{"gene":"USP21","run_date":"2026-06-11T09:02:06","timeline":{"discoveries":[{"year":2011,"finding":"USP21 cleaves polyubiquitin chains and with reduced activity targets ISG15, but is inactive against NEDD8. A crystal structure of USP21 in complex with linear diubiquitin aldehyde revealed a previously unidentified second Ub- and ISG15-binding surface on the USP domain core, explaining polyUb binding and cross-reactivity.","method":"Crystal structure (USP21–diUb aldehyde complex), in vitro cleavage assays with polyUb, ISG15, and NEDD8 substrates","journal":"EMBO reports","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure plus in vitro enzymatic assays with multiple substrates and mutagenesis-level structural rationalization in a single rigorous study","pmids":["21399617"],"is_preprint":false},{"year":2012,"finding":"USP21 localizes to centrosomes and microtubules; microtubule binding is direct and mediated by a novel motif in amino acids 59–75 of the N-terminus. Depletion of USP21 impairs radial microtubule regrowth after cold-induced depolymerization, reduces primary cilium formation, and inhibits NGF-induced neurite outgrowth.","method":"GFP-tagging and systematic subcellular localization survey; in vitro microtubule-binding assay with N-terminal deletion constructs; siRNA knockdown with functional readouts (microtubule regrowth, ciliogenesis, neurite outgrowth)","journal":"Molecular biology of the cell","confidence":"High","confidence_rationale":"Tier 2 / Strong — direct in vitro binding assay identifies motif, combined with multiple KD phenotypes across two cell types in one study","pmids":["22298430"],"is_preprint":false},{"year":2013,"finding":"USP21 interacts with and deubiquitinates GATA3, stabilizing it at the post-translational level. The catalytic mutant C221A reduces this stabilization. FOXP3 transcriptionally activates USP21 upon TCR stimulation, creating a FOXP3–USP21–GATA3 regulatory loop in Treg cells.","method":"Co-immunoprecipitation, ubiquitination assay, catalytic mutant (C221A), siRNA knockdown, FOXP3 chromatin binding/promoter activation assay","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal Co-IP plus in vivo ubiquitination assay and catalytic mutant validation, single lab","pmids":["23395819"],"is_preprint":false},{"year":2013,"finding":"A short variant of USP21 (USP21SV) lacking a nuclear export signal localizes predominantly in the nucleus. Both USP21SV and the long variant (USP21LV) deubiquitylate histone H2A (ubH2A) and activate transcription in vitro.","method":"Identification of USP21SV isoform; differential localization by fluorescence microscopy; in vitro transcription assay with recombinant USP21 variants; ubH2A levels assessed by western blot","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — in vitro transcription reconstitution with recombinant protein, single lab, single study","pmids":["24278184"],"is_preprint":false},{"year":2014,"finding":"USP21 acts as a deubiquitinase for RIG-I, removing Lys63-linked polyubiquitin chains and thereby inhibiting RIG-I-mediated IFN signaling. USP21-deficient MEFs showed elevated RIG-I polyubiquitination, IRF3 phosphorylation, and IFN-α/β production. USP21 KO mice were more resistant to VSV infection with elevated IFN production.","method":"Co-immunoprecipitation; in vivo and in vitro deubiquitination assays; USP21 KO MEFs, peritoneal macrophages, BMDCs; chimeric bone marrow transplant mouse model; comparison with A20 and CYLD","journal":"The Journal of experimental medicine","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal Co-IP, in vitro DUB assay, genetic KO with multiple cell types and in vivo mouse model, replicated across cell types","pmids":["24493797"],"is_preprint":false},{"year":2014,"finding":"USP21 interacts with IL-33, deubiquitinates it to maintain its protein stability, and thereby sustains IL-33-mediated NF-κB p65 promoter activity. Depletion of USP21 reduces IL-33 protein levels and NF-κB p65 transcriptional activity.","method":"Co-immunoprecipitation, in vivo ubiquitination assay, siRNA knockdown, luciferase reporter assay","journal":"International journal of clinical and experimental pathology","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single Co-IP and ubiquitination assay in a single lab, limited mechanistic depth","pmids":["25197364"],"is_preprint":false},{"year":2015,"finding":"SUMOylated BEND3 stabilizes the NoRC component TTF-1-interacting protein 5 (TIP5) via association with USP21 deubiquitinase, linking USP21 to rDNA silencing.","method":"Co-immunoprecipitation, ChIP, functional rDNA transcription assays in mammalian cells","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP establishes complex, functional assays connect USP21 to rDNA silencing, single lab","pmids":["26100909"],"is_preprint":false},{"year":2016,"finding":"USP21 interacts with, deubiquitinates, and stabilizes Nanog in mouse ESCs, preventing its proteasomal degradation. Loss of USP21 causes Nanog degradation, mESC differentiation, and reduced reprogramming efficiency. ERK-mediated phosphorylation of USP21 upon differentiation signals dissociates it from Nanog. USP21 is also recruited to gene promoters by Nanog to deubiquitinate histone H2A at K119, facilitating Nanog-mediated gene expression.","method":"Co-immunoprecipitation, in vitro deubiquitination assay, USP21 KO mESCs, differentiation assays, reprogramming assay, ChIP for H2AK119ub","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal Co-IP, in vitro DUB assay, KO phenotype with multiple orthogonal readouts, mechanistic dissection of ERK regulation, single lab but multiple methods","pmids":["27886188"],"is_preprint":false},{"year":2016,"finding":"USP21 deubiquitinates Nanog (K48-linked ubiquitin chain) and stabilizes it in mouse ESCs, but does not deubiquitinate Oct4 or Sox2. The C-terminal USP domain of USP21 and the C-domain of Nanog mediate this interaction.","method":"Co-immunoprecipitation in vivo and in vitro, domain mapping, in vitro deubiquitination assay, USP21 depletion/differentiation assay","journal":"Signal transduction and targeted therapy","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vitro and in vivo Co-IP plus DUB assay with domain mapping, corroborating independent study (PMID 27886188)","pmids":["29263902"],"is_preprint":false},{"year":2016,"finding":"USP21 deubiquitinates FOXP3, maintaining its protein level in Treg cells and preventing generation of Th1-like Treg cells. Treg-specific USP21 KO mice display spontaneous T-cell activation and excessive Th1 skewing.","method":"Treg-specific USP21 KO mice (Usp21fl/fl × Foxp3-Cre), in vivo ubiquitination assay of FOXP3, flow cytometry analysis of Treg stability","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 / Strong — conditional KO with defined cellular phenotype, in vivo ubiquitination assay, multiple immunological readouts","pmids":["27857073"],"is_preprint":false},{"year":2016,"finding":"USP21 (centrosome-associated) interacts with KCTD6 and Gli1. Both depletion and overexpression of catalytically active USP21 suppress Gli1-dependent transcription. USP21 recruits and stabilizes Gli1 at the centrosome, promoting PKA-mediated phosphorylation of Gli1, thereby suppressing Hedgehog signaling output.","method":"Co-immunoprecipitation, reporter assay for Gli1-dependent transcription, USP21 siRNA depletion and overexpression, immunofluorescence localization, PKA phosphorylation assay","journal":"Journal of cell science","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP, reporter assays, and loss/gain-of-function with spatial localization data, single lab","pmids":["27621083"],"is_preprint":false},{"year":2017,"finding":"USP21 is a deubiquitinating enzyme for STING that removes K27/K63-linked polyubiquitin chains, negatively regulating DNA virus-induced type I IFN production. HSV-1 infection recruits USP21 to STING at late stage via p38-mediated phosphorylation of USP21 at Ser538. p38 inhibition enhances IFN production and protects mice from lethal HSV-1 infection.","method":"Co-immunoprecipitation, in vitro deubiquitination assay, phosphorylation site mapping (Ser538), p38 inhibitor treatment, USP21 KO cells, mouse HSV-1 infection model","journal":"The Journal of experimental medicine","confidence":"High","confidence_rationale":"Tier 2 / Strong — in vitro DUB assay, phosphorylation site identified, KO cells, in vivo mouse model, multiple orthogonal methods","pmids":["28254948"],"is_preprint":false},{"year":2017,"finding":"USP21 regulates Hippo pathway activity by controlling the stability of MARK kinases (which promote Hippo signaling), thereby modulating YAP/TAZ transcriptional co-activator activity.","method":"Co-immunoprecipitation, ubiquitination assay, siRNA knockdown, reporter assays for YAP/TAZ activity in Drosophila and mammalian systems","journal":"Oncotarget","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — Co-IP and ubiquitination assay with functional reporter readouts, corroborated by independent study (PMID 37743467)","pmids":["28969054"],"is_preprint":false},{"year":2017,"finding":"USP21 deubiquitinates EZH2 and stabilizes it in bladder carcinoma, promoting cell proliferation and metastasis.","method":"Co-immunoprecipitation, in vivo ubiquitination assay, siRNA knockdown/overexpression, proliferation and invasion assays","journal":"OncoTargets and therapy","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, single Co-IP and ubiquitination assay, limited mechanistic depth","pmids":["28223825"],"is_preprint":false},{"year":2018,"finding":"USP21 stabilizes MEK2 by decreasing its K48-linked polyubiquitination, thereby activating the ERK signaling pathway and promoting hepatocellular carcinoma growth.","method":"Co-immunoprecipitation, in vivo ubiquitination assay (K48 linkage), siRNA knockdown, ectopic overexpression, in vivo xenograft model","journal":"Cell death & disease","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — K48-specific ubiquitination assay, reciprocal Co-IP, in vivo tumor model, single lab","pmids":["29706623"],"is_preprint":false},{"year":2019,"finding":"USP21 binds and deubiquitinates FOXM1 in vivo and in vitro, increasing its stability and upregulating the FOXM1 transcriptional network. USP21 depletion delays cell cycle progression and sensitizes basal-like breast cancer cells and xenograft tumors to paclitaxel.","method":"RNAi screen, Co-immunoprecipitation, in vitro deubiquitination assay, siRNA depletion, cell cycle analysis, xenograft mouse model","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 2 / Strong — in vitro DUB assay confirms direct enzyme-substrate, Co-IP, KD with cell cycle and in vivo tumor phenotypes, multiple methods","pmids":["30865895"],"is_preprint":false},{"year":2019,"finding":"USP21 deubiquitinates and stabilizes TCF7 (TCF/LEF transcription factor), promoting Wnt pathway activation and cancer cell stemness in pancreatic ductal adenocarcinoma.","method":"Co-immunoprecipitation, in vivo ubiquitination assay, USP21 KD/OE, PDAC mouse models (KrasG12D-driven), patient-derived cell lines","journal":"Genes & development","confidence":"High","confidence_rationale":"Tier 2 / Strong — Co-IP, ubiquitination assay, genetic in vivo PDAC model, multiple cell-based readouts, single lab with multiple orthogonal approaches","pmids":["31488580"],"is_preprint":false},{"year":2019,"finding":"USP21 interacts with and deubiquitinates Goosecoid (GSC), negatively regulating GSC-dependent Sox6 reporter transcription without affecting GSC protein stability.","method":"Co-immunoprecipitation, Sox6 reporter assay, ubiquitination assay, ATDC5 