{"gene":"PURB","run_date":"2026-06-10T06:43:36","timeline":{"discoveries":[{"year":1997,"finding":"PURB (Purbeta, p44) was identified as a component of vascular actin single-stranded DNA-binding factor 2, binding specifically to the purine-rich strand of the MCAT enhancer element of the vascular smooth muscle alpha-actin gene, functioning to repress its transcription in fibroblasts.","method":"Expression library screening, band shift assay, Southwestern blot with cloned and cellular proteins","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal Southwestern and band shift assays with both recombinant and cellular proteins, single lab, two orthogonal methods establishing identity and DNA binding","pmids":["9334258"],"is_preprint":false},{"year":1999,"finding":"PURB (Purbeta) specifically interacts with the mRNA counterpart of vascular smooth muscle alpha-actin exon 3 sequence, participates in mRNP formation in living cells, and its binding to mRNA suppresses translation; deletion mutagenesis mapped the primary structure determinants of Purbeta required for single-stranded DNA binding, mRNA binding, and protein-protein interaction.","method":"Cell-free RNA binding assays with recombinant and fibroblast-derived proteins, translational reporter assays in transfected fibroblasts, deletion mutagenesis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro reconstitution of RNA binding, mutagenesis to map functional domains, cell-based translational reporter assays, multiple orthogonal methods in one study","pmids":["10608902"],"is_preprint":false},{"year":2003,"finding":"The minimal single-stranded DNA/RNA-binding domain of Purbeta was mapped to amino acids 37–263; overexpressed Purbeta (but not Puralpha) inhibits smooth muscle alpha-actin (ACTA2) promoter transcription in A7r5 vascular smooth muscle cells; Purbeta repressor activity requires both the minimal DNA-binding region and a C-terminal domain; MSY1 potentiates and serum response factor (SRF) antagonizes Purbeta inhibitory activity.","method":"Southwestern and Northwestern blotting of purified deletion mutants, quantitative DNA-binding assays, overexpression in A7r5 cells with promoter-reporter assays, co-expression studies","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — deletion mutagenesis combined with quantitative binding assays and cell-based transcriptional reporter assays, multiple orthogonal methods in one study","pmids":["12874279"],"is_preprint":false},{"year":2006,"finding":"Recombinant mouse Purbeta forms a reversible homodimer in the absence of nucleic acid; the homodimer has a markedly asymmetric, non-spherical hydrodynamic shape with a frictional coefficient ratio of 1.60; self-association is characterized by Kd = 1.13 ± 0.27 μM.","method":"Laser light scattering, sedimentation velocity, sedimentation equilibrium analytical ultracentrifugation, dynamic light scattering with purified recombinant protein","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — multiple independent biophysical methods (light scattering, sedimentation velocity, sedimentation equilibrium) on purified protein yielding consistent quantitative results","pmids":["17121857"],"is_preprint":false},{"year":2009,"finding":"Purbeta binds to two distinct G/A-rich sites within the smooth muscle alpha-actin sense-strand enhancer in a sequential, cooperative, monomer-dependent fashion, forming a high-affinity 2:1 protein:DNA complex; intersite cooperativity contributes ~12-fold to complex stability.","method":"Quantitative colorimetric, autoradiographic, and fluorescence DNA-binding assays; statistical mechanical analysis of band shift titrations; DNase I footprint titrations with purified components","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — quantitative in vitro reconstitution with purified components, multiple orthogonal binding assays, thermodynamic modeling","pmids":["19496623"],"is_preprint":false},{"year":2010,"finding":"Limited tryptic digestion of Purβ defined a core ~30 kDa domain (residues 29–305) that retains the ability to self-associate and binds purine-rich cis-elements from the smooth muscle alpha-actin gene with similar specificity but increased affinity compared to full-length Purβ.","method":"Partial proteolysis, epitope mapping, mass spectrometry, size exclusion chromatography, circular dichroism, comparative DNA-binding assays","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — in vitro biochemical characterization with multiple methods but single lab, single study","pmids":["20728429"],"is_preprint":false},{"year":2013,"finding":"Purβ homodimer possesses three separate but unequal single-stranded DNA-binding modules formed by subdomain-specific inter- and intramolecular interactions; stable shRNA-mediated Purβ knockdown in mouse embryo fibroblasts (MEFs) promoted myofibroblast-like morphology, altered actin isoform expression, increased cell migration, and derepressed ACTA2 transcription, confirming Purβ as a suppressor of myofibroblast differentiation.","method":"shRNA knockdown, promoter-reporter assays, recombinant truncation mutant biochemical/biophysical analyses, cell morphology and migration assays, computationally derived structural modeling validated by biochemical assays","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 2 / Moderate — clean stable KO with defined cellular phenotype and orthogonal promoter-reporter and domain-mapping experiments, multiple methods in single rigorous study","pmids":["23724822"],"is_preprint":false},{"year":2013,"finding":"Pur-beta associates with T-oligo (oligonucleotide homologous to the 3'-telomere overhang); knockdown of Pur-beta completely abrogated T-oligo-induced senescence, p53 phosphorylation, pRb hypophosphorylation, and upregulation of E2F1, p21, and p53 in melanoma and NSCLC cells.","method":"Biotinylated T-oligo pulldown with mass spectrometry identification, shRNA knockdown, immunoblotting, senescence-associated beta-galactosidase assay","journal":"OncoTargets and therapy","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — pulldown with MS identification plus clean KD with senescence phenotype, single lab, single study","pmids":["24379680"],"is_preprint":false},{"year":2014,"finding":"In quiescent fibroblasts, Purβ is more effective than Purα in disrupting SRF-DNA interaction at the SMαA core promoter; TGFβ1 signaling dissociates a SRF/Pur protein complex and transiently forms a pSmad3/MRTF-A/Purβ complex during early-phase myofibroblast differentiation; Purβ is subsequently replaced by Purα in the pSmad3/MRTF-A complex in mature myofibroblasts.","method":"Gel shift assays with nuclear extracts, co-immunoprecipitation of signaling complexes, TGFβ1 stimulation experiments in human pulmonary myofibroblasts","journal":"Journal of cellular physiology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — gel shift and Co-IP demonstrating complex formation and dynamic exchange, multiple experiments in single lab","pmids":["24446247"],"is_preprint":false},{"year":2019,"finding":"Purβ is a positive transcriptional regulator of Adcy6; Purβ directly binds the Adcy6 promoter to promote its transcription, thereby activating the glucagon/ADCY6/cAMP/PKA/CREB signaling pathway to increase hepatic glucose production; liver-specific Purβ knockdown in db/db mice ameliorated hyperglycemia via suppression of this pathway.","method":"Adenovirus-mediated knockdown/overexpression in primary hepatocytes and db/db mice, glucose/insulin/lactate tolerance tests, luciferase reporter assays, chromatin immunoprecipitation (ChIP), immunoblotting, RNA-seq","journal":"Molecular metabolism","confidence":"High","confidence_rationale":"Tier 2 / Moderate — ChIP plus luciferase reporter plus in vivo KD with metabolic phenotype, multiple orthogonal methods in single study","pmids":["31918924"],"is_preprint":false},{"year":2019,"finding":"YAP/TAZ regulate Purβ expression in Schwann cells; silencing of Purβ limits the formation of myelin segments, establishing a role for Purβ in Schwann cell myelination downstream of YAP/TAZ mechanosensing.","method":"YAP/TAZ genetic ablation, Purβ siRNA knockdown, myelin segment formation assays in Schwann cells","journal":"Frontiers in molecular neuroscience","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — clean KD with specific myelination phenotype, single lab, limited mechanistic follow-up","pmids":["31379499"],"is_preprint":false},{"year":2018,"finding":"PURB promotes mTOR and SREBP-1c gene transcription by binding their promoters in bovine mammary epithelial cells; PURB is required for Met and Leu-stimulated PI3K/mTOR/SREBP-1c pathway activation and subsequent milk protein and fat synthesis; Met and Leu increase PURB expression and nuclear localization.","method":"Overexpression and knockdown of PURB, luciferase reporter assays, ChIP assays, wortmannin inhibitor experiments, immunoblotting","journal":"Journal of cellular physiology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — ChIP plus reporter assays plus KD/OE with metabolic phenotype, multiple methods, single lab","pmids":["30362557"],"is_preprint":false},{"year":2022,"finding":"PURB forms a complex with Cbx1 and Sp3 that mediates long-term silencing of tissue- and lineage-specific (cardiomyocyte) genes in fibroblasts; knockdown or gene editing of Cbx1, PurB, and Sp3 together induced cardiomyocyte gene expression; the complex localizes nucleosomes to cardiomyocyte genes and promotes H3K27me3 deposition through interaction with the PRC2 complex.","method":"Co-immunoprecipitation, high-throughput DNA sequencing (ChIP-seq), siRNA knockdown, CRISPR gene editing, in vivo reprogramming experiments, histone modification analysis","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal Co-IP plus ChIP-seq plus in vivo KO experiments, multiple orthogonal methods, single lab","pmids":["35605661"],"is_preprint":false},{"year":2025,"finding":"PURB was biochemically purified as a transcriptional corepressor for p53 that acts in an acetylation-dependent manner; PURB recognizes a unique DNA element at the p21 promoter and selectively represses p21 but not PUMA or MDM2 promoters; PURB requires sequence-specific binding to lncRNA HOTAIR to exert its repressive role; the PURB-HOTAIR complex recruits EZH2 histone methyltransferase to target promoters, bridged by PURB's interaction with p53.","method":"Biochemical purification of p53 complex components, knockdown experiments, luciferase promoter-reporter assays, ChIP assays, RNA-protein interaction assays, epistasis experiments with p53 acetylation mutants and promoter element mutations","journal":"Nature structural & molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — biochemical purification of the complex, promoter-element mutagenesis to establish cis-requirement, ChIP, RNA-protein interaction, and epistasis experiments together establishing molecular mechanism, multiple orthogonal methods in single rigorous study","pmids":["40563010"],"is_preprint":false},{"year":2025,"finding":"During thermal stress recovery, PURB is recruited to cytoplasmic HSATIII-based RNA granules (HERALDs) that form with nucleocytoplasmic RNA-binding proteins; these granules are transported along microtubules to distal cellular regions.","method":"RNA immunoprecipitation, live-cell imaging, granule fractionation, fluorescence microscopy during stress recovery","journal":"bioRxiv","confidence":"Low","confidence_rationale":"Tier 3 / Weak — preprint, single lab, limited mechanistic follow-up specifically on PURB's role vs. scaffold role of HSATIII RNA","pmids":["bio_10.1101_2025.11.12.688122"],"is_preprint":true}],"current_model":"PURB (Purbeta) is a sequence-specific single-stranded DNA/RNA-binding protein that homodimerizes to form an asymmetric repressor complex, binding purine-rich cis-elements on the sense strand of target gene enhancers/promoters (notably ACTA2/smooth muscle alpha-actin and p21) to repress transcription; it acts cooperatively with MSY1/YB-1 and dynamically competes with SRF and pSmad3 during myofibroblast differentiation, represses p21 transcription by forming a PURB-HOTAIR-EZH2 complex downstream of p53 acetylation, promotes hepatic glucose production by directly transactivating Adcy6, forms a long-term gene-silencing complex with Cbx1 and Sp3 coupled to PRC2-mediated H3K27me3 deposition, and also binds mRNA to suppress translation, collectively placing PURB as a multifunctional nucleic-acid-binding regulator of transcription, mRNA utilization, and cell identity."},"narrative":{"mechanistic_narrative":"PURB (Purβ) is a sequence-specific single-stranded DNA- and RNA-binding protein that functions as a transcriptional repressor and post-transcriptional regulator controlling cell identity and lineage-specific gene programs [PMID:9334258, PMID:23724822]. It was first defined as a component of vascular actin single-stranded DNA-binding factor 2 that binds the purine-rich strand of the MCAT enhancer of the smooth muscle alpha-actin (ACTA2) gene to repress its transcription [PMID:9334258], and its minimal single-stranded DNA/RNA-binding domain maps to a core central region (residues ~37–263) whose repressor activity additionally requires a C-terminal domain [PMID:12874279]. Purβ forms a reversible, asymmetric homodimer in the absence of nucleic acid [PMID:17121857] and uses three unequal single-stranded DNA-binding modules to engage two G/A-rich sites in the ACTA2 sense-strand enhancer in a sequential, cooperative, 2:1 protein:DNA arrangement [PMID:19496623, PMID:23724822]. At the ACTA2 promoter Purβ acts as a suppressor of myofibroblast differentiation: its activity is potentiated by MSY1 and antagonized by SRF, and TGFβ1 signaling drives dynamic exchange of Pur proteins within SRF- and pSmad3/MRTF-A–containing complexes, with loss of Purβ derepressing ACTA2 and promoting a myofibroblast-like phenotype [PMID:12874279, PMID:23724822, PMID:24446247]. Beyond ACTA2, Purβ enforces long-term silencing of lineage-specific genes by forming a Cbx1/Sp3 complex that positions nucleosomes and couples to PRC2-mediated H3K27me3 deposition [PMID:35605661], and it serves as an acetylation-dependent corepressor for p53 that selectively represses the p21 promoter through sequence-specific binding to the lncRNA HOTAIR and recruitment of EZH2 [PMID:40563010]. Purβ also acts as a positive transcriptional regulator in metabolic contexts, directly binding the Adcy6 promoter to drive hepatic glucose production [PMID:31918924]. In addition to its DNA roles, Purβ binds the smooth muscle alpha-actin mRNA, participates in mRNP formation, and suppresses translation [PMID:10608902].","teleology":[{"year":1997,"claim":"Established PURB's founding identity by showing it binds the purine-rich strand of a smooth muscle gene enhancer to repress transcription, defining it as a single-stranded DNA-binding repressor.","evidence":"Expression library screening, band shift, and Southwestern blotting with recombinant and cellular proteins on the ACTA2 MCAT enhancer","pmids":["9334258"],"confidence":"Medium","gaps":["Did not map the binding domain","Repression mechanism beyond DNA binding undefined"]},{"year":1999,"claim":"Extended