{"gene":"ISG15","run_date":"2026-06-10T01:55:23","timeline":{"discoveries":[{"year":2017,"finding":"Extracellular ISG15 signals through the LFA-1 integrin receptor (CD11a/CD18; αLβ2 integrin) to promote IFN-γ and IL-10 secretion. ISG15 bound directly to the αI domain of CD11a in vitro; LFA-1 inhibition or CD11a knockout blocked cytokine secretion. ISG15 engagement of LFA-1 led to activation of SRC family kinases (SFKs), and SFK inhibition blocked cytokine secretion.","method":"NK-92 cell IFN-γ release assay, direct in vitro binding to αI domain, CD11a-/- mouse splenocytes, LFA-1 inhibitor studies, SFK inhibitor studies, site-directed mutagenesis of ISG15 signaling residues","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — reciprocal functional validation with KO mice, direct in vitro binding, mutagenesis, and kinase inhibitor studies in a single rigorous study","pmids":["29100055"],"is_preprint":false},{"year":1999,"finding":"Pro-ISG15 is synthesized as a precursor that requires processing by a constitutive 100-kDa thiol protease to expose the mature C-terminal glycine (Gly157-Gly158 cleavage). The processing enzyme is stimulated 12-fold by micromolar ubiquitin and is related to yeast Ubp1.","method":"Biochemical purification from human lung fibroblast extracts, in vitro processing assay, thiol protease inhibitor studies, pH rate analysis, tryptic peptide sequencing","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — enzyme purified to homogeneity, in vitro reconstitution with mutagenesis-level cleavage site mapping, single lab but multiple orthogonal biochemical methods","pmids":["10455185"],"is_preprint":false},{"year":2010,"finding":"Free intracellular ISG15 stabilizes USP18 protein by protecting it from proteasomal degradation, and this stabilization of USP18 is the mechanism by which ISG15 downregulates type I IFN-α/β signaling in humans (but not in mice). ISG15-deficient human cells display chronically elevated ISG signaling as a result of reduced USP18 levels.","method":"ISG15-deficient patient fibroblasts, complementation with WT ISG15 vs. ISG15ΔGG (conjugation-deficient), USP18 protein level measurement, IFN signaling readouts","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 / Strong — human patient cells, complementation with conjugation-deficient mutant to separate free from conjugated functions, replicated across multiple labs","pmids":["27193971"],"is_preprint":false},{"year":2012,"finding":"Secreted ISG15 from granulocytes/leukocytes stimulates IFN-γ production by lymphocytes (including NK cells) in response to mycobacterial infection; this extracellular signaling function is essential for antimycobacterial immunity in humans. Loss of intracellular ISGylation did not confer susceptibility to viral infections in these patients.","method":"Human ISG15-deficient patients (inherited loss-of-function mutations), leukocyte secretion assays, NK cell IFN-γ production assays","journal":"Science (New York, N.Y.)","confidence":"High","confidence_rationale":"Tier 2 / Strong — human genetic experiment of nature with well-characterized patients, multiple cell-type functional assays, replicated by subsequent studies","pmids":["22859821"],"is_preprint":false},{"year":2023,"finding":"Cryo-EM structures of human Uba7 (UBE1L) in complex with UBE2L6 and ISG15 adenylate or ISG15 thioester intermediate revealed the molecular basis for ISG15 activation and E1-to-E2 thioester transfer. The structures demonstrate unique overall architecture compared to the ubiquitin conjugation pathway and explain Uba7's exquisite specificity for ISG15 and UBE2L6.","method":"Cryo-EM structure determination, biochemical assays, human cell-based functional validation, mutagenesis of specificity determinants","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 / Moderate — cryo-EM structures at near-atomic resolution combined with biochemical reconstitution and cell-based mutagenesis in a single study","pmids":["37553340"],"is_preprint":false},{"year":2023,"finding":"Cryo-EM and crystal structures of SARS-CoV-2 PLpro complexed with human ISG15 and K48-linked di-ubiquitin revealed that the two UBL domains of ISG15 are differentially engaged by PLpro compared to K48-Ub2, and differential binding stabilities of the two UBL/Ub domains explain PLpro substrate selectivity.","method":"Crystal structures, cryo-EM, solution NMR, cross-linking mass spectrometry, binding affinity measurements","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 / Moderate — multiple orthogonal structural and biophysical methods in a single rigorous study","pmids":["37185902"],"is_preprint":false},{"year":2023,"finding":"A 3.45 Å cryo-EM structure of the UBE1L (Uba7)-UBE2L6 complex bound to activated ISG15 reveals the molecular basis of ISG15 recognition and UBE2L6 recruitment, and identifies the ISG15 C-terminal ubiquitin-like domain as critical for the adenylation reaction. Mutants with altered selectivity for ISG15 vs. ubiquitin conjugation pathways were designed based on the structure.","method":"Cryo-EM structure, biochemical characterization using viral effector proteins (SARS-CoV-2 PLpro, influenza B NS1), mutagenesis of ISG15 and UBE2L6","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 / Moderate — high-resolution cryo-EM structure with structure-guided mutagenesis and biochemical validation, single lab but multiple orthogonal methods","pmids":["38042859"],"is_preprint":false},{"year":2006,"finding":"Elevated ISG15 in tumor cells antagonizes the ubiquitin/26S proteasome pathway by interfering with protein polyubiquitination. siRNA knockdown of ISG15 increased polyubiquitinated protein levels; siRNA knockdown of the E2 enzyme UbcH8 (blocking ISG15 conjugate formation) also increased polyubiquitinated proteins.","method":"siRNA knockdown of ISG15 and UbcH8 in tumor cell lines, western blot analysis of polyubiquitinated proteins","journal":"Cancer research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — two orthogonal siRNA experiments (targeting ISG15 and its E2) with consistent results, single lab","pmids":["16424026"],"is_preprint":false},{"year":2014,"finding":"ISG15 (Isg15) covalently modifies p53 at two sites (N- and C-terminal) via ISGylation. ISGylated p53 is degraded by the 20S proteasome. ISGylation preferentially targets misfolded, dominant-negative p53. Isg15 deletion in normal cells suppresses p53 activity.","method":"Co-immunoprecipitation, ISGylation assays, 20S proteasome degradation assay, Isg15 knockout mouse cells, site-directed mutagenesis","journal":"Cell cycle (Georgetown, Tex.)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct ISGylation shown by Co-IP and in vitro degradation assay, KO cells confirm functional consequence; single lab","pmids":["24844324"],"is_preprint":false},{"year":2019,"finding":"USP18 isopeptidase activity promotes accumulation of misfolded p53 by removing ISG15 from p53; ISG15 is required for degradation of misfolded dominant-negative p53. Depletion of ISG15 causes accumulation of misfolded dominant-negative p53 and enhances HIV-1 replication in myeloid cells.","method":"USP18 isopeptidase activity assays, ISG15 knockdown in primary myeloid cells, p53 conformation assays, HIV-1 replication assays","journal":"mBio","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — enzymatic activity established with conformation-specific p53 detection and HIV replication read-out; single lab, multiple orthogonal approaches","pmids":["31455647"],"is_preprint":false},{"year":2020,"finding":"Intracellular ISGylation inhibits ISG15 secretion, while influenza B NS1 (which sequesters ISG15) and viral de-ISGylases (including SARS-CoV-2 PLpro) have opposing effects on ISG15 secretion. Both epithelial cells and lymphocytes can secrete ISG15 to signal in autocrine or paracrine fashion through LFA-1.","method":"ISG15 secretion assays, ISGylation inhibition experiments, viral effector protein expression, residue mapping of secretion-critical ISG15 residues","journal":"Cell reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — mechanistic dissection of secretion pathway with viral effectors and conjugation mutants; single lab, multiple cell types","pmids":["32553163"],"is_preprint":false},{"year":2021,"finding":"IRF3 drives expression of ISG15, which becomes covalently attached (ISGylated) to glycolytic enzymes in adipocytes, reducing their enzymatic function and decreasing lactate production. Reduced lactate suppresses thermogenic gene expression. ISG15-deficient mice phenocopy adipocyte-specific IRF3-deficient mice with elevated energy expenditure and resistance to diet-induced obesity.","method":"Isg15 knockout mice, IRF3 overexpression/knockout in adipocytes, metabolic measurements (oxygen consumption, energy expenditure), lactate repletion rescue experiments, ISGylation assays of glycolytic enzymes","journal":"The Journal of clinical investigation","confidence":"High","confidence_rationale":"Tier 2 / Strong — ISG15 KO mice phenocopy IRF3 KO, lactate rescue experiment establishes mechanism, ISGylation of specific glycolytic enzyme substrates demonstrated; multiple orthogonal approaches in single study","pmids":["33571167"],"is_preprint":false},{"year":2020,"finding":"ISG15 deletion in pancreatic cancer stem cells (PaCSCs) decreases mitochondrial ISGylation, impairs mitophagy, reduces oxidative phosphorylation, and impairs self-renewal and tumorigenic capacity. Rescue with wild-type ISG15 but not conjugation-deficient ISG15 restored these functions, indicating ISGylation (not free ISG15) is required for mitophagy and metabolic plasticity.","method":"CRISPR-mediated ISG15 KO, complementation with WT vs. conjugation-deficient ISG15, mitochondrial function assays (OXPHOS, mitophagy), in vivo tumor formation assays","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 / Strong — CRISPR KO with complementation using conjugation-deficient mutant to dissect free vs. conjugated ISG15; multiple functional readouts including in vivo; single lab","pmids":["32472071"],"is_preprint":false},{"year":2023,"finding":"ISGylation of STING at residue K289 is required for STING oligomerization and type I IFN induction in response to HIV-1 DNA sensing via cGAS-STING. ISG15 deficiency inhibits STING-dependent antiviral sensing. Removal of STING ISGylation alleviates gain-of-function phenotype in SAVI disease.","method":"ISG15 KO cells, site-directed mutagenesis of STING ISGylation sites (K224, K236, K289, K347, K338, K370), STING oligomerization assays, IFN induction assays, molecular modeling","journal":"Cell reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — site-specific mutagenesis identifies critical K289 residue for oligomerization, KO and molecular modeling support mechanism; single lab","pmids":["37864791"],"is_preprint":false},{"year":2020,"finding":"ISG15 and ISGylation are required for IFN-γ-mediated restriction of Toxoplasma gondii in human cells. ISG15 deletion impaired recruitment of autophagy adaptors p62, NDP52, and LC3 to the parasitophorous vacuole. ISG15 function required conjugation, and ISGylated targets overlapped with the IFN-γ-dependent ATG5 interactome, including adapter p62.","method":"ISG15 KO HeLa cells, proximity biotinylation of ATG5 interactome, immunofluorescence of p62/NDP52/LC3 recruitment, T. gondii growth restriction assays, conjugation-deficient ISG15 rescue","journal":"mBio","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — KO with conjugation-deficient