{"gene":"XRCC4","run_date":"2026-06-11T09:02:06","timeline":{"discoveries":[{"year":1995,"finding":"XRCC4 encodes a novel protein required for completion of V(D)J recombination (both coding and signal joins) and DNA double-strand break repair; XR-1 cells lacking XRCC4 can initiate but not complete V(D)J recombination, and human XRCC4 cDNA complements both defects.","method":"Functional complementation of XRCC4-deficient XR-1 CHO cells with human XRCC4 cDNA; V(D)J recombination substrate assays; DSBR sensitivity assays","journal":"Cell","confidence":"High","confidence_rationale":"Tier 2 / Strong — clean complementation experiment in defined mutant cell line, replicated across multiple assays and subsequently confirmed by many independent labs","pmids":["8548796"],"is_preprint":false},{"year":1997,"finding":"XRCC4 directly interacts with DNA ligase IV and stimulates its ligation activity 5–8-fold in vitro; the two proteins co-immunoprecipitate and interact in yeast two-hybrid, establishing that the primary function of XRCC4 is to stimulate DNA ligase IV for DSB ligation.","method":"Co-immunoprecipitation; yeast two-hybrid; in vitro ligation assay with purified proteins; co-expression in insect cells","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro reconstitution with purified proteins plus co-IP, replicated in subsequent independent studies","pmids":["9242410"],"is_preprint":false},{"year":1997,"finding":"XRCC4 interacts with DNA ligase IV via the unique carboxy-terminal extension of ligase IV that contains tandem BRCT domains; XRCC4 and ligase IV co-purify quantitatively and XRCC4 is a nuclear phosphoprotein that is an in vitro substrate for DNA-PK.","method":"Co-immunoprecipitation; co-purification across chromatographic steps; adenylylation assay; protein mapping/deletion analysis; in vitro DNA-PK phosphorylation assay","journal":"Current biology : CB","confidence":"High","confidence_rationale":"Tier 1 / Strong — multiple orthogonal biochemical methods, replicated by independent labs","pmids":["9259561"],"is_preprint":false},{"year":1997,"finding":"XRCC4 forms a homodimer in vivo; the homodimerization domain maps to amino acids 115–204; a core functional domain (aa 18–204) is required for V(D)J recombination; XRCC4 localizes to the nucleus.","method":"Yeast two-hybrid with deletion mutants; transient V(D)J recombination assay; nuclear localization imaging","journal":"International immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — yeast two-hybrid and functional complementation, single lab, two methods","pmids":["9352367"],"is_preprint":false},{"year":1998,"finding":"DNA ligase IV binds XRCC4 via a region located between (not within) its tandem BRCT domains; this inter-BRCT linker region is necessary and sufficient for XRCC4 binding.","method":"Deletion analysis of DNA ligase IV; co-expression and co-immunoprecipitation; in vitro binding assays","journal":"Current biology : CB","confidence":"High","confidence_rationale":"Tier 1 / Strong — detailed deletion mapping with multiple constructs, confirmed by crystal structure in later work","pmids":["9705934"],"is_preprint":false},{"year":1998,"finding":"A central region of XRCC4 (amino acids 100–250) is required for DNA ligase IV binding; deletions within this region abolish in vivo NHEJ and V(D)J recombination, demonstrating the physical interaction with ligase IV is essential for XRCC4 function in vivo.","method":"XRCC4 deletion mutant analysis; in vitro ligase IV binding assay; in vivo complementation of XRCC4-deficient cells","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — deletion mutagenesis correlated with both in vitro binding and in vivo functional rescue","pmids":["9733770"],"is_preprint":false},{"year":1998,"finding":"XRCC4 is an in vitro substrate of DNA-PK; phosphorylation occurs on serine, threonine, and tyrosine in insect cells and on serine/threonine within the C-terminal 130 aa by DNA-PK in vitro; recombinant XRCC4 facilitates Ku binding to DNA and promotes DNA-PK assembly on DNA ends.","method":"In vitro kinase assay with DNA-PK and recombinant XRCC4; phosphoamino acid analysis; DNA-binding/Ku-assembly assays","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 1-2 / Moderate — in vitro biochemical assays, single lab","pmids":["9430729"],"is_preprint":false},{"year":1998,"finding":"XRCC4 stabilizes DNA ligase IV protein; in XRCC4-deficient XR-1 cells DNA ligase IV protein is nearly absent despite normal mRNA levels, and reintroduction of XRCC4 restores ligase IV to wild-type levels.","method":"Western blot; mRNA analysis by Northern/RT-PCR; XRCC4 cDNA transfection into XR-1 cells","journal":"Mutation research","confidence":"High","confidence_rationale":"Tier 2 / Strong — clear protein stability rescue experiment, replicated in yeast ortholog study","pmids":["10047779"],"is_preprint":false},{"year":1999,"finding":"XRCC4 binds DNA with preference for nicks or broken ends; DNA binding of XRCC4 is correlated with V(D)J recombination complementation activity but is not required for stimulation of DNA ligase IV adenylation, indicating XRCC4 has DNA ligase IV-independent functions.","method":"DNA-binding assays with purified protein; adenylation assay for ligase IV; functional complementation assay in XRCC4-deficient cells; deletion/mutation analysis","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1 / Moderate — reconstituted in vitro assays plus in vivo complementation, single lab but multiple orthogonal methods","pmids":["10202163"],"is_preprint":false},{"year":2000,"finding":"The crystal structure of XRCC4 reveals an elongated dumbbell-shaped tetramer with N-terminal globular head domains containing a beta-sandwich and helix-turn-helix motif, and a C-terminal stalk of >120 Å alpha-helix that mediates ligase IV interaction; the structure suggests coupling of ligase IV association with DNA binding.","method":"X-ray crystallography at 2.7 Å resolution","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1 / Strong — high-resolution crystal structure, consistent with functional data from multiple labs","pmids":["11080143"],"is_preprint":false},{"year":2000,"finding":"XRCC4-ligase IV complex interacts directly with the Ku heterodimer in a DNA-stimulated manner; this interaction recruits XRCC4-ligase IV to DNA ends, increasing the initial ligation rate 20-fold; Ku's interaction is specific for ligase IV and does not extend to other mammalian ligases.","method":"Co-immunoprecipitation; in vitro ligation assay; DNA end-binding assay; preformation of Ku/XRCC4-LigIV complex","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — reconstituted in vitro with purified proteins, multiple binding and activity assays, replicated independently","pmids":["10757784"],"is_preprint":false},{"year":2000,"finding":"DNA ligase IV-XRCC4 complex binds specifically to ends of duplex DNA and can bridge two DNA molecules; Ku and DNA-PKcs bind simultaneously with XRCC4-LigIV at DNA ends; DNA-PKcs stimulates intermolecular ligation while Ku inhibits intramolecular ligation.","method":"DNA end-binding assays; intermolecular/intramolecular ligation assays with purified proteins; simultaneous binding experiments","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro reconstitution with purified components and multiple activity assays","pmids":["10854421"],"is_preprint":false},{"year":2000,"finding":"DNA ligase IV and XRCC4 form a stable mixed tetramer (~300 kDa, 2:2 stoichiometry) with XRCC4 homodimer at the core and ligase IV subunits not in direct contact with each other; this complex functions synergistically with Ku and DNA-PKcs in cell-free end-joining.","method":"Protein expression and purification; cross-linking; size estimation; cell-free end-joining reconstitution assay","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 1-2 / Moderate — biochemical reconstitution, single lab","pmids":["10945980"],"is_preprint":false},{"year":2000,"finding":"p53-deficiency rescues embryonic lethality and neuronal apoptosis caused by XRCC4 deficiency in mice, but does not rescue impaired V(D)J recombination; XRCC4/p53 double-deficient mice develop pro-B cell lymphomas with c-myc/IgH translocations, establishing NHEJ as a genome caretaker.","method":"Double-knockout mouse model; genetic epistasis; tumor pathology; chromosomal translocation analysis","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic epistasis in mouse model, multiple phenotypic readouts, widely replicated","pmids":["10786799"],"is_preprint":false},{"year":2001,"finding":"Crystal structure of human XRCC4 bound to the DNA ligase IV inter-BRCT linker peptide shows that a single ligase IV chain binds asymmetrically to an XRCC4 dimer; binding causes conformational change in XRCC4 helical tails (coiled-coil unwinds to flat surface); interaction is mediated by charged hydrogen bonds and extensive hydrophobic contacts.","method":"X-ray crystallography","journal":"Nature structural biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure with detailed interface characterization, later confirmed by higher-resolution structures","pmids":["11702069"],"is_preprint":false},{"year":2002,"finding":"Direct protein-protein interactions in the NHEJ complex: ligase IV interacts with Ku via its BRCT domains; XRCC4 interacts directly with DNA-PKcs; ligase IV does not bind DNA-PKcs and XRCC4 does not bind Ku directly (or only very weakly).","method":"Pull-down assays; co-immunoprecipitation with defined protein pairs; DNA-independent interaction assays","journal":"DNA repair","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple pairwise binding assays, single lab","pmids":["12509254"],"is_preprint":false},{"year":2003,"finding":"DNA-PK phosphorylates XRCC4 at serines 260 and 318 (mapped by mass spectrometry) within the C-terminal 100 aa, but these phosphorylations are not required for cell survival after IR or for V(D)J recombination, indicating these sites do not directly control NHEJ function.","method":"Mass spectrometry phosphorylation mapping; alanine-substitution mutagenesis; complementation assay in XRCC4-deficient cells; IR survival assay","journal":"DNA repair","confidence":"High","confidence_rationale":"Tier 1-2 / Moderate — MS-based site mapping plus mutagenesis with functional readout; negative result for functional importance is clearly established","pmids":["14599745"],"is_preprint":false},{"year":2003,"finding":"XRCC4 tetramerization and DNA ligase IV binding are mutually exclusive; only the XRCC4 dimer associates with ligase IV (2:1 stoichiometry in solution); the dimer-tetramer equilibrium likely separates two functional states of XRCC4.","method":"Equilibrium sedimentation analysis; mutational analysis of XRCC4; biochemical complex formation assays","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — rigorous biophysical assays plus mutational analysis, single lab","pmids":["14607114"],"is_preprint":false},{"year":2004,"finding":"XRCC4 physically links polynucleotide kinase (PNK) to DNA ligase IV via a CK2-phosphorylation-dependent interaction; the PNK FHA domain binds CK2-phosphorylated XRCC4; disruption of this interaction causes increased radiosensitivity and slower DSB repair kinetics.","method":"Co-immunoprecipitation; FHA domain binding assay; in vitro end-joining assay; CK2 