PURB beyond DNA to show it binds the cognate mRNA, joins mRNP particles, and suppresses translation, revealing a dual nucleic-acid regulatory role.","evidence":"Cell-free RNA binding, translational reporter assays in fibroblasts, and deletion mutagenesis to map functional determinants","pmids":["10608902"],"confidence":"High","gaps":["Physiological scope of translational repression not established","Which endogenous mRNAs are regulated unknown"]},{"year":2003,"claim":"Mapped the minimal ssDNA/RNA-binding domain and showed repressor activity requires both this region and a C-terminal domain, while MSY1 and SRF oppositely modulate Purβ.","evidence":"Deletion-mutant Southwestern/Northwestern blotting, quantitative binding, and promoter-reporter assays in A7r5 cells","pmids":["12874279"],"confidence":"High","gaps":["Structural basis of cofactor modulation unresolved","Purα vs Purβ functional distinction not mechanistically explained"]},{"year":2006,"claim":"Defined the oligomeric state by demonstrating Purβ self-associates into a reversible, highly asymmetric homodimer, providing the structural unit for DNA engagement.","evidence":"Analytical ultracentrifugation and light scattering on purified recombinant protein","pmids":["17121857"],"confidence":"High","gaps":["High-resolution structure absent","Link between dimer asymmetry and DNA binding not directly tested here"]},{"year":2009,"claim":"Resolved the DNA-binding mechanism, showing cooperative, sequential occupancy of two G/A-rich enhancer sites to form a stable 2:1 complex.","evidence":"Quantitative binding assays, DNase I footprint titrations, and statistical-mechanical modeling with purified components","pmids":["19496623"],"confidence":"High","gaps":["In vivo relevance of measured cooperativity untested","Contribution of dimerization to cooperativity not isolated"]},{"year":2010,"claim":"Identified a protease-resistant core domain that retains self-association and higher-affinity purine-rich DNA binding, refining the functional architecture.","evidence":"Limited proteolysis, mass spectrometry, CD, and comparative DNA-binding assays","pmids":["20728429"],"confidence":"Medium","gaps":["Single-study biochemical mapping without structure","Role of removed regions in cells unknown"]},{"year":2013,"claim":"Tied biochemistry to phenotype by demonstrating Purβ knockdown derepresses ACTA2 and drives a migratory, myofibroblast-like state, establishing Purβ as a suppressor of myofibroblast differentiation.","evidence":"Stable shRNA knockdown in MEFs with morphology, migration, promoter-reporter, and domain-mapping assays","pmids":["23724822"],"confidence":"High","gaps":["In vivo fibrosis relevance not tested","Full set of derepressed targets undefined"]},{"year":2014,"claim":"Clarified the dynamics of repression by showing TGFβ1 remodels SRF/Pur and pSmad3/MRTF-A complexes with sequential Purβ-then-Purα exchange during myofibroblast maturation.","evidence":"Gel shift with nuclear extracts and co-immunoprecipitation in TGFβ1-stimulated pulmonary myofibroblasts","pmids":["24446247"],"confidence":"Medium","gaps":["Signal that triggers Pur isoform exchange unknown","Co-IP without reciprocal/structural validation of the transient complex"]},{"year":2013,"claim":"Linked Purβ to the DNA-damage/senescence axis by showing it binds T-oligo and is required for T-oligo-induced senescence and p53/p21/E2F1 activation.","evidence":"Biotinylated T-oligo pulldown with MS, shRNA knockdown, immunoblotting, and SA-β-gal assay in melanoma and NSCLC cells","pmids":["24379680"],"confidence":"Medium","gaps":["Direct molecular role of Purβ in the senescence cascade unclear","Mechanism connecting T-oligo binding to p53 activation undefined"]},{"year":2019,"claim":"Revealed an activating, metabolic role for Purβ by showing it directly transactivates Adcy6 to drive hepatic glucose production, with knockdown ameliorating hyperglycemia in vivo.","evidence":"ChIP, luciferase reporters, and adenoviral knockdown/overexpression in hepatocytes and db/db mice with metabolic phenotyping","pmids":["31918924"],"confidence":"High","gaps":["Switch between repressor and activator modes unexplained","Cofactors at the Adcy6 promoter not identified"]},{"year":2019,"claim":"Placed Purβ downstream of YAP/TAZ mechanosensing in Schwann cells, where it is required for myelin segment formation.","evidence":"YAP/TAZ ablation and Purβ siRNA knockdown with myelination assays in Schwann cells","pmids":["31379499"],"confidence":"Medium","gaps":["Direct Purβ targets in myelination unknown","Whether Purβ acts transcriptionally here untested"]},{"year":2018,"claim":"Showed Purβ can act as a transcriptional activator of mTOR and SREBP-1c, coupling amino-acid signaling to milk protein and fat synthesis.","evidence":"Knockdown/overexpression, ChIP, luciferase reporters, and PI3K inhibition in bovine mammary epithelial cells","pmids":["30362557"],"confidence":"Medium","gaps":["Generality beyond mammary cells unknown","Determinants of activator vs repressor behavior unresolved"]},{"year":2022,"claim":"Defined a chromatin-silencing mechanism in which Purβ partners with Cbx1 and Sp3 to position nucleosomes and recruit PRC2 for H3K27me3-mediated long-term repression of cardiomyocyte genes.","evidence":"Reciprocal Co-IP, ChIP-seq, siRNA/CRISPR editing, and in vivo reprogramming with histone modification analysis","pmids":["35605661"],"confidence":"Medium","gaps":["Direct vs indirect PRC2 recruitment not fully resolved","Stoichiometry of the Cbx1/Sp3/Purβ complex undefined"]},{"year":2025,"claim":"Established Purβ as an acetylation-dependent p53 corepressor that uses lncRNA HOTAIR to recruit EZH2 and selectively silence p21, integrating protein, DNA, and RNA recognition into one repressive mechanism.","evidence":"Biochemical purification of the p53 complex, promoter-element and acetylation-mutant epistasis, ChIP, RNA-protein interaction, and reporter assays","pmids":["40563010"],"confidence":"High","gaps":["Structural basis of the PURB-HOTAIR interaction unknown","Why p21 but not PUMA/MDM2 is targeted not fully explained"]},{"year":2025,"claim":"Implicated Purβ in stress-responsive RNA compartmentalization by showing recruitment to cytoplasmic HSATIII-based RNA granules transported along microtubules.","evidence":"RNA immunoprecipitation, live-cell imaging, and granule fractionation during thermal stress recovery (preprint)","pmids":["bio_10.1101_2025.11.12.688122"],"confidence":"Low","gaps":["Preprint, not peer-reviewed","Functional contribution of PURB vs scaffold HSATIII RNA undetermined"]},{"year":null,"claim":"It remains unresolved what molecular switch determines whether Purβ acts as a repressor or activator at a given promoter, and whether a single structural mechanism unifies its DNA, RNA, and chromatin-silencing functions.","evidence":"","pmids":[],"confidence":"Low","gaps":["No high-resolution structure of a Purβ-DNA or Purβ-RNA complex","Activator vs repressor determinants unknown","Genome-wide direct target catalog incomplete"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0003677","term_label":"DNA binding","supporting_discovery_ids":[0,2,4,6]},{"term_id":"GO:0003723","term_label":"RNA binding","supporting_discovery_ids":[1,13]},{"term_id":"GO:0140110","term_label":"transcription regulator activity","supporting_discovery_ids":[0,2,9,13]},{"term_id":"GO:0045182","term_label":"translation regulator activity","supporting_discovery_ids":[1]}],"localization":[{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[11,12,13]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[1]}],"pathway":[{"term_id":"R-HSA-74160","term_label":"Gene