{"gene":"UBQLN4","run_date":"2026-04-28T21:43:00","timeline":{"discoveries":[{"year":2019,"finding":"UBQLN4 is phosphorylated by ATM kinase and interacts with ubiquitylated MRE11 to mediate early steps of homologous recombination-mediated DNA double-strand break repair (HRR). UBQLN4 removes MRE11 from damaged chromatin via proteasomal degradation, thereby curtailing HRR activity. Loss of UBQLN4 leads to chromatin retention of MRE11 and non-physiological HRR; overexpression represses HRR and favors non-homologous end joining (NHEJ).","method":"Co-immunoprecipitation, in vitro and in vivo DSB repair assays, ATM phosphorylation assays, loss-of-function and overexpression experiments with defined phenotypic readouts","journal":"Cell","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (Co-IP, in vitro/in vivo assays, genetic epistasis), strong mechanistic detail, high-citation foundational paper","pmids":["30612738"],"is_preprint":false},{"year":2019,"finding":"UBQLN4 promotes NHEJ by repressing DNA end-resection in an ATM-dependent manner; the ATM-UBQLN4-MRE11 axis limits excessive end resection after DSBs.","method":"Epistasis analysis and mechanistic commentary elaborating on the UBQLN4-MRE11-ATM interaction","journal":"Molecular & cellular oncology","confidence":"Medium","confidence_rationale":"Tier 3 — commentary/summary with genetic epistasis framing; mechanistic interpretation supported by primary Cell paper","pmids":["31131301"],"is_preprint":false},{"year":2016,"finding":"UBQLN4 recognizes mislocalized transmembrane domain proteins in the cytoplasm (via their exposed transmembrane segments) and targets them to the proteasome for degradation, acting as a BAG6-binding quality control factor for newly synthesized defective polypeptides that fail to reach the ER.","method":"Co-immunoprecipitation, knockdown of SRP54, model truncated transmembrane domain protein degradation assays, proteasome inhibitor experiments","journal":"EMBO reports","confidence":"High","confidence_rationale":"Tier 2 — reciprocal Co-IP, defined substrate, mechanistic KD experiments with specific phenotypic readout, replicated with endogenous substrates","pmids":["27113755"],"is_preprint":false},{"year":2017,"finding":"An ALS-associated UBQLN4 variant impairs proteasomal function and leads to accumulation of beta-catenin (a UBQLN4 substrate), causing aberrant motor axon morphogenesis. Inhibition of beta-catenin function rescues the motor axon phenotype caused by the UBQLN4 variant.","method":"Mouse motor neuron and zebrafish in vivo models, proteasomal function assays, epistasis rescue experiment with beta-catenin inhibition","journal":"eLife","confidence":"High","confidence_rationale":"Tier 2 — genetic epistasis rescue, in vivo model organisms, identification of substrate (beta-catenin), multiple orthogonal approaches","pmids":["28463112"],"is_preprint":false},{"year":2007,"finding":"UBQLN4 (CIP75) interacts with connexin43 (Cx43) via its UBA domain binding to the Cx43 PY motif/multiphosphorylation region (Lys264–Asn302), and its UbL domain interacts with proteasomal subunits S2/RPN1 and S5a/RPN10, mediating Cx43 proteasomal degradation and reducing its half-life.","method":"Yeast two-hybrid, GST pull-down, co-immunoprecipitation, laser confocal microscopy, siRNA knockdown, overexpression with half-life measurements","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal binding methods plus functional degradation assays; replicated in subsequent papers","pmids":["18079109"],"is_preprint":false},{"year":2010,"finding":"UBQLN4 (CIP75) mediates ubiquitin-independent proteasomal degradation of ER-localized Cx43; CIP75 interacts with non-ubiquitinated Cx43 at the ER and brings it to the proteasome, even though CIP75 can also bind poly-ubiquitin chains.","method":"Co-immunoprecipitation, immunofluorescence microscopy, in vitro ubiquitin-binding assays, ubiquitination-deficient Cx43 mutant analysis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1–2 — in vitro binding assay plus Co-IP, mutagenesis (lysine-less Cx43), replicated across labs","pmids":["20940304"],"is_preprint":false},{"year":2014,"finding":"UBQLN4 (CIP75) interacts with ER-localized Cx43 and forms a complex with proteasomal subunits S2/Rpn1 and S5a/Rpn10; CIP75 is essential for Cx43–proteasome interaction and ER dislocation, with deliberate Cx43 misfolding enhancing CIP75 binding.","method":"Subcellular fractionation, co-immunoprecipitation, shRNA knockdown, DTT-induced misfolding experiment","journal":"The Biochemical journal","confidence":"Medium","confidence_rationale":"Tier 2 — fractionation plus Co-IP plus functional KD; single lab but multiple methods","pmids":["24256120"],"is_preprint":false},{"year":2015,"finding":"UBQLN4 (CIP75) UBA domain directly interacts with the carboxyl-terminal domains of Cx40 and Cx45 (in addition to Cx43 and Cx32), mediating their ERAD-associated proteasomal degradation from the ER.","method":"NMR spectroscopy (direct interaction), shRNA knockdown, trafficking inhibitor experiments, co-immunoprecipitation","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 1 — NMR structural interaction data plus functional KD assays","pmids":["25583071"],"is_preprint":false},{"year":2020,"finding":"Ubqln4 binds directly to ER membrane J proteins B12 and B14 via its H domain and STI1 motifs (1-2) in a J-domain-independent manner, and captures SV40 virus emerging