cell functional assays","journal":"Bioscience reports","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, single Co-IP and reporter assay, limited mechanistic depth","pmids":["31253698"],"is_preprint":false},{"year":2020,"finding":"USP21 deubiquitinates and stabilizes YY1, promoting NSCLC cell proliferation, migration, and invasion. YY1 transcriptionally activates SNHG16, and SNHG16 in turn increases USP21 via miR-4500 sponging, forming a regulatory axis.","method":"Co-immunoprecipitation, in vivo ubiquitination assay, siRNA/shRNA knockdown, in vitro and in vivo tumor assays, luciferase reporter","journal":"Experimental & molecular medicine","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — Co-IP plus ubiquitination assay, functional in vitro/in vivo assays, single lab","pmids":["31956270"],"is_preprint":false},{"year":2021,"finding":"USP21 deubiquitinates and stabilizes FOXM1 in cervical cancer; FOXM1 activates the Hippo-YAP1 pathway (promoting nuclear YAP1 translocation), conferring radioresistance. USP21 knockdown enhances radiosensitivity in vitro and in vivo.","method":"Co-immunoprecipitation, ubiquitination assay, siRNA knockdown, colony survival assay, apoptosis assay, xenograft model","journal":"Human cell","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — Co-IP plus ubiquitination assay, multiple functional assays, single lab","pmids":["34825342"],"is_preprint":false},{"year":2021,"finding":"USP21 binds to AIM2 upon DNA stimulation, deubiquitinates it to increase its protein stability, and is required for AIM2 inflammasome assembly (AIM2-ASC complex formation). Depletion of USP21 does not affect AIM2 DNA-binding ability but inhibits inflammasome complex formation.","method":"Co-immunoprecipitation, in vivo ubiquitination assay, USP21 siRNA knockdown, inflammasome assembly assay (ASC speck formation), IL-1β secretion assay","journal":"Journal of immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP, ubiquitination assay, and mechanistic dissection of inflammasome assembly vs. DNA binding, single lab","pmids":["34470856"],"is_preprint":false},{"year":2021,"finding":"USP21 deubiquitinates and stabilizes PD-L1. In vitro deubiquitination assay showed USP21-WT but not catalytic mutant C221A reduces polyubiquitin chains on PD-L1.","method":"Co-immunoprecipitation, in vitro deubiquitination assay, catalytic mutant C221A, siRNA knockdown, overexpression","journal":"American journal of translational research","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — in vitro DUB assay with catalytic mutant control, single lab, limited downstream mechanistic detail","pmids":["34956491"],"is_preprint":false},{"year":2021,"finding":"USP21 inhibits HIV-1 production by deubiquitinating the HIV-1 transactivator Tat (destabilizing it) and by reducing cyclin T1 mRNA levels through increased H3K9 methylation at the cyclin T1 promoter, thereby limiting Tat-dependent transcriptional elongation.","method":"Co-immunoprecipitation, in vivo ubiquitination assay, dominant-negative ubiquitin mutant, chromatin methylation assay (H3K9me), HIV-1 production assay","journal":"Journal of virology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — DUB assay with DN-Ub control, histone methylation assay, two orthogonal mechanisms identified, single lab","pmids":["33827943"],"is_preprint":false},{"year":2021,"finding":"USP21 in skeletal muscle deubiquitinates DNA-PKcs and ACLY, leading to AMPK inhibition. Loss of USP21 promotes oxidative fibre type switching, increases muscle mass, enhances mitochondrial biogenesis, fatty acid oxidation, and thermogenesis, and protects against diet-induced obesity and insulin resistance.","method":"Whole-body and skeletal muscle-specific KO mice, transcriptomics, proteomics, lipidomics, in vivo metabolic phenotyping, Co-IP to identify substrates DNA-PKcs and ACLY, high-fat diet model","journal":"Journal of cachexia, sarcopenia and muscle","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — conditional KO with multiple omics and in vivo metabolic phenotypes, Co-IP identifies substrates, single lab","pmids":["34523817"],"is_preprint":false},{"year":2022,"finding":"USP21 directly interacts with FOXD1, reverting its proteolytic ubiquitination and stabilizing FOXD1 protein. Silencing USP21 enhances FOXD1 polyubiquitination, promotes proteasomal degradation, and attenuates mesenchymal identity in glioblastoma stem cells.","method":"Co-immunoprecipitation, in vitro/in vivo ubiquitination assay, USP21 siRNA knockdown, FOXD1 rescue, GBM xenograft model","journal":"Cell death & disease","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP, ubiquitination assay, rescue experiment, in vivo xenograft, single lab","pmids":["35974001"],"is_preprint":false},{"year":2023,"finding":"USP21 interacts with and deubiquitinates AURKA, preventing its degradation and promoting laryngeal cancer cell proliferation, migration, and invasion. Interaction confirmed by Co-IP and GST pull-down.","method":"Co-immunoprecipitation, GST pull-down, ubiquitination assay, siRNA knockdown, rescue by AURKA overexpression","journal":"The Kaohsiung journal of medical sciences","confidence":"Low","confidence_rationale":"Tier 3 / Weak — Co-IP and GST pull-down with ubiquitination assay, single lab, limited mechanistic depth","pmids":["36919585"],"is_preprint":false},{"year":2023,"finding":"YOD1 interacts with USP21 and deubiquitinates MARK kinases. YOD1 and USP21 mutually deubiquitinate each other; YOD1 regulates USP21 protein stability, but USP21 does not stabilize YOD1. Both cooperate to promote cell proliferation via Hippo pathway modulation.","method":"Co-immunoprecipitation, GST pull-down, western blot ubiquitination assay, cell proliferation assays, YAP/p-YAP western blot","journal":"Cancer cell international","confidence":"Low","confidence_rationale":"Tier 3 / Weak — Co-IP and pull-down, single lab, limited orthogonal validation","pmids":["37743467"],"is_preprint":false},{"year":2023,"finding":"BAY-805 is a potent non-covalent USP21 inhibitor identified via HTS and structure-based optimization. It engages USP21 with high affinity (SPR, CETSA) and activates NF-κB in a cell-based reporter assay, validating USP21's role in NF-κB regulation.","method":"High-throughput screening, structure-based optimization, surface plasmon resonance (SPR), CETSA, NF-κB reporter assay","journal":"Journal of medicinal chemistry","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — structure-based drug design with SPR and CETSA target engagement validation, single lab","pmids":["36802665"],"is_preprint":false},{"year":2024,"finding":"USP21 deubiquitinates and stabilizes HSP90 via K48-linked deubiquitination in cholangiocarcinoma, which increases HIF1A expression, upregulating glycolytic enzyme genes (ENO2, ENO3, ALDOC, ACSS2). USP21 also directly stabilizes ENO1 to promote aerobic glycolysis.","method":"Co-immunoprecipitation, in vitro/in vivo K48-linked deubiquitination assay, siRNA knockdown, glycolysis assays, xenograft model","journal":"International journal of biological sciences","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — K48-specific ubiquitination assay, Co-IP, functional metabolic readouts, single lab","pmids":["38385089"],"is_preprint":false},{"year":2024,"finding":"USP21 deubiquitinates AhR at K432, removing K48-linked polyubiquitin chains and stabilizing AhR protein. Paradoxically, USP21-mediated deubiquitination of AhR at K432 inhibits AhR transcriptional activity in a deubiquitinating-dependent manner, thereby suppressing Th17 cell differentiation.","method":"Co-immunoprecipitation, K48-linked ubiquitination assay, site-directed mutagenesis (K432), siRNA KD, Th17 differentiation assay in vitro and in vivo (Rag1-/- adoptive transfer colitis model)","journal":"Journal of leukocyte biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — site-specific mutagenesis, K48-linkage assay, in vivo adoptive transfer model, single lab","pmids":["38952265"],"is_preprint":false},{"year":2024,"finding":"USP21 binds to and deubiquitinates EGFR, reducing EGFR degradation and enhancing EGFR stability. USP21-KO colon cancer cells show reduced EGF-driven proliferation, migration, colony formation, and tumor spheroid formation.","method":"CRISPR/Cas9 USP21-KO, Co-immunoprecipitation, ubiquitination assay, in vitro cancer progression assays, NSG xenograft model, BAY-805 inhibitor treatment","journal":"Cell death discovery","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — CRISPR KO with defined phenotype, Co-IP and ubiquitination assay, in vivo xenograft, single lab","pmids":["39695128"],"is_preprint":false},{"year":2024,"finding":"USP21 deubiquitinates and stabilizes TET2 in airway epithelial cells, inhibiting cigarette smoke extract-induced TET2 degradation and thereby attenuating apoptosis.","method":"Co-immunoprecipitation, ubiquitination assay, USP21 knockdown, TET2 overexpression, apoptosis assay in vitro and in vivo COPD model","journal":"iScience","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single Co-IP plus ubiquitination assay, single lab, limited mechanistic depth","pmids":["38439981"],"is_preprint":false},{"year":2024,"finding":"USP21 deubiquitinates and stabilizes p65 (NF-κB) via K48-linked deubiquitination in bladder cancer cells, promoting cancer progression. USP21 directly interacts with p65 as confirmed by mechanistic studies.","method":"Co-immunoprecipitation, K48-linked ubiquitination assay, USP21 siRNA knockdown/overexpression, xenograft model, pharmacological inhibition (20-HE)","journal":"Translational oncology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — K48-specific ubiquitination assay, Co-IP, in vivo xenograft, single lab","pmids":["38663220"],"is_preprint":false},{"year":2024,"finding":"USP21 deubiquitinates and stabilizes G3BP1, which activates Wnt/β-Catenin signaling to promote esophageal squamous cell carcinoma proliferation and metastasis. Disulfiram abolishes USP21-mediated G3BP1 stability.","method":"Co-immunoprecipitation, in vivo ubiquitination assay, rescue assay, in vitro and in vivo ESCC tumor models, disulfiram inhibitor treatment","journal":"Oncogenesis","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP, ubiquitination assay, in vivo model, rescue experiments, single lab","pmids":["38906857"],"is_preprint":false},{"year":2024,"finding":"USP21 deubiquitinates and stabilizes YBX1; YBX1 in turn enhances transcription of HIF1A, promoting prostate cancer malignancy via the HIF1 signaling pathway. YBX1 was identified as the primary substrate by Co-IP coupled with mass spectrometry.","method":"Co-IP/mass spectrometry substrate identification, Co-immunoprecipitation, ubiquitination assay, dual-luciferase reporter assay, ChIP, siRNA knockdown, Bay-805 inhibitor, patient-derived organoids","journal":"Journal of translational medicine","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — MS-based substrate discovery, Co-IP, ChIP, reporter assay, multiple orthogonal methods, single lab","pmids":["41131631"],"is_preprint":false},{"year":2024,"finding":"USP21 stabilizes H2BFS (histone H2B family member S) through deubiquitination in hepatocellular carcinoma. METTL3-mediated m6A methylation of USP21 mRNA regulates USP21 expression, linking epitranscriptomic regulation to USP21-mediated substrate stabilization.","method":"MeRIP assay (m6A), Co-immunoprecipitation, ubiquitination assay, siRNA knockdown/overexpression, xenograft model","journal":"Biochemical genetics","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single Co-IP and ubiquitination assay, MeRIP for upstream regulation, single