Affinage

DDI2

Protein DDI1 homolog 2 · UniProt Q5TDH0

Length
399 aa
Mass
44.5 kDa
Annotated
2026-04-28
36 papers in source corpus 15 papers cited in narrative 15 extracted findings

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

DDI2 is a retroviral-like aspartyl endoprotease and ubiquitin shuttling factor that serves as a central regulatory node connecting proteasome homeostasis, stress-adaptive transcription, and protein quality control. Its principal function is the proteolytic cleavage and activation of the transcription factor NRF1/NFE2L1: upon proteasome insufficiency, NRF1 is retrotranslocated from the ER, polyubiquitinated by UBE4A, and then cleaved by DDI2 in a manner requiring both its retroviral protease domain and HDD domain, releasing an active fragment that drives proteasome subunit gene transcription (PMID:27528193, PMID:32521225, PMID:37084817, PMID:34649278). Beyond NRF1, DDI2 cleaves angiomotin to generate a pro-angiogenic C-terminal fragment downstream of a LPA–NF2–TNKS–RNF146 signaling axis (PMID:37350545), shuttles K11/K48-branched ubiquitin conjugates to the proteasome via its UBL domain (PMID:35358511), and partners with p97 to extract misfolded ER cargo such as CCN1 for lysosomal degradation (PMID:41809038). Germline DDI2 knockout in mice is embryonic lethal at E12.5 with insufficient proteasome expression, ubiquitin conjugate accumulation, and activation of the unfolded protein response and integrated stress response, and conditional hepatic loss sensitizes to cadmium toxicity through impaired metallothionein induction (PMID:39328932, PMID:36248746).

Mechanistic history

Synthesis pass · year-by-year structured walk · 11 steps
  1. 2016 High

    Identification of DDI2 as the protease required for NRF1 activation resolved the long-standing question of how proteasome gene transcription is triggered upon proteasome insufficiency.

    Evidence DDI2 KO cells with WT vs. protease-dead add-back, NRF1 cleavage and proteasome gene expression assays

    PMID:27528193

    Open questions at the time
    • How DDI2 recognizes NRF1 as a substrate was unknown
    • Whether catalytic activity is required beyond NRF1 processing was untested
    • No structural basis for DDI2 substrate selectivity
  2. 2020 High

    Reconstitution of DDI2 cleavage in vitro established that DDI2 is a ubiquitin-directed endoprotease that requires dense polyubiquitination of its substrates rather than recognizing a specific sequence motif, and that NRF1 is fully retrotranslocated before cleavage.

    Evidence In vitro cleavage with purified DDI2 and poly-ubiquitinated NRF1; subcellular fractionation in DDI2-depleted cells; mass spectrometry of ubiquitylated substrates

    PMID:31947743 PMID:32521225

    Open questions at the time
    • Identity of the E3 ligase priming NRF1 ubiquitination for DDI2 recognition was unknown
    • Whether DDI2 cleaves substrates beyond NRF1 was unresolved
  3. 2020 High

    Discovery that DDI2 functions as a ubiquitin-conjugate shuttling factor via its UBL domain, delivering K11/K48-branched chains to the proteasome, revealed a non-proteolytic role for DDI2 in proteasome-mediated degradation and explained how nelfinavir disrupts proteostasis through DDI2.

    Evidence Affinity copurification of DDI2 with ubiquitin conjugates and proteasomes, UBL domain deletion mutants, nelfinavir treatment with UPR readouts in DDI2 KO vs. WT cells

    PMID:35358511

    Open questions at the time
    • Whether shuttling and protease functions are coordinately regulated is unclear
    • Structural basis for UBL-proteasome interaction not resolved for DDI2 specifically
  4. 2020 Medium

    Demonstration that the HIV protease inhibitor nelfinavir directly inhibits DDI2 enzymatic activity provided a pharmacological tool and showed DDI2 inhibition potentiates proteasome inhibitor cytotoxicity.

