{"gene":"UBQLN4","run_date":"2026-06-10T10:51:56","timeline":{"discoveries":[{"year":2001,"finding":"UBQLN4 (UBIN) binds to ER-targeting signal sequences of various secretory and ER-luminal proteins (including HSP47) but does not interact with mitochondrial targeting sequences, as determined by yeast two-hybrid screening.","method":"Yeast two-hybrid screening","journal":"Biochemical and biophysical research communications","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single yeast two-hybrid method, single lab, no functional follow-up reported","pmids":["11162551"],"is_preprint":false},{"year":2007,"finding":"UBQLN4 (CIP75) interacts with connexin43 (Cx43) via its UBA domain binding to a region in Cx43 between Lys264 and Asn302 (containing PY motif and multiphosphorylation sites), and its UbL domain interacts with proteasomal subunits S2/RPN1 and S5a/RPN10; overexpression stimulates Cx43 degradation and reduces its half-life, while siRNA knockdown has the opposite effect. CIP75 localizes primarily at the ER, co-localizing with Cx43 in the perinuclear region.","method":"Yeast two-hybrid, GST pull-down, co-immunoprecipitation, laser confocal microscopy, overexpression and siRNA knockdown with Cx43 half-life measurement","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (Y2H, GST pulldown, co-IP, confocal, functional KD/OE), single lab but with rigorous controls across multiple assays","pmids":["18079109"],"is_preprint":false},{"year":2010,"finding":"UBQLN4 (CIP75) mediates ubiquitin-independent proteasomal degradation of ER-localized Cx43: CIP75 interacts with Cx43 at the ER, can bind free monoubiquitin and K48-linked tetraubiquitin chains in vitro, but the Cx43 associated with CIP75 is not ubiquitinated and a lysine-free Cx43 mutant retains the ability to interact with CIP75.","method":"Co-immunoprecipitation, immunofluorescence microscopy, in vitro ubiquitin-binding assay, lysine-free Cx43 mutant analysis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (co-IP, immunofluorescence, in vitro binding, mutagenesis), replicates and extends prior findings from same lab","pmids":["20940304"],"is_preprint":false},{"year":2014,"finding":"UBQLN4 (CIP75) forms a complex with ER-localized Cx43 and proteasomal subunits S2/Rpn1 and S5a/Rpn10; deliberate misfolding of Cx43 by DTT enhanced CIP75 binding; shRNA-mediated knockdown of CIP75 diminished Cx43-proteasome association but still allowed ER dislocation and degradation, indicating CIP75 is essential for Cx43-proteasome interaction but alternate compensatory mechanisms exist for degradation.","method":"Subcellular fractionation, co-immunoprecipitation, DTT-induced misfolding, shRNA knockdown","journal":"The Biochemical journal","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple methods (fractionation, co-IP, functional KD), single lab","pmids":["24256120"],"is_preprint":false},{"year":2015,"finding":"UBQLN4 (CIP75) UBA domain directly interacts with the C-terminal domains of Cx40 and Cx45 (in addition to Cx43), as determined by NMR; shRNA knockdown of CIP75 in HeLa cells increases Cx40 and Cx45 levels; CIP75 preferentially interacts with connexins undergoing ERAD and also interacts with ER-localized Cx32 likely via Cx32 ubiquitination.","method":"NMR spectroscopy, shRNA knockdown with western blot, trafficking inhibitor assays, co-immunoprecipitation","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 1 / Moderate — NMR structural data combined with functional KD and co-IP, single lab but multiple orthogonal methods","pmids":["25583071"],"is_preprint":false},{"year":2016,"finding":"UBQLN4 acts as a BAG6-binding factor that recognizes mislocalized transmembrane domain proteins in the cytoplasm (via their exposed transmembrane segment) and targets them to the proteasome for degradation; UBQLN4 also recognizes endogenous defective proteins induced by SRP54 depletion.","method":"Co-immunoprecipitation (BAG6 binding), truncated transmembrane domain protein degradation assay, SRP54 depletion, proteasome inhibitor treatment","journal":"EMBO reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional degradation assays with substrate and SRP54 knockdown, plus binding partner identification by co-IP, single lab","pmids":["27113755"],"is_preprint":false},{"year":2017,"finding":"An ALS-associated variant of UBQLN4 impairs proteasomal function and causes motor axon morphogenesis defects in mouse motor neurons and zebrafish; beta-catenin (a Wnt signaling effector) was identified as a UBQLN4 substrate, and inhibition of beta-catenin function rescues the UBQLN4 variant-induced motor axon phenotypes.","method":"Expression of ALS-variant in mouse motor neurons and zebrafish (in vivo), proteasomal activity assays, beta-catenin inhibition rescue experiments","journal":"eLife","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vivo rescue experiment plus substrate identification and proteasomal activity assay, single lab","pmids":["28463112"],"is_preprint":false},{"year":2018,"finding":"UBQLN4 (UBIN) shuttles between the nucleus and cytosol in a CRM1-dependent manner via its binding partner POST (which harbors a nuclear export signal); UBIN binds polyubiquitin chains through its UBA domain, and the UBIN-POST complex exports polyubiquitinated proteins from the nucleus to the cytosol for proteasomal degradation, maintaining nuclear protein homeostasis.","method":"Nuclear/cytosol fractionation, CRM1 inhibition, co-immunoprecipitation, proteasome inhibitor and CRM1 inhibitor co-treatment, UBA domain binding assays","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (fractionation, co-IP, pharmacological inhibition, binding assays), clear mechanistic dissection of export complex","pmids":["29666234"],"is_preprint":false},{"year":2019,"finding":"UBQLN4 is phosphorylated by ATM and interacts with ubiquitylated MRE11 to mediate