from the ER to facilitate its escape into the cytosol during infection.","method":"Direct binding assays, domain deletion/mutagenesis (H domain and STI1 motifs), viral infection assays, knockdown experiments","journal":"Journal of virology","confidence":"Medium","confidence_rationale":"Tier 2 — direct binding demonstrated, domain mutagenesis, functional viral infection readout; single lab","pmids":["32161173"],"is_preprint":false},{"year":2021,"finding":"UBQLN4 is a substrate of ATM kinase and, upon DNA damage, interacts with and stabilizes anti-apoptotic proteins BCL2A1 and BCL2L10, thereby preventing mesothelioma cell apoptosis in response to DNA damage.","method":"Mammalian functional genetic screening, co-immunoprecipitation, knockdown experiments with apoptosis readout","journal":"Molecular oncology","confidence":"Medium","confidence_rationale":"Tier 2–3 — Co-IP for binding partners, genetic screening for ATM substrate identification; single lab","pmids":["34245648"],"is_preprint":false},{"year":2021,"finding":"UBQLN4 binds to ubiquitinated MRE11A and promotes its degradation following DNA damage, thereby regulating MRE11A protein levels and promoting cisplatin resistance in esophageal squamous cell carcinoma.","method":"Co-immunoprecipitation, ubiquitination assays after cisplatin treatment, gene copy number analysis, cell line knockdown experiments","journal":"Molecular oncology","confidence":"Medium","confidence_rationale":"Tier 2–3 — Co-IP plus functional degradation assay; independent replication of MRE11-UBQLN4 interaction from original Cell paper","pmids":["33605536"],"is_preprint":false},{"year":2025,"finding":"UBQLN4 promotes proteasomal degradation of GluN2B (NMDA receptor NR2B subunit) at excitatory post-synapses in neurons; reduction of UBQLN4 increases GluN2B levels and seizure susceptibility, while overexpression is protective.","method":"AAV-mediated overexpression and knockdown in vivo (kainic acid epilepsy mouse model), proteasome pathway assays, synaptic fractionation/localization","journal":"Neurobiology of disease","confidence":"Medium","confidence_rationale":"Tier 2 — in vivo loss- and gain-of-function with defined substrate (GluN2B) and proteasomal mechanism; single lab","pmids":["40930427"],"is_preprint":false},{"year":2023,"finding":"Full-length UBQLN4 undergoes liquid-liquid phase separation in vitro; UBQLN4 phase separates at a lower saturation concentration than UBQLN1 and lacks the temperature-dependent phase behavior conferred by the proline-rich (Pxx) region present in UBQLN2. The short N-terminal disordered region inhibits UBQLN4 phase separation via electrostatic interactions.","method":"In vitro phase separation assays, deletion constructs, charge variant analysis, comparison across UBQLN family members","journal":"bioRxiv","confidence":"Low","confidence_rationale":"Tier 3 — in vitro biophysical assay, preprint, single lab, no functional cellular consequence directly linked","pmids":["37808720"],"is_preprint":true}],"current_model":"UBQLN4 is a UbL-UBA domain proteasomal shuttle factor that: (1) is phosphorylated by ATM and interacts with ubiquitylated MRE11 to limit homologous recombination by removing MRE11 from damaged chromatin, thereby favoring NHEJ; (2) recognizes mislocalized or misfolded transmembrane domain proteins (including connexins) in the cytoplasm/ER and targets them for proteasomal degradation via its UBA domain (binding substrates) and UbL domain (engaging proteasomal subunits); (3) regulates beta-catenin and GluN2B levels through proteasomal degradation, linking it to axon morphogenesis and synaptic function; and (4) localizes to the ER-cytosol interface where it binds J proteins to facilitate membrane escape of nonenveloped viruses."},"narrative":{"teleology":[{"year":2007,"claim":"Identification of UBQLN4 as a proteasomal shuttle for connexin43 established its core molecular mechanism: UBA domain binds Cx43 while UbL domain docks on proteasomal subunits RPN1 and RPN10, accelerating Cx43 turnover.","evidence":"Yeast two-hybrid, GST pull-down, co-IP, siRNA knockdown, and half-life measurements in mammalian cells","pmids":["18079109"],"confidence":"High","gaps":["Whether UBQLN4 shuttle activity extends beyond connexins was unknown","Structural basis of UBA–Cx43 interaction not resolved at atomic level"]},{"year":2010,"claim":"Demonstration that UBQLN4 mediates ubiquitin-independent proteasomal degradation of ER-localized Cx43 revealed that substrate ubiquitylation is not always required for UBQLN4 shuttle function.","evidence":"Co-IP with lysine-less Cx43 mutant, ubiquitin-binding assays, and immunofluorescence in mammalian cells","pmids":["20940304"],"confidence":"High","gaps":["How UBQLN4 recognizes non-ubiquitinated substrates structurally remained unclear","Whether ubiquitin-independent mode applies to non-connexin substrates was untested"]},{"year":2014,"claim":"Showing that deliberate Cx43 misfolding enhanced UBQLN4 binding linked UBQLN4 to ERAD-associated quality control, where it facilitates ER dislocation of misfolded substrates to the proteasome.","evidence":"Subcellular fractionation, co-IP, shRNA knockdown, and DTT-induced misfolding in mammalian cells","pmids":["24256120"],"confidence":"Medium","gaps":["Whether UBQLN4 acts upstream or downstream of retrotranslocation machinery was not resolved","Only DTT-induced misfolding tested; physiological