{"gene":"TOPORS","run_date":"2026-04-28T21:42:59","timeline":{"discoveries":[{"year":2004,"finding":"TOPORS functions as a RING-dependent E3 ubiquitin ligase with E2 enzymes UbcH5a, UbcH5c, and UbcH6 (but not UbcH7, CDC34, or UbcH2b), and ubiquitinates p53 both in vitro and in cells, leading to proteasome-dependent p53 degradation; a conserved tryptophan in the RING domain is required for ubiquitination activity.","method":"In vitro ubiquitination assay, active-site mutagenesis, cellular overexpression with proteasome inhibitor, Co-IP","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — reconstituted in vitro ubiquitination with mutagenesis, replicated in cells","pmids":["15247280"],"is_preprint":false},{"year":2005,"finding":"TOPORS acts as a SUMO-1 E3 ligase for p53, enhancing SUMO-1 conjugation to p53 in vivo and in a reconstituted in vitro system; this activity does not depend on the RING finger motif and is accompanied by an increase in endogenous p53 protein levels in HeLa cells.","method":"Reconstituted in vitro SUMO-1 conjugation assay, cellular sumoylation assay, RING domain mutant analysis","journal":"FEBS letters","confidence":"High","confidence_rationale":"Tier 1 — reconstituted in vitro with domain mapping, replicated in cells","pmids":["16122737"],"is_preprint":false},{"year":2004,"finding":"Drosophila Topors (dTopors) ortholog possesses E3 ubiquitin ligase activity in vitro and mediates polyubiquitination and degradation of the Hairy transcriptional repressor; reducing dtopors gene dose antagonizes Hairy-mediated transcriptional repression, establishing a genetic epistasis relationship.","method":"In vitro ubiquitination assay with recombinant protein, genetic dose reduction, transcriptional repression assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — reconstituted in vitro ubiquitination, confirmed by genetic epistasis in Drosophila","pmids":["14871887"],"is_preprint":false},{"year":2009,"finding":"Plk1 phosphorylates TOPORS on Ser718 in vivo; phosphorylation by Plk1 inhibits TOPORS-mediated sumoylation of p53 while enhancing p53 ubiquitination, leading to p53 degradation; a Plk1-unphosphorylatable Topors mutant (S718A) causes dramatic p53 accumulation.","method":"In vivo phosphorylation assay, site-directed mutagenesis (S718A), p53 stability assay, sumoylation and ubiquitination assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1–2 — mutagenesis combined with functional sumoylation/ubiquitination assays and in vivo validation","pmids":["19473992"],"is_preprint":false},{"year":2002,"finding":"TOPORS localizes to PML nuclear bodies in a PML-dependent manner; the punctate nuclear localization requires a basic C-terminal region but not the N-terminal RING domain; TOPORS dynamically relocalizes from nuclear bodies to the nucleoplasm upon treatment with transcription inhibitor DRB or camptothecin.","method":"GFP fusion live imaging, isogenic PML+/+ vs PML−/− MEFs, domain deletion analysis, drug treatment","journal":"Experimental cell research","confidence":"High","confidence_rationale":"Tier 2 — direct localization experiments with functional domain mapping and genetic comparison","pmids":["12083797"],"is_preprint":false},{"year":2003,"finding":"TOPORS is itself a SUMO-1 modification target; it interacts with SUMO-1 and the SUMO-1 conjugating enzyme UBC9 in yeast two-hybrid; Lys560 is identified as a SUMO-1 acceptor site; sumoylation of TOPORS is not required for its localization to nuclear speckles.","method":"Yeast two-hybrid, co-transfection/co-immunoprecipitation, site-directed mutagenesis (K560), subcellular localization","journal":"Experimental cell research","confidence":"Medium","confidence_rationale":"Tier 2 — multiple methods (Y2H, mutagenesis, localization) in a single study","pmids":["14516784"],"is_preprint":false},{"year":2007,"finding":"TOPORS functions as a SUMO-1 E3 ligase for chromatin-modifying proteins; proteomic screen identified mSin3A as a sumoylation substrate for TOPORS, confirmed by transfection studies.","method":"Proteomic screen for SUMO substrates, transfection sumoylation assay","journal":"Journal of proteome research","confidence":"Medium","confidence_rationale":"Tier 2 — proteomic screen plus cellular sumoylation confirmation","pmids":["17803295"],"is_preprint":false},{"year":2007,"finding":"TOPORS induces accumulation of polysumoylated (SUMO-1 chain) forms of DNA topoisomerase I (TOP1) in a reconstituted in vitro system and in human osteosarcoma cells; point mutants of TOP1 identify a limited number of SUMO-1 acceptor sites responsible for the high-molecular weight conjugates.","method":"Reconstituted in vitro SUMO-1 conjugation assay, cellular sumoylation assay, TOP1 point mutant analysis","journal":"FEBS letters","confidence":"High","confidence_rationale":"Tier 1 — reconstituted in vitro with mutagenesis and cellular confirmation","pmids":["17976381"],"is_preprint":false},{"year":2005,"finding":"TOPORS/Topors is a coactivator of p53: overexpressed Topors associates with and stabilizes p53, enhances p53-dependent transcription of p21, MDM2, and Bax promoters, elevates endogenous p21 mRNA, and induces cell cycle arrest and apoptosis; Topors expression is induced by cisplatin and camptothecin.","method":"Co-immunoprecipitation, luciferase reporter assay, RT-PCR, cell cycle analysis, overexpression","journal":"Oncogene","confidence":"Medium","confidence_rationale":"Tier 2 — multiple orthogonal methods (Co-IP, reporter, mRNA, cell cycle) in single study","pmids":["15735665"],"is_preprint":false},{"year":2007,"finding":"TOPORS ubiquitinates NKX3.1 in vitro and in vivo; TOPORS overexpression leads to NKX3.1 proteasomal degradation in prostate cancer cells; siRNA knockdown of TOPORS increases NKX3.1 steady-state levels and prolongs its half-life.","method":"In vitro ubiquitination assay, cellular overexpression, siRNA knockdown, pulse-chase half-life analysis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1–2 — in vitro ubiquitination plus cellular KD/KO with defined substrate stability readout","pmids":["18077445"],"is_preprint":false},{"year":2010,"finding":"TOPORS localizes to the basal bodies/connecting cilium (cilia-centrosomal compartment) of photoreceptors, not only to nuclear bodies; morpholino knockdown of topors in zebrafish causes defective retinal development and failure to form outer segments, rescued by human TOPORS mRNA.","method":"Immunofluorescence/immunolocalization in retina, morpholino knockdown in zebrafish, mRNA rescue experiment","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 2 — direct localization with functional rescue in zebrafish model","pmids":["21159800"],"is_preprint":false},{"year":2010,"finding":"Topors-deficient mice exhibit increased malignant transformation, aneuploidy, and defective chromosomal segregation in primary embryonic fibroblasts; loss of Topors causes mislocalization of HP1alpha and increased transcription from pericentric major satellite DNA, indicating a role in maintenance of pericentric heterochromatin and genomic stability.","method":"Topors knockout mouse, MEF transformation assay, cytogenetics (aneuploidy), immunofluorescence (HP1alpha), RT-PCR (satellite transcription)","journal":"BMC molecular biology","confidence":"High","confidence_rationale":"Tier 2 — clean KO model with multiple defined cellular phenotypes and molecular markers","pmids":["20429939"],"is_preprint":false},{"year":2008,"finding":"Phosphorylation of TOPORS at Ser98 (adjacent to the RING domain) regulates its ubiquitin