expression (Transcription)","supporting_discovery_ids":[0,2,9,13]},{"term_id":"R-HSA-4839726","term_label":"Chromatin organization","supporting_discovery_ids":[12,13]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[6,10,12]}],"complexes":["Purβ homodimer","PURB-Cbx1-Sp3 silencing complex","PURB-HOTAIR-EZH2 p53 corepressor complex","pSmad3/MRTF-A/Purβ complex"],"partners":["MSY1","SRF","SMAD3","MRTF-A","CBX1","SP3","EZH2","TP53"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q96QR8","full_name":"Transcriptional regulator protein Pur-beta","aliases":["Purine-rich element-binding protein B"],"length_aa":312,"mass_kda":33.2,"function":"Transcriptional regulator which can act as an activator or a repressor. Represses the transcription of ACTA2 in fibroblasts and smooth muscle cells via its ability to interact with the purine-rich strand of a MCAT- containing element in the 5' flanking region of the gene. Represses the transcription of MYOCD, capable of repressing all isoforms of MYOCD but the magnitude of the repressive effects is most notable for the SMC- specific isoforms. Promotes hepatic glucose production by activating the transcription of ADCY6, leading to cAMP accumulation, increased PKA activity, CREB activation, and increased transcription of PCK1 and G6PC genes (By similarity). Has capacity to bind repeated elements in single-stranded DNA such as the purine-rich single strand of the PUR element located upstream of the MYC gene (PubMed:1448097). Participates in transcriptional and translational regulation of alpha-MHC expression in cardiac myocytes by binding to the purine-rich negative regulatory (PNR) element Modulates constitutive liver galectin-3 gene transcription by binding to its promoter. May play a role in the dendritic transport of a subset of mRNAs (By similarity)","subcellular_location":"Nucleus","url":"https://www.uniprot.org/uniprotkb/Q96QR8/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/PURB","classification":"Not Classified","n_dependent_lines":1,"n_total_lines":1208,"dependency_fraction":0.0008278145695364238},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/PURB","total_profiled":1310},"omim":[{"mim_id":"618041","title":"PURINE-RICH ELEMENT-BINDING PROTEIN G; PURG","url":"https://www.omim.org/entry/618041"},{"mim_id":"613614","title":"MICRO RNA 499; MIR499","url":"https://www.omim.org/entry/613614"},{"mim_id":"613613","title":"MICRO RNA 208B; MIR208B","url":"https://www.omim.org/entry/613613"},{"mim_id":"608887","title":"PURINE-RICH ELEMENT-BINDING PROTEIN B; PURB","url":"https://www.omim.org/entry/608887"},{"mim_id":"600473","title":"PURINE-RICH ELEMENT-BINDING PROTEIN A; PURA","url":"https://www.omim.org/entry/600473"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Nucleoplasm","reliability":"Supported"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/PURB"},"hgnc":{"alias_symbol":["PURBETA"],"prev_symbol":[]},"alphafold":{"accession":"Q96QR8","domains":[{"cath_id":"3.30.2450.30","chopping":"33-102_118-153_170-195","consensus_level":"high","plddt":87.6505,"start":33,"end":195},{"cath_id":"3.10.450.700","chopping":"223-295","consensus_level":"medium","plddt":89.2221,"start":223,"end":295}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q96QR8","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q96QR8-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q96QR8-F1-predicted_aligned_error_v6.png","plddt_mean":72.69},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=PURB","jax_strain_url":"https://www.jax.org/strain/search?query=PURB"},"sequence":{"accession":"Q96QR8","fasta_url":"https://rest.uniprot.org/uniprotkb/Q96QR8.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q96QR8/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q96QR8"}},"corpus_meta":[{"pmid":"9334258","id":"PMC_9334258","title":"Sequence of cDNAs encoding components of vascular actin single-stranded DNA-binding factor 2 establish identity to Puralpha and Purbeta.","date":"1997","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/9334258","citation_count":84,"is_preprint":false},{"pmid":"2404765","id":"PMC_2404765","title":"Autoregulation of PurR repressor synthesis and involvement of purR in the regulation of purB, purC, purL, purMN and guaBA expression in Escherichia coli.","date":"1990","source":"European journal of biochemistry","url":"https://pubmed.ncbi.nlm.nih.gov/2404765","citation_count":83,"is_preprint":false},{"pmid":"1429435","id":"PMC_1429435","title":"Repression of Escherichia coli purB is by a transcriptional roadblock mechanism.","date":"1992","source":"Journal of bacteriology","url":"https://pubmed.ncbi.nlm.nih.gov/1429435","citation_count":59,"is_preprint":false},{"pmid":"11417483","id":"PMC_11417483","title":"Deletions of PURA, at 5q31, and PURB, at 7p13, in myelodysplastic syndrome and progression to acute myelogenous leukemia.","date":"2001","source":"Leukemia","url":"https://pubmed.ncbi.nlm.nih.gov/11417483","citation_count":55,"is_preprint":false},{"pmid":"10608902","id":"PMC_10608902","title":"The single-stranded DNA-binding proteins, Puralpha, Purbeta, and MSY1 specifically interact with an exon 3-derived mouse vascular smooth muscle alpha-actin messenger RNA sequence.","date":"1999","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/10608902","citation_count":38,"is_preprint":false},{"pmid":"1729205","id":"PMC_1729205","title":"Escherichia coli purB gene: cloning, nucleotide sequence, and regulation by purR.","date":"1992","source":"Journal of bacteriology","url":"https://pubmed.ncbi.nlm.nih.gov/1729205","citation_count":35,"is_preprint":false},{"pmid":"12874279","id":"PMC_12874279","title":"Structure/function analysis of mouse Purbeta, a single-stranded DNA-binding repressor of vascular smooth muscle alpha-actin gene transcription.","date":"2003","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/12874279","citation_count":35,"is_preprint":false},{"pmid":"31918924","id":"PMC_31918924","title":"Purβ promotes hepatic glucose production by increasing Adcy6 transcription.","date":"2019","source":"Molecular metabolism","url":"https://pubmed.ncbi.nlm.nih.gov/31918924","citation_count":27,"is_preprint":false},{"pmid":"30362557","id":"PMC_30362557","title":"PURB is a positive regulator of amino acid-induced milk synthesis in bovine mammary epithelial cells.","date":"2018","source":"Journal of cellular physiology","url":"https://pubmed.ncbi.nlm.nih.gov/30362557","citation_count":27,"is_preprint":false},{"pmid":"17827288","id":"PMC_17827288","title":"The Mesorhizobium loti purB gene is involved in infection thread formation and nodule development in Lotus japonicus.","date":"2007","source":"Journal of bacteriology","url":"https://pubmed.ncbi.nlm.nih.gov/17827288","citation_count":23,"is_preprint":false},{"pmid":"8969519","id":"PMC_8969519","title":"The purB gene of Escherichia coli K-12 is located in an operon.","date":"1996","source":"Microbiology (Reading, England)","url":"https://pubmed.ncbi.nlm.nih.gov/8969519","citation_count":19,"is_preprint":false},{"pmid":"24446247","id":"PMC_24446247","title":"The Purα/Purβ single-strand DNA-binding proteins attenuate smooth-muscle actin gene transactivation in myofibroblasts.","date":"2014","source":"Journal of cellular physiology","url":"https://pubmed.ncbi.nlm.nih.gov/24446247","citation_count":17,"is_preprint":false},{"pmid":"32302712","id":"PMC_32302712","title":"The novel long noncoding RNA Lnc19959.2 modulates triglyceride metabolism-associated genes through the interaction with Purb