phosphorylation assay; siRNA disruption in cells; IR survival/repair kinetics","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — phospho-dependent interaction biochemically characterized, combined with in vivo functional evidence","pmids":["15385968"],"is_preprint":false},{"year":2004,"finding":"Aprataxin (AOA1 gene product) physically interacts with XRCC4 in vitro and in vivo via its divergent FHA domain binding to CK2-phosphorylated XRCC4.","method":"Co-immunoprecipitation in vitro and in vivo; FHA domain-phosphopeptide binding analysis","journal":"DNA repair","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal co-IP and domain mapping, single lab","pmids":["15380105"],"is_preprint":false},{"year":2004,"finding":"DNA polymerase lambda interacts with the XRCC4-DNA ligase IV complex via its N-terminal BRCT domain, and this interaction stimulates pol lambda DNA synthesis activity; pol lambda can perform gap-filling on NHEJ substrates.","method":"Co-immunoprecipitation; in vitro gap-filling synthesis assay; interaction mapping with truncated pol lambda forms","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — co-IP plus in vitro activity assay, single lab","pmids":["15451442"],"is_preprint":false},{"year":2004,"finding":"Recruitment of XRCC4-ligase IV complex to DNA ends requires coordinated assembly of both Ku and DNA-PKcs (p460); interactions of Ku and p460 with XRCC4-LigIV are mainly DNA-dependent; XRCC4 is phosphorylated by DNA-PK upon recruitment but phosphorylation is not required for loading.","method":"In vitro DNA-end pull-down from nuclear extracts; co-immunoprecipitation; wortmannin inhibition; phosphorylation assay","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 2 / Moderate — cell-free biochemical reconstitution with multiple inhibitor controls, single lab","pmids":["12547193"],"is_preprint":false},{"year":2004,"finding":"DNA processing for NHEJ (polymerase and nuclease activities) requires XRCC4/Ligase IV in addition to wortmannin-sensitive kinase activity; XRCC4/LigIV is required for alignment-based gap filling preceding ligation of radiation-induced DSBs.","method":"Two-stage cell-free NHEJ assay; supplementation of XRCC4-deficient extracts with recombinant XRCC4/LigIV; kinase inhibitor (wortmannin) treatment","journal":"The Journal of biological chemistry; Cancer research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — cell-free complementation assay, confirmed by two independent studies","pmids":["17272270","12517771"],"is_preprint":false},{"year":2006,"finding":"XRCC4 is SUMOylated on lysine 210; this modification is required for nuclear accumulation of XRCC4; SUMO-deficient XRCC4 (K210R) remains cytoplasmic, causes radiation sensitivity and failure to complete V(D)J recombination; genetic fusion of SUMO to the C-terminus rescues nuclear localization and function.","method":"Site-directed mutagenesis; in vitro and in vivo SUMO modification assays; subcellular fractionation/immunofluorescence; IR survival assay; V(D)J recombination assay","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — site-specific mutagenesis, biochemical modification assay, rescue experiment, and functional readouts","pmids":["16478998"],"is_preprint":false},{"year":2006,"finding":"XLF (Cernunnos) directly interacts with the XRCC4-Ligase IV complex in vitro and in vivo and is required for efficient NHEJ; siRNA knockdown causes radiosensitivity and NHEJ defect; XLF is absent in patient 2BN cells due to frameshift mutation and its reintroduction corrects the defect.","method":"Co-immunoprecipitation in vitro and in vivo; siRNA knockdown; IR survival; NHEJ reporter assay; complementation of patient cells","journal":"Cell","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods, patient cell complementation, replicated by multiple labs","pmids":["16439205"],"is_preprint":false},{"year":2006,"finding":"Cernunnos/XLF physically interacts with the XRCC4-DNA ligase IV complex and is the mammalian homolog of yeast Nej1, identifying it as a new component of the NHEJ ligation complex.","method":"Co-immunoprecipitation; sequence homology analysis (sensitive sequence analysis methods)","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — co-IP confirming interaction, single lab","pmids":["16571728"],"is_preprint":false},{"year":2006,"finding":"Ku70/80 interacts directly with XRCC4; XRCC4/LigIV accumulation at DSBs depends on Ku70/80 but not DNA-PKcs; Ku heterodimers are in dynamic equilibrium at DNA ends; XRCC4 may serve as a flexible tether between Ku70/80 and ligase IV.","method":"Pulsed near-IR laser micro-irradiation; live-cell imaging; FRAP; co-immunoprecipitation; direct interaction assay (Ku70-XRCC4)","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — direct interaction assay combined with live-cell imaging and FRAP, multiple orthogonal methods","pmids":["17124166"],"is_preprint":false},{"year":2007,"finding":"XRCC4-DNA ligase IV can ligate two DNA ends with fully incompatible 3' overhangs (no base pairing) and can ligate across single-nucleotide gaps; XLF stimulates joining of both compatible and incompatible ends.","method":"In vitro ligation assay with purified XRCC4-LigIV complex; defined DNA substrates with incompatible overhangs and gaps; Ku stimulation assays","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1 / Strong — reconstituted in vitro with purified components and defined substrates","pmids":["17290226"],"is_preprint":false},{"year":2007,"finding":"Ku interacts with DNA ligase IV via its first BRCT domain (residues 644–748); this interaction is enhanced by XRCC4 and dsDNA; DNA-PK kinase activity causes disassembly of the Ku/DNA ligase IV/XRCC4 complex.","method":"Co-immunoprecipitation; deletion mapping of LigIV-Ku interaction; in vitro complex formation; kinase activity assays","journal":"DNA repair","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple biochemical assays, single lab","pmids":["17241822"],"is_preprint":false},{"year":2007,"finding":"XRCC4 plays a role in immunoglobulin class switch recombination (CSR); B-lymphocyte-restricted conditional XRCC4 knockout leads to ~2-fold reduction in CSR, implicating NHEJ in CSR resolution.","method":"Conditional knockout mouse (LoxP-flanked XRCC4, Cre-lentiviral); in vivo and in vitro CSR assays in B cells","journal":"The Journal of experimental medicine","confidence":"High","confidence_rationale":"Tier 2 / Strong — conditional knockout with defined cellular phenotype (CSR reduction)","pmids":["17606631"],"is_preprint":false},{"year":2008,"finding":"Werner protein (WRN) physically interacts with the XRCC4-DNA ligase IV complex (X4L4); X4L4 stimulates WRN exonuclease but not helicase activity; in a DNA end-joining assay, WRN-processed substrates are ligated by X4L4.","method":"Co-immunoprecipitation; in vitro exonuclease/helicase activity assays; DNA end-joining assay","journal":"Biochemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — biochemical interaction and activity assays, single lab","pmids":["18558713"],"is_preprint":false},{"year":2009,"finding":"High-resolution crystal structure of human XRCC4 bound to the C-terminal tandem BRCT repeat of DNA ligase IV reveals an extensive binding interface formed by a helix-loop-helix in the inter-BRCT linker and the second BRCT domain; second BRCT domain contact is necessary for stable binding in cells.","method":"X-ray crystallography (high-resolution); mutagenesis; cellular radiosensitization assays with dominant-negative LigIV fragments","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure plus mutagenesis with cellular functional readout","pmids":["19332554"],"is_preprint":false},{"year":2010,"finding":"CK2-phosphorylated XRCC4 promotes high-affinity interaction with PNKP FHA domain, stimulating PNKP turnover; unphosphorylated XRCC4 also interacts via a lower-affinity site in the PNKP catalytic domain; paradoxically, CK2-phosphorylated XRCC4 inhibits PNKP activity.","method":"Co-immunoprecipitation; in vitro FHA domain binding; PNKP kinase/phosphatase activity assays; CK2 phosphorylation assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — multiple orthogonal biochemical methods defining two modes of interaction and their functional consequences","pmids":["20852255"],"is_preprint":false},{"year":2011,"finding":"XLF and XRCC4 form alternating super-helical filaments through head-domain interactions; XLF Leu-115 ('Leu-lock') inserts into a hydrophobic pocket on XRCC4 (Met-59, Met-61, Lys-65, Lys-99, Phe-106, Leu-108); these filaments form a positively charged channel to bind and align DNA ends for ligation.","method":"X-ray crystallography (crystal + solution structures); SAXS; site-directed mutagenesis; DNA-binding assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure plus mutational validation and biophysical characterization","pmids":["21775435"],"is_preprint":false},{"year":2011,"finding":"XLF and XRCC4 dimers interact through their N-terminal head domains to form alternating left-handed helical filaments (confirmed at 5.5 Å and 8.5 Å resolution); key XRCC4 residues Glu55, Asp58, Met61, and Phe106 are essential for interaction with XLF.","method":"X-ray crystallography; electron microscopy; calorimetry; site-directed mutagenesis","journal":"Proceedings of the National Academy of Sciences of the United States of America; Biochemical Society transactions","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure with mutagenesis and calorimetry confirmation, independently replicated","pmids":["21768349","21936820"],"is_preprint":false},{"year":2011,"finding":"DNA Ligase IV controls nuclear import and sub-nuclear distribution of XRCC4; in LigIV-deficient cells XRCC4 remains cytoplasmic; co-expression with LigIV restores nuclear localization of XRCC4; XRCC4/LigIV complex exchanges faster at DNA damage sites than XRCC4 alone.","method":"Fluorescent fusion protein expression; live-cell imaging; FRAP; LigIV-deficient cell line complementation","journal":"DNA repair","confidence":"High","confidence_rationale":"Tier 2 / Strong — live imaging with FRAP plus genetic loss-of-function, multiple orthogonal approaches","pmids":["21982441"],"is_preprint":false},{"year":2012,"finding":"XRCC4-XLF complexes bridge two independent DNA molecules in a LigIV-independent manner; XRCC4's interaction with XLF is required for coding (but not signal) end joining in V(D)J recombination; DNA-PK phosphorylation of XRCC4/XLF disrupts DNA bridging in vitro.","method":"DNA-binding and bridging assays; crystal structure (3.94 Å); XRCC4 mutants abolishing XLF interaction; V(D)J recombination assays; DNA-PK phosphorylation assay","journal":"Nucleic acids research; Nucleic acids research","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — crystal structure, biochemical bridging assays, and in vivo V(D)J assay with separation-of-function mutants","pmids":["22287571","22228831"],"is_preprint":false},{"year":2014,"finding":"DNA Ligase IV is required for nuclear localization of XRCC4; its C-terminal region (aa 620–800) containing an NLS, BRCT I domain, and XRCC4-interacting region is essential for XRCC4 nuclear accumulation; Ligase IV also regulates XRCC4 protein stability.","method":"Expression of LigIV deletion constructs; subcellular fractionation/imaging; LigIV-deficient cell lines; Western