    Evidence Direct DDI2 activity assay with nelfinavir, NFE2L1 proteolysis and cell viability assays

    PMID:32916277

    Open questions at the time
    • Nelfinavir has off-target effects; a DDI2-selective inhibitor was lacking
    • Therapeutic window for DDI2 inhibition in vivo was not established
  5. 2022 High

    Structure–function dissection showed that both the retroviral protease domain and the HDD domain are individually required for NRF1 cleavage, and that multiple myeloma cells upregulate DDI2 to resist proteasome inhibitor therapy.

    Evidence DDI2 KO with domain-deletion and point-mutant add-back in MM cells; in vivo MM models; bortezomib resistance assays with nelfinavir co-treatment

    PMID:34649278 PMID:35589686

    Open questions at the time
    • Molecular mechanism by which HDD contributes to cleavage is unknown
    • Whether DDI2 upregulation is transcriptionally or post-transcriptionally regulated in resistant MM cells was not determined
  6. 2022 High

    Germline DDI2 KO in mice revealed embryonic lethality at E12.5 with proteasome insufficiency, UPR/ISR activation, and type I interferon signaling, while liver-specific KO linked DDI2–NRF1 to metallothionein expression and cadmium detoxification.

    Evidence Germline and liver-conditional DDI2 KO mice, embryo phenotyping, ubiquitin conjugate and stress pathway profiling, cadmium exposure experiments

    PMID:36248746 PMID:39328932

    Open questions at the time
    • Which tissues are most sensitive to DDI2 loss is incompletely mapped
    • Whether interferon signaling is a direct or indirect consequence of DDI2 loss is unresolved
    • Contribution of non-NRF1 substrates to embryonic lethality is unknown
  7. 2023 High

    Identification of UBE4A as the E3 ligase that polyubiquitinates retrotranslocated NRF1 to prime it for DDI2-mediated cleavage resolved the upstream licensing step in the DDI2–NRF1 pathway.

    Evidence Co-IP, UBE4A KO and ligase-dead mutant, in vitro ubiquitination reconstitution, NRF1 cleavage efficiency assays

    PMID:37084817

    Open questions at the time
    • Whether UBE4A is the sole E3 or acts redundantly with other ligases is not excluded
    • Chain topology (K48, K11/K48 branched) built by UBE4A on NRF1 was not fully characterized
  8. 2023 High

    Discovery that DDI2 cleaves angiomotin (AMOT) downstream of a LPA–NF2–TNKS–RNF146 signaling axis to promote angiogenesis established the first non-NRF1 physiological substrate and a developmental role for DDI2.

    Evidence AMOT cleavage assays, genetic KO in zebrafish and mice, epistasis rescue with AMOT-CT overexpression

    PMID:37350545

    Open questions at the time
    • Whether DDI2 cleaves AMOT in a ubiquitin-dependent manner like NRF1 was not tested
    • Full substrate repertoire of DDI2 remains undefined
  9. 2024 High

    Linking DDI2–NRF1 to ferroptosis resistance revealed that DDI2-mediated proteasome restoration counteracts ferroptosis-induced proteotoxicity, broadening the stress contexts in which this axis is essential.

    Evidence DDI2 KO, RSL3-induced ferroptosis, ubiquitylation site proteomics, nelfinavir sensitization

    PMID:39384955

    Open questions at the time
    • Whether DDI2 has ferroptosis-relevant substrates beyond NRF1 is unknown
    • In vivo relevance of DDI2 in ferroptosis-driven pathologies not tested
  10. 2024 Medium

    Demonstration that early proteasome activity recovery after pulse inhibitor treatment is DDI2-independent defined a temporal boundary: DDI2–NRF1 transcriptional upregulation operates during sustained, not acute, proteasome stress.