lab","pmids":["39680331"],"is_preprint":false},{"year":2024,"finding":"Disulfiram impairs USP21-mediated deubiquitination of MOF at lysine K257, leading to increased MOF ubiquitination and degradation, thereby suppressing Wnt/β-Catenin signaling and ESCC progression.","method":"Co-immunoprecipitation, site-specific ubiquitination assay (K257), disulfiram treatment, in vitro and in vivo ESCC models","journal":"Cancer letters","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — site-specific K257 ubiquitination assay, Co-IP, pharmacological validation, in vivo model, single lab","pmids":["39725149"],"is_preprint":false},{"year":2025,"finding":"Cryo-EM structure of USP21 catalytic domain bound to an H2AK119ub nucleosome revealed a recognition mode distinct from other H2AK119-specific DUBs. The N-terminal intrinsically disordered region (IDR) of USP21 autoinhibits its enzymatic activity. Kinases identified by AlphaFold-Multimer virtual screen phosphorylate the IDR, relieving autoinhibition and activating USP21.","method":"Cryo-EM structure determination of USP21–H2AK119ub nucleosome complex; AlphaFold-Multimer virtual screen; in vitro phosphorylation and DUB activity assays; AlphaFold3 structural modeling of autoinhibition","journal":"Science advances","confidence":"High","confidence_rationale":"Tier 1 / Strong — cryo-EM structure plus biochemical autoinhibition assay plus kinase screen, multiple orthogonal methods in one study","pmids":["41071870"],"is_preprint":false},{"year":2025,"finding":"USP21 deubiquitinates DPYSL2 (removing K48-linked ubiquitin), increasing DPYSL2 centrosomal abundance. Loss of USP21 leads to proteasomal degradation of DPYSL2 at centrosomes and causes ciliary defects in kidney, liver, and trachea of Usp21 KO mice.","method":"Usp21 KO mice (multi-organ phenotyping), Co-immunoprecipitation, K48-linked ubiquitination assay, immunofluorescence localization at centrosome/basal body, proteasome inhibitor rescue","journal":"Journal of genetics and genomics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — KO mouse phenotype, K48-specific ubiquitination assay, spatial localization data, Co-IP, single lab","pmids":["40619097"],"is_preprint":false},{"year":2025,"finding":"USP21 acts as a scaffold (in a deubiquitinase activity-independent manner) to facilitate USP7-Mdm2 interaction, enhancing Mdm2 stability and consequently promoting p53 ubiquitination and degradation, thereby suppressing p53 tumor suppressor activity in colorectal cancer.","method":"Co-immunoprecipitation, ubiquitination assay, catalytic mutant analysis (activity-independent scaffolding), siRNA knockdown, CRC cell lines, in vivo tumor models","journal":"Cell death discovery","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP, ubiquitination assay, catalytic-dead mutant distinguishes scaffolding from DUB activity, single lab","pmids":["42168156"],"is_preprint":false},{"year":2025,"finding":"USP21 deubiquitinates and stabilizes EGFR and Lyn in NSCLC cells, preventing their ubiquitination and degradation and sustaining oncogenic signaling. USP21-KO suppresses tumor growth in xenograft models.","method":"USP21-KO lung cancer cell lines (CRISPR), Co-immunoprecipitation, ubiquitination assay, in vivo xenograft, BAY-805 inhibitor treatment","journal":"Biomarker research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — CRISPR KO, Co-IP, ubiquitination assay, in vivo xenograft, single lab","pmids":["40629473"],"is_preprint":false},{"year":2025,"finding":"USP21 deubiquitinates and stabilizes ALDH2 in vascular smooth muscle cells, promoting VSMC dedifferentiation and phenotypic changes that exacerbate abdominal aortic aneurysm. Pharmacological inhibition of USP21 with disulfiram reduces AAA progression.","method":"USP21 global KO and VSMC-specific KO mice (angiotensin II and PPE models), Co-immunoprecipitation/mass spectrometry substrate identification, proteomic analysis, disulfiram treatment, ALDH2E506K mutant mice","journal":"Cell reports. Medicine","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — conditional KO mice with MS-based substrate identification, in vivo AAA models, pharmacological validation, single lab","pmids":["40925375"],"is_preprint":false},{"year":2026,"finding":"USP21 stabilizes β-catenin by removing K48-linked ubiquitin chains, enabling its nuclear translocation. Nuclear β-catenin binds the ATF3 promoter to upregulate ATF3, which transcriptionally represses CCL4, limiting CD8+ T-cell trafficking via CCL4-CCR5 axis and promoting immune evasion in colorectal cancer.","method":"Syngeneic mouse CRC models (MSI-H and MSS), CRISPR Usp21 KO, BAY-805 treatment, Co-immunoprecipitation, K48-linkage ubiquitination assay, ChIP (β-catenin on ATF3 promoter), huCD34+ humanized mice with anti-PD-1","journal":"Cellular & molecular immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — CRISPR KO, K48-specific ubiquitination assay, ChIP, two independent in vivo models, single lab","pmids":["42091690"],"is_preprint":false},{"year":2026,"finding":"USP21 interacts with SMARCB1 via Co-immunoprecipitation and prevents its ubiquitin-mediated proteasomal degradation under hypoxia in hepatocellular carcinoma cells, stabilizing SMARCB1 to sustain its oncogenic and immunosuppressive activities.","method":"Cycloheximide chase assay, Co-immunoprecipitation, loss/gain-of-function experiments, transcriptomic analysis","journal":"Biochemical and biophysical research communications","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single Co-IP, CHX chase, single lab, limited mechanistic depth","pmids":["41637985"],"is_preprint":false}],"current_model":"USP21 is a cysteine deubiquitinase (catalytic residue C221) that removes polyubiquitin chains—predominantly K48- and K63-linked—from a broad array of substrates including RIG-I, STING, FOXM1, FOXP3, GATA3, Nanog, TCF7, MEK2, EZH2, AIM2, PD-L1, Gli1, AhR, EGFR, Lyn, β-catenin, ALDH2, DPYSL2, and histone H2AK119; its N-terminal region directly binds microtubules and centrosomes (residues 59–75), positioning it at the centrosome where it also regulates Gli1/PKA and cilium formation; the N-terminal IDR autoinhibits catalytic activity in a phosphorylation-reversible manner; and p38-mediated phosphorylation at Ser538 recruits USP21 to STING during viral infection, revealing a general paradigm in which post-translational modification of USP21 itself controls its substrate selectivity and activity."},"narrative":{"mechanistic_narrative":"USP21 is a cysteine deubiquitinase that hydrolyzes polyubiquitin chains—predominantly K48- and K63-linked—and acts more weakly on ISG15 but not NEDD8, with a USP-domain architecture that presents a second ubiquitin/ISG15-binding surface explaining its polyUb processivity and cross-reactivity [PMID:21399617]. Catalysis depends on the active-site cysteine C221, and structural work on the catalytic domain bound to an H2AK119ub nucleosome together with the finding that its N-terminal intrinsically disordered region autoinhibits enzymatic activity establishes a self-regulated enzyme whose activity is relieved by kinase-mediated phosphorylation of the IDR [PMID:41071870, PMID:23395819]. A recurring theme across substrates is that USP21 stabilizes its targets by reversing K48-linked degradative ubiquitination: it sustains protein levels of transcription factors and signaling components including FOXP3 and GATA3 in Treg biology [PMID:27857073, PMID:23395819], Nanog and histone H2AK119 in stem-cell self-renewal [PMID:27886188], TCF7 and β-catenin in Wnt signaling [PMID:31488580, PMID:42091690], FOXM1 in cell-cycle progression [PMID:30865895], MEK2 in ERK signaling [PMID:29706623], and EGFR/Lyn in receptor signaling [PMID:40629473]. In innate immunity USP21 functions as a negative regulator of type I interferon by deubiquitinating RIG-I (K63 chains) and STING (K27/K63 chains), the latter following p38-mediated phosphorylation of USP21 at Ser538 that recruits it to STING during viral infection—illustrating how post-translational modification of USP21 itself directs substrate engagement [PMID:24493797, PMID:28254948]. The protein also localizes to centrosomes and microtubules through a discrete N-terminal motif (residues 59–75), where it controls microtubule regrowth, ciliogenesis, and Gli1/Hedgehog signaling output, and stabilizes the centrosomal substrate DPYSL2 to support cilium formation [PMID:22298430, PMID:27621083, PMID:40619097]. Beyond canonical catalysis, USP21 can act as a deubiquitinase-independent scaffold, promoting the USP7–Mdm2 interaction to drive p53 degradation [PMID:42168156]. Across many tissues these activities position USP21 as a frequently oncogenic and immunoregulatory enzyme, and a non-covalent inhibitor (BAY-805) provides chemical-genetic validation of its functions [PMID:36802665].","teleology":[{"year":2011,"claim":"Established USP21 as a polyubiquitin-cleaving cysteine DUB with defined linkage preferences and a structural basis for ubiquitin/ISG15 cross-reactivity, defining its core enzymatic identity.","evidence":"Crystal structure of USP21–linear diUb aldehyde complex plus in vitro cleavage assays against polyUb, ISG15, and NEDD8","pmids":["21399617"],"confidence":"High","gaps":["Cellular substrate repertoire not addressed","No regulation of the enzyme defined at this stage"]},{"year":2012,"claim":"Localized USP21 to centrosomes and microtubules via a discrete N-terminal motif, linking the enzyme to cytoskeletal and ciliary functions distinct from its catalytic core.","evidence":"Subcellular localization survey, in vitro microtubule-binding with N-terminal deletions, siRNA knockdown with microtubule regrowth, ciliogenesis, and neurite outgrowth readouts","pmids":["22298430"],"confidence":"High","gaps":["Centrosomal substrates not identified here","Whether catalytic activity is required for cytoskeletal effects unresolved"]},{"year":2013,"claim":"Connected USP21 catalysis (C221) to substrate stabilization in adaptive immunity and to histone H2A deubiquitination, broadening its role from in vitro chains to physiological substrates.","evidence":"Co-IP, in vivo ubiquitination assays, C221A catalytic mutant, FOXP3 promoter assays (GATA3); isoform identification and in vitro transcription with ubH2A readout","pmids":["23395819","24278184"],"confidence":"Medium","gaps":["Ubiquitin linkage type on GATA3 not defined","Histone targeting specificity not structurally resolved at this stage"]},{"year":2014,"claim":"Defined USP21 as a negative regulator of innate antiviral signaling by removing K63-linked chains from RIG-I, establishing an in vivo immunoregulatory role.","evidence":"Co-IP, in vitro/in vivo DUB assays, USP21 KO MEFs/macrophages/BMDCs, chimeric mouse VSV infection model","pmids":["24493797"],"confidence":"High","gaps":["Mechanism of USP21 recruitment to RIG-I not defined","Whether the same applies to other RLRs untested"]},{"year":2016,"claim":"Showed USP21 stabilizes the pluripotency factor Nanog (K48 chains) and is itself regulated by ERK phosphorylation, while also acting on promoter-bound H2AK119—coupling substrate selectivity to upstream signaling.","evidence":"Reciprocal Co-IP, in vitro DUB assays, USP21 KO mESC differentiation/reprogramming assays, domain mapping, ChIP for H2AK119ub","pmids":["27886188","29263902"],"confidence":"High","gaps":["Mechanism by which ERK phosphorylation dissociates USP21 from Nanog not structurally defined","Generality of phospho-control across substrates unknown at this stage"]},{"year":2016,"claim":"Established USP21 control of immune-cell identity (FOXP3/Treg stability) and Hedgehog output (Gli1 at the centrosome), demonstrating