{"gene":"PSMD9","run_date":"2026-06-10T06:43:36","timeline":{"discoveries":[{"year":1998,"finding":"Human PSMD9 (p27) was identified as a subunit of the modulator trimer complex that stimulates association of the PA700 regulator with the 20S proteasome to form the active 26S proteasome. Immunoblot analysis showed p27 is associated with both the modulator complex and the 26S proteasome complex.","method":"cDNA cloning, immunoblot analysis with anti-p27 antibody, biochemical fractionation","journal":"Genomics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — biochemical fractionation and immunoblot showing co-association with proteasome complexes, single lab, two orthogonal methods","pmids":["9653651"],"is_preprint":false},{"year":2001,"finding":"Yeast Rpn4 (ortholog of PSMD9) is an extremely short-lived transcriptional activator (t1/2 ~2 min) that directly interacts with RPN2 (a 26S proteasome subunit) and is degraded by the assembled active proteasome, establishing a negative feedback circuit controlling proteasome homeostasis.","method":"Genetic deletion, protein stability assays, direct interaction assay (RPN4-RPN2 co-interaction), cell-cycle analysis","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (stability assays, direct interaction, genetic epistasis), replicated across multiple subsequent studies","pmids":["11248031"],"is_preprint":false},{"year":2001,"finding":"The degradation signal (degron) of yeast Rpn4 was mapped to its N-terminal region, outside the transcription-activation domains.","method":"Domain mapping by deletion analysis","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — domain mapping with functional readout (protein stability), single lab","pmids":["11248031"],"is_preprint":false},{"year":2001,"finding":"Yeast Rpn4 binds the PACE element (5'-GGTGGCAAA-3') found in promoters of proteasomal genes and acts as a transcriptional activator; the protein was purified by affinity chromatography and confirmed by microsequencing.","method":"Gel retardation assay, affinity chromatography purification, microsequencing, reporter gene assay","journal":"Molekuliarnaia biologiia","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro binding assay, protein purification with identity confirmation, reporter gene functional validation; replicated by multiple subsequent studies","pmids":["11443924"],"is_preprint":false},{"year":2004,"finding":"Proteasomal degradation of yeast Rpn4 is mediated by two independent degradation signals: one leading to ubiquitylation on internal lysine(s), and one that is ubiquitin-independent. Both degrons must be inactivated to stabilize Rpn4.","method":"In vivo and in vitro degradation assays, mutagenesis of lysine residues","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro and in vivo assays with mutagenesis, two orthogonal mechanisms demonstrated, single lab with multiple methods","pmids":["15090546"],"is_preprint":false},{"year":2004,"finding":"Ubr2 (E3 ubiquitin ligase) and Rad6 (E2 ubiquitin-conjugating enzyme) mediate ubiquitin-dependent degradation of yeast Rpn4. Rpn4 was demonstrated to be a physiological substrate of Ubr2 through in vivo and in vitro assays. Rad6 directly interacts with Ubr2 and is required for this pathway.","method":"In vivo and in vitro ubiquitylation assays, genetic deletion analysis, synthetic growth defect assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro reconstitution and in vivo assays, identification of E2/E3 pair, multiple orthogonal methods","pmids":["15504724"],"is_preprint":false},{"year":2004,"finding":"Rpn4-induced proteasome expression is required for normal proteasome levels in yeast; when proteasome activity is impaired, proteasome expression increases in an Rpn4-dependent manner. A stable form of Rpn4 elevates proteasome expression, confirming the feedback model.","method":"Genetic epistasis, stable Rpn4 mutant expression, proteasome activity assays","journal":"Biochemical and biophysical research communications","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic epistasis with functional readout, multiple genetic conditions tested, replicated across labs","pmids":["15358214"],"is_preprint":false},{"year":2006,"finding":"Lysine 187 of yeast Rpn4 is the preferred ubiquitination site chosen from multiple susceptible lysines; lysine 187 together with a proximal acidic domain constitutes a portable degradation signal.","method":"In vivo and in vitro ubiquitylation assays, site-directed mutagenesis of lysine residues","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro reconstitution and in vivo mutagenesis, single lab with multiple orthogonal methods","pmids":["16492666"],"is_preprint":false},{"year":2007,"finding":"Phosphorylation of Ser-220 (and to a lesser extent Ser-214) in the N-terminal acidic domain (NAD, aa 211-229) of yeast Rpn4 enhances binding to Ubr2 and is required for efficient ubiquitylation and degradation. The phosphorylation-dependent ubiquitylation signal (NAD) is not the major Ubr2-binding site but is essential for degradation.","method":"Phosphorylation site mutagenesis, in vivo and in vitro ubiquitylation assays, binding assays","journal":"Biochimica et biophysica acta","confidence":"High","confidence_rationale":"Tier 1 / Strong — site-directed mutagenesis with in vitro and in vivo functional validation, two orthogonal methods","pmids":["17532487"],"is_preprint":false},{"year":2007,"finding":"Mub1 (a MYND-domain protein) is an essential cofactor for Rpn4 ubiquitylation: it directly interacts with both Ubr2 and Rpn4, and in vitro reconstitution of Rpn4 ubiquitylation requires Mub1 in addition to Ubr2 and Rad6. Mub1 is itself a short-lived substrate of the Ubr2/Rad6 ligase.","method":"Genome-wide deletion screen, in vitro reconstitution ubiquitylation assay, co-immunoprecipitation/direct interaction assays","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro reconstitution with purified components, genome-wide screen plus direct binding assays, single lab multiple orthogonal methods","pmids":["18070918"],"is_preprint":false},{"year":2008,"finding":"Loss of Rpn4-induced proteasome expression (via disruption of Rpn4-binding site in PRE1 promoter) lowers active proteasome levels, causes G2/M cell-cycle delay, and sensitizes cells to various stresses.","method":"Promoter mutagenesis (PACE site disruption), proteasome activity assay, cell-cycle analysis, stress sensitivity assays","journal":"Genetics","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic approach with defined molecular mechanism and multiple phenotypic readouts, replicated in subsequent work","pmids":["18832351"],"is_preprint":false},{"year":2009,"finding":"Proteasomal degradation of Rpn4 is critical for cell survival under stress: a stabilized Rpn4 mutant retaining transcriptional activity severely reduces viability under genotoxic and proteotoxic stress, an effect abolished by a mutation abrogating transcriptional activity, indicating that overexpression of Rpn4 target genes is detrimental under stress.","method":"Stabilized Rpn4 mutant (degron-defective), transcription-activity point mutation, viability assays, genetic epistasis with proteasome mutations","journal":"Genetics","confidence":"High","confidence_rationale":"Tier 2 / Strong — combined genetic and mutagenesis approaches with clear mechanistic dissection, multiple orthogonal readouts","pmids":["19933873"],"is_preprint":false},{"year":2010,"finding":"Inhibition of Rpn4 proteasomal degradation impairs nonhomologous end-joining (NHEJ) repair of DNA double-strand breaks; NHEJ gene expression is downregulated, Yku70 recruitment