Affinage

FAAP24

Fanconi anemia core complex-associated protein 24 · UniProt Q9BTP7

Length
215 aa
Mass
23.9 kDa
Annotated
2026-04-28
12 papers in source corpus 11 papers cited in narrative 11 extracted findings

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

FAAP24 is an XPF-family homolog that heterodimerizes with FANCM to target the Fanconi anemia (FA) core complex to chromatin at stalled replication forks, while also independently promoting ATR/Chk1 checkpoint signaling in response to DNA interstrand crosslinks. FAAP24 binds both ssDNA and dsDNA through its C-terminal (HhH)2 domain, using distinct surfaces—a canonical HhH motif for dsDNA and an unstructured N-terminal region for ssDNA—that cooperate to recognize junction-containing substrates resembling ICL repair intermediates (PMID:23661679, PMID:23999858, PMID:23932590). FAAP24 stabilizes FANCM and is required for its chromatin association; loss of FAAP24 impairs FANCD2 monoubiquitylation and, through its DNA-binding-dependent recruitment of RPA to ICL-stalled forks, compromises ATR-mediated Chk1 phosphorylation, p53 activation, and Cdc25A degradation (PMID:17289582, PMID:18174376, PMID:20670894, PMID:18995830). A homozygous FAAP24 missense mutation (T212M) in human patients causes impaired FANCD2 monoubiquitylation and delayed CHK1 phosphorylation, confirming its physiological role in both the FA pathway and the DNA damage checkpoint (PMID:27473539).

Mechanistic history

Synthesis pass · year-by-year structured walk · 7 steps
  1. 2007 High

    Identification of FAAP24 as an XPF-family homolog and FA core complex component resolved how FANCM is targeted to fork-like DNA structures and established that FAAP24 is required for damage-induced FANCD2 monoubiquitylation.

    Evidence Co-immunoprecipitation, siRNA depletion, and FANCD2 monoubiquitylation assays in human cells

    PMID:17289582

    Open questions at the time
    • Structural basis of FANCM–FAAP24 interaction unknown
    • Whether FAAP24 has functions independent of the FA core complex not tested
    • DNA-binding specificity of FAAP24 not yet characterized
  2. 2008 High

    Demonstration that FAAP24 depletion destabilizes FANCM and prevents its chromatin association established that FAAP24 is the obligate partner needed for chromatin loading of the FA core complex.

    Evidence Chromatin fractionation and co-immunoprecipitation after siRNA depletion in human cells

    PMID:18174376

    Open questions at the time
    • Whether chromatin targeting depends on FAAP24 DNA binding versus protein stabilization not resolved
    • Contribution of individual FAAP24 domains not mapped
  3. 2008 High

    Discovery that FANCM–FAAP24 interacts with HCLK2 independently of the FA core complex and that FAAP24 loss compromises ATR/Chk1 checkpoint signaling revealed a second, FA-core-independent function in the DNA damage checkpoint.

    Evidence Co-immunoprecipitation, siRNA depletion, phosphorylation assays, and 53BP1 focus formation in human cells

    PMID:18995830

    Open questions at the time
    • Whether FAAP24 directly contacts HCLK2 or acts through FANCM unclear
    • Mechanism linking FAAP24 to ATR activation not defined
  4. 2010 High

    Segregation-of-function experiments showed that FAAP24 DNA-binding activity (not FANCM translocase activity) drives RPA recruitment to ICL-stalled forks, establishing the mechanistic basis for FAAP24's checkpoint role.

    Evidence DNA-binding mutant analysis, RPA focus formation, and ATR checkpoint assays in human cells

    PMID:20670894

    Open questions at the time
    • How FAAP24 DNA binding promotes RPA loading mechanistically unknown
    • Whether FAAP24 directly contacts RPA not tested
  5. 2013 High

    Crystal and NMR structures of the FANCM–FAAP24 complex defined the heterodimer architecture: both subunits contain pseudo-nuclease and (HhH)2 domains; FAAP24's (HhH)2 domain is the primary DNA-contacting module using two distinct surfaces for ssDNA and dsDNA, while the FANCM nuclease domain is catalytically dead, explaining the lack of endonuclease activity despite XPF homology.