ligase activity and E2 (UbcH5a) binding: the phosphomimetic S98D mutant shows increased ubiquitin ligase activity and enhanced UbcH5a binding in vitro and in cells, while S98A shows little change; neither mutant affects SUMO ligase activity, indicating a binary switch between ubiquitination and sumoylation functions.","method":"Mass spectrometry phosphorylation mapping, site-directed mutagenesis (S98A/S98D), in vitro and cellular ubiquitination assays, Co-IP with E2, sumoylation assay","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 — MS-identified site, mutagenesis, and in vitro plus cellular functional assays","pmids":["19053840"],"is_preprint":false},{"year":2009,"finding":"Topors depletion delays mitotic entry and affects mitotic progression; Topors is degraded in response to spindle checkpoint activation; Plk1-mediated phosphorylation of Topors at S718 is essential for nocodazole-induced Topors degradation.","method":"siRNA knockdown, mitotic progression assay, spindle checkpoint activation, phosphorylation-deficient mutant","journal":"Molecular biology reports","confidence":"Medium","confidence_rationale":"Tier 2 — defined cellular phenotype (mitotic delay) with phosphorylation site mutagenesis","pmids":["19821153"],"is_preprint":false},{"year":2005,"finding":"TOPORS mediates sumoylation of p53 and DJ-1; DJ-1 binds TOPORS and p53 in vitro and in vivo; TOPORS-induced sumoylation of p53 abrogates p53 transcriptional activity in a dose-dependent manner; DJ-1 restores repressed p53 activity by releasing the sumoylated form of p53.","method":"Co-immunoprecipitation, in vitro binding assay, cellular sumoylation assay, p53 transcription reporter assay","journal":"International journal of oncology","confidence":"Medium","confidence_rationale":"Tier 2 — reciprocal Co-IP plus functional reporter assay demonstrating pathway antagonism","pmids":["15703819"],"is_preprint":false},{"year":2012,"finding":"TOPORS functions as a novel E3 ubiquitin ligase for H2AX in vitro; TOPORS interacts with H2AX in mammalian cells; overexpression of TOPORS decreases H2AX protein stability; TOPORS dissociates from H2AX under oxidative stress (H2O2, IR) but not replication-inducing stress (doxorubicin, CPT), providing a mechanism to discriminate genotoxic stresses.","method":"Co-immunoprecipitation from cell extracts, in vitro ubiquitination assay, overexpression/protein stability assay","journal":"Journal of biochemical and molecular toxicology","confidence":"Medium","confidence_rationale":"Tier 2 — in vitro ubiquitination assay with cellular Co-IP and stability readout","pmids":["22972498"],"is_preprint":false},{"year":2012,"finding":"TOPORS interacts with the cytoplasmic domain of syndecan-1 (Sdc-1); an 18-amino acid sequence of Topors is required for this interaction; siRNA knockdown of Topors abolishes the inhibitory effect of Sdc-1 on cell growth and PDGF-B induction in arterial smooth muscle cells.","method":"Yeast two-hybrid, ligand blotting, co-precipitation from cell lysates, immunohistochemistry, siRNA knockdown with growth assay","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 — Y2H confirmed by co-precipitation, plus functional KD with defined growth phenotype","pmids":["22912899"],"is_preprint":false},{"year":2016,"finding":"TOPORS interacts with the 26S proteasome regulatory subunit PSMC1 (P26s4/Rpt2); interaction validated by Co-IP from mammalian cell extracts; TOPORS and PSMC1 co-localize at the centrosome in cultured cells (hTERT-RPE1 and 661W) and PSMC1 shows strong immunoreactivity at the RPE/photoreceptor outer segment interface.","method":"Yeast two-hybrid (retinal cDNA library screen), Co-immunoprecipitation, immunofluorescent co-localization","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 3 — Y2H plus reciprocal Co-IP and co-localization without full functional validation","pmids":["26872363"],"is_preprint":false},{"year":2022,"finding":"TOPORS mediates SUMOylation of RAD51 at lysine residues K57 and K70 in response to DNA damage; this SUMOylation is facilitated by ATM-induced phosphorylation of TOPORS at Thr515; SUMOylated RAD51 is required for its chromatin loading and association with BRCA2; knockdown of TOPORS or expression of SUMOylation-deficient RAD51 reduces HR repair efficiency.","method":"Co-immunoprecipitation, in vitro SUMOylation assay, site-directed mutagenesis (K57/K70R, T515A), chromatin fractionation, HR repair assay (DR-GFP), siRNA knockdown","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 1–2 — reconstituted SUMO assay, mutagenesis of both substrate and enzyme, functional HR assay, multiple orthogonal methods","pmids":["35061896"],"is_preprint":false},{"year":2024,"finding":"TOPORS functions as a SUMO-targeted ubiquitin ligase (STUbL) that combines RING-dependent ubiquitin ligase activity with poly-SUMO binding via SUMO-interacting motifs; it is SUMO1-selective and acts in parallel with RNF4 to generate complex ubiquitin landscapes on SUMOylated targets (DPCs and PML), stimulating p97/VCP recruitment and proteasomal degradation; combined loss of TOPORS and RNF4 is synthetic lethal due to accumulation of SUMOylated proteins on chromatin, cell cycle arrest, and apoptosis.","method":"Genome-scale CRISPR screens, in vitro ubiquitination assay, SUMO-interacting motif analysis, Co-IP, proteomics, synthetic lethality assay, DPC repair assay","journal":"Nature structural & molecular biology","confidence":"High","confidence_rationale":"Tier 1 — genome-scale screen, reconstituted in vitro assays, multiple orthogonal methods, clear synthetic lethal epistasis","pmids":["38649616"],"is_preprint":false},{"year":2024,"finding":"TOPORS is recruited to SUMOylated DNMT1-DNA protein crosslinks (DPCs) and promotes their ubiquitination and proteasomal degradation; in TOPORS-knockout cells, SUMOylated DNMT1 accumulates and is not efficiently resolved after decitabine treatment; the RING finger domain of TOPORS is required for this ubiquitination activity (HMA resistance function).","method":"Genome-scale CRISPR/Cas9 screen, Co-IP (DNMT1-DPC proximal proteome), TOPORS KO cellular assay, RING domain mutant analysis, SUMOylation detection","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 2 — genome-scale screen plus mechanistic validation with proximity proteomics and domain mutagenesis","pmids":["38760575"],"is_preprint":false},{"year":2024,"finding":"TOPORS ubiquitinates SUMOylated DNMT1 to promote resolution of DNA-DNMT1 crosslinks in myeloid leukemia cells; TOPORS depletion leads to impaired DNA damage response and accumulation of SUMOylated DNMT1 after HMA treatment; this effect is recapitulated by SUMOylation inhibition (TAK-981).","method":"CRISPR-Cas9 screen, TOPORS KO, Co-IP, SUMOylation detection, xenograft survival assay","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 — genome-scale screen, mechanistic Co-IP, functional rescue in xenograft; replicated in two independent publications same year","pmids":["39198401"],"is_preprint":false},{"year":2025,"finding":"TOPORS is selectively recruited to PML when it reaches a threshold of SUMO1 (but not SUMO2/3) conjugation; arsenic-resistant PML mutant L217F fails to accumulate sufficient SUMO1 to recruit TOPORS, while A216T PML fails to be SUMOylated at all; both TOPORS and RNF4 are required for arsenic-induced polyubiquitination and p97-dependent PML degradation.","method":"PML-/- cell reconstitution with YFP-PML mutants, immunoprecipitation, SUMOylation analysis, arsenic treatment functional assay","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 — defined genetic reconstitution system with biochemical pathway dissection and functional drug response readout","pmids":["40239066"],"is_preprint":false},{"year":2020,"finding":"TOPORS knockdown in mouse hepatocytes increases chromatin interactions between A and B compartments, reduces compartmentalization strength, weakens TAD boundaries at A/B borders, decreases chromatin-lamina interactions (LAD coverage drops from 53.31% to 46.52%), and alters chromatin accessibility predominantly at intergenic regions including enhancers; these changes are associated with PML nuclear bodies.","method":"Hi-C, ChIP-seq (lamina association), ATAC-seq, RNA-seq following siRNA knockdown in mouse hepatocytes","journal":"Biochimica et biophysica acta. Gene regulatory mechanisms","confidence":"Medium","confidence_rationale":"Tier 2 — multiple genome-wide chromatin assays after defined KD with quantitative readouts","pmids":["32113985"],"is_preprint":false},{"year":2022,"finding":"TOPORS binds to the SMAR1 promoter (ChIP analysis) and its occupancy increases upon LPS treatment; LPS induces TOPORS expression via a TLR4-TRIF-dependent pathway, and TOPORS in turn activates transcription of tumor suppressor SMAR1.","method":"ChIP analysis, luciferase reporter assay, siRNA knockdown, LPS stimulation","journal":"Molecular oncology","confidence":"Medium","confidence_rationale":"Tier 2 — direct ChIP evidence of TOPORS promoter binding with functional pathway placement","pmids":["34689394"],"is_preprint":false},{"year":2025,"finding":"TOPORS is identified as a substrate of the deubiquitinase USP7 in TNBC cells; TOPORS interacts with the BRCA1-A DNA damage repair complex, suggesting a USP7-TOPORS-BRCA1-A axis.","method":"Quantitative proteomics after USP7 inhibition, Co-IP (BRCA1-A complex interaction)","journal":"bioRxiv","confidence":"Low","confidence_rationale":"Tier 3 — proteomic identification with single Co-IP, preprint without peer review","pmids":["bio_10.1101_2025.01.28.635372"],"is_preprint":true}],"current_model":"TOPORS is a dual E3 ubiquitin (RING-dependent, SUMO1-selective) and SUMO-1 ligase that functions as a SUMO-targeted ubiquitin ligase (STUbL), ubiquitinating poly-SUMOylated substrates including p53, TOP1, RAD51, NKX3.1, H2AX, and DNMT1-DNA crosslinks to drive their proteasomal degradation; its activities are regulated by phosphorylation (Plk1 at S718 and ATM at T515) and it localizes to PML nuclear bodies and centrosomes/basal bodies of cilia, where it maintains genomic stability, heterochromatin integrity, and photoreceptor function, while acting in parallel with RNF4 as a key effector of SUMO-ubiquitin crosstalk in DNA-protein crosslink repair and stress-induced PML degradation."},"narrative":{"teleology":[{"year":2002,"claim":"Establishing where TOPORS operates: its PML-dependent localization to nuclear bodies and dynamic redistribution upon genotoxic stress defined it as a PML body-resident factor responsive to DNA damage signals.","evidence":"GFP-fusion imaging in PML+/+ vs PML−/− MEFs with domain deletion and drug treatment","pmids":["12083797"],"confidence":"High","gaps":["Molecular basis of PML-dependent recruitment unknown at this stage","Functional consequence of relocalization not determined"]},{"year":2004,"claim":"Identifying TOPORS as an E3 ubiquitin ligase resolved its biochemical function: RING-dependent ubiquitination of p53 driving proteasomal degradation established it as a p53-regulatory enzyme, parallel to but independent of MDM2.","evidence":"Reconstituted in vitro ubiquitination with E2 panel, RING mutagenesis, cellular proteasome inhibitor experiments","pmids":["15247280","14871887"],"confidence":"High","gaps":["In vivo physiological relevance for p53 regulation not yet shown","Relationship between ubiquitination and nuclear body localization unclear"]},{"year":2005,"claim":"The discovery that TOPORS also acts as a SUMO-1 E3 ligase for p53—through a RING-independent mechanism—established it as a dual-function enzyme capable of both ubiquitination and SUMOylation of the same substrate with opposing consequences for p53 stability.","evidence":"Reconstituted in vitro SUMO-1 conjugation, RING mutant retaining SUMO ligase activity, cellular p53 stabilization","pmids":["16122737","15703819"],"confidence":"High","gaps":["Mechanism of RING-independent SUMO ligase activity not defined","How the cell selects between ubiquitination and SUMOylation of p53 was unknown"]},{"year":2007,"claim":"Expanding the substrate repertoire to include TOP1, NKX3.1, and mSin3A showed that TOPORS is not a p53-specific ligase but a general ubiquitin/SUMO E3 acting on diverse chromatin and transcription-associated proteins.","evidence":"In vitro ubiquitination/SUMOylation assays, siRNA knockdown with substrate half-life measurements, proteomic screen","pmids":["17976381","18077445","17803295"],"confidence":"High","gaps":["No unifying substrate-recognition mechanism identified","Relative physiological importance of different substrates unknown"]},{"year":2008,"claim":"Phosphorylation-dependent switching between ubiquitin and SUMO ligase activities was established: Ser98 phosphorylation enhances E2 binding and ubiquitination without affecting SUMOylation, providing the first binary regulatory mechanism for TOPORS's dual activity.","evidence":"Mass spectrometry phosphosite mapping, S98A/S98D mutagenesis with in vitro and cellular ubiquitination and SUMOylation assays","pmids":["19053840"],"confidence":"High","gaps":["Kinase responsible for Ser98 phosphorylation not identified","Whether this switch operates on all substrates not tested"]},{"year":2009,"claim":"Plk1 was identified as a physiological kinase that phosphorylates TOPORS at Ser718, shifting its activity from SUMOylation toward ubiquitination of p53 during mitosis; this also linked TOPORS to cell cycle control, as its depletion delayed mitotic entry.","evidence":"In vivo phosphorylation, S718A mutagenesis with p53 stability/SUMO/ubiquitin readouts, siRNA mitotic progression assay","pmids":["19473992","19821153"],"confidence":"High","gaps":["Full complement of mitotic substrates unknown","Whether Plk1 regulation of TOPORS is direct at the centrosome not tested"]},{"year":2010,"claim":"Genetic loss-of-function studies established TOPORS's physiological roles: knockout mice showed aneuploidy, HP1α mislocalization, and pericentric heterochromatin derepression, while zebrafish morphants failed to form photoreceptor outer segments—revealing functions in genomic stability, heterochromatin maintenance, and ciliary biology.","evidence":"Topors knockout mouse MEFs (cytogenetics, IF, satellite RT-PCR), zebrafish morpholino with human mRNA rescue, retinal immunolocalization to basal bodies","pmids":["20429939","21159800"],"confidence":"High","gaps":["Molecular substrates at the basal body/cilium unidentified","Whether photoreceptor defect is ubiquitin-dependent, SUMO-dependent, or both unknown","Human disease causality not established by these studies"]},{"year":2020,"claim":"TOPORS was shown to regulate higher-order chromatin architecture: its depletion weakened A/B compartment boundaries, reduced lamina-associated domain coverage, and altered chromatin accessibility at enhancers, linking its enzymatic activities to 3D genome organization.","evidence":"Hi-C, ChIP-seq (lamina), ATAC-seq, RNA-seq after siRNA knockdown in mouse hepatocytes","pmids":["32113985"],"confidence":"Medium","gaps":["Whether chromatin architecture changes are direct or secondary