to DSBs is reduced, and Rpn4 and the proteasome are recruited to DSB sites.","method":"Yeast genetics, chromatin immunoprecipitation (Yku70 and Rpn4/proteasome recruitment to DSB), NHEJ assay, synthetic growth defect with checkpoint mutants","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic and biochemical approaches (ChIP, NHEJ assay), single lab, multiple methods","pmids":["20376190"],"is_preprint":false},{"year":2011,"finding":"Dam methylase-based mapping showed that yeast Rpn4 is recruited to proteasomal gene promoters only through direct DNA interactions (not via protein intermediaries).","method":"Dam methylase footprinting in vivo model system","journal":"Molekuliarnaia biologiia","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — novel in vivo DNA-binding assay, single lab, single method","pmids":["21954596"],"is_preprint":false},{"year":2014,"finding":"Human PSMD9 interacts with hnRNPA1 via a PDZ domain–C-terminal motif interaction; this interaction is required for IκBα proteasomal degradation and NF-κB activation (both basal and TNF-α-induced). hnRNPA1 interacts directly with IκBα and with the proteasome upon TNF-α treatment, while PSMD9 associates constitutively with the proteasome. Point mutations in the PSMD9 PDZ domain or deletion of the hnRNPA1 C-terminus disrupt the interaction and reduce NF-κB activity.","method":"Co-immunoprecipitation, PDZ domain mutagenesis, C-terminal deletion of hnRNPA1, NF-κB reporter assay, siRNA knockdown in HEK293 cells","journal":"The FEBS journal","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal co-IP, mutagenesis of both binding partners with functional readout, multiple orthogonal methods in single study","pmids":["24720748"],"is_preprint":false},{"year":2014,"finding":"The central domain of yeast Rpn4 functions as a portable, interspecies proteasomal degradation signal (degron) capable of destabilizing GFP and Alpha-fetoprotein reporter proteins in human HEK293T cells as well as in yeast.","method":"Reporter protein fusion assay in yeast and mammalian cells, proteasome inhibitor treatment","journal":"FEBS letters","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional degron assay in two cell systems, single lab","pmids":["25157437"],"is_preprint":false},{"year":2014,"finding":"PSMD9 interacts with hnRNPA1, S14 (ribosomal protein), CSH1 (growth hormone), E12 (transcription factor), and IL6 receptor via its PDZ domain recognizing C-terminal short linear motifs of client proteins; interactions confirmed with full-length recombinant proteins and in mammalian cells.","method":"C-terminal peptide screening, recombinant protein binding assays, mammalian cell co-immunoprecipitation, structural modeling","journal":"FEBS open bio","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — multiple binding partners confirmed by pulldown and co-IP across two methods, single lab","pmids":["25009770"],"is_preprint":false},{"year":2015,"finding":"A minimal hexamer 'PACE-core' sequence within PACE elements is sufficient to respond to Rpn4 transcriptional activation and is found in promoters of proteasome assembly chaperone genes, extending Rpn4's transcriptional regulon beyond subunit-encoding genes.","method":"Promoter deletion/mutation analysis, reporter gene assays","journal":"FEBS letters","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reporter assays with defined minimal sequences, single lab, single method type","pmids":["25747386"],"is_preprint":false},{"year":2019,"finding":"Rpn4 abundance increases during ER stress first post-transcriptionally, then transcriptionally. Induction of RPN4 transcription during ER stress is triggered by cytosolic mislocalization of secretory proteins and is mediated by multiple signaling pathways; Rpn4 cooperates with the UPR to accelerate clearance of misfolded cytosolic proteins.","method":"Titratable ER stress system, genetic screen, RPN4 expression analysis, epistasis with UPR mutants, protein clearance assays in yeast","journal":"eLife","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic screen plus multiple mechanistic follow-up experiments, epistasis with UPR, single rigorous study with orthogonal methods","pmids":["30865586"],"is_preprint":false},{"year":2019,"finding":"The PSMD9 PDZ domain binds C-terminal peptides with a preference for hydrophobic residues at the P0 position and cysteine at P-2; a low-affinity tetrapeptide was converted to a high-affinity binder (~5 μM) that inhibits PSMD9-hnRNPA1 interaction and NF-κB signaling.","method":"Peptide binding affinity assays, molecular dynamics simulations, NF-κB activity assay with peptide inhibitor","journal":"Biochemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vitro binding with structure-activity analysis and functional validation, single lab","pmids":["31287951"],"is_preprint":false},{"year":2021,"finding":"PSMD9 is required to maintain nucleolar morphology and integrity; PSMD9-null MCF7 cells show disrupted nucleolar structure (by NPM1 immunofluorescence and electron microscopy), accumulation of WT p53, slow growth, and failure of ribosomal proteins RPS25 and RPL15 to localize to the nucleolus. Multiple ribosomal proteins co-purify/pull-down with PSMD9.","method":"PSMD9 knockout in MCF7 cells, NPM1 immunofluorescence, electron microscopy, ribosomal protein co-purification/pulldown, Actinomycin D treatment","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — KO with multiple structural and functional readouts, co-purification of binding partners, single lab","pmids":["34077860"],"is_preprint":false},{"year":2023,"finding":"PSMD9 interacts with proteins carrying an EXKK short linear motif (SLiM) via its coiled-coil N-terminal domain; validated interactions include hnRNPA2B1 (ERKK motif) and PRDX6 (EAKK motif) using purified proteins. PSMD9 KO in HEK293 cells induces ER stress and UPR, reduces aggresome and lipid droplet formation; these defects are rescued by PSMD9 re-expression. PSMD9 also interacts with BIP/GRP78 (EDKK) and FASN (ELKK).","method":"Affinity purification mass spectrometry (AP-MS), in vitro peptide and protein binding assays, PSMD9 KO and rescue in HEK293 cells, UPR and lipid droplet assays","journal":"The FEBS journal","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — AP-MS plus in vitro binding validation, KO rescue experiments with functional readouts, single lab","pmids":["37665644"],"is_preprint":false},{"year":2024,"finding":"PSMD9 directly interacts with the E3 ubiquitin ligase c-Cbl, suppresses EGFR ubiquitination, and influences EGFR endosomal trafficking and degradation, thereby activating ERK1/2 and Akt signaling in hepatocellular carcinoma cells. PSMD9 knockdown sensitizes HCC cells to erlotinib.","method":"Co-immunoprecipitation, immunofluorescence confocal imaging, EGFR ubiquitination assay, siRNA knockdown, in vitro and in vivo tumor models","journal":"Journal of experimental & clinical cancer research : CR","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — co-IP demonstrating direct interaction, ubiquitination assay, functional knockdown with signaling readout, single lab","pmids":["38745188"],"is_preprint":false},{"year":2025,"finding":"PSMD9 directly interacts with DNAJA1 (a mitochondrial chaperone) via the EXKK motif in DNAJA1; the interaction was confirmed by in vitro binding with purified proteins and co-immunoprecipitation from MCF7 cells. Mutations in DNAJA1 disrupting the EXKK motif abolish binding. Upon proteasome inhibition, PSMD9-DNAJA1 interaction is enhanced and DNAJA1 stability increases. PSMD9 depletion leads to elevated mitochondrial membrane potential.","method":"In