Affinage

FEM1B

Protein fem-1 homolog B · UniProt Q9UK73

Length
627 aa
Mass
70.3 kDa
Annotated
2026-06-09
27 papers in source corpus 17 papers cited in narrative 17 extracted findings
Cross-family judge vs UniProt: Affinage preferred faithfulness: 8/8 claims corpus-supported (100%)

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

FEM1B is the substrate-recognition subunit of a CRL2 (Cullin 2-RING) E3 ubiquitin ligase that selects substrates by binding C-terminal degrons and targets them for polyubiquitination and proteasomal degradation (PMID:38670995, PMID:34994556). It recognizes C-degrons terminating in proline through a bipartite mechanism that engages both the C-terminal proline and an upstream aromatic residue, and cryo-EM structures show that the ligase assembles as a dimer whose neddylation state controls activation and substrate turnover (PMID:38670995). Substrate recognition can be conditional: FEM1B reads a heme-formed degron on the transcription factor BACH1, coupling redox state to BACH1 degradation and thereby tuning expression of ferroptosis-protective genes such as SLC7A11 (PMID:42086045), and it engages the reductive-stress factor FNIP1 through a binding surface that includes Cys186 (PMID:34994556). Through its VHL-box, FEM1B binds and ubiquitylates the transcription factor Gli1 to suppress an oncogenic Gli1 autoregulatory loop (PMID:24076122), an axis exploited in colorectal chemoresistance and in skeletal stem cell quiescence where reduced FEM1B stabilizes Gli1 (PMID:38187042, PMID:40016417). Additional characterized substrates include the autophagy regulator SMCR8 (PMID:33892462), the histone mRNA factor SLBP (PMID:39140134), and Ankrd37 (PMID:21723927). Beyond its ligase activity, FEM1B associates with chromatin and is required for Rad9 loading and CHK1 (Ser345) activation during replication stress (PMID:19330022), and it promotes TRAIL-induced extrinsic apoptosis by downregulating TRAF2 and upregulating TRAIL-R2 (PMID:40392678). FEM1B abundance is itself controlled by RACK1-stimulated ubiquitination (PMID:19855191), by METTL3/m6A-dependent mRNA destabilization (PMID:40016417), and by a programmed stop-codon-readthrough event that produces an unstable extended isoform (PMID:39140134). A de novo gain-of-function variant, p.Arg126Gln, causes aberrant FEM1B activation and a syndromic neurodevelopmental disorder, with delayed neuronal migration and patient-cell oxidative stress and type I interferon induction (PMID:38465576).

Mechanistic history

Synthesis pass · year-by-year structured walk · 17 steps
  1. 2004 Medium

    Established the first physical partner and a subcellular context for FEM1B, showing its ANK domain binds PHTF1, which tethers FEM1B to the ER membrane.

    Evidence Yeast two-hybrid, co-IP, domain mapping and co-localization

    PMID:15601915

    Open questions at the time
    • Did not define a catalytic/E3 function
    • No demonstration that PHTF1 is a degradation substrate
  2. 2005 Medium

    A Fem1b knockout linked the gene to a physiological process, revealing a requirement for glucose- and arginine-stimulated insulin secretion in islet beta cells.

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

    PMID:16024793

    Open questions at the time
    • Molecular mechanism connecting FEM1B to insulin secretion unresolved
    • No substrate implicated in the islet phenotype
  3. 2008 Medium

    Connected FEM1B to a developmental program by showing it binds Nkx3.1 and is required for prostate ductal morphogenesis.

    Evidence GST pull-down, co-IP, and KO mouse developmental analysis

    PMID:18816836

    Open questions at the time
    • Whether Nkx3.1 is a ubiquitination substrate not tested
    • Mechanism of developmental defect unknown
  4. 2009 Medium

    Identified a checkpoint role distinct from ligase activity, showing FEM1B associates with chromatin and is needed for Rad9 loading and CHK1 activation under replication stress.