early steps of homologous recombination-mediated DSB repair (HRR); UBQLN4 promotes removal of MRE11 from damaged chromatin via proteasomal degradation. Loss of UBQLN4 leads to chromatin retention of MRE11 and non-physiological HRR activity, while UBQLN4 overexpression represses HRR and favors non-homologous end joining (NHEJ).","method":"Co-immunoprecipitation (UBQLN4-MRE11 interaction), ATM phosphorylation assay, chromatin fractionation, in vitro and in vivo HRR assays, loss-of-function and overexpression studies, patient-derived mutations","journal":"Cell","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — multiple orthogonal methods (co-IP, kinase assay, chromatin fractionation, functional in vitro and in vivo HRR assays), mechanistic dissection replicated in multiple cell lines and patient data","pmids":["30612738"],"is_preprint":false},{"year":2019,"finding":"The UBQLN4-MRE11 interaction is ATM-dependent, indicating that ATM (which initiates HR) also limits excessive end-resection by phosphorylating UBQLN4 to promote MRE11 degradation and repress HR.","method":"Review/commentary of experimental data from PMID:30612738, epistasis analysis","journal":"Molecular & cellular oncology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — commentary synthesizing experimental data from the primary Cell paper; no new experiments reported","pmids":["31131301"],"is_preprint":false},{"year":2020,"finding":"UBQLN4 (Ubqln4) directly binds ER membrane J proteins B12 and B14 at the ER-cytosol interface (in a J-domain-independent manner via its H domain and STI1 motifs 1-2) and captures SV40 virus emerging from the ER, facilitating viral escape from the ER to the cytosol and promoting infection.","method":"Direct binding assays (purified proteins), J-domain mutant analysis, domain deletion mapping (H domain, STI1 motifs), SV40 infection assays with knockdown/overexpression","journal":"Journal of virology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct binding with purified proteins plus domain-mapping mutagenesis and functional infection assay, single lab","pmids":["32161173"],"is_preprint":false},{"year":2021,"finding":"UBQLN4 is a substrate for ATM kinase (phosphorylated by ATM); UBQLN4 interacts with and stabilizes the anti-apoptotic proteins BCL2A1 and BCL2L10, preventing mesothelioma cell apoptosis in response to DNA damage.","method":"Mammalian functional genetic screening for ATM substrates, co-immunoprecipitation (UBQLN4-BCL2A1/BCL2L10), apoptosis assays with UBQLN4 knockdown/overexpression","journal":"Molecular oncology","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — ATM substrate identification by screening plus co-IP and functional apoptosis assay, single lab, single publication","pmids":["34245648"],"is_preprint":false},{"year":2021,"finding":"UBQLN4 binds ubiquitinated MRE11A and promotes its proteasomal degradation following cisplatin-induced DNA damage; MRE11A was found to be ubiquitinated after cisplatin treatment, and UBQLN4 binding to ubiquitinated MRE11A increased MRE11A degradation, thereby regulating MRE11A protein levels and promoting cisplatin resistance.","method":"Co-immunoprecipitation (UBQLN4-ubiquitinated MRE11A), ubiquitination assay after cisplatin treatment, UBQLN4 knockdown with MRE11A protein level measurement, gene copy number analysis","journal":"Molecular oncology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — co-IP with ubiquitination assay and functional KD, corroborates prior findings from PMID:30612738 in a different cancer context","pmids":["33605536"],"is_preprint":false},{"year":2021,"finding":"UBQLN4 activates the Wnt/β-catenin signaling pathway, upregulating β-catenin and c-Myc expression; the transcription factor C/EBPβ directly binds to the UBQLN4 core promoter and activates its transcription.","method":"GSEA, western blot, rescue experiments, dual luciferase reporter assay, ChIP assay, RT-qPCR","journal":"Cell death discovery","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — ChIP and luciferase reporter for transcriptional regulation, rescue experiment for pathway placement, single lab","pmids":["34930912"],"is_preprint":false},{"year":2023,"finding":"Full-length UBQLN4 phase separates in vitro at a lower saturation concentration than UBQLN1; the short N-terminal disordered region of UBQLN4 inhibits its phase separation via electrostatic interactions; UBQLN4 lacks the proline-rich (Pxx) region present in UBQLN2, which accounts for the absence of temperature-dependent phase behavior in UBQLN4.","method":"In vitro phase separation assays with full-length proteins, charge variant mutagenesis, domain deletion constructs","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — in vitro reconstitution of phase separation with mutagenesis, preprint, single lab","pmids":["37808720"],"is_preprint":true},{"year":2025,"finding":"UBQLN4 promotes proteasomal degradation of the NMDA receptor subunit GluN2B, thereby regulating synaptic function; UBQLN4 is localized in neurons at excitatory post-synapses, and its reduction increases seizure susceptibility, while overexpression is protective in a kainic acid-induced chronic epilepsy mouse model.","method":"AAV-mediated overexpression and knockdown in mice, kainic acid epilepsy model, proteasome inhibitor assay to confirm degradation pathway, subcellular localization by immunostaining","journal":"Neurobiology of disease","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vivo gain- and loss-of-function with mechanistic proteasomal degradation assay and substrate identification, single lab","pmids":["40930427"],"is_preprint":false}],"current_model":"UBQLN4 is a UbL-UBA domain proteasomal shuttle factor that (1) recognizes mislocalized/defective transmembrane proteins and ubiquitinated substrates (including MRE11, Cx43/Cx40/Cx45/Cx32, beta-catenin, BCL2A1, BCL2L10, GluN2B) and targets them for proteasomal degradation; (2) is phosphorylated by ATM at DNA double-strand breaks to interact with ubiquitylated MRE11, promote its removal from chromatin, and thereby curtail homologous recombination while favoring non-homologous end joining; (3) forms a nuclear export complex with POST (UBIN-POST) to shuttle polyubiquitinated proteins from the nucleus to the cytosol via CRM1; (4) binds ER membrane J proteins B12/B14 at the ER-cytosol interface; and (5) undergoes phase separation regulated by its N-terminal disordered region."