EI24

EI24 is a p53- and E2F1-regulated endoplasmic reticulum transmembrane protein that serves as an essential component of the basal autophagy pathway and a regulator of ER-mitochondria physiology and cell fate (PMID:8649819, PMID:23074225, PMID:24014029). It was first identified as a direct transcriptional target induced by DNA damage in a p53-dependent manner, and is independently driven by E2F1 binding to its promoter, contributing to survival of p53-deficient cells after irradiation (PMID:8649819, PMID:24014029). Loss of EI24 in vivo impairs autophagic flux, producing accumulation of LC3, p62, and ubiquitin-positive inclusions and causing tissue degeneration (PMID:23074225); mechanistically, EI24 links autophagy to protein degradation by mediating autophagy-dependent turnover of RING-domain E3 ubiquitin ligases (PMID:27541728). At the ER, EI24 binds Bcl-2 through its N-terminus engaging the Bcl-2 BH3 domain and acts at the mitochondrial apoptosis checkpoint, an activity antagonized by Bcl-XL (PMID:10594026, PMID:15781622). EI24 is enriched at ER-mitochondria contact sites (MAM), where its C-terminal domain supports MAM integrity and it assembles with VDAC1, IP3R, and GRP75 to control IP3R-VDAC1 coupling (PMID:31332481). It gates ER stress adaptation by binding the IRE1 kinase domain to restrain UPR activation under basal conditions and dissociating upon stress while limiting IP3R1-mediated calcium depletion (PMID:35005829), and under severe stress translocates to mitochondria to interact with VDAC2 and trigger mitochondrial apoptosis (PMID:38017236). Beyond the secretory/apoptotic axis, EI24 governs mitochondrial bioenergetics: its C-terminal region is essential for ATP synthase support and cristae integrity required for thermogenesis in brown adipocytes (PMID:41326403). EI24 additionally restrains oncogenic signaling, binding TRAF2 to drive its lysosomal degradation and attenuate NF-κB-driven EMT (PMID:24280371), and binding the IGF1R transmembrane domain to suppress IGF1R phosphorylation and improve glucose handling (PMID:40556865). EI24 protein levels are themselves controlled by the E3 ligase WWP1, which ubiquitinates and degrades EI24 to promote hepatocellular carcinoma proliferation (PMID:32252198).

1. 1996 High

   Established EI24's transcriptional origin by showing it is a direct p53 target induced by DNA damage, placing it within the p53 stress-response program.

   Evidence Differential display with p53-null MEF rescue and ectopic p53 overexpression after etoposide/irradiation

   PMID:8649819

   Open questions at the time

   • Did not define the protein's biochemical activity
   • p53 binding site on the EI24 promoter not mapped in this study
2. 2000 High

   Defined an early functional readout — EI24 overexpression triggers apoptosis blocked by Bcl-XL — positioning it at the mitochondrial apoptosis checkpoint.

   Evidence Colony formation and apoptosis assays with Bcl-XL co-expression epistasis

   PMID:10594026

   Open questions at the time

   • Molecular target of the apoptotic activity not identified
   • Endogenous role under physiological stress untested
3. 2005 High

   Provided a molecular mechanism for the apoptotic phenotype by localizing EI24 to the ER and identifying a direct N-terminus-to-BH3 interaction with Bcl-2.

   Evidence Immunofluorescence, fractionation, pull-down, Co-IP, and N-terminal deletion mapping

   PMID:15781622

   Open questions at the time

   • Functional consequence of Bcl-2 sequestration on apoptosis not quantified in vivo
   • Stoichiometry and regulation of the interaction unknown
4. 2012 High

   Established EI24 as an essential basal autophagy component through tissue-specific knockouts showing impaired autophagic flux and toxic protein accumulation.

   Evidence Neural- and liver-specific conditional KO mice with LC3/p62/ubiquitin pathology and flux assays

   PMID:23074225

   Open questions at the time

   • The biochemical step EI24 performs in autophagy not defined
   • Relationship between autophagy role and ER/apoptosis roles unresolved
5. 2012 Medium

   Extended EI24's regulatory reach to kinase stability, showing it shields PKCα from RINCK1-mediated degradation and promotes PKCα membrane localization.

   Evidence Ei24-heterozygous mice in DMBA/TPA model, Co-IP, and domain competition binding

   PMID:22771957

   Open questions at the time

   • Single-lab co-IP/competition without reconstitution
   • Mechanism by which EI24 outcompetes RINCK1 not structurally defined
6. 2013 High

   Showed EI24 transcription is not solely p53-dependent by demonstrating direct E2F1 promoter occupancy and a survival role in p53-deficient cells.

   Evidence ChIP, promoter reporters, and Rb-/-/E2f1-/- MEF epistasis with siRNA knockdown

   PMID:24014029

   Open questions at the time

   • How dual p53/E2F1 inputs are coordinated in vivo unknown
   • Context determining pro-survival vs pro-apoptotic output unresolved
7. 2013 Medium

   Linked EI24 to tumor suppression by showing it binds TRAF2 and drives its lysosomal degradation to attenuate NF-κB-driven EMT and invasion.

