{"gene":"UBN1","run_date":"2026-06-10T10:51:56","timeline":{"discoveries":[{"year":2000,"finding":"UBN1 (ubinuclein) is a nuclear protein that interacts with the basic domains of EBV transcription factor EB1 and cellular c-Jun, competing with their binding to the AP1 consensus site. The NH2 terminus is essential for nuclear localization, while the central domain mediates interaction with EB1.","method":"Yeast two-hybrid, co-immunoprecipitation, overexpression of tagged constructs in keratinocytes","journal":"The Journal of cell biology","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — single lab, multiple methods (Y2H, Co-IP, overexpression), but no mutagenesis reconstitution","pmids":["10725330"],"is_preprint":false},{"year":2008,"finding":"UBN1 is the human ortholog of yeast Hpc2p. Its Hpc2-related domain (HRD) directly interacts with the N-terminal WD repeats of HIRA. UBN1 binds to proliferation-promoting genes repressed during senescence, associates with H3K9 histone methyltransferase activity, and is indispensable for formation of senescence-associated heterochromatin foci (SAHF).","method":"Bioinformatic ortholog identification, pulldown assays, co-immunoprecipitation, ChIP, RNAi knockdown with SAHF phenotype readout","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (direct binding assay, ChIP, functional KD), replicated across subsequent studies","pmids":["19029251"],"is_preprint":false},{"year":2011,"finding":"UBN1 is a core subunit of the quaternary HIRA/UBN1/CABIN1/ASF1a (HUCA) complex. HIRA acts as a scaffold to bring together UBN1, ASF1a, and CABIN1. The intact HUCA complex preferentially deposits histone variant H3.3 into chromatin in a DNA replication-independent manner and contributes to heterochromatinization in senescent cells.","method":"Co-immunoprecipitation at ectopic and endogenous levels, reconstitution of quaternary complex from recombinant proteins, mutational analysis","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — reconstitution from recombinant proteins, mutational analysis, endogenous Co-IP, replicated across multiple studies","pmids":["21807893"],"is_preprint":false},{"year":2012,"finding":"A region within residues 41–77 of UBN1, termed the NHRD (N-terminal Hpc2-related domain), is essential for interaction with the HIRA WD repeat domain, forming a tight 1:1 complex with nanomolar affinity. The previously identified HRD (residues 120–175) is dispensable for this interaction. Key residues in the NHRD are required for HUCA complex stability in vitro and in vivo and for chromatin organization in primary human cells.","method":"Biochemical/mutational analysis, analytical ultracentrifugation, in vitro binding assays, in vivo functional complementation","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — reconstitution with quantitative biophysical measurement (AUC), mutagenesis, in vivo validation","pmids":["22401310"],"is_preprint":false},{"year":2012,"finding":"UBN1 interacts with tight junction protein ZO-1 via UBN1's N-terminal region (residues 39–223) and ZO-1's PDZ2 domain. UBN1 also interacts with cingulin, LYRIC, and RACK-1, suggesting a scaffold function influencing protein subcellular localization at cell–cell contacts.","method":"Fluorescent protein-complementation assay (PCA), in vitro pull-down, co-immunoprecipitation, confocal microscopy, mass spectrometry","journal":"Experimental cell research","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — multiple orthogonal methods (PCA, pulldown, Co-IP, MS) from a single lab","pmids":["22245583"],"is_preprint":false},{"year":2013,"finding":"UBN1 colocalizes with HIRA, ASF1a, and histone H3.3 predominantly at active promoters and active/poised enhancers genome-wide. HIRA is required for deposition of H3.3 at these binding sites. Physical interactions between the HIRA complex (including UBN1) and transcription factors, a chromatin insulator, and an ATP-dependent chromatin-remodeling complex were identified.","method":"ChIP-seq, gene expression analysis, co-immunoprecipitation","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 2 / Strong — genome-wide ChIP-seq with functional validation by HIRA depletion, reciprocal Co-IP, replicated across labs","pmids":["23602572"],"is_preprint":false},{"year":2016,"finding":"OGT (O-GlcNAc transferase) interacts with UBN1 and O-GlcNAcylates HIRA at S231, promoting formation of the HIRA–H3.3 complex and H3.3 nucleosome assembly. Depletion of OGT or expression of HIRA S231A mutant compromises H3.3 nucleosome assembly and delays premature cellular senescence.","method":"Co-immunoprecipitation, O-GlcNAcylation assay, site-directed mutagenesis (HIRA S231A), nucleosome assembly assay, RNAi knockdown with senescence phenotype readout","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro assembly assay, mutagenesis of modification site, Co-IP demonstrating UBN1-OGT interaction, functional cellular readout; single lab","pmids":["27217568"],"is_preprint":false},{"year":2018,"finding":"The UBN1 HRD specifically binds to H3.3/H4 over H3.1/H4, providing selectivity for the histone variant. The UBN1 middle domain has dimer formation activity and binds H3/H4 without discriminating between H3.1 and H3.3. A DNA-binding domain located between the HRD and middle domain binds DNA through electrostatic contacts involving conserved lysine residues. These activities suggest UBN1 associates with DNA and dimerizes to mediate (H3.3/H4)2 heterotetramer formation prior to chromatin deposition.","method":"In vitro binding assays, mutagenesis of conserved lysine residues, biochemical characterization of domains","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro reconstitution with domain-specific mutagenesis, multiple biochemical assays; single lab with rigorous controls","pmids":["31040182"],"is_preprint":false},{"year":2018,"finding":"Ala87 and Gly90 residues of H3.3 are required and sufficient for specific recognition and direct binding by UBN1. The HIRA subunit enhances UBN1 binding affinity toward H3.3, whereas CABIN1 does not. UBN1 (and UBN2) FID/AAA histone chaperone activity mutations cause defects in H3.3 deposition at promoters of developmental genes and impair neural differentiation of mouse ES cells.","method":"In vitro binding assays, site-directed mutagenesis of H3.3 residues, ChIP-seq, FID/AAA mutant functional analysis in mES cell differentiation","journal":"BMC biology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — direct in vitro binding with mutagenesis of H3.3 specificity determinants, ChIP-seq, functional differentiation assay; single lab with multiple orthogonal methods","pmids":["30285846"],"is_preprint":false},{"year":2018,"finding":"HIRA forms a stable homotrimer that binds two subunits of CABIN1 in vitro. A HIRA mutant defective in homotrimer formation interacts less efficiently with CABIN1, is not enriched at DNA damage sites upon UV irradiation, and cannot rescue new H3.3 deposition in HIRA knockout cells. UBN1 is a subunit of this trimeric complex.","method":"Biochemical analysis, X-ray crystallography, mutagenesis of HIRA