misfolding triggers uncharacterized"]},{"year":2015,"claim":"NMR-based demonstration that UBQLN4 UBA domain directly binds C-terminal domains of multiple connexin family members (Cx40, Cx45, Cx32, Cx43) established UBQLN4 as a broad connexin quality control factor.","evidence":"NMR spectroscopy for direct interaction, shRNA knockdown, and co-IP in mammalian cells","pmids":["25583071"],"confidence":"High","gaps":["Selectivity determinants within connexin tails that define UBA binding affinity were not fully mapped","In vivo physiological significance for cardiac gap junction connexins not tested"]},{"year":2016,"claim":"Discovery that UBQLN4 recognizes mislocalized transmembrane domain proteins that fail ER insertion extended its role beyond connexins to a general cytoplasmic quality control pathway for aberrant secretory pathway substrates.","evidence":"SRP54 knockdown to generate mislocalized substrates, co-IP, proteasome inhibitor experiments in mammalian cells","pmids":["27113755"],"confidence":"High","gaps":["Relationship between UBQLN4 and BAG6 in substrate handoff not mechanistically resolved","Whether UBQLN4 acts redundantly with other ubiquilins in this pathway was unclear"]},{"year":2017,"claim":"Identification of beta-catenin as a UBQLN4 degradation substrate in motor neurons, and rescue of axon defects by beta-catenin inhibition, linked UBQLN4 loss-of-function to ALS-associated motor neuron pathology through a defined proteasomal mechanism.","evidence":"ALS-linked UBQLN4 variant in mouse and zebrafish motor neuron models, proteasomal assays, beta-catenin epistasis rescue","pmids":["28463112"],"confidence":"High","gaps":["Whether beta-catenin is a direct binding substrate or requires ubiquitination for UBQLN4 recognition was not determined","Whether other UBQLN4 substrates contribute to ALS pathology was untested"]},{"year":2019,"claim":"Demonstration that ATM phosphorylates UBQLN4 to promote its interaction with ubiquitylated MRE11 and MRE11 chromatin removal established UBQLN4 as a DNA damage response effector that channels DSB repair from HR toward NHEJ.","evidence":"Co-IP, ATM phosphorylation assays, in vitro and in vivo DSB repair assays, loss-of-function and overexpression experiments in mammalian cells","pmids":["30612738","31131301"],"confidence":"High","gaps":["The ATM phosphosite(s) on UBQLN4 and their individual contributions were not fully dissected","Whether UBQLN4's HR-suppressive function operates in all tissue contexts was unknown"]},{"year":2020,"claim":"Binding of UBQLN4 to ER J proteins DNAJB12 and DNAJB14 via its H domain and STI1 motifs, and its role in SV40 ER-to-cytosol escape, revealed an unexpected function at the ER membrane in viral infection.","evidence":"Direct binding assays, domain deletion/mutagenesis, SV40 infection assays, knockdown in mammalian cells","pmids":["32161173"],"confidence":"Medium","gaps":["Whether J protein binding reflects a constitutive ERAD-related function or is specific to viral exploitation was unresolved","Mechanism by which UBQLN4 facilitates membrane penetration of a non-enveloped virus is unclear"]},{"year":2021,"claim":"Independent confirmation that UBQLN4 degrades ubiquitinated MRE11A after DNA damage in esophageal cancer cells, and discovery that UBQLN4 stabilizes anti-apoptotic proteins BCL2A1/BCL2L10 in mesothelioma, expanded its DNA damage response role to therapy resistance and survival signaling.","evidence":"Co-IP and ubiquitination assays after cisplatin treatment; genetic screening and apoptosis assays in mesothelioma cells","pmids":["33605536","34245648"],"confidence":"Medium","gaps":["Whether BCL2A1/BCL2L10 stabilization is proteasome-dependent or involves a distinct mechanism was not established","Generalizability of UBQLN4 cisplatin resistance mechanism across tumor types untested"]},{"year":2025,"claim":"Identification of GluN2B as a neuronal UBQLN4 substrate demonstrated that UBQLN4 regulates excitatory synapse composition and seizure susceptibility through proteasomal control of NMDA receptor subunit levels.","evidence":"AAV-mediated overexpression and knockdown in kainic acid mouse epilepsy model, synaptic fractionation, proteasome pathway assays","pmids":["40930427"],"confidence":"Medium","gaps":["Whether UBQLN4 directly binds GluN2B or acts via an intermediary ubiquitin ligase is unresolved","Contribution of UBQLN4 to synaptic plasticity beyond seizure susceptibility is unknown"]},{"year":null,"claim":"How UBQLN4 achieves substrate selectivity among its diverse clients (connexins, MRE11, beta-catenin, GluN2B, transmembrane domain proteins, BCL2 family members), and how ATM phosphorylation rewires its specificity from quality control to DNA repair, remain central unresolved questions.","evidence":"","pmids":[],"confidence":"High","gaps":["No structural model of full-length UBQLN4 bound to any substrate exists","Relative contributions of ubiquitin-dependent vs. ubiquitin-independent recognition across substrates are unclear","In vivo redundancy with UBQLN1/UBQLN2 in each functional context has not been systematically tested"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[0,2,4,5,7]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,3,11]}],"localization":[{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[5,6,7,8]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[2,4]},{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[0]}],"pathway":[{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[2,4,5,6,7]},{"term_id":"R-HSA-73894","term_label":"DNA