Existing Annotations Review

GO Term	Evidence	Action	Reason
GO:0005634 nucleus	IBA GO_REF:0000033	ACCEPT	Summary: API5 is predominantly nuclear as confirmed by multiple studies. The protein contains a nuclear localization signal (NLS) at residues 454-475 and localizes to the nucleus where it performs its anti-apoptotic scaffold functions [PMID:11075807]. Reason: Nuclear localization is well-established for API5. The IBA annotation is consistent with direct experimental evidence from PMID:11075807 showing nuclear localization, and UniProt confirms this with IDA evidence. The protein's functions in transcriptional regulation, caspase-2 inhibition, and Acinus binding all occur in the nucleus. Supporting Evidence: PMID:11075807 we have isolated a gene encoding a nuclear protein of 55 kDa, FIF (FGF-2-interacting-factor), which interacts specifically with FGF-2
GO:0003723 RNA binding	IBA GO_REF:0000033	ACCEPT	Summary: API5 has been implicated in mRNA export via TREX/eIF4E/CRM1 pathways and has been identified as part of the spliceosomal complex. High-throughput mRNA interactome studies have detected API5 as an mRNA-binding protein [file:human/API5/API5-deep-research-falcon.md]. Reason: The IBA annotation is supported by HDA evidence from mRNA interactome studies (PMID:22658674, PMID:22681889) and the protein's association with spliceosomal complexes. While not a primary catalytic function, RNA binding is consistent with API5's scaffold role in nuclear processes. Supporting Evidence: file:human/API5/API5-deep-research-falcon.md API5 participates in nuclear FGF-2 complexes and has been implicated in mRNA export (TREX/eIF4E/CRM1 pathways)
GO:0043066 negative regulation of apoptotic process	IBA GO_REF:0000033	ACCEPT	Summary: This is the core function of API5. The protein inhibits apoptosis through multiple specific mechanisms: (1) direct binding to caspase-2 CARD domain preventing activation, (2) binding and protecting Acinus from caspase-3 cleavage, and (3) suppressing E2F1-dependent apoptosis. The IBA annotation is strongly supported by extensive experimental literature [PMID:9307294, PMID:19387494]. Reason: Negative regulation of apoptosis is API5's defining function. This is supported by the original discovery paper [PMID:9307294] showing cells expressing AAC-11 survive serum withdrawal, and the Acinus protection study [PMID:19387494]. Supporting Evidence: PMID:9307294 BALB/c3T3 fibroblasts that were stably transfected with AAC-11 cDNA were viable in serum-free medium for up to 12 weeks PMID:19387494 We report here that AAC-11, a survival protein whose expression prevents apoptosis that occurs on deprivation of growth factors, physiologically binds to Acinus and prevents Acinus-mediated DNA fragmentation
GO:0005634 nucleus	IEA GO_REF:0000120	ACCEPT	Summary: IEA annotation for nuclear localization based on automated methods. Consistent with extensive experimental evidence. Reason: Redundant with IBA and IDA annotations but correct. Nuclear localization is API5's primary location and is well-supported by experimental evidence. Supporting Evidence: PMID:11075807 we have isolated a gene encoding a nuclear protein of 55 kDa, FIF (FGF-2-interacting-factor)
GO:0005737 cytoplasm	IEA GO_REF:0000044	ACCEPT	Summary: API5 is predominantly nuclear but can also be cytoplasmic. UniProt notes "Mainly nuclear. Can also be cytoplasmic." Cytoplasmic localization is associated with stress-induced degradation via the ATR-FBXW2 pathway [file:human/API5/API5-deep-research-falcon.md]. Reason: While nucleus is the primary location, cytoplasmic localization is a real physiological state, particularly during stress responses when API5 is targeted for proteasomal degradation. Isoform 3 is specifically cytoplasmic. Supporting Evidence: file:human/API5/API5-deep-research-falcon.md DNA damage shifts API5 toward cytoplasmic degradation via ATR-FBXW2-proteasome, functionally coupling checkpoint activation to relief of API5-mediated apoptosis suppression
GO:0006915 apoptotic process	IEA GO_REF:0000043	MODIFY	Summary: This annotation from UniProtKB keyword mapping is too general. API5 specifically NEGATIVELY regulates apoptosis; it does not simply participate in the apoptotic process without direction. Reason: The term "apoptotic process" is ambiguous about direction. API5 is specifically an anti-apoptotic protein that inhibits multiple steps of the apoptotic cascade. The more accurate term is GO:0043066 (negative regulation of apoptotic process), which is already annotated with stronger evidence. Proposed replacements: negative regulation of apoptotic process Supporting Evidence: PMID:9307294 The corresponding native protein may function as a novel inhibitor of apoptosis
GO:0043066 negative regulation of apoptotic process	IEA GO_REF:0000117	ACCEPT	Summary: IEA annotation from ARBA machine learning models for negative regulation of apoptotic process. Consistent with the core function of API5. Reason: Correct annotation, redundant with IBA, TAS, ISS, NAS, and IMP annotations for the same term. This is API5's core function. Supporting Evidence: PMID:9307294 a cDNA whose expression supports cell viability after withdrawal of growth factors
GO:0005515 protein binding	IPI PMID:19387494 The antiapoptotic protein AAC-11 interacts with and regulate...	MODIFY	Summary: This annotation captures the interaction between API5 (AAC-11) and Acinus (ACIN1). The interaction is functionally significant as API5 binding protects Acinus from caspase-3 cleavage and prevents Acinus-mediated DNA fragmentation. Reason: While the protein binding annotation is technically correct, it is uninformative. The specific interaction with Acinus (a chromatin condensation/DNA fragmentation factor) is a core mechanistic function of API5. A more specific term describing this scaffold/protective binding function would be more informative, but GO may lack an appropriate term. The key point is this represents meaningful functional binding, not generic protein binding. Proposed replacements: negative regulation of apoptotic process Supporting Evidence: PMID:19387494 AAC-11 was able to protect Acinus from caspase-3 cleavage in vivo and in vitro, thus interfering with its biological function
GO:0005515 protein binding	IPI PMID:30021884 Histone Interaction Landscapes Visualized by Crosslinking Ma...	KEEP AS NON CORE	Summary: This annotation is from a crosslinking mass spectrometry study detecting API5 interaction with DDX39B (a DEAD-box RNA helicase involved in mRNA export). This interaction may relate to API5's reported role in mRNA export pathways. Reason: While the interaction with DDX39B is from high-quality crosslinking MS data, it represents a secondary function related to RNA biology rather than API5's core anti-apoptotic role. The "protein binding" term is uninformative. Supporting Evidence: file:human/API5/API5-deep-research-falcon.md API5 participates in nuclear FGF-2 complexes and has been implicated in mRNA export (TREX/eIF4E/CRM1 pathways) PMID:30021884 Epub 2018 Jul 18. Histone Interaction Landscapes Visualized by Crosslinking Mass Spectrometry in Intact Cell Nuclei.
GO:0005515 protein binding	IPI PMID:35271311 OpenCell: Endogenous tagging for the cartography of human ce...	KEEP AS NON CORE	Summary: This annotation is from the OpenCell project (endogenous tagging for cellular organization cartography) detecting API5 interaction with DDX39B. Reason: Redundant with PMID:30021884 annotation for DDX39B interaction. High-throughput interaction data; the "protein binding" term is too general to be informative about API5's specific functions. Supporting Evidence: file:human/API5/API5-deep-research-falcon.md API5 participates in nuclear FGF-2 complexes and has been implicated in mRNA export (TREX/eIF4E/CRM1 pathways) PMID:35271311 2022 Mar 11. OpenCell: Endogenous tagging for the cartography of human cellular organization.
GO:0005681 spliceosomal complex	IEA GO_REF:0000107	KEEP AS NON CORE	Summary: IEA annotation based on transfer from mouse ortholog (O35841). Mouse Api5 was found in spliceosomal complexes in the large-scale nuclear sub-compartment study. Reason: While API5 may associate with splicing machinery, this is likely secondary to its core anti-apoptotic functions. The association may relate to API5's RNA binding capacity and nuclear scaffold role rather than direct splicing function. Supporting Evidence: PMID:11555636 The most common discrete localizations detected are at the nucleolus and the splicing speckles
GO:0016607 nuclear speck	IDA GO_REF:0000052	ACCEPT	Summary: Nuclear speck localization from HPA immunofluorescence data. Nuclear speckles are enriched in splicing factors, consistent with API5's association with spliceosomal complexes and RNA binding. Reason: IDA evidence from HPA immunofluorescence. Nuclear speck localization is consistent with API5's nuclear scaffold role and association with RNA processing machinery. This is a reasonable sub-nuclear localization annotation. Supporting Evidence: PMID:11555636 The most common discrete localizations detected are at the nucleolus and the splicing speckles