},"narrative":{"mechanistic_narrative":"UBQLN4 is a UbL-UBA domain proteasomal shuttle factor that recognizes defective, mislocalized, and ubiquitinated client proteins across multiple cellular compartments and routes them to the proteasome for degradation [PMID:20940304, PMID:27113755, PMID:30612738]. At the endoplasmic reticulum it engages connexins (Cx43, Cx40, Cx45, Cx32) through direct UBA-domain contacts with their C-terminal regions and links them to the proteasome via UbL-domain interactions with the subunits S2/Rpn1 and S5a/Rpn10, accelerating connexin turnover; for Cx43 this degradation can proceed ubiquitin-independently [PMID:18079109, PMID:20940304, PMID:25583071]. In the cytoplasm UBQLN4 partners with BAG6 to capture transmembrane segments of mislocalized proteins for proteasomal clearance [PMID:27113755]. UBQLN4 binds polyubiquitin through its UBA domain and, complexed with POST, exports polyubiquitinated proteins from the nucleus to the cytosol in a CRM1-dependent manner to maintain nuclear proteostasis [PMID:29666234]. In the DNA damage response, ATM phosphorylates UBQLN4, which then binds ubiquitylated MRE11 and drives its removal from damaged chromatin by proteasomal degradation, thereby restraining homologous recombination and favoring non-homologous end joining; this activity also contributes to cisplatin resistance through control of MRE11A levels [PMID:30612738, PMID:33605536]. Additional substrates and binding partners include beta-catenin in motor neurons, the anti-apoptotic proteins BCL2A1/BCL2L10 in mesothelioma, and the NMDA receptor subunit GluN2B at excitatory post-synapses, where UBQLN4 regulates synaptic function and seizure susceptibility [PMID:28463112, PMID:34245648, PMID:40930427]. An ALS-associated UBQLN4 variant impairs proteasomal function and causes motor axon morphogenesis defects, a phenotype rescued by beta-catenin inhibition [PMID:28463112].","teleology":[{"year":2001,"claim":"Established the earliest functional clue that UBQLN4 recognizes ER-targeting signal sequences, distinguishing secretory/ER-luminal clients from mitochondrial targets.","evidence":"Yeast two-hybrid screening against signal sequences including HSP47","pmids":["11162551"],"confidence":"Low","gaps":["Single Y2H method without functional follow-up","No demonstration of degradation or proteasome linkage","Physiological relevance of signal-sequence binding unresolved"]},{"year":2008,"claim":"Defined UBQLN4 as a bridging shuttle factor that links ER-localized Cx43 to the proteasome and controls its turnover, resolving how a connexin is delivered for degradation.","evidence":"Y2H, GST pull-down, co-IP, confocal microscopy and Cx43 half-life measurement under overexpression/knockdown","pmids":["18079109"],"confidence":"High","gaps":["Did not determine whether degradation requires substrate ubiquitination","Generality beyond Cx43 untested at this stage"]},{"year":2010,"claim":"Showed UBQLN4 can mediate ubiquitin-independent proteasomal degradation of Cx43, since CIP75-associated Cx43 is non-ubiquitinated and a lysine-free mutant still binds.","evidence":"Co-IP, immunofluorescence, in vitro ubiquitin-binding assay and lysine-free Cx43 mutant analysis","pmids":["20940304"],"confidence":"High","gaps":["Mechanism of substrate recognition independent of ubiquitin not fully defined","Does not exclude ubiquitin-dependent routes for other substrates"]},{"year":2014,"claim":"Established that UBQLN4 is required for the Cx43-proteasome association but that compensatory routes permit Cx43 dislocation and degradation in its absence.","evidence":"Subcellular fractionation, co-IP, DTT-induced misfolding and shRNA knockdown","pmids":["24256120"],"confidence":"Medium","gaps":["Identity of compensatory degradation machinery unknown","Quantitative contribution of UBQLN4 to total Cx43 flux unresolved"]},{"year":2015,"claim":"Generalized the connexin-targeting role by showing direct UBA-domain binding to Cx40, Cx45 and Cx32 undergoing ERAD, with structural mapping by NMR.","evidence":"NMR spectroscopy, shRNA knockdown with western blot, trafficking inhibitor and co-IP assays","pmids":["25583071"],"confidence":"High","gaps":["Whether all connexin substrates use the same ubiquitin dependence differs (Cx32 via ubiquitination) and is not unified","In vivo physiological consequence of connexin regulation untested"]},{"year":2016,"claim":"Extended UBQLN4 to a cytoplasmic quality-control role recognizing exposed transmembrane segments of mislocalized proteins as a BAG6-binding factor.","evidence":"Co-IP for BAG6, truncated TMD degradation assay, SRP54 depletion and proteasome inhibitor treatment","pmids":["27113755"],"confidence":"Medium","gaps":["Structural basis of TMD recognition not defined","Relative roles of UBQLN4 versus other UBQLNs in this pathway unresolved"]},{"year":2017,"claim":"Linked UBQLN4 to motor neuron disease, identifying beta-catenin as a substrate and demonstrating that an ALS variant impairs proteasome function with rescuable axon defects.","evidence":"ALS-variant expression in mouse motor neurons and zebrafish, proteasomal activity assays and beta-catenin inhibition rescue","pmids":["28463112"],"confidence":"Medium","gaps":["Mechanism by which the variant impairs proteasome function unclear","Direct biochemical UBQLN4-beta-catenin