Repair","supporting_discovery_ids":[0,1,10]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[3,9,10]}],"complexes":[],"partners":["MRE11","GJA1","PSMD1","PSMD4","DNAJB12","DNAJB14","CTNNB1","GRIN2B"],"other_free_text":[]},"mechanistic_narrative":"UBQLN4 is a UbL-UBA domain proteasomal shuttle factor that delivers diverse substrates for proteasomal degradation in contexts ranging from protein quality control to DNA damage repair and synaptic homeostasis. Its UBA domain directly binds the cytoplasmic tails of connexins (Cx43, Cx40, Cx45, Cx32) and mislocalized transmembrane domain proteins, while its UbL domain engages proteasomal subunits S2/RPN1 and S5a/RPN10 to effect substrate degradation, including ubiquitin-independent ERAD of connexins from the ER [PMID:18079109, PMID:20940304, PMID:25583071, PMID:27113755]. Upon DNA double-strand breaks, ATM phosphorylates UBQLN4, enabling it to bind ubiquitylated MRE11, remove MRE11 from damaged chromatin via proteasomal degradation, and thereby suppress homologous recombination in favor of non-homologous end joining [PMID:30612738, PMID:33605536]. UBQLN4 also regulates neuronal signaling by targeting beta-catenin and the NMDA receptor subunit GluN2B for proteasomal degradation; an ALS-associated variant impairs proteasomal function, causing beta-catenin accumulation and aberrant motor axon morphogenesis [PMID:28463112, PMID:40930427]."},"prefetch_data":{"uniprot":{"accession":"Q9NRR5","full_name":"Ubiquilin-4","aliases":["Ataxin-1 interacting ubiquitin-like protein","A1Up","Ataxin-1 ubiquitin-like-interacting protein A1U","Connexin43-interacting protein of 75 kDa","CIP75"],"length_aa":601,"mass_kda":63.9,"function":"Regulator of protein degradation that mediates the proteasomal targeting of misfolded, mislocalized or accumulated proteins (PubMed:15280365, PubMed:27113755, PubMed:29666234, PubMed:30612738). Acts by binding polyubiquitin chains of target proteins via its UBA domain and by interacting with subunits of the proteasome via its ubiquitin-like domain (PubMed:15280365, PubMed:27113755, PubMed:30612738). Key regulator of DNA repair that represses homologous recombination repair: in response to DNA damage, recruited to sites of DNA damage following phosphorylation by ATM and acts by binding and removing ubiquitinated MRE11 from damaged chromatin, leading to MRE11 degradation by the proteasome (PubMed:30612738). MRE11 degradation prevents homologous recombination repair, redirecting double-strand break repair toward non-homologous end joining (NHEJ) (PubMed:30612738). Specifically recognizes and binds mislocalized transmembrane-containing proteins and targets them to proteasomal degradation (PubMed:27113755). Collaborates with DESI1/POST in the export of ubiquitinated proteins from the nucleus to the cytoplasm (PubMed:29666234). Also plays a role in the regulation of the proteasomal degradation of non-ubiquitinated GJA1 (By similarity). Acts as an adapter protein that recruits UBQLN1 to the autophagy machinery (PubMed:23459205). Mediates the association of UBQLN1 with autophagosomes and the autophagy-related protein LC3 (MAP1LC3A/B/C) and may assist in the maturation of autophagosomes to autolysosomes by mediating autophagosome-lysosome fusion (PubMed:23459205)","subcellular_location":"Nucleus; Cytoplasm; Chromosome; Endoplasmic reticulum; Cytoplasm, perinuclear region; Cytoplasmic vesicle, autophagosome","url":"https://www.uniprot.org/uniprotkb/Q9NRR5/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":true,"resolved_as":"","url":"https://depmap.org/portal/gene/UBQLN4","classification":"Common Essential","n_dependent_lines":852,"n_total_lines":1208,"dependency_fraction":0.7052980132450332},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"DYNLL1","stoichiometry":4.0},{"gene":"DYNLL2","stoichiometry":4.0},{"gene":"CAPZB","stoichiometry":0.2},{"gene":"CBX1","stoichiometry":0.2},{"gene":"CSNK2B","stoichiometry":0.2},{"gene":"HDAC1","stoichiometry":0.2},{"gene":"HDAC2","stoichiometry":0.2},{"gene":"HIST2H2BE","stoichiometry":0.2},{"gene":"HMGN5","stoichiometry":0.2},{"gene":"NUMA1","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/UBQLN4","total_profiled":1310},"omim":[{"mim_id":"605440","title":"UBIQUILIN 4; UBQLN4","url":"https://www.omim.org/entry/605440"},{"mim_id":"182350","title":"ATPase, Na+/K+ TRANSPORTING, ALPHA-3 POLYPEPTIDE; ATP1A3","url":"https://www.omim.org/entry/182350"},{"mim_id":"128235","title":"DYSTONIA 12; DYT12","url":"https://www.omim.org/entry/128235"}],"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/UBQLN4"},"hgnc":{"alias_symbol":["A1U","UBIN","CIP75"],"prev_symbol":["C1orf6"]},"alphafold":{"accession":"Q9NRR5","domains":[{"cath_id":"3.10.20.90","chopping":"13-83","consensus_level":"high","plddt":85.301,"start":13,"end":83},{"cath_id":"-","chopping":"191-298","consensus_level":"medium","plddt":68.7094,"start":191,"end":298},{"cath_id":"-","chopping":"394-482","consensus_level":"high","plddt":68.7701,"start":394,"end":482},{"cath_id":"1.10.8.10","chopping":"554-601","consensus_level":"medium","plddt":82.7087,"start":554,"end":601}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9NRR5","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9NRR5-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9NRR5-F1-predicted_aligned_error_v6.png","plddt_mean":60.72},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=UBQLN4","jax_strain_url":"https://www.jax.org/strain/search?query=UBQLN4"},"sequence":{"accession":"Q9NRR5","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9NRR5.