Affinage

API5

Apoptosis inhibitor 5 · UniProt Q9BZZ5

Length
524 aa
Mass
59.0 kDa
Annotated
2026-06-09
31 papers in source corpus 16 papers cited in narrative 16 extracted findings
Cross-family judge vs UniProt: Affinage preferred faithfulness: 7/7 claims corpus-supported (100%)

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

API5 (AAC-11) is an anti-apoptotic factor that couples suppression of programmed cell death to cell-cycle progression and growth-factor signaling (PMID:17112319, PMID:19387494). Its protective activity depends on a leucine-zipper domain that mediates oligomerization and protein–protein interaction: API5 binds Acinus and shields it from caspase-3 cleavage, preventing internucleosomal DNA fragmentation, and a peptide mimicking the leucine-zipper subdomain disrupts this complex to potentiate apoptosis (PMID:19387494). Genetically, API5 acts downstream of E2F1 to restrain E2F-dependent apoptosis (PMID:17112319), and it is required for E2F1-driven transcription of G1/S genes (cyclin E, cyclin A, cyclin D1, Cdk2), facilitating E2F1 promoter occupancy without binding E2F1 directly so that its loss causes G1 arrest (PMID:23940755). A major effector arm operates through FGF2: API5 forms a structurally defined complex with nuclear FGF2, supplying a cryptic NLS to the FGF2 isoform lacking one and regulating bulk and eIF4E-sensitive mRNA nuclear export (c-MYC, cyclin D1) via TREX and eIF4E/LRPPRC (PMID:32383752). API5 also activates FGFR1-mediated PKCδ/ERK and Akt/cMYC signaling to degrade pro-apoptotic BIM, conferring resistance to T cell killing and cisplatin and promoting stem-like and EMT phenotypes in cancer (PMID:24769442, PMID:28883546, PMID:37095445). The protein is regulated post-translationally: p300 acetylates API5 at K251 to stabilize it (reversed by HDAC1), controlling its localization and cell-cycle function (PMID:34385547). Beyond intrinsic cell survival, API5 is secreted by γδ intraepithelial lymphocytes to protect Paneth cells against ATG16L1-associated death (PMID:36198790), and in antiviral immunity SRPK1-mediated phosphorylation at S464 nucleates a complex with p62 that blocks autophagic degradation of RIG-I and MDA5 to sustain RLR signaling (PMID:40641422).

Mechanistic history

Synthesis pass · year-by-year structured walk · 13 steps
  1. 1997 Medium

    Established API5/AAC-11 as a survival protein and identified the leucine-zipper domain as the structural element required for its anti-apoptotic activity, defining the founding mechanistic feature.

    Evidence Expression cloning with site-directed leucine-to-arginine mutagenesis and survival assays after growth-factor withdrawal

    PMID:9307294

    Open questions at the time
    • No molecular partner of the leucine zipper identified at this stage
    • Single-lab functional readout without replication
  2. 2006 High

    Placed API5 in the E2F1 apoptotic network, showing it selectively suppresses E2F-dependent apoptosis (via the Apaf1 apoptosome) without globally blocking E2F transcription, and that this function is evolutionarily conserved.

    Evidence Drosophila in vivo genetic modifier screen, epistasis, and conservation tested in human cells with siRNA

    PMID:17112319

    Open questions at the time
    • Molecular mechanism linking API5 to apoptosome regulation not resolved
    • No direct biochemical target identified
  3. 2009 High

    Defined a direct biochemical target of API5's anti-apoptotic function: binding to Acinus to block its caspase-3 cleavage and DNA fragmentation, and showed the leucine zipper is required for the interaction.

    Evidence Co-IP, in vitro caspase-3 cleavage protection assay, siRNA, and cell-permeable competing peptide

    PMID:19387494

    Open questions at the time
    • Stoichiometry and structural basis of the API5–Acinus complex not determined
    • Whether Acinus binding accounts for all anti-apoptotic activity unknown
  4. 2009 Medium

    Identified upstream regulation of API5 by Pim-2 kinase via NF-κB, linking its phosphorylation to survival signaling in hepatocellular carcinoma.

    Evidence Pim-2 overexpression/knockdown, NF-κB activity and phosphorylation assays with pharmacological inhibition

    PMID:19821157

    Open questions at the time
    • Phosphosite not mapped
    • Direct vs. indirect phosphorylation of API5 not distinguished
  5. 2013 Medium

    Resolved how API5 supports proliferation, showing it is required for E2F1-dependent G1/S gene transcription by promoting E2F1 promoter binding through an indirect (non-physical) mechanism.

