with TRAF2 and modulation of caspase-8 activation (PMID:19330022, PMID:40392678). A recurrent de novo FEM1B missense variant (Arg126Gln) causes a gain-of-function neurodevelopmental disorder characterized by impaired neuronal migration, oxidative stress, and type I interferon pathway activation (PMID:38465576).

1. 2004 Medium

   Identification of PHTF1 as a FEM1B-interacting protein established that FEM1B's ankyrin-repeat domain mediates protein–protein interactions and can be recruited to specific subcellular compartments.

   Evidence Yeast two-hybrid and co-immunoprecipitation with domain mapping and immunofluorescence localization

   PMID:15601915

   Open questions at the time

   • Functional consequence of PHTF1–FEM1B interaction on substrate targeting unknown
   • No independent replication
2. 2005 Medium

   Fem1b knockout mice revealed an unexpected organismal role in glucose homeostasis, demonstrating that FEM1B is required for normal glucose-stimulated insulin secretion from pancreatic islets.

   Evidence Gene-targeted knockout mouse with glucose tolerance and insulin secretion assays

   PMID:16024793

   Open questions at the time

   • Molecular mechanism linking FEM1B to insulin secretion machinery unresolved
   • Whether this reflects substrate degradation or a non-E3-ligase function is unknown
3. 2008 Medium

   Interaction with Nkx3.1 and prostate ductal morphogenesis defects in Fem1b knockout mice connected FEM1B to developmental patterning and sexual dimorphism in mammals, echoing C. elegans Fem1 sex-determination biology.

   Evidence Yeast two-hybrid, GST pull-down, co-immunoprecipitation, and knockout mouse phenotyping

   PMID:18816836

   Open questions at the time

   • Whether FEM1B ubiquitinates Nkx3.1 directly was not tested
   • Single lab observation
4. 2009 Medium

   Two parallel studies established dual functions for FEM1B: as a chromatin-associated adaptor facilitating Rad9–CHK1 coupling in ATR checkpoint signaling, and as a pro-apoptotic protein whose levels are controlled by RACK1-mediated ubiquitination.

   Evidence siRNA knockdown with chromatin fractionation and CHK1 kinase assays (checkpoint); co-immunoprecipitation, ubiquitination assays, and epistasis experiments (RACK1-apoptosis)

   PMID:19330022 PMID:19855191

   Open questions at the time

   • Whether FEM1B's checkpoint role requires its E3 ligase activity is untested
   • RACK1-mediated ubiquitination mechanism (direct E3 or scaffold) not resolved
   • Both findings from single labs
5. 2010 Medium

   Demonstration that proteasome inhibitor-induced apoptosis in colon cancer cells requires FEM1B upregulation placed FEM1B as a functional mediator of apoptosis in malignancy, linking its proteasomal turnover to chemosensitivity.

   Evidence Proteasome inhibitor treatment with morpholino antisense knockdown and apoptosis assay

   PMID:19908242

   Open questions at the time

   • Apoptotic substrates of FEM1B in this context not identified
   • Single lab, single cell type
6. 2011 Medium

   Identification of Ankrd37 as a FEM1B substrate targeted for ubiquitin-mediated degradation expanded the known substrate repertoire, and reciprocally revealed that Ankrd37 promotes FEM1B nuclear translocation.

   Evidence Yeast two-hybrid, co-immunoprecipitation, ubiquitination assay, subcellular localization imaging

   PMID:21723927

   Open questions at the time

   • Physiological consequence of Ankrd37 degradation unknown
   • Single lab, overexpression system
7. 2013 High

   Direct binding and ubiquitylation of Gli1 by FEM1B established a conserved link to C. elegans TRA-1 regulation and demonstrated that FEM1B suppresses Hedgehog pathway output in a VHL-box-dependent manner.

   Evidence Co-immunoprecipitation, GST pull-down, ubiquitylation assay, reporter assay, VHL-box mutagenesis

   PMID:24076122

   Open questions at the time

   • Whether endogenous Gli1 turnover depends on FEM1B in vivo not shown in this study
   • Degron on Gli1 recognized by FEM1B not mapped
8. 2021 High

   Crystal structure of FEM1B bound to the SMCR8 C-degron provided the first atomic-resolution view of how FEM1B recognizes C-terminal degradation signals, establishing FEM1B as a bona fide C-degron reader.

