{"gene":"FAAP24","run_date":"2026-06-09T23:54:43","timeline":{"discoveries":[{"year":2007,"finding":"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.","method":"Co-immunoprecipitation, siRNA knockdown, DNA damage sensitivity assays, FANCD2 monoubiquitylation assay","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal Co-IP, functional siRNA knockdown with multiple orthogonal readouts, replicated by subsequent independent studies","pmids":["17289582"],"is_preprint":false},{"year":2007,"finding":"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.","method":"Immunofluorescence/focus formation assays, co-immunoprecipitation","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 2 / Strong — direct localization experiments with functional consequence, replicated in subsequent studies","pmids":["17289582"],"is_preprint":false},{"year":2008,"finding":"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.","method":"siRNA knockdown, chromatin fractionation, immunoblotting","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 / Strong — chromatin fractionation with functional consequence, replicated across multiple labs","pmids":["18174376"],"is_preprint":false},{"year":2008,"finding":"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.","method":"Co-immunoprecipitation (HCLK2 complex proteomics), siRNA knockdown, phosphorylation assays, focus formation assays","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 2 / Strong — MS-based interactome confirmation plus siRNA with multiple orthogonal checkpoint readouts, replicated by subsequent studies","pmids":["18995830"],"is_preprint":false},{"year":2010,"finding":"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.","method":"siRNA knockdown, RPA focus formation assays, ATR checkpoint activation assays, separation-of-function mutant analysis","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 2 / Strong — separation-of-function mutations combined with multiple functional readouts, mechanistically defines FAAP24 DNA-binding as critical","pmids":["20670894"],"is_preprint":false},{"year":2013,"finding":"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.","method":"X-ray crystallography, in vitro DNA binding assays, site-directed mutagenesis, electron microscopy, ATPase assay","journal":"Structure (London, England : 1993)","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure with mutagenesis validated in vitro and in vivo, multiple orthogonal methods in one study","pmids":["23932590"],"is_preprint":false},{"year":2013,"finding":"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.","method":"X-ray crystallography, site-directed mutagenesis, in vitro DNA binding assays, chromatin association assays","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure with mutagenesis and functional chromatin-binding validation, two independent structural studies reach consistent conclusions","pmids":["24003026"],"is_preprint":false},{"year":2013,"finding":"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.","method":"NMR solution structure determination, in vitro DNA binding assays (various FAAP24 mutations), in vivo checkpoint assays","journal":"Cell research","confidence":"High","confidence_rationale":"Tier 1 / Moderate — NMR structure with separation-of-function mutagenesis validated in vitro and in vivo, single lab but multiple orthogonal methods","pmids":["23999858"],"is_preprint":false},{"year":2013,"finding":"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.","method":"NMR structure determination, NMR titration experiments, site-directed mutagenesis, homology modeling","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 1 / Moderate — NMR structure with NMR titration binding experiments and mutagenesis, single lab but multiple orthogonal methods","pmids":["23661679"],"is_preprint":false},{"year":2013,"finding":"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.","method":"Isogenic knockout cell lines, epistasis analysis, sister chromatid exchange assays, DNA damage sensitivity assays, checkpoint activation assays","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 2 / Strong — isogenic knockout models with epistasis analysis and multiple orthogonal functional readouts","pmids":["23333308"],"is_preprint":false},{"year":2016,"finding":"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.","method":"Whole exome sequencing, patient-derived T cells (HVS-transformed), FANCD2 monoubiquitination assay, CHK1 phosphorylation assay","journal":"Journal of clinical immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional validation in patient-derived cells with two orthogonal readouts, single lab, no in vitro reconstitution of mutant","pmids":["27473539"],"is_preprint":false}],"current_model":"FAAP24 is a component of the FA core complex that heterodimerizes with FANCM via their shared XPF/MUS81-like C-terminal architecture (pseudo-nuclease + (HhH)2 domains); within this complex, FAAP24 directly binds DNA (both ssDNA via its unstructured N-terminus and dsDNA/ICL-junction substrates via its canonical HhH motif) to anchor the FA core complex to chromatin and stalled replication forks, promote FANCD2 monoubiquitination, and—independently of the FA core complex through interaction with HCLK2—recruit RPA to ICL-stalled forks and activate ATR/CHK1-mediated checkpoint signaling."