{"gene":"NABP2","run_date":"2026-06-10T05:19:52","timeline":{"discoveries":[{"year":2008,"finding":"hSSB1 (NABP2) is phosphorylated by ATM kinase at threonine 117 in response to DNA double-strand breaks; this phosphorylation is required for DNA damage-induced stabilization of hSSB1. hSSB1 accumulates in the nucleus and forms distinct foci at DSB sites. Depletion of hSSB1 abrogates ATM activation and phosphorylation of ATM targets after ionizing radiation, establishing hSSB1 as required for ATM-mediated DSB signaling.","method":"Cell-based phosphorylation assays, siRNA depletion, nuclear foci formation (immunofluorescence), ionizing radiation treatment, ATM kinase assay","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal functional assays with siRNA depletion, multiple orthogonal readouts (foci, ATM activation, checkpoint, repair), replicated by subsequent studies","pmids":["18449195"],"is_preprint":false},{"year":2009,"finding":"hSSB1 (NABP2) forms a stable complex with INTS3 and hSSBIP1 (C9ORF80), distinct from the hSSB2 complex. INTS3 depletion decreases the stability of hSSB1 and hSSBIP1, suggesting INTS3 provides a scaffold for proper complex assembly. Both complexes are required for efficient HR-dependent DSB repair and ATM-dependent signaling.","method":"Immunoprecipitation, siRNA depletion, cell survival assays, HR reporter assay, ATM phosphorylation assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal Co-IP, multiple orthogonal functional assays, independently confirmed by subsequent studies","pmids":["19605351"],"is_preprint":false},{"year":2009,"finding":"hSSB1 was shown by tandem affinity purification to copurify with a subset of Integrator complex subunits and MISE (c9orf80/LOC58493). The INTS3-MISE-hSSB1 complex controls ATM activation and RAD51 recruitment to DNA damage foci. INTS3 regulates hSSB1 transcription, thereby controlling hSSB1 function.","method":"Tandem affinity purification of hSSB1 phosphomimetic and phospho-null mutants, mass spectrometry, siRNA depletion, immunofluorescence for RAD51 foci","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — tandem affinity purification with mass spectrometry, multiple orthogonal functional assays, consistent with independent replication","pmids":["19786574"],"is_preprint":false},{"year":2010,"finding":"hSSB1 is rapidly recruited to DSB sites in all interphase cell cycle phases independently of CtIP, MDC1, and the MRN complex. However, hSSB1 depletion prevents MRN complex foci formation and recruitment to DSBs, and leads to defective DSB resection (loss of RPA foci and ssDNA generation), placing hSSB1 upstream of MRN in the DSB response pathway.","method":"siRNA depletion, laser microirradiation, live-cell and fixed immunofluorescence, BrdU ssDNA detection, RPA foci assay","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 2 / Strong — epistasis established by sequential depletion experiments with multiple functional readouts, independently corroborated","pmids":["21051358"],"is_preprint":false},{"year":2011,"finding":"hSSB1 directly binds NBS1 (a component of the MRN complex) in a DNA damage-independent manner, and greatly stimulates MRN endonuclease activity in vitro via its C-terminal tail. NBS1 mutations associated with Nijmegen breakage syndrome show weaker binding to hSSB1.","method":"Direct binding assays (pulldown), in vitro endonuclease assay with purified proteins, C-terminal deletion mutants, NBS1 disease-associated mutants","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro reconstitution of direct binding and stimulation of enzymatic activity, mutagenesis of both proteins, single lab","pmids":["21227926"],"is_preprint":false},{"year":2011,"finding":"hSSB1 directly binds p21 protein and this interaction prevents p21 from ubiquitin-mediated degradation. hSSB1 knockdown-induced abrogation of G1/S and G2/M checkpoints is partially dependent on p21.","method":"Co-immunoprecipitation, siRNA knockdown, cell cycle analysis, ubiquitination assay","journal":"Oncogene","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — Co-IP binding plus functional epistasis, single lab, two complementary methods","pmids":["21242961"],"is_preprint":false},{"year":2012,"finding":"hSSB1 interacts with p53 and protects it from ubiquitin-mediated degradation. hSSB1 also associates with the acetyltransferase p300 and is required for efficient p53 acetylation at lysine 382, promoting transcriptional activation of p21. hSSB1 knockdown-induced G2/M checkpoint abrogation is partially dependent on p53 or p300.","method":"Co-immunoprecipitation, ubiquitination assays, acetylation assays, siRNA knockdown, luciferase reporter assay for p21 transcription, cell cycle analysis","journal":"Cell research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal Co-IP, acetylation/ubiquitination functional assays, epistasis; single lab","pmids":["23184057"],"is_preprint":false},{"year":2013,"finding":"The core hSSB1 complex contains INTS6 in addition to INTS3 and C9orf80. INTS6 directly interacts with INTS3 and forms a stable trimeric complex with INTS3 and hSSB1 both in vitro and in vivo. This complex regulates accumulation of RAD51 and BRCA1 at DNA damage sites and homologous recombination.","method":"Protein affinity purification, Co-immunoprecipitation, in vitro complex reconstitution, siRNA depletion, immunofluorescence for RAD51/BRCA1 foci, HR assay","journal":"Journal of cell science","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vitro and in vivo complex reconstitution, functional HR assay; single lab","pmids":["23986477"],"is_preprint":false},{"year":2014,"finding":"hSSB1 (NABP2) is required for ATR and Chk1 activation and recruitment of Mre11 and Rad51 at hydroxyurea-damaged replication forks. hSSB1-depleted cells fail to repair and restart stalled replication forks, accumulate DNA strand breaks and chromosome aberrations.","method":"siRNA depletion, hydroxyurea/camptothecin treatment, immunofluorescence for replication fork markers, Chk1/ATR phosphorylation assays, DNA fiber assay, chromosome analysis","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (fiber assay, foci, checkpoint kinase), loss-of-function with specific mechanistic readouts, consistent with prior studies","pmids":["24753408"],"is_preprint":false},{"year":2014,"finding":"FBXL5-containing SCF E3 ubiquitin ligase targets hSSB1 for ubiquitination and proteasomal degradation. ATM-mediated phosphorylation of hSSB1 at T117 prevents FBXL5-mediated degradation. Fbxl5 interacts directly with hSSB1.","method":"Co-immunoprecipitation, ubiquitination assays, proteasome inhibitor experiments, ATM kinase assay, overexpression/depletion studies","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 2 / Strong — direct interaction demonstrated, ubiquitination reconstituted, mechanistic link to ATM phosphorylation at defined residue, multiple methods","pmids":["25249620"],"is_preprint":false},{"year":2015,"finding":"hSSB1 (NABP2) forms a complex with hOGG1 (human 8-oxoguanine glycosylase 1) and is required for hOGG1 localization to damaged chromatin. In vitro, hSSB1 directly binds DNA containing 8-oxoguanines