    Evidence Yeast two-hybrid, siRNA, chromatin fractionation, CHK1 kinase and phospho-immunoblot assays

    PMID:19330022

    Open questions at the time
    • Direct biochemical role in Rad9 loading not reconstituted
    • Single lab
    • Relationship to E3 ligase function unclear
  5. 2009 Medium

    Revealed that FEM1B is itself a regulated target, with RACK1 stimulating FEM1B ubiquitination and controlling its pro-apoptotic levels in colon cancer.

    Evidence Reciprocal co-IP, co-expression ubiquitination assay, siRNA and apoptosis rescue

    PMID:19855191

    Open questions at the time
    • E3 ligase mediating FEM1B ubiquitination not identified
    • Single lab
  6. 2010 Medium

    Demonstrated functionally that proteasome-controlled FEM1B levels drive proteasome-inhibitor-induced apoptosis in malignant colon cells.

    Evidence Proteasome inhibitor treatment, morpholino antisense rescue, apoptosis assay

    PMID:19908242

    Open questions at the time
    • Downstream apoptotic effectors not defined
    • Mechanism of FEM1B-driven death unresolved
  7. 2011 Medium

    Provided early evidence of FEM1B acting as a degradation factor by identifying Ankrd37 as a dose-dependent ubiquitination target.

    Evidence Yeast two-hybrid, co-IP, transfection-based degradation and ubiquitination assays

    PMID:21723927

    Open questions at the time
    • Degron on Ankrd37 not mapped
    • CRL2 context not yet established
    • Single lab
  8. 2013 Medium

    Defined a VHL-box-dependent substrate, showing FEM1B binds and ubiquitylates Gli1 to suppress an oncogenic Gli1 autoregulatory loop.

    Evidence Co-IP, in-cell ubiquitylation, reporter assay, VHL-box mutagenesis

    PMID:24076122

    Open questions at the time
    • Full CRL2 reconstitution not shown
    • Degron recognition mechanism unresolved at this stage
  9. 2021 Medium

    Extended the substrate repertoire with structural insight into FEM1B reading the C-degron of the autophagy regulator SMCR8.

    Evidence Structural determination of FEM1B–SMCR8 C-degron complex and co-IP

    PMID:33892462

    Open questions at the time
    • Limited functional validation
    • Cellular consequences of SMCR8 turnover not fully resolved
    • Single lab
  10. 2022 High

    Pinpointed Cys186 as part of the natural substrate-binding site and converted FEM1B into a ligand-addressable E3 for targeted protein degradation.

    Evidence Covalent ligand (EN106) screening, PROTAC synthesis (EN106-JQ1, EN106-dasatinib), target engagement and degradation assays

    PMID:34994556

    Open questions at the time
    • Full structural basis of FNIP1 recognition not solved here
    • In vivo applicability of degraders untested
  11. 2023 Medium

    Placed FEM1B upstream of Gli1 in a clinically relevant axis, showing miR-29a-3p-mediated FEM1B loss stabilizes Gli1 and confers oxaliplatin resistance.

    Evidence miRNA overexpression, FEM1B and GLI1 knockdown, oxaliplatin resistance assay, mouse tumor model

    PMID:38187042

    Open questions at the time
    • Direct miR-29a-3p–FEM1B binding not detailed
    • Single lab
  12. 2024 High

    Solved the mechanistic basis of FEM1B substrate selection, revealing bipartite C-terminal proline degron recognition and NEDD8-controlled dimeric activation of CRL2FEM1B.

    Evidence Cryo-EM structures, in vitro ubiquitination reconstitution, cell-based degradation assays

    PMID:38670995

    Open questions at the time
    • Generality of bipartite recognition across all substrates not exhaustively mapped
    • Dynamics of dimerization in cells not directly observed
  13. 2024 Medium

    Showed FEM1B abundance is set by a programmed stop-codon-readthrough degron and identified SLBP as a substrate controlling histone supply and cell cycle progression.