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9NRR5/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9NRR5"}},"corpus_meta":[{"pmid":"30612738","id":"PMC_30612738","title":"UBQLN4 Represses Homologous Recombination and Is Overexpressed in Aggressive Tumors.","date":"2019","source":"Cell","url":"https://pubmed.ncbi.nlm.nih.gov/30612738","citation_count":105,"is_preprint":false},{"pmid":"27113755","id":"PMC_27113755","title":"UBQLN4 recognizes mislocalized transmembrane domain proteins and targets these to proteasomal degradation.","date":"2016","source":"EMBO reports","url":"https://pubmed.ncbi.nlm.nih.gov/27113755","citation_count":61,"is_preprint":false},{"pmid":"18079109","id":"PMC_18079109","title":"A novel connexin43-interacting protein, CIP75, which belongs to the UbL-UBA protein family, regulates the turnover of connexin43.","date":"2007","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/18079109","citation_count":54,"is_preprint":false},{"pmid":"20940304","id":"PMC_20940304","title":"Ubiquitin-independent proteasomal degradation of endoplasmic reticulum-localized connexin43 mediated by CIP75.","date":"2010","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/20940304","citation_count":45,"is_preprint":false},{"pmid":"28463112","id":"PMC_28463112","title":"A novel ALS-associated variant in UBQLN4 regulates motor axon morphogenesis.","date":"2017","source":"eLife","url":"https://pubmed.ncbi.nlm.nih.gov/28463112","citation_count":42,"is_preprint":false},{"pmid":"29666234","id":"PMC_29666234","title":"Nuclear export of ubiquitinated proteins via the UBIN-POST system.","date":"2018","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/29666234","citation_count":28,"is_preprint":false},{"pmid":"31911755","id":"PMC_31911755","title":"UBQLN4 promotes progression of HCC via activating wnt-β-catenin pathway and is regulated by miR-370.","date":"2020","source":"Cancer cell 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one","url":"https://pubmed.ncbi.nlm.nih.gov/38820386","citation_count":0,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":11476,"output_tokens":3108,"usd":0.040524},"stage2":{"model":"claude-opus-4-6","input_tokens":6482,"output_tokens":2940,"usd":0.158865},"total_usd":0.199389,"stage1_batch_id":"msgbatch_01RPbVc9PDH9zQmYK5Hs6srm","stage2_batch_id":"msgbatch_01JVbVLzeKN2yVrsGp3zC8aM","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2019,\n      \"finding\": \"UBQLN4 is phosphorylated by ATM kinase and interacts with ubiquitylated MRE11 to mediate early steps of homologous recombination-mediated DNA double-strand break repair (HRR). UBQLN4 removes MRE11 from damaged chromatin via proteasomal degradation, thereby curtailing HRR activity. Loss of UBQLN4 leads to chromatin retention of MRE11 and non-physiological HRR; overexpression represses HRR and favors non-homologous end joining (NHEJ).\",\n      \"method\": \"Co-immunoprecipitation, in vitro and in vivo DSB repair assays, ATM phosphorylation assays, loss-of-function and overexpression experiments with defined phenotypic readouts\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (Co-IP, in vitro/in vivo assays, genetic epistasis), strong mechanistic detail, high-citation foundational paper\",\n      \"pmids\": [\"30612738\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"UBQLN4 promotes NHEJ by repressing DNA end-resection in an ATM-dependent manner; the ATM-UBQLN4-MRE11 axis limits excessive end resection after DSBs.\",\n      \"method\": \"Epistasis analysis and mechanistic commentary elaborating on the UBQLN4-MRE11-ATM interaction\",\n      \"journal\": \"Molecular & cellular oncology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — commentary/summary with genetic epistasis framing; mechanistic interpretation supported by primary Cell paper\",\n      \"pmids\": [\"31131301\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"UBQLN4 recognizes mislocalized transmembrane domain proteins in the cytoplasm (via their exposed transmembrane segments) and targets them to the proteasome for degradation, acting as a BAG6-binding quality control factor for newly synthesized defective polypeptides that fail to reach the ER.\",\n      \"method\": \"Co-immunoprecipitation, knockdown of SRP54, model truncated transmembrane domain protein degradation assays, proteasome inhibitor experiments\",\n      \"journal\": \"EMBO reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP, defined substrate, mechanistic KD experiments with specific phenotypic readout, replicated with endogenous substrates\",\n      \"pmids\": [\"27113755\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"An ALS-associated UBQLN4 variant impairs proteasomal function and leads to accumulation of beta-catenin (a UBQLN4 substrate), causing aberrant motor axon morphogenesis. Inhibition of beta-catenin function rescues the motor axon phenotype caused by the UBQLN4 variant.