vitro binding assay with purified proteins, site-directed mutagenesis of DNAJA1, co-immunoprecipitation from MCF7 cells, mitochondrial membrane potential assay","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vitro binding with mutagenesis plus in-cell co-IP, functional mitochondrial readout, single lab","pmids":["40412052"],"is_preprint":false}],"current_model":"PSMD9/Rpn4 functions as an assembly chaperone of the 19S proteasome regulatory particle and, in yeast, as a transcriptional activator that binds PACE elements in proteasome gene promoters; it is itself an extremely short-lived proteasome substrate degraded via two independent pathways (ubiquitin-dependent, requiring Ubr2/Rad6/Mub1 and phosphorylation of Ser-220, and ubiquitin-independent), establishing a negative feedback circuit that controls proteasome homeostasis; in humans, PSMD9 additionally regulates NF-κB signaling through PDZ domain-mediated interaction with hnRNPA1/IκBα, modulates EGFR stability by interacting with c-Cbl, maintains nucleolar integrity through ribosomal protein interactions, and regulates proteostasis including the UPR and lipid droplet formation through an EXKK-motif-driven interaction network."},"narrative":{"mechanistic_narrative":"PSMD9 is a proteasome-associated protein that operates at the interface of proteasome assembly and proteostatic signaling, with the yeast ortholog Rpn4 defining a homeostatic feedback circuit and the human protein acting as a PDZ/coiled-coil adaptor that nucleates client-protein interaction networks [PMID:9653651, PMID:11248031, PMID:24720748]. Human PSMD9 (p27) was first identified as a subunit of a modulator complex that promotes assembly of the 26S proteasome by stimulating association of the PA700 regulator with the 20S core [PMID:9653651]. In yeast, Rpn4 is a short-lived transcriptional activator that binds the PACE element in proteasome gene promoters through direct DNA contacts and is itself degraded by the assembled proteasome, establishing a negative feedback loop that tunes proteasome abundance [PMID:11248031, PMID:11443924, PMID:21954596]; this Rpn4-dependent induction sets normal proteasome levels and, when proteasome activity is impaired, drives compensatory upregulation [PMID:15358214, PMID:18832351]. Rpn4 turnover proceeds through two independent degradation signals — a ubiquitin-independent route and a ubiquitin-dependent route in which the Ubr2 E3 ligase, Rad6 E2 enzyme, and the MYND-domain cofactor Mub1 ubiquitylate a portable degron centered on Lys-187, enhanced by phosphorylation of Ser-220 in an N-terminal acidic domain [PMID:15090546, PMID:15504724, PMID:16492666, PMID:17532487, PMID:18070918]. Rapid Rpn4 degradation is itself essential under genotoxic and proteotoxic stress, where unrestrained target-gene expression is detrimental, and Rpn4 cooperates with the unfolded protein response to clear mislocalized cytosolic proteins [PMID:19933873, PMID:30865586]. In human cells, PSMD9 uses its PDZ domain to recognize C-terminal short linear motifs of clients including hnRNPA1, an interaction required for IκBα degradation and NF-κB activation [PMID:24720748, PMID:25009770, PMID:31287951], and uses an N-terminal coiled-coil domain to bind EXKK-motif clients such as hnRNPA2B1, PRDX6, BIP/GRP78, FASN, and DNAJA1, supporting ER-stress/UPR control, lipid droplet and aggresome formation, and mitochondrial regulation [PMID:37665644, PMID:40412052]. PSMD9 additionally maintains nucleolar integrity through ribosomal protein interactions [PMID:34077860] and stabilizes EGFR by interacting with the E3 ligase c-Cbl to suppress EGFR ubiquitination and sustain ERK/Akt signaling in hepatocellular carcinoma [PMID:38745188].","teleology":[{"year":1998,"claim":"Established human PSMD9 as a proteasome-associated factor, placing it physically within the 26S proteasome assembly machinery rather than as a free cytosolic protein.","evidence":"cDNA cloning and immunoblot of the p27 modulator subunit co-fractionating with PA700 and 26S proteasome","pmids":["9653651"],"confidence":"Medium","gaps":["Does not define the molecular activity of p27 within the modulator","No structural basis for how it stimulates PA700–20S association"]},{"year":2001,"claim":"Defined the yeast ortholog Rpn4 as a short-lived transcriptional activator degraded by the proteasome and binding the proteasomal PACE promoter element, articulating the negative feedback model of proteasome homeostasis.","evidence":"Genetic deletion, protein stability assays, Rpn4–Rpn2 interaction, gel retardation, affinity purification and reporter assays in yeast","pmids":["11248031","11443924"],"confidence":"High","gaps":["Mapping of the degron only localized to the N-terminal region at this stage","Did not identify the E2/E3 machinery"]},{"year":2004,"claim":"Resolved the degradation logic of Rpn4 into two independent degrons and identified the Ubr2/Rad6 ubiquitylation machinery, while genetically confirming the feedback circuit controls proteasome levels.","evidence":"In vivo/in vitro degradation and ubiquitylation assays, lysine mutagenesis, genetic deletion and stable-mutant epistasis in yeast","pmids":["15090546","15504724","15358214"],"confidence":"High","gaps":["Ubiquitin-independent degron mechanism not defined","Additional cofactors of the Ubr2/Rad6 pathway unknown at this point"]},{"year":2006,"claim":"Pinpointed Lys-187 plus a proximal acidic domain as a portable degradation signal, defining the molecular determinants of ubiquitin-targeted Rpn4 turnover.","evidence":"Site-directed lysine mutagenesis with in vivo and in vitro ubiquitylation assays","pmids":["16492666"],"confidence":"High","gaps":["Does not address how lysine selection is regulated","Structural basis of degron recognition not resolved"]},{"year":2007,"claim":"Showed that Ser-220 phosphorylation and the cofactor Mub1 are required for efficient Ubr2-mediated ubiquitylation, completing the reconstituted enzymology of the ubiquitin-dependent pathway.","evidence":"Phosphosite mutagenesis, genome-wide deletion screen, in vitro reconstitution with Ubr2/Rad6/Mub1, binding assays in yeast","pmids":["17532487","18070918"],"confidence":"High","gaps":["Identity of the Rpn4 kinase not established","How phosphorylation and Mub1 cooperate mechanistically unresolved"]},{"year":2008,"claim":"Demonstrated physiological consequence: loss of Rpn4-driven proteasome gene expression lowers proteasome activity and causes cell-cycle and stress phenotypes, validating the regulon's importance.","evidence":"PACE-site promoter mutagenesis, proteasome activity assays, cell-cycle and stress sensitivity analysis in yeast","pmids":["18832351"],"confidence":"High","gaps":["Does not isolate which target genes drive each phenotype"]},{"year":2009,"claim":"Showed that rapid Rpn4 degradation is itself essential under stress, establishing why the feedback loop must be transient rather than constitutive.","evidence":"Degron-defective stabilized Rpn4 mutant combined with transcription-dead point mutation, viability and epistasis assays in yeast","pmids":["19933873"],"confidence":"High","gaps":["Which overexpressed targets cause toxicity not pinned down"]},{"year":2010,"claim":"Connected Rpn4 and the proteasome to DNA double-strand break repair, broadening its role beyond proteasome subunit supply to genome maintenance.","evidence":"Yeast genetics, ChIP of Yku70 and Rpn4/proteasome at DSBs, NHEJ assays","pmids":["20376190"],"confidence":"Medium","gaps":["Direct versus indirect role of proteasome at DSBs unresolved","Single-lab observation"]},{"year":2011,"claim":"Confirmed Rpn4 reaches