rescue, ATG5 interactome, and parasite growth assay; single lab, multiple orthogonal methods","pmids":["33024031"],"is_preprint":false},{"year":2015,"finding":"ISG15 expression restricts Listeria monocytogenes infection in vitro and in vivo. ISG15 induction in non-phagocytic cells upon Listeria infection can be type I IFN-independent, signaling through STING-TBK1-IRF3/IRF7 cytosolic DNA surveillance pathway. ISGylation of ER and Golgi proteins correlates with increased secretion of anti-infection cytokines.","method":"ISG15 overexpression and knockout cells, SILAC proteomics to identify ISGylated proteins, in vivo Listeria infection model, IFN pathway inhibition","journal":"eLife","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — SILAC proteomics combined with KO/OE and in vivo infection model; single lab, multiple orthogonal methods","pmids":["26259872"],"is_preprint":false},{"year":2007,"finding":"UbcH8 (the ISG15 E2 enzyme) competes with RNF125 ubiquitin E3 ligase for RIG-I modification: UbcH8 suppresses RIG-I ubiquitination by RNF125, and this suppression is relieved by ectopic ISG15. Conversely, ISG15 conjugation to RIG-I is suppressed by RNF125. UbcH8 and ISG15 function as regulators of RNF125 E3 ligase activity toward RIG-I.","method":"Co-immunoprecipitation, ectopic expression of UbcH8/ISG15/RNF125, ubiquitination and ISGylation assays for RIG-I","journal":"Molecular immunology","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — Co-IP and overexpression assays demonstrate competitive regulatory circuit, single lab","pmids":["17719635"],"is_preprint":false},{"year":2016,"finding":"HCMV pUL26 interacts with ISG15, UBE1L (E1), and HERC5 (E3), and is itself ISGylated. ISGylation of pUL26 regulated its stability and inhibited its activities (NF-κB suppression and viral growth complementation). Reciprocally, pUL26 suppressed virus-induced ISGylation independent of its own ISGylation. ISGylation inhibits HCMV growth by downregulating viral gene expression and virion release.","method":"Co-immunoprecipitation, ISGylation assays, RNAi knockdown of ISGylation enzymes, UL26-null mutant virus analysis, NF-κB reporter assay","journal":"PLoS pathogens","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal Co-IP, null mutant virus, and functional assays; single lab, multiple orthogonal approaches","pmids":["27564865"],"is_preprint":false},{"year":2022,"finding":"ISG15 conjugation to proteins on nascent DNA mitigates DNA replication stress. Mre11 complex inactivation causes cGAS-STING-TBK1-dependent ISG15 induction and ISGylation of replication fork constituents. ISG15 deletion is associated with replication fork stalling, tonic ATR activation, genomic aberrations, and sensitivity to aphidicolin.","method":"Mre11 complex inactivation, ISG15 deletion mouse cells, iPOND (isolation of proteins on nascent DNA), replication fork stability assays, aphidicolin sensitivity, cGAS-STING signaling assays","journal":"Nature communications","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — ISG15 KO with replication fork assays and genomic stability readouts; single lab, multiple orthogonal methods","pmids":["36216822"],"is_preprint":false},{"year":2019,"finding":"USP18 and ISG15 regulate the SCF-SKP2 ubiquitin E3 ligase complex: free ISG15 disrupts the USP18-SKP2 interaction, liberating USP18 from SKP2-mediated degradation while simultaneously driving SKP2 to degradation and/or ISGylation. USP18 silencing slows HeLa S3 cell progression toward S phase.","method":"Co-immunoprecipitation of USP18-SKP2 complex, ectopic ISG15 expression, cell cycle analysis, USP18 siRNA knockdown","journal":"Scientific reports","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — Co-IP and cell cycle assay; single lab, multiple approaches but primarily overexpression and siRNA","pmids":["30858391"],"is_preprint":false},{"year":2017,"finding":"ISG15 directly conjugates to PTEN (ISGylation of PTEN), and USP18 deISGylase stabilizes PTEN protein. USP18 knockdown destabilizes PTEN while USP18 overexpression stabilizes PTEN. USP18 repression decreased cytoplasmic relative to nuclear PTEN levels.","method":"Co-immunoprecipitation showing ISG15-PTEN conjugation, cycloheximide chase assays, reverse-phase protein arrays, USP18 knockdown/overexpression","journal":"Oncotarget","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — Co-IP demonstrates ISGylation of PTEN, cycloheximide chase shows stability effect; single lab","pmids":["27980214"],"is_preprint":false},{"year":2020,"finding":"ISGylation of viral and host proteins (including antiviral IFIT1/3 proteins) by ISG15 sets non-hematopoietic cells into a state resistant to coxsackievirus B3 (CVB). Concurrently, ISG15 adapts liver metabolism by increasing oxidative capacity and promoting gluconeogenesis. Cells lacking USP18 (thereby stabilizing ISGylation) show increased resistance to CVB.","method":"ISG15 KO and USP18 KO mouse models, shotgun proteomics, metabolic network modeling, coxsackievirus infection assays","journal":"Science advances","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — KO mouse models with proteomics and metabolic modeling; single lab, multiple orthogonal methods","pmids":["32195343"],"is_preprint":false},{"year":2024,"finding":"ISGylation of hexokinase 2 (HK2) and phosphofructokinase (PFK1) during Coxsackievirus B3 infection obstructs their enzymatic activity, preventing infection/IFN-induced upregulation of glycolysis in cardiomyocytes. This preserves mitochondrial oxidative capacity in the heart during infection.","method":"Mass spectrometry ISGylome identification, enzymatic functional testing of HK2 and PFK1, structural modeling, Seahorse metabolic flux analysis in cardiomyocytes and mouse heart tissue, ISG15 KO mice","journal":"Cardiovascular research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — specific ISGylation substrates identified by MS, enzymatic activity assays performed, and Seahorse metabolic profiling; single lab with multiple orthogonal methods","pmids":["38309955"],"is_preprint":false},{"year":2023,"finding":"ISG15 upregulation in pressure-overloaded hearts (driven by type I IFN signaling from pathogenic macrophages) leads to ISGylation of the myofibrillar protein filamin-C, promoting misfolded filamin-C accumulation and impairing cardiomyocyte autophagy, contributing to adverse ventricular remodeling. ISG15 deficiency preserved cardiac function in transverse aortic constriction mice.","method":"Nanoscale LC-MS/MS to identify ISGylation targets, ISG15 KO mice with TAC model, cardiac function measurements, metabolomics, human nonischemic cardiomyopathy tissue analysis","journal":"The Journal of clinical investigation","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — MS-identified substrate validated in KO mouse model with cardiac functional readout; single lab, multiple orthogonal methods","pmids":["37115698"],"is_preprint":false},{"year":2023,"finding":"USP16 functions as an ISG15 cross-reactive deubiquitinase. Recombinant USP16 cleaved pro-ISG15 and ISG15 isopeptide-linked substrates in vitro and from cell lysates; USP16 depletion increased interferon-induced ISGylation. USP16-dependent ISG15 substrates include malate dehydrogenase, superoxide dismutase 1, fructose-bisphosphate aldolase A, and glutamic-oxaloacetic transaminase 1.","method":"ISG15 activity-based profiling, recombinant USP16 in vitro cleavage assays, USP16 depletion and ISGylation measurement, ISG15 interactome analysis","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vitro reconstitution of USP16 deISGylase activity plus cell-based depletion confirmation and substrate identification; single lab","pmids":["38055744"],"is_preprint":false},{"year":2017,"finding":"USP18 is a highly specific protease that cleaves ISG15 from ISGylated proteins. Crystal structures of USP18 and a USP18-ISG15 complex revealed the molecular basis for its unique specificity for ISG15 over ubiquitin. USP18 also interacts with the type I IFN receptor to shut off downstream signaling independent of its enzymatic activity.","method":"Crystal structure of USP18-ISG15 complex, biochemical cleavage assays","journal":"The FEBS journal","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — crystal structure provides mechanistic basis for specificity; single lab structure review article referencing structural data","pmids":["28881486"],"is_preprint":false},{"year":2016,"finding":"ISG15 ISGylation of RSV-infected cells reduces virus growth. Conjugation inhibition assays demonstrated that the antiviral activity against RSV requires protein ISGylation (not free ISG15). High levels of ISG15 present before infection are required for antiviral activity.","method":"ISG15 overexpression, siRNA silencing, ISG15 KO cells, conjugation inhibition assays, RSV growth assays","journal":"Journal of virology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple genetic approaches (OE, KO, conjugation inhibition) with viral growth readout; single lab","pmids":["26763998"],"is_preprint":false},{"year":2023,"finding":"ISGylation of KPNA2 (importin subunit alpha 2) by ISG15 impedes KPNA2 ubiquitination, promoting KPNA2 stability, which in turn maintains cancer stem cell-like characteristics in anaplastic thyroid carcinoma. Overexpression of ISGylation-competent but not non-ISGylatable ISG15 mutant reinforced CSC characteristics.","method":"Mass spectrometry to identify ISGylation substrates, co-immunoprecipitation, ISGylation-deficient ISG15 mutant rescue, KPNA2 ubiquitination assay, xenograft models","journal":"Journal of experimental & clinical cancer research : CR","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — substrate identified by MS and validated by Co-IP, conjugation-deficient mutant used to separate ISGylation-dependent effects; single lab","pmids":["37501099"],"is_preprint":false},{"year":2023,"finding":"ISG15 promotes ISGylation-mediated degradation of glycosylated PD-L1 via K48-linked ubiquitin chains targeting the proteasomal pathway, reducing PD-L1 protein stability in lung adenocarcinoma cells.","method":"Co-immunoprecipitation showing ISG15-PD-L1 conjugation, K48-ubiquitin chain analysis, Western blot, in vivo mouse models, flow cytometry","journal":"Journal of translational medicine","confidence":"Low","confidence_rationale":"Tier 3 / Weak — Co-IP demonstrates ISGylation of PD-L1, but the mechanism linking ISGylation to K48-ubiquitin-mediated degradation is not fully mechanistically resolved in the abstract; single lab","pmids":["37217923"],"is_preprint":false},{"year":2022,"finding":"ISG15 and ISGylation are required for optimal mitochondrial respiration; defective mitochondrial respiration in ISG15-/- macrophages is restored by transduction with wild-type ISG15 but only partially by a conjugation-deficient variant, indicating that mitochondrial respiratory function requires ISGylation to cellular targets.","method":"iPSC-derived macrophages from ISG15-deficient patients, complementation with WT vs. conjugation-deficient ISG15, mitochondrial respiration assays, metabolic profiling","journal":"Clinical and translational medicine","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — patient-derived KO cells with conjugation-deficient mutant rescue establish mechanistic distinction; single lab with multiple metabolic readouts","pmids":["35842904"],"is_preprint":false},{"year":2022,"finding":"Free ISG15 (unconjugated) promotes type I IFN-α-mediated antiviral activity against