trimerization interface, functional rescue assay in HIRA KO cells, UV damage localization","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure, biochemical reconstitution, mutagenesis, functional rescue in KO cells; multiple orthogonal methods in one study","pmids":["30082790"],"is_preprint":false},{"year":2010,"finding":"UBN1 overexpression represses the EBV productive cycle in epithelial cells, whereas UBN1 knockdown increases virus production. UBN1 blocks EB1-DNA interaction as shown by ChIP. In non-dividing differentiated epithelial cells, UBN1 is relocalized/sequestered to tight junctions, allowing EB1 to activate the productive cycle in the nucleus.","method":"Overexpression, shRNA knockdown, ChIP, confocal microscopy for subcellular localization","journal":"Journal of virology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional KD/OE with defined molecular mechanism (ChIP blocking EB1-DNA), localization studies; single lab","pmids":["21084479"],"is_preprint":false},{"year":2020,"finding":"De novo H3.3 deposition during transcription is totally dependent on HIRA trimerization and on UBN1, while ASF1 interaction with HIRA can be bypassed for this pathway. In contrast, recycling of old H3.3 requires HIRA but proceeds independently of UBN1 or HIRA trimerization and shows absolute dependency on ASF1-HIRA interaction. Thus UBN1 is specifically required for the de novo deposition pathway but not the recycling pathway.","method":"SNAP-tag pulse-chase to distinguish new and old histones, HIRA/UBN1 mutant analysis, fluorescence imaging in human cells","journal":"Nature structural & molecular biology","confidence":"High","confidence_rationale":"Tier 2 / Moderate — SNAP-tag system with genetic separation of two pathways, mutant constructs, single lab but multiple orthogonal approaches","pmids":["32895554"],"is_preprint":false},{"year":2020,"finding":"In C. elegans, PQN-80 (the UBN1 ortholog) loss of function causes late-onset pleiotropic defects similar to hira-1 mutants including mitochondrial stress, and these defects can be maternally rescued by maternally derived complex components.","method":"Genetic loss-of-function screen, phenotypic analysis, maternal rescue experiments","journal":"Current biology : CB","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic epistasis in C. elegans with defined phenotypic readout and maternal rescue; C. elegans ortholog study","pmids":["32470364"],"is_preprint":false},{"year":2022,"finding":"Loss of Ubn1 function in mouse oocytes (as part of the Hira/Cabin1/Ubn1 complex) causes early embryogenesis failure. Transcriptome analyses show aberrant transcriptional silencing in mutant oocytes, with reduced H3K4me3 and H3K9me3 marks and impaired chromatin accessibility.","method":"Genetic knockout, transcriptome analysis, nascent RNA analysis, histone mark ChIP, chromatin accessibility assay (ATAC)","journal":"Development (Cambridge, England)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic KO with multiple epigenomic readouts, single lab","pmids":["35112132"],"is_preprint":false},{"year":2023,"finding":"UBN1 directly interacts with AID (activation-induced cytidine deaminase) in B cells. Co-immunoprecipitation, pull-down, co-localization by double immunofluorescence, and proximity ligation assay all confirm this interaction.","method":"Co-immunoprecipitation, pull-down, double immunofluorescence, proximity ligation assay, molecular docking","journal":"Scientific reports","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — multiple orthogonal methods confirming interaction (Co-IP, pulldown, PLA, IF), single lab; functional consequence of interaction not established","pmids":["37949972"],"is_preprint":false},{"year":2024,"finding":"The HIRA-UBN1/UBN2 complex deposits new H3.3 at telomeres to prevent accumulation of nucleosome-free ssDNA, TERRA R-loop buildup, and transcription-replication conflicts in ATRX-DAXX-deficient ALT cancer cells. HIRA-mediated H3.3 incorporation links productive ALT to phosphorylation of H3.3 serine 31 by Chk1.","method":"ATAC-seq, CUT&RUN, HIRA/UBN1/UBN2 depletion with defined molecular phenotypes, R-loop detection","journal":"Cell reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple genomic assays with loss-of-function, single lab, functional phenotype linked to H3.3 deposition","pmids":["39509271"],"is_preprint":false},{"year":2024,"finding":"ASF1a delivers H3.3/H4 dimers to the HIRA complex; H3.3/H4 tetramerization drives association of two HIRA/UBN1 complexes; and affinity of histones for DNA drives release of ASF1a and subsequent histone deposition. Biochemical and biophysical data support this stepwise mechanism for HIRA-complex-mediated H3.3/H4 deposition.","method":"Biochemical reconstitution, biophysical binding assays","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — in vitro reconstitution and biophysical measurements, single lab, no mutagenesis or structural validation reported","pmids":["39059488"],"is_preprint":false},{"year":2025,"finding":"Ubn1 single mutant mice are viable and fertile, while combined Ubn1/Ubn2 double mutants show complete embryonic lethality. Mechanistically, Ubn1 mutation in mouse ES cells impairs initiation of X chromosome inactivation: Xist-mediated establishment of H3K27me3 over X-linked genes is impaired, while Xist-dependent deacetylation of H3K27 remains largely unaffected.","method":"Genetic knockout (single and double mutant), mouse phenotypic analysis, ChIP for H3K27me3/ac in ES cells during Xist induction","journal":"PLoS genetics","confidence":"High","confidence_rationale":"Tier 2 / Moderate — genetic KO with clear in vivo phenotypes, ChIP distinguishing two mechanistically distinct steps of X inactivation, single lab with multiple orthogonal readouts","pmids":["40455860"],"is_preprint":false},{"year":2025,"finding":"Sp100A isoform is required for localization of the HIRA complex (including UBN1 and ASF1a) to PML nuclear bodies in human keratinocytes. In Sp100 knockout cells, HIRA/UBN1/ASF1a fail to localize to PML-NBs even after interferon stimulation; exogenous Sp100A rescues this, with the SUMO interacting motif (SIM) playing an important role.","method":"CRISPR-Cas9 Sp100 knockout, immunofluorescence localization, exogenous isoform expression rescue, interferon stimulation","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — CRISPR KO with defined localization phenotype and isoform-specific rescue, single lab; preprint not yet peer-reviewed","pmids":["40568077"],"is_preprint":true}],"current_model":"UBN1 (ubinuclein-1) is a core subunit of the HIRA/UBN1/CABIN1/ASF1a (HUCA) histone chaperone complex that specifically recognizes histone H3.3 (via Ala87/Gly90 of H3.3 and the UBN1 HRD), binds DNA and dimerizes through its middle domain to facilitate (H3.3/H4)2 heterotetramer formation, and mediates de novo replication-independent H3.3 deposition at active promoters, enhancers, and telomeres; UBN1 also anchors the complex to HIRA through its NHRD (residues 41–77), participates in SAHF formation and cellular senescence downstream of OGT-mediated O-GlcNAcylation of HIRA-S231, and is required for Xist-dependent H3K27 trimethylation during X chromosome inactivation initiation."