{"gene":"COPS4","run_date":"2026-06-09T22:57:19","timeline":{"discoveries":[{"year":2001,"finding":"CSN4 is a component of a smaller cytoplasmic COP9 signalosome subcomplex (~100 kDa, containing CSN4-8 but not CSN1-3) that is distinct from the nuclear 450-kDa holocomplex; this subcomplex arises from nuclear export and is sensitive to leptomycin B treatment.","method":"Glycerol gradient sedimentation and cell fractionation experiments; leptomycin B treatment","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — cell fractionation and gradient sedimentation with pharmacological perturbation, single lab","pmids":["11704659"],"is_preprint":false},{"year":2001,"finding":"CSN4 participates in a pairwise interaction network within the COP9 signalosome; yeast two-hybrid analysis detected Csn4/5/7/6 paired interactions in Arabidopsis, implying a similar quaternary arrangement as in the 26S proteasome lid.","method":"Yeast two-hybrid analysis of all possible paired subunit interactions","journal":"The EMBO journal","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single-method yeast two-hybrid, Arabidopsis orthologs, single lab","pmids":["11742986"],"is_preprint":false},{"year":2002,"finding":"In Drosophila, CSN4 (Drosophila ortholog) forms a complex of similar size to plant and mammalian CSN; null mutations in csn4 are larval lethal and cause defective oocyte/embryo patterning and defects in response to DNA damage, with unique phenotypes reminiscent of ecdysone signaling defects distinct from csn5 mutants.","method":"Gel filtration with CSN subunit antibodies; null mutant generation and phenotypic characterization","journal":"Development (Cambridge, England)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic null mutants with multiple phenotypic readouts and biochemical complex analysis, single lab","pmids":["12223399"],"is_preprint":false},{"year":2002,"finding":"In fission yeast, Csn4 is required for removal of Nedd8 from the cullin Pcu1, and its protein product associates with Csn1 and Csn2; however, csn4 null mutants do not share the DNA damage sensitivity and slow replication phenotypes of csn1/csn2 mutants, indicating subunit-specific functional differences.","method":"Null mutant characterization; co-immunoprecipitation; Nedd8 removal assays","journal":"Molecular biology of the cell","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic null mutants with biochemical deneddylation assay and co-IP, single lab","pmids":["11854407"],"is_preprint":false},{"year":2003,"finding":"In Drosophila oogenesis, CSN4 (with CSN5/Jab1) acts through the CSN to remove Nedd8 from Cullin1 (a subunit of SCF ubiquitin ligase), and genetic epistasis shows that Cyclin E is the major downstream target; CSN4 and Cyclin E mutations reciprocally suppress each other.","method":"Drosophila genetics (CSN4 and CSN5 mutants), genetic epistasis/suppressor analysis, Cyclin E overexpression","journal":"Developmental cell","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic epistasis with multiple alleles and reciprocal suppression, single lab","pmids":["12737805"],"is_preprint":false},{"year":2007,"finding":"The Drosophila CSN4 subunit co-occupies Retinoblastoma target gene promoters with Rbf1 and Rbf2, and physically interacts with Rbf2 during embryogenesis; knockdown of CSN4 leads to increased proteasome-mediated destruction of Rbf1 and Rbf2 and altered cell cycle progression.","method":"Co-immunoprecipitation; chromatin immunoprecipitation (ChIP); targeted RNAi knockdown with Western blot","journal":"Molecular biology of the cell","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal IP and ChIP with functional RNAi readout, single lab, multiple orthogonal methods","pmids":["17251548"],"is_preprint":false},{"year":2009,"finding":"Mass spectrometry-based interaction mapping of reconstituted human CSN identified two symmetrical modules: Csn1/2/3/8 and Csn4/5/6/7, connected by Csn1-Csn6 interactions; the active complex contains a single copy of each of the eight subunits.","method":"Native mass spectrometry of in vitro reconstituted human CSN complex; subcomplex dissociation mapping","journal":"Structure (London, England : 1993)","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro reconstitution of the human complex with native MS structural analysis identifying 35 subcomplexes and interaction map","pmids":["19141280"],"is_preprint":false},{"year":2009,"finding":"CSN4 null mutations in Drosophila prevent normal light-dependent Timeless (TIM) degradation in pacemaker neurons and impair behavioral phase shifts; this places CSN4/CSN in the Jetlag (JET) F-box protein pathway for light-mediated TIM degradation.","method":"Drosophila genetic null mutants; immunofluorescence of TIM in lateral neurons; behavioral phase-shift assays; genetic epistasis with jetlag mutants","journal":"The Journal of neuroscience","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic epistasis and cellular localization readout with multiple behavioral assays, single lab","pmids":["19176824"],"is_preprint":false},{"year":2009,"finding":"Systematic mass spectrometric pulldown of mammalian CSN4 (among other subunits) defined the protein interaction network of the CSN, identifying stable interactions with a subset of CRL complexes including CRL4(Ddb2), and revealing Dda1 as a new CRL4-associated protein.","method":"Mass spectrometry-based interactome mapping of CSN subunits including Csn4 by affinity purification","journal":"Journal of cell science","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — systematic AP-MS of CSN4 among other subunits, single lab, large-scale interactome","pmids":["19295130"],"is_preprint":false},{"year":2010,"finding":"Human CSN4 (or CSN5) knockdown induces proteasome-mediated degradation of the ubiquitin-conjugating enzyme UBC3/Cdc34 via SCF(betaTrCP); the CSN normally protects UBC3 from ubiquitination requiring UBC3's acidic C-terminal extension.","method":"siRNA knockdown of CSN4/CSN5; co-immunoprecipitation; in vitro ubiquitination assay; domain mapping","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — RNAi knockdown with biochemical ubiquitination assay and domain mapping, single lab","pmids":["20378537"],"is_preprint":false},{"year":2010,"finding":"In Neurospora crassa, CSN-4 knockout abolishes deneddylation of cullin proteins (Cul1, Cul3, Cul4), destabilizes Cul1 in SCF complexes and Cul3/BTB proteins in Cul3-BTB E3s, and SKP-1 in SCF complexes; this results in severe defects in growth, conidiation, and circadian rhythm.","method":"Gene knockout mutant generation; neddylation state analysis by immunoblot; phenotypic characterization","journal":"PLoS genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic KO with direct biochemical cullin neddylation assay, multiple substrates tested, single lab","pmids":["21151958"],"is_preprint":false},{"year":2010,"finding":"TorsinA (TA) binds directly to CSN4 in neuroblastoma cells and brain synaptosomes; CSN4 and TA are