pseudorabies virus by facilitating STAT1 and STAT2 nuclear translocation, directly interacting with STAT2, and promoting ISGF3 (STAT1/STAT2/IRF9) complex formation and ISRE-driven gene transcription.","method":"ISG15 KO cell lines and mice, conjugation inhibition assays, co-immunoprecipitation of ISG15-STAT2 interaction, ISGF3 formation assay, ISRE luciferase reporter, nuclear fractionation","journal":"PLoS pathogens","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct ISG15-STAT2 interaction by Co-IP, KO mouse infection model, conjugation inhibition to distinguish free vs. conjugated ISG15; single lab","pmids":["36315588"],"is_preprint":false},{"year":2024,"finding":"PARP12 interacts with ISG15 and upregulates MFN1/2 ISGylation, which reduces MFN1/2 ubiquitination and SUMOylation, thereby inhibiting PINK1/Parkin-dependent mitophagy in chondrocytes and promoting cartilage degradation in osteoarthritis.","method":"Mass spectrometry, co-immunoprecipitation of PARP12-ISG15 interaction, ISGylation, ubiquitination, and SUMOylation assays for MFN1/2, mitophagy assays, rat OA model","journal":"Bone research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — MS identification of substrates, Co-IP, and multiple PTM assays; single lab","pmids":["39465252"],"is_preprint":false},{"year":2023,"finding":"ISG15 restricts Dengue and Zika virus replication by stabilizing USP18 protein. ISG15-dependent USP18 stability prevents NS5-mediated STAT2 degradation. Reconstitution of USP18 in ISG15-deficient cells restored STAT2 stability and restricted virus growth.","method":"ISG15-deficient cells, USP18 reconstitution, STAT2 stability assays, dengue/Zika viral replication assays","journal":"Frontiers in immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — mechanistic rescue experiment with USP18 reconstitution in ISG15-deficient cells with viral replication readout; single lab","pmids":["38384473"],"is_preprint":false},{"year":2023,"finding":"IFNβ restores replication fork stability in BRCA1/2-deficient cells in a manner that strictly depends on ISG15 and ISGylation. Basal ISG15 levels protect nascent DNA from degradation. ISG15 depletion reduces cell proliferation in BRCA1-mutated triple-negative breast cancer cells, and ISG15 upregulation increases resistance to cisplatin in BRCA2-deficient cells.","method":"ISG15 siRNA/KO, IFNβ treatment, DNA fiber assays for replication fork protection, BRCA1/2-deficient mouse embryonic stem cells and human cancer cells, ISGylation complementation","journal":"Nature communications","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple genetic models (siRNA, KO, BRCA-deficient cells), fork protection assays, IFNβ rescue experiment; single lab","pmids":["37783689"],"is_preprint":false},{"year":2022,"finding":"ISG15-deficient skin fibroblasts and keratinocytes show impaired cell migration, reduced collagen and integrin synthesis, increased matrix metalloproteinases, and elevated ROS with reduced ROS scavenger expression. Defective collagen and integrin synthesis was not rescued by conjugation-deficient ISG15, while hyperinflammation was rescued by ruxolitinib.","method":"ISG15-deficient patient fibroblasts, HaCaT keratinocytes, iPSC-derived endothelial cells, complementation with WT vs. conjugation-deficient ISG15, migration assays, 3D epidermis model, ruxolitinib treatment","journal":"The Journal of clinical investigation","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — patient-derived KO cells with conjugation-deficient mutant rescue, multiple cell-type functional assays; single lab","pmids":["34847081"],"is_preprint":false},{"year":2018,"finding":"ISG15 promotes ISGylation of ERK1, and this ISGylation suppresses ovarian cancer progression. Endogenous and exogenous ISG15 suppressed ovarian cancer progression through ISGylation of ERK. ISG15 also activated NK cells and CD8+ T lymphocytes.","method":"Transcriptome profiling of microdissected tumor tissue, ISGylation assays for ERK, functional assays with endogenous/exogenous ISG15 in ovarian cancer cell lines, NK and CD8+ T cell activation assays","journal":"Cancers","confidence":"Low","confidence_rationale":"Tier 3 / Weak — ERK ISGylation shown but mechanistic consequences not fully validated by orthogonal methods; single lab","pmids":["30469497"],"is_preprint":false},{"year":2024,"finding":"DRD4 (dopamine D4 receptor) reduces ISG15 expression, which in turn suppresses NOX4 ISGylation; reduced NOX4 ISGylation promotes its ubiquitination and degradation, counteracting oxidative stress in acute kidney injury.","method":"Transcriptome sequencing, DRD4 KO and overexpression in mouse IRI model, NOX4 ISGylation and ubiquitination assays, ROS measurement","journal":"Redox biology","confidence":"Low","confidence_rationale":"Tier 3 / Weak — ISGylation of NOX4 demonstrated downstream of DRD4-ISG15 axis, but mechanistic link between ISGylation and ubiquitination of NOX4 relies primarily on transcriptome sequencing and limited biochemical validation in abstract; single lab","pmids":["38354631"],"is_preprint":false},{"year":2014,"finding":"ISG15 promotes HCC cell proliferation and migration by maintaining Survivin protein stabilization via sequestering XIAP from interacting with Survivin.","method":"ISG15 siRNA knockdown, xenograft tumor model, co-immunoprecipitation assays for ISG15-XIAP-Survivin interactions","journal":"Oncotarget","confidence":"Low","confidence_rationale":"Tier 3 / Weak — Co-IP shows ISG15-XIAP-Survivin interaction but mechanistic validation is limited to single-lab Co-IP and siRNA","pmids":["25238261"],"is_preprint":false}],"current_model":"ISG15 is an IFN-α/β-induced, ubiquitin-like protein (17 kDa, two UBL domains) that functions through three distinct modes: (1) intracellular covalent ISGylation of hundreds of substrate proteins via a dedicated E1 (UBE1L/Uba7)-E2 (UBE2L6/UbcH8)-E3 (HERC5) cascade—structurally characterized by cryo-EM—which modulates substrates including glycolytic enzymes (HK2, PFK1), STING (at K289 to promote oligomerization and IFN induction), filamin-C, p53 (targeting misfolded forms to the 20S proteasome), PTEN, ERK1, MFN1/2, KPNA2, and replication fork components to regulate antiviral immunity, metabolism, DNA replication fidelity, and autophagy/mitophagy; (2) free intracellular ISG15 that stabilizes USP18 (the major deISGylase that also suppresses IFN signaling) to negatively feedback on type I IFN responses; and (3) secreted extracellular ISG15 that acts as a cytokine by binding the αI domain of the LFA-1 integrin (CD11a/CD18), activating SRC family kinases to drive IFN-γ and IL-10 secretion from NK cells and T cells, providing critical defense against mycobacteria."},"narrative":{"mechanistic_narrative":"ISG15 is an interferon-induced ubiquitin-like protein that operates through three functionally separable modes—covalent ISGylation of intracellular substrates, free intracellular ISG15, and secreted extracellular ISG15—to coordinate antiviral immunity, cellular metabolism, genome stability, and intercellular signaling [PMID:22859821, PMID:33571167, PMID:32472071]. As a covalent modifier, ISG15 is conjugated to substrates via a dedicated E1 (Uba7/UBE1L)–E2 (UBE2L6/UbcH8) cascade whose structural basis for ISG15-specific activation and E1-to-E2 thioester transfer has been resolved by cryo-EM [PMID:37553340, PMID:38042859], after maturation of pro-ISG15 to its active C-terminal diglycine form by a thiol protease [PMID:10455185]. ISGylation tunes diverse pathways: it modifies STING at K289 to drive oligomerization and type I IFN induction [PMID:37864791], modifies replication-fork proteins to stabilize stalled forks and protect nascent DNA, including in BRCA1/2-deficient cells [PMID:36216822, PMID:37783689], and reshapes metabolism by ISGylating glycolytic enzymes such as HK2 and PFK1 to suppress glycolysis while sustaining mitochondrial respiration and mitophagy [PMID:33571167, PMID:32472071, PMID:38309955, PMID:35842904]. ISGylation also controls protein stability and quality control, targeting misfolded p53 to the 20S proteasome [PMID:24844324, PMID:31455647] and modifying substrates including PTEN, KPNA2, filamin-C, and MFN1/2 to alter their turnover and downstream behavior [PMID:27980214, PMID:37115698, PMID:37501099, PMID:39465252]. Free intracellular ISG15 instead acts non-covalently: it stabilizes the deISGylase USP18 to negatively feedback on type I IFN signaling and to restrict flaviviruses by preventing NS5-mediated STAT2 degradation [PMID:27193971, PMID:38384473], and it engages STAT2 to promote ISGF3 assembly and ISRE-driven transcription [PMID:36315588]. Secreted ISG15 functions as a cytokine, binding the αI domain of the LFA-1 integrin to activate SRC-family kinases and drive IFN-γ and IL-10 secretion from lymphocytes, a pathway essential for human antimycobacterial immunity [PMID:29100055, PMID:22859821, PMID:32553163]. DeISGylation is carried out principally by USP18, whose ISG15-specific protease activity is structurally defined [PMID:28881486], with USP16 acting as a cross-reactive deISGylase [PMID:38055744]. Human ISG15 deficiency causes chronically elevated IFN signaling and mycobacterial susceptibility without broad viral susceptibility, distinguishing the secreted/free functions from intracellular ISGylation [PMID:27193971, PMID:22859821].","teleology":[{"year":1999,"claim":"Established how ISG15 becomes a competent conjugation tag, showing that the precursor must be proteolytically matured to expose the reactive C-terminal glycine.","evidence":"Biochemical purification of a 100-kDa thiol protease and in vitro processing assay mapping Gly157-Gly158 cleavage in human fibroblast extracts","pmids":["10455185"],"confidence":"High","gaps":["Identity of the physiological processing protease in vivo not pinned to a single gene","Does not address conjugation machinery"]},{"year":2006,"claim":"Linked ISGylation to crosstalk with the ubiquitin-proteasome system, indicating ISG15 conjugation can antagonize polyubiquitination.","evidence":"siRNA knockdown of ISG15 and its E2 UbcH8 in tumor cells with western blot of polyubiquitinated proteins","pmids":["16424026"],"confidence":"Medium","gaps":["Specific substrates not identified","Mechanism of antagonism not resolved"]},{"year":2007,"claim":"Showed ISGylation participates in innate immune signaling regulation by influencing RIG-I modification in competition with a ubiquitin E3 ligase.","evidence":"Co-IP and ectopic expression of UbcH8/ISG15/RNF125 with RIG-I ubiquitination and ISGylation assays","pmids":["17719635"],"confidence":"Medium","gaps":["Overexpression-based; physiological relevance of competition unclear","No endogenous validation"]},{"year":2012,"claim":"Defined the secreted, extracellular function of ISG15 as essential for human antimycobacterial immunity, separable from intracellular ISGylation.","evidence":"Human ISG15-deficient patients with leukocyte secretion and NK-cell IFN-γ assays","pmids":["22859821"],"confidence":"High","gaps":["Receptor for secreted ISG15 not yet identified in this study","Secretion mechanism unknown"]},{"year":2014,"claim":"Identified misfolded p53 as an ISGylation substrate routed for 20S proteasomal degradation, implicating ISGylation in protein quality control.","evidence":"Co-IP, in vitro 20S degradation assay, site-directed mutagenesis, and Isg15 knockout mouse cells","pmids":["24844324"],"confidence":"Medium","gaps":["E3 ligase directing p53 ISGylation not defined","Single