   Evidence Knockdown/overexpression, reciprocal Co-IP, lysosome inhibitor rescue, NF-κB reporter, metastasis assay

   PMID:24280371

   Open questions at the time

   • Whether degradation is autophagy-dependent versus another lysosomal route not distinguished
   • Single-lab study
8. 2016 Medium

   Generalized the autophagy function into a degradation principle — EI24 directs autophagic turnover of RING-domain E3 ligases, bridging autophagy and the ubiquitin system.

   Evidence Loss-of-function with E3 ligase protein readouts, autophagy inhibitor controls, substrate prediction modeling

   PMID:27541728

   Open questions at the time

   • No biochemical reconstitution of substrate recognition
   • Selectivity determinant for RING-domain ligases unknown
9. 2014 High

   Revealed a non-autophagy nuclear-transport function: an IBB-like domain lets EI24 bind IMPβ1/IMPα2 and competitively block importin-dependent nuclear import.

   Evidence Proteomics, Co-IP, competitive binding with mutant IMPβ1, nuclear import assay, IBBL alanine mutagenesis

   PMID:24821838

   Open questions at the time

   • Physiological cargoes whose import is regulated not identified
   • Interplay with EI24's ER membrane localization unclear
10. 2019 Medium

    Placed EI24 at ER-mitochondria contact sites, identifying a C-terminal-dependent role in MAM integrity and assembly of a VDAC1/IP3R/GRP75 complex.

    Evidence Fractionation, IP-MS, EI24-ΔC rescue, IP3R-VDAC1 Co-IP in KO cells, autophagy marker assays

    PMID:31332481

    Open questions at the time

    • Direct versus scaffolded interactions within the complex not resolved
    • Link between MAM integrity and autophagy flux mechanistically incomplete
11. 2019 Low

    Proposed EI24 promotes autophagic degradation of c-Myc to suppress pancreatic cancer proliferation.

    Evidence EI24 overexpression, autophagy marker Western blots, cell cycle analysis, xenograft

    PMID:30369947

    Open questions at the time

    • Overexpression-only with correlative markers; no flux or rescue experiment demonstrating direct autophagic c-Myc degradation
    • Single lab
12. 2022 High

    Defined EI24 as a gatekeeper of ER stress adaptation that restrains IRE1 basally and limits IP3R1 calcium depletion, with loss causing maladaptive apoptosis.

    Evidence EI24-IRE1 reciprocal Co-IP showing stress-dependent dissociation, KO cells with UPR, calcium and apoptosis readouts

    PMID:35005829

    Open questions at the time

    • Trigger for stress-dependent EI24-IRE1 dissociation not defined
    • How one protein coordinates IRE1 and IP3R1 simultaneously unresolved
13. 2023 Medium

    Showed that under severe (arsenic-induced) stress, p53-driven EI24 relocates from ER to mitochondria and engages VDAC2 to execute mitochondrial apoptosis.

    Evidence ER/mito fractionation, EI24-VDAC2 Co-IP, pifithrin-α and 4-PBA pharmacological inhibition in L-02 cells

    PMID:38017236

    Open questions at the time

    • Mechanism of EI24 translocation not defined
    • Single-lab, single-cell-line study
14. 2025 High

    Established a bioenergetic function — EI24's C-terminus supports ATP synthase and cristae integrity essential for brown-adipocyte thermogenesis independent of UCP1.

    Evidence Adipocyte-specific KO, cold-challenge survival, EM cristae imaging, membrane potential/pH/ATP measurements, C-terminal deletion

    PMID:41326403

    Open questions at the time

    • Direct molecular partner at ATP synthase not identified
    • How an ER/MAM protein supports matrix ATP synthesis mechanistically unclear
15. 2025 Medium

    Identified EI24 as a suppressor of IGF1R signaling by binding the IGF1R transmembrane domain, improving glucose uptake and diabetes resistance.

    Evidence Co-IP, IGF1R phosphorylation assay, Ei24 transgenic mice, glucose tolerance test, STZ diabetes model

    PMID:40556865

    Open questions at the time

    • No mutagenesis confirming the transmembrane-domain interaction
    • Single-lab study
16. 2026 Medium

    Connected EI24 to innate immunity, showing microbiota-driven TLR2/4 induction of EI24 maintains alveolar macrophage homeostasis and restrains phagocytosis/inflammation via metabolic rewiring.

    Evidence Macrophage-specific conditional KO, microbiota manipulation, TLR2/4 inhibitors, phagocytosis/inflammation assays, metabolic profiling

    PMID:41620436

    Open questions at the time

    • Whether the immune role depends on EI24's autophagy or MAM functions unknown
    • Single study

• How EI24 unifies its distinct activities — autophagic E3-ligase degradation, MAM scaffolding, ER-stress gating, mitochondrial bioenergetics, nuclear-import suppression, and receptor signaling regulation — through a single ER-membrane protein remains unresolved.
• No structural model relating its N-terminal, IBBL, and C-terminal functional regions
• Mechanism switching EI24 between pro-survival and pro-apoptotic outputs undefined
• Direct enzymatic activity, if any, unknown