both required for the stability of synaptic proteins snapin and stonin 2; snapin stability is regulated through CSN-associated kinase PKD (protein kinase D) phosphorylation, while stonin 2 stability is regulated through neddylation.","method":"Co-immunoprecipitation; siRNA knockdown; Western blotting; synaptosome fractionation","journal":"The EMBO journal","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal co-IP in cells and synaptosomes with RNAi functional validation, single lab","pmids":["21102408"],"is_preprint":false},{"year":2012,"finding":"A CSN4-5-6-7 subcomplex was reconstituted in vitro: CSN7, CSN4, and CSN6 form a stable heterotrimer (requiring co-expression); CSN5 can be added to reconstitute the quaternary complex. The subcomplex is stabilized by MPN-MPN interactions (CSN5-CSN6), PCI-PCI interactions (CSN4-CSN7), and CSN6 C-terminus interactions with CSN4 and CSN7; CSN8 also interacts with the CSN4-6-7 core.","method":"Bacterial co-expression reconstitution; pairwise and combinatorial interaction analysis; biochemical and biophysical characterization","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro reconstitution with systematic domain interaction mapping and multiple biophysical methods","pmids":["23086934"],"is_preprint":false},{"year":2014,"finding":"CSN4 was identified as a novel binding partner of soluble guanylyl cyclase α1 (sGCα1) in prostate cancer cells; the CSN4-sGCα1 interaction inhibits sGCα1 proteasomal degradation, while CSN4 promotes p53 degradation; these effects are mediated through a CSN4-CSN5-CK2 complex, with CK2 regulating stability of both sGCα1 and p53.","method":"Co-immunoprecipitation; siRNA knockdown of CSN4/CSN5; Western blot for protein stability; CK2 inhibition","journal":"Molecular endocrinology (Baltimore, Md.)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — co-IP and RNAi with functional protein stability readouts, multiple targets, single lab","pmids":["24725084"],"is_preprint":false},{"year":2014,"finding":"CSN4 was identified as a Ca2+-dependent binding partner of the EF-hand protein tescalcin; this interaction involves the PCI domain of CSN4; tescalcin upregulation during differentiation coincides with reduced CSN deneddylation of Cul1 and stabilization of p27Kip1, and tescalcin knockdown increases Cul1 deneddylation and Skp2/c-Jun expression while decreasing p27/p53.","method":"Co-immunoprecipitation; domain mapping; siRNA knockdown; Western blot for deneddylation and downstream targets","journal":"Journal of cell science","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — co-IP with domain specificity and RNAi functional readout, Ca2+-dependence tested, single lab","pmids":["24659803"],"is_preprint":false},{"year":2014,"finding":"3D cryo-EM structural analysis of human CSN reveals a central horseshoe-shaped segment formed by PCI domain subunits; CSN2 and CSN4 densities are better defined in cryo-EM maps compared to negative stain, contributing to the overall structural model of the complex.","method":"Negative stain EM and cryo-EM single-particle analysis; in vitro deneddylation assay","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — cryo-EM structural analysis, single lab, limited resolution details in abstract","pmids":["24973710"],"is_preprint":false},{"year":2014,"finding":"In Drosophila GSCs, CSN4 is specifically required for germline stem cell self-renewal but not differentiation; the differentiation factor Bam sequesters Csn4 from the COP9 complex via protein competition, inactivating COP9's self-renewal function and allowing other Csn proteins to promote differentiation.","method":"Drosophila genetics; GSC clonal analysis; co-immunoprecipitation of Bam-Csn4; loss-of-function phenotyping","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic analysis with multiple alleles, reciprocal Co-IP, and cellular phenotyping replicated across Csn subunits, published in Nature","pmids":["25119050"],"is_preprint":false},{"year":2016,"finding":"Structural and kinetic analysis showed that N-terminal domains of Csn4 (and Csn2) play important roles in sensing neddylated CRL substrates and enabling formation of a high-affinity fully active CSN-CRL complex; Csn4 communicates binding of neddylated substrate to Csn5 to activate deneddylase activity, with large conformational changes occurring upon binding.","method":"Cryo-EM structural analysis; hydrogen-deuterium exchange MS; kinetic binding assays; mutagenesis","journal":"eLife","confidence":"High","confidence_rationale":"Tier 1 / Strong — cryo-EM structure plus kinetic assays plus mutagenesis of specific domains in a single rigorous study","pmids":["27031283"],"is_preprint":false},{"year":2019,"finding":"TCTP (Translationally Controlled Tumor Protein) controls G1/S transition by interacting with CSN4; genetic interaction studies in Arabidopsis, tobacco cells, and Drosophila showed that downregulation of CSN4 delays G1/S transition; loss-of-function of TCTP increases the fraction of deneddylated CUL1, suggesting TCTP negatively interferes with COP9 (CSN4) function to maintain CUL1 neddylation.","method":"Genetic interaction studies; knockdown in multiple organisms; CUL1 neddylation state analysis by immunoblot","journal":"PLoS genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic epistasis in multiple organisms with direct biochemical neddylation readout, single lab","pmids":["30695029"],"is_preprint":false},{"year":2019,"finding":"In breast cancer MDA-MB-231 cells, CSN4 knockdown decreases cellular proliferation, increases sub-G1 population (apoptosis), and alters expression of CDK6 and Caspase3, indicating CSN4 modulates cell cycle progression and apoptosis through these regulators.","method":"Lentivirus-mediated CSN4 knockdown; CCK8 proliferation assay; colony formation; cell cycle analysis; Western blot for CDK6 and Caspase3","journal":"Yi chuan = Hereditas","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, single knockdown approach, no direct mechanistic link established beyond correlation with CDK6/Caspase3 levels","pmids":["30992253"],"is_preprint":false},{"year":2026,"finding":"CSN4 binds directly to DDB1; loss of CSN4 leads to autophagy-mediated destabilization of DDB1 (the DDB1 K1131R mutant is resistant to CSN4 depletion-induced downregulation), reduced CRL4 assembly, and impaired DNA damage-induced ubiquitination; CSN4 deficiency activates autophagy as an alternative degradation pathway for DDB1.","method":"Co-immunoprecipitation; siRNA knockdown; site-directed mutagenesis (DDB1 K1131R); CRL4 assembly assay; ubiquitination assay; autophagy pathway analysis","journal":"Cellular and molecular life sciences : CMLS","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct binding demonstrated by co-IP, mutagenesis of resistance mutant, and functional assembly/ubiquitination assays, single lab","pmids":["41686221"],"is_preprint":false}],"current_model":"COPS4/CSN4 is a PCI-domain subunit of the COP9 signalosome (CSN) that forms the core of a CSN4-5-6-7 submodule (with PCI-PCI interactions between CSN4-CSN7 and MPN-MPN between CSN5-CSN6), senses neddylated cullin-RING ligase (CRL) substrates and communicates to the catalytic CSN5 subunit to activate deneddylation, stabilizes DDB1 and CRL4 assembly in the DNA damage response, protects selected substrates (sGCα1, UBC3/Cdc34, synaptic proteins snapin and stonin 2, Retinoblastoma proteins) from proteasomal degradation, and participates in a smaller cytoplasmic subcomplex (CSN4-8); in Drosophila germline stem cells, CSN4 is specifically sequestered from the COP9 complex by the differentiation factor Bam to switch the complex from self-renewal to differentiation mode."