to heterochromatin loss not resolved","Specific TOPORS substrates mediating lamina attachment unknown"]},{"year":2022,"claim":"TOPORS was established as a DNA damage-responsive SUMO E3 for RAD51, with ATM phosphorylation at Thr515 activating this activity; SUMOylation of RAD51 at K57/K70 was required for chromatin loading, BRCA2 association, and efficient homologous recombination, placing TOPORS in the HR repair pathway.","evidence":"In vitro SUMOylation, K57R/K70R and T515A mutagenesis, chromatin fractionation, DR-GFP HR assay, siRNA","pmids":["35061896"],"confidence":"High","gaps":["Whether TOPORS SUMO ligase activity for RAD51 is redundant with other SUMO E3s not tested","Interplay between RAD51 SUMOylation and subsequent STUbL-mediated turnover not explored"]},{"year":2024,"claim":"The unifying mechanism was revealed: TOPORS functions as a bona fide SUMO-targeted ubiquitin ligase (STUbL), using SUMO-interacting motifs to recognize poly-SUMOylated substrates (DNMT1-DPCs, PML) and ubiquitinate them for p97/VCP-dependent proteasomal extraction; it acts in parallel with RNF4 with SUMO1 selectivity, and their combined loss is synthetic lethal.","evidence":"Genome-scale CRISPR screens in multiple labs, in vitro ubiquitination of SUMOylated substrates, SIM analysis, TOPORS KO DPC accumulation assays, synthetic lethality with RNF4, proximity proteomics","pmids":["38649616","38760575","39198401"],"confidence":"High","gaps":["Structural basis of SUMO1 selectivity over SUMO2/3 not determined","Full STUbL substrate repertoire not catalogued","Whether TOPORS can act as both the SUMO E3 and STUbL on the same substrate sequentially in vivo"]},{"year":2025,"claim":"TOPORS recruitment to PML was shown to require a threshold level of SUMO1 (not SUMO2/3) conjugation on PML, explaining how arsenic-resistant PML mutations evade TOPORS-dependent degradation and establishing the SUMO1-specificity mechanism in a clinically relevant context.","evidence":"PML−/− cell reconstitution with YFP-PML wild-type and arsenic-resistant mutants (L217F, A216T), immunoprecipitation, arsenic functional assay","pmids":["40239066"],"confidence":"High","gaps":["Whether TOPORS SUMO1 preference reflects SIM-SUMO1 affinity or substrate presentation is unresolved","Clinical impact of TOPORS loss on arsenic-based APL therapy not tested"]},{"year":null,"claim":"Key unresolved questions include the structural basis of TOPORS's dual E3 activities and SUMO1 selectivity, whether it acts sequentially as SUMO E3 and STUbL on the same substrate in vivo, the identity of its ciliary/centrosomal substrates relevant to photoreceptor maintenance, and whether TOPORS loss causally contributes to human retinal disease.","evidence":"","pmids":[],"confidence":"Low","gaps":["No crystal or cryo-EM structure of TOPORS or its SUMO/ubiquitin ligase domains","Ciliary/centrosomal substrates unidentified","Human Mendelian disease causation not established despite photoreceptor phenotype in zebrafish"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0016874","term_label":"ligase activity","supporting_discovery_ids":[0,2,9,15,19,20,21]},{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[0,1,7,9,15,18,19,20,21,22]},{"term_id":"GO:0140110","term_label":"transcription regulator activity","supporting_discovery_ids":[8,24]}],"localization":[{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[4,5,11,23]},{"term_id":"GO:0005815","term_label":"microtubule organizing center","supporting_discovery_ids":[10,17]}],"pathway":[{"term_id":"R-HSA-73894","term_label":"DNA Repair","supporting_discovery_ids":[18,19,20,21]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[0,1,7,9,15,19,20,21,22]},{"term_id":"R-HSA-1640170","term_label":"Cell Cycle","supporting_discovery_ids":[3,13]},{"term_id":"R-HSA-4839726","term_label":"Chromatin organization","supporting_discovery_ids":[11,23]},{"term_id":"R-HSA-5357801","term_label":"Programmed Cell Death","supporting_discovery_ids":[8,19]}],"complexes":[],"partners":["TP53","PML","TOP1","RAD51","NKX3.1","RNF4","DNMT1","PLK1"],"other_free_text":[]},"mechanistic_narrative":"TOPORS is a dual-function E3 ligase that catalyzes both RING-dependent ubiquitination and SUMO-1 conjugation of nuclear substrates, functioning as a SUMO-targeted ubiquitin ligase (STUbL) that recognizes poly-SUMOylated proteins through SUMO-interacting motifs and marks them for proteasomal degradation [PMID:15247280, PMID:16122737, PMID:38649616]. Its ubiquitin and SUMO ligase activities are differentially regulated by phosphorylation—Ser98 phosphorylation enhances E2 binding and ubiquitination, while Plk1-mediated Ser718 phosphorylation shifts the balance from sumoylation to ubiquitination of p53, and ATM-mediated Thr515 phosphorylation promotes RAD51 SUMOylation required for homologous recombination [PMID:19053840, PMID:19473992, PMID:35061896]. TOPORS localizes to PML nuclear bodies in a SUMO1-dependent manner and to centrosomes/basal bodies of photoreceptors, where it acts in parallel with RNF4 to ubiquitinate SUMOylated substrates including PML, DNMT1-DNA crosslinks, and chromatin-associated proteins, maintaining genomic stability, pericentric heterochromatin integrity, and higher-order chromatin compartmentalization [PMID:12083797, PMID:20429939, PMID:38760575, PMID:40239066, PMID:32113985]. Loss-of-function in zebrafish causes photoreceptor outer segment failure, and in mice causes aneuploidy and increased malignant transformation, while combined loss with RNF4 is synthetic lethal due to toxic accumulation of SUMOylated chromatin proteins [PMID:21159800, PMID:20429939, PMID:38649616]."},"prefetch_data":{"uniprot":{"accession":"Q9NS56","full_name":"E3 ubiquitin-protein ligase Topors","aliases":["RING-type E3 ubiquitin transferase Topors","SUMO1-protein E3 ligase Topors","Topoisomerase I-binding RING finger protein","Topoisomerase I-binding arginine/serine-rich protein","Tumor suppressor p53-binding protein 3","p53-binding protein 3","p53BP3"],"length_aa":1045,"mass_kda":119.2,"function":"Functions as an E3 ubiquitin-protein ligase and as an E3 SUMO1-protein ligase. Probable tumor suppressor involved in cell growth, cell proliferation and apoptosis that regulates p53/TP53 stability through ubiquitin-dependent degradation. May regulate chromatin modification through sumoylation of several chromatin modification-associated proteins. May be involved in DNA damage-induced cell death through IKBKE sumoylation","subcellular_location":"Nucleus; Nucleus, PML body","url":"https://www.uniprot.org/uniprotkb/Q9NS56/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/TOPORS","classification":"Not Classified","n_dependent_lines":2,"n_total_lines":1208,"dependency_fraction":0.0016556291390728477},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/TOPORS","total_profiled":1310},"omim":[{"mim_id":"609923","title":"RETINITIS PIGMENTOSA 31; RP31","url":"https://www.omim.org/entry/609923"},{"mim_id":"609507","title":"TOPOISOMERASE I-BINDING ARGININE/SERINE-RICH PROTEIN; TOPORS","url":"https://www.omim.org/entry/609507"},{"mim_id":"268000","title":"RETINITIS PIGMENTOSA; RP","url":"https://www.omim.org/entry/268000"}],"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/TOPORS"},"hgnc":{"alias_symbol":["TP53BPL","LUN"],"prev_symbol":["RP31"]},"alphafold":{"accession":"Q9NS56","domains":[{"cath_id":"-","chopping":"114-163","consensus_level":"high","plddt":89.5592,"start":114,"end":163},{"cath_id":"-","chopping":"246-375","consensus_level":"high","plddt":88.1779,"start":246,"end":375}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9NS56","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9NS56-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9NS56-F1-predicted_aligned_error_v6.png","plddt_mean":49.03},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=TOPORS","jax_strain_url":"https://www.jax.org/strain/search?query=TOPORS"},"sequence":{"accession":"Q9NS56","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9NS56.