    Evidence DDI2 KO cells, pulse proteasome inhibitor treatment, transcription and translation inhibitor experiments

    PMID:38619391

    Open questions at the time
    • Identity of the DDI2-independent recovery mechanism is unknown
    • Whether DDI2-independent recovery involves latent proteasome assembly or activator exchange is untested
  11. 2026 Medium

    Discovery that DDI2 partners with p97/VCP to extract misfolded CCN1 from the ER for lysosomal degradation revealed a cargo-receptor-like function bridging the ubiquitin–proteasome system and the autophagy–lysosome pathway.

    Evidence DDI2 KO, reciprocal Co-IP of DDI2–p97, CCN1–LAMP1 colocalization imaging, ROS and autophagy marker analysis, CCN1 KO epistasis

    PMID:41809038

    Open questions at the time
    • Whether DDI2 protease activity is required for the cargo-receptor function is unknown
    • Generality of DDI2-p97 extraction beyond CCN1 not established
    • Single-lab finding awaiting independent confirmation

Open questions

Synthesis pass · forward-looking unresolved questions
  • The full substrate repertoire of DDI2, the structural basis for ubiquitin-directed substrate recognition, and whether its protease and shuttling functions are coordinately regulated remain open questions.
  • No systematic substrate identification screen has been performed
  • No high-resolution structure of DDI2 bound to a ubiquitinated substrate exists
  • Regulatory post-translational modifications of DDI2 itself are uncharacterized

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0140096 catalytic activity, acting on a protein 6 GO:0060090 molecular adaptor activity 2
Localization
GO:0005829 cytosol 2 GO:0005783 endoplasmic reticulum 1
Pathway
R-HSA-392499 Metabolism of proteins 6 R-HSA-74160 Gene expression (Transcription) 3 R-HSA-8953897 Cellular responses to stimuli 3 R-HSA-1266738 Developmental Biology 1 R-HSA-9612973 Autophagy 1
Partners
AMOTCCN1NFE2L1UBE4AVCP