\",\n      \"method\": \"Mouse motor neuron and zebrafish in vivo models, proteasomal function assays, epistasis rescue experiment with beta-catenin inhibition\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis rescue, in vivo model organisms, identification of substrate (beta-catenin), multiple orthogonal approaches\",\n      \"pmids\": [\"28463112\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"UBQLN4 (CIP75) interacts with connexin43 (Cx43) via its UBA domain binding to the Cx43 PY motif/multiphosphorylation region (Lys264–Asn302), and its UbL domain interacts with proteasomal subunits S2/RPN1 and S5a/RPN10, mediating Cx43 proteasomal degradation and reducing its half-life.\",\n      \"method\": \"Yeast two-hybrid, GST pull-down, co-immunoprecipitation, laser confocal microscopy, siRNA knockdown, overexpression with half-life measurements\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal binding methods plus functional degradation assays; replicated in subsequent papers\",\n      \"pmids\": [\"18079109\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"UBQLN4 (CIP75) mediates ubiquitin-independent proteasomal degradation of ER-localized Cx43; CIP75 interacts with non-ubiquitinated Cx43 at the ER and brings it to the proteasome, even though CIP75 can also bind poly-ubiquitin chains.\",\n      \"method\": \"Co-immunoprecipitation, immunofluorescence microscopy, in vitro ubiquitin-binding assays, ubiquitination-deficient Cx43 mutant analysis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vitro binding assay plus Co-IP, mutagenesis (lysine-less Cx43), replicated across labs\",\n      \"pmids\": [\"20940304\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"UBQLN4 (CIP75) interacts with ER-localized Cx43 and forms a complex with proteasomal subunits S2/Rpn1 and S5a/Rpn10; CIP75 is essential for Cx43–proteasome interaction and ER dislocation, with deliberate Cx43 misfolding enhancing CIP75 binding.\",\n      \"method\": \"Subcellular fractionation, co-immunoprecipitation, shRNA knockdown, DTT-induced misfolding experiment\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — fractionation plus Co-IP plus functional KD; single lab but multiple methods\",\n      \"pmids\": [\"24256120\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"UBQLN4 (CIP75) UBA domain directly interacts with the carboxyl-terminal domains of Cx40 and Cx45 (in addition to Cx43 and Cx32), mediating their ERAD-associated proteasomal degradation from the ER.\",\n      \"method\": \"NMR spectroscopy (direct interaction), shRNA knockdown, trafficking inhibitor experiments, co-immunoprecipitation\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — NMR structural interaction data plus functional KD assays\",\n      \"pmids\": [\"25583071\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Ubqln4 binds directly to ER membrane J proteins B12 and B14 via its H domain and STI1 motifs (1-2) in a J-domain-independent manner, and captures SV40 virus emerging from the ER to facilitate its escape into the cytosol during infection.\",\n      \"method\": \"Direct binding assays, domain deletion/mutagenesis (H domain and STI1 motifs), viral infection assays, knockdown experiments\",\n      \"journal\": \"Journal of virology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct binding demonstrated, domain mutagenesis, functional viral infection readout; single lab\",\n      \"pmids\": [\"32161173\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"UBQLN4 is a substrate of ATM kinase and, upon DNA damage, interacts with and stabilizes anti-apoptotic proteins BCL2A1 and BCL2L10, thereby preventing mesothelioma cell apoptosis in response to DNA damage.\",\n      \"method\": \"Mammalian functional genetic screening, co-immunoprecipitation, knockdown experiments with apoptosis readout\",\n      \"journal\": \"Molecular oncology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — Co-IP for binding partners, genetic screening for ATM substrate identification; single lab\",\n      \"pmids\": [\"34245648\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"UBQLN4 binds to ubiquitinated MRE11A and promotes its degradation following DNA damage, thereby regulating MRE11A protein levels and promoting cisplatin resistance in esophageal squamous cell carcinoma.\",\n      \"method\": \"Co-immunoprecipitation, ubiquitination assays after cisplatin treatment, gene copy number analysis, cell line knockdown experiments\",\n      \"journal\": \"Molecular oncology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — Co-IP plus functional degradation assay; independent replication of MRE11-UBQLN4 interaction from original Cell paper\",\n      \"pmids\": [\"33605536\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"UBQLN4 promotes proteasomal degradation of GluN2B (NMDA receptor NR2B subunit) at excitatory post-synapses in neurons; reduction of UBQLN4 increases GluN2B levels and seizure susceptibility, while overexpression is protective.\",\n      \"method\": \"AAV-mediated overexpression and knockdown in vivo (kainic acid epilepsy mouse model), proteasome pathway assays, synaptic fractionation/localization\",\n      \"journal\": \"Neurobiology of disease\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vivo loss- and gain-of-function with defined substrate (GluN2B) and proteasomal mechanism; single lab\",\n      \"pmids\": [\"40930427\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Full-length UBQLN4 undergoes liquid-liquid phase separation in vitro; UBQLN4 phase separates at a lower saturation concentration than UBQLN1 and lacks the temperature-dependent phase behavior conferred by the proline-rich (Pxx) region present in UBQLN2. The short N-terminal disordered region inhibits UBQLN4 phase separation via electrostatic interactions.