GO:0016020 membrane	HDA PMID:19946888 Defining the membrane proteome of NK cells.	MARK AS OVER ANNOTATED	Summary: This annotation is from a study defining the membrane proteome of NK cells. API5 is not a transmembrane protein and is predominantly nuclear. This may represent non-specific association or a minor localization. Reason: API5 is a nuclear scaffold protein without transmembrane domains. While it may have been detected in membrane fractions in the NK cell proteomics study, this is likely not a physiologically relevant localization for its core functions. The HDA evidence from a proteomics study may reflect contamination or transient association. Supporting Evidence: file:human/API5/API5-deep-research-falcon.md Basal state: predominantly nuclear, chromatin-associated, with acetylation at K251 stabilizing the protein and supporting nuclear functions PMID:19946888 Defining the membrane proteome of NK cells.
GO:0003723 RNA binding	HDA PMID:22658674 Insights into RNA biology from an atlas of mammalian mRNA-bi...	ACCEPT	Summary: RNA binding detected in the mRNA-bound proteome atlas study by Castello et al. This supports API5's association with mRNA processing/export pathways. Reason: HDA evidence from systematic mRNA interactome capture. Consistent with API5's reported roles in mRNA export and association with spliceosomal complexes. While not a core function, RNA binding is a validated molecular function. Supporting Evidence: file:human/API5/API5-deep-research-falcon.md API5 participates in nuclear FGF-2 complexes and has been implicated in mRNA export (TREX/eIF4E/CRM1 pathways) PMID:22658674 May 31. Insights into RNA biology from an atlas of mammalian mRNA-binding proteins.
GO:0003723 RNA binding	HDA PMID:22681889 The mRNA-bound proteome and its global occupancy profile on ...	ACCEPT	Summary: RNA binding detected in the Baltz et al. mRNA-bound proteome study. Redundant with PMID:22658674 annotation. Reason: Independent HDA evidence supporting RNA binding function. Consistent with other annotations for this molecular function. Supporting Evidence: file:human/API5/API5-deep-research-falcon.md API5 participates in nuclear FGF-2 complexes and has been implicated in mRNA export (TREX/eIF4E/CRM1 pathways) PMID:22681889 The mRNA-bound proteome and its global occupancy profile on protein-coding transcripts.
GO:0005634 nucleus	IDA PMID:11075807 FIF [fibroblast growth factor-2 (FGF-2)-interacting-factor],...	ACCEPT	Summary: Direct experimental evidence for nuclear localization from the FIF discovery paper. This is the primary localization study for API5. Reason: IDA evidence from the study that characterized FIF (API5) as an FGF-2 interacting factor. Nuclear localization is fundamental to API5's functions in apoptosis regulation and transcriptional control. Supporting Evidence: PMID:11075807 we have isolated a gene encoding a nuclear protein of 55 kDa, FIF (FGF-2-interacting-factor), which interacts specifically with FGF-2
GO:0043066 negative regulation of apoptotic process	TAS PMID:9307294 AAC-11, a novel cDNA that inhibits apoptosis after growth fa...	ACCEPT	Summary: TAS annotation based on the original AAC-11 discovery paper by Tewari et al. This seminal paper identified API5 through functional expression cloning as a gene that prevents apoptosis after growth factor withdrawal. Reason: The original discovery paper provides strong evidence for anti-apoptotic function. Cells expressing AAC-11 survived serum-free conditions for up to 12 weeks. The leucine zipper domain was shown to be essential for this protective function. Supporting Evidence: PMID:9307294 BALB/c3T3 fibroblasts that were stably transfected with AAC-11 cDNA were viable in serum-free medium for up to 12 weeks
GO:0043066 negative regulation of apoptotic process	ISS PMID:9307294 AAC-11, a novel cDNA that inhibits apoptosis after growth fa...	ACCEPT	Summary: ISS annotation based on sequence similarity to mouse Api5, with reference to the original AAC-11 paper showing conservation of anti-apoptotic function. Reason: API5 family is conserved across species and the anti-apoptotic function is preserved. The original paper noted "strong species conservation" of the AAC-11 gene. Supporting Evidence: PMID:9307294 The AAC-11 gene shows strong species conservation and is ubiquitously expressed in embryonic and adult tissues
GO:0005681 spliceosomal complex	ISS PMID:11555636 Large-scale identification of mammalian proteins localized t...	KEEP AS NON CORE	Summary: ISS annotation based on mouse Api5 localization to splicing speckles in the large-scale nuclear sub-compartment identification study. Reason: Association with spliceosomal complex/splicing speckles is supported by independent localization data but represents a secondary function or localization rather than API5's core anti-apoptotic role. Supporting Evidence: PMID:11555636 The most common discrete localizations detected are at the nucleolus and the splicing speckles
GO:0005737 cytoplasm	NAS PMID:11075807 FIF [fibroblast growth factor-2 (FGF-2)-interacting-factor],...	ACCEPT	Summary: NAS annotation for cytoplasmic localization. While API5 is predominantly nuclear, cytoplasmic localization exists, especially for isoform 3 and during stress-induced degradation. Reason: The FIF paper identified multiple isoforms with different localizations. While isoform 3 is specifically cytoplasmic, the full-length protein can shuttle to cytoplasm under certain conditions. Supporting Evidence: PMID:11075807 In contrast to the full-length FIF protein, several deletion constructs were able to transactivate a lac-Z reporter gene
GO:0017134 fibroblast growth factor binding	IPI PMID:11075807 FIF [fibroblast growth factor-2 (FGF-2)-interacting-factor],...	ACCEPT	Summary: IPI annotation for FGF2 binding. The FIF (API5) protein was originally identified as an FGF-2 interacting factor through yeast two-hybrid screening. The interaction is specific for FGF-2 over FGF-1, FGF-3, and FGF-6. Reason: FGF2 binding is well-documented and specific. Two regions of API5 (N-terminal aa 96-107 and C-terminal aa 274-311) are required for FGF2 binding. This interaction may contribute to API5's role in growth factor signaling and survival pathways. Supporting Evidence: PMID:11075807 The region spanning these repeats is, together with a region located in the N-terminal part of the FIF protein, implicated in the binding to FGF-2
GO:0043066 negative regulation of apoptotic process	NAS PMID:10780674 AAC-11 overexpression induces invasion and protects cervical...	ACCEPT	Summary: NAS annotation based on cervical cancer study showing AAC-11 overexpression protects cells from apoptosis. Cells survived serum-free conditions longer when expressing AAC-11. Reason: The study provides additional evidence for anti-apoptotic function in cancer cell context. AAC-11 transfected cells showed extended survival under serum-free conditions. Supporting Evidence: PMID:10780674 Approximately 50% of the AAC-11 transfected cells in serum-free medium died after 2 weeks, compared to 1 week for vector alone or wild-type cells
GO:0043066 negative regulation of apoptotic process	IMP PMID:11075807 FIF [fibroblast growth factor-2 (FGF-2)-interacting-factor],...	ACCEPT	Summary: IMP annotation from the FIF paper showing anti-apoptotic function through mutant phenotype analysis. The C-terminal portion of FIF was shown to have antiapoptotic properties. Reason: IMP evidence from functional studies. The paper discusses that the C-terminal part exhibits antiapoptotic properties, supporting the anti-apoptotic function of API5. Supporting Evidence: PMID:11075807 the COOH-terminal part, but not the full-length FIF protein, has previously been shown to exhibit antiapoptotic properties
GO:0043027 cysteine-type endopeptidase inhibitor activity involved in apoptotic process	IDA PMID:28336776 Apoptosis inhibitor 5 is an endogenous inhibitor of caspase-...	NEW	Summary: API5 directly binds the CARD domain of caspase-2, preventing its dimerization and activation. This is a key mechanism of API5's anti-apoptotic function distinct from Acinus protection [file:human/API5/API5-deep-research-falcon.md]. Reason: The deep research review identifies direct caspase-2 inhibition as a major mechanism of API5 anti-apoptotic function (Imre et al., EMBO Reports 2017, PMID:28336776). This specific molecular function is not captured by the general "negative regulation of apoptotic process" annotations. Supporting Evidence: file:human/API5/API5-deep-research-falcon.md API5 directly binds the CARD of caspase-2, preventing its dimerization/activation; recombinant API5 inhibits full-length but not processed caspase-2 PMID:28336776 Apoptosis inhibitor 5 is an endogenous inhibitor of caspase-2.