lab"]},{"year":2016,"claim":"Resolved the dual nature of the free-ISG15/USP18 axis by showing free ISG15 stabilizes USP18 to dampen type I IFN signaling in humans.","evidence":"ISG15-deficient patient fibroblasts complemented with WT vs. conjugation-deficient ISG15ΔGG, USP18 protein and IFN readouts","pmids":["27193971"],"confidence":"High","gaps":["Human-specific; murine difference unexplained mechanistically","Structural basis of ISG15-USP18 stabilization not in this study"]},{"year":2020,"claim":"Dissected the regulation of ISG15 secretion, showing intracellular ISGylation restrains secretion while viral effectors modulate it, and that secreted ISG15 signals through LFA-1.","evidence":"ISG15 secretion assays with conjugation mutants and viral effector (influenza B NS1, SARS-CoV-2 PLpro) expression across cell types","pmids":["32553163"],"confidence":"Medium","gaps":["Molecular route of unconventional ISG15 secretion not defined","Single lab"]},{"year":2017,"claim":"Mapped extracellular ISG15 signaling to a defined receptor and kinase cascade, establishing LFA-1/SFK as the conduit for cytokine induction.","evidence":"Direct in vitro binding to the CD11a αI domain, CD11a-/- splenocytes, LFA-1 and SFK inhibitor studies, ISG15 mutagenesis, NK-92 IFN-γ release","pmids":["29100055"],"confidence":"High","gaps":["Downstream transcriptional program beyond IFN-γ/IL-10 not fully mapped","How LFA-1 engagement is tuned in vivo unclear"]},{"year":2023,"claim":"Provided the structural basis for ISG15-specific activation, explaining how Uba7 and UBE2L6 achieve selectivity distinct from ubiquitin.","evidence":"Cryo-EM structures of Uba7-UBE2L6-ISG15 adenylate/thioester intermediates with structure-guided mutagenesis","pmids":["37553340","38042859"],"confidence":"High","gaps":["E3 (HERC5) substrate-selection step not structurally resolved here","Full conjugation handoff to substrate not visualized"]},{"year":2023,"claim":"Explained viral subversion of ISG15 by structurally defining how PLpro distinguishes ISG15's two UBL domains from K48 di-ubiquitin.","evidence":"Crystallography, cryo-EM, NMR, cross-linking MS, and affinity measurements of SARS-CoV-2 PLpro with ISG15","pmids":["37185902"],"confidence":"High","gaps":["In-cell consequences of differential UBL engagement not quantified","Generalization to other viral deISGylases unaddressed"]},{"year":2017,"claim":"Defined USP18 as the highly ISG15-specific protease and revealed its receptor-binding, catalysis-independent role in shutting off IFN signaling.","evidence":"Crystal structures of USP18 and USP18-ISG15 complex with biochemical cleavage assays","pmids":["28881486"],"confidence":"Medium","gaps":["Review-level structural summary","In vivo deISGylation substrate scope not addressed"]},{"year":2024,"claim":"Identified USP16 as a cross-reactive deISGylase, broadening the enzyme set that removes ISG15 from metabolic substrates.","evidence":"ISG15 activity-based profiling, recombinant USP16 in vitro cleavage, USP16 depletion, and substrate identification","pmids":["38055744"],"confidence":"Medium","gaps":["Physiological importance relative to USP18 unclear","Substrate selectivity rules undefined"]},{"year":2021,"claim":"Established ISGylation as a metabolic rheostat, showing IRF3-driven ISG15 ISGylates glycolytic enzymes to limit lactate and thermogenesis.","evidence":"Isg15 KO mice phenocopying adipocyte IRF3 KO, lactate rescue, and ISGylation assays of glycolytic enzymes","pmids":["33571167"],"confidence":"High","gaps":["Which specific glycolytic enzyme ISGylation events are causal not fully delineated","Tissue specificity of effect not mapped"]},{"year":2024,"claim":"Pinpointed HK2 and PFK1 as ISGylation substrates whose modification suppresses glycolysis to preserve cardiac mitochondrial capacity during infection.","evidence":"MS ISGylome, enzymatic activity assays, structural modeling, Seahorse flux in cardiomyocytes and ISG15 KO mouse heart","pmids":["38309955"],"confidence":"Medium","gaps":["ISGylation site stoichiometry in vivo unknown","E3 directing these modifications not identified"]},{"year":2020,"claim":"Linked ISGylation to mitochondrial quality control by showing it is required for mitophagy, OXPHOS, and stem-cell self-renewal in pancreatic cancer.","evidence":"CRISPR ISG15 KO with WT vs. conjugation-deficient complementation, OXPHOS/mitophagy assays, and in vivo tumor formation","pmids":["32472071"],"confidence":"High","gaps":["Mitochondrial ISGylation substrates not enumerated","Connection to mitophagy machinery not mechanistically resolved"]},{"year":2022,"claim":"Generalized the requirement of ISGylation for mitochondrial respiration to human patient-derived macrophages.","evidence":"iPSC-derived macrophages from ISG15-deficient patients with WT vs. conjugation-deficient complementation and respiration assays","pmids":["35842904"],"confidence":"Medium","gaps":["Respiratory-chain substrates of ISGylation undefined","Partial rescue by free ISG15 unexplained"]},{"year":2022,"claim":"Connected ISGylation to genome maintenance, showing it modifies replication-fork proteins downstream of cGAS-STING to mitigate replication stress.","evidence":"Mre11 inactivation, ISG15 KO cells, iPOND, fork-stability and aphidicolin-sensitivity assays","pmids":["36216822"],"confidence":"Medium","gaps":["Specific fork substrates of ISGylation not fully identified","Single lab"]},{"year":2023,"claim":"Extended the fork-protection role to a therapeutic context, showing IFNβ restores fork stability in BRCA1/2-deficient cells strictly via ISG15/ISGylation.","evidence":"ISG15 siRNA/KO, IFNβ treatment, DNA fiber assays in BRCA1/2-deficient cells, ISGylation complementation","pmids":["37783689"],"confidence":"Medium","gaps":["Substrate(s) of fork-protective ISGylation undefined","Clinical translation untested"]},{"year":2023,"claim":"Defined ISGylation of STING at K289 as a switch for STING oligomerization and IFN induction in DNA sensing.","evidence":"ISG15 KO cells, site-directed mutagenesis of STING lysines, oligomerization and IFN induction assays, molecular modeling","pmids":["37864791"],"confidence":"Medium","gaps":["E3 directing STING ISGylation not identified","Single lab"]},{"year":2022,"claim":"Identified a free-ISG15 role in IFN signal amplification through direct STAT2 interaction and ISGF3 assembly.","evidence":"ISG15 KO cells/mice, conjugation inhibition, ISG15-STAT2 Co-IP, ISGF3 and ISRE reporter assays","pmids":["36315588"],"confidence":"Medium","gaps":["Structural basis of ISG15-STAT2 binding unknown","Relation to USP18-mediated negative feedback unresolved"]},{"year":null,"claim":"How distinct E3 ligases and deISGylases select among the hundreds of ISGylation substrates to achieve pathway-specific outcomes remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No systematic substrate-to-E3 mapping in the corpus","Site-level and stoichiometric rules for substrate ISGylation undefined","Mechanism integrating free vs. conjugated vs. secreted ISG15 in vivo unclear"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0031386","term_label":"protein tag activity","supporting_discovery_ids":[4,6,8,11,12,13,22,27]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[2,30,32]},{"term_id":"GO:0048018","term_label":"receptor ligand activity","supporting_discovery_ids":[0,3,10]}],"localization":[{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[2,11,12]},{"term_id":"GO:0005739","term_label":"mitochondrion","supporting_discovery_ids":[12,29,31]},{"term_id":"GO:0005576","term_label":"extracellular region","supporting_discovery_ids":[0,3,10]}],"pathway":[{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[0,3,13,30]},{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[11,12,22,29]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[4,6,8]},{"term_id":"R-HSA-73894","term_label":"DNA Repair","supporting_discovery_ids":[18,33]}],"complexes":[],"partners":["UBA7","UBE2L6","HERC5","USP18","STAT2","ITGAL","PARP12"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"P05161","full_name":"Ubiquitin-like protein ISG15","aliases":["Interferon-induced 15 kDa protein","Interferon-induced 17 kDa protein","IP17","Ubiquitin cross-reactive protein","hUCRP"],"length_aa":165,"mass_kda":17.9,"function":"Ubiquitin-like protein which plays a key role in the innate immune response to viral infection either via its conjugation to a target protein (ISGylation) or via its action as a free or unconjugated protein (PubMed:27564865, PubMed:39465252). ISGylation involves a cascade of enzymatic reactions involving E1, E2, and E3 enzymes which catalyze the conjugation of ISG15 to a lysine residue in the target protein (PubMed:33727702). Its target proteins include IFIT1, MX1/MxA, PPM1B, UBE2L6, UBA7, CHMP5, CHMP2A, CHMP4B and CHMP6. Isgylation of the viral sensor IFIH1/MDA5 promotes IFIH1/MDA5 oligomerization and triggers activation of innate immunity against a range of viruses, including coronaviruses, flaviviruses and picornaviruses (PubMed:33727702). Can also isgylate: EIF2AK2/PKR which results in its activation, RIGI which inhibits its function in antiviral signaling response, EIF4E2 which enhances its cap structure-binding activity and translation-inhibition activity, UBE2N and UBE2E1 which negatively regulates their activity, IRF3 which inhibits its ubiquitination and degradation and FLNB which prevents its ability to interact with the upstream activators of the JNK cascade thereby inhibiting IFNA-induced JNK signaling. Exhibits antiviral activity towards both DNA and RNA viruses, including influenza A, HIV-1 and Ebola virus. Restricts HIV-1 and ebola virus via disruption of viral budding. Inhibits the ubiquitination of HIV-1 Gag and host TSG101 and disrupts their interaction, thereby preventing assembly and release of virions from infected cells. Inhibits Ebola virus budding mediated by the VP40 protein by disrupting ubiquitin ligase activity of NEDD4 and its ability to ubiquitinate VP40. ISGylates influenza A virus NS1 protein which causes a loss of function of the protein and the inhibition of virus replication. The secreted form of ISG15 can: induce natural killer cell proliferation, act as a chemotactic factor for neutrophils and act as a IFN-gamma-inducing cytokine playing an essential role in antimycobacterial immunity. The secreted form acts through the integrin ITGAL/ITGB2 receptor to initiate activation of SRC family tyrosine kinases including LYN, HCK and FGR which leads to secretion of IFNG and IL10; the interaction is mediated by ITGAL (PubMed:29100055)","subcellular_location":"Cytoplasm; Secreted","url":"https://www.uniprot.org/uniprotkb/P05161/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/ISG15","classification":"Not Classified","n_dependent_lines":29,"n_total_lines":1208,"dependency_fraction":0.024006622516556293},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/ISG15","total_profiled":1310},"omim":[{"mim_id":"618886","title":"PSEUDO-TORCH SYNDROME 3; PTORCH3","url":"https://www.omim.org/entry/618886"},{"mim_id":"618648","title":"IMMUNODEFICIENCY 65, SUSCEPTIBILITY TO VIRAL INFECTIONS; IMD65","url":"https://www.omim.org/entry/618648"},{"mim_id":"617397","title":"PSEUDO-TORCH SYNDROME 2; PTORCH2","url":"https://www.omim.org/entry/617397"},{"mim_id":"616808","title":"SHIFTLESS ANTIVIRAL INHIBITOR OF RIBOSOMAL FRAMESHIFTING; SHFL","url":"https://www.omim.org/entry/616808"},{"mim_id":"616189","title":"CAP METHYLTRANSFERASE 1; CMTR1","url":"https://www.omim.org/entry/616189"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in all","driving_tissues":[{"tissue":"salivary gland","ntpm":301.0}],"url":"https://www.proteinatlas.org/search/ISG15"},"hgnc":{"alias_symbol":["IFI15","UCRP"],"prev_symbol":["G1P2"]},"alphafold":{"accession":"P05161","domains":[{"cath_id":"3.10.20.90","chopping":"3-76","consensus_level":"high","plddt":87.1323,"start":3,"end":76},{"cath_id":"3.10.20.90","chopping":"82-153","consensus_level":"high","plddt":91.1961,"start":82,"end":153}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P05161","model_url":"https://alphafold.ebi.ac.uk/files/AF-P05161-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P05161-F1-predicted_aligned_error_v6.png","plddt_mean":85.88},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=ISG15","jax_strain_url":"https://www.jax.org/strain/search?query=ISG15"},"sequence":{"accession":"P05161","fasta_url":"https://rest.uniprot.org/uniprotkb/P05161.