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9NS56/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9NS56"}},"corpus_meta":[{"pmid":"15247280","id":"PMC_15247280","title":"Topors 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a conserved tryptophan in the RING domain is required for ubiquitination activity.\",\n      \"method\": \"In vitro ubiquitination assay, active-site mutagenesis, cellular overexpression with proteasome inhibitor, Co-IP\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstituted in vitro ubiquitination with mutagenesis, replicated in cells\",\n      \"pmids\": [\"15247280\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"TOPORS acts as a SUMO-1 E3 ligase for p53, enhancing SUMO-1 conjugation to p53 in vivo and in a reconstituted in vitro system; this activity does not depend on the RING finger motif and is accompanied by an increase in endogenous p53 protein levels in HeLa cells.\",\n      \"method\": \"Reconstituted in vitro SUMO-1 conjugation assay, cellular sumoylation assay, RING domain mutant analysis\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstituted in vitro with domain mapping, replicated in cells\",\n      \"pmids\": [\"16122737\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Drosophila Topors (dTopors) ortholog possesses E3 ubiquitin ligase activity in vitro and mediates polyubiquitination and degradation of the Hairy transcriptional repressor; reducing dtopors gene dose antagonizes Hairy-mediated transcriptional repression, establishing a genetic epistasis relationship.\",\n      \"method\": \"In vitro ubiquitination assay with recombinant protein, genetic dose reduction, transcriptional repression assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstituted in vitro ubiquitination, confirmed by genetic epistasis in Drosophila\",\n      \"pmids\": [\"14871887\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Plk1 phosphorylates TOPORS on Ser718 in vivo; phosphorylation by Plk1 inhibits TOPORS-mediated sumoylation of p53 while enhancing p53 ubiquitination, leading to p53 degradation; a Plk1-unphosphorylatable Topors mutant (S718A) causes dramatic p53 accumulation.\",\n      \"method\": \"In vivo phosphorylation assay, site-directed mutagenesis (S718A), p53 stability assay, sumoylation and ubiquitination assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — mutagenesis combined with functional sumoylation/ubiquitination assays and in vivo validation\",\n      \"pmids\": [\"19473992\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"TOPORS localizes to PML nuclear bodies in a PML-dependent manner; the punctate nuclear localization requires a basic C-terminal region but not the N-terminal RING domain; TOPORS dynamically relocalizes from nuclear bodies to the nucleoplasm upon treatment with transcription inhibitor DRB or camptothecin.\",\n      \"method\": \"GFP fusion live imaging, isogenic PML+/+ vs PML−/− MEFs, domain deletion analysis, drug treatment\",\n      \"journal\": \"Experimental cell research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct localization experiments with functional domain mapping and genetic comparison\",\n      \"pmids\": [\"12083797\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"TOPORS is itself a SUMO-1 modification target; it interacts with SUMO-1 and the SUMO-1 conjugating enzyme UBC9 in yeast two-hybrid; Lys560 is identified as a SUMO-1 acceptor site; sumoylation of TOPORS is not required for its localization to nuclear speckles.\",\n      \"method\": \"Yeast two-hybrid, co-transfection/co-immunoprecipitation, site-directed mutagenesis (K560), subcellular localization\",\n      \"journal\": \"Experimental cell research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple methods (Y2H, mutagenesis, localization) in a single study\",\n      \"pmids\": [\"14516784\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"TOPORS functions as a SUMO-1 E3 ligase for chromatin-modifying proteins; proteomic screen identified mSin3A as a sumoylation substrate for TOPORS, confirmed by transfection studies.\",\n      \"method\": \"Proteomic screen for SUMO substrates, transfection sumoylation assay\",\n      \"journal\": \"Journal of proteome research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — proteomic screen plus cellular sumoylation confirmation\",\n      \"pmids\": [\"17803295\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"TOPORS induces accumulation of polysumoylated (SUMO-1 chain) forms of DNA topoisomerase I (TOP1) in a reconstituted in vitro system and in human osteosarcoma cells; point mutants of TOP1 identify a limited number of SUMO-1 acceptor sites responsible for the high-molecular weight conjugates.\",\n      \"method\": \"Reconstituted in vitro SUMO-1 conjugation assay, cellular sumoylation assay, TOP1 point mutant analysis\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstituted in vitro with mutagenesis and cellular confirmation\",\n      \"pmids\": [\"17976381\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"TOPORS/Topors is a coactivator of p53: overexpressed Topors associates with and stabilizes p53, enhances p53-dependent transcription of p21, MDM2, and Bax promoters, elevates endogenous p21 mRNA, and induces cell cycle arrest and apoptosis; Topors expression is induced by cisplatin and camptothecin.\",\n      \"method\": \"Co-immunoprecipitation, luciferase reporter assay, RT-PCR, cell cycle analysis, overexpression\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (Co-IP, reporter, mRNA, cell cycle) in single study\",\n      \"pmids\": [\"15735665\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"TOPORS ubiquitinates NKX3.1 in vitro and in vivo; TOPORS overexpression leads to NKX3.1 proteasomal degradation in prostate cancer cells; siRNA knockdown of TOPORS increases NKX3.1 steady-state levels and prolongs its half-life.\",\n      \"method\": \"In vitro ubiquitination assay, cellular overexpression, siRNA knockdown, pulse-chase half-life analysis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vitro ubiquitination plus cellular KD/KO with defined substrate stability readout\",\n      \"pmids\": [\"18077445\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"TOPORS localizes to the basal bodies/connecting cilium (cilia-centrosomal compartment) of photoreceptors, not only to nuclear bodies; morpholino knockdown of topors in zebrafish causes defective retinal development and failure to form outer segments, rescued by human TOPORS mRNA.\",\n      \"method\": \"Immunofluorescence/immunolocalization in retina, morpholino knockdown in zebrafish, mRNA rescue experiment\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct localization with functional rescue in zebrafish model\",\n      \"pmids\": [\"21159800\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Topors-deficient mice exhibit increased malignant transformation, aneuploidy, and defective chromosomal segregation in primary embryonic fibroblasts; loss of Topors causes mislocalization of HP1alpha and increased transcription from pericentric major satellite DNA, indicating a role in maintenance of pericentric heterochromatin and genomic stability.