blot for protein levels","journal":"DNA repair","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — cell biology with defined deletion constructs, single lab","pmids":["24984242"],"is_preprint":false},{"year":2015,"finding":"PAXX (paralog of XRCC4 and XLF) interacts directly with Ku and promotes Ku-dependent DNA ligation and assembly of core NHEJ factors; PAXX crystal structure resembles XRCC4; combined PAXX/XLF loss impairs DSB repair additively.","method":"Crystal structure; co-immunoprecipitation; CRISPR-Cas9 and RNAi knockouts; in vitro ligation assay; chromatin fractionation","journal":"Science","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — crystal structure plus reciprocal co-IP, in vitro assay, and genetic loss-of-function with cellular phenotype","pmids":["25574025"],"is_preprint":false},{"year":2016,"finding":"FBXW7 (SCF E3 ligase) promotes K63-linked polyubiquitylation of XRCC4 at lysine 296 after DNA-PKcs phosphorylates XRCC4 at serines 325/326; this ubiquitylation enhances XRCC4 association with Ku70/80 to facilitate NHEJ; a small-molecule inhibitor blocking this ubiquitylation reduces NHEJ.","method":"In vivo ubiquitylation assay; site-directed mutagenesis; co-immunoprecipitation; NHEJ reporter assay; IR sensitivity assay; kinase assay","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — multiple orthogonal methods (ubiquitylation assay, mutagenesis, inhibitor, co-IP, functional assay)","pmids":["26774286"],"is_preprint":false},{"year":2016,"finding":"XRCC4-XLF complexes form mobile 'sliding sleeve' structures around DNA that can bridge two independent DNA molecules and diffuse along DNA; XLF stimulates XRCC4 binding to DNA; XRCC4-XLF bridges can slide along DNA, enabling rapid reconnection of broken ends.","method":"Dual- and quadruple-trap optical tweezers combined with fluorescence microscopy; single-molecule real-time imaging","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 / Strong — rigorous single-molecule biophysical experiment with direct visualization","pmids":["27437582"],"is_preprint":false},{"year":2016,"finding":"APLF is an intrinsically disordered protein that links Ku, DNA-PKcs, XRCC4-LigIV into an extended flexible NHEJ core complex via simultaneous interactions with Ku, DNA-PK, and X4L4; Ku80 C-terminal region connects DNA-PKcs to the X4L4 complex.","method":"SAXS; mutational analysis; in vitro complex reconstitution","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — SAXS structural analysis plus mutagenesis, single lab","pmids":["27875301"],"is_preprint":false},{"year":2017,"finding":"The PNKP-XRCC4-LigIV ternary complex requires CK2-phosphorylation of XRCC4 for stable PNKP binding; only one PNKP protomer binds per XRCC4 dimer; SAXS and HDX reveal multipoint contacts between PNKP and XRCC4-LigIV including a PNKP phosphatase surface (E326K disease mutation impairs PNKP recruitment to damaged DNA).","method":"Recombinant complex purification; SAXS; hydrogen-deuterium exchange; CK2 phosphorylation assay; site-directed mutagenesis; cellular recruitment assay","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 1 / Strong — multiple structural and biochemical methods with cellular validation","pmids":["28453785"],"is_preprint":false},{"year":2017,"finding":"XRCC4-LigIV (X4-LIV) stimulates Artemis nuclease activity at 3' overhangs independently of DNA-PKcs; X4-LIV cannot stimulate Artemis at hairpins or 5' overhangs; X4-LIV and DNA-PKcs interfere with each other in stimulating Artemis, suggesting sequential not concurrent recruitment.","method":"In vitro nuclease assay with purified proteins; in vitro ligation assay; defined DNA substrates","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro reconstitution with purified components and multiple substrate configurations, single lab","pmids":["28696258"],"is_preprint":false},{"year":2017,"finding":"Phospho-mimicking mutations in the C-terminal tails of both XRCC4 and XLF (14 DNA-PK/ATM sites mutated to aspartate) destabilize XRCC4/XLF complexes and reduce DNA bridging activity without affecting LigIV stimulation, suggesting phosphorylation regulates DNA bridging independently of ligation.","method":"Phospho-blocking and -mimicking mutagenesis; DNA bridging assay; LigIV stimulation assay; complex stability analysis","journal":"eLife","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — separation-of-function mutants with multiple orthogonal functional assays","pmids":["28500754"],"is_preprint":false},{"year":2021,"finding":"RIG-I interacts with XRCC4 and impedes formation of the XRCC4/LIG4/XLF complex at DSBs, suppressing NHEJ; reciprocally, XRCC4 promotes RIG-I signaling by enhancing RIG-I oligomerization and ubiquitination, augmenting type I IFN production.","method":"Co-immunoprecipitation; DSB repair assays; RIG-I/XRCC4 knockdown/overexpression; innate immune signaling assays; in vivo viral infection model","journal":"Nature communications","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — co-IP plus functional assays in cells, single lab, novel finding requiring independent replication","pmids":["33846346"],"is_preprint":false}],"current_model":"XRCC4 is a dimeric scaffolding protein that forms a tight, stoichiometric complex with DNA ligase IV (stabilizing it and stimulating its adenylation and ligation activity 5–8-fold), is SUMOylated at K210 for nuclear import, and recruits end-processing enzymes (PNK/PNKP via CK2-phospho-dependent FHA interactions) to DSBs; together with XLF it assembles into dynamic, positively charged super-helical filaments that bridge and align broken DNA ends as sliding sleeves, while its interaction with Ku70/80 (and dependence on Ku/DNA-PKcs for stable chromatin recruitment) positions the entire ligation complex at DSBs, where K63-polyubiquitylation at K296 by SCF-FBXW7 further enhances Ku association and NHEJ efficiency."},"narrative":{"mechanistic_narrative":"XRCC4 is a core scaffolding protein of the non-homologous end-joining (NHEJ) pathway, required for completion of DNA double-strand break repair and V(D)J recombination [PMID:8548796, PMID:10786799]. Its central function is to bind DNA ligase IV through the ligase's inter-BRCT linker and tandem BRCT region, an interaction that stabilizes ligase IV protein (which is nearly absent without XRCC4) and stimulates its ligation activity several-fold [PMID:9242410, PMID:10047779, PMID:19332554]. Structurally, XRCC4 is an elongated dimer with N-terminal globular head domains and a long C-terminal coiled-coil stalk that engages ligase IV; tetramerization and ligase IV binding are mutually exclusive, defining distinct functional states [PMID:11080143, PMID:11702069, PMID:14607114]. The XRCC4-ligase IV complex is recruited to broken ends by the Ku heterodimer in a DNA-stimulated manner, where it cooperates with Ku and DNA-PKcs and can ligate even incompatible or gapped ends [PMID:10757784, PMID:10854421, PMID:17290226]. Through head-domain contacts XRCC4 assembles with XLF into alternating super-helical filaments that form a positively charged channel and act as mobile 'sliding sleeves' bridging and aligning two DNA ends independently of ligase IV [PMID:21775435, PMID:22287571, PMID:22228831, PMID:27437582]. XRCC4 also serves as a platform recruiting end-processing enzymes — PNKP and aprataxin via CK2-phosphorylation-dependent FHA interactions, DNA polymerase lambda, WRN, and Artemis — coupling end processing to ligation [PMID:15385968, PMID:15451442, PMID:20852255, PMID:28696258]. XRCC4 function is regulated by post-translational modification: SUMOylation at K210 drives nuclear accumulation, and FBXW7-mediated K63-polyubiquitylation at K296 (following DNA-PKcs phosphorylation) enhances Ku association and NHEJ efficiency [PMID:16478998, PMID:26774286]. Genetically, p53 loss rescues the embryonic lethality of XRCC4 deficiency in mice, and XRCC4 contributes to immunoglobulin class switch recombination, establishing NHEJ as a genome caretaker [PMID:10786799, PMID:17606631].","teleology":[{"year":1995,"claim":"Established that XRCC4 is genetically required to complete V(D)J recombination and double-strand break repair, defining it as an essential NHEJ factor before any molecular activity was known.","evidence":"Functional complementation of XRCC4-deficient XR-1 CHO cells with human cDNA and V(D)J/DSBR assays","pmids":["8548796"],"confidence":"High","gaps":["Did not reveal the biochemical activity of XRCC4","No interaction partners identified"]},{"year":1997,"claim":"Identified DNA ligase IV as the direct partner of XRCC4 and showed XRCC4 stimulates its ligation activity, assigning XRCC4 a concrete biochemical role in end ligation.","evidence":"Co-IP, yeast two-hybrid, and in vitro ligation assay with purified proteins; mapping to the ligase IV BRCT region","pmids":["9242410","9259561","9352367"],"confidence":"High","gaps":["Did not establish how the complex reaches DNA ends","Stoichiometry of the complex unresolved"]},{"year":1998,"claim":"Mapped the XRCC4–ligase IV interface to the inter-BRCT linker and demonstrated the interaction is essential for NHEJ in vivo, while showing XRCC4 stabilizes ligase IV protein.","evidence":"Deletion mapping with in vitro binding and in vivo complementation; protein/mRNA analysis in XR-1 cells","pmids":["9705934","9733770","10047779"],"confidence":"High","gaps":["Mechanism of ligase IV stabilization not defined","DNA-binding role of XRCC4 still unclear"]},{"year":1999,"claim":"Showed XRCC4 binds DNA nicks and ends and possesses ligase IV-independent functions, indicating roles beyond simply stimulating the ligase.","evidence":"DNA-binding assays, adenylation assays, and complementation with deletion/mutant XRCC4","pmids":["10202163"],"confidence":"High","gaps":["Identity of ligase IV-independent functions not defined"]},{"year":2000,"claim":"Resolved XRCC4 architecture by crystallography and reconstituted its recruitment to DNA ends via Ku and DNA-PKcs, defining how the ligation complex is positioned at breaks.","evidence":"X-ray crystallography of XRCC4; co-IP and in vitro ligation/end-binding assays with Ku, DNA-PKcs, and the XRCC4-LigIV complex; cross-linking and stoichiometry","pmids":["11080143","10757784","10854421","10945980"],"confidence":"High","gaps":["How Ku and XRCC4 physically connect remained ambiguous","Dynamics of recruitment in cells not addressed"]},{"year":2000,"claim":"Demonstrated in mice that NHEJ acts as a genome caretaker, with p53 loss rescuing XRCC4-null lethality but not the recombination defect, linking XRCC4 to tumour suppression.","evidence":"XRCC4/p53 double-knockout mouse; genetic epistasis, tumour pathology, and translocation analysis","pmids":["10786799"],"confidence":"High","gaps":["Molecular basis of apoptosis suppression by p53 loss not defined"]},{"year":2001,"claim":"Defined the atomic XRCC4–ligase IV interface and the asymmetric 2:1 binding mode, with binding inducing conformational change in XRCC4 tails.","evidence":"X-ray crystallography of XRCC4 bound to the ligase IV inter-BRCT peptide; later 2003 sedimentation analysis establishing dimer/tetramer exclusivity","pmids":["11702069","14607114"],"confidence":"High","gaps":["Functional significance of the dimer-tetramer equilibrium in cells unresolved"]},{"year":2003,"claim":"Mapped DNA-PK phosphorylation