    Evidence SCR reporter assays, CRISPR deletion, protein stability and cell cycle analysis, SLBP measurement

    PMID:39140134

    Open questions at the time
    • Trans-factors driving FEM1B readthrough unknown
    • Single lab
  14. 2024 Medium

    Linked FEM1B to human disease, demonstrating that the gain-of-function p.Arg126Gln variant drives aberrant activation, delayed neuronal migration, and a neurodevelopmental phenotype.

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

    PMID:38465576

    Open questions at the time
    • Specific dysregulated substrate driving the neurodevelopmental phenotype not identified
    • Single cohort/lab
  15. 2025 Medium

    Defined a ligase-independent and a ligase-related role in extrinsic apoptosis, with FEM1B downregulating TRAF2 and upregulating TRAIL-R2 to enhance TRAIL-induced caspase-8/3 activation.

    Evidence Co-IP, knockdown/knockout, caspase activity and flow-cytometry apoptosis assays, murine KO

    PMID:40392678

    Open questions at the time
    • Whether TRAF2 is a direct ubiquitination substrate not established
    • Single lab
  16. 2025 Medium

    Connected FEM1B regulation to stem cell biology, showing METTL3/m6A destabilizes Fem1b mRNA to stabilize Gli1 and maintain skeletal stem cell quiescence.

    Evidence Conditional Mettl3 KO mouse, m6A sequencing, RNA stability and Gli1 protein assays

    PMID:40016417

    Open questions at the time
    • Direct m6A reader controlling Fem1b decay not specified
    • Single lab
  17. 2026 High

    Revealed conditional, cofactor-formed degron recognition, showing CRL2FEM1B reads a heme-formed degron on BACH1 to couple redox state to ferroptosis-gene regulation.

    Evidence Co-IP, ubiquitination assays, in vitro and preclinical loss-of-function experiments

    PMID:42086045

    Open questions at the time
    • Structural basis of heme-degron recognition not resolved
    • Breadth of heme-formed substrates unknown

Open questions

Synthesis pass · forward-looking unresolved questions
  • How FEM1B's ligase-independent functions (replication checkpoint, apoptosis) mechanistically relate to its CRL2 substrate-degradation role, and which substrate drives the neurodevelopmental disorder, remain open.
  • No unifying mechanism linking E3 activity to chromatin/checkpoint function
  • Disease-driving substrate unidentified
  • In vivo substrate hierarchy not established

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0140096 catalytic activity, acting on a protein 4 GO:0060090 molecular adaptor activity 3 GO:0016874 ligase activity 2 GO:0140299 molecular sensor activity 1
Localization
GO:0005634 nucleus 1 GO:0005694 chromosome 1 GO:0005783 endoplasmic reticulum 1
Pathway
R-HSA-392499 Metabolism of proteins 4 R-HSA-5357801 Programmed Cell Death 2 R-HSA-74160 Gene expression (Transcription) 2 R-HSA-8953897 Cellular responses to stimuli 2 R-HSA-73894 DNA Repair 1
Partners
ANKRD37BACH1FNIP1GLI1RACK1SLBPSMCR8TRAF2
Complex memberships
CRL2 (Cullin 2-RING) E3 ubiquitin ligase