\",\n      \"method\": \"Topors knockout mouse, MEF transformation assay, cytogenetics (aneuploidy), immunofluorescence (HP1alpha), RT-PCR (satellite transcription)\",\n      \"journal\": \"BMC molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO model with multiple defined cellular phenotypes and molecular markers\",\n      \"pmids\": [\"20429939\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Phosphorylation of TOPORS at Ser98 (adjacent to the RING domain) regulates its ubiquitin ligase activity and E2 (UbcH5a) binding: the phosphomimetic S98D mutant shows increased ubiquitin ligase activity and enhanced UbcH5a binding in vitro and in cells, while S98A shows little change; neither mutant affects SUMO ligase activity, indicating a binary switch between ubiquitination and sumoylation functions.\",\n      \"method\": \"Mass spectrometry phosphorylation mapping, site-directed mutagenesis (S98A/S98D), in vitro and cellular ubiquitination assays, Co-IP with E2, sumoylation assay\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — MS-identified site, mutagenesis, and in vitro plus cellular functional assays\",\n      \"pmids\": [\"19053840\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Topors depletion delays mitotic entry and affects mitotic progression; Topors is degraded in response to spindle checkpoint activation; Plk1-mediated phosphorylation of Topors at S718 is essential for nocodazole-induced Topors degradation.\",\n      \"method\": \"siRNA knockdown, mitotic progression assay, spindle checkpoint activation, phosphorylation-deficient mutant\",\n      \"journal\": \"Molecular biology reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — defined cellular phenotype (mitotic delay) with phosphorylation site mutagenesis\",\n      \"pmids\": [\"19821153\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"TOPORS mediates sumoylation of p53 and DJ-1; DJ-1 binds TOPORS and p53 in vitro and in vivo; TOPORS-induced sumoylation of p53 abrogates p53 transcriptional activity in a dose-dependent manner; DJ-1 restores repressed p53 activity by releasing the sumoylated form of p53.\",\n      \"method\": \"Co-immunoprecipitation, in vitro binding assay, cellular sumoylation assay, p53 transcription reporter assay\",\n      \"journal\": \"International journal of oncology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP plus functional reporter assay demonstrating pathway antagonism\",\n      \"pmids\": [\"15703819\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"TOPORS functions as a novel E3 ubiquitin ligase for H2AX in vitro; TOPORS interacts with H2AX in mammalian cells; overexpression of TOPORS decreases H2AX protein stability; TOPORS dissociates from H2AX under oxidative stress (H2O2, IR) but not replication-inducing stress (doxorubicin, CPT), providing a mechanism to discriminate genotoxic stresses.\",\n      \"method\": \"Co-immunoprecipitation from cell extracts, in vitro ubiquitination assay, overexpression/protein stability assay\",\n      \"journal\": \"Journal of biochemical and molecular toxicology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vitro ubiquitination assay with cellular Co-IP and stability readout\",\n      \"pmids\": [\"22972498\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"TOPORS interacts with the cytoplasmic domain of syndecan-1 (Sdc-1); an 18-amino acid sequence of Topors is required for this interaction; siRNA knockdown of Topors abolishes the inhibitory effect of Sdc-1 on cell growth and PDGF-B induction in arterial smooth muscle cells.\",\n      \"method\": \"Yeast two-hybrid, ligand blotting, co-precipitation from cell lysates, immunohistochemistry, siRNA knockdown with growth assay\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — Y2H confirmed by co-precipitation, plus functional KD with defined growth phenotype\",\n      \"pmids\": [\"22912899\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"TOPORS interacts with the 26S proteasome regulatory subunit PSMC1 (P26s4/Rpt2); interaction validated by Co-IP from mammalian cell extracts; TOPORS and PSMC1 co-localize at the centrosome in cultured cells (hTERT-RPE1 and 661W) and PSMC1 shows strong immunoreactivity at the RPE/photoreceptor outer segment interface.\",\n      \"method\": \"Yeast two-hybrid (retinal cDNA library screen), Co-immunoprecipitation, immunofluorescent co-localization\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — Y2H plus reciprocal Co-IP and co-localization without full functional validation\",\n      \"pmids\": [\"26872363\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"TOPORS mediates SUMOylation of RAD51 at lysine residues K57 and K70 in response to DNA damage; this SUMOylation is facilitated by ATM-induced phosphorylation of TOPORS at Thr515; SUMOylated RAD51 is required for its chromatin loading and association with BRCA2; knockdown of TOPORS or expression of SUMOylation-deficient RAD51 reduces HR repair efficiency.\",\n      \"method\": \"Co-immunoprecipitation, in vitro SUMOylation assay, site-directed mutagenesis (K57/K70R, T515A), chromatin fractionation, HR repair assay (DR-GFP), siRNA knockdown\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — reconstituted SUMO assay, mutagenesis of both substrate and enzyme, functional HR assay, multiple orthogonal methods\",\n      \"pmids\": [\"35061896\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"TOPORS functions as a SUMO-targeted ubiquitin ligase (STUbL) that combines RING-dependent ubiquitin ligase activity with poly-SUMO binding via SUMO-interacting motifs; it is SUMO1-selective and acts in parallel with RNF4 to generate complex ubiquitin landscapes on SUMOylated targets (DPCs and PML), stimulating p97/VCP recruitment and proteasomal degradation; combined loss of TOPORS and RNF4 is synthetic lethal due to accumulation of SUMOylated proteins on chromatin, cell cycle arrest, and apoptosis.\",\n      \"method\": \"Genome-scale CRISPR screens, in vitro ubiquitination assay, SUMO-interacting motif analysis, Co-IP, proteomics, synthetic lethality assay, DPC repair assay\",\n      \"journal\": \"Nature structural & molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — genome-scale screen, reconstituted in vitro assays, multiple orthogonal methods, clear synthetic lethal epistasis\",\n      \"pmids\": [\"38649616\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"TOPORS is recruited to SUMOylated DNMT1-DNA protein crosslinks (DPCs) and promotes their ubiquitination and proteasomal degradation; in TOPORS-knockout cells, SUMOylated DNMT1 accumulates and is not efficiently resolved after decitabine treatment; the RING finger domain of TOPORS is required for this ubiquitination activity (HMA resistance function).\",\n      \"method\": \"Genome-scale CRISPR/Cas9 screen, Co-IP (DNMT1-DPC proximal proteome), TOPORS KO cellular assay, RING domain mutant analysis, SUMOylation detection\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genome-scale screen plus mechanistic validation with proximity proteomics and domain mutagenesis\",\n      \"pmids\": [\"38760575\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"TOPORS ubiquitinates SUMOylated DNMT1 to promote resolution of DNA-DNMT1 crosslinks in myeloid leukemia cells; TOPORS depletion leads to impaired DNA damage response and accumulation of SUMOylated DNMT1 after HMA treatment; this effect is recapitulated by SUMOylation inhibition (TAK-981).