and hnRNPA2B1.","date":"2020","source":"Molecular 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cellular cardiology","url":"https://pubmed.ncbi.nlm.nih.gov/36716953","citation_count":15,"is_preprint":false},{"pmid":"20002573","id":"PMC_20002573","title":"The purB gene controls rhizosphere colonization by Pantoea agglomerans.","date":"2009","source":"Letters in applied microbiology","url":"https://pubmed.ncbi.nlm.nih.gov/20002573","citation_count":15,"is_preprint":false},{"pmid":"20693687","id":"PMC_20693687","title":"Structure of Staphylococcus aureus adenylosuccinate lyase (PurB) and assessment of its potential as a target for structure-based inhibitor discovery.","date":"2010","source":"Acta crystallographica. Section D, Biological crystallography","url":"https://pubmed.ncbi.nlm.nih.gov/20693687","citation_count":15,"is_preprint":false},{"pmid":"3886636","id":"PMC_3886636","title":"Separate regulation of purA and purB loci of Escherichia coli K-12.","date":"1985","source":"Journal of bacteriology","url":"https://pubmed.ncbi.nlm.nih.gov/3886636","citation_count":14,"is_preprint":false},{"pmid":"19496623","id":"PMC_19496623","title":"Mechanism of strand-specific smooth muscle alpha-actin enhancer interaction by purine-rich element binding protein B (Purbeta).","date":"2009","source":"Biochemistry","url":"https://pubmed.ncbi.nlm.nih.gov/19496623","citation_count":13,"is_preprint":false},{"pmid":"17121857","id":"PMC_17121857","title":"Hydrodynamic studies on the quaternary structure of recombinant mouse Purbeta.","date":"2006","source":"The Journal of biological 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neuroscience","url":"https://pubmed.ncbi.nlm.nih.gov/31680860","citation_count":11,"is_preprint":false},{"pmid":"342500","id":"PMC_342500","title":"Genetic and segregation analysis of Escherichia coli strains containing a tandem duplication of the trpD-purB region of the chromosome.","date":"1978","source":"Journal of bacteriology","url":"https://pubmed.ncbi.nlm.nih.gov/342500","citation_count":10,"is_preprint":false},{"pmid":"24379680","id":"PMC_24379680","title":"hnRNP C1/C2 and Pur-beta proteins mediate induction of senescence by oligonucleotides homologous to the telomere overhang.","date":"2013","source":"OncoTargets and therapy","url":"https://pubmed.ncbi.nlm.nih.gov/24379680","citation_count":10,"is_preprint":false},{"pmid":"35605661","id":"PMC_35605661","title":"A novel Cbx1, PurB, and Sp3 complex mediates long-term silencing of tissue- and lineage-specific genes.","date":"2022","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/35605661","citation_count":9,"is_preprint":false},{"pmid":"20728429","id":"PMC_20728429","title":"Isolation and characterization of the core single-stranded DNA-binding domain of purine-rich element binding protein B (Purβ).","date":"2010","source":"Biochemical and biophysical research communications","url":"https://pubmed.ncbi.nlm.nih.gov/20728429","citation_count":9,"is_preprint":false},{"pmid":"37373006","id":"PMC_37373006","title":"Circular Intronic RNA circTTN Inhibits Host Gene Transcription and Myogenesis by Recruiting PURB Proteins to form Heterotypic Complexes.","date":"2023","source":"International journal of molecular sciences","url":"https://pubmed.ncbi.nlm.nih.gov/37373006","citation_count":8,"is_preprint":false},{"pmid":"39482389","id":"PMC_39482389","title":"LINC01320 facilitates cell proliferation and migration of ovarian cancer via regulating PURB/DDB2/NEDD4L/TGF-β axis.","date":"2024","source":"Scientific reports","url":"https://pubmed.ncbi.nlm.nih.gov/39482389","citation_count":6,"is_preprint":false},{"pmid":"40563010","id":"PMC_40563010","title":"The PURB-HOTAIR complex regulates p53-dependent promoter-specific transcriptional activation.","date":"2025","source":"Nature structural & molecular biology","url":"https://pubmed.ncbi.nlm.nih.gov/40563010","citation_count":5,"is_preprint":false},{"pmid":"36318802","id":"PMC_36318802","title":"LncCMRR Plays an Important Role in Cardiac Differentiation by Regulating the Purb/Flk1 Axis.","date":"2023","source":"Stem cells (Dayton, Ohio)","url":"https://pubmed.ncbi.nlm.nih.gov/36318802","citation_count":4,"is_preprint":false},{"pmid":"6409984","id":"PMC_6409984","title":"Bacillus subtilis strains carrying two non-tandem duplications of the trpE-ilvA and the purB-tre regions of the chromosome.","date":"1983","source":"Journal of general microbiology","url":"https://pubmed.ncbi.nlm.nih.gov/6409984","citation_count":3,"is_preprint":false},{"pmid":"39606105","id":"PMC_39606105","title":"Construction, molecular characterization, and safety assessment of purB mutant of Salmonella Gallinarum.","date":"2024","source":"Frontiers in microbiology","url":"https://pubmed.ncbi.nlm.nih.gov/39606105","citation_count":2,"is_preprint":false},{"pmid":null,"id":"bio_10.1101_2025.11.12.688122","title":"Cytoplasmic roles of HSATIII RNAs in RNA granule assembly and production of actin cytoskeleton-associated repeat-containing proteins","date":"2025-11-13","source":"bioRxiv","url":"https://doi.org/10.1101/2025.11.12.688122","citation_count":0,"is_preprint":true}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":17794,"output_tokens":4038,"usd":0.056976,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":11992,"output_tokens":4840,"usd":0.09048,"stage2_stop_reason":"end_turn"},"total_usd":0.147456,"stage1_batch_id":"msgbatch_01CNoDbkChHEkgrJnL7BwtGJ","stage2_batch_id":"msgbatch_016Y4x9E9Wg5VQDANj9quSQD","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1997,\n      \"finding\": \"PURB (Purbeta, p44) was identified as a component of vascular actin single-stranded DNA-binding factor 2, binding specifically to the purine-rich strand of the MCAT enhancer element of the vascular smooth muscle alpha-actin gene, functioning to repress its transcription in fibroblasts.\",\n      \"method\": \"Expression library screening, band shift assay, Southwestern blot with cloned and cellular proteins\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal Southwestern and band shift assays with both recombinant and cellular proteins, single lab, two orthogonal methods establishing identity and DNA binding\",\n      \"pmids\": [\"9334258\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"PURB (Purbeta) specifically interacts with the mRNA counterpart of vascular smooth muscle alpha-actin exon 3 sequence, participates in mRNP formation in living cells, and its binding to mRNA suppresses translation; deletion mutagenesis mapped the primary structure determinants of Purbeta required for single-stranded DNA binding, mRNA binding, and protein-protein interaction.\",\n      \"method\": \"Cell-free RNA binding assays with recombinant and fibroblast-derived proteins, translational reporter assays in transfected fibroblasts, deletion mutagenesis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro reconstitution of RNA binding, mutagenesis to map functional domains, cell-based translational reporter assays, multiple orthogonal methods in one study\",\n      \"pmids\": [\"10608902\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"The minimal single-stranded DNA/RNA-binding domain of Purbeta was mapped to amino acids 37–263; overexpressed Purbeta (but not Puralpha) inhibits smooth muscle alpha-actin (ACTA2) promoter transcription in A7r5 vascular smooth muscle cells; Purbeta repressor activity requires both the minimal DNA-binding region and a C-terminal domain; MSY1 potentiates and serum response factor (SRF) antagonizes Purbeta inhibitory activity.