Core Functions

API5 inhibits apoptosis through multiple specific mechanisms including direct binding to caspase-2 CARD domain preventing caspase-2 dimerization and activation; binding to Acinus to protect it from caspase-3 cleavage, blocking DNA fragmentation; and suppression of E2F1-dependent apoptosis while promoting E2F1-driven G1/S gene transcription.

Molecular Function:

cysteine-type endopeptidase inhibitor activity involved in apoptotic process

Directly Involved In:

negative regulation of apoptotic process

Cellular Locations:

nucleus

Supporting Evidence:

PMID:9307294
BALB/c3T3 fibroblasts that were stably transfected with AAC-11 cDNA were viable in serum-free medium for up to 12 weeks
PMID:19387494
AAC-11 was able to protect Acinus from caspase-3 cleavage in vivo and in vitro

API5/FIF specifically binds FGF2 through two regions (N-terminal aa 96-107 and C-terminal aa 274-311). This interaction may contribute to growth factor signaling and survival pathway activation.

Molecular Function:

fibroblast growth factor binding

Cellular Locations:

nucleus

Supporting Evidence:

PMID:11075807
which interacts specifically with FGF-2 but not with FGF-1, FGF-3, or FGF-6

API5 localizes to nucleus via NLS (aa 454-475) where it performs scaffold functions through HEAT/ARM-like helical repeats, leucine zipper domain, and LxxLL motif. Associates with nuclear speckles and spliceosomal complexes.

Molecular Function:

RNA binding

Cellular Locations:

nucleus nuclear speck

Supporting Evidence:

PMID:11075807
we have isolated a gene encoding a nuclear protein of 55 kDa, FIF (FGF-2-interacting-factor)

References

GO_REF:0000033

Annotation inferences using phylogenetic trees

IBA annotations for nucleus, RNA binding, and negative regulation of apoptotic process

GO_REF:0000043

Gene Ontology annotation based on UniProtKB/Swiss-Prot keyword mapping

IEA annotation for apoptotic process (should be more specific)

GO_REF:0000044

Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location vocabulary mapping

Cytoplasm localization annotation

GO_REF:0000052

Gene Ontology annotation based on curation of immunofluorescence data

Nuclear speck localization from HPA

GO_REF:0000107

Automatic transfer of experimentally verified manual GO annotation data to orthologs using Ensembl Compara

Spliceosomal complex annotation transferred from mouse

GO_REF:0000117

Electronic Gene Ontology annotations created by ARBA machine learning models

Negative regulation of apoptotic process annotation

GO_REF:0000120

Combined Automated Annotation using Multiple IEA Methods

Nucleus localization annotation

PMID:9307294

AAC-11, a novel cDNA that inhibits apoptosis after growth factor withdrawal.

Original discovery of API5/AAC-11 as anti-apoptotic factor

"a cDNA whose expression supports cell viability after withdrawal of growth factors"
Leucine zipper domain essential for protective function

"The protective action of AAC-11 was abolished by mutation of leucines to arginines within the leucine zipper domain"
Strong species conservation

"The AAC-11 gene shows strong species conservation"
Cells survive serum withdrawal for up to 12 weeks when expressing AAC-11

"BALB/c3T3 fibroblasts that were stably transfected with AAC-11 cDNA were viable in serum-free medium for up to 12 weeks"

PMID:10780674

AAC-11 overexpression induces invasion and protects cervical cancer cells from apoptosis.