    Evidence siRNA, ChIP, luciferase reporters, Co-IP (negative for direct E2F1 binding), and cell-cycle flow cytometry

    PMID:23940755

    Open questions at the time
    • The indirect intermediary linking API5 to E2F1 promoter binding not identified
    • How API5 reaches chromatin unknown
  6. 2014 Medium

    Connected API5 to immune evasion by defining an FGF2/FGFR1/PKCδ/ERK axis that degrades BIM to resist antigen-specific T cell killing.

    Evidence RNAi, overexpression, pharmacological pathway blockade, BIM immunoblotting, and T cell killing assays

    PMID:24769442

    Open questions at the time
    • How API5 activates FGF2 signaling mechanistically not resolved
    • Single-lab pathway placement
  7. 2017 Medium

    Extended the FGFR1/BIM axis to chemoresistance and stemness and identified a direct nuclear-receptor partnership, showing API5 binds ERα via its NR box to co-activate estrogen-responsive transcription.

    Evidence siRNA/FGFR1 inhibitors with xenografts (cisplatin, NANOG/stemness); Co-IP with domain mapping and reporter assays for ERα

    PMID:28092370 PMID:28881748 PMID:28883546

    Open questions at the time
    • Whether the transcriptional (ERα/E2F1) and FGFR1-signaling functions are mechanistically linked unknown
    • ERα interaction lacks structural detail
  8. 2018 Medium

    Revealed an extrinsic, immunostimulatory activity of API5 acting as a DAMP-like adjuvant that matures dendritic cells through TLR4/NF-κB, mapped to a region near the leucine zipper.

    Evidence TLR4/NF-κB signaling and DC maturation assays with peptide domain mapping and in vivo vaccination

    PMID:30288341

    Open questions at the time
    • Direct TLR4 binding not demonstrated
    • Mechanism of API5 release for extracellular activity not established here
  9. 2020 High

    Provided the structural and mechanistic basis for a nuclear RNA-handling function, showing API5 binds nuclear FGF2 (crystal structure) to supply a cryptic NLS and to control mRNA nuclear export via TREX and eIF4E/LRPPRC.

    Evidence X-ray crystallography, interface mutagenesis, Co-IP, and mRNA export assays with siRNA

    PMID:32383752

    Open questions at the time
    • How nuclear mRNA-export activity relates to API5's anti-apoptotic and FGFR1-signaling roles unresolved
    • Direct interaction of API5 with TREX/eIF4E components not fully mapped
  10. 2021 Medium

    Identified an acetylation switch controlling API5 abundance, showing p300 acetylates K251 to stabilize the protein while HDAC1 deacetylation destabilizes it, governing its localization and cell-cycle progression.

    Evidence p300/HDAC1 inhibitors, acetylation-specific immunoblotting, K251 analysis, fractionation, and cell-cycle assays

    PMID:34385547

    Open questions at the time
    • Degradation machinery acting on deacetylated API5 not identified
    • Direct enzyme–substrate contacts not structurally defined
  11. 2022 High

    Demonstrated a tissue-protective extracellular role: API5 secreted by γδ intraepithelial lymphocytes safeguards Paneth cells, masking ATG16L1-associated genetic susceptibility to their death.

    Evidence Recombinant API5 reconstitution in vivo and in human organoids, ATG16L1-mutant mice, and lymphocyte–epithelium co-culture

    PMID:36198790

    Open questions at the time
    • Receptor or surface target of secreted API5 on Paneth cells not identified
    • Secretion mechanism from γδ IELs not defined
  12. 2023 Medium

    Showed API5 overexpression is sufficient to drive proliferation and an EMT-like, tumorigenic phenotype in normal breast epithelium through FGF2-driven PDK1-Akt/cMYC and Ras-ERK signaling.

    Evidence 3D acinar cultures, gain/loss-of-function, pathway immunoblotting, and in vivo xenografts

    PMID:37095445

    Open questions at the time
    • Whether API5 acts cell-autonomously vs. via secreted FGF2 not fully separated
    • Single-lab model system
  13. 2025 Medium

    Defined an antiviral function for API5, showing SRPK1 phosphorylation at S464 promotes a p62 complex that blocks API5 ubiquitination at K141 and prevents autophagic degradation of RIG-I and MDA5 to sustain RLR antiviral signaling.