Evidence

Reading pass · 17 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2024 CRL2FEM1B recognizes C-degrons containing a C-terminal proline via a bipartite mechanism that engages both the C-terminal proline and an upstream aromatic residue. Cryo-EM structures revealed the dimeric assembly of the complex and distinct dimerization states of unmodified vs. neddylated CRL2FEM1B, uncovering NEDD8-mediated activation. In vitro ubiquitination and cell-based assays confirmed that polyubiquitination and protein turnover depend on both FEM1B-degron interactions and the dimerization state. Cryo-EM structure determination, in vitro ubiquitination assay, cell-based degradation assays Nature communications High 38670995
2022 A covalent ligand EN106 targets C186 of FEM1B and disrupts recognition of the reductive-stress substrate FNIP1. EN106 can be used as a FEM1B recruiter in PROTAC degraders (e.g., EN106-JQ1 degrades BRD4; EN106-dasatinib degrades BCR-ABL), establishing C186 as part of the natural E3 ligase–substrate binding site. Covalent ligand screening, PROTAC synthesis and cellular degradation assays, target engagement assays Journal of the American Chemical Society High 34994556
2021 CRL2FEM1B recognizes the C-degron of the SMCR8 isoform (an autophagy regulator). Crystal/structural data of FEM1B bound to the SMCR8 C-degron provided mechanistic insight into substrate recognition and regulation of SMCR8 lifetime. Structural determination of FEM1B–SMCR8 C-degron complex, co-immunoprecipitation Biochemical and biophysical research communications Medium 33892462
2026 CRL2FEM1B recruits the transcription factor BACH1 for ubiquitin-mediated degradation by recognizing a degron directly formed by the redox-sensing molecule heme. By degrading BACH1 in response to heme, CRL2FEM1B modulates transcription of ferroptosis-protective genes (e.g., SLC7A11). Loss of CRL2FEM1B stabilizes BACH1 and sensitizes lung tumor cells to ferroptosis inducers. Co-immunoprecipitation, ubiquitination assays, in vitro and in vivo (preclinical) loss-of-function experiments Molecular cell High 42086045
2009 FEM1B interacts with CHK1 (identified by yeast two-hybrid) and is required for chromatin loading of Rad9 and activation of CHK1 (Ser345 phosphorylation) in response to replication stress. FEM1B associates with chromatin (shown by fractionation), and FEM1B depletion by siRNA impairs ATR activity and sensitizes cells to replication block, phenocopying CHK1 loss. Yeast two-hybrid, siRNA knockdown, chromatin fractionation, CHK1 kinase activity assay, phosphorylation (immunoblot) Oncogene Medium 19330022
2009 RACK1 co-immunoprecipitates with endogenous FEM1B in metastatic colon cancer cells, stimulates FEM1B ubiquitination, and downregulates FEM1B protein levels. The N-terminal region of FEM1B is sufficient for RACK1 association. RACK1 knockdown elevates FEM1B and induces apoptosis, which is suppressed by blocking FEM1B upregulation. Co-immunoprecipitation, co-expression ubiquitination assay in HEK293T, siRNA knockdown, apoptosis assay Cancer biology & therapy Medium 19855191
2013 Fem1b directly binds Gli1 (mammalian homolog of nematode TRA-1) via its VHL-box, promotes Gli1 ubiquitylation, suppresses Gli1 transcriptional activity, and attenuates an oncogenic Gli1 autoregulatory loop in cancer cells. These effects are dependent on the VHL-box motif of Fem1b. Co-immunoprecipitation, in-cell ubiquitylation assay, transcriptional reporter assay, domain mutagenesis (VHL-box) Biochemical and biophysical research communications Medium 24076122
2011 Mouse Fem1b binds Ankrd37 (identified by yeast two-hybrid and co-IP), facilitates nuclear translocation of Fem1b in co-transfected cells, and targets Ankrd37 for ubiquitin-mediated degradation in a Fem1b dose-dependent manner. Yeast two-hybrid, co-immunoprecipitation, transfection-based degradation assay, ubiquitination assay Gene Medium 21723927