},"narrative":{"mechanistic_narrative":"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].","teleology":[{"year":2007,"claim":"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","pmids":["17289582"],"confidence":"High","gaps":["Molecular basis of the FANCM interaction not resolved","Direct DNA-binding activity not yet demonstrated"]},{"year":2008,"claim":"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","pmids":["18174376"],"confidence":"High","gaps":["Whether FAAP24 contacts chromatin directly or only via FANCM not distinguished here"]},{"year":2008,"claim":"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","pmids":["18995830"],"confidence":"High","gaps":["Mechanism linking HCLK2 association to ATR activation undefined","Direct vs indirect FAAP24-HCLK2 contact not resolved"]},{"year":2010,"claim":"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","pmids":["20670894"],"confidence":"High","gaps":["Direct FAAP24-RPA contact not established","Order of RPA loading vs FAAP24 DNA binding not resolved"]},{"year":2013,"claim":"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","pmids":["23932590","24003026"],"confidence":"High","gaps":["Structure of full-length complex on a fork substrate not solved","Functional role of the buried FANCM (HhH)2 domain unclear"]},{"year":2013,"claim":"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","pmids":["23999858","23661679"],"confidence":"High","gaps":["Affinity/specificity for physiological ICL intermediates in vivo not quantified"]},{"year":2013,"claim":"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","pmids":["23333308"],"confidence":"High","gaps":["How the two activities are temporally coordinated at a single lesion unresolved"]},{"year":2016,"claim":"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","pmids":["27473539"],"confidence":"Medium","gaps":["No in vitro reconstitution of the T212M mutant","Single-family/two-patient observation","Effect on DNA binding vs FANCM interaction not dissected"]},{"year":null,"claim":"How FAAP24 mechanistically bridges its DNA-junction recognition to RPA loading and ATR activation, and whether it directly contacts RPA or HCLK2, remains unresolved.","evidence":"","pmids":[],"confidence":"High","gaps":["No reconstituted RPA-recruitment assay defining direct partners","Stoichiometry of FAAP24 in checkpoint vs core complex pools unknown"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0003677","term_label":"DNA binding","supporting_discovery_ids":[4,5,6,7,8]},{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[3,4]}],"localization":[{"term_id":"GO:0005694","term_label":"chromosome","supporting_discovery_ids":[2,6]},{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[0,1]}],"pathway":[{"term_id":"R-HSA-73894","term_label":"DNA Repair","supporting_discovery_ids":[0,4,9]},{"term_id":"R-HSA-8953897","term_label":"Cellular responses to stimuli","supporting_discovery_ids":[3,4]}],"complexes":["Fanconi anemia core complex","FANCM-FAAP24 heterodimer"],"partners":["FANCM","HCLK2","RPA"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q9BTP7","full_name":"Fanconi anemia core complex-associated protein 24","aliases":["Fanconi anemia-associated protein of 24 kDa"],"length_aa":215,"mass_kda":23.9,"function":"Plays a role in DNA repair through recruitment of the FA core complex to damaged DNA. Regulates FANCD2 monoubiquitination upon DNA damage. Induces chromosomal instability as well as hypersensitivity to DNA cross-linking agents, when repressed. Targets FANCM/FAAP24 complex to the DNA, preferentially to single strand DNA","subcellular_location":"Nucleus","url":"https://www.uniprot.org/uniprotkb/Q9BTP7/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/FAAP24","classification":"Not Classified","n_dependent_lines":170,"n_total_lines":1208,"dependency_fraction":0.14072847682119205},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/FAAP24","total_profiled":1310},"omim":[{"mim_id":"615128","title":"CENTROMERIC PROTEIN X; CENPX","url":"https://www.omim.org/entry/615128"},{"mim_id":"611140","title":"TELOMERE MAINTENANCE 2; TELO2","url":"https://www.omim.org/entry/611140"},{"mim_id":"610886","title":"ESSENTIAL MEIOTIC STRUCTURE-SPECIFIC ENDONUCLEASE 2; EME2","url":"https://www.omim.org/entry/610886"},{"mim_id":"610884","title":"FA CORE COMPLEX-ASSOCIATED PROTEIN 24; FAAP24","url":"https://www.omim.org/entry/610884"},{"mim_id":"609644","title":"FANCM GENE; FANCM","url":"https://www.omim.org/entry/609644"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Nucleoplasm","reliability":"Supported"},{"location":"Vesicles","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in many","driving_tissues":[],"url":"https://www.proteinatlas.org/search/FAAP24"},"hgnc":{"alias_symbol":["FLJ46828","MGC32020"],"prev_symbol":["C19orf40"]},"alphafold":{"accession":"Q9BTP7","domains":[{"cath_id":"3.40.50.10130","chopping":"17-141","consensus_level":"high","plddt":94.907,"start":17,"end":141},{"cath_id":"1.10.150.20","chopping":"155-211","consensus_level":"high","plddt":92.9902,"start":155,"end":211}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BTP7","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BTP7-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BTP7-F1-predicted_aligned_error_v6.png","plddt_mean":90.06},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=FAAP24","jax_strain_url":"https://www.jax.org/strain/search?query=FAAP24"},"sequence":{"accession":"Q9BTP7","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9BTP7.