Affinage

FAAP24

Fanconi anemia core complex-associated protein 24 · UniProt Q9BTP7

Length
215 aa
Mass
23.9 kDa
Annotated
2026-06-09
12 papers in source corpus 10 papers cited in narrative 11 extracted findings
Cross-family judge vs UniProt: Affinage preferred faithfulness: 6/6 claims corpus-supported (100%)

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

FAAP24 is a DNA-binding component of the Fanconi anemia (FA) core complex that couples recognition of stalled replication forks and interstrand crosslinks (ICLs) to both FA pathway activation and ATR-dependent checkpoint signaling (PMID:17289582, PMID:20670894). It heterodimerizes with the C-terminal region of FANCM through a shared XPF/MUS81-like architecture, forming an S-shaped complex in which both subunits contribute a pseudo-nuclease domain and a (HhH)2 domain; the FANCM nuclease center is catalytically inactive and the heterodimer lacks endonucleolytic activity (PMID:23932590, PMID:24003026). The FAAP24 (HhH)2 domain directly engages DNA—its canonical HhH motif binds dsDNA while its unstructured N-terminus binds ssDNA, allowing both surfaces to recognize ICL-like single/double-strand junctions (PMID:23999858, PMID:23661679). Through this interaction FAAP24 stabilizes FANCM, targets it to damaged-DNA structures, and drives cell cycle-dependent chromatin loading of the FA core complex, thereby promoting FANCD2 monoubiquitination (PMID:17289582, PMID:18174376). Independently of the FA core complex, FAAP24 (with FANCM) associates with the checkpoint protein HCLK2 and uses its DNA-binding activity to recruit RPA to ICL-stalled forks and activate ATR/CHK1 signaling, a function genetically separable from FANCM's translocase-dependent role in lesion incision (PMID:18995830, PMID:20670894, PMID:23333308). A homozygous FAAP24 missense mutation (T212M) in patients impairs FANCD2 monoubiquitination and delays CHK1 phosphorylation, confirming both roles in a disease context (PMID:27473539).

Mechanistic history

Synthesis pass · year-by-year structured walk · 8 steps
  1. 2007 High

    Established that an uncharacterized FANCM-associated protein is itself a functional FA core complex component, defining FAAP24 as required for the DNA-crosslink response.

    Evidence Co-IP, siRNA knockdown with FANCD2 monoubiquitylation and crosslinker-sensitivity readouts in human cells

    PMID:17289582

    Open questions at the time
    • Molecular basis of the FANCM interaction not resolved
    • Direct DNA-binding activity not yet demonstrated
  2. 2008 High

    Showed FAAP24 acts upstream of FA core complex chromatin loading by stabilizing FANCM and enabling its cell cycle-dependent chromatin association.

    Evidence siRNA knockdown with chromatin fractionation and immunoblotting in human cells

    PMID:18174376

    Open questions at the time
    • Whether FAAP24 contacts chromatin directly or only via FANCM not distinguished here
  3. 2008 High

    Revealed an FA-core-complex-independent role: FAAP24/FANCM associate with HCLK2 to support ATR/CHK1 checkpoint signaling, separating checkpoint from repair functions.

    Evidence HCLK2 complex proteomics, siRNA, and checkpoint phosphorylation/focus assays

    PMID:18995830

    Open questions at the time
    • Mechanism linking HCLK2 association to ATR activation undefined
    • Direct vs indirect FAAP24-HCLK2 contact not resolved
  4. 2010 High

    Defined the DNA-binding activity of FAAP24 (not FANCM translocase) as the determinant for recruiting RPA to ICL-stalled forks to trigger ATR signaling.

    Evidence Separation-of-function mutants with RPA focus and ATR activation assays after ICL induction

    PMID:20670894

    Open questions at the time
    • Direct FAAP24-RPA contact not established
    • Order of RPA loading vs FAAP24 DNA binding not resolved
  5. 2013 High

    Atomic structures explained the architecture: FANCM-FAAP24 forms an XPF/MUS81-like heterodimer in which FAAP24's (HhH)2 motif engages DNA while the FANCM nuclease is inactive, accounting for binding without cleavage.