2010 Fem1b protein is downregulated by the proteasome in malignant colon cancer cells; proteasome inhibitor treatment upregulates Fem1b protein, and antisense morpholino blockade of Fem1b upregulation suppresses the resulting apoptosis, demonstrating that Fem1b mediates proteasome inhibitor-induced apoptosis. Proteasome inhibitor treatment, morpholino antisense oligonucleotide, apoptosis assay, transfection over-expression Molecular carcinogenesis Medium 19908242
2004 FEM1B interacts with PHTF1 via the ANK domain of FEM1B (N-terminus of PHTF1 is the binding region), shown by yeast two-hybrid, co-immunoprecipitation, and co-expression. PHTF1 recruits FEM1B to the endoplasmic reticulum membrane. Yeast two-hybrid, co-immunoprecipitation, domain mapping, subcellular co-localization Biology of reproduction Medium 15601915
2008 Mouse Fem1b interacts with the homeodomain protein Nkx3.1 in GST pull-down and co-immunoprecipitation assays; both proteins are co-expressed in neonatal prostate and testis. Fem1b null mice (gene targeting) display defects in prostate ductal morphogenesis and secretory protein expression, similar to Nkx3.1 mutant phenotypes. GST pull-down, co-immunoprecipitation, gene-targeted knockout mouse, histology/developmental analysis Developmental dynamics Medium 18816836
2023 miR-29a-3p downregulates FEM1B expression, leading to stabilization of Gli1 (a FEM1B ubiquitination target); reduced FEM1B levels are sufficient to confer oxaliplatin resistance in colorectal cancer cells, and GLI1 knockdown reverses this resistance, placing FEM1B upstream of Gli1 in a chemoresistance pathway. miRNA overexpression, FEM1B knockdown, GLI1 knockdown, oxaliplatin resistance assay, in vivo mouse tumor model American journal of cancer research Medium 38187042
2025 FEM1B enhances TRAIL-induced apoptosis through the extrinsic caspase-dependent pathway (activating caspase-3 and caspase-8, but not caspase-9). In T lymphocyte lines, FEM1B co-immunoprecipitates with TRAF2 and downregulates TRAF2, diminishing TRAF2's inhibitory effect on caspase-8. In monocyte lines, FEM1B upregulates TRAIL-R2 to promote TRAIL-induced apoptosis. Co-immunoprecipitation, FEM1B knockdown/knockout, caspase activity assays, flow cytometry apoptosis assay, murine KO model FEBS open bio Medium 40392678
2025 METTL3-mediated m6A modification reduces Fem1b mRNA stability in skeletal stem cells, causing lower FEM1B protein levels; reduced FEM1B in turn stabilizes Gli1, maintaining SSC quiescence. Genetic deletion of Mettl3 in murine SSCs elevates Fem1b and destabilizes Gli1, causing quiescence exit. Conditional Mettl3 knockout mouse, m6A sequencing, RNA stability assay, Gli1 protein level measurement The EMBO journal Medium 40016417
2024 Stop codon readthrough (SCR) of FEM1B generates a C-terminally extended, highly unstable isoform. The 81-nt proximal 3'UTR serves as the cis-signal for SCR and encodes the degron responsible for instability of the SCR product. CRISPR deletion of this region increases FEM1B expression, reduces SLBP (a FEM1B-mediated degradation target), reduces replication-dependent histones, and causes cell cycle delay. Stop codon readthrough reporter assays, CRISPR editing, protein stability assays, cell cycle analysis, SLBP expression measurement Journal of cell science Medium 39140134
2005 Mice with targeted inactivation of Fem1b (Fem1b-KO) display abnormal glucose tolerance due predominantly to defective glucose-stimulated and arginine-stimulated insulin secretion. Fem1b is expressed in pancreatic islet beta cells and non-beta cells and in INS-1E cells, implicating it in islet function. Gene-targeted knockout mouse, glucose tolerance test, insulin secretion assay (glucose- and arginine-stimulated) Molecular and cellular biology Medium 16024793
2024 A de novo gain-of-function missense variant FEM1B p.Arg126Gln causes aberrant FEM1B activation. Overexpression of the Arg126Gln variant (but not wild-type) during mouse brain development (in utero electroporation) results in delayed neuronal migration. Patient cells exhibit oxidative stress and induction of type I interferon signaling. In utero electroporation in mouse brain, patient cell oxidative stress assay, interferon signaling measurement Genetics in medicine Medium 38465576