proteasomal promoters through direct DNA binding rather than protein bridging, settling the recruitment mechanism.","evidence":"Dam methylase footprinting in vivo in yeast","pmids":["21954596"],"confidence":"Medium","gaps":["Single method, single lab","Does not address co-activator requirements"]},{"year":2014,"claim":"Defined human PSMD9 as a PDZ-domain adaptor that recognizes C-terminal motifs of clients and links hnRNPA1 to IκBα degradation and NF-κB activation, and showed the yeast degron functions across species.","evidence":"Reciprocal co-IP, PDZ/C-terminal mutagenesis, NF-κB reporter and siRNA in HEK293; peptide screening and recombinant binding; cross-species reporter degron assay","pmids":["24720748","25009770","25157437"],"confidence":"High","gaps":["Whether PSMD9 adaptor function is proteasome-coupled in each case unclear","In vivo relevance of the broad client list not all established"]},{"year":2015,"claim":"Mapped a minimal PACE-core hexamer and extended the Rpn4 regulon to proteasome assembly chaperone genes, refining the transcriptional logic of the feedback circuit.","evidence":"Promoter deletion/mutation and reporter assays in yeast","pmids":["25747386"],"confidence":"Medium","gaps":["Single method type","Does not quantify regulon-wide occupancy"]},{"year":2019,"claim":"Showed Rpn4 is induced both post-transcriptionally and transcriptionally during ER stress and cooperates with the UPR, and refined the PSMD9 PDZ specificity into a druggable inhibitor of NF-κB signaling.","evidence":"Titratable ER stress system, genetic screen and UPR epistasis in yeast; peptide affinity assays, MD simulations and NF-κB inhibitor validation","pmids":["30865586","31287951"],"confidence":"High","gaps":["Signaling pathways triggering RPN4 induction only partially defined","Human relevance of the yeast ER-stress circuit not tested"]},{"year":2021,"claim":"Revealed a nucleolar maintenance role for human PSMD9, linking it to ribosomal protein localization and p53 stability.","evidence":"PSMD9 KO in MCF7 cells, NPM1 immunofluorescence, EM, ribosomal protein pulldown, Actinomycin D treatment","pmids":["34077860"],"confidence":"Medium","gaps":["Mechanism linking PSMD9 to ribosomal protein trafficking unresolved","Whether proteasome activity is required is not addressed"]},{"year":2023,"claim":"Identified an EXKK short linear motif recognized by the PSMD9 coiled-coil domain, defining a second client-recognition surface that controls UPR, lipid droplet, and aggresome biology.","evidence":"AP-MS, in vitro peptide/protein binding, PSMD9 KO and rescue with UPR and lipid droplet assays in HEK293","pmids":["37665644"],"confidence":"Medium","gaps":["How EXKK binding mechanistically alters client fate unclear","Single-lab interaction set"]},{"year":2024,"claim":"Connected PSMD9 to receptor tyrosine kinase signaling by showing it binds c-Cbl, suppresses EGFR ubiquitination, and sustains ERK/Akt in hepatocellular carcinoma.","evidence":"Co-IP, EGFR ubiquitination assay, confocal trafficking imaging, siRNA knockdown, in vitro and in vivo tumor models","pmids":["38745188"],"confidence":"Medium","gaps":["Whether the effect requires PSMD9's proteasomal role not separated","Direct structural basis of PSMD9–c-Cbl binding undefined"]},{"year":2025,"claim":"Extended the EXKK interaction network to the mitochondrial chaperone DNAJA1, linking PSMD9 client binding to chaperone stability and mitochondrial membrane potential.","evidence":"In vitro binding with purified proteins, DNAJA1 motif mutagenesis, co-IP from MCF7, mitochondrial membrane potential assay","pmids":["40412052"],"confidence":"Medium","gaps":["How PSMD9 binding stabilizes DNAJA1 mechanistically unresolved","Physiological consequence of altered membrane potential not defined"]},{"year":null,"claim":"It remains unresolved how the conserved proteasome-assembly/feedback function of PSMD9/Rpn4 mechanistically integrates with the human PSMD9 adaptor activities (PDZ and EXKK client networks) across NF-κB, EGFR, nucleolar, UPR, and mitochondrial pathways.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No structure of human PSMD9 bound to a proteasome subunit","Whether human PSMD9 retains a transcriptional/feedback role like yeast Rpn4 is untested","Whether the diverse human client interactions are proteasome-dependent is unknown"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140110","term_label":"transcription regulator activity","supporting_discovery_ids":[1,3,17]},{"term_id":"GO:0003677","term_label":"DNA binding","supporting_discovery_ids":[3,13]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[14,16,21]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,22]}],"localization":[{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[1,3,13]},{"term_id":"GO:0005730","term_label":"nucleolus","supporting_discovery_ids":[20]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[0,14]}],"pathway":[{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[0,1,4,5]},{"term_id":"R-HSA-74160","term_label":"Gene expression (Transcription)","supporting_discovery_ids":[1,3,17]},{"term_id":"R-HSA-8953897","term_label":"Cellular responses to stimuli","supporting_discovery_ids":[18,21]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[14,22]}],"complexes":["26S proteasome / PA700 modulator complex"],"partners":["RPN2","UBR2","RAD6","MUB1","HNRNPA1","CBL","DNAJA1","PRDX6"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"O00233","full_name":"26S proteasome non-ATPase regulatory subunit 9","aliases":["26S proteasome regulatory subunit p27"],"length_aa":223,"mass_kda":24.7,"function":"Acts as a chaperone during the assembly of the 26S proteasome, specifically of the base subcomplex of the PA700/19S regulatory complex (RC). During the base subcomplex assembly is part of an intermediate PSMD9:PSMC6:PSMC3 module, also known as modulator trimer complex; PSMD9 is released during the further base assembly process","subcellular_location":"","url":"https://www.uniprot.org/uniprotkb/O00233/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/PSMD9","classification":"Not Classified","n_dependent_lines":101,"n_total_lines":1208,"dependency_fraction":0.0836092715231788},"opencell":{"profiled":true,"resolved_as":"","ensg_id":"ENSG00000110801","cell_line_id":"CID000130","localizations":[{"compartment":"cytoplasmic","grade":3},{"compartment":"nucleoplasm","grade":3}],"interactors":[{"gene":"PSMC3","stoichiometry":0.2},{"gene":"PSMC6","stoichiometry":0.2},{"gene":"PSMD12","stoichiometry":0.2},{"gene":"PSMD6","stoichiometry":0.2},{"gene":"KATNA1","stoichiometry":0.2},{"gene":"PCF11","stoichiometry":0.2},{"gene":"PSMD3","stoichiometry":0.2},{"gene":"PSMC4","stoichiometry":0.2},{"gene":"PSMD1","stoichiometry":0.2},{"gene":"KNSTRN","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/target/CID000130","total_profiled":1310},"omim":[{"mim_id":"606223","title":"PROTEASOME 26S SUBUNIT, NON-ATPASE, 2; PSMD2","url":"https://www.omim.org/entry/606223"},{"mim_id":"603146","title":"PROTEASOME 26S SUBUNIT, NON-ATPase, 9; PSMD9","url":"https://www.omim.org/entry/603146"},{"mim_id":"600733","title":"PANCREAS/DUODENUM HOMEOBOX PROTEIN 1; PDX1","url":"https://www.omim.org/entry/600733"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Cytosol","reliability":"Supported"},{"location":"Plasma membrane","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/PSMD9"},"hgnc":{"alias_symbol":["p27","Rpn4"],"prev_symbol":[]},"alphafold":{"accession":"O00233","domains":[{"cath_id":"2.30.42.10","chopping":"138-221","consensus_level":"high","plddt":89.0494,"start":138,"end":221},{"cath_id":"1.10.287","chopping":"21-129","consensus_level":"high","plddt":86.3475,"start":21,"end":129}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/O00233","model_url":"https://alphafold.ebi.ac.uk/files/AF-O00233-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-O00233-F1-predicted_aligned_error_v6.png","plddt_mean":83.0},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=PSMD9","jax_strain_url":"https://www.jax.org/strain/search?query=PSMD9"},"sequence":{"accession":"O00233","fasta_url":"https://rest.uniprot.org/uniprotkb/O00233.