spatially distinct regulatory roles.","evidence":"Treg-specific USP21 KO mice with in vivo FOXP3 ubiquitination assay and flow cytometry; Co-IP, Gli1 reporter, centrosomal IF, PKA phosphorylation assay","pmids":["27857073","27621083"],"confidence":"High","gaps":["How centrosomal localization selects Gli1 vs other substrates unclear","Direct vs indirect effect on PKA phosphorylation of Gli1 not fully resolved"]},{"year":2017,"claim":"Defined the STING-directed antiviral mechanism and showed USP21 substrate recruitment is controlled by p38 phosphorylation at Ser538, formalizing PTM-driven substrate engagement.","evidence":"Co-IP, in vitro DUB assays (K27/K63), Ser538 phospho-site mapping, p38 inhibitor, USP21 KO cells, mouse HSV-1 infection model","pmids":["28254948"],"confidence":"High","gaps":["Whether Ser538 phosphorylation affects catalysis or only localization not separated","Other kinases acting on USP21 not surveyed here"]},{"year":2017,"claim":"Extended USP21 to growth and proliferative signaling by stabilizing MEK2 (K48) and modulating Hippo via MARK kinases, linking the DUB to oncogenic pathways.","evidence":"Co-IP, K48-specific ubiquitination assay, xenograft (MEK2); Co-IP, ubiquitination, YAP/TAZ reporters in Drosophila and mammalian cells (Hippo)","pmids":["29706623","28969054"],"confidence":"Medium","gaps":["Direct enzyme-substrate contacts not structurally mapped","Tissue specificity of Hippo regulation not defined"]},{"year":2019,"claim":"Identified FOXM1 and TCF7 as USP21 substrates driving cell-cycle progression and Wnt-dependent stemness, with in vivo tumor models linking the DUB to cancer phenotypes.","evidence":"RNAi screen, Co-IP, in vitro DUB assay, cell-cycle analysis, xenograft (FOXM1); Co-IP, ubiquitination, KrasG12D PDAC models, patient-derived lines (TCF7)","pmids":["30865895","31488580"],"confidence":"High","gaps":["Ubiquitin chain linkage on FOXM1 not defined","How USP21 is targeted to these transcription factors mechanistically unclear"]},{"year":2021,"claim":"Broadened the substrate landscape to inflammasome assembly (AIM2), immune checkpoint (PD-L1), antiviral restriction of HIV-1 (Tat), and metabolic control in skeletal muscle (DNA-PKcs/ACLY), demonstrating pleiotropic deubiquitinase roles.","evidence":"Co-IP and ubiquitination assays with functional readouts (ASC speck/IL-1β; PD-L1 with C221A control; HIV-1 production with histone methylation assay; muscle-specific KO mice with omics and metabolic phenotyping)","pmids":["34470856","34956491","33827943","34523817"],"confidence":"Medium","gaps":["For several substrates ubiquitin linkage type not defined","Single-lab Co-IP evidence for some interactions"]},{"year":2024,"claim":"Consolidated USP21 as a recurrently oncogenic K48-linkage DUB stabilizing diverse substrates (HSP90/ENO1, AhR, EGFR, p65, G3BP1, YBX1, FOXD1, TET2) across many cancers and tissues, with several upstream regulators (METTL3 m6A) and inhibitors (BAY-805, disulfiram) defined.","evidence":"Co-IP, K48-specific and site-specific ubiquitination assays, CRISPR KO, xenograft models, MS-based substrate ID, pharmacological inhibition","pmids":["38385089","38952265","39695128","38663220","38906857","41131631","35974001","39725149"],"confidence":"Medium","gaps":["Most substrate interactions rest on single-lab Co-IP","Selectivity rules determining which substrates dominate in a given tissue unresolved"]},{"year":2025,"claim":"Resolved the structural basis of nucleosomal H2AK119ub recognition and established that the N-terminal IDR autoinhibits USP21 in a phosphorylation-reversible manner, unifying earlier observations of PTM-controlled activity.","evidence":"Cryo-EM of USP21–H2AK119ub nucleosome, AlphaFold-Multimer kinase screen, in vitro phosphorylation and DUB assays, AlphaFold3 autoinhibition modeling","pmids":["41071870"],"confidence":"High","gaps":["Which physiological kinases relieve autoinhibition in vivo not confirmed","How autoinhibition intersects with substrate-specific recruitment unclear"]},{"year":2025,"claim":"Demonstrated deubiquitinase-independent scaffolding (USP7–Mdm2/p53 axis) and ciliary control via centrosomal DPYSL2 stabilization, refining USP21's catalytic and non-catalytic modes.","evidence":"Co-IP, catalytic-dead mutant analysis, CRC tumor models (scaffold); Usp21 KO mice multi-organ ciliary phenotyping, K48 ubiquitination assay, centrosomal IF (DPYSL2)","pmids":["42168156","40619097"],"confidence":"Medium","gaps":["Structural basis of scaffolding not defined","Relative contribution of catalytic vs scaffold functions in vivo not quantified"]},{"year":2026,"claim":"Linked USP21-driven β-catenin stabilization to immune evasion through an ATF3–CCL4 axis controlling CD8+ T-cell trafficking, integrating its DUB activity with the tumor immune microenvironment.","evidence":"Syngeneic and humanized CRC models, CRISPR KO, BAY-805, K48 ubiquitination assay, ChIP (β-catenin on ATF3 promoter), anti-PD-1 combination","pmids":["42091690"],"confidence":"Medium","gaps":["Direct β-catenin contact site not mapped","Generality across MSI/MSS subtypes only partly addressed"]},{"year":null,"claim":"It remains unresolved how USP21 achieves substrate selectivity among its very broad target set—how localization, IDR phosphorylation, and the second ubiquitin-binding surface are integrated to choose among the dozens of reported substrates in a given cell.","evidence":"No single study in the timeline reconciles the determinants of substrate choice across contexts","pmids":[],"confidence":"Low","gaps":["No unifying model of substrate discrimination","Few substrate interactions validated reciprocally beyond a single lab","In vivo hierarchy of substrates per tissue undefined"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[0,2,4,7,11,15,16,21,37]},{"term_id":"GO:0016787","term_label":"hydrolase activity","supporting_discovery_ids":[0,4,11]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[4,11,9,10]},{"term_id":"GO:0008092","term_label":"cytoskeletal protein binding","supporting_discovery_ids":[1]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[39]}],"localization":[{"term_id":"GO:0005815","term_label":"microtubule organizing center","supporting_discovery_ids":[1,10,38]},{"term_id":"GO:0005856","term_label":"cytoskeleton","supporting_discovery_ids":[1]},{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[3,7]},{"term_id":"GO:0005929","term_label":"cilium","supporting_discovery_ids":[1,38]}],"pathway":[{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[4,9,11,20,29]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[10,12,14,16,42]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[2,7,14,15,16,21,30]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[13,14,15,16,30,39,42]},{"term_id":"R-HSA-1640170","term_label":"Cell Cycle","supporting_discovery_ids":[15,25]}],"complexes":[],"partners":["STING1","DDX58","FOXP3","GATA3","NANOG","GLI1","FOXM1","TCF7"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q9UK80","full_name":"Ubiquitin carboxyl-terminal hydrolase 21","aliases":["Deubiquitinating enzyme 21","Ubiquitin thioesterase 21","Ubiquitin-specific-processing protease 21"],"length_aa":565,"mass_kda":62.7,"function":"Deubiquitinates histone H2A, a specific tag for epigenetic transcriptional repression, thereby acting as a coactivator (By similarity). Deubiquitination of histone H2A releaves the repression of di- and trimethylation of histone H3 at 'Lys-4', resulting in regulation of transcriptional initiation (By similarity). Regulates gene expression via histone H2A deubiquitination (By similarity). Deubiquitinates BAZ2A/TIP5 leading to its stabilization (PubMed:26100909). Also capable of removing NEDD8 from NEDD8 conjugates but has no effect on Sentrin-1 conjugates (PubMed:10799498). Also acts as a negative regulator of the ribosome quality control (RQC) by mediating deubiquitination of 40S ribosomal proteins RPS10/eS10 and RPS20/uS10, thereby antagonizing ZNF598-mediated 40S ubiquitination (PubMed:32011234)","subcellular_location":"Cytoplasm; Nucleus","url":"https://www.uniprot.org/uniprotkb/Q9UK80/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/USP21","classification":"Not Classified","n_dependent_lines":11,"n_total_lines":1208,"dependency_fraction":0.009105960264900662},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"BYSL","stoichiometry":0.2},{"gene":"RACK1","stoichiometry":0.2},{"gene":"RPS16","stoichiometry":0.2},{"gene":"TSR1","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/USP21","total_profiled":1310},"omim":[{"mim_id":"613499","title":"HISTONE GENE CLUSTER 1, H2A HISTONE FAMILY, MEMBER A; HIST1H2AA","url":"https://www.omim.org/entry/613499"},{"mim_id":"604736","title":"UBIQUITIN-SPECIFIC PROTEASE 25; USP25","url":"https://www.omim.org/entry/604736"},{"mim_id":"604729","title":"UBIQUITIN-SPECIFIC PROTEASE 21; USP21","url":"https://www.omim.org/entry/604729"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Cytosol","reliability":"Supported"},{"location":"Nucleoplasm","reliability":"Additional"},{"location":"Plasma membrane","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/USP21"},"hgnc":{"alias_symbol":["USP16"],"prev_symbol":["USP23"]},"alphafold":{"accession":"Q9UK80","domains":[{"cath_id":"-","chopping":"221-318","consensus_level":"medium","plddt":94.5426,"start":221,"end":318}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9UK80","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9UK80-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9UK80-F1-predicted_aligned_error_v6.png","plddt_mean":69.75},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=USP21","jax_strain_url":"https://www.jax.org/strain/search?query=USP21"},"sequence":{"accession":"Q9UK80","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9UK80.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9UK80/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9UK80"}},"corpus_meta":[{"pmid":"24493797","id":"PMC_24493797","title":"USP21 negatively regulates 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p65.","date":"2024","source":"Translational oncology","url":"https://pubmed.ncbi.nlm.nih.gov/38663220","citation_count":4,"is_preprint":false},{"pmid":"34599002","id":"PMC_34599002","title":"Anticipating resistance to KRAS inhibition: a novel role for USP21 in macropinocytosis regulation.","date":"2021","source":"Genes & development","url":"https://pubmed.ncbi.nlm.nih.gov/34599002","citation_count":4,"is_preprint":false},{"pmid":"40925375","id":"PMC_40925375","title":"Targeting USP21 to inhibit abdominal aortic aneurysm progression by suppressing the phenotypic transition of vascular smooth muscle cells.","date":"2025","source":"Cell reports. 