\",\n      \"method\": \"Southwestern and Northwestern blotting of purified deletion mutants, quantitative DNA-binding assays, overexpression in A7r5 cells with promoter-reporter assays, co-expression studies\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — deletion mutagenesis combined with quantitative binding assays and cell-based transcriptional reporter assays, multiple orthogonal methods in one study\",\n      \"pmids\": [\"12874279\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Recombinant mouse Purbeta forms a reversible homodimer in the absence of nucleic acid; the homodimer has a markedly asymmetric, non-spherical hydrodynamic shape with a frictional coefficient ratio of 1.60; self-association is characterized by Kd = 1.13 ± 0.27 μM.\",\n      \"method\": \"Laser light scattering, sedimentation velocity, sedimentation equilibrium analytical ultracentrifugation, dynamic light scattering with purified recombinant protein\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — multiple independent biophysical methods (light scattering, sedimentation velocity, sedimentation equilibrium) on purified protein yielding consistent quantitative results\",\n      \"pmids\": [\"17121857\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Purbeta binds to two distinct G/A-rich sites within the smooth muscle alpha-actin sense-strand enhancer in a sequential, cooperative, monomer-dependent fashion, forming a high-affinity 2:1 protein:DNA complex; intersite cooperativity contributes ~12-fold to complex stability.\",\n      \"method\": \"Quantitative colorimetric, autoradiographic, and fluorescence DNA-binding assays; statistical mechanical analysis of band shift titrations; DNase I footprint titrations with purified components\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — quantitative in vitro reconstitution with purified components, multiple orthogonal binding assays, thermodynamic modeling\",\n      \"pmids\": [\"19496623\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Limited tryptic digestion of Purβ defined a core ~30 kDa domain (residues 29–305) that retains the ability to self-associate and binds purine-rich cis-elements from the smooth muscle alpha-actin gene with similar specificity but increased affinity compared to full-length Purβ.\",\n      \"method\": \"Partial proteolysis, epitope mapping, mass spectrometry, size exclusion chromatography, circular dichroism, comparative DNA-binding assays\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — in vitro biochemical characterization with multiple methods but single lab, single study\",\n      \"pmids\": [\"20728429\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Purβ homodimer possesses three separate but unequal single-stranded DNA-binding modules formed by subdomain-specific inter- and intramolecular interactions; stable shRNA-mediated Purβ knockdown in mouse embryo fibroblasts (MEFs) promoted myofibroblast-like morphology, altered actin isoform expression, increased cell migration, and derepressed ACTA2 transcription, confirming Purβ as a suppressor of myofibroblast differentiation.\",\n      \"method\": \"shRNA knockdown, promoter-reporter assays, recombinant truncation mutant biochemical/biophysical analyses, cell morphology and migration assays, computationally derived structural modeling validated by biochemical assays\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — clean stable KO with defined cellular phenotype and orthogonal promoter-reporter and domain-mapping experiments, multiple methods in single rigorous study\",\n      \"pmids\": [\"23724822\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Pur-beta associates with T-oligo (oligonucleotide homologous to the 3'-telomere overhang); knockdown of Pur-beta completely abrogated T-oligo-induced senescence, p53 phosphorylation, pRb hypophosphorylation, and upregulation of E2F1, p21, and p53 in melanoma and NSCLC cells.\",\n      \"method\": \"Biotinylated T-oligo pulldown with mass spectrometry identification, shRNA knockdown, immunoblotting, senescence-associated beta-galactosidase assay\",\n      \"journal\": \"OncoTargets and therapy\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — pulldown with MS identification plus clean KD with senescence phenotype, single lab, single study\",\n      \"pmids\": [\"24379680\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"In quiescent fibroblasts, Purβ is more effective than Purα in disrupting SRF-DNA interaction at the SMαA core promoter; TGFβ1 signaling dissociates a SRF/Pur protein complex and transiently forms a pSmad3/MRTF-A/Purβ complex during early-phase myofibroblast differentiation; Purβ is subsequently replaced by Purα in the pSmad3/MRTF-A complex in mature myofibroblasts.\",\n      \"method\": \"Gel shift assays with nuclear extracts, co-immunoprecipitation of signaling complexes, TGFβ1 stimulation experiments in human pulmonary myofibroblasts\",\n      \"journal\": \"Journal of cellular physiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — gel shift and Co-IP demonstrating complex formation and dynamic exchange, multiple experiments in single lab\",\n      \"pmids\": [\"24446247\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Purβ is a positive transcriptional regulator of Adcy6; Purβ directly binds the Adcy6 promoter to promote its transcription, thereby activating the glucagon/ADCY6/cAMP/PKA/CREB signaling pathway to increase hepatic glucose production; liver-specific Purβ knockdown in db/db mice ameliorated hyperglycemia via suppression of this pathway.\",\n      \"method\": \"Adenovirus-mediated knockdown/overexpression in primary hepatocytes and db/db mice, glucose/insulin/lactate tolerance tests, luciferase reporter assays, chromatin immunoprecipitation (ChIP), immunoblotting, RNA-seq\",\n      \"journal\": \"Molecular metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — ChIP plus luciferase reporter plus in vivo KD with metabolic phenotype, multiple orthogonal methods in single study\",\n      \"pmids\": [\"31918924\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"YAP/TAZ regulate Purβ expression in Schwann cells; silencing of Purβ limits the formation of myelin segments, establishing a role for Purβ in Schwann cell myelination downstream of YAP/TAZ mechanosensing.\",\n      \"method\": \"YAP/TAZ genetic ablation, Purβ siRNA knockdown, myelin segment formation assays in Schwann cells\",\n      \"journal\": \"Frontiers in molecular neuroscience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — clean KD with specific myelination phenotype, single lab, limited mechanistic follow-up\",\n      \"pmids\": [\"31379499\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"PURB promotes mTOR and SREBP-1c gene transcription by binding their promoters in bovine mammary epithelial cells; PURB is required for Met and Leu-stimulated PI3K/mTOR/SREBP-1c pathway activation and subsequent milk protein and fat synthesis; Met and Leu increase PURB expression and nuclear localization.\",\n      \"method\": \"Overexpression and knockdown of PURB, luciferase reporter assays, ChIP assays, wortmannin inhibitor experiments, immunoblotting\",\n      \"journal\": \"Journal of cellular physiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — ChIP plus reporter assays plus KD/OE with metabolic phenotype, multiple methods, single lab\",\n      \"pmids\": [\"30362557\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"PURB forms a complex with Cbx1 and Sp3 that mediates long-term silencing of tissue- and lineage-specific (cardiomyocyte) genes in fibroblasts; knockdown or gene editing of Cbx1, PurB, and Sp3 together induced cardiomyocyte gene expression; the complex localizes nucleosomes to cardiomyocyte genes and promotes H3K27me3 deposition through interaction with the PRC2 complex.