},"narrative":{"mechanistic_narrative":"COPS4/CSN4 is a PCI-domain subunit of the COP9 signalosome (CSN), the eight-subunit complex that deneddylates cullin-RING ubiquitin ligases (CRLs) to regulate their activity and substrate turnover [PMID:19141280, PMID:21151958]. Within the CSN, CSN4 anchors one of two symmetrical structural modules: it forms a stable CSN4-5-6-7 core stabilized by a PCI-PCI contact with CSN7 and bridged to the CSN5-CSN6 MPN dimer, with CSN8 also docking onto this core [PMID:19141280, PMID:23086934]. CSN4 contributes to the central horseshoe of PCI subunits in the assembled complex, and its N-terminal domains sense neddylated CRL substrate and relay that binding through large conformational changes to activate the catalytic CSN5 subunit [PMID:24973710, PMID:27031283]. Through these activities CSN4/CSN governs deneddylation of multiple cullins (Cul1, Cul3, Cul4) and the stability of CRL components such as SKP1, controlling SCF and CRL3 function across development and the circadian clock [PMID:21151958, PMID:19176824]. Beyond catalytic deneddylation, CSN4 protects selected proteins from degradation — the ubiquitin-conjugating enzyme UBC3/Cdc34 from SCF(betaTrCP)-mediated turnover, the synaptic proteins snapin and stonin 2 in complex with TorsinA, soluble guanylyl cyclase alpha1, and Retinoblastoma proteins Rbf1/Rbf2 at target promoters — while also promoting p53 degradation [PMID:20378537, PMID:21102408, PMID:24725084, PMID:17251548]. CSN4 binds directly to DDB1 to support CRL4 assembly and DNA-damage-induced ubiquitination, with its loss diverting DDB1 to autophagic degradation [PMID:41686221]. In Drosophila germline stem cells, the differentiation factor Bam sequesters CSN4 from the COP9 complex by protein competition, switching the complex from a self-renewal to a differentiation mode [PMID:25119050].","teleology":[{"year":2001,"claim":"Established that CSN4 exists not only in the large nuclear CSN holocomplex but also in a smaller cytoplasmic subcomplex, raising the question of compartment-specific CSN forms.","evidence":"Glycerol gradient sedimentation, cell fractionation, and leptomycin B treatment defining a ~100 kDa CSN4-8 cytoplasmic subcomplex","pmids":["11704659"],"confidence":"Medium","gaps":["Function of the cytoplasmic CSN4-8 subcomplex not defined","Single lab, pharmacological perturbation only"]},{"year":2001,"claim":"Defined the pairwise subunit interaction network placing CSN4 adjacent to CSN5/6/7, foreshadowing the modular architecture of the complex.","evidence":"Yeast two-hybrid analysis of all paired subunit interactions in Arabidopsis","pmids":["11742986"],"confidence":"Low","gaps":["Single-method Y2H in plant orthologs","Does not establish direct contacts in the assembled human complex"]},{"year":2002,"claim":"Genetic and biochemical analyses in Drosophila and fission yeast showed CSN4 is essential and required for cullin deneddylation, but with subunit-specific phenotypes distinct from CSN1/CSN2/CSN5, indicating non-redundant roles among CSN subunits.","evidence":"Null mutant phenotyping, gel filtration, co-IP with Csn1/Csn2, and Nedd8-removal assays on Pcu1","pmids":["12223399","11854407"],"confidence":"Medium","gaps":["Molecular basis of subunit-specific phenotypes unresolved","Whether CSN4 acts catalytically or structurally not separated"]},{"year":2003,"claim":"Connected CSN4-mediated deneddylation of Cullin1 to a defined downstream output by identifying Cyclin E as the major target through reciprocal genetic suppression.","evidence":"Drosophila genetic epistasis and suppressor analysis with Cyclin E overexpression","pmids":["12737805"],"confidence":"Medium","gaps":["Direct biochemical link between CSN4 and Cyclin E turnover not shown","Restricted to oogenesis context"]},{"year":2007,"claim":"Revealed a non-canonical, promoter-associated role: CSN4 co-occupies Rb target genes and protects Rbf1/Rbf2 from proteasomal destruction, linking CSN4 to transcriptional and cell-cycle control.","evidence":"Co-IP, ChIP, and RNAi knockdown with Western blot in Drosophila embryos","pmids":["17251548"],"confidence":"Medium","gaps":["Whether protection occurs via holocomplex deneddylation or a separate activity unclear","Single lab"]},{"year":2009,"claim":"Resolved the architecture of the human CSN into two symmetrical modules and placed CSN4 in the CSN4/5/6/7 module, defining the structural neighborhood through which it acts.","evidence":"Native mass spectrometry of reconstituted human CSN with subcomplex dissociation mapping","pmids":["19141280"],"confidence":"High","gaps":["Static interaction map without catalytic state","Does not address conformational dynamics during deneddylation"]},{"year":2009,"claim":"Extended CSN4/CSN function into circadian biology and mapped its physical engagement with specific CRL complexes.","evidence":"Drosophila null mutants with TIM immunofluorescence and behavioral phase-shift assays; AP-MS interactome of mammalian CSN4 identifying CRL4(Ddb2) and Dda1","pmids":["19176824","19295130"],"confidence":"Medium","gaps":["Direct enzymatic action of CSN4 on the JET/TIM CRL not shown","Interactome correlative for many partners"]},{"year":2010,"claim":"Demonstrated that CSN4 protects substrates beyond cullins — stabilizing UBC3/Cdc34 and the TorsinA-associated synaptic proteins snapin and stonin 2 — establishing a protective function distinct from bulk deneddylation.","evidence":"siRNA knockdown, co-IP, in vitro ubiquitination, domain mapping (UBC3); reciprocal co-IP in cells and synaptosomes with RNAi (TorsinA/snapin/stonin 2); Neurospora csn-4 knockout with cullin neddylation assays","pmids":["20378537","21102408","21151958"],"confidence":"Medium","gaps":["Mechanism distinguishing protection from deneddylation incomplete","Tissue-specificity of synaptic role not generalized"]},{"year":2012,"claim":"Reconstituted the CSN4-5-6-7 submodule in vitro and mapped the specific PCI-PCI (CSN4-CSN7) and MPN-MPN contacts that build it, providing a molecular blueprint for CSN4's structural role.","evidence":"Bacterial co-expression reconstitution with combinatorial interaction and biophysical analysis","pmids":["23086934"],"confidence":"High","gaps":["Subcomplex behavior may differ from holocomplex","Does not address substrate