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/O00233/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/O00233"}},"corpus_meta":[{"pmid":"11248031","id":"PMC_11248031","title":"RPN4 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Immunoblot analysis showed p27 is associated with both the modulator complex and the 26S proteasome complex.\",\n      \"method\": \"cDNA cloning, immunoblot analysis with anti-p27 antibody, biochemical fractionation\",\n      \"journal\": \"Genomics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — biochemical fractionation and immunoblot showing co-association with proteasome complexes, single lab, two orthogonal methods\",\n      \"pmids\": [\"9653651\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Yeast Rpn4 (ortholog of PSMD9) is an extremely short-lived transcriptional activator (t1/2 ~2 min) that directly interacts with RPN2 (a 26S proteasome subunit) and is degraded by the assembled active proteasome, establishing a negative feedback circuit controlling proteasome homeostasis.\",\n      \"method\": \"Genetic deletion, protein stability assays, direct interaction assay (RPN4-RPN2 co-interaction), cell-cycle analysis\",\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 (stability assays, direct interaction, genetic epistasis), replicated across multiple subsequent studies\",\n      \"pmids\": [\"11248031\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"The degradation signal (degron) of yeast Rpn4 was mapped to its N-terminal region, outside the transcription-activation domains.\",\n      \"method\": \"Domain mapping by deletion analysis\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — domain mapping with functional readout (protein stability), single lab\",\n      \"pmids\": [\"11248031\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Yeast Rpn4 binds the PACE element (5'-GGTGGCAAA-3') found in promoters of proteasomal genes and acts as a transcriptional activator; the protein was purified by affinity chromatography and confirmed by microsequencing.\",\n      \"method\": \"Gel retardation assay, affinity chromatography purification, microsequencing, reporter gene assay\",\n      \"journal\": \"Molekuliarnaia biologiia\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro binding assay, protein purification with identity confirmation, reporter gene functional validation; replicated by multiple subsequent studies\",\n      \"pmids\": [\"11443924\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Proteasomal degradation of yeast Rpn4 is mediated by two independent degradation signals: one leading to ubiquitylation on internal lysine(s), and one that is ubiquitin-independent. Both degrons must be inactivated to stabilize Rpn4.\",\n      \"method\": \"In vivo and in vitro degradation assays, mutagenesis of lysine residues\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro and in vivo assays with mutagenesis, two orthogonal mechanisms demonstrated, single lab with multiple methods\",\n      \"pmids\": [\"15090546\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Ubr2 (E3 ubiquitin ligase) and Rad6 (E2 ubiquitin-conjugating enzyme) mediate ubiquitin-dependent degradation of yeast Rpn4. Rpn4 was demonstrated to be a physiological substrate of Ubr2 through in vivo and in vitro assays. Rad6 directly interacts with Ubr2 and is required for this pathway.\",\n      \"method\": \"In vivo and in vitro ubiquitylation assays, genetic deletion analysis, synthetic growth defect assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro reconstitution and in vivo assays, identification of E2/E3 pair, multiple orthogonal methods\",\n      \"pmids\": [\"15504724\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Rpn4-induced proteasome expression is required for normal proteasome levels in yeast; when proteasome activity is impaired, proteasome expression increases in an Rpn4-dependent manner. A stable form of Rpn4 elevates proteasome expression, confirming the feedback model.\",\n      \"method\": \"Genetic epistasis, stable Rpn4 mutant expression, proteasome activity assays\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic epistasis with functional readout, multiple genetic conditions tested, replicated across labs\",\n      \"pmids\": [\"15358214\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Lysine 187 of yeast Rpn4 is the preferred ubiquitination site chosen from multiple susceptible lysines; lysine 187 together with a proximal acidic domain constitutes a portable degradation signal.\",\n      \"method\": \"In vivo and in vitro ubiquitylation assays, site-directed mutagenesis of lysine residues\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro reconstitution and in vivo mutagenesis, single lab with multiple orthogonal methods\",\n      \"pmids\": [\"16492666\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Phosphorylation of Ser-220 (and to a lesser extent Ser-214) in the N-terminal acidic domain (NAD, aa 211-229) of yeast Rpn4 enhances binding to Ubr2 and is required for efficient ubiquitylation and degradation. The phosphorylation-dependent ubiquitylation signal (NAD) is not the major Ubr2-binding site but is essential for degradation.\",\n      \"method\": \"Phosphorylation site mutagenesis, in vivo and in vitro ubiquitylation assays, binding assays\",\n      \"journal\": \"Biochimica et biophysica acta\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — site-directed mutagenesis with in vitro and in vivo functional validation, two orthogonal methods\",\n      \"pmids\": [\"17532487\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Mub1 (a MYND-domain protein) is an essential cofactor for Rpn4 ubiquitylation: it directly interacts with both Ubr2 and Rpn4, and in vitro reconstitution of Rpn4 ubiquitylation requires Mub1 in addition to Ubr2 and Rad6. Mub1 is itself a short-lived substrate of the Ubr2/Rad6 ligase.\",\n      \"method\": \"Genome-wide deletion screen, in vitro reconstitution ubiquitylation assay, co-immunoprecipitation/direct interaction assays\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro reconstitution with purified components, genome-wide screen plus direct binding assays, single lab multiple orthogonal methods\",\n      \"pmids\": [\"18070918\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Loss of Rpn4-induced proteasome expression (via disruption of Rpn4-binding site in PRE1 promoter) lowers active proteasome levels, causes G2/M cell-cycle delay, and sensitizes cells to various stresses.\",\n      \"method\": \"Promoter mutagenesis (PACE site disruption), proteasome activity assay, cell-cycle analysis, stress sensitivity assays\",\n      \"journal\": \"Genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic approach with defined molecular mechanism and multiple phenotypic readouts, replicated in subsequent work\",\n      \"pmids\": [\"18832351\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Proteasomal degradation of Rpn4 is critical for cell survival under stress: a stabilized Rpn4 mutant retaining transcriptional activity severely reduces viability under genotoxic and proteotoxic stress, an effect abolished by a mutation abrogating transcriptional activity, indicating that overexpression of Rpn4 target genes is detrimental under stress.