Evidence

Reading pass · 15 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2016 DDI2 (aspartyl protease) is required to proteolytically cleave and activate Nrf1 in response to proteasome dysfunction; protease-defective DDI2 cannot rescue the cleavage defect, demonstrating the catalytic activity is essential. DDI2 knockout cells, add-back of wild-type vs. protease-dead DDI2, western blot for cleaved vs. full-length Nrf1, proteasome gene expression assays eLife High 27528193
2020 DDI2 functions as a ubiquitin-directed endoprotease that cleaves NRF1 in vitro only when NRF1 is highly poly-ubiquitylated, and can cleave ubiquitylated high-molecular-weight proteins accumulating upon proteasome inhibition; no deubiquitylase activity was detected. Purified DDI2 in vitro cleavage assay with poly-ubiquitylated NRF1; DDI2 KO cells; mass spectrometry identification of ubiquitylated substrates Molecular cell High 32521225
2020 NRF1 can be completely retrotranslocated into the cytosol, where it is then cleaved and activated by DDI2; protease-dead DDI2 point mutant recapitulates DDI2 depletion phenotypes, confirming the protease activity drives NRF1 activation. Subcellular fractionation, DDI2 depletion and protease-dead add-back in MDA-MB-231 cells, NRF1 cleavage/nuclear translocation assays International journal of molecular sciences Medium 31947743
2020 DDI2 binds ubiquitin conjugates through its ubiquitin-like (UBL) domain (also required for proteasome binding), and acts as a shuttling factor delivering K11/K48 branched-chain ubiquitylated proteins to the proteasome; blocking DDI2 endoprotease activity (genetically or with nelfinavir) increased Ub-conjugate binding but decreased proteasome association and degradation, and nelfinavir required DDI2 to induce the unfolded protein response. Affinity copurification of Ddi2 with Ub conjugates and proteasomes, UBL domain deletion mutants, nelfinavir treatment, UPR assays in Ddi2 KO vs. WT cells The Journal of biological chemistry High 35358511
2022 Both the protease domain and the HDD domain of DDI2 are required for NRF1 cleavage and activation; DDI2 KO in multiple myeloma cells blocks NRF1 cleavage and nuclear translocation, causing impaired proteasome activity recovery, and is rescued by WT but not catalytically dead DDI2. DDI2 KO (CRISPR), domain deletion/point-mutant add-back, NRF1 nuclear translocation assays, proteasome activity assays, in vitro and in vivo MM models Blood advances High 34649278
2022 Upon prolonged bortezomib treatment, MM cells upregulate DDI2 expression and consequently activate NRF1; both the protease and HDD domains of DDI2 are required for NRF1 activation, and partial inhibition of the DDI2 protease domain with nelfinavir increases bortezomib susceptibility. DDI2 domain mutant add-back, nelfinavir treatment, NRF1 activation assays, bortezomib resistance assays Cell death & disease High 35589686
2020 Nelfinavir (an HIV protease inhibitor) directly inhibits DDI2 enzymatic activity, blocking NRF1 (NFE2L1) proteolysis and potentiating cytotoxicity of proteasome inhibitors in cancer cells. Direct DDI2 activity assay with nelfinavir, NFE2L1 proteolysis assay, cell cytotoxicity assays Cellular signalling Medium 32916277
2023 E3 ubiquitin ligase UBE4A catalyzes polyubiquitination of retrotranslocated NRF1 and promotes its cleavage by DDI2; UBE4A depletion reduces ubiquitin chain length on NRF1 and decreases DDI2-mediated cleavage efficiency, and recombinant UBE4A promotes NRF1 ubiquitination in vitro. Co-IP, UBE4A KO/ligase-dead mutant, in vitro ubiquitination assay with recombinant UBE4A, NRF1 cleavage efficiency assays Biochimica et biophysica acta. Gene regulatory mechanisms High 37084817