    Evidence X-ray crystallography and NMR of FANCM C-term–FAAP24, mutagenesis, DNA binding assays, and chromatin association assays

    PMID:23661679 PMID:23932590 PMID:23999858 PMID:24003026

    Open questions at the time
    • Full-length complex structure not available
    • How ssDNA and dsDNA binding surfaces cooperate on branched substrates in vivo not resolved
  6. 2013 High

    Epistasis analysis in isogenic knockouts separated FANCM and FAAP24 functions: FAAP24 specifically promotes ATR-mediated checkpoint activation after crosslinking, while FANCM drives recombination-independent ICL repair through its translocase activity; both cooperatively suppress sister chromatid exchange.

    Evidence Isogenic knockout models, checkpoint assays, and sister chromatid exchange assays in human cells

    PMID:23333308

    Open questions at the time
    • Molecular mechanism by which FAAP24 activates ATR independently of FANCM translocase activity not fully elucidated
    • Whether FAAP24 has additional partners beyond FANCM and HCLK2 not explored
  7. 2016 Medium

    A homozygous FAAP24-T212M patient mutation validated the dual physiological roles of FAAP24 in FA pathway activation and checkpoint signaling, confirming cell-based models in a human disease context.

    Evidence Whole exome sequencing of patient, FANCD2 monoubiquitylation and CHK1 phosphorylation assays in patient-derived T cells

    PMID:27473539

    Open questions at the time
    • Single patient/family reported; independent replication in additional families lacking
    • Structural consequence of T212M mutation on FAAP24 fold or DNA binding not characterized
    • Full clinical spectrum and genotype-phenotype correlation not established

Open questions

Synthesis pass · forward-looking unresolved questions
  • The mechanism by which FAAP24 DNA binding facilitates RPA loading at ICL-stalled forks, whether FAAP24 directly contacts RPA or additional checkpoint factors, and a full-length structure of the FANCM–FAAP24 complex bound to branched DNA remain unresolved.
  • No reconstitution of FAAP24-dependent RPA loading with purified components
  • Direct interaction between FAAP24 and RPA not tested biochemically
  • Full-length FANCM–FAAP24 structure on branched DNA not available

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0003677 DNA binding 6
Localization
GO:0005694 chromosome 2
Pathway
R-HSA-73894 DNA Repair 3 R-HSA-69306 DNA Replication 1
Partners
FANCD2FANCMHCLK2
Complex memberships
FA core complexFANCM-FAAP24 heterodimer