    Evidence X-ray crystallography and NMR of FANCM-CTD/FAAP24 with DNA, plus mutagenesis validated in vitro and in vivo

    PMID:23932590 PMID:24003026

    Open questions at the time
    • Structure of full-length complex on a fork substrate not solved
    • Functional role of the buried FANCM (HhH)2 domain unclear
  6. 2013 High

    Demonstrated FAAP24 has intrinsic, separable DNA-binding modes—canonical HhH for dsDNA and an unstructured N-terminus for ssDNA—enabling recognition of ICL-like junctions and dual roles distinct from FANCM binding.

    Evidence NMR solution structures, NMR titrations, segregation-of-function mutagenesis, in vitro and in vivo checkpoint assays

    PMID:23661679 PMID:23999858

    Open questions at the time
    • Affinity/specificity for physiological ICL intermediates in vivo not quantified
  7. 2013 High

    Genetically separated FAAP24 and FANCM functions: FAAP24 drives ATR checkpoint activation while FANCM mediates translocase-dependent ICL incision, with both cooperating in FA pathway activation and SCE suppression.

    Evidence Isogenic knockout cell lines with epistasis, SCE, sensitivity, and checkpoint assays

    PMID:23333308

    Open questions at the time
    • How the two activities are temporally coordinated at a single lesion unresolved
  8. 2016 Medium

    Linked FAAP24 dysfunction to human disease, showing a patient missense mutation impairs both FA pathway and checkpoint outputs in vivo.

    Evidence Whole-exome sequencing and patient-derived T cells assayed for FANCD2 ubiquitination and CHK1 phosphorylation

    PMID:27473539

    Open questions at the time
    • No in vitro reconstitution of the T212M mutant
    • Single-family/two-patient observation
    • Effect on DNA binding vs FANCM interaction not dissected

Open questions

Synthesis pass · forward-looking unresolved questions
  • How FAAP24 mechanistically bridges its DNA-junction recognition to RPA loading and ATR activation, and whether it directly contacts RPA or HCLK2, remains unresolved.
  • No reconstituted RPA-recruitment assay defining direct partners
  • Stoichiometry of FAAP24 in checkpoint vs core complex pools unknown

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0003677 DNA binding 5 GO:0060089 molecular transducer activity 2
Localization
GO:0005634 nucleus 2 GO:0005694 chromosome 2
Pathway
R-HSA-73894 DNA Repair 3 R-HSA-8953897 Cellular responses to stimuli 2
Partners
FANCMHCLK2RPA
Complex memberships
FANCM-FAAP24 heterodimerFanconi anemia core complex