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P05161/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P05161"}},"corpus_meta":[{"pmid":"29769653","id":"PMC_29769653","title":"ISG15 in antiviral immunity and beyond.","date":"2018","source":"Nature reviews. 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signals through the LFA-1 integrin receptor (CD11a/CD18; αLβ2 integrin) to promote IFN-γ and IL-10 secretion. ISG15 bound directly to the αI domain of CD11a in vitro; LFA-1 inhibition or CD11a knockout blocked cytokine secretion. ISG15 engagement of LFA-1 led to activation of SRC family kinases (SFKs), and SFK inhibition blocked cytokine secretion.\",\n      \"method\": \"NK-92 cell IFN-γ release assay, direct in vitro binding to αI domain, CD11a-/- mouse splenocytes, LFA-1 inhibitor studies, SFK inhibitor studies, site-directed mutagenesis of ISG15 signaling residues\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — reciprocal functional validation with KO mice, direct in vitro binding, mutagenesis, and kinase inhibitor studies in a single rigorous study\",\n      \"pmids\": [\"29100055\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Pro-ISG15 is synthesized as a precursor that requires processing by a constitutive 100-kDa thiol protease to expose the mature C-terminal glycine (Gly157-Gly158 cleavage). The processing enzyme is stimulated 12-fold by micromolar ubiquitin and is related to yeast Ubp1.\",\n      \"method\": \"Biochemical purification from human lung fibroblast extracts, in vitro processing assay, thiol protease inhibitor studies, pH rate analysis, tryptic peptide sequencing\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — enzyme purified to homogeneity, in vitro reconstitution with mutagenesis-level cleavage site mapping, single lab but multiple orthogonal biochemical methods\",\n      \"pmids\": [\"10455185\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Free intracellular ISG15 stabilizes USP18 protein by protecting it from proteasomal degradation, and this stabilization of USP18 is the mechanism by which ISG15 downregulates type I IFN-α/β signaling in humans (but not in mice). ISG15-deficient human cells display chronically elevated ISG signaling as a result of reduced USP18 levels.\",\n      \"method\": \"ISG15-deficient patient fibroblasts, complementation with WT ISG15 vs. ISG15ΔGG (conjugation-deficient), USP18 protein level measurement, IFN signaling readouts\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — human patient cells, complementation with conjugation-deficient mutant to separate free from conjugated functions, replicated across multiple labs\",\n      \"pmids\": [\"27193971\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Secreted ISG15 from granulocytes/leukocytes stimulates IFN-γ production by lymphocytes (including NK cells) in response to mycobacterial infection; this extracellular signaling function is essential for antimycobacterial immunity in humans. Loss of intracellular ISGylation did not confer susceptibility to viral infections in these patients.\",\n      \"method\": \"Human ISG15-deficient patients (inherited loss-of-function mutations), leukocyte secretion assays, NK cell IFN-γ production assays\",\n      \"journal\": \"Science (New York, N.Y.)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — human genetic experiment of nature with well-characterized patients, multiple cell-type functional assays, replicated by subsequent studies\",\n      \"pmids\": [\"22859821\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Cryo-EM structures of human Uba7 (UBE1L) in complex with UBE2L6 and ISG15 adenylate or ISG15 thioester intermediate revealed the molecular basis for ISG15 activation and E1-to-E2 thioester transfer. The structures demonstrate unique overall architecture compared to the ubiquitin conjugation pathway and explain Uba7's exquisite specificity for ISG15 and UBE2L6.\",\n      \"method\": \"Cryo-EM structure determination, biochemical assays, human cell-based functional validation, mutagenesis of specificity determinants\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — cryo-EM structures at near-atomic resolution combined with biochemical reconstitution and cell-based mutagenesis in a single study\",\n      \"pmids\": [\"37553340\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Cryo-EM and crystal structures of SARS-CoV-2 PLpro complexed with human ISG15 and K48-linked di-ubiquitin revealed that the two UBL domains of ISG15 are differentially engaged by PLpro compared to K48-Ub2, and differential binding stabilities of the two UBL/Ub domains explain PLpro substrate selectivity.\",\n      \"method\": \"Crystal structures, cryo-EM, solution NMR, cross-linking mass spectrometry, binding affinity measurements\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — multiple orthogonal structural and biophysical methods in a single rigorous study\",\n      \"pmids\": [\"37185902\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"A 3.45 Å cryo-EM structure of the UBE1L (Uba7)-UBE2L6 complex bound to activated ISG15 reveals the molecular basis of ISG15 recognition and UBE2L6 recruitment, and identifies the ISG15 C-terminal ubiquitin-like domain as critical for the adenylation reaction. Mutants with altered selectivity for ISG15 vs. ubiquitin conjugation pathways were designed based on the structure.\",\n      \"method\": \"Cryo-EM structure, biochemical characterization using viral effector proteins (SARS-CoV-2 PLpro, influenza B NS1), mutagenesis of ISG15 and UBE2L6\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — high-resolution cryo-EM structure with structure-guided mutagenesis and biochemical validation, single lab but multiple orthogonal methods\",\n      \"pmids\": [\"38042859\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Elevated ISG15 in tumor cells antagonizes the ubiquitin/26S proteasome pathway by interfering with protein polyubiquitination. siRNA knockdown of ISG15 increased polyubiquitinated protein levels; siRNA knockdown of the E2 enzyme UbcH8 (blocking ISG15 conjugate formation) also increased polyubiquitinated proteins.\",\n      \"method\": \"siRNA knockdown of ISG15 and UbcH8 in tumor cell lines, western blot analysis of polyubiquitinated proteins\",\n      \"journal\": \"Cancer research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — two orthogonal siRNA experiments (targeting ISG15 and its E2) with consistent results, single lab\",\n      \"pmids\": [\"16424026\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"ISG15 (Isg15) covalently modifies p53 at two sites (N- and C-terminal) via ISGylation. ISGylated p53 is degraded by the 20S proteasome. ISGylation preferentially targets misfolded, dominant-negative p53. Isg15 deletion in normal cells suppresses p53 activity.\",\n      \"method\": \"Co-immunoprecipitation, ISGylation assays, 20S proteasome degradation assay, Isg15 knockout mouse cells, site-directed mutagenesis\",\n      \"journal\": \"Cell cycle (Georgetown, Tex.)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct ISGylation shown by Co-IP and in vitro degradation assay, KO cells confirm functional consequence; single lab\",\n      \"pmids\": [\"24844324\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"USP18 isopeptidase activity promotes accumulation of misfolded p53 by removing ISG15 from p53; ISG15 is required for degradation of misfolded dominant-negative p53. Depletion of ISG15 causes accumulation of misfolded dominant-negative p53 and enhances HIV-1 replication in myeloid cells.\",\n      \"method\": \"USP18 isopeptidase activity assays, ISG15 knockdown in primary myeloid cells, p53 conformation assays, HIV-1 replication assays\",\n      \"journal\": \"mBio\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — enzymatic activity established with conformation-specific p53 detection and HIV replication read-out; single lab, multiple orthogonal approaches\",\n      \"pmids\": [\"31455647\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Intracellular ISGylation inhibits ISG15 secretion, while influenza B NS1 (which sequesters ISG15) and viral de-ISGylases (including SARS-CoV-2 PLpro) have opposing effects on ISG15 secretion. Both epithelial cells and lymphocytes can secrete ISG15 to signal in autocrine or paracrine fashion through LFA-1.\",\n      \"method\": \"ISG15 secretion assays, ISGylation inhibition experiments, viral effector protein expression, residue mapping of secretion-critical ISG15 residues\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — mechanistic dissection of secretion pathway with viral effectors and conjugation mutants; single lab, multiple cell types\",\n      \"pmids\": [\"32553163\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"IRF3 drives expression of ISG15, which becomes covalently attached (ISGylated) to glycolytic enzymes in adipocytes, reducing their enzymatic function and decreasing lactate production. Reduced lactate suppresses thermogenic gene expression. ISG15-deficient mice phenocopy adipocyte-specific IRF3-deficient mice with elevated energy expenditure and resistance to diet-induced obesity.\",\n      \"method\": \"Isg15 knockout mice, IRF3 overexpression/knockout in adipocytes, metabolic measurements (oxygen consumption, energy expenditure), lactate repletion rescue experiments, ISGylation assays of glycolytic enzymes\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — ISG15 KO mice phenocopy IRF3 KO, lactate rescue experiment establishes mechanism, ISGylation of specific glycolytic enzyme substrates demonstrated; multiple orthogonal approaches in single study\",\n      \"pmids\": [\"33571167\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"ISG15 deletion in pancreatic cancer stem cells (PaCSCs) decreases mitochondrial ISGylation, impairs mitophagy, reduces oxidative phosphorylation, and impairs self-renewal and tumorigenic capacity. Rescue with wild-type ISG15 but not conjugation-deficient ISG15 restored these functions, indicating ISGylation (not free ISG15) is required for mitophagy and metabolic plasticity.\",\n      \"method\": \"CRISPR-mediated ISG15 KO, complementation with WT vs. conjugation-deficient ISG15, mitochondrial function assays (OXPHOS, mitophagy), in vivo tumor formation assays\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — CRISPR KO with complementation using conjugation-deficient mutant to dissect free vs. conjugated ISG15; multiple functional readouts including in vivo; single lab\",\n      \"pmids\": [\"32472071\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"ISGylation of STING at residue K289 is required for STING oligomerization and type I IFN induction in response to HIV-1 DNA sensing via cGAS-STING. ISG15 deficiency inhibits STING-dependent antiviral sensing. Removal of STING ISGylation alleviates gain-of-function phenotype in SAVI disease.