interaction not detailed"]},{"year":2018,"claim":"Revealed a nuclear proteostasis function: UBQLN4 binds polyubiquitin and, with POST, exports polyubiquitinated proteins from nucleus to cytosol via CRM1.","evidence":"Nuclear/cytosol fractionation, CRM1 inhibition, co-IP, combined proteasome/CRM1 inhibitor treatment and UBA binding assays","pmids":["29666234"],"confidence":"High","gaps":["Range of nuclear substrates exported is undefined","Whether DSB-related MRE11 handling uses the same export route is not connected"]},{"year":2019,"claim":"Placed UBQLN4 in the DNA double-strand break response, showing ATM phosphorylates it to drive proteasomal removal of ubiquitylated MRE11 from chromatin and bias repair from HR toward NHEJ.","evidence":"Co-IP, ATM phosphorylation assay, chromatin fractionation, in vitro and in vivo HRR assays, loss/gain-of-function and patient mutations; with synthesizing commentary","pmids":["30612738","31131301"],"confidence":"High","gaps":["E3 ligase that ubiquitylates MRE11 not identified","How phosphorylation switches UBQLN4 substrate selectivity is unresolved"]},{"year":2020,"claim":"Identified a membrane-proximal interaction whereby UBQLN4 binds ER J proteins B12/B14 via its H domain and STI1 motifs and is hijacked by SV40 for ER escape.","evidence":"Direct binding with purified proteins, J-domain mutant and domain-deletion mapping, SV40 infection assays","pmids":["32161173"],"confidence":"Medium","gaps":["Endogenous (non-viral) function of B12/B14 binding unknown","Relationship to proteasomal shuttling activity unclear"]},{"year":2021,"claim":"Expanded UBQLN4's cancer roles by showing it stabilizes anti-apoptotic BCL2A1/BCL2L10, controls MRE11A levels to confer cisplatin resistance, and is transcriptionally driven by C/EBPbeta to activate Wnt/beta-catenin signaling.","evidence":"ATM-substrate genetic screen, co-IP, apoptosis assays, ubiquitination/degradation assays after cisplatin, ChIP and luciferase reporter assays","pmids":["34245648","33605536","34930912"],"confidence":"Medium","gaps":["How UBQLN4 stabilizes rather than degrades BCL2A1/BCL2L10 mechanistically unclear","Direct versus indirect activation of Wnt/beta-catenin not fully separated"]},{"year":2023,"claim":"Characterized UBQLN4 biophysics, showing it phase separates in vitro at a lower saturation concentration than UBQLN1 and that its N-terminal disordered region inhibits condensation electrostatically.","evidence":"In vitro phase separation assays, charge-variant mutagenesis and domain-deletion constructs (preprint)","pmids":["37808720"],"confidence":"Medium","gaps":["Preprint not peer-reviewed","Cellular relevance of phase separation to proteostasis function untested"]},{"year":2025,"claim":"Established a synaptic function by showing UBQLN4 drives proteasomal degradation of GluN2B at excitatory post-synapses and modulates seizure susceptibility in vivo.","evidence":"AAV-mediated overexpression/knockdown in mice, kainic acid epilepsy model, proteasome inhibitor assay and immunostaining","pmids":["40930427"],"confidence":"Medium","gaps":["Whether GluN2B degradation is ubiquitin-dependent not defined","Direct UBQLN4-GluN2B binding interface not mapped"]},{"year":null,"claim":"It remains unknown how UBQLN4 selects among its diverse substrates across the ER, cytosol, nucleus, and synapse, and how phosphorylation, phase separation, and partner binding integrate to switch between degradative and stabilizing outcomes.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No unifying model of substrate-selectivity switching","E3 ligases acting upstream of most substrates unidentified","In vivo relevance of phase separation to proteostasis untested"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[2,5,8,12,15]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[2,7,8]},{"term_id":"GO:0031386","term_label":"protein tag activity","supporting_discovery_ids":[2,7]}],"localization":[{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[1,2,3,10]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[5,7]},{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[7,8]},{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[15]}],"pathway":[{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[2,5,7]},{"term_id":"R-HSA-73894","term_label":"DNA Repair","supporting_discovery_ids":[8,12]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[6,13]}],"complexes":["UBIN-POST nuclear export complex"],"partners":["POST","MRE11","BAG6","GJA1","BCL2A1","BCL2L10","DNAJB12","DNAJB14"],"other_free_text":[]}},"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 international","url":"https://pubmed.ncbi.nlm.nih.gov/31911755","citation_count":22,"is_preprint":false},{"pmid":"33605536","id":"PMC_33605536","title":"Regulation of MRE11A by UBQLN4 leads to cisplatin resistance in patients with esophageal squamous cell carcinoma.","date":"2021","source":"Molecular oncology","url":"https://pubmed.ncbi.nlm.nih.gov/33605536","citation_count":19,"is_preprint":false},{"pmid":"11162551","id":"PMC_11162551","title":"Molecular cloning of a novel ubiquitin-like protein, UBIN, that binds to ER targeting signal sequences.","date":"2001","source":"Biochemical and biophysical research communications","url":"https://pubmed.ncbi.nlm.nih.gov/11162551","citation_count":19,"is_preprint":false},{"pmid":"34930912","id":"PMC_34930912","title":"UBQLN4 is activated by C/EBPβ and exerts oncogenic effects on colorectal cancer via the Wnt/β-catenin signaling pathway.","date":"2021","source":"Cell death 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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":12249,"output_tokens":4251,"usd":0.050256,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":12061,"output_tokens":4482,"usd":0.086178,"stage2_stop_reason":"end_turn"},"total_usd":0.136434,"stage1_batch_id":"msgbatch_017qqVz33Ga63SCDwzb35XPb","stage2_batch_id":"msgbatch_01Scr2Ei9x4HJMiRGxwrmvLK","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2001,\n      \"finding\": \"UBQLN4 (UBIN) binds to ER-targeting signal sequences of various secretory and ER-luminal proteins (including HSP47) but does not interact with mitochondrial targeting sequences, as determined by yeast two-hybrid screening.