Evidence

Reading pass · 11 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2007 FAAP24 associates with the C-terminal region of FANCM, shares homology with the XPF family of flap/fork endonucleases, and is a component of the FA core complex. FAAP24 is required for normal levels of FANCD2 monoubiquitylation following DNA damage, and depletion by siRNA causes cellular hypersensitivity to DNA crosslinking agents and chromosomal instability. Co-immunoprecipitation, siRNA knockdown, DNA damage sensitivity assays, FANCD2 monoubiquitylation assay Molecular cell High 17289582
2007 FAAP24 targets FANCM to structures that mimic DNA replication/repair intermediates (e.g., stalled replication forks), suggesting the FANCM/FAAP24 complex plays a key role in recruitment of the FA core complex to damaged DNA. Immunofluorescence/focus formation assays, co-immunoprecipitation Molecular cell High 17289582
2008 Depletion of FAAP24 disrupts chromatin association of FANCM and destabilizes FANCM protein, leading to defective recruitment of the FA core complex to chromatin. The FANCM/FAAP24 interaction is essential for cell cycle-dependent chromatin loading of the FA core complex. siRNA knockdown, chromatin fractionation, immunoblotting Blood High 18174376
2008 FANCM and FAAP24 interact with the checkpoint protein HCLK2 independently of the FA core complex. Downregulation of FAAP24 compromises ATR/Chk1-mediated checkpoint signaling, leading to defective Chk1, p53, and FANCE phosphorylation; 53BP1 focus formation; and Cdc25A degradation, demonstrating a role for FAAP24 in ATR checkpoint signaling that is independent of its role in the FA core complex. Co-immunoprecipitation (HCLK2 complex proteomics), siRNA knockdown, phosphorylation assays, focus formation assays Molecular cell High 18995830
2010 The DNA-binding activity of FAAP24 (but not the DNA translocase activity of FANCM) is required for FANCM/FAAP24-dependent recruitment of RPA to ICL-stalled replication forks, which in turn is required for efficient ATR-mediated checkpoint activation in response to interstrand crosslinks. siRNA knockdown, RPA focus formation assays, ATR checkpoint activation assays, separation-of-function mutant analysis Molecular cell High 20670894
2013 Crystal structure of the FANCM C-terminal domain (CTD) bound to FAAP24 and DNA reveals an S-shaped heterodimer similar to XPF/MUS81 endonucleases. The FAAP24 (HhH)2 domain directly engages DNA while the FANCM (HhH)2 domain is buried. The FANCM pseudo-nuclease domain contains a metal center and a second DNA contact; mutations in either impair dsDNA binding in vitro and FANCM-FAAP24 function in vivo. The complex lacks endonucleolytic activity. X-ray crystallography, in vitro DNA binding assays, site-directed mutagenesis, electron microscopy, ATPase assay Structure (London, England : 1993) High 23932590
2013 Crystal structure of FANCM C-terminal segment in complex with FAAP24 shows that both subunits contain an N-terminal nuclease domain and a C-terminal (HhH)2 domain, forming an architecture similar to ApXPF. The FANCM nuclease domain is catalytically inactive due to variations in key active-site residues. The first HhH motif of FAAP24 is a DNA-binding site critical for targeting FANCM-FAAP24 to chromatin. X-ray crystallography, site-directed mutagenesis, in vitro DNA binding assays, chromatin association assays Nucleic acids research High 24003026
2013 FAAP24 possesses intrinsic single-stranded DNA (ssDNA)-binding activity mediated by its (HhH)2 domain. The DNA-binding and FANCM-interacting functions of FAAP24, while both requiring the C-terminal (HhH)2 domain, can be separated by segregation-of-function mutations, demonstrating dual independent roles: one through FANCM/FAAP24 heterodimer formation and one via ssDNA binding for checkpoint activation. NMR solution structure determination, in vitro DNA binding assays (various FAAP24 mutations), in vivo checkpoint assays Cell research High 23999858
2013 NMR structure of the FAAP24 HhH domain reveals a canonical hairpin motif followed by a distorted motif. The canonical HhH motif is required for dsDNA binding, whereas the unstructured N-terminus interacts with ssDNA. Both surfaces together bind ICL-like single/double-strand junction DNA substrates. NMR structure determination, NMR titration experiments, site-directed mutagenesis, homology modeling Nucleic acids research High 23661679
2013 FAAP24 and FANCM have non-overlapping (non-epistatic) functions: FAAP24 specifically promotes ATR-mediated checkpoint activation in response to DNA crosslinking agents, while FANCM (but not FAAP24) participates in recombination-independent ICL repair by facilitating lesion incision through its translocase activity. Both cooperate to suppress sister chromatid exchange and activate the FA pathway. Isogenic knockout cell lines, epistasis analysis, sister chromatid exchange assays, DNA damage sensitivity assays, checkpoint activation assays Molecular cell High 23333308
2016 A homozygous missense mutation in FAAP24 (T212M) found in two human patients results in impaired FANCD2 monoubiquitination and delayed CHK1 phosphorylation, confirming in a disease context that FAAP24 is required for both FA pathway activation and ATR/CHK1 checkpoint signaling. Whole exome sequencing, patient-derived T cells (HVS-transformed), FANCD2 monoubiquitination assay, CHK1 phosphorylation assay 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 154 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 69 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 15 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

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