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international","url":"https://pubmed.ncbi.nlm.nih.gov/41258964","citation_count":1,"is_preprint":false},{"pmid":"40320442","id":"PMC_40320442","title":"Exploring bioactive phytoconstituents as USP21 inhibitors for therapeutic development against cancer.","date":"2025","source":"Scientific reports","url":"https://pubmed.ncbi.nlm.nih.gov/40320442","citation_count":1,"is_preprint":false},{"pmid":"39270458","id":"PMC_39270458","title":"Molecular hybridization assisted multi-technique approach for designing USP21 inhibitors to halt catalytic triad-mediated nucleophilic attack and suppress pancreatic ductal adenocarcinoma progression: A molecular dynamics study.","date":"2024","source":"Computers in biology and medicine","url":"https://pubmed.ncbi.nlm.nih.gov/39270458","citation_count":1,"is_preprint":false},{"pmid":"42091690","id":"PMC_42091690","title":"USP21 drives immune evasion in colorectal cancer via deubiquitination and stabilization of β-catenin.","date":"2026","source":"Cellular & molecular immunology","url":"https://pubmed.ncbi.nlm.nih.gov/42091690","citation_count":0,"is_preprint":false},{"pmid":"41637985","id":"PMC_41637985","title":"USP21-mediated SMARCB1 stabilization under hypoxia may influence tumor progression and immune response in HCC.","date":"2026","source":"Biochemical and biophysical research communications","url":"https://pubmed.ncbi.nlm.nih.gov/41637985","citation_count":0,"is_preprint":false},{"pmid":"41096992","id":"PMC_41096992","title":"Usp21 Knockout Causes Abnormal Lipid Metabolism in Mouse and Its Polymorphism Correlates with Hypercholesterolemia in Outpatients.","date":"2025","source":"International journal of molecular sciences","url":"https://pubmed.ncbi.nlm.nih.gov/41096992","citation_count":0,"is_preprint":false},{"pmid":"42168156","id":"PMC_42168156","title":"USP21 functions as an oncogenic regulator of the Mdm2-p53 axis in colorectal cancer.","date":"2026","source":"Cell death discovery","url":"https://pubmed.ncbi.nlm.nih.gov/42168156","citation_count":0,"is_preprint":false},{"pmid":"40205504","id":"PMC_40205504","title":"USP21 is involved in the development of chronic hepatitis B by modulating the immune microenvironment.","date":"2025","source":"European journal of medical research","url":"https://pubmed.ncbi.nlm.nih.gov/40205504","citation_count":0,"is_preprint":false},{"pmid":null,"id":"bio_10.1101_2024.09.17.613539","title":"Decoding polyubiquitin regulation of K  <sub>V</sub>  7. 1 functional expression with engineered linkage-selective deubiquitinases","date":"2024-09-17","source":"bioRxiv","url":"https://doi.org/10.1101/2024.09.17.613539","citation_count":0,"is_preprint":true}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":32993,"output_tokens":10589,"usd":0.128907,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":21014,"output_tokens":5539,"usd":0.121773,"stage2_stop_reason":"end_turn"},"total_usd":0.25068,"stage1_batch_id":"msgbatch_01UT1AahFPCAyn2uVweS7A9z","stage2_batch_id":"msgbatch_01HuKWtKNT4Pqj1bc8eEP5RV","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2011,\n      \"finding\": \"USP21 cleaves polyubiquitin chains and with reduced activity targets ISG15, but is inactive against NEDD8. A crystal structure of USP21 in complex with linear diubiquitin aldehyde revealed a previously unidentified second Ub- and ISG15-binding surface on the USP domain core, explaining polyUb binding and cross-reactivity.\",\n      \"method\": \"Crystal structure (USP21–diUb aldehyde complex), in vitro cleavage assays with polyUb, ISG15, and NEDD8 substrates\",\n      \"journal\": \"EMBO reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure plus in vitro enzymatic assays with multiple substrates and mutagenesis-level structural rationalization in a single rigorous study\",\n      \"pmids\": [\"21399617\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"USP21 localizes to centrosomes and microtubules; microtubule binding is direct and mediated by a novel motif in amino acids 59–75 of the N-terminus. Depletion of USP21 impairs radial microtubule regrowth after cold-induced depolymerization, reduces primary cilium formation, and inhibits NGF-induced neurite outgrowth.\",\n      \"method\": \"GFP-tagging and systematic subcellular localization survey; in vitro microtubule-binding assay with N-terminal deletion constructs; siRNA knockdown with functional readouts (microtubule regrowth, ciliogenesis, neurite outgrowth)\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — direct in vitro binding assay identifies motif, combined with multiple KD phenotypes across two cell types in one study\",\n      \"pmids\": [\"22298430\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"USP21 interacts with and deubiquitinates GATA3, stabilizing it at the post-translational level. The catalytic mutant C221A reduces this stabilization. FOXP3 transcriptionally activates USP21 upon TCR stimulation, creating a FOXP3–USP21–GATA3 regulatory loop in Treg cells.\",\n      \"method\": \"Co-immunoprecipitation, ubiquitination assay, catalytic mutant (C221A), siRNA knockdown, FOXP3 chromatin binding/promoter activation assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal Co-IP plus in vivo ubiquitination assay and catalytic mutant validation, single lab\",\n      \"pmids\": [\"23395819\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"A short variant of USP21 (USP21SV) lacking a nuclear export signal localizes predominantly in the nucleus. Both USP21SV and the long variant (USP21LV) deubiquitylate histone H2A (ubH2A) and activate transcription in vitro.\",\n      \"method\": \"Identification of USP21SV isoform; differential localization by fluorescence microscopy; in vitro transcription assay with recombinant USP21 variants; ubH2A levels assessed by western blot\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — in vitro transcription reconstitution with recombinant protein, single lab, single study\",\n      \"pmids\": [\"24278184\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"USP21 acts as a deubiquitinase for RIG-I, removing Lys63-linked polyubiquitin chains and thereby inhibiting RIG-I-mediated IFN signaling. USP21-deficient MEFs showed elevated RIG-I polyubiquitination, IRF3 phosphorylation, and IFN-α/β production. USP21 KO mice were more resistant to VSV infection with elevated IFN production.\",\n      \"method\": \"Co-immunoprecipitation; in vivo and in vitro deubiquitination assays; USP21 KO MEFs, peritoneal macrophages, BMDCs; chimeric bone marrow transplant mouse model; comparison with A20 and CYLD\",\n      \"journal\": \"The Journal of experimental medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal Co-IP, in vitro DUB assay, genetic KO with multiple cell types and in vivo mouse model, replicated across cell types\",\n      \"pmids\": [\"24493797\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"USP21 interacts with IL-33, deubiquitinates it to maintain its protein stability, and thereby sustains IL-33-mediated NF-κB p65 promoter activity. Depletion of USP21 reduces IL-33 protein levels and NF-κB p65 transcriptional activity.\",\n      \"method\": \"Co-immunoprecipitation, in vivo ubiquitination assay, siRNA knockdown, luciferase reporter assay\",\n      \"journal\": \"International journal of clinical and experimental pathology\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single Co-IP and ubiquitination assay in a single lab, limited mechanistic depth\",\n      \"pmids\": [\"25197364\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"SUMOylated BEND3 stabilizes the NoRC component TTF-1-interacting protein 5 (TIP5) via association with USP21 deubiquitinase, linking USP21 to rDNA silencing.\",\n      \"method\": \"Co-immunoprecipitation, ChIP, functional rDNA transcription assays in mammalian cells\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP establishes complex, functional assays connect USP21 to rDNA silencing, single lab\",\n      \"pmids\": [\"26100909\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"USP21 interacts with, deubiquitinates, and stabilizes Nanog in mouse ESCs, preventing its proteasomal degradation. Loss of USP21 causes Nanog degradation, mESC differentiation, and reduced reprogramming efficiency. ERK-mediated phosphorylation of USP21 upon differentiation signals dissociates it from Nanog. USP21 is also recruited to gene promoters by Nanog to deubiquitinate histone H2A at K119, facilitating Nanog-mediated gene expression.\",\n      \"method\": \"Co-immunoprecipitation, in vitro deubiquitination assay, USP21 KO mESCs, differentiation assays, reprogramming assay, ChIP for H2AK119ub\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal Co-IP, in vitro DUB assay, KO phenotype with multiple orthogonal readouts, mechanistic dissection of ERK regulation, single lab but multiple methods\",\n      \"pmids\": [\"27886188\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"USP21 deubiquitinates Nanog (K48-linked ubiquitin chain) and stabilizes it in mouse ESCs, but does not deubiquitinate Oct4 or Sox2. The C-terminal USP domain of USP21 and the C-domain of Nanog mediate this interaction.\",\n      \"method\": \"Co-immunoprecipitation in vivo and in vitro, domain mapping, in vitro deubiquitination assay, USP21 depletion/differentiation assay\",\n      \"journal\": \"Signal transduction and targeted therapy\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vitro and in vivo Co-IP plus DUB assay with domain mapping, corroborating independent study (PMID 27886188)\",\n      \"pmids\": [\"29263902\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"USP21 deubiquitinates FOXP3, maintaining its protein level in Treg cells and preventing generation of Th1-like Treg cells. Treg-specific USP21 KO mice display spontaneous T-cell activation and excessive Th1 skewing.\",\n      \"method\": \"Treg-specific USP21 KO mice (Usp21fl/fl × Foxp3-Cre), in vivo ubiquitination assay of FOXP3, flow cytometry analysis of Treg stability\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — conditional KO with defined cellular phenotype, in vivo ubiquitination assay, multiple immunological readouts\",\n      \"pmids\": [\"27857073\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"USP21 (centrosome-associated) interacts with KCTD6 and Gli1. Both depletion and overexpression of catalytically active USP21 suppress Gli1-dependent transcription. USP21 recruits and stabilizes Gli1 at the centrosome, promoting PKA-mediated phosphorylation of Gli1, thereby suppressing Hedgehog signaling output.\",\n      \"method\": \"Co-immunoprecipitation, reporter assay for Gli1-dependent transcription, USP21 siRNA depletion and overexpression, immunofluorescence localization, PKA phosphorylation assay\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP, reporter assays, and loss/gain-of-function with spatial localization data, single lab\",\n      \"pmids\": [\"27621083\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"USP21 is a deubiquitinating enzyme for STING that removes K27/K63-linked polyubiquitin chains, negatively regulating DNA virus-induced type I IFN production. HSV-1 infection recruits USP21 to STING at late stage via p38-mediated phosphorylation of USP21 at Ser538. p38 inhibition enhances IFN production and protects mice from lethal HSV-1 infection.\",\n      \"method\": \"Co-immunoprecipitation, in vitro deubiquitination assay, phosphorylation site mapping (Ser538), p38 inhibitor treatment, USP21 KO cells, mouse HSV-1 infection model\",\n      \"journal\": \"The Journal of experimental medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — in vitro DUB assay, phosphorylation site identified, KO cells, in vivo mouse model, multiple orthogonal methods\",\n      \"pmids\": [\"28254948\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"USP21 regulates Hippo pathway activity by controlling the stability of MARK kinases (which promote Hippo signaling), thereby modulating YAP/TAZ transcriptional co-activator activity.\",\n      \"method\": \"Co-immunoprecipitation, ubiquitination assay, siRNA knockdown, reporter assays for YAP/TAZ activity in Drosophila and mammalian systems\",\n      \"journal\": \"Oncotarget\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — Co-IP and ubiquitination assay with functional reporter readouts, corroborated by independent study (PMID 37743467)\",\n      \"pmids\": [\"28969054\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"USP21 deubiquitinates EZH2 and stabilizes it in bladder carcinoma, promoting cell proliferation and metastasis.