\",\n      \"method\": \"Yeast two-hybrid screening\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single yeast two-hybrid method, single lab, no functional follow-up reported\",\n      \"pmids\": [\"11162551\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"UBQLN4 (CIP75) interacts with connexin43 (Cx43) via its UBA domain binding to a region in Cx43 between Lys264 and Asn302 (containing PY motif and multiphosphorylation sites), and its UbL domain interacts with proteasomal subunits S2/RPN1 and S5a/RPN10; overexpression stimulates Cx43 degradation and reduces its half-life, while siRNA knockdown has the opposite effect. CIP75 localizes primarily at the ER, co-localizing with Cx43 in the perinuclear region.\",\n      \"method\": \"Yeast two-hybrid, GST pull-down, co-immunoprecipitation, laser confocal microscopy, overexpression and siRNA knockdown with Cx43 half-life measurement\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (Y2H, GST pulldown, co-IP, confocal, functional KD/OE), single lab but with rigorous controls across multiple assays\",\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 Cx43 at the ER, can bind free monoubiquitin and K48-linked tetraubiquitin chains in vitro, but the Cx43 associated with CIP75 is not ubiquitinated and a lysine-free Cx43 mutant retains the ability to interact with CIP75.\",\n      \"method\": \"Co-immunoprecipitation, immunofluorescence microscopy, in vitro ubiquitin-binding assay, lysine-free Cx43 mutant analysis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (co-IP, immunofluorescence, in vitro binding, mutagenesis), replicates and extends prior findings from same lab\",\n      \"pmids\": [\"20940304\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"UBQLN4 (CIP75) forms a complex with ER-localized Cx43 and proteasomal subunits S2/Rpn1 and S5a/Rpn10; deliberate misfolding of Cx43 by DTT enhanced CIP75 binding; shRNA-mediated knockdown of CIP75 diminished Cx43-proteasome association but still allowed ER dislocation and degradation, indicating CIP75 is essential for Cx43-proteasome interaction but alternate compensatory mechanisms exist for degradation.\",\n      \"method\": \"Subcellular fractionation, co-immunoprecipitation, DTT-induced misfolding, shRNA knockdown\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple methods (fractionation, co-IP, functional KD), single lab\",\n      \"pmids\": [\"24256120\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"UBQLN4 (CIP75) UBA domain directly interacts with the C-terminal domains of Cx40 and Cx45 (in addition to Cx43), as determined by NMR; shRNA knockdown of CIP75 in HeLa cells increases Cx40 and Cx45 levels; CIP75 preferentially interacts with connexins undergoing ERAD and also interacts with ER-localized Cx32 likely via Cx32 ubiquitination.\",\n      \"method\": \"NMR spectroscopy, shRNA knockdown with western blot, trafficking inhibitor assays, co-immunoprecipitation\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — NMR structural data combined with functional KD and co-IP, single lab but multiple orthogonal methods\",\n      \"pmids\": [\"25583071\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"UBQLN4 acts as a BAG6-binding factor that recognizes mislocalized transmembrane domain proteins in the cytoplasm (via their exposed transmembrane segment) and targets them to the proteasome for degradation; UBQLN4 also recognizes endogenous defective proteins induced by SRP54 depletion.\",\n      \"method\": \"Co-immunoprecipitation (BAG6 binding), truncated transmembrane domain protein degradation assay, SRP54 depletion, proteasome inhibitor treatment\",\n      \"journal\": \"EMBO reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional degradation assays with substrate and SRP54 knockdown, plus binding partner identification by co-IP, single lab\",\n      \"pmids\": [\"27113755\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"An ALS-associated variant of UBQLN4 impairs proteasomal function and causes motor axon morphogenesis defects in mouse motor neurons and zebrafish; beta-catenin (a Wnt signaling effector) was identified as a UBQLN4 substrate, and inhibition of beta-catenin function rescues the UBQLN4 variant-induced motor axon phenotypes.\",\n      \"method\": \"Expression of ALS-variant in mouse motor neurons and zebrafish (in vivo), proteasomal activity assays, beta-catenin inhibition rescue experiments\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vivo rescue experiment plus substrate identification and proteasomal activity assay, single lab\",\n      \"pmids\": [\"28463112\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"UBQLN4 (UBIN) shuttles between the nucleus and cytosol in a CRM1-dependent manner via its binding partner POST (which harbors a nuclear export signal); UBIN binds polyubiquitin chains through its UBA domain, and the UBIN-POST complex exports polyubiquitinated proteins from the nucleus to the cytosol for proteasomal degradation, maintaining nuclear protein homeostasis.\",\n      \"method\": \"Nuclear/cytosol fractionation, CRM1 inhibition, co-immunoprecipitation, proteasome inhibitor and CRM1 inhibitor co-treatment, UBA domain binding assays\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (fractionation, co-IP, pharmacological inhibition, binding assays), clear mechanistic dissection of export complex\",\n      \"pmids\": [\"29666234\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"UBQLN4 is phosphorylated by ATM and interacts with ubiquitylated MRE11 to mediate early steps of homologous recombination-mediated DSB repair (HRR); UBQLN4 promotes removal of MRE11 from damaged chromatin via proteasomal degradation. Loss of UBQLN4 leads to chromatin retention of MRE11 and non-physiological HRR activity, while UBQLN4 overexpression represses HRR and favors non-homologous end joining (NHEJ).