AAC-11 overexpression protects cervical cancer cells from apoptosis

"The effect of AAC-11 on cultured cervical cancer cells was associated with antiapoptotic process"
Extended survival under serum-free conditions

"Approximately 50% of the AAC-11 transfected cells in serum-free medium died after 2 weeks"
Also promotes invasion (secondary function)

"Invasion of matrigel and adhesion to laminin by AAC-11 transfected CUMC-6 cells were increased"

PMID:11075807

FIF [fibroblast growth factor-2 (FGF-2)-interacting-factor], a nuclear putatively antiapoptotic factor, interacts specifically with FGF-2.

Identified FIF (API5) as nuclear 55 kDa protein

"we have isolated a gene encoding a nuclear protein of 55 kDa, FIF (FGF-2-interacting-factor)"
Specific interaction with FGF-2 but not FGF-1, FGF-3, or FGF-6

"which interacts specifically with FGF-2 but not with FGF-1, FGF-3, or FGF-6"
C-terminal part has antiapoptotic properties

"the COOH-terminal part, but not the full-length FIF protein, has previously been shown to exhibit antiapoptotic properties"
Multiple isoforms with different localizations

"In contrast to the full-length FIF protein, several deletion constructs were able to transactivate a lac-Z reporter gene"

PMID:11555636

Large-scale identification of mammalian proteins localized to nuclear sub-compartments.

Mouse Api5 localizes to splicing speckles

"The most common discrete localizations detected are at the nucleolus and the splicing speckles"

PMID:19387494

The antiapoptotic protein AAC-11 interacts with and regulates Acinus-mediated DNA fragmentation.

AAC-11 binds Acinus and prevents DNA fragmentation

"AAC-11, a survival protein whose expression prevents apoptosis that occurs on deprivation of growth factors, physiologically binds to Acinus and prevents Acinus-mediated DNA fragmentation"
Protects Acinus from caspase-3 cleavage

"AAC-11 was able to protect Acinus from caspase-3 cleavage in vivo and in vitro"
Leucine zipper domain required for Acinus binding

"AAC-11 possesses a leucine-zipper domain that dictates, upon oligomerization, its interaction with Acinus"
AAC-11 depletion sensitizes cells to chemotherapy

"AAC-11 depletion markedly increased cellular sensitivity to anticancer drugs"

PMID:19946888

Defining the membrane proteome of NK cells.

High-throughput proteomics detected API5 in membrane fraction

"Mass spectrometric analysis identified 1843 proteins with high confidence scores"

PMID:22658674

Insights into RNA biology from an atlas of mammalian mRNA-binding proteins.

API5 detected as mRNA-binding protein

"We identify 860 proteins that qualify as RBPs by biochemical and statistical criteria, adding more than 300 RBPs to those previously known"

PMID:22681889

The mRNA-bound proteome and its global occupancy profile on protein-coding transcripts.

API5 detected in mRNA-bound proteome

"Application to a human embryonic kidney cell line identified close to 800 proteins"

PMID:28336776

Apoptosis inhibitor 5 is an endogenous inhibitor of caspase-2.

API5 directly binds caspase-2 CARD domain

"API5 protein directly binds to the caspase recruitment domain (CARD) of caspase-2"
Prevents caspase-2 dimerization and activation

"impedes dimerization and activation of caspase-2"
Key mechanism of anti-apoptotic function

"recombinant API5 directly inhibits full length but not processed caspase-2"

PMID:30021884

Histone Interaction Landscapes Visualized by Crosslinking Mass Spectrometry in Intact Cell Nuclei.

API5-DDX39B interaction detected

"we use crosslinking mass spectrometry (XL-MS) to chart the protein-protein interactions in intact human nuclei"

PMID:35271311

OpenCell: Endogenous tagging for the cartography of human cellular organization.

API5-DDX39B interaction confirmed

"We combined genome engineering, confocal live-cell imaging, mass spectrometry, and data science to systematically map the localization and interactions of human proteins"

file:human/API5/API5-deep-research-falcon.md

Deep research summary for API5

Comprehensive review of API5 anti-apoptotic mechanisms

"API5 is a nuclear scaffold protein that inhibits apoptosis through multiple mechanisms"

Suggested Experiments

Experiment: Structure-function analysis of API5 domains to dissect contributions of caspase-2 binding, Acinus binding, and FGF2 binding to overall anti-apoptotic function.

Experiment: Systematic analysis of API5 interactome during apoptotic stress vs. normal conditions to identify context-dependent interaction partners.

📚 Additional Documentation

Deep Research Falcon

(API5-deep-research-falcon.md)

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gene_id: API5
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uniprot_accession: Q9BZZ5
protein_description: 'RecName: Full=Apoptosis inhibitor 5 {ECO:0000305}; Short=API-5;
AltName: Full=Antiapoptosis clone 11 protein; Short=AAC-11; AltName: Full=Cell
migration-inducing gene 8 protein; AltName: Full=Fibroblast growth factor 2-interacting
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gene_info: Name=API5 {ECO:0000312|HGNC:HGNC:594}; ORFNames=MIG8;
organism_full: Homo sapiens (Human).
protein_family: Belongs to the API5 family. .
protein_domains: API5. (IPR008383); ARM-like. (IPR011989); ARM-type_fold. (IPR016024);
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Question

Gene Research for Functional Annotation

⚠️ CRITICAL: Gene/Protein Identification Context

BEFORE YOU BEGIN RESEARCH: You MUST verify you are researching the CORRECT gene/protein. Gene symbols can be ambiguous, especially for less well-characterized genes from non-model organisms.

Target Gene/Protein Identity (from UniProt):

UniProt Accession: Q9BZZ5
Protein Description: RecName: Full=Apoptosis inhibitor 5 {ECO:0000305}; Short=API-5; AltName: Full=Antiapoptosis clone 11 protein; Short=AAC-11; AltName: Full=Cell migration-inducing gene 8 protein; AltName: Full=Fibroblast growth factor 2-interacting factor; Short=FIF; AltName: Full=Protein XAGL;
Gene Information: Name=API5 {ECO:0000312|HGNC:HGNC:594}; ORFNames=MIG8;
Organism (full): Homo sapiens (Human).
Protein Family: Belongs to the API5 family. .
Key Domains: API5. (IPR008383); ARM-like. (IPR011989); ARM-type_fold. (IPR016024); API5 (PF05918)

MANDATORY VERIFICATION STEPS:

Check if the gene symbol "API5" matches the protein description above
Verify the organism is correct: Homo sapiens (Human).
Check if protein family/domains align with what you find in literature
If you find literature for a DIFFERENT gene with the same or similar symbol, STOP

If Gene Symbol is Ambiguous or You Cannot Find Relevant Literature:

DO NOT PROCEED WITH RESEARCH ON A DIFFERENT GENE. Instead:
- State clearly: "The gene symbol 'API5' is ambiguous or literature is limited for this specific protein"
- Explain what you found (e.g., "Found extensive literature on a different gene with the same symbol in a different organism")
- Describe the protein based ONLY on the UniProt information provided above
- Suggest that the protein function can be inferred from domain/family information

Research Target:

Please provide a comprehensive research report on the gene API5 (gene ID: API5, UniProt: Q9BZZ5) in human.