\",\n      \"method\": \"Stabilized Rpn4 mutant (degron-defective), transcription-activity point mutation, viability assays, genetic epistasis with proteasome mutations\",\n      \"journal\": \"Genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — combined genetic and mutagenesis approaches with clear mechanistic dissection, multiple orthogonal readouts\",\n      \"pmids\": [\"19933873\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Inhibition of Rpn4 proteasomal degradation impairs nonhomologous end-joining (NHEJ) repair of DNA double-strand breaks; NHEJ gene expression is downregulated, Yku70 recruitment to DSBs is reduced, and Rpn4 and the proteasome are recruited to DSB sites.\",\n      \"method\": \"Yeast genetics, chromatin immunoprecipitation (Yku70 and Rpn4/proteasome recruitment to DSB), NHEJ assay, synthetic growth defect with checkpoint mutants\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic and biochemical approaches (ChIP, NHEJ assay), single lab, multiple methods\",\n      \"pmids\": [\"20376190\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Dam methylase-based mapping showed that yeast Rpn4 is recruited to proteasomal gene promoters only through direct DNA interactions (not via protein intermediaries).\",\n      \"method\": \"Dam methylase footprinting in vivo model system\",\n      \"journal\": \"Molekuliarnaia biologiia\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — novel in vivo DNA-binding assay, single lab, single method\",\n      \"pmids\": [\"21954596\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Human PSMD9 interacts with hnRNPA1 via a PDZ domain–C-terminal motif interaction; this interaction is required for IκBα proteasomal degradation and NF-κB activation (both basal and TNF-α-induced). hnRNPA1 interacts directly with IκBα and with the proteasome upon TNF-α treatment, while PSMD9 associates constitutively with the proteasome. Point mutations in the PSMD9 PDZ domain or deletion of the hnRNPA1 C-terminus disrupt the interaction and reduce NF-κB activity.\",\n      \"method\": \"Co-immunoprecipitation, PDZ domain mutagenesis, C-terminal deletion of hnRNPA1, NF-κB reporter assay, siRNA knockdown in HEK293 cells\",\n      \"journal\": \"The FEBS journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal co-IP, mutagenesis of both binding partners with functional readout, multiple orthogonal methods in single study\",\n      \"pmids\": [\"24720748\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"The central domain of yeast Rpn4 functions as a portable, interspecies proteasomal degradation signal (degron) capable of destabilizing GFP and Alpha-fetoprotein reporter proteins in human HEK293T cells as well as in yeast.\",\n      \"method\": \"Reporter protein fusion assay in yeast and mammalian cells, proteasome inhibitor treatment\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional degron assay in two cell systems, single lab\",\n      \"pmids\": [\"25157437\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"PSMD9 interacts with hnRNPA1, S14 (ribosomal protein), CSH1 (growth hormone), E12 (transcription factor), and IL6 receptor via its PDZ domain recognizing C-terminal short linear motifs of client proteins; interactions confirmed with full-length recombinant proteins and in mammalian cells.\",\n      \"method\": \"C-terminal peptide screening, recombinant protein binding assays, mammalian cell co-immunoprecipitation, structural modeling\",\n      \"journal\": \"FEBS open bio\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — multiple binding partners confirmed by pulldown and co-IP across two methods, single lab\",\n      \"pmids\": [\"25009770\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"A minimal hexamer 'PACE-core' sequence within PACE elements is sufficient to respond to Rpn4 transcriptional activation and is found in promoters of proteasome assembly chaperone genes, extending Rpn4's transcriptional regulon beyond subunit-encoding genes.\",\n      \"method\": \"Promoter deletion/mutation analysis, reporter gene assays\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reporter assays with defined minimal sequences, single lab, single method type\",\n      \"pmids\": [\"25747386\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Rpn4 abundance increases during ER stress first post-transcriptionally, then transcriptionally. Induction of RPN4 transcription during ER stress is triggered by cytosolic mislocalization of secretory proteins and is mediated by multiple signaling pathways; Rpn4 cooperates with the UPR to accelerate clearance of misfolded cytosolic proteins.\",\n      \"method\": \"Titratable ER stress system, genetic screen, RPN4 expression analysis, epistasis with UPR mutants, protein clearance assays in yeast\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic screen plus multiple mechanistic follow-up experiments, epistasis with UPR, single rigorous study with orthogonal methods\",\n      \"pmids\": [\"30865586\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"The PSMD9 PDZ domain binds C-terminal peptides with a preference for hydrophobic residues at the P0 position and cysteine at P-2; a low-affinity tetrapeptide was converted to a high-affinity binder (~5 μM) that inhibits PSMD9-hnRNPA1 interaction and NF-κB signaling.\",\n      \"method\": \"Peptide binding affinity assays, molecular dynamics simulations, NF-κB activity assay with peptide inhibitor\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vitro binding with structure-activity analysis and functional validation, single lab\",\n      \"pmids\": [\"31287951\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"PSMD9 is required to maintain nucleolar morphology and integrity; PSMD9-null MCF7 cells show disrupted nucleolar structure (by NPM1 immunofluorescence and electron microscopy), accumulation of WT p53, slow growth, and failure of ribosomal proteins RPS25 and RPL15 to localize to the nucleolus. Multiple ribosomal proteins co-purify/pull-down with PSMD9.\",\n      \"method\": \"PSMD9 knockout in MCF7 cells, NPM1 immunofluorescence, electron microscopy, ribosomal protein co-purification/pulldown, Actinomycin D treatment\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — KO with multiple structural and functional readouts, co-purification of binding partners, single lab\",\n      \"pmids\": [\"34077860\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"PSMD9 interacts with proteins carrying an EXKK short linear motif (SLiM) via its coiled-coil N-terminal domain; validated interactions include hnRNPA2B1 (ERKK motif) and PRDX6 (EAKK motif) using purified proteins. PSMD9 KO in HEK293 cells induces ER stress and UPR, reduces aggresome and lipid droplet formation; these defects are rescued by PSMD9 re-expression. PSMD9 also interacts with BIP/GRP78 (EDKK) and FASN (ELKK).\",\n      \"method\": \"Affinity purification mass spectrometry (AP-MS), in vitro peptide and protein binding assays, PSMD9 KO and rescue in HEK293 cells, UPR and lipid droplet assays\",\n      \"journal\": \"The FEBS journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — AP-MS plus in vitro binding validation, KO rescue experiments with functional readouts, single lab\",\n      \"pmids\": [\"37665644\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"PSMD9 directly interacts with the E3 ubiquitin ligase c-Cbl, suppresses EGFR ubiquitination, and influences EGFR endosomal trafficking and degradation, thereby activating ERK1/2 and Akt signaling in hepatocellular carcinoma cells. PSMD9 knockdown sensitizes HCC cells to erlotinib.