\",\n      \"method\": \"Co-immunoprecipitation (UBQLN4-MRE11 interaction), ATM phosphorylation assay, chromatin fractionation, in vitro and in vivo HRR assays, loss-of-function and overexpression studies, patient-derived mutations\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — multiple orthogonal methods (co-IP, kinase assay, chromatin fractionation, functional in vitro and in vivo HRR assays), mechanistic dissection replicated in multiple cell lines and patient data\",\n      \"pmids\": [\"30612738\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"The UBQLN4-MRE11 interaction is ATM-dependent, indicating that ATM (which initiates HR) also limits excessive end-resection by phosphorylating UBQLN4 to promote MRE11 degradation and repress HR.\",\n      \"method\": \"Review/commentary of experimental data from PMID:30612738, epistasis analysis\",\n      \"journal\": \"Molecular & cellular oncology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — commentary synthesizing experimental data from the primary Cell paper; no new experiments reported\",\n      \"pmids\": [\"31131301\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"UBQLN4 (Ubqln4) directly binds ER membrane J proteins B12 and B14 at the ER-cytosol interface (in a J-domain-independent manner via its H domain and STI1 motifs 1-2) and captures SV40 virus emerging from the ER, facilitating viral escape from the ER to the cytosol and promoting infection.\",\n      \"method\": \"Direct binding assays (purified proteins), J-domain mutant analysis, domain deletion mapping (H domain, STI1 motifs), SV40 infection assays with knockdown/overexpression\",\n      \"journal\": \"Journal of virology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct binding with purified proteins plus domain-mapping mutagenesis and functional infection assay, single lab\",\n      \"pmids\": [\"32161173\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"UBQLN4 is a substrate for ATM kinase (phosphorylated by ATM); UBQLN4 interacts with and stabilizes the anti-apoptotic proteins BCL2A1 and BCL2L10, preventing mesothelioma cell apoptosis in response to DNA damage.\",\n      \"method\": \"Mammalian functional genetic screening for ATM substrates, co-immunoprecipitation (UBQLN4-BCL2A1/BCL2L10), apoptosis assays with UBQLN4 knockdown/overexpression\",\n      \"journal\": \"Molecular oncology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — ATM substrate identification by screening plus co-IP and functional apoptosis assay, single lab, single publication\",\n      \"pmids\": [\"34245648\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"UBQLN4 binds ubiquitinated MRE11A and promotes its proteasomal degradation following cisplatin-induced DNA damage; MRE11A was found to be ubiquitinated after cisplatin treatment, and UBQLN4 binding to ubiquitinated MRE11A increased MRE11A degradation, thereby regulating MRE11A protein levels and promoting cisplatin resistance.\",\n      \"method\": \"Co-immunoprecipitation (UBQLN4-ubiquitinated MRE11A), ubiquitination assay after cisplatin treatment, UBQLN4 knockdown with MRE11A protein level measurement, gene copy number analysis\",\n      \"journal\": \"Molecular oncology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — co-IP with ubiquitination assay and functional KD, corroborates prior findings from PMID:30612738 in a different cancer context\",\n      \"pmids\": [\"33605536\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"UBQLN4 activates the Wnt/β-catenin signaling pathway, upregulating β-catenin and c-Myc expression; the transcription factor C/EBPβ directly binds to the UBQLN4 core promoter and activates its transcription.\",\n      \"method\": \"GSEA, western blot, rescue experiments, dual luciferase reporter assay, ChIP assay, RT-qPCR\",\n      \"journal\": \"Cell death discovery\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — ChIP and luciferase reporter for transcriptional regulation, rescue experiment for pathway placement, single lab\",\n      \"pmids\": [\"34930912\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Full-length UBQLN4 phase separates in vitro at a lower saturation concentration than UBQLN1; the short N-terminal disordered region of UBQLN4 inhibits its phase separation via electrostatic interactions; UBQLN4 lacks the proline-rich (Pxx) region present in UBQLN2, which accounts for the absence of temperature-dependent phase behavior in UBQLN4.\",\n      \"method\": \"In vitro phase separation assays with full-length proteins, charge variant mutagenesis, domain deletion constructs\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — in vitro reconstitution of phase separation with mutagenesis, preprint, single lab\",\n      \"pmids\": [\"37808720\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"UBQLN4 promotes proteasomal degradation of the NMDA receptor subunit GluN2B, thereby regulating synaptic function; UBQLN4 is localized in neurons at excitatory post-synapses, and its reduction increases seizure susceptibility, while overexpression is protective in a kainic acid-induced chronic epilepsy mouse model.