The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.

You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.

We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.

We are interested in where in or outside the cell the gene product carries out its function.

We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.

Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.

Output

Question: You are an expert researcher providing comprehensive, well-cited information.

Provide detailed information focusing on:
1. Key concepts and definitions with current understanding
2. Recent developments and latest research (prioritize 2023-2024 sources)
3. Current applications and real-world implementations
4. Expert opinions and analysis from authoritative sources
5. Relevant statistics and data from recent studies

Format as a comprehensive research report with proper citations. Include URLs and publication dates where available.
Always prioritize recent, authoritative sources and provide specific citations for all major claims.

Gene Research for Functional Annotation

⚠️ CRITICAL: Gene/Protein Identification Context

BEFORE YOU BEGIN RESEARCH: You MUST verify you are researching the CORRECT gene/protein. Gene symbols can be ambiguous, especially for less well-characterized genes from non-model organisms.

Target Gene/Protein Identity (from UniProt):

UniProt Accession: Q9BZZ5
Protein Description: RecName: Full=Apoptosis inhibitor 5 {ECO:0000305}; Short=API-5; AltName: Full=Antiapoptosis clone 11 protein; Short=AAC-11; AltName: Full=Cell migration-inducing gene 8 protein; AltName: Full=Fibroblast growth factor 2-interacting factor; Short=FIF; AltName: Full=Protein XAGL;
Gene Information: Name=API5 {ECO:0000312|HGNC:HGNC:594}; ORFNames=MIG8;
Organism (full): Homo sapiens (Human).
Protein Family: Belongs to the API5 family. .
Key Domains: API5. (IPR008383); ARM-like. (IPR011989); ARM-type_fold. (IPR016024); API5 (PF05918)

MANDATORY VERIFICATION STEPS:

Check if the gene symbol "API5" matches the protein description above
Verify the organism is correct: Homo sapiens (Human).
Check if protein family/domains align with what you find in literature
If you find literature for a DIFFERENT gene with the same or similar symbol, STOP

If Gene Symbol is Ambiguous or You Cannot Find Relevant Literature:

Research Target:

Please provide a comprehensive research report on the gene API5 (gene ID: API5, UniProt: Q9BZZ5) in human.

The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.

We are interested in where in or outside the cell the gene product carries out its function.

We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.

Research report: Human API5 (AAC-11/FIF; UniProt Q9BZZ5)

Executive verification of identity
- Gene/protein: API5 (apoptosis inhibitor 5), also known as AAC-11 and FIF, matches UniProt Q9BZZ5 and is described as a ~55–58 kDa, largely nuclear, helical repeat protein that functions as a scaffold; it lacks catalytic activity and belongs to the API5 family. Structural analyses and curated reviews describe an elongated, all-helical architecture with N‑terminal HEAT-like and C‑terminal ARM-like repeats, plus an LxxLL motif, a heptad leucine-repeat/leucine zipper segment, and a nuclear localization sequence, consistent with InterPro entries API5/ARM-like folds provided in the prompt (IPR008383; IPR011989; IPR016024; PF05918). Organism: Homo sapiens. Synonyms (AAC‑11/FIF) are consistently used across the literature and map to human API5. Stop conditions for symbol ambiguity are not triggered. (abbas2024apoptosisinhibitor5 pages 2-4, sharma2021interplaybetweenp300 pages 1-2)

1) Key concepts and definitions with current understanding
- Primary role: API5 is a nuclear scaffold protein that inhibits apoptosis through multiple mechanisms and coordinates transcriptional and post-transcriptional programs affecting cell-cycle progression and stress responses. It is not an enzyme or transporter; its activity is mediated by modular helical-repeat interfaces that bind partner proteins. (abbas2024apoptosisinhibitor5 pages 2-4, sharma2021interplaybetweenp300 pages 1-2)
- Canonical anti-apoptotic mechanisms:
• Caspase-2: API5 directly binds the CARD of caspase-2, preventing its dimerization/activation; recombinant API5 inhibits full-length but not processed caspase-2, and API5 depletion increases caspase-2 activation and sensitizes cells to caspase‑2–dependent death (EMBO Reports, 2017; DOI 10.15252/embr.201643744). URL: https://doi.org/10.15252/embr.201643744 (published online 23 Mar 2017). (imre2017apoptosisinhibitor5 pages 1-3)
• Acinus/DNA fragmentation: API5 (AAC‑11) binds Acinus, protects it from caspase‑3 cleavage, and blocks Acinus-mediated apoptotic DNA fragmentation; an AAC‑11 LZ-mimicking peptide disrupts the complex and potentiates chemotherapy-induced death (EMBO J., 2009). URL: https://doi.org/10.1038/emboj.2009.106 (published online 23 Apr 2009). (rigou2009theantiapoptoticprotein pages 1-2)
• E2F1 axis: API5 suppresses E2F1‑dependent apoptosis while positively contributing to E2F1 transcriptional activation of G1/S genes by facilitating E2F1 promoter occupancy, thus promoting proliferation (PLoS ONE, 2013; DOI 10.1371/journal.pone.0071443). URL: https://doi.org/10.1371/journal.pone.0071443 (published 7 Aug 2013). (navarro2013api5contributesto pages 1-2, abbas2024apoptosisinhibitor5 pages 7-8, abbas2024apoptosisinhibitor5 pages 4-6)
- Post-translational regulation:
• Acetylation: K251 acetylation by p300 stabilizes API5; HDAC1 reverses this mark, lowering stability and altering localization; API5 levels peak in G1/G1‑S and support cell-cycle progression (Scientific Reports, 2021; DOI 10.1038/s41598-021-95941-4). URL: https://doi.org/10.1038/s41598-021-95941-4 (published 12 Aug 2021). (sharma2021interplaybetweenp300 pages 1-2)
• DNA-damage turnover: Upon genotoxic stress, ATR phosphorylates API5 (reported S138), promoting cytoplasmic relocalization and SCF‑FBXW2–mediated ubiquitination and proteasomal degradation (bioRxiv preprint, 8 Aug 2021; DOI 10.1101/2021.08.08.455545). URL: https://doi.org/10.1101/2021.08.08.455545 (posted 8 Aug 2021). (sharma2021atrfacilitatesthe pages 1-4, sharma2021atrfacilitatesthe pages 31-33)
- Subcellular localization: Predominantly nuclear; acetylation status and stress signaling modulate stability and nucleocytoplasmic distribution (p300/HDAC1; ATR–FBXW2 axis). (sharma2021interplaybetweenp300 pages 1-2, sharma2021atrfacilitatesthe pages 1-4)
- Additional cellular programs: API5 participates in nuclear FGF-2 complexes and has been implicated in mRNA export (TREX/eIF4E/CRM1 pathways) in some reports; it also acts as a cofactor for ERα via its LxxLL “NR box,” influencing estrogen-responsive transcription. (abbas2024apoptosisinhibitor5 pages 7-8, basset2017api5anew pages 1-2)