sites on XRCC4 (S260, S318) and showed they are dispensable for survival and V(D)J recombination, ruling out these modifications as direct controllers of NHEJ.","evidence":"Mass spectrometry site mapping plus alanine mutagenesis with complementation and IR survival assays","pmids":["14599745"],"confidence":"High","gaps":["Other phosphorylation sites and their roles not yet examined","Function of XRCC4 phosphorylation left open"]},{"year":2004,"claim":"Established XRCC4 as a recruitment hub for end-processing enzymes, linking PNK, aprataxin, and polymerase lambda to the ligation complex.","evidence":"Co-IP, CK2-dependent FHA-domain binding, in vitro activity and end-joining assays; siRNA disruption with IR survival/repair kinetics","pmids":["15385968","15380105","15451442","12547193","17272270","12517771"],"confidence":"High","gaps":["Order of enzyme recruitment relative to ligation not fully defined","Some interactions characterized in single labs"]},{"year":2006,"claim":"Showed SUMOylation at K210 governs XRCC4 nuclear import and identified XLF and direct Ku70/80 binding as components governing complex assembly and chromatin recruitment.","evidence":"SUMO modification assays with K210R mutant and rescue; XLF co-IP and patient-cell complementation; laser micro-irradiation, FRAP, and Ku70-XRCC4 interaction assays","pmids":["16478998","16439205","16571728","17124166","12509254"],"confidence":"High","gaps":["SUMO E3 ligase for XRCC4 not identified","Relative contributions of Ku-XRCC4 versus Ku-ligase IV contacts to recruitment unresolved"]},{"year":2007,"claim":"Demonstrated the catalytic versatility of XRCC4-ligase IV — joining incompatible ends and gaps — and extended XRCC4's biological role to immunoglobulin class switch recombination.","evidence":"In vitro ligation with defined incompatible/gapped substrates; conditional B-cell XRCC4 knockout with CSR assays; LigIV-Ku BRCT mapping","pmids":["17290226","17606631","17241822"],"confidence":"High","gaps":["Mechanism of incompatible-end accommodation at structural level not defined"]},{"year":2008,"claim":"Extended the XRCC4-ligase IV interactome to WRN, coupling WRN exonuclease processing to downstream ligation.","evidence":"Co-IP and in vitro exonuclease/helicase and end-joining assays","pmids":["18558713"],"confidence":"Medium","gaps":["Single-lab biochemistry without cellular validation","Physiological context of WRN-X4L4 cooperation unclear"]},{"year":2011,"claim":"Revealed that XRCC4 and XLF assemble into alternating super-helical filaments forming a positively charged DNA-binding channel, providing a structural basis for end bridging and alignment.","evidence":"Crystallography, EM, SAXS, calorimetry, and mutagenesis of XLF-XRCC4 head-domain contacts; FRAP and imaging of LigIV control of XRCC4 nuclear import","pmids":["21775435","21768349","21936820","21982441"],"confidence":"High","gaps":["Filament length and stoichiometry on real broken DNA in cells not defined"]},{"year":2012,"claim":"Showed XRCC4-XLF bridges two DNA molecules independently of ligase IV and that this bridging is needed specifically for coding-end joining and is regulated by DNA-PK phosphorylation.","evidence":"DNA bridging assays, crystallography, separation-of-function XRCC4 mutants, and V(D)J recombination assays","pmids":["22287571","22228831"],"confidence":"High","gaps":["Why bridging is selectively required for coding but not signal ends not explained"]},{"year":2016,"claim":"Visualized XRCC4-XLF bridges as mobile 'sliding sleeves' on single DNA molecules and identified FBXW7-mediated K63-ubiquitylation at K296 as a regulator enhancing Ku association.","evidence":"Optical tweezers with single-molecule fluorescence; in vivo ubiquitylation assays, mutagenesis, small-molecule inhibitor, and NHEJ reporter assays; SAXS of APLF-linked core complex","pmids":["27437582","26774286","27875301"],"confidence":"High","gaps":["How sliding sleeves are converted to a static ligation-competent state unresolved","Deubiquitylase counteracting K296 ubiquitylation not identified"]},{"year":2017,"claim":"Defined how XRCC4-ligase IV stimulates and coordinates end-processing enzymes (PNKP, Artemis) and showed C-terminal tail phosphorylation tunes DNA bridging independently of ligation.","evidence":"SAXS/HDX of PNKP-X4L4 complex; in vitro Artemis nuclease assays; phospho-mimic mutagenesis with bridging and LigIV-stimulation assays","pmids":["28453785","28696258","28500754"],"confidence":"High","gaps":["Temporal coupling of processing to ligation in cells not directly observed"]},{"year":2015,"claim":"Identified PAXX as an XRCC4 paralog that promotes Ku-dependent ligation and core NHEJ assembly, expanding the XRCC4-superfamily scaffold network.","evidence":"Crystal structure, co-IP, CRISPR/RNAi knockout, chromatin fractionation, and in vitro ligation","pmids":["25574025"],"confidence":"High","gaps":["Functional redundancy/division of labour between XRCC4, XLF and PAXX incompletely resolved"]},{"year":2021,"claim":"Linked XRCC4 to innate immunity, with RIG-I suppressing NHEJ complex formation while XRCC4 reciprocally augments RIG-I signaling.","evidence":"Co-IP, DSB repair and innate immune signaling assays, and an in vivo viral infection model","pmids":["33846346"],"confidence":"Medium","gaps":["Single-lab finding awaiting independent replication","Physiological significance of the XRCC4-RIG-I axis unresolved"]},{"year":null,"claim":"How the dynamic, sliding XRCC4-XLF filament transitions to a fixed, ligation-competent synaptic state and how the many post-translational modifications are temporally integrated during repair remain open.","evidence":"","pmids":[],"confidence":"High","gaps":["No real-time model linking bridging, processing, and ligation steps","Coordination of SUMO, phosphorylation, and ubiquitylation signals over the repair cycle undefined"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[1,7,18,26]},{"term_id":"GO:0003677","term_label":"DNA binding","supporting_discovery_ids":[8,11,33,40]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[1,7,43]},{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[9,33,34]}],"localization":[{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[3,23,35]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[23,35]},{"term_id":"GO:0000228","term_label":"nuclear chromosome","supporting_discovery_ids":[26,36]}],"pathway":[{"term_id":"R-HSA-73894","term_label":"DNA Repair","supporting_discovery_ids":[0,10,27,40]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[0,29,45]},{"term_id":"R-HSA-1640170","term_label":"Cell Cycle","supporting_discovery_ids":[13]}],"complexes":["XRCC4-DNA ligase IV complex","XRCC4-XLF filament","NHEJ core complex (Ku/DNA-PKcs/X4L4)"],"partners":["LIG4","XLF","XRCC5","XRCC6","PRKDC","PNKP","APTX","PAXX"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q13426","full_name":"DNA repair protein XRCC4","aliases":["X-ray repair cross-complementing protein 4"],"length_aa":336,"mass_kda":38.3,"function":"DNA non-homologous end joining (NHEJ) core factor, required for double-strand break repair and V(D)J recombination (PubMed:10757784, PubMed:10854421, PubMed:12517771, PubMed:16412978, PubMed:17124166, PubMed:17290226, PubMed:22228831, PubMed:25597996, PubMed:25742519, PubMed:25934149, PubMed:26100018, PubMed:26774286, PubMed:8548796). Acts as a scaffold protein that regulates recruitment of other proteins to DNA double-strand breaks (DSBs) (PubMed:15385968, PubMed:20852255, PubMed:26774286, PubMed:27437582). Associates with NHEJ1/XLF to form alternating helical filaments that bridge DNA and act like a bandage, holding together the broken DNA until it is repaired (PubMed:21768349, PubMed:21775435, PubMed:22287571, PubMed:26100018, PubMed:27437582, PubMed:28500754). The XRCC4-NHEJ1/XLF subcomplex binds to the DNA fragments of a DSB in a highly diffusive manner and robustly bridges two independent DNA molecules, holding the broken DNA fragments in close proximity to one other (PubMed:27437582). The mobility of the bridges ensures that the ends remain accessible for further processing by other repair factors (PubMed:27437582). Plays a key role in the NHEJ ligation step of the broken DNA during DSB repair via direct interaction with DNA ligase IV (LIG4): the LIG4-XRCC4 subcomplex reseals the DNA breaks after the gap filling is completed (PubMed:10757784, PubMed:10854421, PubMed:12517771, PubMed:17290226, PubMed:19837014, PubMed:9242410). XRCC4 stabilizes LIG4, regulates its subcellular localization and enhances LIG4's joining activity (PubMed:10757784, PubMed:10854421, PubMed:12517771, PubMed:17290226, PubMed:21982441, PubMed:22228831, PubMed:9242410). Binding of the LIG4-XRCC4 subcomplex to DNA ends is dependent on the assembly of the DNA-dependent protein kinase complex DNA-PK to these DNA ends (PubMed:10757784, PubMed:10854421). Promotes displacement of PNKP from processed strand break termini (PubMed:20852255, PubMed:28453785) Acts as an activator of the phospholipid scramblase activity of XKR4 (PubMed:33725486). This form, which is generated upon caspase-3 (CASP3) cleavage, translocates into the cytoplasm and interacts with XKR4, thereby promoting phosphatidylserine scramblase activity of XKR4 and leading to phosphatidylserine exposure on apoptotic cell surface (PubMed:33725486)","subcellular_location":"Cytoplasm","url":"https://www.uniprot.org/uniprotkb/Q13426/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/XRCC4","classification":"Not Classified","n_dependent_lines":151,"n_total_lines":1208,"dependency_fraction":0.125},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"CSNK1A1","stoichiometry":4.0},{"gene":"DYNLL1","stoichiometry":0.2},{"gene":"DYNLL2","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/XRCC4","total_profiled":1310},"omim":[{"mim_id":"621368","title":"X KELL BLOOD GROUP PRECURSOR-RELATED FAMILY, MEMBER 4; XKR4","url":"https://www.omim.org/entry/621368"},{"mim_id":"620676","title":"COILED-COIL DOMAIN-CONTAINING PROTEIN 61; CCDC61","url":"https://www.omim.org/entry/620676"},{"mim_id":"618703","title":"ZINC FINGER PROTEIN 281; ZNF281","url":"https://www.omim.org/entry/618703"},{"mim_id":"616541","title":"SHORT STATURE, MICROCEPHALY, AND ENDOCRINE DYSFUNCTION; SSMED","url":"https://www.omim.org/entry/616541"},{"mim_id":"616315","title":"PAXX NONHOMOLOGOUS END JOINING FACTOR; PAXX","url":"https://www.omim.org/entry/616315"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Enhanced","locations":[{"location":"Nucleoplasm","reliability":"Enhanced"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/XRCC4"},"hgnc":{"alias_symbol":["hXRCC4"],"prev_symbol":[]},"alphafold":{"accession":"Q13426","domains":[{"cath_id":"2.170.210.10","chopping":"1-117","consensus_level":"high","plddt":93.7198,"start":1,"end":117},{"cath_id":"1.20.5","chopping":"119-186","consensus_level":"medium","plddt":97.9634,"start":119,"end":186}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q13426","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q13426-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q13426-F1-predicted_aligned_error_v6.png","plddt_mean":74.81},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=XRCC4","jax_strain_url":"https://www.jax.org/strain/search?query=XRCC4"},"sequence":{"accession":"Q13426","fasta_url":"https://rest.uniprot.org/uniprotkb/Q13426.