\",\n      \"method\": \"Co-immunoprecipitation, in vivo ubiquitination assay, siRNA knockdown/overexpression, proliferation and invasion assays\",\n      \"journal\": \"OncoTargets and therapy\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, single Co-IP and ubiquitination assay, limited mechanistic depth\",\n      \"pmids\": [\"28223825\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"USP21 stabilizes MEK2 by decreasing its K48-linked polyubiquitination, thereby activating the ERK signaling pathway and promoting hepatocellular carcinoma growth.\",\n      \"method\": \"Co-immunoprecipitation, in vivo ubiquitination assay (K48 linkage), siRNA knockdown, ectopic overexpression, in vivo xenograft model\",\n      \"journal\": \"Cell death & disease\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — K48-specific ubiquitination assay, reciprocal Co-IP, in vivo tumor model, single lab\",\n      \"pmids\": [\"29706623\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"USP21 binds and deubiquitinates FOXM1 in vivo and in vitro, increasing its stability and upregulating the FOXM1 transcriptional network. USP21 depletion delays cell cycle progression and sensitizes basal-like breast cancer cells and xenograft tumors to paclitaxel.\",\n      \"method\": \"RNAi screen, Co-immunoprecipitation, in vitro deubiquitination assay, siRNA depletion, cell cycle analysis, xenograft mouse model\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — in vitro DUB assay confirms direct enzyme-substrate, Co-IP, KD with cell cycle and in vivo tumor phenotypes, multiple methods\",\n      \"pmids\": [\"30865895\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"USP21 deubiquitinates and stabilizes TCF7 (TCF/LEF transcription factor), promoting Wnt pathway activation and cancer cell stemness in pancreatic ductal adenocarcinoma.\",\n      \"method\": \"Co-immunoprecipitation, in vivo ubiquitination assay, USP21 KD/OE, PDAC mouse models (KrasG12D-driven), patient-derived cell lines\",\n      \"journal\": \"Genes & development\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — Co-IP, ubiquitination assay, genetic in vivo PDAC model, multiple cell-based readouts, single lab with multiple orthogonal approaches\",\n      \"pmids\": [\"31488580\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"USP21 interacts with and deubiquitinates Goosecoid (GSC), negatively regulating GSC-dependent Sox6 reporter transcription without affecting GSC protein stability.\",\n      \"method\": \"Co-immunoprecipitation, Sox6 reporter assay, ubiquitination assay, ATDC5 cell functional assays\",\n      \"journal\": \"Bioscience reports\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, single Co-IP and reporter assay, limited mechanistic depth\",\n      \"pmids\": [\"31253698\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"USP21 deubiquitinates and stabilizes YY1, promoting NSCLC cell proliferation, migration, and invasion. YY1 transcriptionally activates SNHG16, and SNHG16 in turn increases USP21 via miR-4500 sponging, forming a regulatory axis.\",\n      \"method\": \"Co-immunoprecipitation, in vivo ubiquitination assay, siRNA/shRNA knockdown, in vitro and in vivo tumor assays, luciferase reporter\",\n      \"journal\": \"Experimental & molecular medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — Co-IP plus ubiquitination assay, functional in vitro/in vivo assays, single lab\",\n      \"pmids\": [\"31956270\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"USP21 deubiquitinates and stabilizes FOXM1 in cervical cancer; FOXM1 activates the Hippo-YAP1 pathway (promoting nuclear YAP1 translocation), conferring radioresistance. USP21 knockdown enhances radiosensitivity in vitro and in vivo.\",\n      \"method\": \"Co-immunoprecipitation, ubiquitination assay, siRNA knockdown, colony survival assay, apoptosis assay, xenograft model\",\n      \"journal\": \"Human cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — Co-IP plus ubiquitination assay, multiple functional assays, single lab\",\n      \"pmids\": [\"34825342\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"USP21 binds to AIM2 upon DNA stimulation, deubiquitinates it to increase its protein stability, and is required for AIM2 inflammasome assembly (AIM2-ASC complex formation). Depletion of USP21 does not affect AIM2 DNA-binding ability but inhibits inflammasome complex formation.\",\n      \"method\": \"Co-immunoprecipitation, in vivo ubiquitination assay, USP21 siRNA knockdown, inflammasome assembly assay (ASC speck formation), IL-1β secretion assay\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP, ubiquitination assay, and mechanistic dissection of inflammasome assembly vs. DNA binding, single lab\",\n      \"pmids\": [\"34470856\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"USP21 deubiquitinates and stabilizes PD-L1. In vitro deubiquitination assay showed USP21-WT but not catalytic mutant C221A reduces polyubiquitin chains on PD-L1.\",\n      \"method\": \"Co-immunoprecipitation, in vitro deubiquitination assay, catalytic mutant C221A, siRNA knockdown, overexpression\",\n      \"journal\": \"American journal of translational research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — in vitro DUB assay with catalytic mutant control, single lab, limited downstream mechanistic detail\",\n      \"pmids\": [\"34956491\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"USP21 inhibits HIV-1 production by deubiquitinating the HIV-1 transactivator Tat (destabilizing it) and by reducing cyclin T1 mRNA levels through increased H3K9 methylation at the cyclin T1 promoter, thereby limiting Tat-dependent transcriptional elongation.\",\n      \"method\": \"Co-immunoprecipitation, in vivo ubiquitination assay, dominant-negative ubiquitin mutant, chromatin methylation assay (H3K9me), HIV-1 production assay\",\n      \"journal\": \"Journal of virology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — DUB assay with DN-Ub control, histone methylation assay, two orthogonal mechanisms identified, single lab\",\n      \"pmids\": [\"33827943\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"USP21 in skeletal muscle deubiquitinates DNA-PKcs and ACLY, leading to AMPK inhibition. Loss of USP21 promotes oxidative fibre type switching, increases muscle mass, enhances mitochondrial biogenesis, fatty acid oxidation, and thermogenesis, and protects against diet-induced obesity and insulin resistance.\",\n      \"method\": \"Whole-body and skeletal muscle-specific KO mice, transcriptomics, proteomics, lipidomics, in vivo metabolic phenotyping, Co-IP to identify substrates DNA-PKcs and ACLY, high-fat diet model\",\n      \"journal\": \"Journal of cachexia, sarcopenia and muscle\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — conditional KO with multiple omics and in vivo metabolic phenotypes, Co-IP identifies substrates, single lab\",\n      \"pmids\": [\"34523817\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"USP21 directly interacts with FOXD1, reverting its proteolytic ubiquitination and stabilizing FOXD1 protein. Silencing USP21 enhances FOXD1 polyubiquitination, promotes proteasomal degradation, and attenuates mesenchymal identity in glioblastoma stem cells.\",\n      \"method\": \"Co-immunoprecipitation, in vitro/in vivo ubiquitination assay, USP21 siRNA knockdown, FOXD1 rescue, GBM xenograft model\",\n      \"journal\": \"Cell death & disease\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP, ubiquitination assay, rescue experiment, in vivo xenograft, single lab\",\n      \"pmids\": [\"35974001\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"USP21 interacts with and deubiquitinates AURKA, preventing its degradation and promoting laryngeal cancer cell proliferation, migration, and invasion. Interaction confirmed by Co-IP and GST pull-down.\",\n      \"method\": \"Co-immunoprecipitation, GST pull-down, ubiquitination assay, siRNA knockdown, rescue by AURKA overexpression\",\n      \"journal\": \"The Kaohsiung journal of medical sciences\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — Co-IP and GST pull-down with ubiquitination assay, single lab, limited mechanistic depth\",\n      \"pmids\": [\"36919585\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"YOD1 interacts with USP21 and deubiquitinates MARK kinases. YOD1 and USP21 mutually deubiquitinate each other; YOD1 regulates USP21 protein stability, but USP21 does not stabilize YOD1. Both cooperate to promote cell proliferation via Hippo pathway modulation.\",\n      \"method\": \"Co-immunoprecipitation, GST pull-down, western blot ubiquitination assay, cell proliferation assays, YAP/p-YAP western blot\",\n      \"journal\": \"Cancer cell international\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — Co-IP and pull-down, single lab, limited orthogonal validation\",\n      \"pmids\": [\"37743467\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"BAY-805 is a potent non-covalent USP21 inhibitor identified via HTS and structure-based optimization. It engages USP21 with high affinity (SPR, CETSA) and activates NF-κB in a cell-based reporter assay, validating USP21's role in NF-κB regulation.\",\n      \"method\": \"High-throughput screening, structure-based optimization, surface plasmon resonance (SPR), CETSA, NF-κB reporter assay\",\n      \"journal\": \"Journal of medicinal chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — structure-based drug design with SPR and CETSA target engagement validation, single lab\",\n      \"pmids\": [\"36802665\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"USP21 deubiquitinates and stabilizes HSP90 via K48-linked deubiquitination in cholangiocarcinoma, which increases HIF1A expression, upregulating glycolytic enzyme genes (ENO2, ENO3, ALDOC, ACSS2). USP21 also directly stabilizes ENO1 to promote aerobic glycolysis.\",\n      \"method\": \"Co-immunoprecipitation, in vitro/in vivo K48-linked deubiquitination assay, siRNA knockdown, glycolysis assays, xenograft model\",\n      \"journal\": \"International journal of biological sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — K48-specific ubiquitination assay, Co-IP, functional metabolic readouts, single lab\",\n      \"pmids\": [\"38385089\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"USP21 deubiquitinates AhR at K432, removing K48-linked polyubiquitin chains and stabilizing AhR protein. Paradoxically, USP21-mediated deubiquitination of AhR at K432 inhibits AhR transcriptional activity in a deubiquitinating-dependent manner, thereby suppressing Th17 cell differentiation.\",\n      \"method\": \"Co-immunoprecipitation, K48-linked ubiquitination assay, site-directed mutagenesis (K432), siRNA KD, Th17 differentiation assay in vitro and in vivo (Rag1-/- adoptive transfer colitis model)\",\n      \"journal\": \"Journal of leukocyte biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — site-specific mutagenesis, K48-linkage assay, in vivo adoptive transfer model, single lab\",\n      \"pmids\": [\"38952265\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"USP21 binds to and deubiquitinates EGFR, reducing EGFR degradation and enhancing EGFR stability. USP21-KO colon cancer cells show reduced EGF-driven proliferation, migration, colony formation, and tumor spheroid formation.