    Evidence Phosphosite/ubiquitination-site mapping, Co-IP, autophagy flux assays, RIG-I/MDA5 stability, and antiviral reporters

    PMID:40641422

    Open questions at the time
    • Limited independent replication
    • How the cytosolic antiviral pool of API5 relates to its nuclear/secreted functions unknown

Open questions

Synthesis pass · forward-looking unresolved questions
  • It remains unresolved how API5's distinct activities — Acinus-binding anti-apoptosis, E2F1/ERα transcriptional support, nuclear FGF2/mRNA export, secreted Paneth-cell protection, and cytosolic antiviral RIG-I/MDA5 stabilization — are coordinated within one protein and partitioned across subcellular compartments.
  • No unifying structural or regulatory model linking nuclear, cytosolic, and secreted functions
  • Compartment-specific determinants of which interaction dominates not defined

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0098772 molecular function regulator activity 2 GO:0140110 transcription regulator activity 2 GO:0003723 RNA binding 1
Localization
GO:0005634 nucleus 2 GO:0005576 extracellular region 1 GO:0005829 cytosol 1
Pathway
R-HSA-162582 Signal Transduction 2 R-HSA-1640170 Cell Cycle 2 R-HSA-168256 Immune System 2 R-HSA-5357801 Programmed Cell Death 2 R-HSA-8953854 Metabolism of RNA 1
Partners
ACIN1E2F1ESR1FGF2SQSTM1