sensing"]},{"year":2014,"claim":"Identified CSN4 as a regulated hub interacting with sGCalpha1, tescalcin (Ca2+-dependent, PCI-domain mediated), and contributing to the cryo-EM PCI horseshoe, connecting partner binding to modulation of CRL deneddylation and substrate stability.","evidence":"Co-IP, domain mapping, siRNA knockdown, CK2 inhibition (sGCalpha1/p53); Ca2+-dependent co-IP and deneddylation readouts (tescalcin); negative-stain and cryo-EM of human CSN","pmids":["24725084","24659803","24973710"],"confidence":"Medium","gaps":["How partner binding allosterically tunes CSN activity not mechanistically resolved","Cancer-cell-context dependence"]},{"year":2014,"claim":"Showed that CSN4 can be selectively extracted from the CSN by a competing factor to reprogram complex function, demonstrating subunit sequestration as a regulatory switch in stem-cell fate.","evidence":"Drosophila GSC clonal genetics with multiple alleles and Bam-Csn4 co-IP","pmids":["25119050"],"confidence":"High","gaps":["Whether analogous sequestration occurs in mammals unknown","Stoichiometry of Bam-CSN4 competition not quantified"]},{"year":2016,"claim":"Defined CSN4's catalytic-regulatory role: its N-terminal domains sense neddylated CRL substrate and transmit binding through conformational change to activate CSN5, explaining how the complex achieves substrate-stimulated deneddylation.","evidence":"Cryo-EM, hydrogen-deuterium exchange MS, kinetic binding assays, and mutagenesis of CSN4 domains","pmids":["27031283"],"confidence":"High","gaps":["Real-time dynamics of the sensing-to-activation relay not visualized","Generalization across diverse CRL substrates incomplete"]},{"year":2019,"claim":"Tied CSN4 to G1/S control through TCTP, which antagonizes CSN4/COP9 function to maintain CUL1 neddylation, and correlated CSN4 levels with proliferation and apoptosis regulators in cancer cells.","evidence":"Multi-organism genetic interaction and knockdown with CUL1 neddylation immunoblots (TCTP); lentiviral knockdown with proliferation, cell-cycle, and CDK6/Caspase3 readouts in MDA-MB-231","pmids":["30695029","30992253"],"confidence":"Medium","gaps":["Breast cancer findings are correlative without direct mechanism","Direct TCTP-CSN4 binding interface not mapped"]},{"year":2026,"claim":"Established a direct CSN4-DDB1 interaction required for CRL4 assembly and DNA-damage ubiquitination, and revealed that CSN4 loss diverts DDB1 to autophagic degradation.","evidence":"Co-IP, siRNA knockdown, DDB1 K1131R resistance mutant, CRL4 assembly and ubiquitination assays, autophagy analysis","pmids":["41686221"],"confidence":"Medium","gaps":["How CSN4 binding blocks autophagic targeting of DDB1 unresolved","Single lab"]},{"year":null,"claim":"It remains unresolved how CSN4 mechanistically separates its substrate-protection function from canonical CSN deneddylation, and whether the mammalian CSN undergoes subunit-sequestration switching analogous to Bam-mediated CSN4 extraction in Drosophila.","evidence":"","pmids":[],"confidence":"Low","gaps":["No structural model of CSN4 in a protection-competent versus deneddylation-competent state","No mammalian equivalent of Bam-CSN4 competition identified","Substrate selectivity rules for protection undefined"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[10,3,17]},{"term_id":"GO:0140299","term_label":"molecular sensor activity","supporting_discovery_ids":[17]},{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[6,12,15]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[9,13,20]}],"localization":[{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[0,5]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[0]}],"pathway":[{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[10,9,20]},{"term_id":"R-HSA-1640170","term_label":"Cell Cycle","supporting_discovery_ids":[4,5,18]},{"term_id":"R-HSA-73894","term_label":"DNA Repair","supporting_discovery_ids":[20]},{"term_id":"R-HSA-9909396","term_label":"Circadian clock","supporting_discovery_ids":[7,10]}],"complexes":["COP9 signalosome (CSN)","CSN4-5-6-7 submodule","cytoplasmic CSN4-8 subcomplex","CRL4 (DDB1) ligase"],"partners":["CSN5","CSN6","CSN7","DDB1","TORSINA","RBF2","TESCALCIN","SGCALPHA1"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q9BT78","full_name":"COP9 signalosome complex subunit 4","aliases":["JAB1-containing signalosome subunit 4"],"length_aa":406,"mass_kda":46.3,"function":"Component of the COP9 signalosome complex (CSN), a complex involved in various cellular and developmental processes. The CSN complex is an essential regulator of the ubiquitin (Ubl) conjugation pathway by mediating the deneddylation of the cullin subunits of SCF-type E3 ligase complexes, leading to decrease the Ubl ligase activity of SCF-type complexes such as SCF, CSA or DDB2. Also involved in the deneddylation of non-cullin subunits such as STON2. The complex is also involved in phosphorylation of p53/TP53, c-jun/JUN, IkappaBalpha/NFKBIA, ITPK1, IRF8/ICSBP and SNAPIN, possibly via its association with CK2 and PKD kinases. CSN-dependent phosphorylation of TP53 and JUN promotes and protects degradation by the Ubl system, respectively","subcellular_location":"Cytoplasm; Nucleus; Cytoplasmic vesicle, secretory vesicle, synaptic vesicle","url":"https://www.uniprot.org/uniprotkb/Q9BT78/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":true,"resolved_as":"","url":"https://depmap.org/portal/gene/COPS4","classification":"Common Essential","n_dependent_lines":1175,"n_total_lines":1208,"dependency_fraction":0.972682119205298},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"DDB1","stoichiometry":4.0}],"url":"https://opencell.sf.czbiohub.org/search/COPS4","total_profiled":1310},"omim":[{"mim_id":"616008","title":"COP9 SIGNALOSOME, SUBUNIT 4; COPS4","url":"https://www.omim.org/entry/616008"},{"mim_id":"614729","title":"COP9 SIGNALOSOME, SUBUNIT 6; COPS6","url":"https://www.omim.org/entry/614729"},{"mim_id":"604508","title":"COP9 SIGNALOSOME, SUBUNIT 2; COPS2","url":"https://www.omim.org/entry/604508"},{"mim_id":"603134","title":"CULLIN 1; CUL1","url":"https://www.omim.org/entry/603134"},{"mim_id":"601934","title":"G PROTEIN PATHWAY SUPPRESSOR 1; GPS1","url":"https://www.omim.org/entry/601934"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Nuclear speckles","reliability":"Supported"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/COPS4"},"hgnc":{"alias_symbol":["CSN4","SGN4"],"prev_symbol":[]},"alphafold":{"accession":"Q9BT78","domains":[{"cath_id":"-","chopping":"142-212","consensus_level":"medium","plddt":97.81,"start":142,"end":212},{"cath_id":"1.10.10.10","chopping":"318-360","consensus_level":"high","plddt":94.5867,"start":318,"end":360},{"cath_id":"1.25.40","chopping":"1-117","consensus_level":"medium","plddt":94.6448,"start":1,"end":117}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BT78","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BT78-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BT78-F1-predicted_aligned_error_v6.png","plddt_mean":94.69},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=COPS4","jax_strain_url":"https://www.jax.org/strain/search?query=COPS4"},"sequence":{"accession":"Q9BT78","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9BT78.