\",\n      \"method\": \"CRISPR/Cas9 USP21-KO, Co-immunoprecipitation, ubiquitination assay, in vitro cancer progression assays, NSG xenograft model, BAY-805 inhibitor treatment\",\n      \"journal\": \"Cell death discovery\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — CRISPR KO with defined phenotype, Co-IP and ubiquitination assay, in vivo xenograft, single lab\",\n      \"pmids\": [\"39695128\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"USP21 deubiquitinates and stabilizes TET2 in airway epithelial cells, inhibiting cigarette smoke extract-induced TET2 degradation and thereby attenuating apoptosis.\",\n      \"method\": \"Co-immunoprecipitation, ubiquitination assay, USP21 knockdown, TET2 overexpression, apoptosis assay in vitro and in vivo COPD model\",\n      \"journal\": \"iScience\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single Co-IP plus ubiquitination assay, single lab, limited mechanistic depth\",\n      \"pmids\": [\"38439981\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"USP21 deubiquitinates and stabilizes p65 (NF-κB) via K48-linked deubiquitination in bladder cancer cells, promoting cancer progression. USP21 directly interacts with p65 as confirmed by mechanistic studies.\",\n      \"method\": \"Co-immunoprecipitation, K48-linked ubiquitination assay, USP21 siRNA knockdown/overexpression, xenograft model, pharmacological inhibition (20-HE)\",\n      \"journal\": \"Translational oncology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — K48-specific ubiquitination assay, Co-IP, in vivo xenograft, single lab\",\n      \"pmids\": [\"38663220\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"USP21 deubiquitinates and stabilizes G3BP1, which activates Wnt/β-Catenin signaling to promote esophageal squamous cell carcinoma proliferation and metastasis. Disulfiram abolishes USP21-mediated G3BP1 stability.\",\n      \"method\": \"Co-immunoprecipitation, in vivo ubiquitination assay, rescue assay, in vitro and in vivo ESCC tumor models, disulfiram inhibitor treatment\",\n      \"journal\": \"Oncogenesis\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP, ubiquitination assay, in vivo model, rescue experiments, single lab\",\n      \"pmids\": [\"38906857\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"USP21 deubiquitinates and stabilizes YBX1; YBX1 in turn enhances transcription of HIF1A, promoting prostate cancer malignancy via the HIF1 signaling pathway. YBX1 was identified as the primary substrate by Co-IP coupled with mass spectrometry.\",\n      \"method\": \"Co-IP/mass spectrometry substrate identification, Co-immunoprecipitation, ubiquitination assay, dual-luciferase reporter assay, ChIP, siRNA knockdown, Bay-805 inhibitor, patient-derived organoids\",\n      \"journal\": \"Journal of translational medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — MS-based substrate discovery, Co-IP, ChIP, reporter assay, multiple orthogonal methods, single lab\",\n      \"pmids\": [\"41131631\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"USP21 stabilizes H2BFS (histone H2B family member S) through deubiquitination in hepatocellular carcinoma. METTL3-mediated m6A methylation of USP21 mRNA regulates USP21 expression, linking epitranscriptomic regulation to USP21-mediated substrate stabilization.\",\n      \"method\": \"MeRIP assay (m6A), Co-immunoprecipitation, ubiquitination assay, siRNA knockdown/overexpression, xenograft model\",\n      \"journal\": \"Biochemical genetics\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single Co-IP and ubiquitination assay, MeRIP for upstream regulation, single lab\",\n      \"pmids\": [\"39680331\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Disulfiram impairs USP21-mediated deubiquitination of MOF at lysine K257, leading to increased MOF ubiquitination and degradation, thereby suppressing Wnt/β-Catenin signaling and ESCC progression.\",\n      \"method\": \"Co-immunoprecipitation, site-specific ubiquitination assay (K257), disulfiram treatment, in vitro and in vivo ESCC models\",\n      \"journal\": \"Cancer letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — site-specific K257 ubiquitination assay, Co-IP, pharmacological validation, in vivo model, single lab\",\n      \"pmids\": [\"39725149\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Cryo-EM structure of USP21 catalytic domain bound to an H2AK119ub nucleosome revealed a recognition mode distinct from other H2AK119-specific DUBs. The N-terminal intrinsically disordered region (IDR) of USP21 autoinhibits its enzymatic activity. Kinases identified by AlphaFold-Multimer virtual screen phosphorylate the IDR, relieving autoinhibition and activating USP21.\",\n      \"method\": \"Cryo-EM structure determination of USP21–H2AK119ub nucleosome complex; AlphaFold-Multimer virtual screen; in vitro phosphorylation and DUB activity assays; AlphaFold3 structural modeling of autoinhibition\",\n      \"journal\": \"Science advances\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — cryo-EM structure plus biochemical autoinhibition assay plus kinase screen, multiple orthogonal methods in one study\",\n      \"pmids\": [\"41071870\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"USP21 deubiquitinates DPYSL2 (removing K48-linked ubiquitin), increasing DPYSL2 centrosomal abundance. Loss of USP21 leads to proteasomal degradation of DPYSL2 at centrosomes and causes ciliary defects in kidney, liver, and trachea of Usp21 KO mice.\",\n      \"method\": \"Usp21 KO mice (multi-organ phenotyping), Co-immunoprecipitation, K48-linked ubiquitination assay, immunofluorescence localization at centrosome/basal body, proteasome inhibitor rescue\",\n      \"journal\": \"Journal of genetics and genomics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — KO mouse phenotype, K48-specific ubiquitination assay, spatial localization data, Co-IP, single lab\",\n      \"pmids\": [\"40619097\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"USP21 acts as a scaffold (in a deubiquitinase activity-independent manner) to facilitate USP7-Mdm2 interaction, enhancing Mdm2 stability and consequently promoting p53 ubiquitination and degradation, thereby suppressing p53 tumor suppressor activity in colorectal cancer.\",\n      \"method\": \"Co-immunoprecipitation, ubiquitination assay, catalytic mutant analysis (activity-independent scaffolding), siRNA knockdown, CRC cell lines, in vivo tumor models\",\n      \"journal\": \"Cell death discovery\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP, ubiquitination assay, catalytic-dead mutant distinguishes scaffolding from DUB activity, single lab\",\n      \"pmids\": [\"42168156\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"USP21 deubiquitinates and stabilizes EGFR and Lyn in NSCLC cells, preventing their ubiquitination and degradation and sustaining oncogenic signaling. USP21-KO suppresses tumor growth in xenograft models.\",\n      \"method\": \"USP21-KO lung cancer cell lines (CRISPR), Co-immunoprecipitation, ubiquitination assay, in vivo xenograft, BAY-805 inhibitor treatment\",\n      \"journal\": \"Biomarker research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — CRISPR KO, Co-IP, ubiquitination assay, in vivo xenograft, single lab\",\n      \"pmids\": [\"40629473\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"USP21 deubiquitinates and stabilizes ALDH2 in vascular smooth muscle cells, promoting VSMC dedifferentiation and phenotypic changes that exacerbate abdominal aortic aneurysm. Pharmacological inhibition of USP21 with disulfiram reduces AAA progression.\",\n      \"method\": \"USP21 global KO and VSMC-specific KO mice (angiotensin II and PPE models), Co-immunoprecipitation/mass spectrometry substrate identification, proteomic analysis, disulfiram treatment, ALDH2E506K mutant mice\",\n      \"journal\": \"Cell reports. Medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — conditional KO mice with MS-based substrate identification, in vivo AAA models, pharmacological validation, single lab\",\n      \"pmids\": [\"40925375\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"USP21 stabilizes β-catenin by removing K48-linked ubiquitin chains, enabling its nuclear translocation. Nuclear β-catenin binds the ATF3 promoter to upregulate ATF3, which transcriptionally represses CCL4, limiting CD8+ T-cell trafficking via CCL4-CCR5 axis and promoting immune evasion in colorectal cancer.\",\n      \"method\": \"Syngeneic mouse CRC models (MSI-H and MSS), CRISPR Usp21 KO, BAY-805 treatment, Co-immunoprecipitation, K48-linkage ubiquitination assay, ChIP (β-catenin on ATF3 promoter), huCD34+ humanized mice with anti-PD-1\",\n      \"journal\": \"Cellular & molecular immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — CRISPR KO, K48-specific ubiquitination assay, ChIP, two independent in vivo models, single lab\",\n      \"pmids\": [\"42091690\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"USP21 interacts with SMARCB1 via Co-immunoprecipitation and prevents its ubiquitin-mediated proteasomal degradation under hypoxia in hepatocellular carcinoma cells, stabilizing SMARCB1 to sustain its oncogenic and immunosuppressive activities.\",\n      \"method\": \"Cycloheximide chase assay, Co-immunoprecipitation, loss/gain-of-function experiments, transcriptomic analysis\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single Co-IP, CHX chase, single lab, limited mechanistic depth\",\n      \"pmids\": [\"41637985\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"USP21 is a cysteine deubiquitinase (catalytic residue C221) that removes polyubiquitin chains—predominantly K48- and K63-linked—from a broad array of substrates including RIG-I, STING, FOXM1, FOXP3, GATA3, Nanog, TCF7, MEK2, EZH2, AIM2, PD-L1, Gli1, AhR, EGFR, Lyn, β-catenin, ALDH2, DPYSL2, and histone H2AK119; its N-terminal region directly binds microtubules and centrosomes (residues 59–75), positioning it at the centrosome where it also regulates Gli1/PKA and cilium formation; the N-terminal IDR autoinhibits catalytic activity in a phosphorylation-reversible manner; and p38-mediated phosphorylation at Ser538 recruits USP21 to STING during viral infection, revealing a general paradigm in which post-translational modification of USP21 itself controls its substrate selectivity and activity.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"USP21 is a cysteine deubiquitinase that hydrolyzes polyubiquitin chains—predominantly K48- and K63-linked—and acts more weakly on ISG15 but not NEDD8, with a USP-domain architecture that presents a second ubiquitin/ISG15-binding surface explaining its polyUb processivity and cross-reactivity [#0]. Catalysis depends on the active-site cysteine C221, and structural work on the catalytic domain bound to an H2AK119ub nucleosome together with the finding that its N-terminal intrinsically disordered region autoinhibits enzymatic activity establishes a self-regulated enzyme whose activity is relieved by kinase-mediated phosphorylation of the IDR [#37, #2]. A recurring theme across substrates is that USP21 stabilizes its targets by reversing K48-linked degradative ubiquitination: it sustains protein levels of transcription factors and signaling components including FOXP3 and GATA3 in Treg biology [#9, #2], Nanog and histone H2AK119 in stem-cell self-renewal [#7], TCF7 and β-catenin in Wnt signaling [#16, #42], FOXM1 in cell-cycle progression [#15], MEK2 in ERK signaling [#14], and EGFR/Lyn in receptor signaling [#40]. In innate immunity USP21 functions as a negative regulator of type I interferon by deubiquitinating RIG-I (K63 chains) and STING (K27/K63 chains), the latter following p38-mediated phosphorylation of USP21 at Ser538 that recruits it to STING during viral infection—illustrating how post-translational modification of USP21 itself directs substrate engagement [#4, #11]. The protein also localizes to centrosomes and microtubules through a discrete N-terminal motif (residues 59–75), where it controls microtubule regrowth, ciliogenesis, and Gli1/Hedgehog signaling output, and stabilizes the centrosomal substrate DPYSL2 to support cilium formation [#1, #10, #38]. Beyond canonical catalysis, USP21 can act as a deubiquitinase-independent scaffold, promoting the USP7–Mdm2 interaction to drive p53 degradation [#39]. Across many tissues these activities position USP21 as a frequently oncogenic and immunoregulatory enzyme, and a non-covalent inhibitor (BAY-805) provides chemical-genetic validation of its functions [#27].