Evidence

Reading pass · 11 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2007 FAAP24 shares homology with the XPF family of flap/fork endonucleases, associates with the C-terminal region of FANCM, and is a component of the FA core complex. FAAP24 targets FANCM to DNA structures that mimic replication/repair intermediates and is required for normal levels of FANCD2 monoubiquitylation following DNA damage. Co-immunoprecipitation, siRNA depletion, FANCD2 monoubiquitylation assay, cellular sensitivity assays Molecular cell High 17289582
2008 FANCM and FAAP24 interact with the checkpoint protein HCLK2 independently of the FA core complex, and downregulation of FAAP24 compromises ATR/Chk1-mediated checkpoint signaling, leading to defective Chk1, p53, and FANCE phosphorylation; 53BP1 focus formation; and Cdc25A degradation. siRNA depletion, co-immunoprecipitation, phosphorylation assays, 53BP1 focus formation assays Molecular cell High 18995830
2008 Depletion of FAAP24 disrupts the chromatin association of FANCM and destabilizes FANCM, leading to defective recruitment of the FA core complex to chromatin, establishing that the FANCM/FAAP24 interaction is essential for chromatin-loading activity. siRNA depletion, chromatin fractionation, co-immunoprecipitation Blood High 18174376
2010 The DNA-binding activity of FAAP24 (but not the DNA translocase activity of FANCM) is specifically required for recruitment of RPA to ICL-stalled replication forks, and this FANCM/FAAP24-dependent RPA recruitment is required for efficient ATR-mediated checkpoint activation in response to ICL. siRNA depletion, RPA focus formation assays, DNA-binding mutant analysis, ATR checkpoint assays Molecular cell High 20670894
2013 Crystal structure of the FANCM C-terminal domain bound to FAAP24 and DNA reveals that the FAAP24 (HhH)2 domain engages DNA while the FANCM (HhH)2 domain is buried; mutations in the FANCM pseudo-nuclease domain metal center impair dsDNA binding in vitro and FANCM-FAAP24 function in vivo. The complex lacks endonucleolytic activity despite XPF/MUS81 homology. X-ray crystallography, in vitro DNA binding assays, site-directed mutagenesis, electron microscopy Structure High 23932590
2013 Crystal structure of the C-terminal segment of FANCM in complex with FAAP24 shows both subunits contain a nuclease domain and tandem (HhH)2 domain; the FANCM nuclease domain is catalytically inactive due to variations in key active-site residues; the first HhH motif of FAAP24 is the primary DNA-binding site required for chromatin targeting. X-ray crystallography, site-directed mutagenesis, in vitro DNA binding assays, chromatin association assays Nucleic acids research High 24003026
2013 FAAP24 possesses single-stranded DNA (ssDNA)-binding activity mediated by its C-terminal (HhH)2 domain; the DNA-binding and FANCM-interacting functions of FAAP24, while both requiring the (HhH)2 domain, can be separated by segregation-of-function mutations. ssDNA-binding activity of FAAP24 is required independently for optimized checkpoint activation in response to crosslinking lesions. NMR structure determination, in vitro DNA binding assays, site-directed mutagenesis, in vivo checkpoint assays Cell research High 23999858
2013 The HhH domain of FAAP24 uses two distinct DNA-binding surfaces: the canonical HhH motif binds dsDNA, while the unstructured N-terminus binds ssDNA; both surfaces cooperate to bind ICL-like single/double-strand junction-containing DNA substrates. NMR structure determination, NMR titration experiments, site-directed mutagenesis, DNA binding assays Nucleic acids research High 23661679
2013 FAAP24 specifically promotes ATR-mediated checkpoint activation in response to DNA crosslinking agents, whereas FANCM participates in recombination-independent ICL repair by facilitating recruitment of lesion incision activities (requiring its translocase activity); FANCM and FAAP24 cooperatively suppress sister chromatid exchange and activate the FA pathway. Isogenic knockout cell models, epistasis analysis, checkpoint assays, sister chromatid exchange assays Molecular cell High 23333308
2009 FANCM and FAAP24 interact with HCLK2, and the DNA translocase activity of FANCM is essential for efficient ATR checkpoint signaling, a function distinct from FA core complex targeting; this interaction was identified by proteomic analysis of HCLK2 complexes. Proteomic/mass spectrometry analysis of HCLK2 complexes, co-immunoprecipitation Cell cycle Medium 19282663
2016 A homozygous missense mutation in FAAP24 (T212M) in human patients results in impaired FANCD2 monoubiquitination and delayed CHK1 phosphorylation in patient T cells, confirming the dual roles of FAAP24 in FA pathway activation and ATR-mediated checkpoint signaling in a physiological context. Patient-derived cell analysis, FANCD2 monoubiquitination assay, CHK1 phosphorylation assay, whole exome sequencing Journal of clinical immunology Medium 27473539

Source papers

Stage 0 corpus · 12 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2007 Identification of FAAP24, a Fanconi anemia core complex protein that interacts with FANCM. Molecular cell 250 17289582
2008 FANCM and FAAP24 function in ATR-mediated checkpoint signaling independently of the Fanconi anemia core complex. Molecular cell 167 18995830
2008 Cell cycle-dependent chromatin loading of the Fanconi anemia core complex by FANCM/FAAP24. Blood 153 18174376
2010 The FANCM/FAAP24 complex is required for the DNA interstrand crosslink-induced checkpoint response. Molecular cell 105 20670894
2013 FANCM and FAAP24 maintain genome stability via cooperative as well as unique functions. Molecular cell 68 23333308
2013 Architecture and DNA recognition elements of the Fanconi anemia FANCM-FAAP24 complex. Structure (London, England : 1993) 27 23932590
2009 FANCM-FAAP24 and FANCJ: FA proteins that metabolize DNA. Mutation research 25 19379763
2013 Structural insights into the functions of the FANCM-FAAP24 complex in DNA repair. Nucleic acids research 14 24003026
2016 Fatal Lymphoproliferative Disease in Two Siblings Lacking Functional FAAP24. Journal of clinical immunology 13 27473539
2009 FANCM-FAAP24 and HCLK2: roles in ATR signalling and the Fanconi anemia pathway. Cell cycle (Georgetown, Tex.) 13 19282663
2013 Structure analysis of FAAP24 reveals single-stranded DNA-binding activity and domain functions in DNA damage response. Cell research 7 23999858
2013 The Fanconi anemia associated protein FAAP24 uses two substrate specific binding surfaces for DNA recognition. Nucleic acids research 4 23661679