and enhances hOGG1 enzymatic activity, establishing hSSB1 as a participant in base excision repair of oxidative DNA lesions.","method":"Co-immunoprecipitation, siRNA depletion, chromatin fractionation, in vitro DNA binding assays (EMSA), in vitro OGG1 activity assay with purified proteins","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro reconstitution of direct binding and enzymatic stimulation, complemented by cellular localization assays; single lab but multiple orthogonal methods","pmids":["26261212"],"is_preprint":false},{"year":2015,"finding":"SSB1 (NABP2) binds specifically to G-strand telomeric DNA in vitro and associates with telomeres in vivo. SSB1 interacts with the TERT catalytic subunit and regulates its recruitment to telomeres. SSB1 deletion reduces TERT interaction with telomeres and leads to G-overhang loss.","method":"In vitro DNA binding assay (EMSA with telomeric oligonucleotides), Co-immunoprecipitation (SSB1-TERT), ChIP at telomeres, SSB1 deletion/depletion experiments, telomere G-overhang assay","journal":"Cancer research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct in vitro binding, Co-IP of TERT, ChIP at telomeres, functional G-overhang loss; single lab","pmids":["25589350"],"is_preprint":false},{"year":2016,"finding":"hSSB1 forms stable oligomers under oxidizing conditions, and this oligomerization is required for its function in base excision repair (8-oxoguanine removal) but is not required for DSB repair by homologous recombination. Monomeric hSSB1 shows decreased affinity for oxidized DNA.","method":"Size exclusion chromatography, EMSA with oxidized DNA, siRNA rescue experiments with oligomerization-deficient mutants, 8-oxoG repair assay, HR assay","journal":"Scientific reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional separation using oligomerization mutants with multiple repair pathway readouts; single lab","pmids":["27273218"],"is_preprint":false},{"year":2016,"finding":"ssDNA recognition by hSSB1 in solution is mediated by base-stacking of four key aromatic residues within the OB domain. This DNA binding mode in solution differs from that observed in the crystal structure of the SOSS1 complex.","method":"Solution-state NMR (chemical shift perturbation mapping), biophysical assays (ITC, fluorescence), functional DNA binding assays, comparison with crystal structure","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 1 / Moderate — NMR structural analysis with mutagenesis and biophysical validation, multiple orthogonal methods; single lab","pmids":["27387285"],"is_preprint":false},{"year":2017,"finding":"hSSB1 forms a tetramer structurally similar to E. coli SSB, stabilized by cysteines C81 and C99 and charged/hydrophobic residues. The tetramer can still interact with INTS3, showing oligomerization does not preclude SOSS1 complex formation.","method":"Solution-state NMR, size exclusion chromatography, mutagenesis of C81/C99, Co-immunoprecipitation with INTS3, biophysical experiments","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 1 / Moderate — NMR-based structural model with mutagenesis and biophysical validation; single lab but multiple orthogonal methods","pmids":["28609781"],"is_preprint":false},{"year":2017,"finding":"hSSB1 is directly phosphorylated by DNA-PK at serine residue 134. This modification is suppressed in undamaged cells by PPP-family protein phosphatases but is enhanced following replication fork disruption, promoting cellular survival.","method":"In vitro kinase assay with purified DNA-PK, phospho-specific antibodies, siRNA depletion of phosphatases, site-directed mutagenesis (S134A), cell survival assays after HU treatment","journal":"DNA repair","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro kinase reconstitution with mutagenesis at defined residue, functional phosphatase epistasis; single lab","pmids":["28448822"],"is_preprint":false},{"year":2017,"finding":"hSSB1 forms a complex with BLM helicase in cells, and this interaction is altered in response to ionizing radiation. hSSB1 depletion leads to proteasome-mediated degradation of BLM and defective recruitment of BLM to chromatin following IR-induced DSBs and HU-induced stalled replication forks.","method":"Co-immunoprecipitation, siRNA depletion, proteasome inhibitor treatment, chromatin fractionation, immunofluorescence for BLM foci","journal":"BMC molecular biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP, protein stability assay, chromatin recruitment assay; single lab, multiple methods","pmids":["28506294"],"is_preprint":false},{"year":2015,"finding":"The C-terminal tail of hSSB1 (NABP2) is essential for DNA binding ability and protein stability. Deletion of the C-terminus diminishes DNA binding. Both hNABP1 and hNABP2 exist as monomers in solution under the tested conditions, and hNABP2 has a disordered C-terminus.","method":"EMSA, ITC, circular dichroism spectroscopy, sucrose gradient centrifugation, C-terminal deletion mutants","journal":"Biochimica et biophysica acta","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — in vitro biochemical reconstitution with deletion mutants and multiple biophysical methods; single lab","pmids":["26550690"],"is_preprint":false},{"year":2015,"finding":"When RPA is depleted, hSSB1/hSSB2 and their partner INTS3 form sub-nuclear foci, associate with the ATR-ATRIP complex, and recruit it to sites of genomic stress. This alternate ATR activation via hSSB1/2-INTS3 requires TopBP1 and the Rad9-Rad1-Hus1 complex.","method":"siRNA depletion (RPA, hSSB1/2, INTS3), immunofluorescence for ATRIP/ATR foci, Co-immunoprecipitation, Chk1 phosphorylation assay","journal":"Nucleic acids research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic epistasis by sequential depletion, Co-IP, functional checkpoint readout; single lab","pmids":["25916848"],"is_preprint":false},{"year":2020,"finding":"hSSB1 undergoes SUMOylation at K79 and K94, enhanced by DNA damage, catalyzed by PIAS2α and reversed by SENP2. SUMOylation stabilizes hSSB1 protein and enhances recruitment of NBS1 to DNA damage sites. Cells with SUMOylation-defective hSSB1 are sensitive to ionizing radiation.","method":"In vivo SUMOylation assay, site-directed mutagenesis (K79R, K94R), Co-immunoprecipitation with PIAS2α/SENP2, NBS1 foci immunofluorescence, cell survival assay after IR, UBC9 knockout","journal":"Signal transduction and targeted therapy","confidence":"High","confidence_rationale":"Tier 2 / Strong — identification of writer (PIAS2α) and eraser (SENP2), mutagenesis of modified residues, multiple functional readouts, mechanistically defined","pmids":["32576812"],"is_preprint":false}],"current_model":"hSSB1 (NABP2/OBFC2B) is a single OB-domain ssDNA-binding protein that acts upstream of the MRN complex and ATM at DNA double-strand breaks: ATM phosphorylates hSSB1 at T117 to stabilize it (counteracting FBXL5-mediated ubiquitin–proteasome degradation), while SUMOylation at K79/K94 further stabilizes it and promotes NBS1 recruitment; hSSB1 directly binds NBS1 and stimulates MRN endonuclease activity via its C-terminal tail, facilitating DSB resection and RAD51/BRCA1 loading for homologous recombination; it also functions at stalled replication forks (requiring DNA-PK-mediated S134 phosphorylation) and in base excision repair of 8-oxoguanine by forming a complex with hOGG1, with its oligomeric state (mediated by C81/C99) specifically required for BER but not for DSB repair; hSSB1 resides in the SOSS1 heterotrimeric complex with INTS3 and C9orf80, interacts with BLM helicase to stabilize it, associates with telomeres and TERT to maintain G-overhangs, and modulates p53 and p21 stability and activity to regulate checkpoint responses."