{"gene":"DDI2","run_date":"2026-06-09T23:54:41","timeline":{"discoveries":[{"year":2016,"finding":"DDI2 (aspartyl protease) is required to cleave and activate the transcription factor Nrf1 (NFE2L1) in response to proteasome dysfunction. Deletion of DDI2 reduced the cleaved form of Nrf1 and increased the full-length cytosolic form, resulting in poor upregulation of proteasomes. These defects were restored by wild-type DDI2 but not by protease-defective DDI2, establishing that DDI2's protease activity is essential for Nrf1 processing.","method":"DDI2 gene deletion (KO), add-back of wild-type vs. protease-dead DDI2, Western blot for Nrf1 forms, proteasome activity assays","journal":"eLife","confidence":"High","confidence_rationale":"Tier 2 / Strong — clean KO with defined molecular phenotype, mutagenesis rescue experiment, independently replicated across multiple labs","pmids":["27528193"],"is_preprint":false},{"year":2020,"finding":"DDI2 is a ubiquitin-directed endoprotease: it cleaves NRF1 in vitro only when NRF1 is highly poly-ubiquitylated. Purified DDI2 can cleave high-molecular-weight ubiquitylated proteins in cell extracts. No evidence for DDI2 acting as a de-ubiquitylating enzyme was found.","method":"DDI2 KO cells, in vitro protease assay with purified DDI2 and poly-ubiquitylated NRF1, mass spectrometry, cell-based accumulation of ubiquitin conjugates","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro reconstitution with purified components, multiple orthogonal methods (in vitro assay, KO cells, MS), single lab but rigorous","pmids":["32521225"],"is_preprint":false},{"year":2020,"finding":"NRF1 can be completely retrotranslocated from the ER into the cytosol, where it is then cleaved and activated by DDI2. Expression of a protease-dead point mutant of DDI2 recapitulates the loss-of-function effects on NRF1 activation, confirming that DDI2's protease activity drives cytosolic NRF1 processing.","method":"Cell fractionation, DDI2 depletion (siRNA/KD), protease-dead DDI2 point mutant expression, NRF1 cleavage assays in MDA-MB-231 cells","journal":"International journal of molecular sciences","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — defined cellular localization of cleavage event with mutagenesis, single lab, two orthogonal methods","pmids":["31947743"],"is_preprint":false},{"year":2022,"finding":"DDI2 functions as a ubiquitin-shuttling factor: its ubiquitin-like (UBL) domain mediates binding to ubiquitin conjugates (K11/K48 branched chains) and to the proteasome. Adding Ub conjugates to cell extracts increases Ddi2 association with proteasomes; adding Ddi2 increases Ub conjugate binding to purified proteasomes. Blocking DDI2 endoprotease activity (genetically or with nelfinavir) increases its binding to Ub conjugates but decreases its binding to proteasomes, reducing protein degradation.","method":"Affinity co-purification, deletion of UBL domain, nelfinavir treatment, purified proteasome binding assays, Ub conjugate accumulation assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1–2 / Moderate — reconstitution with purified proteasomes, multiple domain-deletion and pharmacological perturbation experiments, single lab","pmids":["35358511"],"is_preprint":false},{"year":2022,"finding":"Both the protease domain and the HDD domain of DDI2 are required to activate NRF1 in multiple myeloma cells. DDI2 expression is upregulated upon prolonged bortezomib treatment, contributing to bortezomib resistance via enhanced NRF1 activation.","method":"DDI2 KO in MM cells, domain mutant add-back experiments, NRF1 cleavage assays, nelfinavir (partial DDI2 protease inhibitor) treatment","journal":"Cell death & disease","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — KO plus domain-specific mutant rescue, single lab, two orthogonal methods","pmids":["35589686"],"is_preprint":false},{"year":2022,"finding":"DDI2 KO in multiple myeloma cells blocks NRF1 cleavage and nuclear translocation, impairing proteasome activity recovery upon irreversible proteasome inhibition. Add-back of wild-type but not catalytically dead DDI2 fully rescues these phenotypes, confirming DDI2 catalytic activity is necessary.","method":"DDI2 KO (CRISPR), wild-type vs. catalytically dead DDI2 add-back, NRF1 localization by immunofluorescence, proteasome activity assays, in vitro and in vivo MM models","journal":"Blood advances","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal rescue with catalytic mutant, in vitro and in vivo validation, independently replicated concept across multiple labs","pmids":["34649278"],"is_preprint":false},{"year":2020,"finding":"Nelfinavir (an HIV protease inhibitor) directly inhibits DDI2 activity, blocking NFE2L1 (NRF1) proteolysis and potentiating cytotoxicity of proteasome inhibitors in cancer cells.","method":"DDI2 protease activity assay with nelfinavir, NFE2L1 cleavage assays in cells, cell viability assays","journal":"Cellular signalling","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct in vitro inhibition assay plus cellular validation, single lab, two orthogonal methods","pmids":["32916277"],"is_preprint":false},{"year":2023,"finding":"E3 ubiquitin ligase UBE4A catalyzes polyubiquitination of retrotranslocated NRF1 and promotes its cleavage by DDI2. UBE4A interacts with NRF1, and in vitro recombinant UBE4A promotes ubiquitination of retrotranslocated NRF1. Depletion of UBE4A reduces ubiquitin modification on NRF1, shortens polyubiquitin chain length, decreases DDI2-mediated cleavage efficiency, and reduces proteasomal subunit transcription.","method":"Co-IP (UBE4A–NRF1 interaction), in vitro ubiquitination assay, UBE4A KO/depletion, ligase-dead mutant expression, NRF1 cleavage assays, RT-qPCR for proteasome subunits","journal":"Biochimica et biophysica acta. Gene regulatory mechanisms","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vitro reconstitution of ubiquitination, co-IP, KO phenotype, single lab with multiple orthogonal methods","pmids":["37084817"],"is_preprint":false},{"year":2023,"finding":"DDI2 proteolytically cleaves angiomotin (AMOT) to generate an AMOT-CT fragment that promotes angiogenesis. AMOT cleavage by DDI2 is regulated upstream by a signaling axis: NF2 controls AMOT membrane localization, TNKS1/2 catalyzes poly-ADP ribosylation of AMOT, and RNF146 catalyzes AMOT ubiquitination — all required for DDI2-mediated AMOT cleavage. Genetic inactivation of AMOT cleavage regulators