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q13426/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q13426"}},"corpus_meta":[{"pmid":"16439205","id":"PMC_16439205","title":"XLF interacts with the XRCC4-DNA ligase IV complex to promote DNA nonhomologous end-joining.","date":"2006","source":"Cell","url":"https://pubmed.ncbi.nlm.nih.gov/16439205","citation_count":596,"is_preprint":false},{"pmid":"9242410","id":"PMC_9242410","title":"Activity of DNA ligase IV stimulated by complex formation with XRCC4 protein in mammalian cells.","date":"1997","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/9242410","citation_count":521,"is_preprint":false},{"pmid":"10786799","id":"PMC_10786799","title":"Interplay of p53 and DNA-repair protein XRCC4 in tumorigenesis, genomic stability and development.","date":"2000","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/10786799","citation_count":482,"is_preprint":false},{"pmid":"9259561","id":"PMC_9259561","title":"Mammalian DNA double-strand break repair protein XRCC4 interacts with DNA ligase IV.","date":"1997","source":"Current biology : CB","url":"https://pubmed.ncbi.nlm.nih.gov/9259561","citation_count":373,"is_preprint":false},{"pmid":"8548796","id":"PMC_8548796","title":"The XRCC4 gene encodes a novel protein involved in DNA double-strand break repair and V(D)J recombination.","date":"1995","source":"Cell","url":"https://pubmed.ncbi.nlm.nih.gov/8548796","citation_count":371,"is_preprint":false},{"pmid":"17124166","id":"PMC_17124166","title":"Dynamic assembly of end-joining complexes requires interaction between Ku70/80 and XRCC4.","date":"2006","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/17124166","citation_count":322,"is_preprint":false},{"pmid":"10757784","id":"PMC_10757784","title":"Ku recruits the XRCC4-ligase IV complex to DNA ends.","date":"2000","source":"Molecular and cellular biology","url":"https://pubmed.ncbi.nlm.nih.gov/10757784","citation_count":298,"is_preprint":false},{"pmid":"25574025","id":"PMC_25574025","title":"DNA repair. 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double-strand break repair; XR-1 cells lacking XRCC4 can initiate but not complete V(D)J recombination, and human XRCC4 cDNA complements both defects.\",\n      \"method\": \"Functional complementation of XRCC4-deficient XR-1 CHO cells with human XRCC4 cDNA; V(D)J recombination substrate assays; DSBR sensitivity assays\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — clean complementation experiment in defined mutant cell line, replicated across multiple assays and subsequently confirmed by many independent labs\",\n      \"pmids\": [\"8548796\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"XRCC4 directly interacts with DNA ligase IV and stimulates its ligation activity 5–8-fold in vitro; the two proteins co-immunoprecipitate and interact in yeast two-hybrid, establishing that the primary function of XRCC4 is to stimulate DNA ligase IV for DSB ligation.\",\n      \"method\": \"Co-immunoprecipitation; yeast two-hybrid; in vitro ligation assay with purified proteins; co-expression in insect cells\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro reconstitution with purified proteins plus co-IP, replicated in subsequent independent studies\",\n      \"pmids\": [\"9242410\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"XRCC4 interacts with DNA ligase IV via the unique carboxy-terminal extension of ligase IV that contains tandem BRCT domains; XRCC4 and ligase IV co-purify quantitatively and XRCC4 is a nuclear phosphoprotein that is an in vitro substrate for DNA-PK.\",\n      \"method\": \"Co-immunoprecipitation; co-purification across chromatographic steps; adenylylation assay; protein mapping/deletion analysis; in vitro DNA-PK phosphorylation assay\",\n      \"journal\": \"Current biology : CB\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — multiple orthogonal biochemical methods, replicated by independent labs\",\n      \"pmids\": [\"9259561\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"XRCC4 forms a homodimer in vivo; the homodimerization domain maps to amino acids 115–204; a core functional domain (aa 18–204) is required for V(D)J recombination; XRCC4 localizes to the nucleus.\",\n      \"method\": \"Yeast two-hybrid with deletion mutants; transient V(D)J recombination assay; nuclear localization imaging\",\n      \"journal\": \"International immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — yeast two-hybrid and functional complementation, single lab, two methods\",\n      \"pmids\": [\"9352367\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"DNA ligase IV binds XRCC4 via a region located between (not within) its tandem BRCT domains; this inter-BRCT linker region is necessary and sufficient for XRCC4 binding.\",\n      \"method\": \"Deletion analysis of DNA ligase IV; co-expression and co-immunoprecipitation; in vitro binding assays\",\n      \"journal\": \"Current biology : CB\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — detailed deletion mapping with multiple constructs, confirmed by crystal structure in later work\",\n      \"pmids\": [\"9705934\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"A central region of XRCC4 (amino acids 100–250) is required for DNA ligase IV binding; deletions within this region abolish in vivo NHEJ and V(D)J recombination, demonstrating the physical interaction with ligase IV is essential for XRCC4 function in vivo.\",\n      \"method\": \"XRCC4 deletion mutant analysis; in vitro ligase IV binding assay; in vivo complementation of XRCC4-deficient cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — deletion mutagenesis correlated with both in vitro binding and in vivo functional rescue\",\n      \"pmids\": [\"9733770\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"XRCC4 is an in vitro substrate of DNA-PK; phosphorylation occurs on serine, threonine, and tyrosine in insect cells and on serine/threonine within the C-terminal 130 aa by DNA-PK in vitro; recombinant XRCC4 facilitates Ku binding to DNA and promotes DNA-PK assembly on DNA ends.\",\n      \"method\": \"In vitro kinase assay with DNA-PK and recombinant XRCC4; phosphoamino acid analysis; DNA-binding/Ku-assembly assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — in vitro biochemical assays, single lab\",\n      \"pmids\": [\"9430729\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"XRCC4 stabilizes DNA ligase IV protein; in XRCC4-deficient XR-1 cells DNA ligase IV protein is nearly absent despite normal mRNA levels, and reintroduction of XRCC4 restores ligase IV to wild-type levels.\",\n      \"method\": \"Western blot; mRNA analysis by Northern/RT-PCR; XRCC4 cDNA transfection into XR-1 cells\",\n      \"journal\": \"Mutation research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — clear protein stability rescue experiment, replicated in yeast ortholog study\",\n      \"pmids\": [\"10047779\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"XRCC4 binds DNA with preference for nicks or broken ends; DNA binding of XRCC4 is correlated with V(D)J recombination complementation activity but is not required for stimulation of DNA ligase IV adenylation, indicating XRCC4 has DNA ligase IV-independent functions.\",\n      \"method\": \"DNA-binding assays with purified protein; adenylation assay for ligase IV; functional complementation assay in XRCC4-deficient cells; deletion/mutation analysis\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — reconstituted in vitro assays plus in vivo complementation, single lab but multiple orthogonal methods\",\n      \"pmids\": [\"10202163\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"The crystal structure of XRCC4 reveals an elongated dumbbell-shaped tetramer with N-terminal globular head domains containing a beta-sandwich and helix-turn-helix motif, and a C-terminal stalk of >120 Å alpha-helix that mediates ligase IV interaction; the structure suggests coupling of ligase IV association with DNA binding.\",\n      \"method\": \"X-ray crystallography at 2.7 Å resolution\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — high-resolution crystal structure, consistent with functional data from multiple labs\",\n      \"pmids\": [\"11080143\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"XRCC4-ligase IV complex interacts directly with the Ku heterodimer in a DNA-stimulated manner; this interaction recruits XRCC4-ligase IV to DNA ends, increasing the initial ligation rate 20-fold; Ku's interaction is specific for ligase IV and does not extend to other mammalian ligases.\",\n      \"method\": \"Co-immunoprecipitation; in vitro ligation assay; DNA end-binding assay; preformation of Ku/XRCC4-LigIV complex\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — reconstituted in vitro with purified proteins, multiple binding and activity assays, replicated independently\",\n      \"pmids\": [\"10757784\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"DNA ligase IV-XRCC4 complex binds specifically to ends of duplex DNA and can bridge two DNA molecules; Ku and DNA-PKcs bind simultaneously with XRCC4-LigIV at DNA ends; DNA-PKcs stimulates intermolecular ligation while Ku inhibits intramolecular ligation.\",\n      \"method\": \"DNA end-binding assays; intermolecular/intramolecular ligation assays with purified proteins; simultaneous binding experiments\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro reconstitution with purified components and multiple activity assays\",\n      \"pmids\": [\"10854421\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"DNA ligase IV and XRCC4 form a stable mixed tetramer (~300 kDa, 2:2 stoichiometry) with XRCC4 homodimer at the core and ligase IV subunits not in direct contact with each other; this complex functions synergistically with Ku and DNA-PKcs in cell-free end-joining.\",\n      \"method\": \"Protein expression and purification; cross-linking; size estimation; cell-free end-joining reconstitution assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — biochemical reconstitution, single lab\",\n      \"pmids\": [\"10945980\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"p53-deficiency rescues embryonic lethality and neuronal apoptosis caused by XRCC4 deficiency in mice, but does not rescue impaired V(D)J recombination; XRCC4/p53 double-deficient mice develop pro-B cell lymphomas with c-myc/IgH translocations, establishing NHEJ as a genome caretaker.\",\n      \"method\": \"Double-knockout mouse model; genetic epistasis; tumor pathology; chromosomal translocation analysis\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic epistasis in mouse model, multiple phenotypic readouts, widely replicated\",\n      \"pmids\": [\"10786799\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Crystal structure of human XRCC4 bound to the DNA ligase IV inter-BRCT linker peptide shows that a single ligase IV chain binds asymmetrically to an XRCC4 dimer; binding causes conformational change in XRCC4 helical tails (coiled-coil unwinds to flat surface); interaction is mediated by charged hydrogen bonds and extensive hydrophobic contacts.