},"narrative":{"mechanistic_narrative":"NABP2 (hSSB1/OBFC2B) is a single-stranded DNA-binding protein that acts as an apical regulator of the genome stability response, operating upstream of the MRN complex and ATM at DNA double-strand breaks (DSBs) [PMID:18449195, PMID:21051358]. It is rapidly recruited to break sites independently of CtIP, MDC1, and MRN, and its loss abrogates ATM activation, MRN focus formation, and resection-dependent ssDNA/RPA generation, placing it at the head of the DSB signaling cascade [PMID:18449195, PMID:21051358]. Mechanistically, hSSB1 directly binds NBS1 and stimulates MRN endonuclease activity through its C-terminal tail to drive resection and subsequent RAD51/BRCA1 loading for homologous recombination [PMID:21227926, PMID:23986477]. Beyond DSBs, hSSB1 acts at hydroxyurea-stalled replication forks to enable ATR/Chk1 activation and fork restart [PMID:24753408], and participates in base excision repair by complexing with hOGG1 and binding 8-oxoguanine-containing DNA, a BER role that uniquely requires its redox-dependent oligomerization via C81/C99 whereas HR does not [PMID:26261212, PMID:27273218, PMID:28609781]. hSSB1 stability is heavily regulated by post-translational modification: ATM phosphorylation at T117 counteracts FBXL5-SCF-mediated ubiquitin-proteasome degradation [PMID:18449195, PMID:25249620], DNA-PK phosphorylates S134 to promote survival after fork disruption [PMID:28448822], and PIAS2α-catalyzed SUMOylation at K79/K94 stabilizes the protein and promotes NBS1 recruitment [PMID:32576812]. It functions as a stable subunit of the SOSS1 complex with INTS3, C9orf80, and INTS6, which is required for RAD51/BRCA1 recruitment and HR [PMID:19605351, PMID:19786574, PMID:23986477]. hSSB1 also stabilizes BLM helicase [PMID:28506294], binds G-strand telomeric DNA and recruits TERT to maintain G-overhangs [PMID:25589350], and modulates checkpoint responses by protecting p53 and p21 from degradation and promoting p300-mediated p53 acetylation [PMID:21242961, PMID:23184057].","teleology":[{"year":2008,"claim":"Established hSSB1 as an essential, ATM-regulated component of the DSB response, resolving whether a human single-OB-domain SSB participates in damage signaling.","evidence":"Cell-based phosphorylation assays, siRNA depletion, DSB foci, and ATM kinase readouts after ionizing radiation","pmids":["18449195"],"confidence":"High","gaps":["Did not define the upstream recruitment determinants","Did not resolve how T117 phosphorylation stabilizes the protein"]},{"year":2009,"claim":"Defined hSSB1 as a subunit of a stable heterotrimeric complex (with INTS3 and C9orf80/MISE), explaining how its stability and HR function are scaffolded.","evidence":"Co-IP, tandem affinity purification with mass spectrometry, HR reporter and ATM signaling assays","pmids":["19605351","19786574"],"confidence":"High","gaps":["Stoichiometry and structural architecture of the complex unresolved","Whether INTS3 acts only transcriptionally vs. post-translationally was ambiguous"]},{"year":2010,"claim":"Placed hSSB1 epistatically upstream of MRN, showing it is recruited independently of CtIP/MDC1/MRN yet is required for MRN recruitment and resection.","evidence":"Sequential siRNA depletion, laser microirradiation, BrdU ssDNA and RPA foci assays","pmids":["21051358"],"confidence":"High","gaps":["The direct molecular trigger for hSSB1 recruitment to breaks not identified"]},{"year":2011,"claim":"Provided the biochemical mechanism for the hSSB1-MRN axis: direct NBS1 binding and C-terminal-tail-dependent stimulation of MRN endonuclease activity.","evidence":"In vitro reconstitution with purified proteins, pulldowns, C-terminal deletion and NBS1 disease mutants","pmids":["21227926"],"confidence":"High","gaps":["Single lab reconstitution","Structural basis of NBS1 contact not solved"]},{"year":2011,"claim":"Linked hSSB1 to checkpoint control by showing it stabilizes p21 against degradation, partly explaining checkpoint defects on depletion.","evidence":"Co-IP, siRNA knockdown, ubiquitination and cell cycle analysis","pmids":["21242961"],"confidence":"Medium","gaps":["Single lab","Direct vs. indirect protection of p21 not fully distinguished"]},{"year":2012,"claim":"Extended hSSB1's checkpoint role to the p53 axis, protecting p53 and promoting p300-mediated K382 acetylation to activate p21 transcription.","evidence":"Co-IP, ubiquitination/acetylation assays, luciferase reporter, cell cycle analysis","pmids":["23184057"],"confidence":"Medium","gaps":["Single lab","Whether p53/p300 effects are separable from direct repair functions unclear"]},{"year":2014,"claim":"Demonstrated a distinct role at stalled replication forks, where hSSB1 enables ATR/Chk1 activation and Mre11/Rad51 recruitment for fork restart.","evidence":"siRNA depletion, HU/CPT treatment, DNA fiber assay, checkpoint kinase and foci assays","pmids":["24753408"],"confidence":"High","gaps":["Molecular distinction between DSB and fork functions not defined here"]},{"year":2014,"claim":"Identified the degradation machinery (FBXL5-SCF) controlling hSSB1 abundance and the protective role of T117 phosphorylation, defining its stability switch.","evidence":"Co-IP, ubiquitination assays, proteasome inhibition, ATM kinase assay","pmids":["25249620"],"confidence":"High","gaps":["How phosphorylation sterically/biochemically blocks FBXL5 binding not resolved"]},{"year":2015,"claim":"Established a base excision repair function via complex formation with hOGG1 and direct binding to 8-oxoguanine DNA, broadening hSSB1 beyond DSB/fork repair.","evidence":"Co-IP, chromatin fractionation, EMSA and in vitro OGG1 activity assays","pmids":["26261212"],"confidence":"High","gaps":["Single lab","In vivo contribution of BER role to oxidative damage survival not quantified"]},{"year":2015,"claim":"Revealed a telomeric function: G-strand DNA binding and TERT recruitment required to maintain G-overhangs.","evidence":"EMSA with telomeric oligos, Co-IP with TERT, telomere ChIP and G-overhang assays","pmids":["25589350"],"confidence":"Medium","gaps":["Single lab","Relationship between telomeric and DSB roles unclear"]},{"year":2015,"claim":"Showed an RPA-independent ATR activation route, with hSSB1/2-INTS3 recruiting ATR-ATRIP via TopBP1 and the 9-1-1 complex under genomic stress.","evidence":"Sequential siRNA depletion, ATRIP/ATR foci, Co-IP, Chk1 phosphorylation","pmids":["25916848"],"confidence":"Medium","gaps":["Single lab","Physiological conditions favoring this alternate route not defined"]},{"year":2015,"claim":"Mapped the C-terminal tail as essential