in zebrafish and mice causes defective angiogenesis rescued by AMOT-CT overexpression.","method":"In vitro cleavage assay (DDI2 + AMOT), genetic KO in zebrafish and mice, rescue with AMOT-CT, co-IP/biochemical epistasis for NF2-TNKS1/2-RNF146-DDI2 axis","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — in vitro cleavage reconstitution, genetic epistasis in two vertebrate models, multiple orthogonal methods, single lab","pmids":["37350545"],"is_preprint":false},{"year":2024,"finding":"DDI2-mediated NRF1 (NFE2L1) proteolytic cleavage is critical for ferroptosis-induced feedback regulation of proteasome function. Cells lacking DDI2 cannot activate NFE2L1 in response to RSL3-induced ferroptosis, showing global hyperubiquitylation and diminished proteasomal activity. Nelfinavir (DDI2 inhibitor) sensitizes cells to ferroptosis.","method":"DDI2 KO cells, RSL3-induced ferroptosis, ubiquitylome proteomics, NRF1 cleavage assays, proteasome activity assays, nelfinavir treatment","journal":"Cell death and differentiation","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — KO with defined molecular phenotype, proteomic analysis, pharmacological inhibition, single lab","pmids":["39384955"],"is_preprint":false},{"year":2022,"finding":"DDI2 KO mice die at embryonic day E12.5 with severe developmental failure, characterized by insufficient proteasome expression, proteotoxic stress, accumulation of high-molecular-weight ubiquitin conjugates, induction of the unfolded protein response, and activation of cell death pathways. In DDI2 surrogate KO cells, proteotoxic stress activates the integrated stress response and induces a type I interferon signature.","method":"Conditional/germline DDI2 KO in mice, molecular characterization of embryos (ubiquitin conjugates, UPR markers, proteasome activity), surrogate KO cell lines, transcriptomics","journal":"iScience","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vivo KO with defined molecular phenotypes, multiple readouts, single lab","pmids":["39328932"],"is_preprint":false},{"year":2022,"finding":"Liver-specific DDI2 KO mice demonstrate that DDI2 contributes to metallothionein (MT) expression in hepatocytes at baseline and upon cadmium (Cd) exposure through DDI2-mediated NRF1 proteolytic maturation. Cd exposure inhibits proteasome activity, resulting in DDI2-mediated NRF1 cleavage; DDI2 deficiency sensitizes cells to Cd toxicity. NRF2 does not contribute to MT production in this context.","method":"Liver-specific Ddi2 KO mice, cadmium exposure, MT expression assays, proteasome activity assays, genetic analysis comparing NRF1 vs NRF2 contribution","journal":"iScience","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — conditional in vivo KO with multiple molecular readouts, single lab, multiple orthogonal methods","pmids":["36248746"],"is_preprint":false},{"year":2024,"finding":"Early recovery of proteasome activity after pulse treatment with proteasome inhibitors is DDI2-independent: it occurs before transcription of proteasomal genes is upregulated but requires protein translation. This establishes a DDI2- and transcription-independent pathway for rapid proteasome activity recovery.","method":"DDI2 KO cells, time-course proteasome activity assays after pulse treatment with proteasome inhibitors, translation inhibitor experiments","journal":"eLife","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — clean KO with time-resolved functional readout, single lab, peer-reviewed negative/dissociative result","pmids":["38619391"],"is_preprint":false},{"year":2026,"finding":"Loss of DDI2 leads to proteotoxic accumulation of the secretory protein CCN1, which is normally extracted from the ER by a DDI2-p97 complex and directed to lysosomes. In the absence of DDI2, CCN1 builds up, generates reactive oxygen species, and triggers compensatory autophagy. DDI2 functions as a selective cargo receptor linking the UPS and the autophagy-lysosome pathway.","method":"DDI2 KO in human and murine cells, CCN1 KO rescue, DDI2-p97 co-IP complex identification, CCN1-LAMP1 colocalization (immunofluorescence), ROS measurement, autophagy flux assays","journal":"iScience","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — KO with defined molecular pathway, co-IP for complex, CCN1 KO rescue, single lab with multiple orthogonal methods","pmids":["41809038"],"is_preprint":false},{"year":2020,"finding":"Comparative structural analysis of retroviral and retroviral-like protease domains shows that DDI2 contains a retroviral protease-like domain and that the mode of dimerization and density of intermonomeric contacts differ between DDI1/DDI2 and canonical retroviral proteases, correlating with evolutionary relationships.","method":"Structural bioinformatics analysis of PDB entries, multiple sequence and structure alignments","journal":"International journal of molecular sciences","confidence":"Low","confidence_rationale":"Tier 4 / Weak — computational/structural comparison only, no functional experiment on DDI2 protein","pmids":["32079302"],"is_preprint":false}],"current_model":"DDI2 is an aspartyl (retroviral-like) endoprotease that functions as a ubiquitin-directed protease and shuttling factor: it binds K11/K48-branched polyubiquitin conjugates via its UBL domain, delivers substrates to the 26S proteasome, and cleaves highly polyubiquitylated substrates — most prominently the ER-resident transcription factor NRF1 (NFE2L1) in the cytosol after its retrotranslocation, thereby activating NRF1 to drive proteasome subunit gene transcription as a compensatory response to proteotoxic stress; DDI2 also cleaves angiomotin (AMOT) in a ubiquitin/PARylation-dependent manner to promote angiogenesis, and forms a complex with p97 to extract and route the secretory protein CCN1 from the ER to lysosomes, with DDI2 ablation causing embryonic lethality and triggering compensatory autophagy via CCN1 accumulation and ROS production."