\",\n      \"method\": \"X-ray crystallography\",\n      \"journal\": \"Nature structural biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure with detailed interface characterization, later confirmed by higher-resolution structures\",\n      \"pmids\": [\"11702069\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Direct protein-protein interactions in the NHEJ complex: ligase IV interacts with Ku via its BRCT domains; XRCC4 interacts directly with DNA-PKcs; ligase IV does not bind DNA-PKcs and XRCC4 does not bind Ku directly (or only very weakly).\",\n      \"method\": \"Pull-down assays; co-immunoprecipitation with defined protein pairs; DNA-independent interaction assays\",\n      \"journal\": \"DNA repair\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple pairwise binding assays, single lab\",\n      \"pmids\": [\"12509254\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"DNA-PK phosphorylates XRCC4 at serines 260 and 318 (mapped by mass spectrometry) within the C-terminal 100 aa, but these phosphorylations are not required for cell survival after IR or for V(D)J recombination, indicating these sites do not directly control NHEJ function.\",\n      \"method\": \"Mass spectrometry phosphorylation mapping; alanine-substitution mutagenesis; complementation assay in XRCC4-deficient cells; IR survival assay\",\n      \"journal\": \"DNA repair\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — MS-based site mapping plus mutagenesis with functional readout; negative result for functional importance is clearly established\",\n      \"pmids\": [\"14599745\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"XRCC4 tetramerization and DNA ligase IV binding are mutually exclusive; only the XRCC4 dimer associates with ligase IV (2:1 stoichiometry in solution); the dimer-tetramer equilibrium likely separates two functional states of XRCC4.\",\n      \"method\": \"Equilibrium sedimentation analysis; mutational analysis of XRCC4; biochemical complex formation assays\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — rigorous biophysical assays plus mutational analysis, single lab\",\n      \"pmids\": [\"14607114\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"XRCC4 physically links polynucleotide kinase (PNK) to DNA ligase IV via a CK2-phosphorylation-dependent interaction; the PNK FHA domain binds CK2-phosphorylated XRCC4; disruption of this interaction causes increased radiosensitivity and slower DSB repair kinetics.\",\n      \"method\": \"Co-immunoprecipitation; FHA domain binding assay; in vitro end-joining assay; CK2 phosphorylation assay; siRNA disruption in cells; IR survival/repair kinetics\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — phospho-dependent interaction biochemically characterized, combined with in vivo functional evidence\",\n      \"pmids\": [\"15385968\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Aprataxin (AOA1 gene product) physically interacts with XRCC4 in vitro and in vivo via its divergent FHA domain binding to CK2-phosphorylated XRCC4.\",\n      \"method\": \"Co-immunoprecipitation in vitro and in vivo; FHA domain-phosphopeptide binding analysis\",\n      \"journal\": \"DNA repair\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal co-IP and domain mapping, single lab\",\n      \"pmids\": [\"15380105\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"DNA polymerase lambda interacts with the XRCC4-DNA ligase IV complex via its N-terminal BRCT domain, and this interaction stimulates pol lambda DNA synthesis activity; pol lambda can perform gap-filling on NHEJ substrates.\",\n      \"method\": \"Co-immunoprecipitation; in vitro gap-filling synthesis assay; interaction mapping with truncated pol lambda forms\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — co-IP plus in vitro activity assay, single lab\",\n      \"pmids\": [\"15451442\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Recruitment of XRCC4-ligase IV complex to DNA ends requires coordinated assembly of both Ku and DNA-PKcs (p460); interactions of Ku and p460 with XRCC4-LigIV are mainly DNA-dependent; XRCC4 is phosphorylated by DNA-PK upon recruitment but phosphorylation is not required for loading.\",\n      \"method\": \"In vitro DNA-end pull-down from nuclear extracts; co-immunoprecipitation; wortmannin inhibition; phosphorylation assay\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — cell-free biochemical reconstitution with multiple inhibitor controls, single lab\",\n      \"pmids\": [\"12547193\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"DNA processing for NHEJ (polymerase and nuclease activities) requires XRCC4/Ligase IV in addition to wortmannin-sensitive kinase activity; XRCC4/LigIV is required for alignment-based gap filling preceding ligation of radiation-induced DSBs.\",\n      \"method\": \"Two-stage cell-free NHEJ assay; supplementation of XRCC4-deficient extracts with recombinant XRCC4/LigIV; kinase inhibitor (wortmannin) treatment\",\n      \"journal\": \"The Journal of biological chemistry; Cancer research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — cell-free complementation assay, confirmed by two independent studies\",\n      \"pmids\": [\"17272270\", \"12517771\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"XRCC4 is SUMOylated on lysine 210; this modification is required for nuclear accumulation of XRCC4; SUMO-deficient XRCC4 (K210R) remains cytoplasmic, causes radiation sensitivity and failure to complete V(D)J recombination; genetic fusion of SUMO to the C-terminus rescues nuclear localization and function.\",\n      \"method\": \"Site-directed mutagenesis; in vitro and in vivo SUMO modification assays; subcellular fractionation/immunofluorescence; IR survival assay; V(D)J recombination assay\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — site-specific mutagenesis, biochemical modification assay, rescue experiment, and functional readouts\",\n      \"pmids\": [\"16478998\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"XLF (Cernunnos) directly interacts with the XRCC4-Ligase IV complex in vitro and in vivo and is required for efficient NHEJ; siRNA knockdown causes radiosensitivity and NHEJ defect; XLF is absent in patient 2BN cells due to frameshift mutation and its reintroduction corrects the defect.\",\n      \"method\": \"Co-immunoprecipitation in vitro and in vivo; siRNA knockdown; IR survival; NHEJ reporter assay; complementation of patient cells\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods, patient cell complementation, replicated by multiple labs\",\n      \"pmids\": [\"16439205\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Cernunnos/XLF physically interacts with the XRCC4-DNA ligase IV complex and is the mammalian homolog of yeast Nej1, identifying it as a new component of the NHEJ ligation complex.\",\n      \"method\": \"Co-immunoprecipitation; sequence homology analysis (sensitive sequence analysis methods)\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — co-IP confirming interaction, single lab\",\n      \"pmids\": [\"16571728\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Ku70/80 interacts directly with XRCC4; XRCC4/LigIV accumulation at DSBs depends on Ku70/80 but not DNA-PKcs; Ku heterodimers are in dynamic equilibrium at DNA ends; XRCC4 may serve as a flexible tether between Ku70/80 and ligase IV.\",\n      \"method\": \"Pulsed near-IR laser micro-irradiation; live-cell imaging; FRAP; co-immunoprecipitation; direct interaction assay (Ku70-XRCC4)\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — direct interaction assay combined with live-cell imaging and FRAP, multiple orthogonal methods\",\n      \"pmids\": [\"17124166\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"XRCC4-DNA ligase IV can ligate two DNA ends with fully incompatible 3' overhangs (no base pairing) and can ligate across single-nucleotide gaps; XLF stimulates joining of both compatible and incompatible ends.\",\n      \"method\": \"In vitro ligation assay with purified XRCC4-LigIV complex; defined DNA substrates with incompatible overhangs and gaps; Ku stimulation assays\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — reconstituted in vitro with purified components and defined substrates\",\n      \"pmids\": [\"17290226\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Ku interacts with DNA ligase IV via its first BRCT domain (residues 644–748); this interaction is enhanced by XRCC4 and dsDNA; DNA-PK kinase activity causes disassembly of the Ku/DNA ligase IV/XRCC4 complex.\",\n      \"method\": \"Co-immunoprecipitation; deletion mapping of LigIV-Ku interaction; in vitro complex formation; kinase activity assays\",\n      \"journal\": \"DNA repair\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple biochemical assays, single lab\",\n      \"pmids\": [\"17241822\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"XRCC4 plays a role in immunoglobulin class switch recombination (CSR); B-lymphocyte-restricted conditional XRCC4 knockout leads to ~2-fold reduction in CSR, implicating NHEJ in CSR resolution.\",\n      \"method\": \"Conditional knockout mouse (LoxP-flanked XRCC4, Cre-lentiviral); in vivo and in vitro CSR assays in B cells\",\n      \"journal\": \"The Journal of experimental medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — conditional knockout with defined cellular phenotype (CSR reduction)\",\n      \"pmids\": [\"17606631\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Werner protein (WRN) physically interacts with the XRCC4-DNA ligase IV complex (X4L4); X4L4 stimulates WRN exonuclease but not helicase activity; in a DNA end-joining assay, WRN-processed substrates are ligated by X4L4.\",\n      \"method\": \"Co-immunoprecipitation; in vitro exonuclease/helicase activity assays; DNA end-joining assay\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — biochemical interaction and activity assays, single lab\",\n      \"pmids\": [\"18558713\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"High-resolution crystal structure of human XRCC4 bound to the C-terminal tandem BRCT repeat of DNA ligase IV reveals an extensive binding interface formed by a helix-loop-helix in the inter-BRCT linker and the second BRCT domain; second BRCT domain contact is necessary for stable binding in cells.\",\n      \"method\": \"X-ray crystallography (high-resolution); mutagenesis; cellular radiosensitization assays with dominant-negative LigIV fragments\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure plus mutagenesis with cellular functional readout\",\n      \"pmids\": [\"19332554\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"CK2-phosphorylated XRCC4 promotes high-affinity interaction with PNKP FHA domain, stimulating PNKP turnover; unphosphorylated XRCC4 also interacts via a lower-affinity site in the PNKP catalytic domain; paradoxically, CK2-phosphorylated XRCC4 inhibits PNKP activity.\",\n      \"method\": \"Co-immunoprecipitation; in vitro FHA domain binding; PNKP kinase/phosphatase activity assays; CK2 phosphorylation assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — multiple orthogonal biochemical methods defining two modes of interaction and their functional consequences\",\n      \"pmids\": [\"20852255\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"XLF and XRCC4 form alternating super-helical filaments through head-domain interactions; XLF Leu-115 ('Leu-lock') inserts into a hydrophobic pocket on XRCC4 (Met-59, Met-61, Lys-65, Lys-99, Phe-106, Leu-108); these filaments form a positively charged channel to bind and align DNA ends for ligation.