2023 DDI2 proteolytically cleaves angiomotin (AMOT) to generate a C-terminal fragment (AMOT-CT) that promotes angiogenesis; this cleavage is regulated by a signaling axis: LPA stimulation → NF2 → TNKS1/2 (poly-ADP ribosylation) → RNF146 (ubiquitination) → AMOT membrane localization, then DDI2 cleavage; genetic inactivation of AMOT cleavage regulators causes defective angiogenesis in zebrafish and mice rescued by AMOT-CT overexpression. AMOT cleavage assay, Co-IP, genetic KO in zebrafish and mice, rescue experiments with AMOT-CT, LPA stimulation assays The EMBO journal High 37350545
2024 DDI2-mediated cleavage of NFE2L1 (NRF1) is a critical step in the ferroptosis-induced feedback loop for proteasome restoration; cells lacking DDI2 cannot activate NFE2L1 in response to RSL3-induced ferroptosis and show global hyperubiquitylation and increased cell death; nelfinavir (DDI2 inhibitor) sensitizes cells to ferroptosis. DDI2 KO, proteomic ubiquitylation site mapping, RSL3-induced ferroptosis assays, nelfinavir treatment, NFE2L1 activation readouts Cell death and differentiation High 39384955
2022 DDI2 knockout in mice is embryonic lethal at E12.5, with insufficient proteasome expression, accumulation of high-molecular-weight ubiquitin conjugates, induction of the unfolded protein response (UPR), and cell death; surrogate KO cells show activation of the integrated stress response and type I interferon signaling. Germline DDI2 KO mice, embryo phenotyping, ubiquitin conjugate accumulation assays, UPR and ISR marker analysis, interferon signature profiling iScience High 39328932
2022 Liver-specific DDI2 KO demonstrates that DDI2-mediated NRF1 proteolytic maturation controls metallothionein (MT) expression at baseline and upon cadmium exposure; cadmium inhibits proteasome activity, triggering DDI2-mediated NRF1 cleavage; DDI2 deficiency sensitizes cells to cadmium toxicity. Liver-specific Ddi2 KO mice, cadmium exposure, MT expression analysis, proteasome activity assays, NRF1 cleavage assays iScience High 36248746
2024 Rapid early recovery of proteasome activity after pulse proteasome inhibitor treatment is DDI2-independent and occurs before proteasomal gene transcription is upregulated, but requires protein translation; this reveals a DDI2/transcription-independent pathway for proteasome activity recovery. DDI2 KO cells, pulse proteasome inhibitor treatment, proteasome activity assays, transcription and translation inhibitor experiments eLife Medium 38619391
2026 DDI2 forms a complex with p97 to extract misfolded CCN1 from the ER and directs it to lysosomes for degradation; loss of DDI2 causes CCN1 accumulation, ROS production, and compensatory autophagy induction; DDI2 acts as a selective cargo receptor linking the UPS and autophagy-lysosome pathway. DDI2 KO in human and murine cells, Co-IP of DDI2-p97, CCN1-LAMP1 colocalization by imaging, ROS assays, autophagy marker analysis, CCN1 KO epistasis iScience Medium 41809038
2020 The Ddi1/DDI2 retroviral protease domain structure aligns with retroviral proteases; comparative structural analysis reveals that dimerization interface organization is the main determinant of intermonomeric interactions and correlates with evolutionary relationships. Comparative structural analysis using PDB/PDBsum, multiple sequence and structure alignments of Ddi1, Ddi2, and retroviral proteases International journal of molecular sciences Low 32079302