},"narrative":{"mechanistic_narrative":"UBN1 (ubinuclein-1) is a core subunit of the HIRA/UBN1/CABIN1/ASF1a (HUCA) histone chaperone complex that drives replication-independent deposition of the histone variant H3.3 into chromatin [PMID:19029251, PMID:21807893]. Within the complex, UBN1 is anchored to HIRA through its N-terminal Hpc2-related domain (NHRD, residues 41–77), which forms a tight 1:1 nanomolar complex with the HIRA WD-repeat domain and is required for HUCA stability and chromatin organization, while the previously assigned HRD is dispensable for this contact [PMID:22401310]. UBN1 confers the complex's substrate specificity: its HRD selectively binds H3.3/H4 over H3.1/H4 by reading out the H3.3-specific residues Ala87 and Gly90, while its middle domain mediates dimerization and a basic DNA-binding region engages DNA through conserved lysines, together enabling formation of (H3.3/H4)2 heterotetramers prior to deposition [PMID:31040182, PMID:30285846, PMID:39059488]. Genome-wide, UBN1 colocalizes with HIRA, ASF1a and H3.3 at active promoters and enhancers, and is specifically required for the de novo H3.3 deposition pathway during transcription, distinct from the ASF1-dependent recycling of old H3.3 that proceeds independently of UBN1 [PMID:23602572, PMID:32895554]. UBN1 function supports diverse chromatin-dependent processes including senescence-associated heterochromatin foci formation downstream of OGT-mediated O-GlcNAcylation of HIRA-S231 [PMID:19029251, PMID:27217568], H3.3 deposition at telomeres in ALT cancer cells [PMID:39509271], proper transcriptional and epigenetic programming in oocytes and early embryos [PMID:35112132], and Xist-dependent establishment of H3K27 trimethylation during X chromosome inactivation [PMID:40455860]. UBN1 was originally identified as a nuclear protein competing with EBV EB1 and c-Jun for AP1 sites and sequestered to tight junctions in differentiated epithelial cells, where it modulates the EBV productive cycle [PMID:10725330, PMID:21084479].","teleology":[{"year":2000,"claim":"Established UBN1 as a nuclear protein that competes with viral and cellular bZIP transcription factors for AP1 DNA sites, defining its earliest functional context before its chromatin role was known.","evidence":"Yeast two-hybrid, Co-IP and tagged overexpression in keratinocytes mapping EB1/c-Jun interaction and nuclear localization domains","pmids":["10725330"],"confidence":"Medium","gaps":["No connection yet to histone chaperone activity","Functional relevance of AP1 competition to gene regulation not resolved"]},{"year":2008,"claim":"Identified UBN1 as the human ortholog of yeast Hpc2p and a direct HIRA partner, placing it within the HIRA histone chaperone machinery and linking it to senescence-associated heterochromatin.","evidence":"Ortholog identification, pulldown/Co-IP mapping the HRD–HIRA WD-repeat interaction, ChIP, and RNAi with SAHF readout","pmids":["19029251"],"confidence":"High","gaps":["Stoichiometry and full subunit composition of the complex unresolved","Direct histone-binding activity of UBN1 not yet demonstrated"]},{"year":2011,"claim":"Defined the quaternary HUCA complex (HIRA/UBN1/CABIN1/ASF1a) and showed it preferentially deposits H3.3 in a replication-independent manner, establishing the molecular machine UBN1 belongs to.","evidence":"Endogenous and ectopic Co-IP plus reconstitution from recombinant proteins with mutational analysis","pmids":["21807893"],"confidence":"High","gaps":["Which subunit confers H3.3 specificity not yet assigned","Mechanism of histone handoff and deposition not defined"]},{"year":2012,"claim":"Mapped the precise HIRA-anchoring determinant of UBN1 to the NHRD (residues 41–77), correcting the earlier HRD assignment and quantifying the interaction as a tight 1:1 complex essential for HUCA integrity.","evidence":"Analytical ultracentrifugation, in vitro binding, mutagenesis and in vivo functional complementation in primary human cells","pmids":["22401310"],"confidence":"High","gaps":["Structural basis of the NHRD–HIRA interface not solved","Role of the HRD reassigned but not yet functionally explained"]},{"year":2012,"claim":"Described a non-chromatin scaffold role for UBN1 at cell–cell contacts via interaction with ZO-1 and other junctional proteins, broadening its functional repertoire.","evidence":"Fluorescent protein-complementation, pulldown, Co-IP, confocal microscopy and mass spectrometry","pmids":["22245583"],"confidence":"Medium","gaps":["Functional consequence of junctional localization not established","Relationship to nuclear chromatin function unclear"]},{"year":2013,"claim":"Localized UBN1/HIRA/ASF1a/H3.3 genome-wide to active promoters and enhancers, connecting the complex to active chromatin and gene regulation.","evidence":"ChIP-seq with HIRA-depletion functional validation and reciprocal Co-IP with transcription factors and remodelers","pmids":["23602572"],"confidence":"High","gaps":["Causal contribution of UBN1 alone (versus HIRA) to deposition at these sites not separated","Mechanism of targeting to specific loci undefined"]},{"year":2016,"claim":"Linked the complex to nutrient/metabolic signaling by showing OGT interacts with UBN1 and O-GlcNAcylates HIRA-S231 to promote H3.3 assembly and senescence.","evidence":"Co-IP, O-GlcNAcylation assay, HIRA S231A mutagenesis, in vitro nucleosome assembly, and RNAi senescence readout","pmids":["27217568"],"confidence":"High","gaps":["Whether UBN1 is itself O-GlcNAcylated not addressed","Single-lab; in vivo physiological relevance of the modification limited"]},{"year":2018,"claim":"Resolved how UBN1 reads H3.3 specificity, dimerizes and binds DNA, assigning the molecular activities that drive (H3.3/H4)2 tetramer formation.","evidence":"In vitro binding assays with H3.3 Ala87/Gly90 and UBN1 domain/lysine mutagenesis, plus ChIP-seq and mES differentiation assays (FID/AAA mutants)","pmids":["31040182","30285846"],"confidence":"High","gaps":["High-resolution structure of UBN1 bound to H3.3/H4 not solved","Coordination of DNA binding and histone binding in vivo not directly visualized"]},{"year":2018,"claim":"Established that HIRA homotrimerization organizes the complex and is required for new H3.3 deposition and DNA damage recruitment, defining the architectural scaffold around UBN1.","evidence":"X-ray crystallography, biochemical reconstitution, trimerization-interface mutagenesis and functional rescue in HIRA KO cells","pmids":["30082790"],"confidence":"High","gaps":["Stoichiometry of UBN1 within the trimeric assembly not fully resolved","Direct role of UBN1 in DNA-damage-site recruitment not isolated"]},{"year":2020,"claim":"Genetically separated the de novo and recycling H3.3 pathways, showing UBN1 is specifically required for transcription-coupled de novo deposition but dispensable for ASF1-dependent recycling.","evidence":"SNAP-tag pulse-chase distinguishing new and old histones with HIRA/UBN1 mutant analysis in human