2) Recent developments and latest research (2023–2024 priority)
- 2024 review synthesis: A comprehensive review (Biomolecules, 2024) consolidates API5’s structural modules (HEAT/ARM-like repeats, LxxLL, heptad leucine repeat), anti-apoptotic mechanisms (Acinus, caspase-2, E2F1), post-translational regulation (K251 acetylation; SUMOylation), and oncologic relevance (aggressive behavior, resistance, prognosis). It also summarizes links to innate immune signaling (TLR4) and viral interference with API5 SUMOylation that dampens IFN-β responses. URL: https://doi.org/10.3390/biom14010136 (published Jan 2024). (abbas2024apoptosisinhibitor5 pages 2-4, abbas2024apoptosisinhibitor5 pages 7-8, abbas2024apoptosisinhibitor5 pages 4-6, abbas2024apoptosisinhibitor5 pages 10-11)
- 2023 primary data (breast cancer): In BMC Cancer (2023), perturbation of Api5 in breast models showed that overexpression drives proliferation, EMT-like phenotypes, migration, and polarity disruption; mechanistically, FGF2-driven PDK1–AKT/cMYC and RAS–ERK signaling were implicated; Api5 knockdown reduced FGF2 signaling and in vivo tumorigenicity. URL: https://doi.org/10.1186/s12885-023-10866-7 (published 21 Apr 2023). (kuttanamkuzhi2023alteredexpressionof pages 1-2)
- 2022–2024 immunology: Nature (2022) identified API5 as a protective γδ intraepithelial lymphocyte effector; recombinant API5 protected Paneth cells ex vivo/in vivo and masked genetic susceptibility (ATG16L1 T300A), highlighting a secreted, tissue-protective role for API5 beyond tumor biology. URL: https://doi.org/10.1038/s41586-022-05259-y (published 12 Oct 2022; PMC 2023). (matsuzawaishimoto2022theγδiel pages 1-3)

3) Current applications and real-world implementations
- Peptide therapeutics derived from AAC‑11 (API5) heptad leucine repeat:
• RT53 peptide (AAC‑11 LZ fused to a cell-penetrating sequence) selectively kills cancer cells, induces bona fide immunogenic cell death (ICD) marked by calreticulin exposure, ATP and HMGB1 release, and elicits antitumor immunity in vaccination and intratumoral settings (PLoS ONE, 2018). URL: https://doi.org/10.1371/journal.pone.0201220 (published 6 Aug 2018). (pasquereaukotula2018theanticancerpeptide pages 1-2)
• Clinical/preclinical relevance in chemoresistance and angiogenesis: In TNBC patient-derived xenografts, an “anti‑API‑5” peptide inhibited growth of a chemoresistant model via caspase‑3 activation and anti‑angiogenic effects; in 78 TNBC biopsies, higher endothelial API5 associated with non‑pCR and greater microvessel density (Oncotarget, 2019). URL: https://doi.org/10.18632/oncotarget.27312 (published 12 Nov 2019). (bousquet2019highexpressionof pages 1-2)
• HIV targeting: AAC‑11–derived LZ peptides preferentially killed metabolically active, HIV‑susceptible CD4+ T cells in vitro, rendering surviving cells resistant to HIV replication, suggesting a strategy to deplete “permissive” reservoirs (J. Virol., 2020). URL: https://doi.org/10.1128/JVI.00611-20 (published 24 Jun 2020). ()
- Vaccine adjuvancy (human DAMP): Purified API5 activates/matures dendritic cells through TLR4–NF‑κB, enhancing antigen‑specific CD8+ T‑cell responses and tumor protection in mice, positioning API5 as a human-origin adjuvant candidate for DC vaccines (OncoImmunology, 2018). URL: https://doi.org/10.1080/2162402X.2018.1472187 (published online 15 Aug 2018). (kim2018anovelfunction pages 1-3)

4) Expert opinions and analysis from authoritative sources
- Mechanistic anti-apoptotic schema: Recent reviews emphasize four complementary API5 mechanisms—(i) blocking Acinus-mediated DNA fragmentation; (ii) direct inhibition of initiator caspase-2; (iii) suppressing E2F1‑dependent apoptosis while promoting E2F1-driven G1/S gene expression; and (iv) enforcing survival via FGF2→FGFR1→ERK/PKCδ signaling that degrades BIM—collectively explaining chemoresistance and immune escape. (abbas2024apoptosisinhibitor5 pages 4-6, imre2017apoptosisinhibitor5 pages 1-3, navarro2013api5contributesto pages 1-2, abbas2024apoptosisinhibitor5 pages 10-11, jang2017api5inducescisplatin pages 1-2)
- Structural/biophysical rationale: HEAT/ARM-like repeats and an LZ region provide adaptable protein–protein interaction surfaces, consistent with API5’s role as a multipartner scaffold (FGF2, Acinus, ERα, caspase-2), a view reiterated in 2024 synthesis. (abbas2024apoptosisinhibitor5 pages 2-4, sharma2021interplaybetweenp300 pages 1-2)
- Regulation under stress: The 2021 studies collectively suggest a regulatory model where p300/HDAC1 set a basal stability/localization state via K251 acetylation, while ATR–FBXW2 mediates degradation upon DNA damage—consistent with API5 acting as a survival determinant that is actively removed to permit apoptotic execution after genotoxic stress (preprint caveat noted). (sharma2021interplaybetweenp300 pages 1-2, sharma2021atrfacilitatesthe pages 1-4, sharma2021atrfacilitatesthe pages 31-33)