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9BT78/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BT78"}},"corpus_meta":[{"pmid":"11704659","id":"PMC_11704659","title":"The cytoplasmic shuttling and subsequent degradation of p27Kip1 mediated by Jab1/CSN5 and the COP9 signalosome complex.","date":"2001","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/11704659","citation_count":252,"is_preprint":false},{"pmid":"11742986","id":"PMC_11742986","title":"Subunit interaction maps for the regulatory particle of the 26S proteasome and the COP9 signalosome.","date":"2001","source":"The EMBO journal","url":"https://pubmed.ncbi.nlm.nih.gov/11742986","citation_count":198,"is_preprint":false},{"pmid":"19141280","id":"PMC_19141280","title":"Symmetrical modularity of the COP9 signalosome complex suggests its multifunctionality.","date":"2009","source":"Structure (London, England : 1993)","url":"https://pubmed.ncbi.nlm.nih.gov/19141280","citation_count":132,"is_preprint":false},{"pmid":"12223399","id":"PMC_12223399","title":"COP9 signalosome subunits 4 and 5 regulate multiple pleiotropic pathways in Drosophila 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this subcomplex arises from nuclear export and is sensitive to leptomycin B treatment.\",\n      \"method\": \"Glycerol gradient sedimentation and cell fractionation experiments; leptomycin B treatment\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — cell fractionation and gradient sedimentation with pharmacological perturbation, single lab\",\n      \"pmids\": [\"11704659\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"CSN4 participates in a pairwise interaction network within the COP9 signalosome; yeast two-hybrid analysis detected Csn4/5/7/6 paired interactions in Arabidopsis, implying a similar quaternary arrangement as in the 26S proteasome lid.\",\n      \"method\": \"Yeast two-hybrid analysis of all possible paired subunit interactions\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single-method yeast two-hybrid, Arabidopsis orthologs, single lab\",\n      \"pmids\": [\"11742986\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"In Drosophila, CSN4 (Drosophila ortholog) forms a complex of similar size to plant and mammalian CSN; null mutations in csn4 are larval lethal and cause defective oocyte/embryo patterning and defects in response to DNA damage, with unique phenotypes reminiscent of ecdysone signaling defects distinct from csn5 mutants.\",\n      \"method\": \"Gel filtration with CSN subunit antibodies; null mutant generation and phenotypic characterization\",\n      \"journal\": \"Development (Cambridge, England)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic null mutants with multiple phenotypic readouts and biochemical complex analysis, single lab\",\n      \"pmids\": [\"12223399\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"In fission yeast, Csn4 is required for removal of Nedd8 from the cullin Pcu1, and its protein product associates with Csn1 and Csn2; however, csn4 null mutants do not share the DNA damage sensitivity and slow replication phenotypes of csn1/csn2 mutants, indicating subunit-specific functional differences.\",\n      \"method\": \"Null mutant characterization; co-immunoprecipitation; Nedd8 removal assays\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic null mutants with biochemical deneddylation assay and co-IP, single lab\",\n      \"pmids\": [\"11854407\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"In Drosophila oogenesis, CSN4 (with CSN5/Jab1) acts through the CSN to remove Nedd8 from Cullin1 (a subunit of SCF ubiquitin ligase), and genetic epistasis shows that Cyclin E is the major downstream target; CSN4 and Cyclin E mutations reciprocally suppress each other.\",\n      \"method\": \"Drosophila genetics (CSN4 and CSN5 mutants), genetic epistasis/suppressor analysis, Cyclin E overexpression\",\n      \"journal\": \"Developmental cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis with multiple alleles and reciprocal suppression, single lab\",\n      \"pmids\": [\"12737805\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"The Drosophila CSN4 subunit co-occupies Retinoblastoma target gene promoters with Rbf1 and Rbf2, and physically interacts with Rbf2 during embryogenesis; knockdown of CSN4 leads to increased proteasome-mediated destruction of Rbf1 and Rbf2 and altered cell cycle progression.\",\n      \"method\": \"Co-immunoprecipitation; chromatin immunoprecipitation (ChIP); targeted RNAi knockdown with Western blot\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal IP and ChIP with functional RNAi readout, single lab, multiple orthogonal methods\",\n      \"pmids\": [\"17251548\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Mass spectrometry-based interaction mapping of reconstituted human CSN identified two symmetrical modules: Csn1/2/3/8 and Csn4/5/6/7, connected by Csn1-Csn6 interactions; the active complex contains a single copy of each of the eight subunits.\",\n      \"method\": \"Native mass spectrometry of in vitro reconstituted human CSN complex; subcomplex dissociation mapping\",\n      \"journal\": \"Structure (London, England : 1993)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro reconstitution of the human complex with native MS structural analysis identifying 35 subcomplexes and interaction map\",\n      \"pmids\": [\"19141280\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"CSN4 null mutations in Drosophila prevent normal light-dependent Timeless (TIM) degradation in pacemaker neurons and impair behavioral phase shifts; this places CSN4/CSN in the Jetlag (JET) F-box protein pathway for light-mediated TIM degradation.\",\n      \"method\": \"Drosophila genetic null mutants; immunofluorescence of TIM in lateral neurons; behavioral phase-shift assays; genetic epistasis with jetlag mutants\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis and cellular localization readout with multiple behavioral assays, single lab\",\n      \"pmids\": [\"19176824\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Systematic mass spectrometric pulldown of mammalian CSN4 (among other subunits) defined the protein interaction network of the CSN, identifying stable interactions with a subset of CRL complexes including CRL4(Ddb2), and revealing Dda1 as a new CRL4-associated protein.\",\n      \"method\": \"Mass spectrometry-based interactome mapping of CSN subunits including Csn4 by affinity purification\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — systematic AP-MS of CSN4 among other subunits, single lab, large-scale interactome\",\n      \"pmids\": [\"19295130\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Human CSN4 (or CSN5) knockdown induces proteasome-mediated degradation of the ubiquitin-conjugating enzyme UBC3/Cdc34 via SCF(betaTrCP); the CSN normally protects UBC3 from ubiquitination requiring UBC3's acidic C-terminal extension.