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9BTP7/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BTP7"}},"corpus_meta":[{"pmid":"17289582","id":"PMC_17289582","title":"Identification of FAAP24, a Fanconi anemia core complex protein that interacts with FANCM.","date":"2007","source":"Molecular cell","url":"https://pubmed.ncbi.nlm.nih.gov/17289582","citation_count":250,"is_preprint":false},{"pmid":"18995830","id":"PMC_18995830","title":"FANCM and FAAP24 function in ATR-mediated checkpoint signaling independently of the Fanconi anemia core complex.","date":"2008","source":"Molecular cell","url":"https://pubmed.ncbi.nlm.nih.gov/18995830","citation_count":167,"is_preprint":false},{"pmid":"18174376","id":"PMC_18174376","title":"Cell cycle-dependent chromatin loading of the Fanconi anemia core complex by FANCM/FAAP24.","date":"2008","source":"Blood","url":"https://pubmed.ncbi.nlm.nih.gov/18174376","citation_count":154,"is_preprint":false},{"pmid":"20670894","id":"PMC_20670894","title":"The FANCM/FAAP24 complex is required for the DNA interstrand crosslink-induced checkpoint response.","date":"2010","source":"Molecular cell","url":"https://pubmed.ncbi.nlm.nih.gov/20670894","citation_count":105,"is_preprint":false},{"pmid":"23333308","id":"PMC_23333308","title":"FANCM and FAAP24 maintain genome stability via cooperative as well as unique functions.","date":"2013","source":"Molecular cell","url":"https://pubmed.ncbi.nlm.nih.gov/23333308","citation_count":69,"is_preprint":false},{"pmid":"23932590","id":"PMC_23932590","title":"Architecture and DNA recognition elements of the Fanconi anemia FANCM-FAAP24 complex.","date":"2013","source":"Structure (London, England : 1993)","url":"https://pubmed.ncbi.nlm.nih.gov/23932590","citation_count":27,"is_preprint":false},{"pmid":"19379763","id":"PMC_19379763","title":"FANCM-FAAP24 and FANCJ: FA proteins that metabolize DNA.","date":"2009","source":"Mutation research","url":"https://pubmed.ncbi.nlm.nih.gov/19379763","citation_count":25,"is_preprint":false},{"pmid":"24003026","id":"PMC_24003026","title":"Structural insights into the functions of the FANCM-FAAP24 complex in DNA repair.","date":"2013","source":"Nucleic acids research","url":"https://pubmed.ncbi.nlm.nih.gov/24003026","citation_count":15,"is_preprint":false},{"pmid":"19282663","id":"PMC_19282663","title":"FANCM-FAAP24 and HCLK2: roles in ATR signalling and the Fanconi anemia pathway.","date":"2009","source":"Cell cycle (Georgetown, Tex.)","url":"https://pubmed.ncbi.nlm.nih.gov/19282663","citation_count":13,"is_preprint":false},{"pmid":"27473539","id":"PMC_27473539","title":"Fatal Lymphoproliferative Disease in Two Siblings Lacking Functional FAAP24.","date":"2016","source":"Journal of clinical immunology","url":"https://pubmed.ncbi.nlm.nih.gov/27473539","citation_count":13,"is_preprint":false},{"pmid":"23999858","id":"PMC_23999858","title":"Structure analysis of FAAP24 reveals single-stranded DNA-binding activity and domain functions in DNA damage response.","date":"2013","source":"Cell research","url":"https://pubmed.ncbi.nlm.nih.gov/23999858","citation_count":7,"is_preprint":false},{"pmid":"23661679","id":"PMC_23661679","title":"The Fanconi anemia associated protein FAAP24 uses two substrate specific binding surfaces for DNA recognition.","date":"2013","source":"Nucleic acids research","url":"https://pubmed.ncbi.nlm.nih.gov/23661679","citation_count":4,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":7115,"output_tokens":2768,"usd":0.031433,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":10045,"output_tokens":2905,"usd":0.061425,"stage2_stop_reason":"end_turn"},"total_usd":0.092858,"stage1_batch_id":"msgbatch_01AsR7g8dGTEiEw9U83bJeWn","stage2_batch_id":"msgbatch_01YV5ZyxtFNUZEjxRcpd9jzb","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2007,\n      \"finding\": \"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.\",\n      \"method\": \"Co-immunoprecipitation, siRNA knockdown, DNA damage sensitivity assays, FANCD2 monoubiquitylation assay\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal Co-IP, functional siRNA knockdown with multiple orthogonal readouts, replicated by subsequent independent studies\",\n      \"pmids\": [\"17289582\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"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.\",\n      \"method\": \"Immunofluorescence/focus formation assays, co-immunoprecipitation\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — direct localization experiments with functional consequence, replicated in subsequent studies\",\n      \"pmids\": [\"17289582\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"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.