5) Relevant statistics and data from recent studies
- Breast cancer (2023): In silico analyses (TCGA, GENT2) linked elevated API5 transcripts with poorer prognosis; experimental overexpression in 3D MCF10A acini increased proliferation and motility and induced partial EMT‑like features; API5 knockdown reduced proliferation and in vivo tumorigenicity in malignant models (BMC Cancer, 2023; detailed hazard ratios not provided in the excerpt; study provides mechanistic and phenotypic effect sizes across models). URL: https://doi.org/10.1186/s12885-023-10866-7 (21 Apr 2023). (kuttanamkuzhi2023alteredexpressionof pages 1-2)
- Cervical cancer (2014): Tissue microarray (173 primary cancers, 306 CINs, 429 normals) showed stepwise API5 increase during progression; API5 correlated with stage (P=0.004), grade (P<0.001), chemo‑radiation response (P=0.004), and pERK1/2; multivariate: API5+ (P=0.039) and combined API5+/pERK1/2+ (P=0.032) were independent predictors of OS (BMC Cancer, 2014). URL: https://doi.org/10.1186/1471-2407-14-545 (published 29 Jul 2014). (cho2014apoptosisinhibitor5overexpression pages 1-2)
- TNBC cohort and xenografts (2019): In 78 TNBC biopsies, higher endothelial API5 in non‑pCR vs pCR (70% vs 40% positive endothelial cells; p=0.01); higher microvessel density in non‑pCR (17% vs 7%; p<0.05). Anti‑API5 peptide therapy in resistant PDXs achieved tumor inhibition with caspase‑3 activation and decreased angiogenesis (Oncotarget, 2019). URL: https://doi.org/10.18632/oncotarget.27312 (12 Nov 2019). (bousquet2019highexpressionof pages 1-2)
- Cisplatin resistance (2017): API5high cells acquired cisplatin resistance via FGFR1 signaling that degraded BIM; FGFR1 inhibition (siRNA or SSR128129E) restored sensitivity in vitro and in xenografts (Exp. Mol. Med., 2017; published 8 Sep 2017). URL: https://doi.org/10.1038/emm.2017.130. (jang2017api5inducescisplatin pages 1-2)
- Metastasis (2015): API5 promoted ERK‑dependent MMP-9 activity, invasion, and pulmonary metastasis; mutating critical leucines in the LZ region abrogated these effects (BMB Reports, 2015; 18 Jun 2015). URL: https://doi.org/10.5483/bmbrep.2015.48.6.139. (jang2017api5inducescisplatin pages 1-2)
- ERα cofactor and prognosis (2017): In ERα+ breast cancer meta-analysis (n=1228), high Api5 associated with shorter recurrence-free survival (HR≈1.91, 95% CI 1.57–2.33; p=8.4×10−11); Api5 engages ERα via LxxLL to coactivate estrogen-responsive genes and promotes tumorigenicity in xenografts (Oncotarget, 2017). URL: https://doi.org/10.18632/oncotarget.17281 (20 Apr 2017). (basset2017api5anew pages 1-2)
- Immune biology (2022): Recombinant API5 protected Paneth cells in vivo and in human organoids carrying ATG16L1 risk allele, defining a protective γδ IEL effector function with therapeutic implications in intestinal barrier diseases (Nature, 2022). URL: https://doi.org/10.1038/s41586-022-05259-y (12 Oct 2022). (matsuzawaishimoto2022theγδiel pages 1-3)

Functional pathways and precise molecular placement
- Apoptosis control: API5 acts upstream of apoptotic execution by preventing caspase‑2 activation (initiator-like), shielding Acinus from caspase‑3 cleavage (blocks DNA laddering), and dampening E2F1‑driven apoptotic programs while maintaining cell-cycle gene transcription—integrating transcriptional control with caspase regulation. (imre2017apoptosisinhibitor5 pages 1-3, rigou2009theantiapoptoticprotein pages 1-2, navarro2013api5contributesto pages 1-2, abbas2024apoptosisinhibitor5 pages 4-6)
- Growth factor signaling: API5 elevates FGF2 and activates FGFR1→PKCδ→ERK signaling, promoting survival by BIM degradation; in multiple models, API5 also engages PI3K–AKT/cMYC via FGF2, linking it to proliferation and stemness programs (Kuttanamkuzhi 2023; Jang 2017). (kuttanamkuzhi2023alteredexpressionof pages 1-2, jang2017api5inducescisplatin pages 1-2, abbas2024apoptosisinhibitor5 pages 10-11)
- MAPK/ERK and metastasis: API5’s LZ region is required for ERK activation that drives MMP‑9 and invasion/metastasis; this provides a structural–functional bridge to pro-metastatic signaling. (jang2017api5inducescisplatin pages 1-2)
- Hormone signaling: Through an LxxLL NR box, API5 binds ERα (C domain), coactivates estrogen-responsive genes upon E2 stimulation, and predicts poorer RFS in ERα+ cohorts, marking an ER‑modulating scaffold function. (basset2017api5anew pages 1-2)
- Innate and adaptive immunity: API5 can function as a DAMP-like signal engaging TLR4 on DCs to induce NF‑κB activation, DC maturation, and antigen-specific CD8+ T cells in vivo; viral proteins can subvert API5 SUMOylation to reduce IFN‑β signaling, and influenza A NP antagonizes API5 to enhance E2F1‑dependent apoptosis for viral replication (OncoImmunology 2018; summarized in 2024 review). (kim2018anovelfunction pages 1-3, abbas2024apoptosisinhibitor5 pages 10-11)

Subcellular localization and context dependence
- Basal state: predominantly nuclear, chromatin-associated, with acetylation at K251 stabilizing the protein and supporting nuclear functions in transcription/mRNA export. (sharma2021interplaybetweenp300 pages 1-2, abbas2024apoptosisinhibitor5 pages 7-8)
- Stress state: DNA damage shifts API5 toward cytoplasmic degradation via ATR–FBXW2–proteasome, functionally coupling checkpoint activation to relief of API5-mediated apoptosis suppression (preprint). (sharma2021atrfacilitatesthe pages 1-4, sharma2021atrfacilitatesthe pages 31-33)

Recent implementation notes and translational opportunities
- Direct targeting of API5 interactions: AAC‑11 LZ‑mimetic peptides (e.g., RT53) disrupt API5 complexes and have demonstrated cancer-selective cytotoxicity and ICD, supporting development of peptides or small molecules that destabilize API5 scaffolding. (pasquereaukotula2018theanticancerpeptide pages 1-2, rigou2009theantiapoptoticprotein pages 1-2)
- Pathway-interdiction strategies downstream of API5: In API5high cancers, FGFR1 blockade resensitizes to cisplatin; ERK/MMP axes and ERα coactivation may provide additional combinatorial targets. (jang2017api5inducescisplatin pages 1-2, basset2017api5anew pages 1-2)
- Immunotherapeutic adjuvants: API5 protein fragments proximal to the LZ region act as TLR4-dependent DC adjuvants in murine vaccine models, suggesting a human-source adjuvant paradigm requiring safety/translation studies. (kim2018anovelfunction pages 1-3)
- Non-oncology: The γδ IEL–API5 axis indicates potential for recombinant API5 to protect epithelial barrier cell types (e.g., Paneth cells) where genetic susceptibility and viral infections intersect—an emerging therapeutic angle in IBD-like conditions. (matsuzawaishimoto2022theγδiel pages 1-3)

Limitations and open questions
- Some regulatory findings (ATR–FBXW2 pathway) are from a 2021 preprint and require peer‑reviewed confirmation; nevertheless, they fit with established p300/HDAC1 control of API5 stability/localization. (sharma2021atrfacilitatesthe pages 1-4, sharma2021atrfacilitatesthe pages 31-33, sharma2021interplaybetweenp300 pages 1-2)
- While API5 is frequently overexpressed in cancers with adverse associations, prospective clinical validation and standardized assays are needed for use as a prognostic or predictive biomarker.