\",\n      \"method\": \"AAV-mediated overexpression and knockdown in mice, kainic acid epilepsy model, proteasome inhibitor assay to confirm degradation pathway, subcellular localization by immunostaining\",\n      \"journal\": \"Neurobiology of disease\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vivo gain- and loss-of-function with mechanistic proteasomal degradation assay and substrate identification, single lab\",\n      \"pmids\": [\"40930427\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"UBQLN4 is a UbL-UBA domain proteasomal shuttle factor that (1) recognizes mislocalized/defective transmembrane proteins and ubiquitinated substrates (including MRE11, Cx43/Cx40/Cx45/Cx32, beta-catenin, BCL2A1, BCL2L10, GluN2B) and targets them for proteasomal degradation; (2) is phosphorylated by ATM at DNA double-strand breaks to interact with ubiquitylated MRE11, promote its removal from chromatin, and thereby curtail homologous recombination while favoring non-homologous end joining; (3) forms a nuclear export complex with POST (UBIN-POST) to shuttle polyubiquitinated proteins from the nucleus to the cytosol via CRM1; (4) binds ER membrane J proteins B12/B14 at the ER-cytosol interface; and (5) undergoes phase separation regulated by its N-terminal disordered region.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"UBQLN4 is a UbL-UBA domain proteasomal shuttle factor that recognizes defective, mislocalized, and ubiquitinated client proteins across multiple cellular compartments and routes them to the proteasome for degradation [#2, #5, #8]. At the endoplasmic reticulum it engages connexins (Cx43, Cx40, Cx45, Cx32) through direct UBA-domain contacts with their C-terminal regions and links them to the proteasome via UbL-domain interactions with the subunits S2/Rpn1 and S5a/Rpn10, accelerating connexin turnover; for Cx43 this degradation can proceed ubiquitin-independently [#1, #2, #4]. In the cytoplasm UBQLN4 partners with BAG6 to capture transmembrane segments of mislocalized proteins for proteasomal clearance [#5]. UBQLN4 binds polyubiquitin through its UBA domain and, complexed with POST, exports polyubiquitinated proteins from the nucleus to the cytosol in a CRM1-dependent manner to maintain nuclear proteostasis [#7]. In the DNA damage response, ATM phosphorylates UBQLN4, which then binds ubiquitylated MRE11 and drives its removal from damaged chromatin by proteasomal degradation, thereby restraining homologous recombination and favoring non-homologous end joining; this activity also contributes to cisplatin resistance through control of MRE11A levels [#8, #12]. Additional substrates and binding partners include beta-catenin in motor neurons, the anti-apoptotic proteins BCL2A1/BCL2L10 in mesothelioma, and the NMDA receptor subunit GluN2B at excitatory post-synapses, where UBQLN4 regulates synaptic function and seizure susceptibility [#6, #11, #15]. An ALS-associated UBQLN4 variant impairs proteasomal function and causes motor axon morphogenesis defects, a phenotype rescued by beta-catenin inhibition [#6].\"\n,\n  \"teleology\": [\n    {\n      \"year\": 2001,\n      \"claim\": \"Established the earliest functional clue that UBQLN4 recognizes ER-targeting signal sequences, distinguishing secretory/ER-luminal clients from mitochondrial targets.\",\n      \"evidence\": \"Yeast two-hybrid screening against signal sequences including HSP47\",\n      \"pmids\": [\"11162551\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"Single Y2H method without functional follow-up\", \"No demonstration of degradation or proteasome linkage\", \"Physiological relevance of signal-sequence binding unresolved\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Defined UBQLN4 as a bridging shuttle factor that links ER-localized Cx43 to the proteasome and controls its turnover, resolving how a connexin is delivered for degradation.\",\n      \"evidence\": \"Y2H, GST pull-down, co-IP, confocal microscopy and Cx43 half-life measurement under overexpression/knockdown\",\n      \"pmids\": [\"18079109\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not determine whether degradation requires substrate ubiquitination\", \"Generality beyond Cx43 untested at this stage\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Showed UBQLN4 can mediate ubiquitin-independent proteasomal degradation of Cx43, since CIP75-associated Cx43 is non-ubiquitinated and a lysine-free mutant still binds.\",\n      \"evidence\": \"Co-IP, immunofluorescence, in vitro ubiquitin-binding assay and lysine-free Cx43 mutant analysis\",\n      \"pmids\": [\"20940304\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of substrate recognition independent of ubiquitin not fully defined\", \"Does not exclude ubiquitin-dependent routes for other substrates\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Established that UBQLN4 is required for the Cx43-proteasome association but that compensatory routes permit Cx43 dislocation and degradation in its absence.\",\n      \"evidence\": \"Subcellular fractionation, co-IP, DTT-induced misfolding and shRNA knockdown\",\n      \"pmids\": [\"24256120\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Identity of compensatory degradation machinery unknown\", \"Quantitative contribution of UBQLN4 to total Cx43 flux unresolved\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Generalized the connexin-targeting role by showing direct UBA-domain binding to Cx40, Cx45 and Cx32 undergoing ERAD, with structural mapping by NMR.\",\n      \"evidence\": \"NMR spectroscopy, shRNA knockdown with western blot, trafficking inhibitor and co-IP assays\",\n      \"pmids\": [\"25583071\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether all connexin substrates use the same ubiquitin dependence differs (Cx32 via ubiquitination) and is not unified\", \"In vivo physiological consequence of connexin regulation untested\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Extended UBQLN4 to a cytoplasmic quality-control role recognizing exposed transmembrane segments of mislocalized proteins as a BAG6-binding factor.