Evidence

Reading pass · 16 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
1997 AAC-11 (API5) encodes a ~55 kDa protein containing a leucine zipper domain that is required for its anti-apoptotic function; mutation of leucines to arginines within the leucine zipper abolished protection from apoptosis after growth factor withdrawal. Functional expression cloning, site-directed mutagenesis of leucine zipper, stable transfection assay in serum-free medium Cancer research Medium 9307294
2006 Api5/Aac11 acts downstream of E2F to suppress E2F-dependent apoptosis without generally blocking E2F-dependent transcription; this function involves the dArk/Apaf1 apoptosome-dependent activation of caspases and is sensitive to dIAP1 levels. The interaction is conserved from Drosophila to humans. Drosophila in vivo genetic modifier screen, epistasis analysis, cultured cell apoptosis assays, siRNA depletion PLoS genetics High 17112319
2009 AAC-11 (API5) physically binds to Acinus and prevents Acinus-mediated internucleosomal DNA fragmentation; AAC-11 also protects Acinus from caspase-3 cleavage both in vivo and in vitro. This interaction requires the leucine-zipper domain of AAC-11 for oligomerization. A cell-permeable peptide mimicking the leucine-zipper subdomain disrupts the AAC-11–Acinus complex and potentiates drug-mediated apoptosis. Co-immunoprecipitation, in vitro caspase-3 cleavage assay, siRNA depletion, cell-permeable peptide competition, yeast two-hybrid (Rain JC co-author implies Y2H screen) The EMBO journal High 19387494
2009 Pim-2 kinase activates API5 through the NF-κB pathway, leading to increased API5 phosphorylation and inhibition of apoptosis in hepatocellular carcinoma cells; NF-κB inhibition with parthenolide reverses API5 phosphorylation and the anti-apoptotic effect. Pim-2 overexpression and siRNA knockdown in liver cell lines, NF-κB activity assay, phosphorylation detection by Western blot, apoptosis assay, pharmacological inhibition Pathology oncology research : POR Medium 19821157
2013 Api5 is required for E2F1-dependent transcriptional activation of G1/S transition genes (cyclin E, cyclin A, cyclin D1, Cdk2); although Api5 does not physically interact with E2F1, it facilitates E2F1 binding to target gene promoters through an indirect mechanism, and its depletion causes G1 cell cycle arrest. siRNA knockdown, luciferase reporter assay, chromatin immunoprecipitation (ChIP), co-immunoprecipitation (negative for direct E2F1 interaction), flow cytometry cell cycle analysis PloS one Medium 23940755
2014 API5 mediates tumor immune escape by activating FGF2 signaling through a FGFR1/PKCδ/ERK effector pathway, which triggers degradation of the pro-apoptotic molecule BIM, thereby conferring resistance to antigen-specific T cell-mediated apoptosis. RNAi silencing, overexpression, pharmacological blockade of FGF2/PKCδ/ERK, Western blot for BIM, functional T cell killing assays Cancer research Medium 24769442
2017 API5 directly interacts with ERα (estrogen receptor alpha) via its Nuclear Receptor (NR) box, which drives interaction with the C domain of ERα; this interaction promotes ERα target gene transcription upon estrogen treatment. Co-immunoprecipitation, domain mapping, luciferase reporter assays, in vivo xenograft model Oncotarget Medium 28881748
2017 API5 confers cisplatin resistance through FGFR1 signaling activation, leading to BIM degradation; FGFR1 inhibition (siRNA or small-molecule inhibitor) disrupts cisplatin resistance in API5-high cancer cells in vitro and in vivo. siRNA knockdown, FGFR1 inhibitor, Western blot for BIM, in vivo xenograft model, cisplatin resistance assays Experimental & molecular medicine Medium 28883546
2017 API5 confers cancer stem cell-like properties through an E2F1-dependent FGF2 expression axis that activates FGFR1 signaling to upregulate NANOG. siRNA knockdown, FGFR1 inhibition, sphere-forming assays, flow cytometry for CD44, qPCR/Western blot for NANOG and FGF2 Oncogenesis Medium 28092370
2018 API5 activates dendritic cells through TLR4-dependent NF-κB signaling; the protein sequence fragment proximal to its leucine zipper motif is responsible for this adjuvant/DAMP-like activity. TLR4 signaling assays, NF-κB activation measurement, DC maturation assays, domain mapping using API5 peptide fragments, in vivo vaccination experiments Oncoimmunology Medium 30288341
2020 API5 forms a complex with nuclear FGF2; the crystal structure of this complex was determined and revealed critical residues driving the interaction. API5 provides a nuclear localization function for the FGF2 isoform lacking a canonical NLS by harboring a cryptic NLS in FGF2. The API5–FGF2 complex regulates mRNA nuclear export through both the TREX and eIF4E/LRPPRC mRNA export complexes, controlling bulk mRNA export as well as specific mRNAs (c-MYC, cyclin D1) containing eIF4E sensitivity elements. X-ray crystallography, Co-immunoprecipitation, mutagenesis of critical interface residues, mRNA export assays, siRNA knockdown Nucleic acids research High 32383752
2021 API5 is acetylated at lysine 251 (K251) by the acetyltransferase p300, and deacetylated by HDAC1; K251 acetylation maintains API5 protein stability, whereas the deacetylated form is unstable. This acetylation/deacetylation switch also regulates the subcellular localization of API5 and is required for cell cycle progression. p300 and HDAC1 inhibitor treatment, co-immunoprecipitation, acetylation-specific immunoblotting, site-directed analysis of K251, subcellular fractionation/localization, cell cycle analysis Scientific reports Medium 34385547
2022 API5 is secreted by γδ intraepithelial lymphocytes (γδ IELs) and acts as a Paneth cell-protective factor; recombinant API5 protected Paneth cells in vivo in ATG16L1-mutant mice and ex vivo in human organoids carrying the ATG16L1 risk allele. Viral infection inhibited API5 secretion from γδ IELs, implicating API5 in the mechanism by which γδ IELs mask genetic susceptibility to Paneth cell death. Ex vivo lymphocyte-epithelium co-culture system, recombinant protein administration in vivo and ex vivo, ATG16L1-mutant mouse model, human organoid culture Nature High 36198790
2023 In non-tumorigenic breast epithelial 3D acinar cultures, Api5 overexpression activates FGF2 signaling through the PDK1-Akt/cMYC and Ras-ERK pathways to promote proliferation and an EMT-like phenotype; Api5 knockdown downregulates FGF2 signaling and reduces in vivo tumorigenic potential. 3D acinar culture, siRNA knockdown, overexpression, Western blot for pathway components (PDK1, Akt, cMYC, ERK), in vivo xenograft BMC cancer Medium 37095445
2015 Influenza A virus nucleoprotein (NP) directly interacts with API5 and suppresses its expression, leading to upregulation of APAF1 and cleavage of caspases 9 and 3 (E2F1-dependent apoptosis pathway). Overexpression of API5 in NP-expressing or IAV-infected cells decreased viral titers and NP levels, while API5 silencing increased viral replication. Co-immunoprecipitation (NP–API5 direct interaction), siRNA knockdown, overexpression, annexin V/7AAD apoptosis assay, Western blot for caspases and APAF1, viral titer measurement Cell death & disease Medium 26673663
2025 SRPK1-dependent phosphorylation of API5 at serine 464 (S464) is required for antiviral immune responses; phosphorylated API5 forms a complex with the autophagic receptor p62, preventing API5 ubiquitination at K141, thereby reducing p62 aggregation and inhibiting autophagic degradation of cytosolic RNA sensors RIG-I and MDA5 to sustain RLR-mediated antiviral responses. Phosphorylation site identification (S464), Co-immunoprecipitation (API5–p62 complex), ubiquitination assays, autophagy flux assays, RIG-I/MDA5 stability measurement, antiviral reporter assays Advanced science Medium 40641422