},"narrative":{"mechanistic_narrative":"DDI2 is a ubiquitin-directed aspartyl endoprotease that couples recognition of polyubiquitylated substrates to their proteolytic processing, serving as a central effector of the cellular response to proteotoxic stress [PMID:27528193, PMID:32521225]. Its ubiquitin-like (UBL) domain binds K11/K48-branched polyubiquitin conjugates and the 26S proteasome, allowing DDI2 to act as a shuttling factor that delivers ubiquitylated cargo for degradation; blocking its endoprotease activity traps it on ubiquitin conjugates and reduces its proteasome association, impairing protein turnover [PMID:35358511]. Its best-defined catalytic function is the cleavage of the transcription factor NRF1 (NFE2L1): after NRF1 is retrotranslocated from the ER into the cytosol and rendered highly polyubiquitylated, DDI2 cleaves it to generate the active, nuclear form that drives proteasome subunit gene transcription, providing a compensatory bounce-back when the proteasome is inhibited [PMID:27528193, PMID:32521225, PMID:31947743]. This activity requires both the protease and HDD domains and depends on upstream polyubiquitylation by the E3 ligase UBE4A [PMID:35589686, PMID:37084817]. Through NRF1 processing, DDI2 governs proteasome recovery in multiple contexts, including bortezomib-treated multiple myeloma cells, ferroptotic stress, and cadmium-induced metallothionein expression in hepatocytes [PMID:34649278, PMID:39384955, PMID:36248746]. Beyond NRF1, DDI2 cleaves angiomotin (AMOT) in a poly-ADP-ribosylation/ubiquitination-dependent manner downstream of an NF2–TNKS1/2–RNF146 axis to release a pro-angiogenic AMOT-CT fragment [PMID:37350545], and it forms a complex with p97 that extracts the secretory protein CCN1 from the ER for lysosomal routing, linking the ubiquitin-proteasome system to the autophagy-lysosome pathway [PMID:41809038]. DDI2 is essential for development, as germline knockout mice die at E12.5 with proteotoxic stress, ubiquitin conjugate accumulation, and UPR activation [PMID:39328932]. The pharmacological inhibitor nelfinavir directly blocks DDI2 activity and sensitizes cells to proteasome inhibitors and ferroptosis [PMID:32916277, PMID:39384955].","teleology":[{"year":2016,"claim":"Established that DDI2 is the protease responsible for activating the transcription factor NRF1 under proteasome dysfunction, defining its role in the proteasome bounce-back response.","evidence":"DDI2 knockout with wild-type versus protease-dead add-back, Western blot for NRF1 forms and proteasome activity assays","pmids":["27528193"],"confidence":"High","gaps":["Did not establish whether DDI2 cleaves NRF1 directly or via an intermediate","Substrate recognition mechanism unresolved","Site of cleavage within the cell not localized"]},{"year":2020,"claim":"Demonstrated that DDI2 is a ubiquitin-directed endoprotease that cleaves NRF1 only when highly polyubiquitylated, ruling out deubiquitinase activity and defining the substrate signal.","evidence":"In vitro protease assay with purified DDI2 and polyubiquitylated NRF1, KO cells, mass spectrometry","pmids":["32521225"],"confidence":"High","gaps":["Structural basis of ubiquitin-dependent activation not resolved","Did not define the minimal ubiquitin chain architecture required"]},{"year":2020,"claim":"Localized the DDI2-dependent NRF1 cleavage event to the cytosol following complete retrotranslocation from the ER, defining the compartment of activation.","evidence":"Cell fractionation, DDI2 depletion and protease-dead mutant expression, NRF1 cleavage assays in MDA-MB-231 cells","pmids":["31947743"],"confidence":"Medium","gaps":["Machinery driving complete NRF1 retrotranslocation not identified","Single cell line"]},{"year":2020,"claim":"Identified nelfinavir as a direct pharmacological inhibitor of DDI2, providing a tool to block NRF1 processing and potentiate proteasome inhibitor cytotoxicity.","evidence":"In vitro DDI2 activity assay with nelfinavir, cellular NFE2L1 cleavage and viability assays","pmids":["32916277"],"confidence":"Medium","gaps":["Inhibitor is only partially effective and not DDI2-selective","Binding mode to DDI2 not structurally defined"]},{"year":2022,"claim":"Defined DDI2 as a ubiquitin-shuttling factor whose UBL domain links K11/K48-branched conjugates to the proteasome, explaining how protease activity and substrate delivery are coupled.","evidence":"Affinity co-purification, UBL domain deletion, nelfinavir treatment, purified proteasome binding assays","pmids":["35358511"],"confidence":"High","gaps":["Stoichiometry of DDI2-proteasome engagement unresolved","Whether shuttling is general or substrate-restricted not determined"]},{"year":2022,"claim":"Showed both protease and HDD domains are required for NRF1 activation and that DDI2 upregulation drives bortezomib resistance, establishing therapeutic relevance in multiple myeloma.","evidence":"DDI2 KO in MM cells, domain-mutant add-back, NRF1 cleavage assays, nelfinavir treatment","pmids":["35589686"],"confidence":"Medium","gaps":["Mechanistic role of the HDD domain not defined","Single lab"]},{"year":2022,"claim":"Confirmed via reciprocal catalytic-mutant rescue in vitro and in vivo that DDI2 catalytic activity is required for NRF1 nuclear translocation and proteasome recovery after irreversible inhibition.","evidence":"CRISPR KO, wild-type versus catalytically dead add-back, NRF1 immunofluorescence, proteasome activity, in vivo MM models","pmids":["34649278"],"confidence":"High","gaps":["Did not address non-NRF1 substrates in this setting"]},{"year":2022,"claim":"Established the essential developmental requirement for DDI2, linking its loss to embryonic lethality with proteotoxic stress, UPR, and interferon signatures.","evidence":"Germline/conditional DDI2 KO mice, embryo molecular characterization, surrogate KO cells, transcriptomics","pmids":["39328932"],"confidence":"Medium","gaps":["Which substrate(s) underlie lethality not pinpointed","Origin of the type I interferon signature unresolved"]},{"year":2022,"claim":"Demonstrated a physiological role for DDI2-NRF1 in hepatic metallothionein induction and cadmium detoxification, distinct from NRF2.","evidence":"Liver-specific Ddi2 KO mice, cadmium exposure, metallothionein and proteasome activity assays","pmids":["36248746"],"confidence":"Medium","gaps":["Tissue-specificity of this response not broadly mapped"]},{"year":2023,"claim":"Identified UBE4A as the E3 ligase that polyubiquitylates retrotranslocated NRF1 to license DDI2 cleavage, defining the upstream substrate-priming step.","evidence":"Co-IP, in vitro ubiquitination assay, UBE4A KO and ligase-dead mutant, NRF1 cleavage assays, RT-qPCR","pmids":["37084817"],"confidence":"Medium","gaps":["Whether other E3 ligases contribute not excluded","Chain branching specificity not fully resolved"]},{"year":2023,"claim":"Expanded DDI2 substrate repertoire beyond NRF1 by showing it cleaves angiomotin to release a