for DNA binding and protein stability and characterized the monomeric solution state with a disordered C-terminus.","evidence":"EMSA, ITC, CD spectroscopy, sucrose gradient, C-terminal deletion mutants","pmids":["26550690"],"confidence":"Medium","gaps":["Single lab","Functional state in cells vs. in vitro not reconciled"]},{"year":2016,"claim":"Separated repair pathway requirements at the level of oligomeric state: redox-driven oligomerization is needed for BER but dispensable for HR.","evidence":"Size exclusion chromatography, EMSA with oxidized DNA, siRNA rescue with oligomerization mutants, 8-oxoG and HR assays","pmids":["27273218"],"confidence":"Medium","gaps":["Single lab","In vivo trigger of oxidative oligomerization not established"]},{"year":2016,"claim":"Defined the structural ssDNA recognition mode in solution via base-stacking of four OB-domain aromatic residues, distinct from the crystallographic SOSS1 mode.","evidence":"Solution-state NMR chemical shift mapping, ITC, fluorescence and DNA binding assays","pmids":["27387285"],"confidence":"High","gaps":["Single lab","Functional significance of the solution/crystal discrepancy not resolved"]},{"year":2017,"claim":"Confirmed an SSB-like tetramer stabilized by C81/C99 that retains INTS3 binding, integrating oligomerization with SOSS1 assembly; further defined the core complex including INTS6.","evidence":"Solution NMR, SEC, C81/C99 mutagenesis, INTS3/INTS6 Co-IP and in vitro reconstitution, RAD51/BRCA1 foci and HR assays","pmids":["28609781","23986477"],"confidence":"High","gaps":["Single lab structural model","Cellular conditions selecting tetramer vs. monomer not defined"]},{"year":2017,"claim":"Added a second regulatory kinase, DNA-PK, phosphorylating S134 to promote survival after fork disruption, with PPP-family phosphatases setting the baseline.","evidence":"In vitro DNA-PK kinase assay, S134A mutagenesis, phosphatase depletion, survival assays","pmids":["28448822"],"confidence":"High","gaps":["Single lab","Downstream effector of S134 phosphorylation not identified"]},{"year":2017,"claim":"Identified hSSB1 as a stabilizer of BLM helicase, required for BLM chromatin recruitment after DSBs and fork stalling.","evidence":"Co-IP, siRNA depletion, proteasome inhibition, chromatin fractionation, BLM foci","pmids":["28506294"],"confidence":"Medium","gaps":["Single lab","Whether stabilization is via direct binding or indirect not fully resolved"]},{"year":2020,"claim":"Completed the post-translational regulatory map by defining SUMOylation at K79/K94 (writer PIAS2α, eraser SENP2) that stabilizes hSSB1 and promotes NBS1 recruitment.","evidence":"In vivo SUMOylation assays, K79R/K94R mutagenesis, PIAS2α/SENP2 Co-IP, NBS1 foci, IR survival, UBC9 knockout","pmids":["32576812"],"confidence":"High","gaps":["Crosstalk among T117 phosphorylation, S134 phosphorylation, and SUMOylation not integrated","Whether SUMOylation acts on chromatin-bound vs. soluble pools unknown"]},{"year":null,"claim":"How hSSB1 is initially targeted to DSBs and stalled forks, and how its many modifications and oligomeric states are coordinated across HR, BER, replication, and telomere maintenance, remains unresolved.","evidence":"","pmids":[],"confidence":"High","gaps":["Direct recruitment mechanism to damage sites not identified","Integrated model linking phospho/SUMO regulation to pathway choice missing","In vivo significance of telomeric and p53/p21 roles relative to core HR function not quantified"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0003677","term_label":"DNA binding","supporting_discovery_ids":[0,10,11,13,17]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[4,10]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[3,4]}],"localization":[{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[0]},{"term_id":"GO:0000228","term_label":"nuclear chromosome","supporting_discovery_ids":[3,10]}],"pathway":[{"term_id":"R-HSA-73894","term_label":"DNA Repair","supporting_discovery_ids":[0,3,4,10]},{"term_id":"R-HSA-69306","term_label":"DNA Replication","supporting_discovery_ids":[8]},{"term_id":"R-HSA-1640170","term_label":"Cell Cycle","supporting_discovery_ids":[5,6]},{"term_id":"R-HSA-8953897","term_label":"Cellular responses to stimuli","supporting_discovery_ids":[0,8,18]}],"complexes":["SOSS1 complex (INTS3-hSSB1-C9orf80/INTS6)"],"partners":["INTS3","C9ORF80","INTS6","NBS1","HOGG1","TERT","BLM","FBXL5"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q9BQ15","full_name":"SOSS complex subunit B1","aliases":["Nucleic acid-binding protein 2","Oligonucleotide/oligosaccharide-binding fold-containing protein 2B","Sensor of single-strand DNA complex subunit B1","Sensor of ssDNA subunit B1","SOSS-B1","Single-stranded DNA-binding protein 1","hSSB1"],"length_aa":211,"mass_kda":22.3,"function":"Component of the SOSS complex, a multiprotein complex that functions downstream of the MRN complex to promote DNA repair and G2/M checkpoint (PubMed:25249620). In the SOSS complex, acts as a sensor of single-stranded DNA that binds to single-stranded DNA, in particular to polypyrimidines. The SOSS complex associates with DNA lesions and influences diverse endpoints in the cellular DNA damage response including cell-cycle checkpoint activation, recombinational repair and maintenance of genomic stability. Required for efficient homologous recombination-dependent repair of double-strand breaks (DSBs) and ATM-dependent signaling pathways","subcellular_location":"Nucleus","url":"https://www.uniprot.org/uniprotkb/Q9BQ15/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/NABP2","classification":"Not Classified","n_dependent_lines":60,"n_total_lines":1208,"dependency_fraction":0.04966887417218543},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"HIST2H2BE","stoichiometry":0.2},{"gene":"POLR2B","stoichiometry":0.2},{"gene":"SSRP1","stoichiometry":0.2},{"gene":"SUPT5H","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/NABP2","total_profiled":1310},"omim":[{"mim_id":"612104","title":"NUCLEIC ACID-BINDING PROTEIN 2; NABP2","url":"https://www.omim.org/entry/612104"},{"mim_id":"117650","title":"CEREBROCOSTOMANDIBULAR SYNDROME; CCMS","url":"https://www.omim.org/entry/117650"}],"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/NABP2"},"hgnc":{"alias_symbol":["MGC2731","SSB1","hSSB1","SOSS-B1"],"prev_symbol":["OBFC2B"]},"alphafold":{"accession":"Q9BQ15","domains":[{"cath_id":"2.40.50.140","chopping":"6-96","consensus_level":"high","plddt":97.613,"start":6,"end":96}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BQ15","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BQ15-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BQ15-F1-predicted_aligned_error_v6.png","plddt_mean":76.81},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=NABP2","jax_strain_url":"https://www.jax.org/strain/search?query=NABP2"},"sequence":{"accession":"Q9BQ15","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9BQ15.