\",\n      \"method\": \"X-ray crystallography (crystal + solution structures); SAXS; site-directed mutagenesis; DNA-binding assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure plus mutational validation and biophysical characterization\",\n      \"pmids\": [\"21775435\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"XLF and XRCC4 dimers interact through their N-terminal head domains to form alternating left-handed helical filaments (confirmed at 5.5 Å and 8.5 Å resolution); key XRCC4 residues Glu55, Asp58, Met61, and Phe106 are essential for interaction with XLF.\",\n      \"method\": \"X-ray crystallography; electron microscopy; calorimetry; site-directed mutagenesis\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America; Biochemical Society transactions\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure with mutagenesis and calorimetry confirmation, independently replicated\",\n      \"pmids\": [\"21768349\", \"21936820\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"DNA Ligase IV controls nuclear import and sub-nuclear distribution of XRCC4; in LigIV-deficient cells XRCC4 remains cytoplasmic; co-expression with LigIV restores nuclear localization of XRCC4; XRCC4/LigIV complex exchanges faster at DNA damage sites than XRCC4 alone.\",\n      \"method\": \"Fluorescent fusion protein expression; live-cell imaging; FRAP; LigIV-deficient cell line complementation\",\n      \"journal\": \"DNA repair\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — live imaging with FRAP plus genetic loss-of-function, multiple orthogonal approaches\",\n      \"pmids\": [\"21982441\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"XRCC4-XLF complexes bridge two independent DNA molecules in a LigIV-independent manner; XRCC4's interaction with XLF is required for coding (but not signal) end joining in V(D)J recombination; DNA-PK phosphorylation of XRCC4/XLF disrupts DNA bridging in vitro.\",\n      \"method\": \"DNA-binding and bridging assays; crystal structure (3.94 Å); XRCC4 mutants abolishing XLF interaction; V(D)J recombination assays; DNA-PK phosphorylation assay\",\n      \"journal\": \"Nucleic acids research; Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — crystal structure, biochemical bridging assays, and in vivo V(D)J assay with separation-of-function mutants\",\n      \"pmids\": [\"22287571\", \"22228831\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"DNA Ligase IV is required for nuclear localization of XRCC4; its C-terminal region (aa 620–800) containing an NLS, BRCT I domain, and XRCC4-interacting region is essential for XRCC4 nuclear accumulation; Ligase IV also regulates XRCC4 protein stability.\",\n      \"method\": \"Expression of LigIV deletion constructs; subcellular fractionation/imaging; LigIV-deficient cell lines; Western blot for protein levels\",\n      \"journal\": \"DNA repair\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — cell biology with defined deletion constructs, single lab\",\n      \"pmids\": [\"24984242\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"PAXX (paralog of XRCC4 and XLF) interacts directly with Ku and promotes Ku-dependent DNA ligation and assembly of core NHEJ factors; PAXX crystal structure resembles XRCC4; combined PAXX/XLF loss impairs DSB repair additively.\",\n      \"method\": \"Crystal structure; co-immunoprecipitation; CRISPR-Cas9 and RNAi knockouts; in vitro ligation assay; chromatin fractionation\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — crystal structure plus reciprocal co-IP, in vitro assay, and genetic loss-of-function with cellular phenotype\",\n      \"pmids\": [\"25574025\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"FBXW7 (SCF E3 ligase) promotes K63-linked polyubiquitylation of XRCC4 at lysine 296 after DNA-PKcs phosphorylates XRCC4 at serines 325/326; this ubiquitylation enhances XRCC4 association with Ku70/80 to facilitate NHEJ; a small-molecule inhibitor blocking this ubiquitylation reduces NHEJ.\",\n      \"method\": \"In vivo ubiquitylation assay; site-directed mutagenesis; co-immunoprecipitation; NHEJ reporter assay; IR sensitivity assay; kinase assay\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — multiple orthogonal methods (ubiquitylation assay, mutagenesis, inhibitor, co-IP, functional assay)\",\n      \"pmids\": [\"26774286\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"XRCC4-XLF complexes form mobile 'sliding sleeve' structures around DNA that can bridge two independent DNA molecules and diffuse along DNA; XLF stimulates XRCC4 binding to DNA; XRCC4-XLF bridges can slide along DNA, enabling rapid reconnection of broken ends.\",\n      \"method\": \"Dual- and quadruple-trap optical tweezers combined with fluorescence microscopy; single-molecule real-time imaging\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — rigorous single-molecule biophysical experiment with direct visualization\",\n      \"pmids\": [\"27437582\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"APLF is an intrinsically disordered protein that links Ku, DNA-PKcs, XRCC4-LigIV into an extended flexible NHEJ core complex via simultaneous interactions with Ku, DNA-PK, and X4L4; Ku80 C-terminal region connects DNA-PKcs to the X4L4 complex.\",\n      \"method\": \"SAXS; mutational analysis; in vitro complex reconstitution\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — SAXS structural analysis plus mutagenesis, single lab\",\n      \"pmids\": [\"27875301\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"The PNKP-XRCC4-LigIV ternary complex requires CK2-phosphorylation of XRCC4 for stable PNKP binding; only one PNKP protomer binds per XRCC4 dimer; SAXS and HDX reveal multipoint contacts between PNKP and XRCC4-LigIV including a PNKP phosphatase surface (E326K disease mutation impairs PNKP recruitment to damaged DNA).\",\n      \"method\": \"Recombinant complex purification; SAXS; hydrogen-deuterium exchange; CK2 phosphorylation assay; site-directed mutagenesis; cellular recruitment assay\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — multiple structural and biochemical methods with cellular validation\",\n      \"pmids\": [\"28453785\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"XRCC4-LigIV (X4-LIV) stimulates Artemis nuclease activity at 3' overhangs independently of DNA-PKcs; X4-LIV cannot stimulate Artemis at hairpins or 5' overhangs; X4-LIV and DNA-PKcs interfere with each other in stimulating Artemis, suggesting sequential not concurrent recruitment.\",\n      \"method\": \"In vitro nuclease assay with purified proteins; in vitro ligation assay; defined DNA substrates\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro reconstitution with purified components and multiple substrate configurations, single lab\",\n      \"pmids\": [\"28696258\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Phospho-mimicking mutations in the C-terminal tails of both XRCC4 and XLF (14 DNA-PK/ATM sites mutated to aspartate) destabilize XRCC4/XLF complexes and reduce DNA bridging activity without affecting LigIV stimulation, suggesting phosphorylation regulates DNA bridging independently of ligation.\",\n      \"method\": \"Phospho-blocking and -mimicking mutagenesis; DNA bridging assay; LigIV stimulation assay; complex stability analysis\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — separation-of-function mutants with multiple orthogonal functional assays\",\n      \"pmids\": [\"28500754\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"RIG-I interacts with XRCC4 and impedes formation of the XRCC4/LIG4/XLF complex at DSBs, suppressing NHEJ; reciprocally, XRCC4 promotes RIG-I signaling by enhancing RIG-I oligomerization and ubiquitination, augmenting type I IFN production.\",\n      \"method\": \"Co-immunoprecipitation; DSB repair assays; RIG-I/XRCC4 knockdown/overexpression; innate immune signaling assays; in vivo viral infection model\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — co-IP plus functional assays in cells, single lab, novel finding requiring independent replication\",\n      \"pmids\": [\"33846346\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"XRCC4 is a dimeric scaffolding protein that forms a tight, stoichiometric complex with DNA ligase IV (stabilizing it and stimulating its adenylation and ligation activity 5–8-fold), is SUMOylated at K210 for nuclear import, and recruits end-processing enzymes (PNK/PNKP via CK2-phospho-dependent FHA interactions) to DSBs; together with XLF it assembles into dynamic, positively charged super-helical filaments that bridge and align broken DNA ends as sliding sleeves, while its interaction with Ku70/80 (and dependence on Ku/DNA-PKcs for stable chromatin recruitment) positions the entire ligation complex at DSBs, where K63-polyubiquitylation at K296 by SCF-FBXW7 further enhances Ku association and NHEJ efficiency.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"XRCC4 is a core scaffolding protein of the non-homologous end-joining (NHEJ) pathway, required for completion of DNA double-strand break repair and V(D)J recombination [#0, #13]. Its central function is to bind DNA ligase IV through the ligase's inter-BRCT linker and tandem BRCT region, an interaction that stabilizes ligase IV protein (which is nearly absent without XRCC4) and stimulates its ligation activity several-fold [#1, #7, #31]. Structurally, XRCC4 is an elongated dimer with N-terminal globular head domains and a long C-terminal coiled-coil stalk that engages ligase IV; tetramerization and ligase IV binding are mutually exclusive, defining distinct functional states [#9, #14, #17]. The XRCC4-ligase IV complex is recruited to broken ends by the Ku heterodimer in a DNA-stimulated manner, where it cooperates with Ku and DNA-PKcs and can ligate even incompatible or gapped ends [#10, #11, #27]. Through head-domain contacts XRCC4 assembles with XLF into alternating super-helical filaments that form a positively charged channel and act as mobile 'sliding sleeves' bridging and aligning two DNA ends independently of ligase IV [#33, #36, #40]. XRCC4 also serves as a platform recruiting end-processing enzymes — PNKP and aprataxin via CK2-phosphorylation-dependent FHA interactions, DNA polymerase lambda, WRN, and Artemis — coupling end processing to ligation [#18, #20, #32, #43]. XRCC4 function is regulated by post-translational modification: SUMOylation at K210 drives nuclear accumulation, and FBXW7-mediated K63-polyubiquitylation at K296 (following DNA-PKcs phosphorylation) enhances Ku association and NHEJ efficiency [#23, #39]. Genetically, p53 loss rescues the embryonic lethality of XRCC4 deficiency in mice, and XRCC4 contributes to immunoglobulin class switch recombination, establishing NHEJ as a genome caretaker [#13, #29].\",\n  \"teleology\": [\n    {\n      \"year\": 1995,\n      \"claim\": \"Established that XRCC4 is genetically required to complete V(D)J recombination and double-strand break repair, defining it as an essential NHEJ factor before any molecular activity was known.