\",\n      \"method\": \"CRISPR-Cas9 screen, TOPORS KO, Co-IP, SUMOylation detection, xenograft survival assay\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genome-scale screen, mechanistic Co-IP, functional rescue in xenograft; replicated in two independent publications same year\",\n      \"pmids\": [\"39198401\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"TOPORS is selectively recruited to PML when it reaches a threshold of SUMO1 (but not SUMO2/3) conjugation; arsenic-resistant PML mutant L217F fails to accumulate sufficient SUMO1 to recruit TOPORS, while A216T PML fails to be SUMOylated at all; both TOPORS and RNF4 are required for arsenic-induced polyubiquitination and p97-dependent PML degradation.\",\n      \"method\": \"PML-/- cell reconstitution with YFP-PML mutants, immunoprecipitation, SUMOylation analysis, arsenic treatment functional assay\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — defined genetic reconstitution system with biochemical pathway dissection and functional drug response readout\",\n      \"pmids\": [\"40239066\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"TOPORS knockdown in mouse hepatocytes increases chromatin interactions between A and B compartments, reduces compartmentalization strength, weakens TAD boundaries at A/B borders, decreases chromatin-lamina interactions (LAD coverage drops from 53.31% to 46.52%), and alters chromatin accessibility predominantly at intergenic regions including enhancers; these changes are associated with PML nuclear bodies.\",\n      \"method\": \"Hi-C, ChIP-seq (lamina association), ATAC-seq, RNA-seq following siRNA knockdown in mouse hepatocytes\",\n      \"journal\": \"Biochimica et biophysica acta. Gene regulatory mechanisms\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple genome-wide chromatin assays after defined KD with quantitative readouts\",\n      \"pmids\": [\"32113985\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"TOPORS binds to the SMAR1 promoter (ChIP analysis) and its occupancy increases upon LPS treatment; LPS induces TOPORS expression via a TLR4-TRIF-dependent pathway, and TOPORS in turn activates transcription of tumor suppressor SMAR1.\",\n      \"method\": \"ChIP analysis, luciferase reporter assay, siRNA knockdown, LPS stimulation\",\n      \"journal\": \"Molecular oncology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct ChIP evidence of TOPORS promoter binding with functional pathway placement\",\n      \"pmids\": [\"34689394\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"TOPORS is identified as a substrate of the deubiquitinase USP7 in TNBC cells; TOPORS interacts with the BRCA1-A DNA damage repair complex, suggesting a USP7-TOPORS-BRCA1-A axis.\",\n      \"method\": \"Quantitative proteomics after USP7 inhibition, Co-IP (BRCA1-A complex interaction)\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — proteomic identification with single Co-IP, preprint without peer review\",\n      \"pmids\": [\"bio_10.1101_2025.01.28.635372\"],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"TOPORS is a dual E3 ubiquitin (RING-dependent, SUMO1-selective) and SUMO-1 ligase that functions as a SUMO-targeted ubiquitin ligase (STUbL), ubiquitinating poly-SUMOylated substrates including p53, TOP1, RAD51, NKX3.1, H2AX, and DNMT1-DNA crosslinks to drive their proteasomal degradation; its activities are regulated by phosphorylation (Plk1 at S718 and ATM at T515) and it localizes to PML nuclear bodies and centrosomes/basal bodies of cilia, where it maintains genomic stability, heterochromatin integrity, and photoreceptor function, while acting in parallel with RNF4 as a key effector of SUMO-ubiquitin crosstalk in DNA-protein crosslink repair and stress-induced PML degradation.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"TOPORS is a dual-function E3 ligase that catalyzes both RING-dependent ubiquitination and SUMO-1 conjugation of nuclear substrates, functioning as a SUMO-targeted ubiquitin ligase (STUbL) that recognizes poly-SUMOylated proteins through SUMO-interacting motifs and marks them for proteasomal degradation [PMID:15247280, PMID:16122737, PMID:38649616]. Its ubiquitin and SUMO ligase activities are differentially regulated by phosphorylation—Ser98 phosphorylation enhances E2 binding and ubiquitination, while Plk1-mediated Ser718 phosphorylation shifts the balance from sumoylation to ubiquitination of p53, and ATM-mediated Thr515 phosphorylation promotes RAD51 SUMOylation required for homologous recombination [PMID:19053840, PMID:19473992, PMID:35061896]. TOPORS localizes to PML nuclear bodies in a SUMO1-dependent manner and to centrosomes/basal bodies of photoreceptors, where it acts in parallel with RNF4 to ubiquitinate SUMOylated substrates including PML, DNMT1-DNA crosslinks, and chromatin-associated proteins, maintaining genomic stability, pericentric heterochromatin integrity, and higher-order chromatin compartmentalization [PMID:12083797, PMID:20429939, PMID:38760575, PMID:40239066, PMID:32113985]. Loss-of-function in zebrafish causes photoreceptor outer segment failure, and in mice causes aneuploidy and increased malignant transformation, while combined loss with RNF4 is synthetic lethal due to toxic accumulation of SUMOylated chromatin proteins [PMID:21159800, PMID:20429939, PMID:38649616].\",\n  \"teleology\": [\n    {\n      \"year\": 2002,\n      \"claim\": \"Establishing where TOPORS operates: its PML-dependent localization to nuclear bodies and dynamic redistribution upon genotoxic stress defined it as a PML body-resident factor responsive to DNA damage signals.\",\n      \"evidence\": \"GFP-fusion imaging in PML+/+ vs PML−/− MEFs with domain deletion and drug treatment\",\n      \"pmids\": [\"12083797\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular basis of PML-dependent recruitment unknown at this stage\", \"Functional consequence of relocalization not determined\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Identifying TOPORS as an E3 ubiquitin ligase resolved its biochemical function: RING-dependent ubiquitination of p53 driving proteasomal degradation established it as a p53-regulatory enzyme, parallel to but independent of MDM2.\",\n      \"evidence\": \"Reconstituted in vitro ubiquitination with E2 panel, RING mutagenesis, cellular proteasome inhibitor experiments\",\n      \"pmids\": [\"15247280\", \"14871887\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo physiological relevance for p53 regulation not yet shown\", \"Relationship between ubiquitination and nuclear body localization unclear\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"The discovery that TOPORS also acts as a SUMO-1 E3 ligase for p53—through a RING-independent mechanism—established it as a dual-function enzyme capable of both ubiquitination and SUMOylation of the same substrate with opposing consequences for p53 stability.\",\n      \"evidence\": \"Reconstituted in vitro SUMO-1 conjugation, RING mutant retaining SUMO ligase activity, cellular p53 stabilization\",\n      \"pmids\": [\"16122737\", \"15703819\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of RING-independent SUMO ligase activity not defined\", \"How the cell selects between ubiquitination and SUMOylation of p53 was unknown\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Expanding the substrate repertoire to include TOP1, NKX3.1, and mSin3A showed that TOPORS is not a p53-specific ligase but a general ubiquitin/SUMO E3 acting on diverse chromatin and transcription-associated proteins.