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9BQ15/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BQ15"}},"corpus_meta":[{"pmid":"18449195","id":"PMC_18449195","title":"Single-stranded DNA-binding protein hSSB1 is critical for genomic stability.","date":"2008","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/18449195","citation_count":199,"is_preprint":false},{"pmid":"2823109","id":"PMC_2823109","title":"Saccharomyces cerevisiae SSB1 protein and its relationship to nucleolar RNA-binding proteins.","date":"1987","source":"Molecular and cellular biology","url":"https://pubmed.ncbi.nlm.nih.gov/2823109","citation_count":111,"is_preprint":false},{"pmid":"19605351","id":"PMC_19605351","title":"HSSB1 and hSSB2 form similar multiprotein complexes that participate in DNA damage response.","date":"2009","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/19605351","citation_count":95,"is_preprint":false},{"pmid":"6384214","id":"PMC_6384214","title":"Characterization of the structural and functional defect in the Escherichia coli single-stranded DNA binding protein encoded by the ssb-1 mutant gene. 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\"discoveries\": [\n    {\n      \"year\": 2008,\n      \"finding\": \"hSSB1 (NABP2) is phosphorylated by ATM kinase at threonine 117 in response to DNA double-strand breaks; this phosphorylation is required for DNA damage-induced stabilization of hSSB1. hSSB1 accumulates in the nucleus and forms distinct foci at DSB sites. Depletion of hSSB1 abrogates ATM activation and phosphorylation of ATM targets after ionizing radiation, establishing hSSB1 as required for ATM-mediated DSB signaling.\",\n      \"method\": \"Cell-based phosphorylation assays, siRNA depletion, nuclear foci formation (immunofluorescence), ionizing radiation treatment, ATM kinase assay\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal functional assays with siRNA depletion, multiple orthogonal readouts (foci, ATM activation, checkpoint, repair), replicated by subsequent studies\",\n      \"pmids\": [\"18449195\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"hSSB1 (NABP2) forms a stable complex with INTS3 and hSSBIP1 (C9ORF80), distinct from the hSSB2 complex. INTS3 depletion decreases the stability of hSSB1 and hSSBIP1, suggesting INTS3 provides a scaffold for proper complex assembly. Both complexes are required for efficient HR-dependent DSB repair and ATM-dependent signaling.\",\n      \"method\": \"Immunoprecipitation, siRNA depletion, cell survival assays, HR reporter assay, ATM phosphorylation assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal Co-IP, multiple orthogonal functional assays, independently confirmed by subsequent studies\",\n      \"pmids\": [\"19605351\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"hSSB1 was shown by tandem affinity purification to copurify with a subset of Integrator complex subunits and MISE (c9orf80/LOC58493). The INTS3-MISE-hSSB1 complex controls ATM activation and RAD51 recruitment to DNA damage foci. INTS3 regulates hSSB1 transcription, thereby controlling hSSB1 function.\",\n      \"method\": \"Tandem affinity purification of hSSB1 phosphomimetic and phospho-null mutants, mass spectrometry, siRNA depletion, immunofluorescence for RAD51 foci\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — tandem affinity purification with mass spectrometry, multiple orthogonal functional assays, consistent with independent replication\",\n      \"pmids\": [\"19786574\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"hSSB1 is rapidly recruited to DSB sites in all interphase cell cycle phases independently of CtIP, MDC1, and the MRN complex. However, hSSB1 depletion prevents MRN complex foci formation and recruitment to DSBs, and leads to defective DSB resection (loss of RPA foci and ssDNA generation), placing hSSB1 upstream of MRN in the DSB response pathway.\",\n      \"method\": \"siRNA depletion, laser microirradiation, live-cell and fixed immunofluorescence, BrdU ssDNA detection, RPA foci assay\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — epistasis established by sequential depletion experiments with multiple functional readouts, independently corroborated\",\n      \"pmids\": [\"21051358\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"hSSB1 directly binds NBS1 (a component of the MRN complex) in a DNA damage-independent manner, and greatly stimulates MRN endonuclease activity in vitro via its C-terminal tail. NBS1 mutations associated with Nijmegen breakage syndrome show weaker binding to hSSB1.\",\n      \"method\": \"Direct binding assays (pulldown), in vitro endonuclease assay with purified proteins, C-terminal deletion mutants, NBS1 disease-associated mutants\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro reconstitution of direct binding and stimulation of enzymatic activity, mutagenesis of both proteins, single lab\",\n      \"pmids\": [\"21227926\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"hSSB1 directly binds p21 protein and this interaction prevents p21 from ubiquitin-mediated degradation. hSSB1 knockdown-induced abrogation of G1/S and G2/M checkpoints is partially dependent on p21.\",\n      \"method\": \"Co-immunoprecipitation, siRNA knockdown, cell cycle analysis, ubiquitination assay\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — Co-IP binding plus functional epistasis, single lab, two complementary methods\",\n      \"pmids\": [\"21242961\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"hSSB1 interacts with p53 and protects it from ubiquitin-mediated degradation. hSSB1 also associates with the acetyltransferase p300 and is required for efficient p53 acetylation at lysine 382, promoting transcriptional activation of p21. hSSB1 knockdown-induced G2/M checkpoint abrogation is partially dependent on p53 or p300.\",\n      \"method\": \"Co-immunoprecipitation, ubiquitination assays, acetylation assays, siRNA knockdown, luciferase reporter assay for p21 transcription, cell cycle analysis\",\n      \"journal\": \"Cell research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal Co-IP, acetylation/ubiquitination functional assays, epistasis; single lab\",\n      \"pmids\": [\"23184057\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"The core hSSB1 complex contains INTS6 in addition to INTS3 and C9orf80. INTS6 directly interacts with INTS3 and forms a stable trimeric complex with INTS3 and hSSB1 both in vitro and in vivo. This complex regulates accumulation of RAD51 and BRCA1 at DNA damage sites and homologous recombination.\",\n      \"method\": \"Protein affinity purification, Co-immunoprecipitation, in vitro complex reconstitution, siRNA depletion, immunofluorescence for RAD51/BRCA1 foci, HR assay\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vitro and in vivo complex reconstitution, functional HR assay; single lab\",\n      \"pmids\": [\"23986477\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"hSSB1 (NABP2) is required for ATR and Chk1 activation and recruitment of Mre11 and Rad51 at hydroxyurea-damaged replication forks. hSSB1-depleted cells fail to repair and restart stalled replication forks, accumulate DNA strand breaks and chromosome aberrations.