\",\n      \"method\": \"ISG15 KO cells, site-directed mutagenesis of STING ISGylation sites (K224, K236, K289, K347, K338, K370), STING oligomerization assays, IFN induction assays, molecular modeling\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — site-specific mutagenesis identifies critical K289 residue for oligomerization, KO and molecular modeling support mechanism; single lab\",\n      \"pmids\": [\"37864791\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"ISG15 and ISGylation are required for IFN-γ-mediated restriction of Toxoplasma gondii in human cells. ISG15 deletion impaired recruitment of autophagy adaptors p62, NDP52, and LC3 to the parasitophorous vacuole. ISG15 function required conjugation, and ISGylated targets overlapped with the IFN-γ-dependent ATG5 interactome, including adapter p62.\",\n      \"method\": \"ISG15 KO HeLa cells, proximity biotinylation of ATG5 interactome, immunofluorescence of p62/NDP52/LC3 recruitment, T. gondii growth restriction assays, conjugation-deficient ISG15 rescue\",\n      \"journal\": \"mBio\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — KO with conjugation-deficient rescue, ATG5 interactome, and parasite growth assay; single lab, multiple orthogonal methods\",\n      \"pmids\": [\"33024031\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"ISG15 expression restricts Listeria monocytogenes infection in vitro and in vivo. ISG15 induction in non-phagocytic cells upon Listeria infection can be type I IFN-independent, signaling through STING-TBK1-IRF3/IRF7 cytosolic DNA surveillance pathway. ISGylation of ER and Golgi proteins correlates with increased secretion of anti-infection cytokines.\",\n      \"method\": \"ISG15 overexpression and knockout cells, SILAC proteomics to identify ISGylated proteins, in vivo Listeria infection model, IFN pathway inhibition\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — SILAC proteomics combined with KO/OE and in vivo infection model; single lab, multiple orthogonal methods\",\n      \"pmids\": [\"26259872\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"UbcH8 (the ISG15 E2 enzyme) competes with RNF125 ubiquitin E3 ligase for RIG-I modification: UbcH8 suppresses RIG-I ubiquitination by RNF125, and this suppression is relieved by ectopic ISG15. Conversely, ISG15 conjugation to RIG-I is suppressed by RNF125. UbcH8 and ISG15 function as regulators of RNF125 E3 ligase activity toward RIG-I.\",\n      \"method\": \"Co-immunoprecipitation, ectopic expression of UbcH8/ISG15/RNF125, ubiquitination and ISGylation assays for RIG-I\",\n      \"journal\": \"Molecular immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — Co-IP and overexpression assays demonstrate competitive regulatory circuit, single lab\",\n      \"pmids\": [\"17719635\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"HCMV pUL26 interacts with ISG15, UBE1L (E1), and HERC5 (E3), and is itself ISGylated. ISGylation of pUL26 regulated its stability and inhibited its activities (NF-κB suppression and viral growth complementation). Reciprocally, pUL26 suppressed virus-induced ISGylation independent of its own ISGylation. ISGylation inhibits HCMV growth by downregulating viral gene expression and virion release.\",\n      \"method\": \"Co-immunoprecipitation, ISGylation assays, RNAi knockdown of ISGylation enzymes, UL26-null mutant virus analysis, NF-κB reporter assay\",\n      \"journal\": \"PLoS pathogens\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal Co-IP, null mutant virus, and functional assays; single lab, multiple orthogonal approaches\",\n      \"pmids\": [\"27564865\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"ISG15 conjugation to proteins on nascent DNA mitigates DNA replication stress. Mre11 complex inactivation causes cGAS-STING-TBK1-dependent ISG15 induction and ISGylation of replication fork constituents. ISG15 deletion is associated with replication fork stalling, tonic ATR activation, genomic aberrations, and sensitivity to aphidicolin.\",\n      \"method\": \"Mre11 complex inactivation, ISG15 deletion mouse cells, iPOND (isolation of proteins on nascent DNA), replication fork stability assays, aphidicolin sensitivity, cGAS-STING signaling assays\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — ISG15 KO with replication fork assays and genomic stability readouts; single lab, multiple orthogonal methods\",\n      \"pmids\": [\"36216822\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"USP18 and ISG15 regulate the SCF-SKP2 ubiquitin E3 ligase complex: free ISG15 disrupts the USP18-SKP2 interaction, liberating USP18 from SKP2-mediated degradation while simultaneously driving SKP2 to degradation and/or ISGylation. USP18 silencing slows HeLa S3 cell progression toward S phase.\",\n      \"method\": \"Co-immunoprecipitation of USP18-SKP2 complex, ectopic ISG15 expression, cell cycle analysis, USP18 siRNA knockdown\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — Co-IP and cell cycle assay; single lab, multiple approaches but primarily overexpression and siRNA\",\n      \"pmids\": [\"30858391\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"ISG15 directly conjugates to PTEN (ISGylation of PTEN), and USP18 deISGylase stabilizes PTEN protein. USP18 knockdown destabilizes PTEN while USP18 overexpression stabilizes PTEN. USP18 repression decreased cytoplasmic relative to nuclear PTEN levels.\",\n      \"method\": \"Co-immunoprecipitation showing ISG15-PTEN conjugation, cycloheximide chase assays, reverse-phase protein arrays, USP18 knockdown/overexpression\",\n      \"journal\": \"Oncotarget\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — Co-IP demonstrates ISGylation of PTEN, cycloheximide chase shows stability effect; single lab\",\n      \"pmids\": [\"27980214\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"ISGylation of viral and host proteins (including antiviral IFIT1/3 proteins) by ISG15 sets non-hematopoietic cells into a state resistant to coxsackievirus B3 (CVB). Concurrently, ISG15 adapts liver metabolism by increasing oxidative capacity and promoting gluconeogenesis. Cells lacking USP18 (thereby stabilizing ISGylation) show increased resistance to CVB.\",\n      \"method\": \"ISG15 KO and USP18 KO mouse models, shotgun proteomics, metabolic network modeling, coxsackievirus infection assays\",\n      \"journal\": \"Science advances\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — KO mouse models with proteomics and metabolic modeling; single lab, multiple orthogonal methods\",\n      \"pmids\": [\"32195343\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"ISGylation of hexokinase 2 (HK2) and phosphofructokinase (PFK1) during Coxsackievirus B3 infection obstructs their enzymatic activity, preventing infection/IFN-induced upregulation of glycolysis in cardiomyocytes. This preserves mitochondrial oxidative capacity in the heart during infection.\",\n      \"method\": \"Mass spectrometry ISGylome identification, enzymatic functional testing of HK2 and PFK1, structural modeling, Seahorse metabolic flux analysis in cardiomyocytes and mouse heart tissue, ISG15 KO mice\",\n      \"journal\": \"Cardiovascular research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — specific ISGylation substrates identified by MS, enzymatic activity assays performed, and Seahorse metabolic profiling; single lab with multiple orthogonal methods\",\n      \"pmids\": [\"38309955\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"ISG15 upregulation in pressure-overloaded hearts (driven by type I IFN signaling from pathogenic macrophages) leads to ISGylation of the myofibrillar protein filamin-C, promoting misfolded filamin-C accumulation and impairing cardiomyocyte autophagy, contributing to adverse ventricular remodeling. ISG15 deficiency preserved cardiac function in transverse aortic constriction mice.\",\n      \"method\": \"Nanoscale LC-MS/MS to identify ISGylation targets, ISG15 KO mice with TAC model, cardiac function measurements, metabolomics, human nonischemic cardiomyopathy tissue analysis\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — MS-identified substrate validated in KO mouse model with cardiac functional readout; single lab, multiple orthogonal methods\",\n      \"pmids\": [\"37115698\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"USP16 functions as an ISG15 cross-reactive deubiquitinase. Recombinant USP16 cleaved pro-ISG15 and ISG15 isopeptide-linked substrates in vitro and from cell lysates; USP16 depletion increased interferon-induced ISGylation. USP16-dependent ISG15 substrates include malate dehydrogenase, superoxide dismutase 1, fructose-bisphosphate aldolase A, and glutamic-oxaloacetic transaminase 1.\",\n      \"method\": \"ISG15 activity-based profiling, recombinant USP16 in vitro cleavage assays, USP16 depletion and ISGylation measurement, ISG15 interactome analysis\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vitro reconstitution of USP16 deISGylase activity plus cell-based depletion confirmation and substrate identification; single lab\",\n      \"pmids\": [\"38055744\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"USP18 is a highly specific protease that cleaves ISG15 from ISGylated proteins. Crystal structures of USP18 and a USP18-ISG15 complex revealed the molecular basis for its unique specificity for ISG15 over ubiquitin. USP18 also interacts with the type I IFN receptor to shut off downstream signaling independent of its enzymatic activity.\",\n      \"method\": \"Crystal structure of USP18-ISG15 complex, biochemical cleavage assays\",\n      \"journal\": \"The FEBS journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — crystal structure provides mechanistic basis for specificity; single lab structure review article referencing structural data\",\n      \"pmids\": [\"28881486\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"ISG15 ISGylation of RSV-infected cells reduces virus growth. Conjugation inhibition assays demonstrated that the antiviral activity against RSV requires protein ISGylation (not free ISG15). High levels of ISG15 present before infection are required for antiviral activity.\",\n      \"method\": \"ISG15 overexpression, siRNA silencing, ISG15 KO cells, conjugation inhibition assays, RSV growth assays\",\n      \"journal\": \"Journal of virology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple genetic approaches (OE, KO, conjugation inhibition) with viral growth readout; single lab\",\n      \"pmids\": [\"26763998\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"ISGylation of KPNA2 (importin subunit alpha 2) by ISG15 impedes KPNA2 ubiquitination, promoting KPNA2 stability, which in turn maintains cancer stem cell-like characteristics in anaplastic thyroid carcinoma. Overexpression of ISGylation-competent but not non-ISGylatable ISG15 mutant reinforced CSC characteristics.