\",\n      \"method\": \"Co-immunoprecipitation, high-throughput DNA sequencing (ChIP-seq), siRNA knockdown, CRISPR gene editing, in vivo reprogramming experiments, histone modification analysis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal Co-IP plus ChIP-seq plus in vivo KO experiments, multiple orthogonal methods, single lab\",\n      \"pmids\": [\"35605661\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"PURB was biochemically purified as a transcriptional corepressor for p53 that acts in an acetylation-dependent manner; PURB recognizes a unique DNA element at the p21 promoter and selectively represses p21 but not PUMA or MDM2 promoters; PURB requires sequence-specific binding to lncRNA HOTAIR to exert its repressive role; the PURB-HOTAIR complex recruits EZH2 histone methyltransferase to target promoters, bridged by PURB's interaction with p53.\",\n      \"method\": \"Biochemical purification of p53 complex components, knockdown experiments, luciferase promoter-reporter assays, ChIP assays, RNA-protein interaction assays, epistasis experiments with p53 acetylation mutants and promoter element mutations\",\n      \"journal\": \"Nature structural & molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — biochemical purification of the complex, promoter-element mutagenesis to establish cis-requirement, ChIP, RNA-protein interaction, and epistasis experiments together establishing molecular mechanism, multiple orthogonal methods in single rigorous study\",\n      \"pmids\": [\"40563010\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"During thermal stress recovery, PURB is recruited to cytoplasmic HSATIII-based RNA granules (HERALDs) that form with nucleocytoplasmic RNA-binding proteins; these granules are transported along microtubules to distal cellular regions.\",\n      \"method\": \"RNA immunoprecipitation, live-cell imaging, granule fractionation, fluorescence microscopy during stress recovery\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — preprint, single lab, limited mechanistic follow-up specifically on PURB's role vs. scaffold role of HSATIII RNA\",\n      \"pmids\": [\"bio_10.1101_2025.11.12.688122\"],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"PURB (Purbeta) is a sequence-specific single-stranded DNA/RNA-binding protein that homodimerizes to form an asymmetric repressor complex, binding purine-rich cis-elements on the sense strand of target gene enhancers/promoters (notably ACTA2/smooth muscle alpha-actin and p21) to repress transcription; it acts cooperatively with MSY1/YB-1 and dynamically competes with SRF and pSmad3 during myofibroblast differentiation, represses p21 transcription by forming a PURB-HOTAIR-EZH2 complex downstream of p53 acetylation, promotes hepatic glucose production by directly transactivating Adcy6, forms a long-term gene-silencing complex with Cbx1 and Sp3 coupled to PRC2-mediated H3K27me3 deposition, and also binds mRNA to suppress translation, collectively placing PURB as a multifunctional nucleic-acid-binding regulator of transcription, mRNA utilization, and cell identity.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"PURB (Purβ) is a sequence-specific single-stranded DNA- and RNA-binding protein that functions as a transcriptional repressor and post-transcriptional regulator controlling cell identity and lineage-specific gene programs [#0, #6]. It was first defined as a component of vascular actin single-stranded DNA-binding factor 2 that binds the purine-rich strand of the MCAT enhancer of the smooth muscle alpha-actin (ACTA2) gene to repress its transcription [#0], and its minimal single-stranded DNA/RNA-binding domain maps to a core central region (residues ~37–263) whose repressor activity additionally requires a C-terminal domain [#2]. Purβ forms a reversible, asymmetric homodimer in the absence of nucleic acid [#3] and uses three unequal single-stranded DNA-binding modules to engage two G/A-rich sites in the ACTA2 sense-strand enhancer in a sequential, cooperative, 2:1 protein:DNA arrangement [#4, #6]. At the ACTA2 promoter Purβ acts as a suppressor of myofibroblast differentiation: its activity is potentiated by MSY1 and antagonized by SRF, and TGFβ1 signaling drives dynamic exchange of Pur proteins within SRF- and pSmad3/MRTF-A–containing complexes, with loss of Purβ derepressing ACTA2 and promoting a myofibroblast-like phenotype [#2, #6, #8]. Beyond ACTA2, Purβ enforces long-term silencing of lineage-specific genes by forming a Cbx1/Sp3 complex that positions nucleosomes and couples to PRC2-mediated H3K27me3 deposition [#12], and it serves as an acetylation-dependent corepressor for p53 that selectively represses the p21 promoter through sequence-specific binding to the lncRNA HOTAIR and recruitment of EZH2 [#13]. Purβ also acts as a positive transcriptional regulator in metabolic contexts, directly binding the Adcy6 promoter to drive hepatic glucose production [#9]. In addition to its DNA roles, Purβ binds the smooth muscle alpha-actin mRNA, participates in mRNP formation, and suppresses translation [#1].\",\n  \"teleology\": [\n    {\n      \"year\": 1997,\n      \"claim\": \"Established PURB's founding identity by showing it binds the purine-rich strand of a smooth muscle gene enhancer to repress transcription, defining it as a single-stranded DNA-binding repressor.\",\n      \"evidence\": \"Expression library screening, band shift, and Southwestern blotting with recombinant and cellular proteins on the ACTA2 MCAT enhancer\",\n      \"pmids\": [\"9334258\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Did not map the binding domain\", \"Repression mechanism beyond DNA binding undefined\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Extended PURB beyond DNA to show it binds the cognate mRNA, joins mRNP particles, and suppresses translation, revealing a dual nucleic-acid regulatory role.\",\n      \"evidence\": \"Cell-free RNA binding, translational reporter assays in fibroblasts, and deletion mutagenesis to map functional determinants\",\n      \"pmids\": [\"10608902\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological scope of translational repression not established\", \"Which endogenous mRNAs are regulated unknown\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Mapped the minimal ssDNA/RNA-binding domain and showed repressor activity requires both this region and a C-terminal domain, while MSY1 and SRF oppositely modulate Purβ.\",\n      \"evidence\": \"Deletion-mutant Southwestern/Northwestern blotting, quantitative binding, and promoter-reporter assays in A7r5 cells\",\n      \"pmids\": [\"12874279\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of cofactor modulation unresolved\", \"Purα vs Purβ functional distinction not mechanistically explained\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Defined the oligomeric state by demonstrating Purβ self-associates into a reversible, highly asymmetric homodimer, providing the structural unit for DNA engagement.\",\n      \"evidence\": \"Analytical ultracentrifugation and light scattering on purified recombinant protein\",\n      \"pmids\": [\"17121857\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"High-resolution structure absent\", \"Link between dimer asymmetry and DNA binding not directly tested here\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Resolved the DNA-binding mechanism, showing cooperative, sequential occupancy of two G/A-rich enhancer sites to form a stable 2:1 complex.