\",\n      \"method\": \"In vitro phase separation assays, deletion constructs, charge variant analysis, comparison across UBQLN family members\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — in vitro biophysical assay, preprint, single lab, no functional cellular consequence directly linked\",\n      \"pmids\": [\"37808720\"],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"UBQLN4 is a UbL-UBA domain proteasomal shuttle factor that: (1) is phosphorylated by ATM and interacts with ubiquitylated MRE11 to limit homologous recombination by removing MRE11 from damaged chromatin, thereby favoring NHEJ; (2) recognizes mislocalized or misfolded transmembrane domain proteins (including connexins) in the cytoplasm/ER and targets them for proteasomal degradation via its UBA domain (binding substrates) and UbL domain (engaging proteasomal subunits); (3) regulates beta-catenin and GluN2B levels through proteasomal degradation, linking it to axon morphogenesis and synaptic function; and (4) localizes to the ER-cytosol interface where it binds J proteins to facilitate membrane escape of nonenveloped viruses.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"UBQLN4 is a UbL-UBA domain proteasomal shuttle factor that delivers diverse substrates for proteasomal degradation in contexts ranging from protein quality control to DNA damage repair and synaptic homeostasis. Its UBA domain directly binds the cytoplasmic tails of connexins (Cx43, Cx40, Cx45, Cx32) and mislocalized transmembrane domain proteins, while its UbL domain engages proteasomal subunits S2/RPN1 and S5a/RPN10 to effect substrate degradation, including ubiquitin-independent ERAD of connexins from the ER [PMID:18079109, PMID:20940304, PMID:25583071, PMID:27113755]. Upon DNA double-strand breaks, ATM phosphorylates UBQLN4, enabling it to bind ubiquitylated MRE11, remove MRE11 from damaged chromatin via proteasomal degradation, and thereby suppress homologous recombination in favor of non-homologous end joining [PMID:30612738, PMID:33605536]. UBQLN4 also regulates neuronal signaling by targeting beta-catenin and the NMDA receptor subunit GluN2B for proteasomal degradation; an ALS-associated variant impairs proteasomal function, causing beta-catenin accumulation and aberrant motor axon morphogenesis [PMID:28463112, PMID:40930427].\",\n  \"teleology\": [\n    {\n      \"year\": 2007,\n      \"claim\": \"Identification of UBQLN4 as a proteasomal shuttle for connexin43 established its core molecular mechanism: UBA domain binds Cx43 while UbL domain docks on proteasomal subunits RPN1 and RPN10, accelerating Cx43 turnover.\",\n      \"evidence\": \"Yeast two-hybrid, GST pull-down, co-IP, siRNA knockdown, and half-life measurements in mammalian cells\",\n      \"pmids\": [\"18079109\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether UBQLN4 shuttle activity extends beyond connexins was unknown\", \"Structural basis of UBA–Cx43 interaction not resolved at atomic level\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Demonstration that UBQLN4 mediates ubiquitin-independent proteasomal degradation of ER-localized Cx43 revealed that substrate ubiquitylation is not always required for UBQLN4 shuttle function.\",\n      \"evidence\": \"Co-IP with lysine-less Cx43 mutant, ubiquitin-binding assays, and immunofluorescence in mammalian cells\",\n      \"pmids\": [\"20940304\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How UBQLN4 recognizes non-ubiquitinated substrates structurally remained unclear\", \"Whether ubiquitin-independent mode applies to non-connexin substrates was untested\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Showing that deliberate Cx43 misfolding enhanced UBQLN4 binding linked UBQLN4 to ERAD-associated quality control, where it facilitates ER dislocation of misfolded substrates to the proteasome.\",\n      \"evidence\": \"Subcellular fractionation, co-IP, shRNA knockdown, and DTT-induced misfolding in mammalian cells\",\n      \"pmids\": [\"24256120\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether UBQLN4 acts upstream or downstream of retrotranslocation machinery was not resolved\", \"Only DTT-induced misfolding tested; physiological misfolding triggers uncharacterized\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"NMR-based demonstration that UBQLN4 UBA domain directly binds C-terminal domains of multiple connexin family members (Cx40, Cx45, Cx32, Cx43) established UBQLN4 as a broad connexin quality control factor.\",\n      \"evidence\": \"NMR spectroscopy for direct interaction, shRNA knockdown, and co-IP in mammalian cells\",\n      \"pmids\": [\"25583071\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Selectivity determinants within connexin tails that define UBA binding affinity were not fully mapped\", \"In vivo physiological significance for cardiac gap junction connexins not tested\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Discovery that UBQLN4 recognizes mislocalized transmembrane domain proteins that fail ER insertion extended its role beyond connexins to a general cytoplasmic quality control pathway for aberrant secretory pathway substrates.