\",\n      \"evidence\": \"Co-IP for BAG6, truncated TMD degradation assay, SRP54 depletion and proteasome inhibitor treatment\",\n      \"pmids\": [\"27113755\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Structural basis of TMD recognition not defined\", \"Relative roles of UBQLN4 versus other UBQLNs in this pathway unresolved\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Linked UBQLN4 to motor neuron disease, identifying beta-catenin as a substrate and demonstrating that an ALS variant impairs proteasome function with rescuable axon defects.\",\n      \"evidence\": \"ALS-variant expression in mouse motor neurons and zebrafish, proteasomal activity assays and beta-catenin inhibition rescue\",\n      \"pmids\": [\"28463112\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism by which the variant impairs proteasome function unclear\", \"Direct biochemical UBQLN4-beta-catenin interaction not detailed\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Revealed a nuclear proteostasis function: UBQLN4 binds polyubiquitin and, with POST, exports polyubiquitinated proteins from nucleus to cytosol via CRM1.\",\n      \"evidence\": \"Nuclear/cytosol fractionation, CRM1 inhibition, co-IP, combined proteasome/CRM1 inhibitor treatment and UBA binding assays\",\n      \"pmids\": [\"29666234\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Range of nuclear substrates exported is undefined\", \"Whether DSB-related MRE11 handling uses the same export route is not connected\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Placed UBQLN4 in the DNA double-strand break response, showing ATM phosphorylates it to drive proteasomal removal of ubiquitylated MRE11 from chromatin and bias repair from HR toward NHEJ.\",\n      \"evidence\": \"Co-IP, ATM phosphorylation assay, chromatin fractionation, in vitro and in vivo HRR assays, loss/gain-of-function and patient mutations; with synthesizing commentary\",\n      \"pmids\": [\"30612738\", \"31131301\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"E3 ligase that ubiquitylates MRE11 not identified\", \"How phosphorylation switches UBQLN4 substrate selectivity is unresolved\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Identified a membrane-proximal interaction whereby UBQLN4 binds ER J proteins B12/B14 via its H domain and STI1 motifs and is hijacked by SV40 for ER escape.\",\n      \"evidence\": \"Direct binding with purified proteins, J-domain mutant and domain-deletion mapping, SV40 infection assays\",\n      \"pmids\": [\"32161173\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Endogenous (non-viral) function of B12/B14 binding unknown\", \"Relationship to proteasomal shuttling activity unclear\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Expanded UBQLN4's cancer roles by showing it stabilizes anti-apoptotic BCL2A1/BCL2L10, controls MRE11A levels to confer cisplatin resistance, and is transcriptionally driven by C/EBPbeta to activate Wnt/beta-catenin signaling.\",\n      \"evidence\": \"ATM-substrate genetic screen, co-IP, apoptosis assays, ubiquitination/degradation assays after cisplatin, ChIP and luciferase reporter assays\",\n      \"pmids\": [\"34245648\", \"33605536\", \"34930912\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"How UBQLN4 stabilizes rather than degrades BCL2A1/BCL2L10 mechanistically unclear\", \"Direct versus indirect activation of Wnt/beta-catenin not fully separated\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Characterized UBQLN4 biophysics, showing it phase separates in vitro at a lower saturation concentration than UBQLN1 and that its N-terminal disordered region inhibits condensation electrostatically.\",\n      \"evidence\": \"In vitro phase separation assays, charge-variant mutagenesis and domain-deletion constructs (preprint)\",\n      \"pmids\": [\"37808720\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Preprint not peer-reviewed\", \"Cellular relevance of phase separation to proteostasis function untested\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Established a synaptic function by showing UBQLN4 drives proteasomal degradation of GluN2B at excitatory post-synapses and modulates seizure susceptibility in vivo.\",\n      \"evidence\": \"AAV-mediated overexpression/knockdown in mice, kainic acid epilepsy model, proteasome inhibitor assay and immunostaining\",\n      \"pmids\": [\"40930427\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether GluN2B degradation is ubiquitin-dependent not defined\", \"Direct UBQLN4-GluN2B binding interface not mapped\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"It remains unknown how UBQLN4 selects among its diverse substrates across the ER, cytosol, nucleus, and synapse, and how phosphorylation, phase separation, and partner binding integrate to switch between degradative and stabilizing outcomes.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No unifying model of substrate-selectivity switching\", \"E3 ligases acting upstream of most substrates unidentified\", \"In vivo relevance of phase separation to proteostasis untested\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [2, 5, 8, 12, 15]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [2, 7, 8]},\n      {\"term_id\": \"GO:0031386\", \"supporting_discovery_ids\": [2, 7]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [1, 2, 3, 10]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [5, 7]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [7, 8]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [15]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [2, 5, 7]},\n      {\"term_id\": \"R-HSA-73894\", \"supporting_discovery_ids\": [8, 12]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [6, 13]}\n    ],\n    \"complexes\": [\"UBIN-POST nuclear export complex\"],\n    \"partners\": [\"POST\", \"MRE11\", \"BAG6\", \"GJA1\", \"BCL2A1\", \"BCL2L10\", \"DNAJB12\", \"DNAJB14\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":7,"faith_total":7,"faith_pct":100.0}}