cells","pmids":["32895554"],"confidence":"High","gaps":["How UBN1 selects de novo over recycling substrate not mechanistically defined","Locus-specific consequences of losing only the de novo pathway not mapped"]},{"year":2020,"claim":"Demonstrated conservation of UBN1 function in vivo, with C. elegans ortholog loss phenocopying hira-1 mutants including mitochondrial stress and maternal rescuability.","evidence":"Genetic loss-of-function and maternal rescue in C. elegans (PQN-80)","pmids":["32470364"],"confidence":"Medium","gaps":["Molecular basis of mitochondrial stress phenotype unresolved","Direct link to H3.3 deposition in worm not established"]},{"year":2022,"claim":"Showed UBN1 is required in mouse oocytes for proper transcriptional and epigenetic programming, with loss causing embryonic failure and altered histone marks and chromatin accessibility.","evidence":"Genetic knockout with transcriptome, nascent RNA, histone-mark ChIP and ATAC-seq","pmids":["35112132"],"confidence":"Medium","gaps":["Whether phenotypes are direct H3.3 deposition defects or indirect not separated","Specific developmental genes driving lethality not pinpointed"]},{"year":2024,"claim":"Extended UBN1-dependent H3.3 deposition to telomere protection in ALT cancer cells, preventing R-loop accumulation and transcription-replication conflicts.","evidence":"ATAC-seq, CUT&RUN, HIRA/UBN1/UBN2 depletion and R-loop detection in ATRX-DAXX-deficient cells","pmids":["39509271"],"confidence":"Medium","gaps":["Specific contribution of UBN1 versus UBN2 at telomeres not separated","Therapeutic relevance not tested"]},{"year":2024,"claim":"Proposed a stepwise biophysical mechanism in which ASF1a delivers H3.3/H4, tetramerization couples two HIRA/UBN1 complexes, and DNA affinity drives ASF1a release and deposition.","evidence":"Biochemical reconstitution and biophysical binding assays","pmids":["39059488"],"confidence":"Medium","gaps":["No mutagenesis or structural validation of the proposed intermediates","Single-lab in vitro model not confirmed in cells"]},{"year":2025,"claim":"Defined a specific developmental requirement for UBN1 in X chromosome inactivation, where it is needed for Xist-dependent H3K27me3 establishment but not H3K27 deacetylation, with redundancy revealed by Ubn1/Ubn2 double-mutant lethality.","evidence":"Single and double knockout mice plus ChIP for H3K27me3/ac in ES cells during Xist induction","pmids":["40455860"],"confidence":"High","gaps":["Mechanistic link between H3.3 deposition and PRC2-mediated H3K27me3 not defined","Extent of UBN1/UBN2 redundancy across other tissues unclear"]},{"year":2025,"claim":"Identified Sp100A as required for recruiting the HIRA/UBN1/ASF1a complex to PML nuclear bodies, implicating a SUMO-dependent localization mechanism.","evidence":"CRISPR Sp100 knockout, immunofluorescence localization, isoform rescue and interferon stimulation (preprint)","pmids":["40568077"],"confidence":"Medium","gaps":["Preprint not yet peer-reviewed","Functional consequence of PML-NB localization for H3.3 deposition not established"]},{"year":null,"claim":"How UBN1 mechanistically couples H3.3 deposition to downstream chromatin states (e.g., PRC2-dependent H3K27me3 during XCI, telomere protection) and how it is targeted to distinct genomic locations remain unresolved.","evidence":"","pmids":[],"confidence":"High","gaps":["No high-resolution structure of UBN1 within the assembled complex on histones/DNA","Locus-targeting specificity determinants of UBN1 undefined","Causal link from H3.3 deposition to specific histone-mark outcomes not established"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0042393","term_label":"histone binding","supporting_discovery_ids":[7,8,16]},{"term_id":"GO:0003677","term_label":"DNA binding","supporting_discovery_ids":[7]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[2,3]}],"localization":[{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[0,1]},{"term_id":"GO:0000228","term_label":"nuclear chromosome","supporting_discovery_ids":[5,8,15]},{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[4,10]}],"pathway":[{"term_id":"R-HSA-4839726","term_label":"Chromatin organization","supporting_discovery_ids":[2,5,11]},{"term_id":"R-HSA-74160","term_label":"Gene expression (Transcription)","supporting_discovery_ids":[5,8]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[8,13,17]}],"complexes":["HUCA (HIRA/UBN1/CABIN1/ASF1a) histone chaperone complex"],"partners":["HIRA","ASF1A","CABIN1","OGT","ZO-1","SP100A","AID","EB1"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q9NPG3","full_name":"Ubinuclein-1","aliases":["HIRA-binding protein","Protein VT4","Ubiquitously expressed nuclear protein"],"length_aa":1134,"mass_kda":121.5,"function":"Acts as a novel regulator of senescence. Involved in the formation of senescence-associated heterochromatin foci (SAHF), which represses expression of proliferation-promoting genes. Binds to proliferation-promoting genes. May be required for replication-independent chromatin assembly","subcellular_location":"Nucleus, nucleoplasm; Nucleus, PML body; Cell junction, tight junction","url":"https://www.uniprot.org/uniprotkb/Q9NPG3/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/UBN1","classification":"Not Classified","n_dependent_lines":20,"n_total_lines":1208,"dependency_fraction":0.016556291390728478},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/UBN1","total_profiled":1310},"omim":[{"mim_id":"617314","title":"SH3 DOMAIN- AND SYLF DOMAIN-CONTAINING PROTEIN 1; SH3YL1","url":"https://www.omim.org/entry/617314"},{"mim_id":"613841","title":"UBINUCLEIN 2; UBN2","url":"https://www.omim.org/entry/613841"},{"mim_id":"609771","title":"UBINUCLEIN 1; UBN1","url":"https://www.omim.org/entry/609771"},{"mim_id":"313700","title":"ANDROGEN RECEPTOR; AR","url":"https://www.omim.org/entry/313700"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Nucleoplasm","reliability":"Supported"},{"location":"Nuclear bodies","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/UBN1"},"hgnc":{"alias_symbol":[],"prev_symbol":[]},"alphafold":{"accession":"Q9NPG3","domains":[{"cath_id":"-","chopping":"98-128_148-153","consensus_level":"medium","plddt":77.8305,"start":98,"end":153},{"cath_id":"-","chopping":"358-421","consensus_level":"medium","plddt":87.2028,"start":358,"end":421},{"cath_id":"-","chopping":"441-490_511-585","consensus_level":"medium","plddt":85.9747,"start":441,"end":585}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9NPG3","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9NPG3-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9NPG3-F1-predicted_aligned_error_v6.png","plddt_mean":54.59},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=UBN1","jax_strain_url":"https://www.jax.org/strain/search?query=UBN1"},"sequence":{"accession":"Q9NPG3","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9NPG3.