\",\n      \"evidence\": \"SRP54 knockdown to generate mislocalized substrates, co-IP, proteasome inhibitor experiments in mammalian cells\",\n      \"pmids\": [\"27113755\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Relationship between UBQLN4 and BAG6 in substrate handoff not mechanistically resolved\", \"Whether UBQLN4 acts redundantly with other ubiquilins in this pathway was unclear\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Identification of beta-catenin as a UBQLN4 degradation substrate in motor neurons, and rescue of axon defects by beta-catenin inhibition, linked UBQLN4 loss-of-function to ALS-associated motor neuron pathology through a defined proteasomal mechanism.\",\n      \"evidence\": \"ALS-linked UBQLN4 variant in mouse and zebrafish motor neuron models, proteasomal assays, beta-catenin epistasis rescue\",\n      \"pmids\": [\"28463112\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether beta-catenin is a direct binding substrate or requires ubiquitination for UBQLN4 recognition was not determined\", \"Whether other UBQLN4 substrates contribute to ALS pathology was untested\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Demonstration that ATM phosphorylates UBQLN4 to promote its interaction with ubiquitylated MRE11 and MRE11 chromatin removal established UBQLN4 as a DNA damage response effector that channels DSB repair from HR toward NHEJ.\",\n      \"evidence\": \"Co-IP, ATM phosphorylation assays, in vitro and in vivo DSB repair assays, loss-of-function and overexpression experiments in mammalian cells\",\n      \"pmids\": [\"30612738\", \"31131301\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"The ATM phosphosite(s) on UBQLN4 and their individual contributions were not fully dissected\", \"Whether UBQLN4's HR-suppressive function operates in all tissue contexts was unknown\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Binding of UBQLN4 to ER J proteins DNAJB12 and DNAJB14 via its H domain and STI1 motifs, and its role in SV40 ER-to-cytosol escape, revealed an unexpected function at the ER membrane in viral infection.\",\n      \"evidence\": \"Direct binding assays, domain deletion/mutagenesis, SV40 infection assays, knockdown in mammalian cells\",\n      \"pmids\": [\"32161173\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether J protein binding reflects a constitutive ERAD-related function or is specific to viral exploitation was unresolved\", \"Mechanism by which UBQLN4 facilitates membrane penetration of a non-enveloped virus is unclear\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Independent confirmation that UBQLN4 degrades ubiquitinated MRE11A after DNA damage in esophageal cancer cells, and discovery that UBQLN4 stabilizes anti-apoptotic proteins BCL2A1/BCL2L10 in mesothelioma, expanded its DNA damage response role to therapy resistance and survival signaling.\",\n      \"evidence\": \"Co-IP and ubiquitination assays after cisplatin treatment; genetic screening and apoptosis assays in mesothelioma cells\",\n      \"pmids\": [\"33605536\", \"34245648\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether BCL2A1/BCL2L10 stabilization is proteasome-dependent or involves a distinct mechanism was not established\", \"Generalizability of UBQLN4 cisplatin resistance mechanism across tumor types untested\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Identification of GluN2B as a neuronal UBQLN4 substrate demonstrated that UBQLN4 regulates excitatory synapse composition and seizure susceptibility through proteasomal control of NMDA receptor subunit levels.\",\n      \"evidence\": \"AAV-mediated overexpression and knockdown in kainic acid mouse epilepsy model, synaptic fractionation, proteasome pathway assays\",\n      \"pmids\": [\"40930427\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether UBQLN4 directly binds GluN2B or acts via an intermediary ubiquitin ligase is unresolved\", \"Contribution of UBQLN4 to synaptic plasticity beyond seizure susceptibility is unknown\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How UBQLN4 achieves substrate selectivity among its diverse clients (connexins, MRE11, beta-catenin, GluN2B, transmembrane domain proteins, BCL2 family members), and how ATM phosphorylation rewires its specificity from quality control to DNA repair, remain central unresolved questions.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No structural model of full-length UBQLN4 bound to any substrate exists\", \"Relative contributions of ubiquitin-dependent vs. ubiquitin-independent recognition across substrates are unclear\", \"In vivo redundancy with UBQLN1/UBQLN2 in each functional context has not been systematically tested\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [0, 2, 4, 5, 7]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 3, 11]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [5, 6, 7, 8]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [2, 4]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [0]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [2, 4, 5, 6, 7]},\n      {\"term_id\": \"R-HSA-73894\", \"supporting_discovery_ids\": [0, 1, 10]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [3, 9, 10]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"MRE11\",\n      \"GJA1\",\n      \"PSMD1\",\n      \"PSMD4\",\n      \"DNAJB12\",\n      \"DNAJB14\",\n      \"CTNNB1\",\n      \"GRIN2B\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}