pro-angiogenic fragment downstream of a PARylation/ubiquitination axis.","evidence":"In vitro cleavage assay, genetic KO in zebrafish and mice, AMOT-CT rescue, biochemical epistasis of NF2-TNKS1/2-RNF146","pmids":["37350545"],"confidence":"High","gaps":["Whether AMOT cleavage requires proteasome shuttling like NRF1 not addressed","Structural basis of substrate discrimination unknown"]},{"year":2024,"claim":"Connected DDI2-NRF1 signaling to ferroptosis, showing DDI2 is required for feedback proteasome recovery and that its inhibition sensitizes cells to ferroptotic death.","evidence":"DDI2 KO cells, RSL3-induced ferroptosis, ubiquitylome proteomics, NRF1 cleavage and proteasome activity assays, nelfinavir","pmids":["39384955"],"confidence":"Medium","gaps":["Mechanistic link between hyperubiquitylation and ferroptosis sensitivity not fully resolved"]},{"year":2024,"claim":"Dissociated rapid post-inhibitor proteasome recovery from DDI2, showing a transcription-independent, translation-dependent pathway operates before the DDI2-NRF1 program.","evidence":"DDI2 KO cells, time-course proteasome activity assays after pulse inhibition, translation inhibitor experiments","pmids":["38619391"],"confidence":"Medium","gaps":["Identity of the DDI2-independent recovery factors unknown"]},{"year":2026,"claim":"Revealed a non-NRF1 role for DDI2 as a p97-associated cargo receptor that extracts secretory CCN1 from the ER to lysosomes, bridging the UPS and autophagy-lysosome systems.","evidence":"DDI2 KO in human and murine cells, CCN1 KO rescue, DDI2-p97 co-IP, CCN1-LAMP1 colocalization, ROS and autophagy flux assays","pmids":["41809038"],"confidence":"Medium","gaps":["Whether CCN1 routing requires DDI2 protease activity not established","Structure of the DDI2-p97 complex unknown"]},{"year":null,"claim":"The structural basis by which DDI2 recognizes polyubiquitin and selects diverse substrates (NRF1, AMOT, CCN1), and how protease versus shuttling/cargo-receptor functions are partitioned, remains unresolved.","evidence":"","pmids":[],"confidence":"Low","gaps":["No experimental structure of substrate-bound DDI2","Rules governing substrate selectivity undefined","Determinants distinguishing cleavage from shuttling unknown"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[0,1,5,8]},{"term_id":"GO:0016787","term_label":"hydrolase activity","supporting_discovery_ids":[0,1]},{"term_id":"GO:0140097","term_label":"catalytic activity, acting on DNA","supporting_discovery_ids":[1]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[3]},{"term_id":"GO:0038024","term_label":"cargo receptor activity","supporting_discovery_ids":[13]}],"localization":[{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[1,2]}],"pathway":[{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[0,1,3]},{"term_id":"R-HSA-74160","term_label":"Gene expression (Transcription)","supporting_discovery_ids":[0,5,7]},{"term_id":"R-HSA-8953897","term_label":"Cellular responses to stimuli","supporting_discovery_ids":[9,10]}],"complexes":["DDI2-p97 complex"],"partners":["NFE2L1","AMOT","UBE4A","CCN1","VCP"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q5TDH0","full_name":"Protein DDI1 homolog 2","aliases":[],"length_aa":399,"mass_kda":44.5,"function":"Aspartic protease that mediates the cleavage of NFE2L1/NRF1 at 'Leu-104', thereby promoting release of NFE2L1/NRF1 from the endoplasmic reticulum membrane (PubMed:27528193, PubMed:27676298). Ubiquitination of NFE2L1/NRF1 is a prerequisite for cleavage, suggesting that DDI2 specifically recognizes and binds ubiquitinated NFE2L1/NRF1 (PubMed:27528193). Seems to act as a proteasomal shuttle which links the proteasome and replication fork proteins like RTF2 (Probable). Required, with DDI1, for cellular survival following replication stress. Together or redudantly with DDI1, removes RTF2 from stalled forks to allow cell cycle progression after replication stress and maintains genome integrity (PubMed:29290612)","subcellular_location":"Cytoplasm, cytosol; Chromosome","url":"https://www.uniprot.org/uniprotkb/Q5TDH0/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/DDI2","classification":"Not Classified","n_dependent_lines":169,"n_total_lines":1208,"dependency_fraction":0.13990066225165562},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/DDI2","total_profiled":1310},"omim":[{"mim_id":"621498","title":"REPLICATION TERMINATION FACTOR 2; RTF2","url":"https://www.omim.org/entry/621498"},{"mim_id":"620871","title":"DNA DAMAGE-INDUCIBLE 1 HOMOLOG 2; DDI2","url":"https://www.omim.org/entry/620871"},{"mim_id":"603753","title":"UBIQUITINATION FACTOR E4A; UBE4A","url":"https://www.omim.org/entry/603753"},{"mim_id":"300410","title":"ANGIOMOTIN; AMOT","url":"https://www.omim.org/entry/300410"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Enhanced","locations":[{"location":"Nucleoplasm","reliability":"Enhanced"},{"location":"Cytosol","reliability":"Enhanced"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/DDI2"},"hgnc":{"alias_symbol":["MGC14844"],"prev_symbol":["RSC1A1"]},"alphafold":{"accession":"Q5TDH0","domains":[{"cath_id":"3.10.20.90","chopping":"1-75","consensus_level":"high","plddt":88.8256,"start":1,"end":75},{"cath_id":"-","chopping":"137-176","consensus_level":"medium","plddt":86.235,"start":137,"end":176},{"cath_id":"2.40.70.10","chopping":"236-363","consensus_level":"high","plddt":95.5773,"start":236,"end":363}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q5TDH0","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q5TDH0-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q5TDH0-F1-predicted_aligned_error_v6.png","plddt_mean":79.56},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=DDI2","jax_strain_url":"https://www.jax.org/strain/search?query=DDI2"},"sequence":{"accession":"Q5TDH0","fasta_url":"https://rest.uniprot.org/uniprotkb/Q5TDH0.