\",\n      \"method\": \"siRNA knockdown of CSN4/CSN5; co-immunoprecipitation; in vitro ubiquitination assay; domain mapping\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — RNAi knockdown with biochemical ubiquitination assay and domain mapping, single lab\",\n      \"pmids\": [\"20378537\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"In Neurospora crassa, CSN-4 knockout abolishes deneddylation of cullin proteins (Cul1, Cul3, Cul4), destabilizes Cul1 in SCF complexes and Cul3/BTB proteins in Cul3-BTB E3s, and SKP-1 in SCF complexes; this results in severe defects in growth, conidiation, and circadian rhythm.\",\n      \"method\": \"Gene knockout mutant generation; neddylation state analysis by immunoblot; phenotypic characterization\",\n      \"journal\": \"PLoS genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic KO with direct biochemical cullin neddylation assay, multiple substrates tested, single lab\",\n      \"pmids\": [\"21151958\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"TorsinA (TA) binds directly to CSN4 in neuroblastoma cells and brain synaptosomes; CSN4 and TA are both required for the stability of synaptic proteins snapin and stonin 2; snapin stability is regulated through CSN-associated kinase PKD (protein kinase D) phosphorylation, while stonin 2 stability is regulated through neddylation.\",\n      \"method\": \"Co-immunoprecipitation; siRNA knockdown; Western blotting; synaptosome fractionation\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal co-IP in cells and synaptosomes with RNAi functional validation, single lab\",\n      \"pmids\": [\"21102408\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"A CSN4-5-6-7 subcomplex was reconstituted in vitro: CSN7, CSN4, and CSN6 form a stable heterotrimer (requiring co-expression); CSN5 can be added to reconstitute the quaternary complex. The subcomplex is stabilized by MPN-MPN interactions (CSN5-CSN6), PCI-PCI interactions (CSN4-CSN7), and CSN6 C-terminus interactions with CSN4 and CSN7; CSN8 also interacts with the CSN4-6-7 core.\",\n      \"method\": \"Bacterial co-expression reconstitution; pairwise and combinatorial interaction analysis; biochemical and biophysical characterization\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro reconstitution with systematic domain interaction mapping and multiple biophysical methods\",\n      \"pmids\": [\"23086934\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"CSN4 was identified as a novel binding partner of soluble guanylyl cyclase α1 (sGCα1) in prostate cancer cells; the CSN4-sGCα1 interaction inhibits sGCα1 proteasomal degradation, while CSN4 promotes p53 degradation; these effects are mediated through a CSN4-CSN5-CK2 complex, with CK2 regulating stability of both sGCα1 and p53.\",\n      \"method\": \"Co-immunoprecipitation; siRNA knockdown of CSN4/CSN5; Western blot for protein stability; CK2 inhibition\",\n      \"journal\": \"Molecular endocrinology (Baltimore, Md.)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — co-IP and RNAi with functional protein stability readouts, multiple targets, single lab\",\n      \"pmids\": [\"24725084\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"CSN4 was identified as a Ca2+-dependent binding partner of the EF-hand protein tescalcin; this interaction involves the PCI domain of CSN4; tescalcin upregulation during differentiation coincides with reduced CSN deneddylation of Cul1 and stabilization of p27Kip1, and tescalcin knockdown increases Cul1 deneddylation and Skp2/c-Jun expression while decreasing p27/p53.\",\n      \"method\": \"Co-immunoprecipitation; domain mapping; siRNA knockdown; Western blot for deneddylation and downstream targets\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — co-IP with domain specificity and RNAi functional readout, Ca2+-dependence tested, single lab\",\n      \"pmids\": [\"24659803\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"3D cryo-EM structural analysis of human CSN reveals a central horseshoe-shaped segment formed by PCI domain subunits; CSN2 and CSN4 densities are better defined in cryo-EM maps compared to negative stain, contributing to the overall structural model of the complex.\",\n      \"method\": \"Negative stain EM and cryo-EM single-particle analysis; in vitro deneddylation assay\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — cryo-EM structural analysis, single lab, limited resolution details in abstract\",\n      \"pmids\": [\"24973710\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"In Drosophila GSCs, CSN4 is specifically required for germline stem cell self-renewal but not differentiation; the differentiation factor Bam sequesters Csn4 from the COP9 complex via protein competition, inactivating COP9's self-renewal function and allowing other Csn proteins to promote differentiation.\",\n      \"method\": \"Drosophila genetics; GSC clonal analysis; co-immunoprecipitation of Bam-Csn4; loss-of-function phenotyping\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic analysis with multiple alleles, reciprocal Co-IP, and cellular phenotyping replicated across Csn subunits, published in Nature\",\n      \"pmids\": [\"25119050\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Structural and kinetic analysis showed that N-terminal domains of Csn4 (and Csn2) play important roles in sensing neddylated CRL substrates and enabling formation of a high-affinity fully active CSN-CRL complex; Csn4 communicates binding of neddylated substrate to Csn5 to activate deneddylase activity, with large conformational changes occurring upon binding.\",\n      \"method\": \"Cryo-EM structural analysis; hydrogen-deuterium exchange MS; kinetic binding assays; mutagenesis\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — cryo-EM structure plus kinetic assays plus mutagenesis of specific domains in a single rigorous study\",\n      \"pmids\": [\"27031283\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"TCTP (Translationally Controlled Tumor Protein) controls G1/S transition by interacting with CSN4; genetic interaction studies in Arabidopsis, tobacco cells, and Drosophila showed that downregulation of CSN4 delays G1/S transition; loss-of-function of TCTP increases the fraction of deneddylated CUL1, suggesting TCTP negatively interferes with COP9 (CSN4) function to maintain CUL1 neddylation.\",\n      \"method\": \"Genetic interaction studies; knockdown in multiple organisms; CUL1 neddylation state analysis by immunoblot\",\n      \"journal\": \"PLoS genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis in multiple organisms with direct biochemical neddylation readout, single lab\",\n      \"pmids\": [\"30695029\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"In breast cancer MDA-MB-231 cells, CSN4 knockdown decreases cellular proliferation, increases sub-G1 population (apoptosis), and alters expression of CDK6 and Caspase3, indicating CSN4 modulates cell cycle progression and apoptosis through these regulators.