\",\n      \"evidence\": \"Functional complementation of XRCC4-deficient XR-1 CHO cells with human cDNA and V(D)J/DSBR assays\",\n      \"pmids\": [\"8548796\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not reveal the biochemical activity of XRCC4\", \"No interaction partners identified\"]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"Identified DNA ligase IV as the direct partner of XRCC4 and showed XRCC4 stimulates its ligation activity, assigning XRCC4 a concrete biochemical role in end ligation.\",\n      \"evidence\": \"Co-IP, yeast two-hybrid, and in vitro ligation assay with purified proteins; mapping to the ligase IV BRCT region\",\n      \"pmids\": [\"9242410\", \"9259561\", \"9352367\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not establish how the complex reaches DNA ends\", \"Stoichiometry of the complex unresolved\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Mapped the XRCC4–ligase IV interface to the inter-BRCT linker and demonstrated the interaction is essential for NHEJ in vivo, while showing XRCC4 stabilizes ligase IV protein.\",\n      \"evidence\": \"Deletion mapping with in vitro binding and in vivo complementation; protein/mRNA analysis in XR-1 cells\",\n      \"pmids\": [\"9705934\", \"9733770\", \"10047779\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of ligase IV stabilization not defined\", \"DNA-binding role of XRCC4 still unclear\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Showed XRCC4 binds DNA nicks and ends and possesses ligase IV-independent functions, indicating roles beyond simply stimulating the ligase.\",\n      \"evidence\": \"DNA-binding assays, adenylation assays, and complementation with deletion/mutant XRCC4\",\n      \"pmids\": [\"10202163\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of ligase IV-independent functions not defined\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Resolved XRCC4 architecture by crystallography and reconstituted its recruitment to DNA ends via Ku and DNA-PKcs, defining how the ligation complex is positioned at breaks.\",\n      \"evidence\": \"X-ray crystallography of XRCC4; co-IP and in vitro ligation/end-binding assays with Ku, DNA-PKcs, and the XRCC4-LigIV complex; cross-linking and stoichiometry\",\n      \"pmids\": [\"11080143\", \"10757784\", \"10854421\", \"10945980\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How Ku and XRCC4 physically connect remained ambiguous\", \"Dynamics of recruitment in cells not addressed\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Demonstrated in mice that NHEJ acts as a genome caretaker, with p53 loss rescuing XRCC4-null lethality but not the recombination defect, linking XRCC4 to tumour suppression.\",\n      \"evidence\": \"XRCC4/p53 double-knockout mouse; genetic epistasis, tumour pathology, and translocation analysis\",\n      \"pmids\": [\"10786799\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular basis of apoptosis suppression by p53 loss not defined\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Defined the atomic XRCC4–ligase IV interface and the asymmetric 2:1 binding mode, with binding inducing conformational change in XRCC4 tails.\",\n      \"evidence\": \"X-ray crystallography of XRCC4 bound to the ligase IV inter-BRCT peptide; later 2003 sedimentation analysis establishing dimer/tetramer exclusivity\",\n      \"pmids\": [\"11702069\", \"14607114\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional significance of the dimer-tetramer equilibrium in cells unresolved\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Mapped DNA-PK phosphorylation sites on XRCC4 (S260, S318) and showed they are dispensable for survival and V(D)J recombination, ruling out these modifications as direct controllers of NHEJ.\",\n      \"evidence\": \"Mass spectrometry site mapping plus alanine mutagenesis with complementation and IR survival assays\",\n      \"pmids\": [\"14599745\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Other phosphorylation sites and their roles not yet examined\", \"Function of XRCC4 phosphorylation left open\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Established XRCC4 as a recruitment hub for end-processing enzymes, linking PNK, aprataxin, and polymerase lambda to the ligation complex.\",\n      \"evidence\": \"Co-IP, CK2-dependent FHA-domain binding, in vitro activity and end-joining assays; siRNA disruption with IR survival/repair kinetics\",\n      \"pmids\": [\"15385968\", \"15380105\", \"15451442\", \"12547193\", \"17272270\", \"12517771\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Order of enzyme recruitment relative to ligation not fully defined\", \"Some interactions characterized in single labs\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Showed SUMOylation at K210 governs XRCC4 nuclear import and identified XLF and direct Ku70/80 binding as components governing complex assembly and chromatin recruitment.\",\n      \"evidence\": \"SUMO modification assays with K210R mutant and rescue; XLF co-IP and patient-cell complementation; laser micro-irradiation, FRAP, and Ku70-XRCC4 interaction assays\",\n      \"pmids\": [\"16478998\", \"16439205\", \"16571728\", \"17124166\", \"12509254\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"SUMO E3 ligase for XRCC4 not identified\", \"Relative contributions of Ku-XRCC4 versus Ku-ligase IV contacts to recruitment unresolved\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Demonstrated the catalytic versatility of XRCC4-ligase IV — joining incompatible ends and gaps — and extended XRCC4's biological role to immunoglobulin class switch recombination.\",\n      \"evidence\": \"In vitro ligation with defined incompatible/gapped substrates; conditional B-cell XRCC4 knockout with CSR assays; LigIV-Ku BRCT mapping\",\n      \"pmids\": [\"17290226\", \"17606631\", \"17241822\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of incompatible-end accommodation at structural level not defined\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Extended the XRCC4-ligase IV interactome to WRN, coupling WRN exonuclease processing to downstream ligation.\",\n      \"evidence\": \"Co-IP and in vitro exonuclease/helicase and end-joining assays\",\n      \"pmids\": [\"18558713\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab biochemistry without cellular validation\", \"Physiological context of WRN-X4L4 cooperation unclear\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Revealed that XRCC4 and XLF assemble into alternating super-helical filaments forming a positively charged DNA-binding channel, providing a structural basis for end bridging and alignment.\",\n      \"evidence\": \"Crystallography, EM, SAXS, calorimetry, and mutagenesis of XLF-XRCC4 head-domain contacts; FRAP and imaging of LigIV control of XRCC4 nuclear import\",\n      \"pmids\": [\"21775435\", \"21768349\", \"21936820\", \"21982441\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Filament length and stoichiometry on real broken DNA in cells not defined\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Showed XRCC4-XLF bridges two DNA molecules independently of ligase IV and that this bridging is needed specifically for coding-end joining and is regulated by DNA-PK phosphorylation.\",\n      \"evidence\": \"DNA bridging assays, crystallography, separation-of-function XRCC4 mutants, and V(D)J recombination assays\",\n      \"pmids\": [\"22287571\", \"22228831\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Why bridging is selectively required for coding but not signal ends not explained\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Visualized XRCC4-XLF bridges as mobile 'sliding sleeves' on single DNA molecules and identified FBXW7-mediated K63-ubiquitylation at K296 as a regulator enhancing Ku association.\",\n      \"evidence\": \"Optical tweezers with single-molecule fluorescence; in vivo ubiquitylation assays, mutagenesis, small-molecule inhibitor, and NHEJ reporter assays; SAXS of APLF-linked core complex\",\n      \"pmids\": [\"27437582\", \"26774286\", \"27875301\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How sliding sleeves are converted to a static ligation-competent state unresolved\", \"Deubiquitylase counteracting K296 ubiquitylation not identified\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Defined how XRCC4-ligase IV stimulates and coordinates end-processing enzymes (PNKP, Artemis) and showed C-terminal tail phosphorylation tunes DNA bridging independently of ligation.\",\n      \"evidence\": \"SAXS/HDX of PNKP-X4L4 complex; in vitro Artemis nuclease assays; phospho-mimic mutagenesis with bridging and LigIV-stimulation assays\",\n      \"pmids\": [\"28453785\", \"28696258\", \"28500754\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Temporal coupling of processing to ligation in cells not directly observed\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Identified PAXX as an XRCC4 paralog that promotes Ku-dependent ligation and core NHEJ assembly, expanding the XRCC4-superfamily scaffold network.\",\n      \"evidence\": \"Crystal structure, co-IP, CRISPR/RNAi knockout, chromatin fractionation, and in vitro ligation\",\n      \"pmids\": [\"25574025\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional redundancy/division of labour between XRCC4, XLF and PAXX incompletely resolved\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Linked XRCC4 to innate immunity, with RIG-I suppressing NHEJ complex formation while XRCC4 reciprocally augments RIG-I signaling.\",\n      \"evidence\": \"Co-IP, DSB repair and innate immune signaling assays, and an in vivo viral infection model\",\n      \"pmids\": [\"33846346\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab finding awaiting independent replication\", \"Physiological significance of the XRCC4-RIG-I axis unresolved\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How the dynamic, sliding XRCC4-XLF filament transitions to a fixed, ligation-competent synaptic state and how the many post-translational modifications are temporally integrated during repair remain open.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No real-time model linking bridging, processing, and ligation steps\", \"Coordination of SUMO, phosphorylation, and ubiquitylation signals over the repair cycle undefined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [1, 7, 18, 26]},\n      {\"term_id\": \"GO:0003677\", \"supporting_discovery_ids\": [8, 11, 33, 40]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [1, 7, 43]},\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [9, 33, 34]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [3, 23, 35]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [23, 35]},\n      {\"term_id\": \"GO:0000228\", \"supporting_discovery_ids\": [26, 36]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-73894\", \"supporting_discovery_ids\": [0, 10, 27, 40]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [0, 29, 45]},\n      {\"term_id\": \"R-HSA-1640170\", \"supporting_discovery_ids\": [13]}\n    ],\n    \"complexes\": [\"XRCC4-DNA ligase IV complex\", \"XRCC4-XLF filament\", \"NHEJ core complex (Ku/DNA-PKcs/X4L4)\"],\n    \"partners\": [\"LIG4\", \"XLF\", \"XRCC5\", \"XRCC6\", \"PRKDC\", \"PNKP\", \"APTX\", \"PAXX\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":8,"faith_total":8,"faith_pct":100.0}}