   Evidence X-ray crystal structure determination with biochemical binding assay

   PMID:33892462

   Open questions at the time

   • Only one substrate C-degron characterized structurally at this point
   • Full-length substrate recognition mechanism not addressed
9. 2022 High

   Covalent targeting of FEM1B C186 with EN106 disrupted FNIP1 recognition, establishing FEM1B as the E3 ligase responsible for the reductive stress response and demonstrating FEM1B's utility as a PROTAC recruitment handle.

   Evidence Covalent ligand screening, competitive binding assay, PROTAC-mediated degradation of BRD4 and BCR-ABL in cells

   PMID:34994556

   Open questions at the time

   • Structural basis of EN106-mediated disruption not determined
   • Long-term selectivity of EN106 in vivo untested
10. 2023 Medium

    miR-29a-3p was shown to downregulate FEM1B, relieving Gli1 degradation and conferring oxaliplatin resistance, revealing a microRNA-based regulatory layer controlling the FEM1B–Gli1 axis in chemoresistance.

    Evidence miRNA overexpression, genetic knockdown, oxaliplatin resistance assay, mouse tumor model

    PMID:38187042

    Open questions at the time

    • Direct miR-29a-3p binding site on FEM1B 3'UTR not validated by luciferase reporter in this study
    • Single lab
11. 2024 High

    Cryo-EM structures of CRL2-FEM1B with different substrates revealed a bipartite degron recognition mechanism (C-terminal ψ-Pro plus upstream aromatic residue) and showed that NEDD8-dependent dimerization is required for full E3 ligase activity, providing the comprehensive structural framework for substrate polyubiquitination.

    Evidence Cryo-EM structure determination, in vitro ubiquitination assay, cell-based degradation assay, mutagenesis

    PMID:38670995

    Open questions at the time

    • How dimerization is regulated in vivo under different signaling conditions is unknown
    • Not all known substrates have been mapped to the bipartite degron model
12. 2024 Medium

    Discovery of Hominini-specific stop codon readthrough (SCR) generating an unstable FEM1B isoform revealed an additional post-translational regulatory mechanism; CRISPR deletion of the SCR element increased FEM1B levels, reduced SLBP, and delayed the cell cycle.

    Evidence CRISPR editing, SCR reporter assay, western blot, cell cycle analysis

    PMID:39140134

    Open questions at the time

    • Physiological triggers of SCR frequency changes unknown
    • Single lab
13. 2024 Medium

    A recurrent de novo FEM1B R126Q variant was identified as a gain-of-function cause of a neurodevelopmental disorder, linking FEM1B hyperactivation to impaired neuronal migration, oxidative stress, and interferon pathway induction.

    Evidence In utero electroporation in mouse brain, patient cell analysis, oxidative stress and interferon signaling assays

    PMID:38465576

    Open questions at the time

    • Specific substrate(s) hyperactivated by R126Q not identified
    • Number of families limited; awaits larger cohort replication
14. 2025 Medium

    Mettl3-mediated m6A modification was shown to control Fem1b mRNA stability, connecting epitranscriptomic regulation to the FEM1B–Gli1 degradation axis in skeletal stem cell quiescence and differentiation.

    Evidence Conditional Mettl3 knockout mouse, m6A-seq, mRNA stability assay

    PMID:40016417

    Open questions at the time

    • Specific m6A reader mediating Fem1b mRNA decay not identified
    • Single lab
15. 2025 Medium

    FEM1B was shown to enhance TRAIL-induced extrinsic apoptosis in a cell-type-specific manner: in T cells via TRAF2 interaction and downregulation relieving caspase-8 inhibition, and in monocytes via TRAIL-R2 upregulation.

    Evidence FEM1B knockdown/knockout, co-immunoprecipitation, caspase activity assays, TRAIL-induced apoptosis assay

    PMID:40392678

    Open questions at the time

    • Whether FEM1B ubiquitinates TRAF2 directly not demonstrated
    • Mechanism of TRAIL-R2 upregulation in monocytes unclear

• Key unresolved questions include: the structural and mechanistic basis for FEM1B's non-E3-ligase functions (checkpoint signaling, apoptosis), the identity of hyperactivated substrates in the R126Q neurodevelopmental disorder, and whether all known substrates conform to the bipartite C-degron recognition model.
• No structure of FEM1B in complex with Rad9 or CHK1
• Complete substrate landscape not mapped by unbiased proteomics
• In vivo significance of SCR regulation under physiological stress unclear