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q5TDH0/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q5TDH0"}},"corpus_meta":[{"pmid":"27528193","id":"PMC_27528193","title":"The aspartyl protease DDI2 activates Nrf1 to compensate for proteasome dysfunction.","date":"2016","source":"eLife","url":"https://pubmed.ncbi.nlm.nih.gov/27528193","citation_count":152,"is_preprint":false},{"pmid":"32521225","id":"PMC_32521225","title":"DDI2 Is a Ubiquitin-Directed Endoprotease Responsible for Cleavage of Transcription Factor NRF1.","date":"2020","source":"Molecular cell","url":"https://pubmed.ncbi.nlm.nih.gov/32521225","citation_count":68,"is_preprint":false},{"pmid":"15601832","id":"PMC_15601832","title":"Mice without the regulator gene Rsc1A1 exhibit increased Na+-D-glucose cotransport in small intestine and develop obesity.","date":"2005","source":"Molecular and cellular biology","url":"https://pubmed.ncbi.nlm.nih.gov/15601832","citation_count":50,"is_preprint":false},{"pmid":"16788146","id":"PMC_16788146","title":"RS1 (RSC1A1) regulates the exocytotic pathway of Na+-D-glucose cotransporter SGLT1.","date":"2006","source":"American journal of physiology. Renal physiology","url":"https://pubmed.ncbi.nlm.nih.gov/16788146","citation_count":36,"is_preprint":false},{"pmid":"31947743","id":"PMC_31947743","title":"Disabling the Protease DDI2 Attenuates the Transcriptional Activity of NRF1 and Potentiates Proteasome Inhibitor Cytotoxicity.","date":"2020","source":"International journal of molecular sciences","url":"https://pubmed.ncbi.nlm.nih.gov/31947743","citation_count":35,"is_preprint":false},{"pmid":"14724758","id":"PMC_14724758","title":"Downregulation of the Na(+)- D-glucose cotransporter SGLT1 by protein RS1 (RSC1A1) is dependent on dynamin and protein kinase C.","date":"2003","source":"The Journal of membrane biology","url":"https://pubmed.ncbi.nlm.nih.gov/14724758","citation_count":35,"is_preprint":false},{"pmid":"34649278","id":"PMC_34649278","title":"Multiple myeloma cells depend on the DDI2/NRF1-mediated proteasome stress response for survival.","date":"2022","source":"Blood advances","url":"https://pubmed.ncbi.nlm.nih.gov/34649278","citation_count":28,"is_preprint":false},{"pmid":"17686765","id":"PMC_17686765","title":"Tripeptides of RS1 (RSC1A1) inhibit a monosaccharide-dependent exocytotic pathway of Na+-D-glucose cotransporter SGLT1 with high affinity.","date":"2007","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/17686765","citation_count":28,"is_preprint":false},{"pmid":"16788147","id":"PMC_16788147","title":"Transporter regulator RS1 (RSC1A1) coats the trans-Golgi network and migrates into the nucleus.","date":"2006","source":"American journal of physiology. 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Deletion of DDI2 reduced the cleaved form of Nrf1 and increased the full-length cytosolic form, resulting in poor upregulation of proteasomes. These defects were restored by wild-type DDI2 but not by protease-defective DDI2, establishing that DDI2's protease activity is essential for Nrf1 processing.\",\n      \"method\": \"DDI2 gene deletion (KO), add-back of wild-type vs. protease-dead DDI2, Western blot for Nrf1 forms, proteasome activity assays\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — clean KO with defined molecular phenotype, mutagenesis rescue experiment, independently replicated across multiple labs\",\n      \"pmids\": [\"27528193\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"DDI2 is a ubiquitin-directed endoprotease: it cleaves NRF1 in vitro only when NRF1 is highly poly-ubiquitylated. Purified DDI2 can cleave high-molecular-weight ubiquitylated proteins in cell extracts. No evidence for DDI2 acting as a de-ubiquitylating enzyme was found.\",\n      \"method\": \"DDI2 KO cells, in vitro protease assay with purified DDI2 and poly-ubiquitylated NRF1, mass spectrometry, cell-based accumulation of ubiquitin conjugates\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro reconstitution with purified components, multiple orthogonal methods (in vitro assay, KO cells, MS), single lab but rigorous\",\n      \"pmids\": [\"32521225\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"NRF1 can be completely retrotranslocated from the ER into the cytosol, where it is then cleaved and activated by DDI2. Expression of a protease-dead point mutant of DDI2 recapitulates the loss-of-function effects on NRF1 activation, confirming that DDI2's protease activity drives cytosolic NRF1 processing.\",\n      \"method\": \"Cell fractionation, DDI2 depletion (siRNA/KD), protease-dead DDI2 point mutant expression, NRF1 cleavage assays in MDA-MB-231 cells\",\n      \"journal\": \"International journal of molecular sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — defined cellular localization of cleavage event with mutagenesis, single lab, two orthogonal methods\",\n      \"pmids\": [\"31947743\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"DDI2 functions as a ubiquitin-shuttling factor: its ubiquitin-like (UBL) domain mediates binding to ubiquitin conjugates (K11/K48 branched chains) and to the proteasome. Adding Ub conjugates to cell extracts increases Ddi2 association with proteasomes; adding Ddi2 increases Ub conjugate binding to purified proteasomes. Blocking DDI2 endoprotease activity (genetically or with nelfinavir) increases its binding to Ub conjugates but decreases its binding to proteasomes, reducing protein degradation.\",\n      \"method\": \"Affinity co-purification, deletion of UBL domain, nelfinavir treatment, purified proteasome binding assays, Ub conjugate accumulation assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — reconstitution with purified proteasomes, multiple domain-deletion and pharmacological perturbation experiments, single lab\",\n      \"pmids\": [\"35358511\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Both the protease domain and the HDD domain of DDI2 are required to activate NRF1 in multiple myeloma cells. DDI2 expression is upregulated upon prolonged bortezomib treatment, contributing to bortezomib resistance via enhanced NRF1 activation.\",\n      \"method\": \"DDI2 KO in MM cells, domain mutant add-back experiments, NRF1 cleavage assays, nelfinavir (partial DDI2 protease inhibitor) treatment\",\n      \"journal\": \"Cell death & disease\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — KO plus domain-specific mutant rescue, single lab, two orthogonal methods\",\n      \"pmids\": [\"35589686\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"DDI2 KO in multiple myeloma cells blocks NRF1 cleavage and nuclear translocation, impairing proteasome activity recovery upon irreversible proteasome inhibition. Add-back of wild-type but not catalytically dead DDI2 fully rescues these phenotypes, confirming DDI2 catalytic activity is necessary.