\",\n      \"method\": \"Co-immunoprecipitation, immunofluorescence confocal imaging, EGFR ubiquitination assay, siRNA knockdown, in vitro and in vivo tumor models\",\n      \"journal\": \"Journal of experimental & clinical cancer research : CR\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — co-IP demonstrating direct interaction, ubiquitination assay, functional knockdown with signaling readout, single lab\",\n      \"pmids\": [\"38745188\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"PSMD9 directly interacts with DNAJA1 (a mitochondrial chaperone) via the EXKK motif in DNAJA1; the interaction was confirmed by in vitro binding with purified proteins and co-immunoprecipitation from MCF7 cells. Mutations in DNAJA1 disrupting the EXKK motif abolish binding. Upon proteasome inhibition, PSMD9-DNAJA1 interaction is enhanced and DNAJA1 stability increases. PSMD9 depletion leads to elevated mitochondrial membrane potential.\",\n      \"method\": \"In vitro binding assay with purified proteins, site-directed mutagenesis of DNAJA1, co-immunoprecipitation from MCF7 cells, mitochondrial membrane potential assay\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vitro binding with mutagenesis plus in-cell co-IP, functional mitochondrial readout, single lab\",\n      \"pmids\": [\"40412052\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"PSMD9/Rpn4 functions as an assembly chaperone of the 19S proteasome regulatory particle and, in yeast, as a transcriptional activator that binds PACE elements in proteasome gene promoters; it is itself an extremely short-lived proteasome substrate degraded via two independent pathways (ubiquitin-dependent, requiring Ubr2/Rad6/Mub1 and phosphorylation of Ser-220, and ubiquitin-independent), establishing a negative feedback circuit that controls proteasome homeostasis; in humans, PSMD9 additionally regulates NF-κB signaling through PDZ domain-mediated interaction with hnRNPA1/IκBα, modulates EGFR stability by interacting with c-Cbl, maintains nucleolar integrity through ribosomal protein interactions, and regulates proteostasis including the UPR and lipid droplet formation through an EXKK-motif-driven interaction network.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"PSMD9 is a proteasome-associated protein that operates at the interface of proteasome assembly and proteostatic signaling, with the yeast ortholog Rpn4 defining a homeostatic feedback circuit and the human protein acting as a PDZ/coiled-coil adaptor that nucleates client-protein interaction networks [#0, #1, #14]. Human PSMD9 (p27) was first identified as a subunit of a modulator complex that promotes assembly of the 26S proteasome by stimulating association of the PA700 regulator with the 20S core [#0]. In yeast, Rpn4 is a short-lived transcriptional activator that binds the PACE element in proteasome gene promoters through direct DNA contacts and is itself degraded by the assembled proteasome, establishing a negative feedback loop that tunes proteasome abundance [#1, #3, #13]; this Rpn4-dependent induction sets normal proteasome levels and, when proteasome activity is impaired, drives compensatory upregulation [#6, #10]. Rpn4 turnover proceeds through two independent degradation signals — a ubiquitin-independent route and a ubiquitin-dependent route in which the Ubr2 E3 ligase, Rad6 E2 enzyme, and the MYND-domain cofactor Mub1 ubiquitylate a portable degron centered on Lys-187, enhanced by phosphorylation of Ser-220 in an N-terminal acidic domain [#4, #5, #7, #8, #9]. Rapid Rpn4 degradation is itself essential under genotoxic and proteotoxic stress, where unrestrained target-gene expression is detrimental, and Rpn4 cooperates with the unfolded protein response to clear mislocalized cytosolic proteins [#11, #18]. In human cells, PSMD9 uses its PDZ domain to recognize C-terminal short linear motifs of clients including hnRNPA1, an interaction required for IκBα degradation and NF-κB activation [#14, #16, #19], and uses an N-terminal coiled-coil domain to bind EXKK-motif clients such as hnRNPA2B1, PRDX6, BIP/GRP78, FASN, and DNAJA1, supporting ER-stress/UPR control, lipid droplet and aggresome formation, and mitochondrial regulation [#21, #23]. PSMD9 additionally maintains nucleolar integrity through ribosomal protein interactions [#20] and stabilizes EGFR by interacting with the E3 ligase c-Cbl to suppress EGFR ubiquitination and sustain ERK/Akt signaling in hepatocellular carcinoma [#22].\",\n  \"teleology\": [\n    {\n      \"year\": 1998,\n      \"claim\": \"Established human PSMD9 as a proteasome-associated factor, placing it physically within the 26S proteasome assembly machinery rather than as a free cytosolic protein.\",\n      \"evidence\": \"cDNA cloning and immunoblot of the p27 modulator subunit co-fractionating with PA700 and 26S proteasome\",\n      \"pmids\": [\"9653651\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Does not define the molecular activity of p27 within the modulator\", \"No structural basis for how it stimulates PA700–20S association\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Defined the yeast ortholog Rpn4 as a short-lived transcriptional activator degraded by the proteasome and binding the proteasomal PACE promoter element, articulating the negative feedback model of proteasome homeostasis.\",\n      \"evidence\": \"Genetic deletion, protein stability assays, Rpn4–Rpn2 interaction, gel retardation, affinity purification and reporter assays in yeast\",\n      \"pmids\": [\"11248031\", \"11443924\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mapping of the degron only localized to the N-terminal region at this stage\", \"Did not identify the E2/E3 machinery\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Resolved the degradation logic of Rpn4 into two independent degrons and identified the Ubr2/Rad6 ubiquitylation machinery, while genetically confirming the feedback circuit controls proteasome levels.\",\n      \"evidence\": \"In vivo/in vitro degradation and ubiquitylation assays, lysine mutagenesis, genetic deletion and stable-mutant epistasis in yeast\",\n      \"pmids\": [\"15090546\", \"15504724\", \"15358214\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Ubiquitin-independent degron mechanism not defined\", \"Additional cofactors of the Ubr2/Rad6 pathway unknown at this point\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Pinpointed Lys-187 plus a proximal acidic domain as a portable degradation signal, defining the molecular determinants of ubiquitin-targeted Rpn4 turnover.\",\n      \"evidence\": \"Site-directed lysine mutagenesis with in vivo and in vitro ubiquitylation assays\",\n      \"pmids\": [\"16492666\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Does not address how lysine selection is regulated\", \"Structural basis of degron recognition not resolved\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Showed that Ser-220 phosphorylation and the cofactor Mub1 are required for efficient Ubr2-mediated ubiquitylation, completing the reconstituted enzymology of the ubiquitin-dependent pathway.\",\n      \"evidence\": \"Phosphosite mutagenesis, genome-wide deletion screen, in vitro reconstitution with Ubr2/Rad6/Mub1, binding assays in yeast\",\n      \"pmids\": [\"17532487\", \"18070918\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the Rpn4 kinase not established\", \"How phosphorylation and Mub1 cooperate mechanistically unresolved\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Demonstrated physiological consequence: loss of Rpn4-driven proteasome gene expression lowers proteasome activity and causes cell-cycle and stress phenotypes, validating the regulon's importance.