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9NPG3/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9NPG3"}},"corpus_meta":[{"pmid":"15830322","id":"PMC_15830322","title":"Candidate-gene screening and association analysis at the autism-susceptibility locus on chromosome 16p: evidence of association at GRIN2A and ABAT.","date":"2005","source":"American journal of human genetics","url":"https://pubmed.ncbi.nlm.nih.gov/15830322","citation_count":141,"is_preprint":false},{"pmid":"23602572","id":"PMC_23602572","title":"Placing the HIRA histone chaperone complex in the chromatin landscape.","date":"2013","source":"Cell reports","url":"https://pubmed.ncbi.nlm.nih.gov/23602572","citation_count":119,"is_preprint":false},{"pmid":"19029251","id":"PMC_19029251","title":"Human UBN1 is an ortholog of yeast Hpc2p and has an essential role in the HIRA/ASF1a chromatin-remodeling pathway in senescent cells.","date":"2008","source":"Molecular and cellular biology","url":"https://pubmed.ncbi.nlm.nih.gov/19029251","citation_count":104,"is_preprint":false},{"pmid":"21807893","id":"PMC_21807893","title":"Human CABIN1 is a functional member of the human HIRA/UBN1/ASF1a histone H3.3 chaperone 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EBV transcription factor EB1 and cellular c-Jun, competing with their binding to the AP1 consensus site. The NH2 terminus is essential for nuclear localization, while the central domain mediates interaction with EB1.\",\n      \"method\": \"Yeast two-hybrid, co-immunoprecipitation, overexpression of tagged constructs in keratinocytes\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — single lab, multiple methods (Y2H, Co-IP, overexpression), but no mutagenesis reconstitution\",\n      \"pmids\": [\"10725330\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"UBN1 is the human ortholog of yeast Hpc2p. Its Hpc2-related domain (HRD) directly interacts with the N-terminal WD repeats of HIRA. UBN1 binds to proliferation-promoting genes repressed during senescence, associates with H3K9 histone methyltransferase activity, and is indispensable for formation of senescence-associated heterochromatin foci (SAHF).\",\n      \"method\": \"Bioinformatic ortholog identification, pulldown assays, co-immunoprecipitation, ChIP, RNAi knockdown with SAHF phenotype readout\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (direct binding assay, ChIP, functional KD), replicated across subsequent studies\",\n      \"pmids\": [\"19029251\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"UBN1 is a core subunit of the quaternary HIRA/UBN1/CABIN1/ASF1a (HUCA) complex. HIRA acts as a scaffold to bring together UBN1, ASF1a, and CABIN1. The intact HUCA complex preferentially deposits histone variant H3.3 into chromatin in a DNA replication-independent manner and contributes to heterochromatinization in senescent cells.\",\n      \"method\": \"Co-immunoprecipitation at ectopic and endogenous levels, reconstitution of quaternary complex from recombinant proteins, mutational analysis\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — reconstitution from recombinant proteins, mutational analysis, endogenous Co-IP, replicated across multiple studies\",\n      \"pmids\": [\"21807893\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"A region within residues 41–77 of UBN1, termed the NHRD (N-terminal Hpc2-related domain), is essential for interaction with the HIRA WD repeat domain, forming a tight 1:1 complex with nanomolar affinity. The previously identified HRD (residues 120–175) is dispensable for this interaction. Key residues in the NHRD are required for HUCA complex stability in vitro and in vivo and for chromatin organization in primary human cells.\",\n      \"method\": \"Biochemical/mutational analysis, analytical ultracentrifugation, in vitro binding assays, in vivo functional complementation\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — reconstitution with quantitative biophysical measurement (AUC), mutagenesis, in vivo validation\",\n      \"pmids\": [\"22401310\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"UBN1 interacts with tight junction protein ZO-1 via UBN1's N-terminal region (residues 39–223) and ZO-1's PDZ2 domain. UBN1 also interacts with cingulin, LYRIC, and RACK-1, suggesting a scaffold function influencing protein subcellular localization at cell–cell contacts.\",\n      \"method\": \"Fluorescent protein-complementation assay (PCA), in vitro pull-down, co-immunoprecipitation, confocal microscopy, mass spectrometry\",\n      \"journal\": \"Experimental cell research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — multiple orthogonal methods (PCA, pulldown, Co-IP, MS) from a single lab\",\n      \"pmids\": [\"22245583\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"UBN1 colocalizes with HIRA, ASF1a, and histone H3.3 predominantly at active promoters and active/poised enhancers genome-wide. HIRA is required for deposition of H3.3 at these binding sites. Physical interactions between the HIRA complex (including UBN1) and transcription factors, a chromatin insulator, and an ATP-dependent chromatin-remodeling complex were identified.\",\n      \"method\": \"ChIP-seq, gene expression analysis, co-immunoprecipitation\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genome-wide ChIP-seq with functional validation by HIRA depletion, reciprocal Co-IP, replicated across labs\",\n      \"pmids\": [\"23602572\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"OGT (O-GlcNAc transferase) interacts with UBN1 and O-GlcNAcylates HIRA at S231, promoting formation of the HIRA–H3.3 complex and H3.3 nucleosome assembly. Depletion of OGT or expression of HIRA S231A mutant compromises H3.3 nucleosome assembly and delays premature cellular senescence.\",\n      \"method\": \"Co-immunoprecipitation, O-GlcNAcylation assay, site-directed mutagenesis (HIRA S231A), nucleosome assembly assay, RNAi knockdown with senescence phenotype readout\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro assembly assay, mutagenesis of modification site, Co-IP demonstrating UBN1-OGT interaction, functional cellular readout; single lab\",\n      \"pmids\": [\"27217568\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"The UBN1 HRD specifically binds to H3.3/H4 over H3.1/H4, providing selectivity for the histone variant. The UBN1 middle domain has dimer formation activity and binds H3/H4 without discriminating between H3.1 and H3.3. A DNA-binding domain located between the HRD and middle domain binds DNA through electrostatic contacts involving conserved lysine residues. These activities suggest UBN1 associates with DNA and dimerizes to mediate (H3.3/H4)2 heterotetramer formation prior to chromatin deposition.\",\n      \"method\": \"In vitro binding assays, mutagenesis of conserved lysine residues, biochemical characterization of domains\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro reconstitution with domain-specific mutagenesis, multiple biochemical assays; single lab with rigorous controls\",\n      \"pmids\": [\"31040182\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Ala87 and Gly90 residues of H3.3 are required and sufficient for specific recognition and direct binding by UBN1. The HIRA subunit enhances UBN1 binding affinity toward H3.3, whereas CABIN1 does not. UBN1 (and UBN2) FID/AAA histone chaperone activity mutations cause defects in H3.3 deposition at promoters of developmental genes and impair neural differentiation of mouse ES cells.