\",\n      \"method\": \"DDI2 KO (CRISPR), wild-type vs. catalytically dead DDI2 add-back, NRF1 localization by immunofluorescence, proteasome activity assays, in vitro and in vivo MM models\",\n      \"journal\": \"Blood advances\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal rescue with catalytic mutant, in vitro and in vivo validation, independently replicated concept across multiple labs\",\n      \"pmids\": [\"34649278\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Nelfinavir (an HIV protease inhibitor) directly inhibits DDI2 activity, blocking NFE2L1 (NRF1) proteolysis and potentiating cytotoxicity of proteasome inhibitors in cancer cells.\",\n      \"method\": \"DDI2 protease activity assay with nelfinavir, NFE2L1 cleavage assays in cells, cell viability assays\",\n      \"journal\": \"Cellular signalling\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct in vitro inhibition assay plus cellular validation, single lab, two orthogonal methods\",\n      \"pmids\": [\"32916277\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"E3 ubiquitin ligase UBE4A catalyzes polyubiquitination of retrotranslocated NRF1 and promotes its cleavage by DDI2. UBE4A interacts with NRF1, and in vitro recombinant UBE4A promotes ubiquitination of retrotranslocated NRF1. Depletion of UBE4A reduces ubiquitin modification on NRF1, shortens polyubiquitin chain length, decreases DDI2-mediated cleavage efficiency, and reduces proteasomal subunit transcription.\",\n      \"method\": \"Co-IP (UBE4A–NRF1 interaction), in vitro ubiquitination assay, UBE4A KO/depletion, ligase-dead mutant expression, NRF1 cleavage assays, RT-qPCR for proteasome subunits\",\n      \"journal\": \"Biochimica et biophysica acta. Gene regulatory mechanisms\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vitro reconstitution of ubiquitination, co-IP, KO phenotype, single lab with multiple orthogonal methods\",\n      \"pmids\": [\"37084817\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"DDI2 proteolytically cleaves angiomotin (AMOT) to generate an AMOT-CT fragment that promotes angiogenesis. AMOT cleavage by DDI2 is regulated upstream by a signaling axis: NF2 controls AMOT membrane localization, TNKS1/2 catalyzes poly-ADP ribosylation of AMOT, and RNF146 catalyzes AMOT ubiquitination — all required for DDI2-mediated AMOT cleavage. Genetic inactivation of AMOT cleavage regulators in zebrafish and mice causes defective angiogenesis rescued by AMOT-CT overexpression.\",\n      \"method\": \"In vitro cleavage assay (DDI2 + AMOT), genetic KO in zebrafish and mice, rescue with AMOT-CT, co-IP/biochemical epistasis for NF2-TNKS1/2-RNF146-DDI2 axis\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — in vitro cleavage reconstitution, genetic epistasis in two vertebrate models, multiple orthogonal methods, single lab\",\n      \"pmids\": [\"37350545\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"DDI2-mediated NRF1 (NFE2L1) proteolytic cleavage is critical for ferroptosis-induced feedback regulation of proteasome function. Cells lacking DDI2 cannot activate NFE2L1 in response to RSL3-induced ferroptosis, showing global hyperubiquitylation and diminished proteasomal activity. Nelfinavir (DDI2 inhibitor) sensitizes cells to ferroptosis.\",\n      \"method\": \"DDI2 KO cells, RSL3-induced ferroptosis, ubiquitylome proteomics, NRF1 cleavage assays, proteasome activity assays, nelfinavir treatment\",\n      \"journal\": \"Cell death and differentiation\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — KO with defined molecular phenotype, proteomic analysis, pharmacological inhibition, single lab\",\n      \"pmids\": [\"39384955\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"DDI2 KO mice die at embryonic day E12.5 with severe developmental failure, characterized by insufficient proteasome expression, proteotoxic stress, accumulation of high-molecular-weight ubiquitin conjugates, induction of the unfolded protein response, and activation of cell death pathways. In DDI2 surrogate KO cells, proteotoxic stress activates the integrated stress response and induces a type I interferon signature.\",\n      \"method\": \"Conditional/germline DDI2 KO in mice, molecular characterization of embryos (ubiquitin conjugates, UPR markers, proteasome activity), surrogate KO cell lines, transcriptomics\",\n      \"journal\": \"iScience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vivo KO with defined molecular phenotypes, multiple readouts, single lab\",\n      \"pmids\": [\"39328932\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Liver-specific DDI2 KO mice demonstrate that DDI2 contributes to metallothionein (MT) expression in hepatocytes at baseline and upon cadmium (Cd) exposure through DDI2-mediated NRF1 proteolytic maturation. Cd exposure inhibits proteasome activity, resulting in DDI2-mediated NRF1 cleavage; DDI2 deficiency sensitizes cells to Cd toxicity. NRF2 does not contribute to MT production in this context.\",\n      \"method\": \"Liver-specific Ddi2 KO mice, cadmium exposure, MT expression assays, proteasome activity assays, genetic analysis comparing NRF1 vs NRF2 contribution\",\n      \"journal\": \"iScience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — conditional in vivo KO with multiple molecular readouts, single lab, multiple orthogonal methods\",\n      \"pmids\": [\"36248746\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Early recovery of proteasome activity after pulse treatment with proteasome inhibitors is DDI2-independent: it occurs before transcription of proteasomal genes is upregulated but requires protein translation. This establishes a DDI2- and transcription-independent pathway for rapid proteasome activity recovery.\",\n      \"method\": \"DDI2 KO cells, time-course proteasome activity assays after pulse treatment with proteasome inhibitors, translation inhibitor experiments\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — clean KO with time-resolved functional readout, single lab, peer-reviewed negative/dissociative result\",\n      \"pmids\": [\"38619391\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"Loss of DDI2 leads to proteotoxic accumulation of the secretory protein CCN1, which is normally extracted from the ER by a DDI2-p97 complex and directed to lysosomes. In the absence of DDI2, CCN1 builds up, generates reactive oxygen species, and triggers compensatory autophagy. DDI2 functions as a selective cargo receptor linking the UPS and the autophagy-lysosome pathway.