\",\n      \"evidence\": \"PACE-site promoter mutagenesis, proteasome activity assays, cell-cycle and stress sensitivity analysis in yeast\",\n      \"pmids\": [\"18832351\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Does not isolate which target genes drive each phenotype\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Showed that rapid Rpn4 degradation is itself essential under stress, establishing why the feedback loop must be transient rather than constitutive.\",\n      \"evidence\": \"Degron-defective stabilized Rpn4 mutant combined with transcription-dead point mutation, viability and epistasis assays in yeast\",\n      \"pmids\": [\"19933873\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Which overexpressed targets cause toxicity not pinned down\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Connected Rpn4 and the proteasome to DNA double-strand break repair, broadening its role beyond proteasome subunit supply to genome maintenance.\",\n      \"evidence\": \"Yeast genetics, ChIP of Yku70 and Rpn4/proteasome at DSBs, NHEJ assays\",\n      \"pmids\": [\"20376190\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct versus indirect role of proteasome at DSBs unresolved\", \"Single-lab observation\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Confirmed Rpn4 reaches proteasomal promoters through direct DNA binding rather than protein bridging, settling the recruitment mechanism.\",\n      \"evidence\": \"Dam methylase footprinting in vivo in yeast\",\n      \"pmids\": [\"21954596\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single method, single lab\", \"Does not address co-activator requirements\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Defined human PSMD9 as a PDZ-domain adaptor that recognizes C-terminal motifs of clients and links hnRNPA1 to IκBα degradation and NF-κB activation, and showed the yeast degron functions across species.\",\n      \"evidence\": \"Reciprocal co-IP, PDZ/C-terminal mutagenesis, NF-κB reporter and siRNA in HEK293; peptide screening and recombinant binding; cross-species reporter degron assay\",\n      \"pmids\": [\"24720748\", \"25009770\", \"25157437\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether PSMD9 adaptor function is proteasome-coupled in each case unclear\", \"In vivo relevance of the broad client list not all established\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Mapped a minimal PACE-core hexamer and extended the Rpn4 regulon to proteasome assembly chaperone genes, refining the transcriptional logic of the feedback circuit.\",\n      \"evidence\": \"Promoter deletion/mutation and reporter assays in yeast\",\n      \"pmids\": [\"25747386\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single method type\", \"Does not quantify regulon-wide occupancy\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Showed Rpn4 is induced both post-transcriptionally and transcriptionally during ER stress and cooperates with the UPR, and refined the PSMD9 PDZ specificity into a druggable inhibitor of NF-κB signaling.\",\n      \"evidence\": \"Titratable ER stress system, genetic screen and UPR epistasis in yeast; peptide affinity assays, MD simulations and NF-κB inhibitor validation\",\n      \"pmids\": [\"30865586\", \"31287951\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Signaling pathways triggering RPN4 induction only partially defined\", \"Human relevance of the yeast ER-stress circuit not tested\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Revealed a nucleolar maintenance role for human PSMD9, linking it to ribosomal protein localization and p53 stability.\",\n      \"evidence\": \"PSMD9 KO in MCF7 cells, NPM1 immunofluorescence, EM, ribosomal protein pulldown, Actinomycin D treatment\",\n      \"pmids\": [\"34077860\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism linking PSMD9 to ribosomal protein trafficking unresolved\", \"Whether proteasome activity is required is not addressed\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Identified an EXKK short linear motif recognized by the PSMD9 coiled-coil domain, defining a second client-recognition surface that controls UPR, lipid droplet, and aggresome biology.\",\n      \"evidence\": \"AP-MS, in vitro peptide/protein binding, PSMD9 KO and rescue with UPR and lipid droplet assays in HEK293\",\n      \"pmids\": [\"37665644\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"How EXKK binding mechanistically alters client fate unclear\", \"Single-lab interaction set\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Connected PSMD9 to receptor tyrosine kinase signaling by showing it binds c-Cbl, suppresses EGFR ubiquitination, and sustains ERK/Akt in hepatocellular carcinoma.\",\n      \"evidence\": \"Co-IP, EGFR ubiquitination assay, confocal trafficking imaging, siRNA knockdown, in vitro and in vivo tumor models\",\n      \"pmids\": [\"38745188\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether the effect requires PSMD9's proteasomal role not separated\", \"Direct structural basis of PSMD9–c-Cbl binding undefined\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Extended the EXKK interaction network to the mitochondrial chaperone DNAJA1, linking PSMD9 client binding to chaperone stability and mitochondrial membrane potential.\",\n      \"evidence\": \"In vitro binding with purified proteins, DNAJA1 motif mutagenesis, co-IP from MCF7, mitochondrial membrane potential assay\",\n      \"pmids\": [\"40412052\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"How PSMD9 binding stabilizes DNAJA1 mechanistically unresolved\", \"Physiological consequence of altered membrane potential not defined\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"It remains unresolved how the conserved proteasome-assembly/feedback function of PSMD9/Rpn4 mechanistically integrates with the human PSMD9 adaptor activities (PDZ and EXKK client networks) across NF-κB, EGFR, nucleolar, UPR, and mitochondrial pathways.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No structure of human PSMD9 bound to a proteasome subunit\", \"Whether human PSMD9 retains a transcriptional/feedback role like yeast Rpn4 is untested\", \"Whether the diverse human client interactions are proteasome-dependent is unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140110\", \"supporting_discovery_ids\": [1, 3, 17]},\n      {\"term_id\": \"GO:0003677\", \"supporting_discovery_ids\": [3, 13]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [14, 16, 21]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 22]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [1, 3, 13]},\n      {\"term_id\": \"GO:0005730\", \"supporting_discovery_ids\": [20]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [0, 14]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [0, 1, 4, 5]},\n      {\"term_id\": \"R-HSA-74160\", \"supporting_discovery_ids\": [1, 3, 17]},\n      {\"term_id\": \"R-HSA-8953897\", \"supporting_discovery_ids\": [18, 21]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [14, 22]}\n    ],\n    \"complexes\": [\"26S proteasome / PA700 modulator complex\"],\n    \"partners\": [\"RPN2\", \"UBR2\", \"RAD6\", \"MUB1\", \"HNRNPA1\", \"CBL\", \"DNAJA1\", \"PRDX6\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":7,"faith_pct":85.71428571428571}}