IER5

IER5 is an immediate-early response gene product that functions as a substrate-specifying adapter for the PP2A/B55α phosphatase, coupling stress and developmental signals to dephosphorylation of key regulatory proteins including HSF1, RB/p107, S6K, and MDM2. Its N-terminal SERTA-like domain binds and occludes the B55α substrate recruitment surface (cryo-EM structure), while simultaneously engaging PP2A substrates to direct their dephosphorylation; a minimal construct comprising this domain plus the bipartite NLS is sufficient to rescue differentiation in IER5-knockout keratinocytes (PMID:40209703, PMID:25816751, PMID:26496226). Downstream, IER5-dependent dephosphorylation activates HSF1 to drive chaperone expression, hypophosphorylates RB to repress cell-cycle genes, and destabilizes MDM2 to stabilize p53 (PMID:26754925, PMID:36047562, PMID:40081547). IER5 also directly represses Cdc25B transcription by displacing the NF-YB/p300 coactivator complex from the Cdc25B promoter, enforcing G2/M arrest after irradiation, and is a direct transcriptional target of Notch signaling required for squamous epithelial differentiation (PMID:22132193, PMID:32936072).

1. 1999 Medium

   Identification of IER5 as an intronless immediate-early gene encoding a nuclear, proline-rich protein with PEST-like sequences established it as a rapidly inducible and likely short-lived signaling effector, but its function was unknown.

   Evidence Genomic cloning, promoter analysis, and induction kinetics in serum-stimulated cells

   PMID:10049588

   Open questions at the time

   • No function or binding partners identified
   • PEST-mediated degradation not directly tested
   • Expression limited to cell line models
2. 2009 Medium

   Loss-of-function experiments revealed that IER5 suppresses proliferation and sensitizes cells to radiation by modulating G2/M checkpoint control, placing it functionally at the cell cycle–DNA damage interface.

   Evidence siRNA knockdown in HeLa cells with flow cytometry and clonogenic survival assays

   PMID:19238419

   Open questions at the time

   • Molecular targets of IER5 in cell cycle regulation unidentified
   • No direct DNA damage repair mechanism established
3. 2011 Medium

   The mechanism for G2/M arrest was linked to direct transcriptional repression of Cdc25B, where IER5 binds the Cdc25B promoter and evicts the NF-YB/p300 coactivator complex, providing the first mechanistic target for IER5's checkpoint function.

   Evidence ChIP, co-IP, and luciferase reporter assays in AML cells with IER5 overexpression

   PMID:22132193

   Open questions at the time

   • Whether IER5 has enzymatic or purely scaffolding activity at promoters was unclear
   • Physiological relevance beyond overexpression not tested
4. 2015 High

   Discovery that IER5 physically interacts with PP2A/B55 and directs dephosphorylation of HSF1 and S6K established IER5 as a phosphatase adapter rather than a conventional transcription factor, fundamentally reframing its mechanism of action.

   Evidence Co-IP of IER5 with PP2A/B55 subunits; deletion mapping of N-terminal B55-binding region; HSF1 and S6K dephosphorylation assays

   PMID:25816751 PMID:26496226

   Open questions at the time

   • Structural basis of B55 interaction unknown
   • Whether IER5 redirects or inhibits PP2A was unresolved
5. 2016 High

   A ternary IER5–HSF1–PP2A complex was demonstrated to generate hypo-phosphorylated, transcriptionally active HSF1, closing a positive feedback loop (HSF1 drives IER5 transcription, IER5 activates HSF1), and IER5 was identified as a p53 target gene associated with super-enhancers.

   Evidence Co-IP of ternary complex, phosphorylation mapping, ChIP of HSF1 on IER5 promoter, super-enhancer analysis

   PMID:25355627 PMID:26754925

   Open questions at the time

   • Stoichiometry and kinetics of ternary complex not determined
   • Super-enhancer regulation not mechanistically dissected
6. 2017 Medium

   IER5 was connected to DNA double-strand break repair via NHEJ through physical interaction with PARP1 and Ku70, broadening its role beyond cell cycle control to include direct participation in the DNA damage response.

   Evidence Mass spectrometry identification followed by co-IP validation; γH2AX foci assay after IER5 knockdown

   PMID:29104487

   Open questions at the time

   • Mechanism by which IER5 promotes NHEJ not defined
   • Whether PP2A is involved in the NHEJ function is unknown
   • Single study without independent replication
7. 2019 High

   Mapping of a bipartite NLS (aa 217–244) and demonstration that nuclear import is required for HSF1 dephosphorylation resolved how IER5 accesses its nuclear substrates and confirmed that its phosphatase-adapter function is compartment-dependent.

   Evidence NLS mutagenesis, importin-α/β co-IP, nuclear import and phosphorylation assays

   PMID:31669744

   Open questions at the time

   • Whether cytoplasmic IER5 has any function remained untested
   • Regulation of IER5 nuclear-cytoplasmic shuttling not explored
8. 2020 High

   Genetic epistasis placed IER5 downstream of Notch and upstream of PP2A/B55α in squamous epithelial differentiation, and reconstituted IER5–B55α interaction with purified proteins confirmed a direct binding event independent of other cellular factors.

   Evidence Conditional Notch activation, IER5 KO/KD differentiation assays, co-IP with purified recombinant proteins in keratinocytes

   PMID:32936072

   Open questions at the time

   • PP2A substrates critical for differentiation downstream of IER5 not identified
   • Whether IER5 functions outside squamous lineage differentiation via Notch is unknown
9. 2022 Medium

   Extension of the PP2A adapter model to RB family proteins showed that IER5 bridges PP2A/B55 to RB and p107, causing their dephosphorylation and repression of cell cycle gene expression, providing a mechanistic explanation for IER5-mediated growth arrest.

   Evidence Co-IP of IER5 with RB/p107, knockdown with phosphorylation analysis, ChIP on RB target promoters

   PMID:36047562

   Open questions at the time

   • Relative contributions of Cdc25B repression vs. RB dephosphorylation to G2/M arrest not delineated
   • In vivo validation lacking
10. 2025 High

    Cryo-EM structure of IER5-N50 bound to PP2A/B55α revealed that IER5 occludes the B55α substrate recruitment groove via a SERTA-like domain fold, and a minimal IER5 construct (N50 + NLS) rescues keratinocyte differentiation in IER5 KO cells, defining the minimal functional unit. Concurrently, IER5 was shown to stabilize p53 by directing PP2A/B55-dependent dephosphorylation of MDM2 at Ser166, triggering MDM2 ubiquitination and nuclear depletion.

    Evidence Cryo-EM at near-atomic resolution, in vitro pTau dephosphorylation inhibition, interface mutagenesis, KO rescue in keratinocytes, MDM2 phosphorylation/ubiquitination assays

    PMID:40081547 PMID:40209703

    Open questions at the time

    • How IER5 simultaneously occludes B55 substrate surface yet redirects PP2A toward specific substrates is mechanistically paradoxical and unresolved
    • Full-length IER5 structure remains undetermined
    • In vivo mouse phenotype of IER5 loss not reported

• The central paradox—how IER5 inhibits general B55 substrate access while simultaneously promoting dephosphorylation of specific substrates—remains structurally unresolved, and whether distinct IER5 pools serve inhibitory versus adapter roles in different signaling contexts is unknown.
• No full-length IER5 structure with a substrate simultaneously bound
• No in vivo genetic model characterizing organismal function
• Relative importance of individual substrate dephosphorylation events in physiological contexts untested