\",\n      \"method\": \"DDI2 KO in human and murine cells, CCN1 KO rescue, DDI2-p97 co-IP complex identification, CCN1-LAMP1 colocalization (immunofluorescence), ROS measurement, autophagy flux assays\",\n      \"journal\": \"iScience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — KO with defined molecular pathway, co-IP for complex, CCN1 KO rescue, single lab with multiple orthogonal methods\",\n      \"pmids\": [\"41809038\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Comparative structural analysis of retroviral and retroviral-like protease domains shows that DDI2 contains a retroviral protease-like domain and that the mode of dimerization and density of intermonomeric contacts differ between DDI1/DDI2 and canonical retroviral proteases, correlating with evolutionary relationships.\",\n      \"method\": \"Structural bioinformatics analysis of PDB entries, multiple sequence and structure alignments\",\n      \"journal\": \"International journal of molecular sciences\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 4 / Weak — computational/structural comparison only, no functional experiment on DDI2 protein\",\n      \"pmids\": [\"32079302\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"DDI2 is an aspartyl (retroviral-like) endoprotease that functions as a ubiquitin-directed protease and shuttling factor: it binds K11/K48-branched polyubiquitin conjugates via its UBL domain, delivers substrates to the 26S proteasome, and cleaves highly polyubiquitylated substrates — most prominently the ER-resident transcription factor NRF1 (NFE2L1) in the cytosol after its retrotranslocation, thereby activating NRF1 to drive proteasome subunit gene transcription as a compensatory response to proteotoxic stress; DDI2 also cleaves angiomotin (AMOT) in a ubiquitin/PARylation-dependent manner to promote angiogenesis, and forms a complex with p97 to extract and route the secretory protein CCN1 from the ER to lysosomes, with DDI2 ablation causing embryonic lethality and triggering compensatory autophagy via CCN1 accumulation and ROS production.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"DDI2 is a ubiquitin-directed aspartyl endoprotease that couples recognition of polyubiquitylated substrates to their proteolytic processing, serving as a central effector of the cellular response to proteotoxic stress [#0, #1]. Its ubiquitin-like (UBL) domain binds K11/K48-branched polyubiquitin conjugates and the 26S proteasome, allowing DDI2 to act as a shuttling factor that delivers ubiquitylated cargo for degradation; blocking its endoprotease activity traps it on ubiquitin conjugates and reduces its proteasome association, impairing protein turnover [#3]. Its best-defined catalytic function is the cleavage of the transcription factor NRF1 (NFE2L1): after NRF1 is retrotranslocated from the ER into the cytosol and rendered highly polyubiquitylated, DDI2 cleaves it to generate the active, nuclear form that drives proteasome subunit gene transcription, providing a compensatory bounce-back when the proteasome is inhibited [#0, #1, #2]. This activity requires both the protease and HDD domains and depends on upstream polyubiquitylation by the E3 ligase UBE4A [#4, #7]. Through NRF1 processing, DDI2 governs proteasome recovery in multiple contexts, including bortezomib-treated multiple myeloma cells, ferroptotic stress, and cadmium-induced metallothionein expression in hepatocytes [#5, #9, #11]. Beyond NRF1, DDI2 cleaves angiomotin (AMOT) in a poly-ADP-ribosylation/ubiquitination-dependent manner downstream of an NF2–TNKS1/2–RNF146 axis to release a pro-angiogenic AMOT-CT fragment [#8], and it forms a complex with p97 that extracts the secretory protein CCN1 from the ER for lysosomal routing, linking the ubiquitin-proteasome system to the autophagy-lysosome pathway [#13]. DDI2 is essential for development, as germline knockout mice die at E12.5 with proteotoxic stress, ubiquitin conjugate accumulation, and UPR activation [#10]. The pharmacological inhibitor nelfinavir directly blocks DDI2 activity and sensitizes cells to proteasome inhibitors and ferroptosis [#6, #9].\",\n  \"teleology\": [\n    {\n      \"year\": 2016,\n      \"claim\": \"Established that DDI2 is the protease responsible for activating the transcription factor NRF1 under proteasome dysfunction, defining its role in the proteasome bounce-back response.\",\n      \"evidence\": \"DDI2 knockout with wild-type versus protease-dead add-back, Western blot for NRF1 forms and proteasome activity assays\",\n      \"pmids\": [\"27528193\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not establish whether DDI2 cleaves NRF1 directly or via an intermediate\", \"Substrate recognition mechanism unresolved\", \"Site of cleavage within the cell not localized\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Demonstrated that DDI2 is a ubiquitin-directed endoprotease that cleaves NRF1 only when highly polyubiquitylated, ruling out deubiquitinase activity and defining the substrate signal.\",\n      \"evidence\": \"In vitro protease assay with purified DDI2 and polyubiquitylated NRF1, KO cells, mass spectrometry\",\n      \"pmids\": [\"32521225\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of ubiquitin-dependent activation not resolved\", \"Did not define the minimal ubiquitin chain architecture required\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Localized the DDI2-dependent NRF1 cleavage event to the cytosol following complete retrotranslocation from the ER, defining the compartment of activation.\",\n      \"evidence\": \"Cell fractionation, DDI2 depletion and protease-dead mutant expression, NRF1 cleavage assays in MDA-MB-231 cells\",\n      \"pmids\": [\"31947743\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Machinery driving complete NRF1 retrotranslocation not identified\", \"Single cell line\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Identified nelfinavir as a direct pharmacological inhibitor of DDI2, providing a tool to block NRF1 processing and potentiate proteasome inhibitor cytotoxicity.\",\n      \"evidence\": \"In vitro DDI2 activity assay with nelfinavir, cellular NFE2L1 cleavage and viability assays\",\n      \"pmids\": [\"32916277\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Inhibitor is only partially effective and not DDI2-selective\", \"Binding mode to DDI2 not structurally defined\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Defined DDI2 as a ubiquitin-shuttling factor whose UBL domain links K11/K48-branched conjugates to the proteasome, explaining how protease activity and substrate delivery are coupled.