\",\n      \"method\": \"Lentivirus-mediated CSN4 knockdown; CCK8 proliferation assay; colony formation; cell cycle analysis; Western blot for CDK6 and Caspase3\",\n      \"journal\": \"Yi chuan = Hereditas\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, single knockdown approach, no direct mechanistic link established beyond correlation with CDK6/Caspase3 levels\",\n      \"pmids\": [\"30992253\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"CSN4 binds directly to DDB1; loss of CSN4 leads to autophagy-mediated destabilization of DDB1 (the DDB1 K1131R mutant is resistant to CSN4 depletion-induced downregulation), reduced CRL4 assembly, and impaired DNA damage-induced ubiquitination; CSN4 deficiency activates autophagy as an alternative degradation pathway for DDB1.\",\n      \"method\": \"Co-immunoprecipitation; siRNA knockdown; site-directed mutagenesis (DDB1 K1131R); CRL4 assembly assay; ubiquitination assay; autophagy pathway analysis\",\n      \"journal\": \"Cellular and molecular life sciences : CMLS\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct binding demonstrated by co-IP, mutagenesis of resistance mutant, and functional assembly/ubiquitination assays, single lab\",\n      \"pmids\": [\"41686221\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"COPS4/CSN4 is a PCI-domain subunit of the COP9 signalosome (CSN) that forms the core of a CSN4-5-6-7 submodule (with PCI-PCI interactions between CSN4-CSN7 and MPN-MPN between CSN5-CSN6), senses neddylated cullin-RING ligase (CRL) substrates and communicates to the catalytic CSN5 subunit to activate deneddylation, stabilizes DDB1 and CRL4 assembly in the DNA damage response, protects selected substrates (sGCα1, UBC3/Cdc34, synaptic proteins snapin and stonin 2, Retinoblastoma proteins) from proteasomal degradation, and participates in a smaller cytoplasmic subcomplex (CSN4-8); in Drosophila germline stem cells, CSN4 is specifically sequestered from the COP9 complex by the differentiation factor Bam to switch the complex from self-renewal to differentiation mode.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"COPS4/CSN4 is a PCI-domain subunit of the COP9 signalosome (CSN), the eight-subunit complex that deneddylates cullin-RING ubiquitin ligases (CRLs) to regulate their activity and substrate turnover [#6, #10]. Within the CSN, CSN4 anchors one of two symmetrical structural modules: it forms a stable CSN4-5-6-7 core stabilized by a PCI-PCI contact with CSN7 and bridged to the CSN5-CSN6 MPN dimer, with CSN8 also docking onto this core [#6, #12]. CSN4 contributes to the central horseshoe of PCI subunits in the assembled complex, and its N-terminal domains sense neddylated CRL substrate and relay that binding through large conformational changes to activate the catalytic CSN5 subunit [#15, #17]. Through these activities CSN4/CSN governs deneddylation of multiple cullins (Cul1, Cul3, Cul4) and the stability of CRL components such as SKP1, controlling SCF and CRL3 function across development and the circadian clock [#10, #7]. Beyond catalytic deneddylation, CSN4 protects selected proteins from degradation — the ubiquitin-conjugating enzyme UBC3/Cdc34 from SCF(betaTrCP)-mediated turnover, the synaptic proteins snapin and stonin 2 in complex with TorsinA, soluble guanylyl cyclase alpha1, and Retinoblastoma proteins Rbf1/Rbf2 at target promoters — while also promoting p53 degradation [#9, #11, #13, #5]. CSN4 binds directly to DDB1 to support CRL4 assembly and DNA-damage-induced ubiquitination, with its loss diverting DDB1 to autophagic degradation [#20]. In Drosophila germline stem cells, the differentiation factor Bam sequesters CSN4 from the COP9 complex by protein competition, switching the complex from a self-renewal to a differentiation mode [#16].\",\n  \"teleology\": [\n    {\n      \"year\": 2001,\n      \"claim\": \"Established that CSN4 exists not only in the large nuclear CSN holocomplex but also in a smaller cytoplasmic subcomplex, raising the question of compartment-specific CSN forms.\",\n      \"evidence\": \"Glycerol gradient sedimentation, cell fractionation, and leptomycin B treatment defining a ~100 kDa CSN4-8 cytoplasmic subcomplex\",\n      \"pmids\": [\"11704659\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Function of the cytoplasmic CSN4-8 subcomplex not defined\", \"Single lab, pharmacological perturbation only\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Defined the pairwise subunit interaction network placing CSN4 adjacent to CSN5/6/7, foreshadowing the modular architecture of the complex.\",\n      \"evidence\": \"Yeast two-hybrid analysis of all paired subunit interactions in Arabidopsis\",\n      \"pmids\": [\"11742986\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"Single-method Y2H in plant orthologs\", \"Does not establish direct contacts in the assembled human complex\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Genetic and biochemical analyses in Drosophila and fission yeast showed CSN4 is essential and required for cullin deneddylation, but with subunit-specific phenotypes distinct from CSN1/CSN2/CSN5, indicating non-redundant roles among CSN subunits.\",\n      \"evidence\": \"Null mutant phenotyping, gel filtration, co-IP with Csn1/Csn2, and Nedd8-removal assays on Pcu1\",\n      \"pmids\": [\"12223399\", \"11854407\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Molecular basis of subunit-specific phenotypes unresolved\", \"Whether CSN4 acts catalytically or structurally not separated\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Connected CSN4-mediated deneddylation of Cullin1 to a defined downstream output by identifying Cyclin E as the major target through reciprocal genetic suppression.\",\n      \"evidence\": \"Drosophila genetic epistasis and suppressor analysis with Cyclin E overexpression\",\n      \"pmids\": [\"12737805\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct biochemical link between CSN4 and Cyclin E turnover not shown\", \"Restricted to oogenesis context\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Revealed a non-canonical, promoter-associated role: CSN4 co-occupies Rb target genes and protects Rbf1/Rbf2 from proteasomal destruction, linking CSN4 to transcriptional and cell-cycle control.\",\n      \"evidence\": \"Co-IP, ChIP, and RNAi knockdown with Western blot in Drosophila embryos\",\n      \"pmids\": [\"17251548\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether protection occurs via holocomplex deneddylation or a separate activity unclear\", \"Single lab\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Resolved the architecture of the human CSN into two symmetrical modules and placed CSN4 in the CSN4/5/6/7 module, defining the structural neighborhood through which it acts.