\",\n      \"method\": \"siRNA depletion, hydroxyurea/camptothecin treatment, immunofluorescence for replication fork markers, Chk1/ATR phosphorylation assays, DNA fiber assay, chromosome analysis\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (fiber assay, foci, checkpoint kinase), loss-of-function with specific mechanistic readouts, consistent with prior studies\",\n      \"pmids\": [\"24753408\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"FBXL5-containing SCF E3 ubiquitin ligase targets hSSB1 for ubiquitination and proteasomal degradation. ATM-mediated phosphorylation of hSSB1 at T117 prevents FBXL5-mediated degradation. Fbxl5 interacts directly with hSSB1.\",\n      \"method\": \"Co-immunoprecipitation, ubiquitination assays, proteasome inhibitor experiments, ATM kinase assay, overexpression/depletion studies\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — direct interaction demonstrated, ubiquitination reconstituted, mechanistic link to ATM phosphorylation at defined residue, multiple methods\",\n      \"pmids\": [\"25249620\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"hSSB1 (NABP2) forms a complex with hOGG1 (human 8-oxoguanine glycosylase 1) and is required for hOGG1 localization to damaged chromatin. In vitro, hSSB1 directly binds DNA containing 8-oxoguanines and enhances hOGG1 enzymatic activity, establishing hSSB1 as a participant in base excision repair of oxidative DNA lesions.\",\n      \"method\": \"Co-immunoprecipitation, siRNA depletion, chromatin fractionation, in vitro DNA binding assays (EMSA), in vitro OGG1 activity assay with purified proteins\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro reconstitution of direct binding and enzymatic stimulation, complemented by cellular localization assays; single lab but multiple orthogonal methods\",\n      \"pmids\": [\"26261212\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"SSB1 (NABP2) binds specifically to G-strand telomeric DNA in vitro and associates with telomeres in vivo. SSB1 interacts with the TERT catalytic subunit and regulates its recruitment to telomeres. SSB1 deletion reduces TERT interaction with telomeres and leads to G-overhang loss.\",\n      \"method\": \"In vitro DNA binding assay (EMSA with telomeric oligonucleotides), Co-immunoprecipitation (SSB1-TERT), ChIP at telomeres, SSB1 deletion/depletion experiments, telomere G-overhang assay\",\n      \"journal\": \"Cancer research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct in vitro binding, Co-IP of TERT, ChIP at telomeres, functional G-overhang loss; single lab\",\n      \"pmids\": [\"25589350\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"hSSB1 forms stable oligomers under oxidizing conditions, and this oligomerization is required for its function in base excision repair (8-oxoguanine removal) but is not required for DSB repair by homologous recombination. Monomeric hSSB1 shows decreased affinity for oxidized DNA.\",\n      \"method\": \"Size exclusion chromatography, EMSA with oxidized DNA, siRNA rescue experiments with oligomerization-deficient mutants, 8-oxoG repair assay, HR assay\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional separation using oligomerization mutants with multiple repair pathway readouts; single lab\",\n      \"pmids\": [\"27273218\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"ssDNA recognition by hSSB1 in solution is mediated by base-stacking of four key aromatic residues within the OB domain. This DNA binding mode in solution differs from that observed in the crystal structure of the SOSS1 complex.\",\n      \"method\": \"Solution-state NMR (chemical shift perturbation mapping), biophysical assays (ITC, fluorescence), functional DNA binding assays, comparison with crystal structure\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — NMR structural analysis with mutagenesis and biophysical validation, multiple orthogonal methods; single lab\",\n      \"pmids\": [\"27387285\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"hSSB1 forms a tetramer structurally similar to E. coli SSB, stabilized by cysteines C81 and C99 and charged/hydrophobic residues. The tetramer can still interact with INTS3, showing oligomerization does not preclude SOSS1 complex formation.\",\n      \"method\": \"Solution-state NMR, size exclusion chromatography, mutagenesis of C81/C99, Co-immunoprecipitation with INTS3, biophysical experiments\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — NMR-based structural model with mutagenesis and biophysical validation; single lab but multiple orthogonal methods\",\n      \"pmids\": [\"28609781\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"hSSB1 is directly phosphorylated by DNA-PK at serine residue 134. This modification is suppressed in undamaged cells by PPP-family protein phosphatases but is enhanced following replication fork disruption, promoting cellular survival.\",\n      \"method\": \"In vitro kinase assay with purified DNA-PK, phospho-specific antibodies, siRNA depletion of phosphatases, site-directed mutagenesis (S134A), cell survival assays after HU treatment\",\n      \"journal\": \"DNA repair\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro kinase reconstitution with mutagenesis at defined residue, functional phosphatase epistasis; single lab\",\n      \"pmids\": [\"28448822\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"hSSB1 forms a complex with BLM helicase in cells, and this interaction is altered in response to ionizing radiation. hSSB1 depletion leads to proteasome-mediated degradation of BLM and defective recruitment of BLM to chromatin following IR-induced DSBs and HU-induced stalled replication forks.\",\n      \"method\": \"Co-immunoprecipitation, siRNA depletion, proteasome inhibitor treatment, chromatin fractionation, immunofluorescence for BLM foci\",\n      \"journal\": \"BMC molecular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP, protein stability assay, chromatin recruitment assay; single lab, multiple methods\",\n      \"pmids\": [\"28506294\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"The C-terminal tail of hSSB1 (NABP2) is essential for DNA binding ability and protein stability. Deletion of the C-terminus diminishes DNA binding. Both hNABP1 and hNABP2 exist as monomers in solution under the tested conditions, and hNABP2 has a disordered C-terminus.\",\n      \"method\": \"EMSA, ITC, circular dichroism spectroscopy, sucrose gradient centrifugation, C-terminal deletion mutants\",\n      \"journal\": \"Biochimica et biophysica acta\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro biochemical reconstitution with deletion mutants and multiple biophysical methods; single lab\",\n      \"pmids\": [\"26550690\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"When RPA is depleted, hSSB1/hSSB2 and their partner INTS3 form sub-nuclear foci, associate with the ATR-ATRIP complex, and recruit it to sites of genomic stress. This alternate ATR activation via hSSB1/2-INTS3 requires TopBP1 and the Rad9-Rad1-Hus1 complex.