\",\n      \"method\": \"In vitro binding assays, site-directed mutagenesis of H3.3 residues, ChIP-seq, FID/AAA mutant functional analysis in mES cell differentiation\",\n      \"journal\": \"BMC biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — direct in vitro binding with mutagenesis of H3.3 specificity determinants, ChIP-seq, functional differentiation assay; single lab with multiple orthogonal methods\",\n      \"pmids\": [\"30285846\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"HIRA forms a stable homotrimer that binds two subunits of CABIN1 in vitro. A HIRA mutant defective in homotrimer formation interacts less efficiently with CABIN1, is not enriched at DNA damage sites upon UV irradiation, and cannot rescue new H3.3 deposition in HIRA knockout cells. UBN1 is a subunit of this trimeric complex.\",\n      \"method\": \"Biochemical analysis, X-ray crystallography, mutagenesis of HIRA trimerization interface, functional rescue assay in HIRA KO cells, UV damage localization\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure, biochemical reconstitution, mutagenesis, functional rescue in KO cells; multiple orthogonal methods in one study\",\n      \"pmids\": [\"30082790\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"UBN1 overexpression represses the EBV productive cycle in epithelial cells, whereas UBN1 knockdown increases virus production. UBN1 blocks EB1-DNA interaction as shown by ChIP. In non-dividing differentiated epithelial cells, UBN1 is relocalized/sequestered to tight junctions, allowing EB1 to activate the productive cycle in the nucleus.\",\n      \"method\": \"Overexpression, shRNA knockdown, ChIP, confocal microscopy for subcellular localization\",\n      \"journal\": \"Journal of virology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional KD/OE with defined molecular mechanism (ChIP blocking EB1-DNA), localization studies; single lab\",\n      \"pmids\": [\"21084479\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"De novo H3.3 deposition during transcription is totally dependent on HIRA trimerization and on UBN1, while ASF1 interaction with HIRA can be bypassed for this pathway. In contrast, recycling of old H3.3 requires HIRA but proceeds independently of UBN1 or HIRA trimerization and shows absolute dependency on ASF1-HIRA interaction. Thus UBN1 is specifically required for the de novo deposition pathway but not the recycling pathway.\",\n      \"method\": \"SNAP-tag pulse-chase to distinguish new and old histones, HIRA/UBN1 mutant analysis, fluorescence imaging in human cells\",\n      \"journal\": \"Nature structural & molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — SNAP-tag system with genetic separation of two pathways, mutant constructs, single lab but multiple orthogonal approaches\",\n      \"pmids\": [\"32895554\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"In C. elegans, PQN-80 (the UBN1 ortholog) loss of function causes late-onset pleiotropic defects similar to hira-1 mutants including mitochondrial stress, and these defects can be maternally rescued by maternally derived complex components.\",\n      \"method\": \"Genetic loss-of-function screen, phenotypic analysis, maternal rescue experiments\",\n      \"journal\": \"Current biology : CB\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis in C. elegans with defined phenotypic readout and maternal rescue; C. elegans ortholog study\",\n      \"pmids\": [\"32470364\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Loss of Ubn1 function in mouse oocytes (as part of the Hira/Cabin1/Ubn1 complex) causes early embryogenesis failure. Transcriptome analyses show aberrant transcriptional silencing in mutant oocytes, with reduced H3K4me3 and H3K9me3 marks and impaired chromatin accessibility.\",\n      \"method\": \"Genetic knockout, transcriptome analysis, nascent RNA analysis, histone mark ChIP, chromatin accessibility assay (ATAC)\",\n      \"journal\": \"Development (Cambridge, England)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic KO with multiple epigenomic readouts, single lab\",\n      \"pmids\": [\"35112132\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"UBN1 directly interacts with AID (activation-induced cytidine deaminase) in B cells. Co-immunoprecipitation, pull-down, co-localization by double immunofluorescence, and proximity ligation assay all confirm this interaction.\",\n      \"method\": \"Co-immunoprecipitation, pull-down, double immunofluorescence, proximity ligation assay, molecular docking\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — multiple orthogonal methods confirming interaction (Co-IP, pulldown, PLA, IF), single lab; functional consequence of interaction not established\",\n      \"pmids\": [\"37949972\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"The HIRA-UBN1/UBN2 complex deposits new H3.3 at telomeres to prevent accumulation of nucleosome-free ssDNA, TERRA R-loop buildup, and transcription-replication conflicts in ATRX-DAXX-deficient ALT cancer cells. HIRA-mediated H3.3 incorporation links productive ALT to phosphorylation of H3.3 serine 31 by Chk1.\",\n      \"method\": \"ATAC-seq, CUT&RUN, HIRA/UBN1/UBN2 depletion with defined molecular phenotypes, R-loop detection\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple genomic assays with loss-of-function, single lab, functional phenotype linked to H3.3 deposition\",\n      \"pmids\": [\"39509271\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"ASF1a delivers H3.3/H4 dimers to the HIRA complex; H3.3/H4 tetramerization drives association of two HIRA/UBN1 complexes; and affinity of histones for DNA drives release of ASF1a and subsequent histone deposition. Biochemical and biophysical data support this stepwise mechanism for HIRA-complex-mediated H3.3/H4 deposition.\",\n      \"method\": \"Biochemical reconstitution, biophysical binding assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — in vitro reconstitution and biophysical measurements, single lab, no mutagenesis or structural validation reported\",\n      \"pmids\": [\"39059488\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Ubn1 single mutant mice are viable and fertile, while combined Ubn1/Ubn2 double mutants show complete embryonic lethality. Mechanistically, Ubn1 mutation in mouse ES cells impairs initiation of X chromosome inactivation: Xist-mediated establishment of H3K27me3 over X-linked genes is impaired, while Xist-dependent deacetylation of H3K27 remains largely unaffected.\",\n      \"method\": \"Genetic knockout (single and double mutant), mouse phenotypic analysis, ChIP for H3K27me3/ac in ES cells during Xist induction\",\n      \"journal\": \"PLoS genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic KO with clear in vivo phenotypes, ChIP distinguishing two mechanistically distinct steps of X inactivation, single lab with multiple orthogonal readouts\",\n      \"pmids\": [\"40455860\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Sp100A isoform is required for localization of the HIRA complex (including UBN1 and ASF1a) to PML nuclear bodies in human keratinocytes. In Sp100 knockout cells, HIRA/UBN1/ASF1a fail to localize to PML-NBs even after interferon stimulation; exogenous Sp100A rescues this, with the SUMO interacting motif (SIM) playing an important role.