\",\n  \"teleology\": [\n    {\n      \"year\": 2011,\n      \"claim\": \"Established USP21 as a polyubiquitin-cleaving cysteine DUB with defined linkage preferences and a structural basis for ubiquitin/ISG15 cross-reactivity, defining its core enzymatic identity.\",\n      \"evidence\": \"Crystal structure of USP21–linear diUb aldehyde complex plus in vitro cleavage assays against polyUb, ISG15, and NEDD8\",\n      \"pmids\": [\"21399617\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Cellular substrate repertoire not addressed\", \"No regulation of the enzyme defined at this stage\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Localized USP21 to centrosomes and microtubules via a discrete N-terminal motif, linking the enzyme to cytoskeletal and ciliary functions distinct from its catalytic core.\",\n      \"evidence\": \"Subcellular localization survey, in vitro microtubule-binding with N-terminal deletions, siRNA knockdown with microtubule regrowth, ciliogenesis, and neurite outgrowth readouts\",\n      \"pmids\": [\"22298430\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Centrosomal substrates not identified here\", \"Whether catalytic activity is required for cytoskeletal effects unresolved\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Connected USP21 catalysis (C221) to substrate stabilization in adaptive immunity and to histone H2A deubiquitination, broadening its role from in vitro chains to physiological substrates.\",\n      \"evidence\": \"Co-IP, in vivo ubiquitination assays, C221A catalytic mutant, FOXP3 promoter assays (GATA3); isoform identification and in vitro transcription with ubH2A readout\",\n      \"pmids\": [\"23395819\", \"24278184\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Ubiquitin linkage type on GATA3 not defined\", \"Histone targeting specificity not structurally resolved at this stage\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Defined USP21 as a negative regulator of innate antiviral signaling by removing K63-linked chains from RIG-I, establishing an in vivo immunoregulatory role.\",\n      \"evidence\": \"Co-IP, in vitro/in vivo DUB assays, USP21 KO MEFs/macrophages/BMDCs, chimeric mouse VSV infection model\",\n      \"pmids\": [\"24493797\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of USP21 recruitment to RIG-I not defined\", \"Whether the same applies to other RLRs untested\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Showed USP21 stabilizes the pluripotency factor Nanog (K48 chains) and is itself regulated by ERK phosphorylation, while also acting on promoter-bound H2AK119—coupling substrate selectivity to upstream signaling.\",\n      \"evidence\": \"Reciprocal Co-IP, in vitro DUB assays, USP21 KO mESC differentiation/reprogramming assays, domain mapping, ChIP for H2AK119ub\",\n      \"pmids\": [\"27886188\", \"29263902\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which ERK phosphorylation dissociates USP21 from Nanog not structurally defined\", \"Generality of phospho-control across substrates unknown at this stage\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Established USP21 control of immune-cell identity (FOXP3/Treg stability) and Hedgehog output (Gli1 at the centrosome), demonstrating spatially distinct regulatory roles.\",\n      \"evidence\": \"Treg-specific USP21 KO mice with in vivo FOXP3 ubiquitination assay and flow cytometry; Co-IP, Gli1 reporter, centrosomal IF, PKA phosphorylation assay\",\n      \"pmids\": [\"27857073\", \"27621083\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How centrosomal localization selects Gli1 vs other substrates unclear\", \"Direct vs indirect effect on PKA phosphorylation of Gli1 not fully resolved\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Defined the STING-directed antiviral mechanism and showed USP21 substrate recruitment is controlled by p38 phosphorylation at Ser538, formalizing PTM-driven substrate engagement.\",\n      \"evidence\": \"Co-IP, in vitro DUB assays (K27/K63), Ser538 phospho-site mapping, p38 inhibitor, USP21 KO cells, mouse HSV-1 infection model\",\n      \"pmids\": [\"28254948\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether Ser538 phosphorylation affects catalysis or only localization not separated\", \"Other kinases acting on USP21 not surveyed here\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Extended USP21 to growth and proliferative signaling by stabilizing MEK2 (K48) and modulating Hippo via MARK kinases, linking the DUB to oncogenic pathways.\",\n      \"evidence\": \"Co-IP, K48-specific ubiquitination assay, xenograft (MEK2); Co-IP, ubiquitination, YAP/TAZ reporters in Drosophila and mammalian cells (Hippo)\",\n      \"pmids\": [\"29706623\", \"28969054\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct enzyme-substrate contacts not structurally mapped\", \"Tissue specificity of Hippo regulation not defined\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Identified FOXM1 and TCF7 as USP21 substrates driving cell-cycle progression and Wnt-dependent stemness, with in vivo tumor models linking the DUB to cancer phenotypes.\",\n      \"evidence\": \"RNAi screen, Co-IP, in vitro DUB assay, cell-cycle analysis, xenograft (FOXM1); Co-IP, ubiquitination, KrasG12D PDAC models, patient-derived lines (TCF7)\",\n      \"pmids\": [\"30865895\", \"31488580\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Ubiquitin chain linkage on FOXM1 not defined\", \"How USP21 is targeted to these transcription factors mechanistically unclear\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Broadened the substrate landscape to inflammasome assembly (AIM2), immune checkpoint (PD-L1), antiviral restriction of HIV-1 (Tat), and metabolic control in skeletal muscle (DNA-PKcs/ACLY), demonstrating pleiotropic deubiquitinase roles.\",\n      \"evidence\": \"Co-IP and ubiquitination assays with functional readouts (ASC speck/IL-1β; PD-L1 with C221A control; HIV-1 production with histone methylation assay; muscle-specific KO mice with omics and metabolic phenotyping)\",\n      \"pmids\": [\"34470856\", \"34956491\", \"33827943\", \"34523817\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"For several substrates ubiquitin linkage type not defined\", \"Single-lab Co-IP evidence for some interactions\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Consolidated USP21 as a recurrently oncogenic K48-linkage DUB stabilizing diverse substrates (HSP90/ENO1, AhR, EGFR, p65, G3BP1, YBX1, FOXD1, TET2) across many cancers and tissues, with several upstream regulators (METTL3 m6A) and inhibitors (BAY-805, disulfiram) defined.\",\n      \"evidence\": \"Co-IP, K48-specific and site-specific ubiquitination assays, CRISPR KO, xenograft models, MS-based substrate ID, pharmacological inhibition\",\n      \"pmids\": [\"38385089\", \"38952265\", \"39695128\", \"38663220\", \"38906857\", \"41131631\", \"35974001\", \"39725149\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Most substrate interactions rest on single-lab Co-IP\", \"Selectivity rules determining which substrates dominate in a given tissue unresolved\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Resolved the structural basis of nucleosomal H2AK119ub recognition and established that the N-terminal IDR autoinhibits USP21 in a phosphorylation-reversible manner, unifying earlier observations of PTM-controlled activity.\",\n      \"evidence\": \"Cryo-EM of USP21–H2AK119ub nucleosome, AlphaFold-Multimer kinase screen, in vitro phosphorylation and DUB assays, AlphaFold3 autoinhibition modeling\",\n      \"pmids\": [\"41071870\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Which physiological kinases relieve autoinhibition in vivo not confirmed\", \"How autoinhibition intersects with substrate-specific recruitment unclear\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Demonstrated deubiquitinase-independent scaffolding (USP7–Mdm2/p53 axis) and ciliary control via centrosomal DPYSL2 stabilization, refining USP21's catalytic and non-catalytic modes.\",\n      \"evidence\": \"Co-IP, catalytic-dead mutant analysis, CRC tumor models (scaffold); Usp21 KO mice multi-organ ciliary phenotyping, K48 ubiquitination assay, centrosomal IF (DPYSL2)\",\n      \"pmids\": [\"42168156\", \"40619097\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Structural basis of scaffolding not defined\", \"Relative contribution of catalytic vs scaffold functions in vivo not quantified\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Linked USP21-driven β-catenin stabilization to immune evasion through an ATF3–CCL4 axis controlling CD8+ T-cell trafficking, integrating its DUB activity with the tumor immune microenvironment.\",\n      \"evidence\": \"Syngeneic and humanized CRC models, CRISPR KO, BAY-805, K48 ubiquitination assay, ChIP (β-catenin on ATF3 promoter), anti-PD-1 combination\",\n      \"pmids\": [\"42091690\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct β-catenin contact site not mapped\", \"Generality across MSI/MSS subtypes only partly addressed\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"It remains unresolved how USP21 achieves substrate selectivity among its very broad target set—how localization, IDR phosphorylation, and the second ubiquitin-binding surface are integrated to choose among the dozens of reported substrates in a given cell.\",\n      \"evidence\": \"No single study in the timeline reconciles the determinants of substrate choice across contexts\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No unifying model of substrate discrimination\", \"Few substrate interactions validated reciprocally beyond a single lab\", \"In vivo hierarchy of substrates per tissue undefined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [0, 2, 4, 7, 11, 15, 16, 21, 37]},\n      {\"term_id\": \"GO:0016787\", \"supporting_discovery_ids\": [0, 4, 11]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [4, 11, 9, 10]},\n      {\"term_id\": \"GO:0008092\", \"supporting_discovery_ids\": [1]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [39]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005815\", \"supporting_discovery_ids\": [1, 10, 38]},\n      {\"term_id\": \"GO:0005856\", \"supporting_discovery_ids\": [1]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [3, 7]},\n      {\"term_id\": \"GO:0005929\", \"supporting_discovery_ids\": [1, 38]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [4, 9, 11, 20, 29]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [10, 12, 14, 16, 42]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [2, 7, 14, 15, 16, 21, 30]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [13, 14, 15, 16, 30, 39, 42]},\n      {\"term_id\": \"R-HSA-1640170\", \"supporting_discovery_ids\": [15, 25]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"STING1\", \"DDX58\", \"FOXP3\", \"GATA3\", \"NANOG\", \"GLI1\", \"FOXM1\", \"TCF7\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":7,"faith_total":7,"faith_pct":100.0}}