\",\n      \"evidence\": \"Affinity co-purification, UBL domain deletion, nelfinavir treatment, purified proteasome binding assays\",\n      \"pmids\": [\"35358511\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Stoichiometry of DDI2-proteasome engagement unresolved\", \"Whether shuttling is general or substrate-restricted not determined\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Showed both protease and HDD domains are required for NRF1 activation and that DDI2 upregulation drives bortezomib resistance, establishing therapeutic relevance in multiple myeloma.\",\n      \"evidence\": \"DDI2 KO in MM cells, domain-mutant add-back, NRF1 cleavage assays, nelfinavir treatment\",\n      \"pmids\": [\"35589686\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanistic role of the HDD domain not defined\", \"Single lab\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Confirmed via reciprocal catalytic-mutant rescue in vitro and in vivo that DDI2 catalytic activity is required for NRF1 nuclear translocation and proteasome recovery after irreversible inhibition.\",\n      \"evidence\": \"CRISPR KO, wild-type versus catalytically dead add-back, NRF1 immunofluorescence, proteasome activity, in vivo MM models\",\n      \"pmids\": [\"34649278\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not address non-NRF1 substrates in this setting\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Established the essential developmental requirement for DDI2, linking its loss to embryonic lethality with proteotoxic stress, UPR, and interferon signatures.\",\n      \"evidence\": \"Germline/conditional DDI2 KO mice, embryo molecular characterization, surrogate KO cells, transcriptomics\",\n      \"pmids\": [\"39328932\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Which substrate(s) underlie lethality not pinpointed\", \"Origin of the type I interferon signature unresolved\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Demonstrated a physiological role for DDI2-NRF1 in hepatic metallothionein induction and cadmium detoxification, distinct from NRF2.\",\n      \"evidence\": \"Liver-specific Ddi2 KO mice, cadmium exposure, metallothionein and proteasome activity assays\",\n      \"pmids\": [\"36248746\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Tissue-specificity of this response not broadly mapped\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Identified UBE4A as the E3 ligase that polyubiquitylates retrotranslocated NRF1 to license DDI2 cleavage, defining the upstream substrate-priming step.\",\n      \"evidence\": \"Co-IP, in vitro ubiquitination assay, UBE4A KO and ligase-dead mutant, NRF1 cleavage assays, RT-qPCR\",\n      \"pmids\": [\"37084817\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether other E3 ligases contribute not excluded\", \"Chain branching specificity not fully resolved\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Expanded DDI2 substrate repertoire beyond NRF1 by showing it cleaves angiomotin to release a pro-angiogenic fragment downstream of a PARylation/ubiquitination axis.\",\n      \"evidence\": \"In vitro cleavage assay, genetic KO in zebrafish and mice, AMOT-CT rescue, biochemical epistasis of NF2-TNKS1/2-RNF146\",\n      \"pmids\": [\"37350545\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether AMOT cleavage requires proteasome shuttling like NRF1 not addressed\", \"Structural basis of substrate discrimination unknown\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Connected DDI2-NRF1 signaling to ferroptosis, showing DDI2 is required for feedback proteasome recovery and that its inhibition sensitizes cells to ferroptotic death.\",\n      \"evidence\": \"DDI2 KO cells, RSL3-induced ferroptosis, ubiquitylome proteomics, NRF1 cleavage and proteasome activity assays, nelfinavir\",\n      \"pmids\": [\"39384955\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanistic link between hyperubiquitylation and ferroptosis sensitivity not fully resolved\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Dissociated rapid post-inhibitor proteasome recovery from DDI2, showing a transcription-independent, translation-dependent pathway operates before the DDI2-NRF1 program.\",\n      \"evidence\": \"DDI2 KO cells, time-course proteasome activity assays after pulse inhibition, translation inhibitor experiments\",\n      \"pmids\": [\"38619391\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Identity of the DDI2-independent recovery factors unknown\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Revealed a non-NRF1 role for DDI2 as a p97-associated cargo receptor that extracts secretory CCN1 from the ER to lysosomes, bridging the UPS and autophagy-lysosome systems.\",\n      \"evidence\": \"DDI2 KO in human and murine cells, CCN1 KO rescue, DDI2-p97 co-IP, CCN1-LAMP1 colocalization, ROS and autophagy flux assays\",\n      \"pmids\": [\"41809038\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether CCN1 routing requires DDI2 protease activity not established\", \"Structure of the DDI2-p97 complex unknown\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The structural basis by which DDI2 recognizes polyubiquitin and selects diverse substrates (NRF1, AMOT, CCN1), and how protease versus shuttling/cargo-receptor functions are partitioned, remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No experimental structure of substrate-bound DDI2\", \"Rules governing substrate selectivity undefined\", \"Determinants distinguishing cleavage from shuttling unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [0, 1, 5, 8]},\n      {\"term_id\": \"GO:0016787\", \"supporting_discovery_ids\": [0, 1]},\n      {\"term_id\": \"GO:0140097\", \"supporting_discovery_ids\": [1]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [3]},\n      {\"term_id\": \"GO:0038024\", \"supporting_discovery_ids\": [13]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [1, 2]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [0, 1, 3]},\n      {\"term_id\": \"R-HSA-74160\", \"supporting_discovery_ids\": [0, 5, 7]},\n      {\"term_id\": \"R-HSA-8953897\", \"supporting_discovery_ids\": [9, 10]}\n    ],\n    \"complexes\": [\"DDI2-p97 complex\"],\n    \"partners\": [\"NFE2L1\", \"AMOT\", \"UBE4A\", \"CCN1\", \"VCP\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":8,"faith_total":8,"faith_pct":100.0}}