\",\n      \"method\": \"siRNA depletion (RPA, hSSB1/2, INTS3), immunofluorescence for ATRIP/ATR foci, Co-immunoprecipitation, Chk1 phosphorylation assay\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis by sequential depletion, Co-IP, functional checkpoint readout; single lab\",\n      \"pmids\": [\"25916848\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"hSSB1 undergoes SUMOylation at K79 and K94, enhanced by DNA damage, catalyzed by PIAS2α and reversed by SENP2. SUMOylation stabilizes hSSB1 protein and enhances recruitment of NBS1 to DNA damage sites. Cells with SUMOylation-defective hSSB1 are sensitive to ionizing radiation.\",\n      \"method\": \"In vivo SUMOylation assay, site-directed mutagenesis (K79R, K94R), Co-immunoprecipitation with PIAS2α/SENP2, NBS1 foci immunofluorescence, cell survival assay after IR, UBC9 knockout\",\n      \"journal\": \"Signal transduction and targeted therapy\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — identification of writer (PIAS2α) and eraser (SENP2), mutagenesis of modified residues, multiple functional readouts, mechanistically defined\",\n      \"pmids\": [\"32576812\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"hSSB1 (NABP2/OBFC2B) is a single OB-domain ssDNA-binding protein that acts upstream of the MRN complex and ATM at DNA double-strand breaks: ATM phosphorylates hSSB1 at T117 to stabilize it (counteracting FBXL5-mediated ubiquitin–proteasome degradation), while SUMOylation at K79/K94 further stabilizes it and promotes NBS1 recruitment; hSSB1 directly binds NBS1 and stimulates MRN endonuclease activity via its C-terminal tail, facilitating DSB resection and RAD51/BRCA1 loading for homologous recombination; it also functions at stalled replication forks (requiring DNA-PK-mediated S134 phosphorylation) and in base excision repair of 8-oxoguanine by forming a complex with hOGG1, with its oligomeric state (mediated by C81/C99) specifically required for BER but not for DSB repair; hSSB1 resides in the SOSS1 heterotrimeric complex with INTS3 and C9orf80, interacts with BLM helicase to stabilize it, associates with telomeres and TERT to maintain G-overhangs, and modulates p53 and p21 stability and activity to regulate checkpoint responses.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"NABP2 (hSSB1/OBFC2B) is a single-stranded DNA-binding protein that acts as an apical regulator of the genome stability response, operating upstream of the MRN complex and ATM at DNA double-strand breaks (DSBs) [#0, #3]. It is rapidly recruited to break sites independently of CtIP, MDC1, and MRN, and its loss abrogates ATM activation, MRN focus formation, and resection-dependent ssDNA/RPA generation, placing it at the head of the DSB signaling cascade [#0, #3]. Mechanistically, hSSB1 directly binds NBS1 and stimulates MRN endonuclease activity through its C-terminal tail to drive resection and subsequent RAD51/BRCA1 loading for homologous recombination [#4, #7]. Beyond DSBs, hSSB1 acts at hydroxyurea-stalled replication forks to enable ATR/Chk1 activation and fork restart [#8], and participates in base excision repair by complexing with hOGG1 and binding 8-oxoguanine-containing DNA, a BER role that uniquely requires its redox-dependent oligomerization via C81/C99 whereas HR does not [#10, #12, #14]. hSSB1 stability is heavily regulated by post-translational modification: ATM phosphorylation at T117 counteracts FBXL5-SCF-mediated ubiquitin-proteasome degradation [#0, #9], DNA-PK phosphorylates S134 to promote survival after fork disruption [#15], and PIAS2\\u03b1-catalyzed SUMOylation at K79/K94 stabilizes the protein and promotes NBS1 recruitment [#19]. It functions as a stable subunit of the SOSS1 complex with INTS3, C9orf80, and INTS6, which is required for RAD51/BRCA1 recruitment and HR [#1, #2, #7]. hSSB1 also stabilizes BLM helicase [#16], binds G-strand telomeric DNA and recruits TERT to maintain G-overhangs [#11], and modulates checkpoint responses by protecting p53 and p21 from degradation and promoting p300-mediated p53 acetylation [#5, #6].\",\n  \"teleology\": [\n    {\n      \"year\": 2008,\n      \"claim\": \"Established hSSB1 as an essential, ATM-regulated component of the DSB response, resolving whether a human single-OB-domain SSB participates in damage signaling.\",\n      \"evidence\": \"Cell-based phosphorylation assays, siRNA depletion, DSB foci, and ATM kinase readouts after ionizing radiation\",\n      \"pmids\": [\"18449195\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not define the upstream recruitment determinants\", \"Did not resolve how T117 phosphorylation stabilizes the protein\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Defined hSSB1 as a subunit of a stable heterotrimeric complex (with INTS3 and C9orf80/MISE), explaining how its stability and HR function are scaffolded.\",\n      \"evidence\": \"Co-IP, tandem affinity purification with mass spectrometry, HR reporter and ATM signaling assays\",\n      \"pmids\": [\"19605351\", \"19786574\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Stoichiometry and structural architecture of the complex unresolved\", \"Whether INTS3 acts only transcriptionally vs. post-translationally was ambiguous\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Placed hSSB1 epistatically upstream of MRN, showing it is recruited independently of CtIP/MDC1/MRN yet is required for MRN recruitment and resection.\",\n      \"evidence\": \"Sequential siRNA depletion, laser microirradiation, BrdU ssDNA and RPA foci assays\",\n      \"pmids\": [\"21051358\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"The direct molecular trigger for hSSB1 recruitment to breaks not identified\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Provided the biochemical mechanism for the hSSB1-MRN axis: direct NBS1 binding and C-terminal-tail-dependent stimulation of MRN endonuclease activity.\",\n      \"evidence\": \"In vitro reconstitution with purified proteins, pulldowns, C-terminal deletion and NBS1 disease mutants\",\n      \"pmids\": [\"21227926\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Single lab reconstitution\", \"Structural basis of NBS1 contact not solved\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Linked hSSB1 to checkpoint control by showing it stabilizes p21 against degradation, partly explaining checkpoint defects on depletion.\",\n      \"evidence\": \"Co-IP, siRNA knockdown, ubiquitination and cell cycle analysis\",\n      \"pmids\": [\"21242961\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab\", \"Direct vs. indirect protection of p21 not fully distinguished\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Extended hSSB1's checkpoint role to the p53 axis, protecting p53 and promoting p300-mediated K382 acetylation to activate p21 transcription.\",\n      \"evidence\": \"Co-IP, ubiquitination/acetylation assays, luciferase reporter, cell cycle analysis\",\n      \"pmids\": [\"23184057\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab\", \"Whether p53/p300 effects are separable from direct repair functions unclear\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Demonstrated a distinct role at stalled replication forks, where hSSB1 enables ATR/Chk1 activation and Mre11/Rad51 recruitment for fork restart.