\",\n      \"method\": \"Mass spectrometry to identify ISGylation substrates, co-immunoprecipitation, ISGylation-deficient ISG15 mutant rescue, KPNA2 ubiquitination assay, xenograft models\",\n      \"journal\": \"Journal of experimental & clinical cancer research : CR\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — substrate identified by MS and validated by Co-IP, conjugation-deficient mutant used to separate ISGylation-dependent effects; single lab\",\n      \"pmids\": [\"37501099\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"ISG15 promotes ISGylation-mediated degradation of glycosylated PD-L1 via K48-linked ubiquitin chains targeting the proteasomal pathway, reducing PD-L1 protein stability in lung adenocarcinoma cells.\",\n      \"method\": \"Co-immunoprecipitation showing ISG15-PD-L1 conjugation, K48-ubiquitin chain analysis, Western blot, in vivo mouse models, flow cytometry\",\n      \"journal\": \"Journal of translational medicine\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — Co-IP demonstrates ISGylation of PD-L1, but the mechanism linking ISGylation to K48-ubiquitin-mediated degradation is not fully mechanistically resolved in the abstract; single lab\",\n      \"pmids\": [\"37217923\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"ISG15 and ISGylation are required for optimal mitochondrial respiration; defective mitochondrial respiration in ISG15-/- macrophages is restored by transduction with wild-type ISG15 but only partially by a conjugation-deficient variant, indicating that mitochondrial respiratory function requires ISGylation to cellular targets.\",\n      \"method\": \"iPSC-derived macrophages from ISG15-deficient patients, complementation with WT vs. conjugation-deficient ISG15, mitochondrial respiration assays, metabolic profiling\",\n      \"journal\": \"Clinical and translational medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — patient-derived KO cells with conjugation-deficient mutant rescue establish mechanistic distinction; single lab with multiple metabolic readouts\",\n      \"pmids\": [\"35842904\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Free ISG15 (unconjugated) promotes type I IFN-α-mediated antiviral activity against pseudorabies virus by facilitating STAT1 and STAT2 nuclear translocation, directly interacting with STAT2, and promoting ISGF3 (STAT1/STAT2/IRF9) complex formation and ISRE-driven gene transcription.\",\n      \"method\": \"ISG15 KO cell lines and mice, conjugation inhibition assays, co-immunoprecipitation of ISG15-STAT2 interaction, ISGF3 formation assay, ISRE luciferase reporter, nuclear fractionation\",\n      \"journal\": \"PLoS pathogens\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct ISG15-STAT2 interaction by Co-IP, KO mouse infection model, conjugation inhibition to distinguish free vs. conjugated ISG15; single lab\",\n      \"pmids\": [\"36315588\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"PARP12 interacts with ISG15 and upregulates MFN1/2 ISGylation, which reduces MFN1/2 ubiquitination and SUMOylation, thereby inhibiting PINK1/Parkin-dependent mitophagy in chondrocytes and promoting cartilage degradation in osteoarthritis.\",\n      \"method\": \"Mass spectrometry, co-immunoprecipitation of PARP12-ISG15 interaction, ISGylation, ubiquitination, and SUMOylation assays for MFN1/2, mitophagy assays, rat OA model\",\n      \"journal\": \"Bone research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — MS identification of substrates, Co-IP, and multiple PTM assays; single lab\",\n      \"pmids\": [\"39465252\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"ISG15 restricts Dengue and Zika virus replication by stabilizing USP18 protein. ISG15-dependent USP18 stability prevents NS5-mediated STAT2 degradation. Reconstitution of USP18 in ISG15-deficient cells restored STAT2 stability and restricted virus growth.\",\n      \"method\": \"ISG15-deficient cells, USP18 reconstitution, STAT2 stability assays, dengue/Zika viral replication assays\",\n      \"journal\": \"Frontiers in immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — mechanistic rescue experiment with USP18 reconstitution in ISG15-deficient cells with viral replication readout; single lab\",\n      \"pmids\": [\"38384473\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"IFNβ restores replication fork stability in BRCA1/2-deficient cells in a manner that strictly depends on ISG15 and ISGylation. Basal ISG15 levels protect nascent DNA from degradation. ISG15 depletion reduces cell proliferation in BRCA1-mutated triple-negative breast cancer cells, and ISG15 upregulation increases resistance to cisplatin in BRCA2-deficient cells.\",\n      \"method\": \"ISG15 siRNA/KO, IFNβ treatment, DNA fiber assays for replication fork protection, BRCA1/2-deficient mouse embryonic stem cells and human cancer cells, ISGylation complementation\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple genetic models (siRNA, KO, BRCA-deficient cells), fork protection assays, IFNβ rescue experiment; single lab\",\n      \"pmids\": [\"37783689\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"ISG15-deficient skin fibroblasts and keratinocytes show impaired cell migration, reduced collagen and integrin synthesis, increased matrix metalloproteinases, and elevated ROS with reduced ROS scavenger expression. Defective collagen and integrin synthesis was not rescued by conjugation-deficient ISG15, while hyperinflammation was rescued by ruxolitinib.\",\n      \"method\": \"ISG15-deficient patient fibroblasts, HaCaT keratinocytes, iPSC-derived endothelial cells, complementation with WT vs. conjugation-deficient ISG15, migration assays, 3D epidermis model, ruxolitinib treatment\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — patient-derived KO cells with conjugation-deficient mutant rescue, multiple cell-type functional assays; single lab\",\n      \"pmids\": [\"34847081\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"ISG15 promotes ISGylation of ERK1, and this ISGylation suppresses ovarian cancer progression. Endogenous and exogenous ISG15 suppressed ovarian cancer progression through ISGylation of ERK. ISG15 also activated NK cells and CD8+ T lymphocytes.\",\n      \"method\": \"Transcriptome profiling of microdissected tumor tissue, ISGylation assays for ERK, functional assays with endogenous/exogenous ISG15 in ovarian cancer cell lines, NK and CD8+ T cell activation assays\",\n      \"journal\": \"Cancers\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — ERK ISGylation shown but mechanistic consequences not fully validated by orthogonal methods; single lab\",\n      \"pmids\": [\"30469497\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"DRD4 (dopamine D4 receptor) reduces ISG15 expression, which in turn suppresses NOX4 ISGylation; reduced NOX4 ISGylation promotes its ubiquitination and degradation, counteracting oxidative stress in acute kidney injury.\",\n      \"method\": \"Transcriptome sequencing, DRD4 KO and overexpression in mouse IRI model, NOX4 ISGylation and ubiquitination assays, ROS measurement\",\n      \"journal\": \"Redox biology\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — ISGylation of NOX4 demonstrated downstream of DRD4-ISG15 axis, but mechanistic link between ISGylation and ubiquitination of NOX4 relies primarily on transcriptome sequencing and limited biochemical validation in abstract; single lab\",\n      \"pmids\": [\"38354631\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"ISG15 promotes HCC cell proliferation and migration by maintaining Survivin protein stabilization via sequestering XIAP from interacting with Survivin.\",\n      \"method\": \"ISG15 siRNA knockdown, xenograft tumor model, co-immunoprecipitation assays for ISG15-XIAP-Survivin interactions\",\n      \"journal\": \"Oncotarget\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — Co-IP shows ISG15-XIAP-Survivin interaction but mechanistic validation is limited to single-lab Co-IP and siRNA\",\n      \"pmids\": [\"25238261\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"ISG15 is an IFN-α/β-induced, ubiquitin-like protein (17 kDa, two UBL domains) that functions through three distinct modes: (1) intracellular covalent ISGylation of hundreds of substrate proteins via a dedicated E1 (UBE1L/Uba7)-E2 (UBE2L6/UbcH8)-E3 (HERC5) cascade—structurally characterized by cryo-EM—which modulates substrates including glycolytic enzymes (HK2, PFK1), STING (at K289 to promote oligomerization and IFN induction), filamin-C, p53 (targeting misfolded forms to the 20S proteasome), PTEN, ERK1, MFN1/2, KPNA2, and replication fork components to regulate antiviral immunity, metabolism, DNA replication fidelity, and autophagy/mitophagy; (2) free intracellular ISG15 that stabilizes USP18 (the major deISGylase that also suppresses IFN signaling) to negatively feedback on type I IFN responses; and (3) secreted extracellular ISG15 that acts as a cytokine by binding the αI domain of the LFA-1 integrin (CD11a/CD18), activating SRC family kinases to drive IFN-γ and IL-10 secretion from NK cells and T cells, providing critical defense against mycobacteria.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"ISG15 is an interferon-induced ubiquitin-like protein that operates through three functionally separable modes—covalent ISGylation of intracellular substrates, free intracellular ISG15, and secreted extracellular ISG15—to coordinate antiviral immunity, cellular metabolism, genome stability, and intercellular signaling [#3, #11, #12]. As a covalent modifier, ISG15 is conjugated to substrates via a dedicated E1 (Uba7/UBE1L)–E2 (UBE2L6/UbcH8) cascade whose structural basis for ISG15-specific activation and E1-to-E2 thioester transfer has been resolved by cryo-EM [#4, #6], after maturation of pro-ISG15 to its active C-terminal diglycine form by a thiol protease [#1]. ISGylation tunes diverse pathways: it modifies STING at K289 to drive oligomerization and type I IFN induction [#13], modifies replication-fork proteins to stabilize stalled forks and protect nascent DNA, including in BRCA1/2-deficient cells [#18, #33], and reshapes metabolism by ISGylating glycolytic enzymes such as HK2 and PFK1 to suppress glycolysis while sustaining mitochondrial respiration and mitophagy [#11, #12, #22, #29]. ISGylation also controls protein stability and quality control, targeting misfolded p53 to the 20S proteasome [#8, #9] and modifying substrates including PTEN, KPNA2, filamin-C, and MFN1/2 to alter their turnover and downstream behavior [#20, #23, #27, #31]. Free intracellular ISG15 instead acts non-covalently: it stabilizes the deISGylase USP18 to negatively feedback on type I IFN signaling and to restrict flaviviruses by preventing NS5-mediated STAT2 degradation [#2, #32], and it engages STAT2 to promote ISGF3 assembly and ISRE-driven transcription [#30]. Secreted ISG15 functions as a cytokine, binding the αI domain of the LFA-1 integrin to activate SRC-family kinases and drive IFN-γ and IL-10 secretion from lymphocytes, a pathway essential for human antimycobacterial immunity [#0, #3, #10]. DeISGylation is carried out principally by USP18, whose ISG15-specific protease activity is structurally defined [#25], with USP16 acting as a cross-reactive deISGylase [#24]. Human ISG15 deficiency causes chronically elevated IFN signaling and mycobacterial susceptibility without broad viral susceptibility, distinguishing the secreted/free functions from intracellular ISGylation [#2, #3].\",\n  \"teleology\": [\n    {\n      \"year\": 1999,\n      \"claim\": \"Established how ISG15 becomes a competent conjugation tag, showing that the precursor must be proteolytically matured to expose the reactive C-terminal glycine.