\",\n      \"method\": \"siRNA knockdown, chromatin fractionation, immunoblotting\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — chromatin fractionation with functional consequence, replicated across multiple labs\",\n      \"pmids\": [\"18174376\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"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.\",\n      \"method\": \"Co-immunoprecipitation (HCLK2 complex proteomics), siRNA knockdown, phosphorylation assays, focus formation assays\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — MS-based interactome confirmation plus siRNA with multiple orthogonal checkpoint readouts, replicated by subsequent studies\",\n      \"pmids\": [\"18995830\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"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.\",\n      \"method\": \"siRNA knockdown, RPA focus formation assays, ATR checkpoint activation assays, separation-of-function mutant analysis\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — separation-of-function mutations combined with multiple functional readouts, mechanistically defines FAAP24 DNA-binding as critical\",\n      \"pmids\": [\"20670894\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"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.\",\n      \"method\": \"X-ray crystallography, in vitro DNA binding assays, site-directed mutagenesis, electron microscopy, ATPase assay\",\n      \"journal\": \"Structure (London, England : 1993)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure with mutagenesis validated in vitro and in vivo, multiple orthogonal methods in one study\",\n      \"pmids\": [\"23932590\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"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.\",\n      \"method\": \"X-ray crystallography, site-directed mutagenesis, in vitro DNA binding assays, chromatin association assays\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure with mutagenesis and functional chromatin-binding validation, two independent structural studies reach consistent conclusions\",\n      \"pmids\": [\"24003026\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"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.\",\n      \"method\": \"NMR solution structure determination, in vitro DNA binding assays (various FAAP24 mutations), in vivo checkpoint assays\",\n      \"journal\": \"Cell research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — NMR structure with separation-of-function mutagenesis validated in vitro and in vivo, single lab but multiple orthogonal methods\",\n      \"pmids\": [\"23999858\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"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.\",\n      \"method\": \"NMR structure determination, NMR titration experiments, site-directed mutagenesis, homology modeling\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — NMR structure with NMR titration binding experiments and mutagenesis, single lab but multiple orthogonal methods\",\n      \"pmids\": [\"23661679\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"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.\",\n      \"method\": \"Isogenic knockout cell lines, epistasis analysis, sister chromatid exchange assays, DNA damage sensitivity assays, checkpoint activation assays\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — isogenic knockout models with epistasis analysis and multiple orthogonal functional readouts\",\n      \"pmids\": [\"23333308\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"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.\",\n      \"method\": \"Whole exome sequencing, patient-derived T cells (HVS-transformed), FANCD2 monoubiquitination assay, CHK1 phosphorylation assay\",\n      \"journal\": \"Journal of clinical immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional validation in patient-derived cells with two orthogonal readouts, single lab, no in vitro reconstitution of mutant\",\n      \"pmids\": [\"27473539\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"FAAP24 is a component of the FA core complex that heterodimerizes with FANCM via their shared XPF/MUS81-like C-terminal architecture (pseudo-nuclease + (HhH)2 domains); within this complex, FAAP24 directly binds DNA (both ssDNA via its unstructured N-terminus and dsDNA/ICL-junction substrates via its canonical HhH motif) to anchor the FA core complex to chromatin and stalled replication forks, promote FANCD2 monoubiquitination, and—independently of the FA core complex through interaction with HCLK2—recruit RPA to ICL-stalled forks and activate ATR/CHK1-mediated checkpoint signaling.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"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 [#0, #4]. 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 [#5, #6]. 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 [#7, #8]. 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 [#1, #2]. 