Source papers

Stage 0 corpus · 36 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2016 The aspartyl protease DDI2 activates Nrf1 to compensate for proteasome dysfunction. eLife 150 27528193
2020 DDI2 Is a Ubiquitin-Directed Endoprotease Responsible for Cleavage of Transcription Factor NRF1. Molecular cell 64 32521225
2005 Mice without the regulator gene Rsc1A1 exhibit increased Na+-D-glucose cotransport in small intestine and develop obesity. Molecular and cellular biology 50 15601832
2006 RS1 (RSC1A1) regulates the exocytotic pathway of Na+-D-glucose cotransporter SGLT1. American journal of physiology. Renal physiology 36 16788146
2003 Downregulation of the Na(+)- D-glucose cotransporter SGLT1 by protein RS1 (RSC1A1) is dependent on dynamin and protein kinase C. The Journal of membrane biology 35 14724758
2020 Disabling the Protease DDI2 Attenuates the Transcriptional Activity of NRF1 and Potentiates Proteasome Inhibitor Cytotoxicity. International journal of molecular sciences 33 31947743
2022 Multiple myeloma cells depend on the DDI2/NRF1-mediated proteasome stress response for survival. Blood advances 28 34649278
2007 Tripeptides of RS1 (RSC1A1) inhibit a monosaccharide-dependent exocytotic pathway of Na+-D-glucose cotransporter SGLT1 with high affinity. The Journal of biological chemistry 28 17686765
2006 Transporter regulator RS1 (RSC1A1) coats the trans-Golgi network and migrates into the nucleus. American journal of physiology. Renal physiology 27 16788147
2020 Nelfinavir inhibits human DDI2 and potentiates cytotoxicity of proteasome inhibitors. Cellular signalling 21 32916277
2015 Phosphorylation of RS1 (RSC1A1) Steers Inhibition of Different Exocytotic Pathways for Glucose Transporter SGLT1 and Nucleoside Transporter CNT1, and an RS1-Derived Peptide Inhibits Glucose Absorption. Molecular pharmacology 21 26464324
2022 The aspartyl protease DDI2 drives adaptation to proteasome inhibition in multiple myeloma. Cell death & disease 20 35589686
2015 Two duplicated genes DDI2 and DDI3 in budding yeast encode a cyanamide hydratase and are induced by cyanamide. The Journal of biological chemistry 17 25847245
2024 Activating the NFE2L1-ubiquitin-proteasome system by DDI2 protects from ferroptosis. Cell death and differentiation 16 39384955
2019 MicroRNA-3607 inhibits the tumorigenesis of colorectal cancer by targeting DDI2 and regulating the DNA damage repair pathway. Apoptosis : an international journal on programmed cell death 15 31134446
2022 LncRNA FAM13A-AS1 Regulates Proliferation and Apoptosis of Cervical Cancer Cells by Targeting miRNA-205-3p/DDI2 Axis. Journal of oncology 14 35783157
2018 RS1 (Rsc1A1) deficiency limits cerebral SGLT1 expression and delays brain damage after experimental traumatic brain injury. Journal of neurochemistry 13 30022488
2009 Novel shuttling domain in a regulator (RSC1A1) of transporter SGLT1 steers cell cycle-dependent nuclear location. Traffic (Copenhagen, Denmark) 13 19765263
2022 Mammalian Ddi2 is a shuttling factor containing a retroviral protease domain that influences binding of ubiquitylated proteins and proteasomal degradation. The Journal of biological chemistry 12 35358511
2023 Proteolytic activation of angiomotin by DDI2 promotes angiogenesis. The EMBO journal 10 37350545
2020 Dimer Interface Organization is a Main Determinant of Intermonomeric Interactions and Correlates with Evolutionary Relationships of Retroviral and Retroviral-Like Ddi1 and Ddi2 Proteases. International journal of molecular sciences 10 32079302
2019 Fine-tuning the expression of target genes using a DDI2 promoter gene switch in budding yeast. Scientific reports 10 31467340
2016 Protein RS1 (RSC1A1) Downregulates the Exocytotic Pathway of Glucose Transporter SGLT1 at Low Intracellular Glucose via Inhibition of Ornithine Decarboxylase. Molecular pharmacology 9 27555600
2023 UBE4A catalyzes NRF1 ubiquitination and facilitates DDI2-mediated NRF1 cleavage. Biochimica et biophysica acta. Gene regulatory mechanisms 8 37084817
2022 The protease DDI2 regulates NRF1 activation in response to cadmium toxicity. iScience 8 36248746
2019 Structure of Ddi2, a highly inducible detoxifying metalloenzyme from Saccharomyces cerevisiae. The Journal of biological chemistry 8 31152065
2023 In-silico identification of novel DDI2 inhibitor in glioblastoma via repurposing FDA approved drugs using molecular docking and MD simulation study. Journal of biomolecular structure & dynamics 6 37139547
2015 The tomato DDI2, a PCNA ortholog, associating with DDB1-CUL4 complex is required for UV-damaged DNA repair and plant tolerance to UV stress. Plant science : an international journal of experimental plant biology 6 25900570
2022 DDI2 promotes tumor metastasis and resists antineoplastic drugs-induced apoptosis in colorectal cancer. Apoptosis : an international journal on programmed cell death 5 36520320
2024 DDI2 protease controls embryonic development and inflammation via TCF11/NRF1. iScience 3 39328932
2022 Transcriptional activation of budding yeast DDI2/3 through chemical modifications of Fzf1. Cell biology and toxicology 3 35809138
2018 A Modified Tripeptide Motif of RS1 (RSC1A1) Down-Regulates Exocytotic Pathways of Human Na+-d-glucose Cotransporters SGLT1, SGLT2, and Glucose Sensor SGLT3 in the Presence of Glucose. Molecular pharmacology 3 30355744
2025 Deficiency of DDI2 suppresses liver cancer progression by worsening cell survival conditions. Free radical biology & medicine 1 40049338
2024 Early recovery of proteasome activity in cells pulse-treated with proteasome inhibitors is independent of DDI2. eLife 1 38619391
2026 Loss of DDI2 rewires proteostasis through CCN1-driven compensatory autophagy. iScience 0 41809038
2023 Early recovery of proteasome activity in cells pulse-treated with proteasome inhibitors is independent of DDI2. bioRxiv : the preprint server for biology 0 37577495