Source papers

Stage 0 corpus · 27 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2022 Discovery of a Covalent FEM1B Recruiter for Targeted Protein Degradation Applications. Journal of the American Chemical Society 166 34994556
2023 Glucose-responsive, antioxidative HA-PBA-FA/EN106 hydrogel enhanced diabetic wound healing through modulation of FEM1b-FNIP1 axis and promoting angiogenesis. Bioactive materials 59 37521275
2009 RACK1 downregulates levels of the pro-apoptotic protein Fem1b in apoptosis-resistant colon cancer cells. Cancer biology & therapy 28 19855191
2005 Abnormal glucose homeostasis and pancreatic islet function in mice with inactivation of the Fem1b gene. Molecular and cellular biology 25 16024793
2000 Sequence, organization, and expression of the human FEM1B gene. Biochemical and biophysical research communications 22 10623617
2008 FEM1A and FEM1B: novel candidate genes for polycystic ovary syndrome. Human reproduction (Oxford, England) 21 18757445
2011 Mouse Fem1b interacts with and induces ubiquitin-mediated degradation of Ankrd37. Gene 20 21723927
2010 Fem1b, a proapoptotic protein, mediates proteasome inhibitor-induced apoptosis of human colon cancer cells. Molecular carcinogenesis 20 19908242
2024 Mechanism of Ψ-Pro/C-degron recognition by the CRL2FEM1B ubiquitin ligase. Nature communications 19 38670995
2013 Fem1b promotes ubiquitylation and suppresses transcriptional activity of Gli1. Biochemical and biophysical research communications 18 24076122
2021 Structural insights into SMCR8 C-degron recognition by FEM1B. Biochemical and biophysical research communications 14 33892462
2025 Development of the First-in-Class FEM1B-Recruiting Histone Deacetylase Degraders. Journal of medicinal chemistry 10 39804678
2004 Putative homeodomain transcription factor 1 interacts with the feminization factor homolog fem1b in male germ cells. Biology of reproduction 10 15601915
2008 Mouse Fem1b interacts with the Nkx3.1 homeoprotein and is required for proper male secondary sexual development. Developmental dynamics : an official publication of the American Association of Anatomists 9 18816836
2023 Induction of resistance to oxaliplatin in cancer by a microRNA/Fem1B/Gli1 pathway. American journal of cancer research 7 38187042
2016 Molecular cloning and expression analysis of Fem1b from oriental river prawn Macrobrachium nipponense. Genetics and molecular research : GMR 6 27323097
2009 Human FEM1B is required for Rad9 recruitment and CHK1 activation in response to replication stress. Oncogene 6 19330022
2024 A recurrent missense variant in the E3 ubiquitin ligase substrate recognition subunit FEM1B causes a rare syndromic neurodevelopmental disorder. Genetics in medicine : official journal of the American College of Medical Genetics 5 38465576
2000 Rapid communication: the human FEM1B gene maps to chromosome 15q22 and is excluded as the gene for Bardet-Biedl syndrome, type 4. The American journal of the medical sciences 3 10768616
2022 miR-29b Regulates Lung Cancer Progression by Downregulating FEM1B and Inhibiting the FOX01/AKT Pathway. Computational and mathematical methods in medicine 2 36003920
2025 An epitranscriptomic program maintains skeletal stem cell quiescence via a METTL3-FEM1B-GLI1 axis. The EMBO journal 1 40016417
2010 Fem1b antigen in the stool of ApcMin mice as a biomarker of early Wnt signaling activation in intestinal neoplasia. Cancer epidemiology 1 20952268
2026 CRL2FEM1B uses heme to recruit BACH1 for degradation and regulate ferroptosis in lung cancer. Molecular cell 0 42086045
2025 FEM1B enhances TRAIL-induced apoptosis in T lymphocytes and monocytes. FEBS open bio 0 40392678
2024 Hominini-specific regulation of the cell cycle by stop codon readthrough of FEM1B. Journal of cell science 0 39140134
2023 Retracted: miR-29b Regulates Lung Cancer Progression by Downregulating FEM1B and Inhibiting the FOX01/AKT Pathway. Computational and mathematical methods in medicine 0 37829534
2009 WITHDRAWN: Fem1b interacts with Ankrd37 in mouse testis and induces its degradation by ubiquitin-mediated proteolysis pathway. Cellular signalling 0 19171189

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