\",\n      \"evidence\": \"In vitro ubiquitination/SUMOylation assays, siRNA knockdown with substrate half-life measurements, proteomic screen\",\n      \"pmids\": [\"17976381\", \"18077445\", \"17803295\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No unifying substrate-recognition mechanism identified\", \"Relative physiological importance of different substrates unknown\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Phosphorylation-dependent switching between ubiquitin and SUMO ligase activities was established: Ser98 phosphorylation enhances E2 binding and ubiquitination without affecting SUMOylation, providing the first binary regulatory mechanism for TOPORS's dual activity.\",\n      \"evidence\": \"Mass spectrometry phosphosite mapping, S98A/S98D mutagenesis with in vitro and cellular ubiquitination and SUMOylation assays\",\n      \"pmids\": [\"19053840\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Kinase responsible for Ser98 phosphorylation not identified\", \"Whether this switch operates on all substrates not tested\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Plk1 was identified as a physiological kinase that phosphorylates TOPORS at Ser718, shifting its activity from SUMOylation toward ubiquitination of p53 during mitosis; this also linked TOPORS to cell cycle control, as its depletion delayed mitotic entry.\",\n      \"evidence\": \"In vivo phosphorylation, S718A mutagenesis with p53 stability/SUMO/ubiquitin readouts, siRNA mitotic progression assay\",\n      \"pmids\": [\"19473992\", \"19821153\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Full complement of mitotic substrates unknown\", \"Whether Plk1 regulation of TOPORS is direct at the centrosome not tested\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Genetic loss-of-function studies established TOPORS's physiological roles: knockout mice showed aneuploidy, HP1α mislocalization, and pericentric heterochromatin derepression, while zebrafish morphants failed to form photoreceptor outer segments—revealing functions in genomic stability, heterochromatin maintenance, and ciliary biology.\",\n      \"evidence\": \"Topors knockout mouse MEFs (cytogenetics, IF, satellite RT-PCR), zebrafish morpholino with human mRNA rescue, retinal immunolocalization to basal bodies\",\n      \"pmids\": [\"20429939\", \"21159800\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular substrates at the basal body/cilium unidentified\", \"Whether photoreceptor defect is ubiquitin-dependent, SUMO-dependent, or both unknown\", \"Human disease causality not established by these studies\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"TOPORS was shown to regulate higher-order chromatin architecture: its depletion weakened A/B compartment boundaries, reduced lamina-associated domain coverage, and altered chromatin accessibility at enhancers, linking its enzymatic activities to 3D genome organization.\",\n      \"evidence\": \"Hi-C, ChIP-seq (lamina), ATAC-seq, RNA-seq after siRNA knockdown in mouse hepatocytes\",\n      \"pmids\": [\"32113985\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether chromatin architecture changes are direct or secondary to heterochromatin loss not resolved\", \"Specific TOPORS substrates mediating lamina attachment unknown\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"TOPORS was established as a DNA damage-responsive SUMO E3 for RAD51, with ATM phosphorylation at Thr515 activating this activity; SUMOylation of RAD51 at K57/K70 was required for chromatin loading, BRCA2 association, and efficient homologous recombination, placing TOPORS in the HR repair pathway.\",\n      \"evidence\": \"In vitro SUMOylation, K57R/K70R and T515A mutagenesis, chromatin fractionation, DR-GFP HR assay, siRNA\",\n      \"pmids\": [\"35061896\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether TOPORS SUMO ligase activity for RAD51 is redundant with other SUMO E3s not tested\", \"Interplay between RAD51 SUMOylation and subsequent STUbL-mediated turnover not explored\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"The unifying mechanism was revealed: TOPORS functions as a bona fide SUMO-targeted ubiquitin ligase (STUbL), using SUMO-interacting motifs to recognize poly-SUMOylated substrates (DNMT1-DPCs, PML) and ubiquitinate them for p97/VCP-dependent proteasomal extraction; it acts in parallel with RNF4 with SUMO1 selectivity, and their combined loss is synthetic lethal.\",\n      \"evidence\": \"Genome-scale CRISPR screens in multiple labs, in vitro ubiquitination of SUMOylated substrates, SIM analysis, TOPORS KO DPC accumulation assays, synthetic lethality with RNF4, proximity proteomics\",\n      \"pmids\": [\"38649616\", \"38760575\", \"39198401\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of SUMO1 selectivity over SUMO2/3 not determined\", \"Full STUbL substrate repertoire not catalogued\", \"Whether TOPORS can act as both the SUMO E3 and STUbL on the same substrate sequentially in vivo\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"TOPORS recruitment to PML was shown to require a threshold level of SUMO1 (not SUMO2/3) conjugation on PML, explaining how arsenic-resistant PML mutations evade TOPORS-dependent degradation and establishing the SUMO1-specificity mechanism in a clinically relevant context.\",\n      \"evidence\": \"PML−/− cell reconstitution with YFP-PML wild-type and arsenic-resistant mutants (L217F, A216T), immunoprecipitation, arsenic functional assay\",\n      \"pmids\": [\"40239066\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether TOPORS SUMO1 preference reflects SIM-SUMO1 affinity or substrate presentation is unresolved\", \"Clinical impact of TOPORS loss on arsenic-based APL therapy not tested\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the structural basis of TOPORS's dual E3 activities and SUMO1 selectivity, whether it acts sequentially as SUMO E3 and STUbL on the same substrate in vivo, the identity of its ciliary/centrosomal substrates relevant to photoreceptor maintenance, and whether TOPORS loss causally contributes to human retinal disease.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No crystal or cryo-EM structure of TOPORS or its SUMO/ubiquitin ligase domains\", \"Ciliary/centrosomal substrates unidentified\", \"Human Mendelian disease causation not established despite photoreceptor phenotype in zebrafish\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0016874\", \"supporting_discovery_ids\": [0, 2, 9, 15, 19, 20, 21]},\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [0, 1, 7, 9, 15, 18, 19, 20, 21, 22]},\n      {\"term_id\": \"GO:0140110\", \"supporting_discovery_ids\": [8, 24]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [4, 5, 11, 23]},\n      {\"term_id\": \"GO:0005815\", \"supporting_discovery_ids\": [10, 17]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-73894\", \"supporting_discovery_ids\": [18, 19, 20, 21]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [0, 1, 7, 9, 15, 19, 20, 21, 22]},\n      {\"term_id\": \"R-HSA-1640170\", \"supporting_discovery_ids\": [3, 13]},\n      {\"term_id\": \"R-HSA-4839726\", \"supporting_discovery_ids\": [11, 23]},\n      {\"term_id\": \"R-HSA-5357801\", \"supporting_discovery_ids\": [8, 19]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"TP53\",\n      \"PML\",\n      \"TOP1\",\n      \"RAD51\",\n      \"NKX3.1\",\n      \"RNF4\",\n      \"DNMT1\",\n      \"PLK1\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}