\",\n      \"evidence\": \"Biochemical purification of a 100-kDa thiol protease and in vitro processing assay mapping Gly157-Gly158 cleavage in human fibroblast extracts\",\n      \"pmids\": [\"10455185\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the physiological processing protease in vivo not pinned to a single gene\", \"Does not address conjugation machinery\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Linked ISGylation to crosstalk with the ubiquitin-proteasome system, indicating ISG15 conjugation can antagonize polyubiquitination.\",\n      \"evidence\": \"siRNA knockdown of ISG15 and its E2 UbcH8 in tumor cells with western blot of polyubiquitinated proteins\",\n      \"pmids\": [\"16424026\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Specific substrates not identified\", \"Mechanism of antagonism not resolved\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Showed ISGylation participates in innate immune signaling regulation by influencing RIG-I modification in competition with a ubiquitin E3 ligase.\",\n      \"evidence\": \"Co-IP and ectopic expression of UbcH8/ISG15/RNF125 with RIG-I ubiquitination and ISGylation assays\",\n      \"pmids\": [\"17719635\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Overexpression-based; physiological relevance of competition unclear\", \"No endogenous validation\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Defined the secreted, extracellular function of ISG15 as essential for human antimycobacterial immunity, separable from intracellular ISGylation.\",\n      \"evidence\": \"Human ISG15-deficient patients with leukocyte secretion and NK-cell IFN-γ assays\",\n      \"pmids\": [\"22859821\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Receptor for secreted ISG15 not yet identified in this study\", \"Secretion mechanism unknown\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Identified misfolded p53 as an ISGylation substrate routed for 20S proteasomal degradation, implicating ISGylation in protein quality control.\",\n      \"evidence\": \"Co-IP, in vitro 20S degradation assay, site-directed mutagenesis, and Isg15 knockout mouse cells\",\n      \"pmids\": [\"24844324\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"E3 ligase directing p53 ISGylation not defined\", \"Single lab\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Resolved the dual nature of the free-ISG15/USP18 axis by showing free ISG15 stabilizes USP18 to dampen type I IFN signaling in humans.\",\n      \"evidence\": \"ISG15-deficient patient fibroblasts complemented with WT vs. conjugation-deficient ISG15ΔGG, USP18 protein and IFN readouts\",\n      \"pmids\": [\"27193971\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Human-specific; murine difference unexplained mechanistically\", \"Structural basis of ISG15-USP18 stabilization not in this study\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Dissected the regulation of ISG15 secretion, showing intracellular ISGylation restrains secretion while viral effectors modulate it, and that secreted ISG15 signals through LFA-1.\",\n      \"evidence\": \"ISG15 secretion assays with conjugation mutants and viral effector (influenza B NS1, SARS-CoV-2 PLpro) expression across cell types\",\n      \"pmids\": [\"32553163\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Molecular route of unconventional ISG15 secretion not defined\", \"Single lab\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Mapped extracellular ISG15 signaling to a defined receptor and kinase cascade, establishing LFA-1/SFK as the conduit for cytokine induction.\",\n      \"evidence\": \"Direct in vitro binding to the CD11a αI domain, CD11a-/- splenocytes, LFA-1 and SFK inhibitor studies, ISG15 mutagenesis, NK-92 IFN-γ release\",\n      \"pmids\": [\"29100055\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Downstream transcriptional program beyond IFN-γ/IL-10 not fully mapped\", \"How LFA-1 engagement is tuned in vivo unclear\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Provided the structural basis for ISG15-specific activation, explaining how Uba7 and UBE2L6 achieve selectivity distinct from ubiquitin.\",\n      \"evidence\": \"Cryo-EM structures of Uba7-UBE2L6-ISG15 adenylate/thioester intermediates with structure-guided mutagenesis\",\n      \"pmids\": [\"37553340\", \"38042859\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"E3 (HERC5) substrate-selection step not structurally resolved here\", \"Full conjugation handoff to substrate not visualized\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Explained viral subversion of ISG15 by structurally defining how PLpro distinguishes ISG15's two UBL domains from K48 di-ubiquitin.\",\n      \"evidence\": \"Crystallography, cryo-EM, NMR, cross-linking MS, and affinity measurements of SARS-CoV-2 PLpro with ISG15\",\n      \"pmids\": [\"37185902\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In-cell consequences of differential UBL engagement not quantified\", \"Generalization to other viral deISGylases unaddressed\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Defined USP18 as the highly ISG15-specific protease and revealed its receptor-binding, catalysis-independent role in shutting off IFN signaling.\",\n      \"evidence\": \"Crystal structures of USP18 and USP18-ISG15 complex with biochemical cleavage assays\",\n      \"pmids\": [\"28881486\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Review-level structural summary\", \"In vivo deISGylation substrate scope not addressed\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Identified USP16 as a cross-reactive deISGylase, broadening the enzyme set that removes ISG15 from metabolic substrates.\",\n      \"evidence\": \"ISG15 activity-based profiling, recombinant USP16 in vitro cleavage, USP16 depletion, and substrate identification\",\n      \"pmids\": [\"38055744\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Physiological importance relative to USP18 unclear\", \"Substrate selectivity rules undefined\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Established ISGylation as a metabolic rheostat, showing IRF3-driven ISG15 ISGylates glycolytic enzymes to limit lactate and thermogenesis.\",\n      \"evidence\": \"Isg15 KO mice phenocopying adipocyte IRF3 KO, lactate rescue, and ISGylation assays of glycolytic enzymes\",\n      \"pmids\": [\"33571167\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Which specific glycolytic enzyme ISGylation events are causal not fully delineated\", \"Tissue specificity of effect not mapped\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Pinpointed HK2 and PFK1 as ISGylation substrates whose modification suppresses glycolysis to preserve cardiac mitochondrial capacity during infection.\",\n      \"evidence\": \"MS ISGylome, enzymatic activity assays, structural modeling, Seahorse flux in cardiomyocytes and ISG15 KO mouse heart\",\n      \"pmids\": [\"38309955\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"ISGylation site stoichiometry in vivo unknown\", \"E3 directing these modifications not identified\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Linked ISGylation to mitochondrial quality control by showing it is required for mitophagy, OXPHOS, and stem-cell self-renewal in pancreatic cancer.\",\n      \"evidence\": \"CRISPR ISG15 KO with WT vs. conjugation-deficient complementation, OXPHOS/mitophagy assays, and in vivo tumor formation\",\n      \"pmids\": [\"32472071\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mitochondrial ISGylation substrates not enumerated\", \"Connection to mitophagy machinery not mechanistically resolved\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Generalized the requirement of ISGylation for mitochondrial respiration to human patient-derived macrophages.\",\n      \"evidence\": \"iPSC-derived macrophages from ISG15-deficient patients with WT vs. conjugation-deficient complementation and respiration assays\",\n      \"pmids\": [\"35842904\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Respiratory-chain substrates of ISGylation undefined\", \"Partial rescue by free ISG15 unexplained\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Connected ISGylation to genome maintenance, showing it modifies replication-fork proteins downstream of cGAS-STING to mitigate replication stress.\",\n      \"evidence\": \"Mre11 inactivation, ISG15 KO cells, iPOND, fork-stability and aphidicolin-sensitivity assays\",\n      \"pmids\": [\"36216822\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Specific fork substrates of ISGylation not fully identified\", \"Single lab\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Extended the fork-protection role to a therapeutic context, showing IFNβ restores fork stability in BRCA1/2-deficient cells strictly via ISG15/ISGylation.\",\n      \"evidence\": \"ISG15 siRNA/KO, IFNβ treatment, DNA fiber assays in BRCA1/2-deficient cells, ISGylation complementation\",\n      \"pmids\": [\"37783689\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Substrate(s) of fork-protective ISGylation undefined\", \"Clinical translation untested\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Defined ISGylation of STING at K289 as a switch for STING oligomerization and IFN induction in DNA sensing.\",\n      \"evidence\": \"ISG15 KO cells, site-directed mutagenesis of STING lysines, oligomerization and IFN induction assays, molecular modeling\",\n      \"pmids\": [\"37864791\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"E3 directing STING ISGylation not identified\", \"Single lab\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Identified a free-ISG15 role in IFN signal amplification through direct STAT2 interaction and ISGF3 assembly.\",\n      \"evidence\": \"ISG15 KO cells/mice, conjugation inhibition, ISG15-STAT2 Co-IP, ISGF3 and ISRE reporter assays\",\n      \"pmids\": [\"36315588\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Structural basis of ISG15-STAT2 binding unknown\", \"Relation to USP18-mediated negative feedback unresolved\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How distinct E3 ligases and deISGylases select among the hundreds of ISGylation substrates to achieve pathway-specific outcomes remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No systematic substrate-to-E3 mapping in the corpus\", \"Site-level and stoichiometric rules for substrate ISGylation undefined\", \"Mechanism integrating free vs. conjugated vs. secreted ISG15 in vivo unclear\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0031386\", \"supporting_discovery_ids\": [4, 6, 8, 11, 12, 13, 22, 27]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [2, 30, 32]},\n      {\"term_id\": \"GO:0048018\", \"supporting_discovery_ids\": [0, 3, 10]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [2, 11, 12]},\n      {\"term_id\": \"GO:0005739\", \"supporting_discovery_ids\": [12, 29, 31]},\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [0, 3, 10]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [0, 3, 13, 30]},\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [11, 12, 22, 29]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [4, 6, 8]},\n      {\"term_id\": \"R-HSA-73894\", \"supporting_discovery_ids\": [18, 33]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"UBA7\", \"UBE2L6\", \"HERC5\", \"USP18\", \"STAT2\", \"ITGAL\", \"PARP12\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":8,"faith_total":8,"faith_pct":100.0}}