\",\n      \"evidence\": \"Native mass spectrometry of reconstituted human CSN with subcomplex dissociation mapping\",\n      \"pmids\": [\"19141280\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Static interaction map without catalytic state\", \"Does not address conformational dynamics during deneddylation\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Extended CSN4/CSN function into circadian biology and mapped its physical engagement with specific CRL complexes.\",\n      \"evidence\": \"Drosophila null mutants with TIM immunofluorescence and behavioral phase-shift assays; AP-MS interactome of mammalian CSN4 identifying CRL4(Ddb2) and Dda1\",\n      \"pmids\": [\"19176824\", \"19295130\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct enzymatic action of CSN4 on the JET/TIM CRL not shown\", \"Interactome correlative for many partners\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Demonstrated that CSN4 protects substrates beyond cullins — stabilizing UBC3/Cdc34 and the TorsinA-associated synaptic proteins snapin and stonin 2 — establishing a protective function distinct from bulk deneddylation.\",\n      \"evidence\": \"siRNA knockdown, co-IP, in vitro ubiquitination, domain mapping (UBC3); reciprocal co-IP in cells and synaptosomes with RNAi (TorsinA/snapin/stonin 2); Neurospora csn-4 knockout with cullin neddylation assays\",\n      \"pmids\": [\"20378537\", \"21102408\", \"21151958\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism distinguishing protection from deneddylation incomplete\", \"Tissue-specificity of synaptic role not generalized\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Reconstituted the CSN4-5-6-7 submodule in vitro and mapped the specific PCI-PCI (CSN4-CSN7) and MPN-MPN contacts that build it, providing a molecular blueprint for CSN4's structural role.\",\n      \"evidence\": \"Bacterial co-expression reconstitution with combinatorial interaction and biophysical analysis\",\n      \"pmids\": [\"23086934\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Subcomplex behavior may differ from holocomplex\", \"Does not address substrate sensing\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Identified CSN4 as a regulated hub interacting with sGCalpha1, tescalcin (Ca2+-dependent, PCI-domain mediated), and contributing to the cryo-EM PCI horseshoe, connecting partner binding to modulation of CRL deneddylation and substrate stability.\",\n      \"evidence\": \"Co-IP, domain mapping, siRNA knockdown, CK2 inhibition (sGCalpha1/p53); Ca2+-dependent co-IP and deneddylation readouts (tescalcin); negative-stain and cryo-EM of human CSN\",\n      \"pmids\": [\"24725084\", \"24659803\", \"24973710\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"How partner binding allosterically tunes CSN activity not mechanistically resolved\", \"Cancer-cell-context dependence\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Showed that CSN4 can be selectively extracted from the CSN by a competing factor to reprogram complex function, demonstrating subunit sequestration as a regulatory switch in stem-cell fate.\",\n      \"evidence\": \"Drosophila GSC clonal genetics with multiple alleles and Bam-Csn4 co-IP\",\n      \"pmids\": [\"25119050\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether analogous sequestration occurs in mammals unknown\", \"Stoichiometry of Bam-CSN4 competition not quantified\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Defined CSN4's catalytic-regulatory role: its N-terminal domains sense neddylated CRL substrate and transmit binding through conformational change to activate CSN5, explaining how the complex achieves substrate-stimulated deneddylation.\",\n      \"evidence\": \"Cryo-EM, hydrogen-deuterium exchange MS, kinetic binding assays, and mutagenesis of CSN4 domains\",\n      \"pmids\": [\"27031283\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Real-time dynamics of the sensing-to-activation relay not visualized\", \"Generalization across diverse CRL substrates incomplete\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Tied CSN4 to G1/S control through TCTP, which antagonizes CSN4/COP9 function to maintain CUL1 neddylation, and correlated CSN4 levels with proliferation and apoptosis regulators in cancer cells.\",\n      \"evidence\": \"Multi-organism genetic interaction and knockdown with CUL1 neddylation immunoblots (TCTP); lentiviral knockdown with proliferation, cell-cycle, and CDK6/Caspase3 readouts in MDA-MB-231\",\n      \"pmids\": [\"30695029\", \"30992253\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Breast cancer findings are correlative without direct mechanism\", \"Direct TCTP-CSN4 binding interface not mapped\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Established a direct CSN4-DDB1 interaction required for CRL4 assembly and DNA-damage ubiquitination, and revealed that CSN4 loss diverts DDB1 to autophagic degradation.\",\n      \"evidence\": \"Co-IP, siRNA knockdown, DDB1 K1131R resistance mutant, CRL4 assembly and ubiquitination assays, autophagy analysis\",\n      \"pmids\": [\"41686221\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"How CSN4 binding blocks autophagic targeting of DDB1 unresolved\", \"Single lab\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"It remains unresolved how CSN4 mechanistically separates its substrate-protection function from canonical CSN deneddylation, and whether the mammalian CSN undergoes subunit-sequestration switching analogous to Bam-mediated CSN4 extraction in Drosophila.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No structural model of CSN4 in a protection-competent versus deneddylation-competent state\", \"No mammalian equivalent of Bam-CSN4 competition identified\", \"Substrate selectivity rules for protection undefined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [10, 3, 17]},\n      {\"term_id\": \"GO:0140299\", \"supporting_discovery_ids\": [17]},\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [6, 12, 15]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [9, 13, 20]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [0, 5]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [0]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [10, 9, 20]},\n      {\"term_id\": \"R-HSA-1640170\", \"supporting_discovery_ids\": [4, 5, 18]},\n      {\"term_id\": \"R-HSA-73894\", \"supporting_discovery_ids\": [20]},\n      {\"term_id\": \"R-HSA-9909396\", \"supporting_discovery_ids\": [7, 10]}\n    ],\n    \"complexes\": [\n      \"COP9 signalosome (CSN)\",\n      \"CSN4-5-6-7 submodule\",\n      \"cytoplasmic CSN4-8 subcomplex\",\n      \"CRL4 (DDB1) ligase\"\n    ],\n    \"partners\": [\n      \"CSN5\",\n      \"CSN6\",\n      \"CSN7\",\n      \"DDB1\",\n      \"TorsinA\",\n      \"Rbf2\",\n      \"tescalcin\",\n      \"sGCalpha1\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":7,"faith_total":7,"faith_pct":100.0}}