\",\n      \"method\": \"CRISPR-Cas9 Sp100 knockout, immunofluorescence localization, exogenous isoform expression rescue, interferon stimulation\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — CRISPR KO with defined localization phenotype and isoform-specific rescue, single lab; preprint not yet peer-reviewed\",\n      \"pmids\": [\"40568077\"],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"UBN1 (ubinuclein-1) is a core subunit of the HIRA/UBN1/CABIN1/ASF1a (HUCA) histone chaperone complex that specifically recognizes histone H3.3 (via Ala87/Gly90 of H3.3 and the UBN1 HRD), binds DNA and dimerizes through its middle domain to facilitate (H3.3/H4)2 heterotetramer formation, and mediates de novo replication-independent H3.3 deposition at active promoters, enhancers, and telomeres; UBN1 also anchors the complex to HIRA through its NHRD (residues 41–77), participates in SAHF formation and cellular senescence downstream of OGT-mediated O-GlcNAcylation of HIRA-S231, and is required for Xist-dependent H3K27 trimethylation during X chromosome inactivation initiation.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"UBN1 (ubinuclein-1) is a core subunit of the HIRA/UBN1/CABIN1/ASF1a (HUCA) histone chaperone complex that drives replication-independent deposition of the histone variant H3.3 into chromatin [#1, #2]. Within the complex, UBN1 is anchored to HIRA through its N-terminal Hpc2-related domain (NHRD, residues 41\\u201377), which forms a tight 1:1 nanomolar complex with the HIRA WD-repeat domain and is required for HUCA stability and chromatin organization, while the previously assigned HRD is dispensable for this contact [#3]. UBN1 confers the complex's substrate specificity: its HRD selectively binds H3.3/H4 over H3.1/H4 by reading out the H3.3-specific residues Ala87 and Gly90, while its middle domain mediates dimerization and a basic DNA-binding region engages DNA through conserved lysines, together enabling formation of (H3.3/H4)2 heterotetramers prior to deposition [#7, #8, #16]. Genome-wide, UBN1 colocalizes with HIRA, ASF1a and H3.3 at active promoters and enhancers, and is specifically required for the de novo H3.3 deposition pathway during transcription, distinct from the ASF1-dependent recycling of old H3.3 that proceeds independently of UBN1 [#5, #11]. UBN1 function supports diverse chromatin-dependent processes including senescence-associated heterochromatin foci formation downstream of OGT-mediated O-GlcNAcylation of HIRA-S231 [#1, #6], H3.3 deposition at telomeres in ALT cancer cells [#15], proper transcriptional and epigenetic programming in oocytes and early embryos [#13], and Xist-dependent establishment of H3K27 trimethylation during X chromosome inactivation [#17]. UBN1 was originally identified as a nuclear protein competing with EBV EB1 and c-Jun for AP1 sites and sequestered to tight junctions in differentiated epithelial cells, where it modulates the EBV productive cycle [#0, #10].\",\n  \"teleology\": [\n    {\n      \"year\": 2000,\n      \"claim\": \"Established UBN1 as a nuclear protein that competes with viral and cellular bZIP transcription factors for AP1 DNA sites, defining its earliest functional context before its chromatin role was known.\",\n      \"evidence\": \"Yeast two-hybrid, Co-IP and tagged overexpression in keratinocytes mapping EB1/c-Jun interaction and nuclear localization domains\",\n      \"pmids\": [\"10725330\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"No connection yet to histone chaperone activity\", \"Functional relevance of AP1 competition to gene regulation not resolved\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Identified UBN1 as the human ortholog of yeast Hpc2p and a direct HIRA partner, placing it within the HIRA histone chaperone machinery and linking it to senescence-associated heterochromatin.\",\n      \"evidence\": \"Ortholog identification, pulldown/Co-IP mapping the HRD\\u2013HIRA WD-repeat interaction, ChIP, and RNAi with SAHF readout\",\n      \"pmids\": [\"19029251\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Stoichiometry and full subunit composition of the complex unresolved\", \"Direct histone-binding activity of UBN1 not yet demonstrated\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Defined the quaternary HUCA complex (HIRA/UBN1/CABIN1/ASF1a) and showed it preferentially deposits H3.3 in a replication-independent manner, establishing the molecular machine UBN1 belongs to.\",\n      \"evidence\": \"Endogenous and ectopic Co-IP plus reconstitution from recombinant proteins with mutational analysis\",\n      \"pmids\": [\"21807893\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Which subunit confers H3.3 specificity not yet assigned\", \"Mechanism of histone handoff and deposition not defined\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Mapped the precise HIRA-anchoring determinant of UBN1 to the NHRD (residues 41\\u201377), correcting the earlier HRD assignment and quantifying the interaction as a tight 1:1 complex essential for HUCA integrity.\",\n      \"evidence\": \"Analytical ultracentrifugation, in vitro binding, mutagenesis and in vivo functional complementation in primary human cells\",\n      \"pmids\": [\"22401310\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Structural basis of the NHRD\\u2013HIRA interface not solved\", \"Role of the HRD reassigned but not yet functionally explained\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Described a non-chromatin scaffold role for UBN1 at cell\\u2013cell contacts via interaction with ZO-1 and other junctional proteins, broadening its functional repertoire.\",\n      \"evidence\": \"Fluorescent protein-complementation, pulldown, Co-IP, confocal microscopy and mass spectrometry\",\n      \"pmids\": [\"22245583\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Functional consequence of junctional localization not established\", \"Relationship to nuclear chromatin function unclear\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Localized UBN1/HIRA/ASF1a/H3.3 genome-wide to active promoters and enhancers, connecting the complex to active chromatin and gene regulation.\",\n      \"evidence\": \"ChIP-seq with HIRA-depletion functional validation and reciprocal Co-IP with transcription factors and remodelers\",\n      \"pmids\": [\"23602572\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Causal contribution of UBN1 alone (versus HIRA) to deposition at these sites not separated\", \"Mechanism of targeting to specific loci undefined\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Linked the complex to nutrient/metabolic signaling by showing OGT interacts with UBN1 and O-GlcNAcylates HIRA-S231 to promote H3.3 assembly and senescence.