\",\n      \"evidence\": \"Quantitative binding assays, DNase I footprint titrations, and statistical-mechanical modeling with purified components\",\n      \"pmids\": [\"19496623\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo relevance of measured cooperativity untested\", \"Contribution of dimerization to cooperativity not isolated\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Identified a protease-resistant core domain that retains self-association and higher-affinity purine-rich DNA binding, refining the functional architecture.\",\n      \"evidence\": \"Limited proteolysis, mass spectrometry, CD, and comparative DNA-binding assays\",\n      \"pmids\": [\"20728429\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-study biochemical mapping without structure\", \"Role of removed regions in cells unknown\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Tied biochemistry to phenotype by demonstrating Purβ knockdown derepresses ACTA2 and drives a migratory, myofibroblast-like state, establishing Purβ as a suppressor of myofibroblast differentiation.\",\n      \"evidence\": \"Stable shRNA knockdown in MEFs with morphology, migration, promoter-reporter, and domain-mapping assays\",\n      \"pmids\": [\"23724822\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo fibrosis relevance not tested\", \"Full set of derepressed targets undefined\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Clarified the dynamics of repression by showing TGFβ1 remodels SRF/Pur and pSmad3/MRTF-A complexes with sequential Purβ-then-Purα exchange during myofibroblast maturation.\",\n      \"evidence\": \"Gel shift with nuclear extracts and co-immunoprecipitation in TGFβ1-stimulated pulmonary myofibroblasts\",\n      \"pmids\": [\"24446247\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Signal that triggers Pur isoform exchange unknown\", \"Co-IP without reciprocal/structural validation of the transient complex\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Linked Purβ to the DNA-damage/senescence axis by showing it binds T-oligo and is required for T-oligo-induced senescence and p53/p21/E2F1 activation.\",\n      \"evidence\": \"Biotinylated T-oligo pulldown with MS, shRNA knockdown, immunoblotting, and SA-β-gal assay in melanoma and NSCLC cells\",\n      \"pmids\": [\"24379680\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct molecular role of Purβ in the senescence cascade unclear\", \"Mechanism connecting T-oligo binding to p53 activation undefined\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Revealed an activating, metabolic role for Purβ by showing it directly transactivates Adcy6 to drive hepatic glucose production, with knockdown ameliorating hyperglycemia in vivo.\",\n      \"evidence\": \"ChIP, luciferase reporters, and adenoviral knockdown/overexpression in hepatocytes and db/db mice with metabolic phenotyping\",\n      \"pmids\": [\"31918924\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Switch between repressor and activator modes unexplained\", \"Cofactors at the Adcy6 promoter not identified\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Placed Purβ downstream of YAP/TAZ mechanosensing in Schwann cells, where it is required for myelin segment formation.\",\n      \"evidence\": \"YAP/TAZ ablation and Purβ siRNA knockdown with myelination assays in Schwann cells\",\n      \"pmids\": [\"31379499\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct Purβ targets in myelination unknown\", \"Whether Purβ acts transcriptionally here untested\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Showed Purβ can act as a transcriptional activator of mTOR and SREBP-1c, coupling amino-acid signaling to milk protein and fat synthesis.\",\n      \"evidence\": \"Knockdown/overexpression, ChIP, luciferase reporters, and PI3K inhibition in bovine mammary epithelial cells\",\n      \"pmids\": [\"30362557\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Generality beyond mammary cells unknown\", \"Determinants of activator vs repressor behavior unresolved\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Defined a chromatin-silencing mechanism in which Purβ partners with Cbx1 and Sp3 to position nucleosomes and recruit PRC2 for H3K27me3-mediated long-term repression of cardiomyocyte genes.\",\n      \"evidence\": \"Reciprocal Co-IP, ChIP-seq, siRNA/CRISPR editing, and in vivo reprogramming with histone modification analysis\",\n      \"pmids\": [\"35605661\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct vs indirect PRC2 recruitment not fully resolved\", \"Stoichiometry of the Cbx1/Sp3/Purβ complex undefined\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Established Purβ as an acetylation-dependent p53 corepressor that uses lncRNA HOTAIR to recruit EZH2 and selectively silence p21, integrating protein, DNA, and RNA recognition into one repressive mechanism.\",\n      \"evidence\": \"Biochemical purification of the p53 complex, promoter-element and acetylation-mutant epistasis, ChIP, RNA-protein interaction, and reporter assays\",\n      \"pmids\": [\"40563010\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of the PURB-HOTAIR interaction unknown\", \"Why p21 but not PUMA/MDM2 is targeted not fully explained\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Implicated Purβ in stress-responsive RNA compartmentalization by showing recruitment to cytoplasmic HSATIII-based RNA granules transported along microtubules.\",\n      \"evidence\": \"RNA immunoprecipitation, live-cell imaging, and granule fractionation during thermal stress recovery (preprint)\",\n      \"pmids\": [\"bio_10.1101_2025.11.12.688122\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"Preprint, not peer-reviewed\", \"Functional contribution of PURB vs scaffold HSATIII RNA undetermined\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"It remains unresolved what molecular switch determines whether Purβ acts as a repressor or activator at a given promoter, and whether a single structural mechanism unifies its DNA, RNA, and chromatin-silencing functions.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No high-resolution structure of a Purβ-DNA or Purβ-RNA complex\", \"Activator vs repressor determinants unknown\", \"Genome-wide direct target catalog incomplete\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0003677\", \"supporting_discovery_ids\": [0, 2, 4, 6]},\n      {\"term_id\": \"GO:0003723\", \"supporting_discovery_ids\": [1, 13]},\n      {\"term_id\": \"GO:0140110\", \"supporting_discovery_ids\": [0, 2, 9, 13]},\n      {\"term_id\": \"GO:0045182\", \"supporting_discovery_ids\": [1]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [11, 12, 13]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [1]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-74160\", \"supporting_discovery_ids\": [0, 2, 9, 13]},\n      {\"term_id\": \"R-HSA-4839726\", \"supporting_discovery_ids\": [12, 13]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [6, 10, 12]}\n    ],\n    \"complexes\": [\n      \"Pur\\u03b2 homodimer\",\n      \"PURB-Cbx1-Sp3 silencing complex\",\n      \"PURB-HOTAIR-EZH2 p53 corepressor complex\",\n      \"pSmad3/MRTF-A/Pur\\u03b2 complex\"\n    ],\n    \"partners\": [\n      \"MSY1\",\n      \"SRF\",\n      \"SMAD3\",\n      \"MRTF-A\",\n      \"CBX1\",\n      \"SP3\",\n      \"EZH2\",\n      \"TP53\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":7,"faith_total":7,"faith_pct":100.0}}