Source papers

Stage 0 corpus · 31 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2006 Functional identification of Api5 as a suppressor of E2F-dependent apoptosis in vivo. PLoS genetics 101 17112319
1997 AAC-11, a novel cDNA that inhibits apoptosis after growth factor withdrawal. Cancer research 83 9307294
2009 The antiapoptotic protein AAC-11 interacts with and regulates Acinus-mediated DNA fragmentation. The EMBO journal 64 19387494
2022 The γδ IEL effector API5 masks genetic susceptibility to Paneth cell death. Nature 54 36198790
2014 API5 confers tumoral immune escape through FGF2-dependent cell survival pathway. Cancer research 45 24769442
2015 Nucleoprotein of influenza A virus negatively impacts antiapoptotic protein API5 to enhance E2F1-dependent apoptosis and virus replication. Cell death & disease 44 26673663
2016 A Cell-Penetrating Peptide Targeting AAC-11 Specifically Induces Cancer Cells Death. Cancer research 43 27406828
2000 AAC-11 overexpression induces invasion and protects cervical cancer cells from apoptosis. Laboratory investigation; a journal of technical methods and pathology 42 10780674
2014 Tumor suppressors miR-143 and miR-145 and predicted target proteins API5, ERK5, K-RAS, and IRS-1 are differentially expressed in proximal and distal colon. American journal of physiology. Gastrointestinal and liver physiology 41 25477374
2009 Pim-2 activates API-5 to inhibit the apoptosis of hepatocellular carcinoma cells through NF-kappaB pathway. Pathology oncology research : POR 40 19821157
2020 Regulation of mRNA export through API5 and nuclear FGF2 interaction. Nucleic acids research 35 32383752
2017 API5 confers cancer stem cell-like properties through the FGF2-NANOG axis. Oncogenesis 27 28092370
2013 Api5 contributes to E2F1 control of the G1/S cell cycle phase transition. PloS one 25 23940755
2017 API5 induces cisplatin resistance through FGFR signaling in human cancer cells. Experimental & molecular medicine 21 28883546
2018 A novel function of API5 (apoptosis inhibitor 5), TLR4-dependent activation of antigen presenting cells. Oncoimmunology 20 30288341
2012 Apoptosis inhibitor 5 (API-5; AAC-11; FIF) is upregulated in human carcinomas in vivo. Oncology letters 20 22741017
2014 MicroRNA-1 promotes apoptosis of hepatocarcinoma cells by targeting apoptosis inhibitor-5 (API-5). FEBS letters 18 25433291
2010 Targeting AAC-11 in cancer therapy. Expert opinion on therapeutic targets 14 20001210
2022 Circ_0060055 Promotes the Growth, Invasion, and Radioresistance of Glioblastoma by Targeting MiR-197-3p/API5 Axis. Neurotoxicity research 13 35849320
2011 Tocotrienol-treated MCF-7 human breast cancer cells show down-regulation of API5 and up-regulation of MIG6 genes. Cancer genomics & proteomics 13 21289334
2021 Evolution and Structure of API5 and Its Roles in Anti-Apoptosis. Protein and peptide letters 12 33319655
2017 Api5 a new cofactor of estrogen receptor alpha involved in breast cancer outcome. Oncotarget 12 28881748
2020 Prophylactic and therapeutic antileukemic effects induced by the AAC-11-derived Peptide RT53. Oncoimmunology 10 32158621
2019 High expression of apoptosis protein (Api-5) in chemoresistant triple-negative breast cancers: an innovative target. Oncotarget 9 31762939
2022 Validation of AAC-11-Derived Peptide Anti-Tumor Activity in a Single Graft Sézary Patient-Derived Xenograft Mouse Model. Cells 7 36230895
2023 Altered expression of anti-apoptotic protein Api5 affects breast tumorigenesis. BMC cancer 5 37095445
2021 Interplay between p300 and HDAC1 regulate acetylation and stability of Api5 to regulate cell proliferation. Scientific reports 5 34385547
2021 PAK1-Dependent Antitumor Effect of AAC-11‒Derived Peptides on Sézary Syndrome Malignant CD4+ T Lymphocytes. The Journal of investigative dermatology 4 33745910
2025 API5 Phosphorylation Promotes Antiviral Immunity by Inhibiting Degradation of Cytosolic RNA Sensor RLRs. Advanced science (Weinheim, Baden-Wurttemberg, Germany) 2 40641422
2023 API5-Hsp20 axis regulate apoptosis and viral infection in mud crab (Scylla paramamosain). Frontiers in microbiology 2 38143869
2026 Mammosphere Assay Reveals Api5-Induced Stemness in Non-Tumorigenic Breast Epithelial Cell Lines. Journal of visualized experiments : JoVE 0 41838736

Missed literature

Know a paper Affinage missed for API5? Flag it for the maintainers and the community.

No submissions yet.