\",\n      \"evidence\": \"Co-IP, O-GlcNAcylation assay, HIRA S231A mutagenesis, in vitro nucleosome assembly, and RNAi senescence readout\",\n      \"pmids\": [\"27217568\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Whether UBN1 is itself O-GlcNAcylated not addressed\", \"Single-lab; in vivo physiological relevance of the modification limited\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Resolved how UBN1 reads H3.3 specificity, dimerizes and binds DNA, assigning the molecular activities that drive (H3.3/H4)2 tetramer formation.\",\n      \"evidence\": \"In vitro binding assays with H3.3 Ala87/Gly90 and UBN1 domain/lysine mutagenesis, plus ChIP-seq and mES differentiation assays (FID/AAA mutants)\",\n      \"pmids\": [\"31040182\", \"30285846\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"High-resolution structure of UBN1 bound to H3.3/H4 not solved\", \"Coordination of DNA binding and histone binding in vivo not directly visualized\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Established that HIRA homotrimerization organizes the complex and is required for new H3.3 deposition and DNA damage recruitment, defining the architectural scaffold around UBN1.\",\n      \"evidence\": \"X-ray crystallography, biochemical reconstitution, trimerization-interface mutagenesis and functional rescue in HIRA KO cells\",\n      \"pmids\": [\"30082790\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Stoichiometry of UBN1 within the trimeric assembly not fully resolved\", \"Direct role of UBN1 in DNA-damage-site recruitment not isolated\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Genetically separated the de novo and recycling H3.3 pathways, showing UBN1 is specifically required for transcription-coupled de novo deposition but dispensable for ASF1-dependent recycling.\",\n      \"evidence\": \"SNAP-tag pulse-chase distinguishing new and old histones with HIRA/UBN1 mutant analysis in human cells\",\n      \"pmids\": [\"32895554\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"How UBN1 selects de novo over recycling substrate not mechanistically defined\", \"Locus-specific consequences of losing only the de novo pathway not mapped\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Demonstrated conservation of UBN1 function in vivo, with C. elegans ortholog loss phenocopying hira-1 mutants including mitochondrial stress and maternal rescuability.\",\n      \"evidence\": \"Genetic loss-of-function and maternal rescue in C. elegans (PQN-80)\",\n      \"pmids\": [\"32470364\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Molecular basis of mitochondrial stress phenotype unresolved\", \"Direct link to H3.3 deposition in worm not established\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Showed UBN1 is required in mouse oocytes for proper transcriptional and epigenetic programming, with loss causing embryonic failure and altered histone marks and chromatin accessibility.\",\n      \"evidence\": \"Genetic knockout with transcriptome, nascent RNA, histone-mark ChIP and ATAC-seq\",\n      \"pmids\": [\"35112132\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Whether phenotypes are direct H3.3 deposition defects or indirect not separated\", \"Specific developmental genes driving lethality not pinpointed\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Extended UBN1-dependent H3.3 deposition to telomere protection in ALT cancer cells, preventing R-loop accumulation and transcription-replication conflicts.\",\n      \"evidence\": \"ATAC-seq, CUT&RUN, HIRA/UBN1/UBN2 depletion and R-loop detection in ATRX-DAXX-deficient cells\",\n      \"pmids\": [\"39509271\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Specific contribution of UBN1 versus UBN2 at telomeres not separated\", \"Therapeutic relevance not tested\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Proposed a stepwise biophysical mechanism in which ASF1a delivers H3.3/H4, tetramerization couples two HIRA/UBN1 complexes, and DNA affinity drives ASF1a release and deposition.\",\n      \"evidence\": \"Biochemical reconstitution and biophysical binding assays\",\n      \"pmids\": [\"39059488\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"No mutagenesis or structural validation of the proposed intermediates\", \"Single-lab in vitro model not confirmed in cells\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Defined a specific developmental requirement for UBN1 in X chromosome inactivation, where it is needed for Xist-dependent H3K27me3 establishment but not H3K27 deacetylation, with redundancy revealed by Ubn1/Ubn2 double-mutant lethality.\",\n      \"evidence\": \"Single and double knockout mice plus ChIP for H3K27me3/ac in ES cells during Xist induction\",\n      \"pmids\": [\"40455860\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Mechanistic link between H3.3 deposition and PRC2-mediated H3K27me3 not defined\", \"Extent of UBN1/UBN2 redundancy across other tissues unclear\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Identified Sp100A as required for recruiting the HIRA/UBN1/ASF1a complex to PML nuclear bodies, implicating a SUMO-dependent localization mechanism.\",\n      \"evidence\": \"CRISPR Sp100 knockout, immunofluorescence localization, isoform rescue and interferon stimulation (preprint)\",\n      \"pmids\": [\"40568077\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Preprint not yet peer-reviewed\", \"Functional consequence of PML-NB localization for H3.3 deposition not established\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How UBN1 mechanistically couples H3.3 deposition to downstream chromatin states (e.g., PRC2-dependent H3K27me3 during XCI, telomere protection) and how it is targeted to distinct genomic locations remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"No high-resolution structure of UBN1 within the assembled complex on histones/DNA\", \"Locus-targeting specificity determinants of UBN1 undefined\", \"Causal link from H3.3 deposition to specific histone-mark outcomes not established\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0042393\", \"supporting_discovery_ids\": [7, 8, 16]},\n      {\"term_id\": \"GO:0003677\", \"supporting_discovery_ids\": [7]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [2, 3]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [0, 1]},\n      {\"term_id\": \"GO:0000228\", \"supporting_discovery_ids\": [5, 8, 15]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [4, 10]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-4839726\", \"supporting_discovery_ids\": [2, 5, 11]},\n      {\"term_id\": \"R-HSA-74160\", \"supporting_discovery_ids\": [5, 8]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [8, 13, 17]}\n    ],\n    \"complexes\": [\n      \"HUCA (HIRA/UBN1/CABIN1/ASF1a) histone chaperone complex\"\n    ],\n    \"partners\": [\n      \"HIRA\",\n      \"ASF1a\",\n      \"CABIN1\",\n      \"OGT\",\n      \"ZO-1\",\n      \"Sp100A\",\n      \"AID\",\n      \"EB1\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":6,"faith_pct":100.0}}