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 [#3, #4, #9]. A homozygous FAAP24 missense mutation (T212M) in patients impairs FANCD2 monoubiquitination and delays CHK1 phosphorylation, confirming both roles in a disease context [#10].\",\n  \"teleology\": [\n    {\n      \"year\": 2007,\n      \"claim\": \"Established that an uncharacterized FANCM-associated protein is itself a functional FA core complex component, defining FAAP24 as required for the DNA-crosslink response.\",\n      \"evidence\": \"Co-IP, siRNA knockdown with FANCD2 monoubiquitylation and crosslinker-sensitivity readouts in human cells\",\n      \"pmids\": [\"17289582\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular basis of the FANCM interaction not resolved\", \"Direct DNA-binding activity not yet demonstrated\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Showed FAAP24 acts upstream of FA core complex chromatin loading by stabilizing FANCM and enabling its cell cycle-dependent chromatin association.\",\n      \"evidence\": \"siRNA knockdown with chromatin fractionation and immunoblotting in human cells\",\n      \"pmids\": [\"18174376\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether FAAP24 contacts chromatin directly or only via FANCM not distinguished here\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Revealed an FA-core-complex-independent role: FAAP24/FANCM associate with HCLK2 to support ATR/CHK1 checkpoint signaling, separating checkpoint from repair functions.\",\n      \"evidence\": \"HCLK2 complex proteomics, siRNA, and checkpoint phosphorylation/focus assays\",\n      \"pmids\": [\"18995830\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism linking HCLK2 association to ATR activation undefined\", \"Direct vs indirect FAAP24-HCLK2 contact not resolved\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Defined the DNA-binding activity of FAAP24 (not FANCM translocase) as the determinant for recruiting RPA to ICL-stalled forks to trigger ATR signaling.\",\n      \"evidence\": \"Separation-of-function mutants with RPA focus and ATR activation assays after ICL induction\",\n      \"pmids\": [\"20670894\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct FAAP24-RPA contact not established\", \"Order of RPA loading vs FAAP24 DNA binding not resolved\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"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.\",\n      \"evidence\": \"X-ray crystallography and NMR of FANCM-CTD/FAAP24 with DNA, plus mutagenesis validated in vitro and in vivo\",\n      \"pmids\": [\"23932590\", \"24003026\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structure of full-length complex on a fork substrate not solved\", \"Functional role of the buried FANCM (HhH)2 domain unclear\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"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.\",\n      \"evidence\": \"NMR solution structures, NMR titrations, segregation-of-function mutagenesis, in vitro and in vivo checkpoint assays\",\n      \"pmids\": [\"23999858\", \"23661679\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Affinity/specificity for physiological ICL intermediates in vivo not quantified\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"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.\",\n      \"evidence\": \"Isogenic knockout cell lines with epistasis, SCE, sensitivity, and checkpoint assays\",\n      \"pmids\": [\"23333308\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How the two activities are temporally coordinated at a single lesion unresolved\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Linked FAAP24 dysfunction to human disease, showing a patient missense mutation impairs both FA pathway and checkpoint outputs in vivo.\",\n      \"evidence\": \"Whole-exome sequencing and patient-derived T cells assayed for FANCD2 ubiquitination and CHK1 phosphorylation\",\n      \"pmids\": [\"27473539\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No in vitro reconstitution of the T212M mutant\", \"Single-family/two-patient observation\", \"Effect on DNA binding vs FANCM interaction not dissected\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How FAAP24 mechanistically bridges its DNA-junction recognition to RPA loading and ATR activation, and whether it directly contacts RPA or HCLK2, remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No reconstituted RPA-recruitment assay defining direct partners\", \"Stoichiometry of FAAP24 in checkpoint vs core complex pools unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0003677\", \"supporting_discovery_ids\": [4, 5, 6, 7, 8]},\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [3, 4]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005694\", \"supporting_discovery_ids\": [2, 6]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [0, 1]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-73894\", \"supporting_discovery_ids\": [0, 4, 9]},\n      {\"term_id\": \"R-HSA-8953897\", \"supporting_discovery_ids\": [3, 4]}\n    ],\n    \"complexes\": [\"Fanconi anemia core complex\", \"FANCM-FAAP24 heterodimer\"],\n    \"partners\": [\"FANCM\", \"HCLK2\", \"RPA\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":6,"faith_pct":100.0}}