\",\n      \"evidence\": \"siRNA depletion, HU/CPT treatment, DNA fiber assay, checkpoint kinase and foci assays\",\n      \"pmids\": [\"24753408\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular distinction between DSB and fork functions not defined here\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Identified the degradation machinery (FBXL5-SCF) controlling hSSB1 abundance and the protective role of T117 phosphorylation, defining its stability switch.\",\n      \"evidence\": \"Co-IP, ubiquitination assays, proteasome inhibition, ATM kinase assay\",\n      \"pmids\": [\"25249620\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How phosphorylation sterically/biochemically blocks FBXL5 binding not resolved\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Established a base excision repair function via complex formation with hOGG1 and direct binding to 8-oxoguanine DNA, broadening hSSB1 beyond DSB/fork repair.\",\n      \"evidence\": \"Co-IP, chromatin fractionation, EMSA and in vitro OGG1 activity assays\",\n      \"pmids\": [\"26261212\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Single lab\", \"In vivo contribution of BER role to oxidative damage survival not quantified\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Revealed a telomeric function: G-strand DNA binding and TERT recruitment required to maintain G-overhangs.\",\n      \"evidence\": \"EMSA with telomeric oligos, Co-IP with TERT, telomere ChIP and G-overhang assays\",\n      \"pmids\": [\"25589350\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab\", \"Relationship between telomeric and DSB roles unclear\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Showed an RPA-independent ATR activation route, with hSSB1/2-INTS3 recruiting ATR-ATRIP via TopBP1 and the 9-1-1 complex under genomic stress.\",\n      \"evidence\": \"Sequential siRNA depletion, ATRIP/ATR foci, Co-IP, Chk1 phosphorylation\",\n      \"pmids\": [\"25916848\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab\", \"Physiological conditions favoring this alternate route not defined\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Mapped the C-terminal tail as essential for DNA binding and protein stability and characterized the monomeric solution state with a disordered C-terminus.\",\n      \"evidence\": \"EMSA, ITC, CD spectroscopy, sucrose gradient, C-terminal deletion mutants\",\n      \"pmids\": [\"26550690\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab\", \"Functional state in cells vs. in vitro not reconciled\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Separated repair pathway requirements at the level of oligomeric state: redox-driven oligomerization is needed for BER but dispensable for HR.\",\n      \"evidence\": \"Size exclusion chromatography, EMSA with oxidized DNA, siRNA rescue with oligomerization mutants, 8-oxoG and HR assays\",\n      \"pmids\": [\"27273218\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab\", \"In vivo trigger of oxidative oligomerization not established\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Defined the structural ssDNA recognition mode in solution via base-stacking of four OB-domain aromatic residues, distinct from the crystallographic SOSS1 mode.\",\n      \"evidence\": \"Solution-state NMR chemical shift mapping, ITC, fluorescence and DNA binding assays\",\n      \"pmids\": [\"27387285\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Single lab\", \"Functional significance of the solution/crystal discrepancy not resolved\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Confirmed an SSB-like tetramer stabilized by C81/C99 that retains INTS3 binding, integrating oligomerization with SOSS1 assembly; further defined the core complex including INTS6.\",\n      \"evidence\": \"Solution NMR, SEC, C81/C99 mutagenesis, INTS3/INTS6 Co-IP and in vitro reconstitution, RAD51/BRCA1 foci and HR assays\",\n      \"pmids\": [\"28609781\", \"23986477\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Single lab structural model\", \"Cellular conditions selecting tetramer vs. monomer not defined\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Added a second regulatory kinase, DNA-PK, phosphorylating S134 to promote survival after fork disruption, with PPP-family phosphatases setting the baseline.\",\n      \"evidence\": \"In vitro DNA-PK kinase assay, S134A mutagenesis, phosphatase depletion, survival assays\",\n      \"pmids\": [\"28448822\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Single lab\", \"Downstream effector of S134 phosphorylation not identified\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Identified hSSB1 as a stabilizer of BLM helicase, required for BLM chromatin recruitment after DSBs and fork stalling.\",\n      \"evidence\": \"Co-IP, siRNA depletion, proteasome inhibition, chromatin fractionation, BLM foci\",\n      \"pmids\": [\"28506294\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab\", \"Whether stabilization is via direct binding or indirect not fully resolved\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Completed the post-translational regulatory map by defining SUMOylation at K79/K94 (writer PIAS2\\u03b1, eraser SENP2) that stabilizes hSSB1 and promotes NBS1 recruitment.\",\n      \"evidence\": \"In vivo SUMOylation assays, K79R/K94R mutagenesis, PIAS2\\u03b1/SENP2 Co-IP, NBS1 foci, IR survival, UBC9 knockout\",\n      \"pmids\": [\"32576812\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Crosstalk among T117 phosphorylation, S134 phosphorylation, and SUMOylation not integrated\", \"Whether SUMOylation acts on chromatin-bound vs. soluble pools unknown\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How hSSB1 is initially targeted to DSBs and stalled forks, and how its many modifications and oligomeric states are coordinated across HR, BER, replication, and telomere maintenance, remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct recruitment mechanism to damage sites not identified\", \"Integrated model linking phospho/SUMO regulation to pathway choice missing\", \"In vivo significance of telomeric and p53/p21 roles relative to core HR function not quantified\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0003677\", \"supporting_discovery_ids\": [0, 10, 11, 13, 17]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [4, 10]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [3, 4]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [0]},\n      {\"term_id\": \"GO:0000228\", \"supporting_discovery_ids\": [3, 10]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-73894\", \"supporting_discovery_ids\": [0, 3, 4, 10]},\n      {\"term_id\": \"R-HSA-69306\", \"supporting_discovery_ids\": [8]},\n      {\"term_id\": \"R-HSA-1640170\", \"supporting_discovery_ids\": [5, 6]},\n      {\"term_id\": \"R-HSA-8953897\", \"supporting_discovery_ids\": [0, 8, 18]}\n    ],\n    \"complexes\": [\n      \"SOSS1 complex (INTS3-hSSB1-C9orf80/INTS6)\"\n    ],\n    \"partners\": [\n      \"INTS3\",\n      \"C9orf80\",\n      \"INTS6\",\n      \"NBS1\",\n      \"hOGG1\",\n      \"TERT\",\n      \"BLM\",\n      \"FBXL5\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":5,"faith_total":5,"faith_pct":100.0}}