References with URLs and dates (selection used above)
- Abbas H. et al., Biomolecules 2024; Apoptosis Inhibitor 5: A Multifaceted Regulator of Cell Fate. URL: https://doi.org/10.3390/biom14010136 (Jan 2024). (abbas2024apoptosisinhibitor5 pages 2-4, abbas2024apoptosisinhibitor5 pages 7-8, abbas2024apoptosisinhibitor5 pages 4-6, abbas2024apoptosisinhibitor5 pages 10-11)
- Kuttanamkuzhi A. et al., BMC Cancer 2023; Altered expression of anti-apoptotic protein Api5 affects breast tumorigenesis. URL: https://doi.org/10.1186/s12885-023-10866-7 (Apr 2023). (kuttanamkuzhi2023alteredexpressionof pages 1-2)
- Matsuzawa-Ishimoto Y. et al., Nature 2022; γδ IEL effector API5 masks genetic susceptibility to Paneth cell death. URL: https://doi.org/10.1038/s41586-022-05259-y (Oct 2022). (matsuzawaishimoto2022theγδiel pages 1-3)
- Sharma V.K. & Lahiri M., Sci. Reports 2021; p300/HDAC1 regulate K251 acetylation and API5 stability/localization. URL: https://doi.org/10.1038/s41598-021-95941-4 (Aug 2021). (sharma2021interplaybetweenp300 pages 1-2)
- Sharma V.K. et al., bioRxiv 2021; ATR–FBXW2 mediates DNA damage-induced API5 degradation. URL: https://doi.org/10.1101/2021.08.08.455545 (Aug 2021). (sharma2021atrfacilitatesthe pages 1-4, sharma2021atrfacilitatesthe pages 31-33)
- Imre G. et al., EMBO Reports 2017; API5 is an endogenous inhibitor of caspase‑2. URL: https://doi.org/10.15252/embr.201643744 (Mar 2017). (imre2017apoptosisinhibitor5 pages 1-3)
- Rigou P. et al., EMBO J. 2009; AAC‑11 regulates Acinus-mediated DNA fragmentation. URL: https://doi.org/10.1038/emboj.2009.106 (Apr 2009). (rigou2009theantiapoptoticprotein pages 1-2)
- Navarro M.G.-J. et al., PLoS ONE 2013; Api5 contributes to E2F1 control of G1/S. URL: https://doi.org/10.1371/journal.pone.0071443 (Aug 2013). (navarro2013api5contributesto pages 1-2)
- Jang H.S. et al., Exp Mol Med 2017; API5 induces cisplatin resistance via FGFR1/BIM. URL: https://doi.org/10.1038/emm.2017.130 (Sep 2017). (jang2017api5inducescisplatin pages 1-2)
- Cho H. et al., BMC Cancer 2014; API5 overexpression predicts poor prognosis in cervical cancer. URL: https://doi.org/10.1186/1471-2407-14-545 (Jul 2014). (cho2014apoptosisinhibitor5overexpression pages 1-2)
- Bousquet G. et al., Oncotarget 2019; High API5 in chemoresistant TNBC; peptide efficacy. URL: https://doi.org/10.18632/oncotarget.27312 (Nov 2019). (bousquet2019highexpressionof pages 1-2)
- Pasquereau‑Kotula E. et al., PLoS ONE 2018; RT53 induces immunogenic cell death. URL: https://doi.org/10.1371/journal.pone.0201220 (Aug 2018). (pasquereaukotula2018theanticancerpeptide pages 1-2)
- Kim Y.S. et al., OncoImmunology 2018; API5 as TLR4‑dependent DC adjuvant. URL: https://doi.org/10.1080/2162402X.2018.1472187 (Aug 2018). (kim2018anovelfunction pages 1-3)
- Basset C. et al., Oncotarget 2017; Api5 is an ERα cofactor; RFS meta-analysis. URL: https://doi.org/10.18632/oncotarget.17281 (Apr 2017). (basset2017api5anew pages 1-2)

Conclusion
Human API5 (AAC‑11/FIF; Q9BZZ5) is a conserved, nuclear scaffold with HEAT/ARM-like repeats and regulatory motifs (LxxLL, LZ, NLS) that orchestrates multiple anti-apoptotic mechanisms and pro-survival transcriptional programs. Recent peer‑reviewed and 2023–2024 literature reinforce API5’s placement as a nexus connecting caspase control (caspase‑2; Acinus), E2F1-driven cell-cycle regulation, growth factor signaling (FGF2/FGFR1→ERK/AKT), hormone receptor coactivation (ERα), and immune modulation (TLR4 activation; γδ IEL‑mediated epithelial protection). Translational progress includes AAC‑11–derived peptides (e.g., RT53) inducing ICD and resensitization strategies targeting FGFR1 in API5high tumors. Outstanding questions include full validation of the ATR–FBXW2 turnover pathway and prospective biomarker qualification, but API5 remains a compelling functional node and therapeutic target in oncology and mucosal immunobiology. (imre2017apoptosisinhibitor5 pages 1-3, rigou2009theantiapoptoticprotein pages 1-2, navarro2013api5contributesto pages 1-2, jang2017api5inducescisplatin pages 1-2, cho2014apoptosisinhibitor5overexpression pages 1-2, bousquet2019highexpressionof pages 1-2, abbas2024apoptosisinhibitor5 pages 2-4, kuttanamkuzhi2023alteredexpressionof pages 1-2, matsuzawaishimoto2022theγδiel pages 1-3, sharma2021interplaybetweenp300 pages 1-2, kim2018anovelfunction pages 1-3, sharma2021atrfacilitatesthe pages 1-4, sharma2021atrfacilitatesthe pages 31-33)

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(kim2018anovelfunction pages 1-3): Young Seob Kim, Hyun Jin Park, Jung Hwa Park, Eun Ji Hong, Gun-Young Jang, In Duk Jung, Hee Dong Han, Seung-Hyun Lee, Manh-Cuong Vo, Je-Jung Lee, Andrew Yang, Emily Farmer, T.-C. Wu, Tae Heung Kang, and Yeong-Min Park. A novel function of api5 (apoptosis inhibitor 5), tlr4-dependent activation of antigen presenting cells. OncoImmunology, 7:e1472187, Aug 2018. URL: https://doi.org/10.1080/2162402x.2018.1472187, doi:10.1080/2162402x.2018.1472187. This article has 20 citations and is from a domain leading peer-reviewed journal.
(jang2017api5inducescisplatin pages 1-2): Han Sol Jang, Seon Rang Woo, Kwon-Ho Song, Hanbyoul Cho, Doo Byung Chay, Soon-Oh Hong, Hyo-Jung Lee, Se Jin Oh, Joon-Yong Chung, Jae-Hoon Kim, and Tae Woo Kim. Api5 induces cisplatin resistance through fgfr signaling in human cancer cells. Experimental & Molecular Medicine, 49:e374-e374, Sep 2017. URL: https://doi.org/10.1038/emm.2017.130, doi:10.1038/emm.2017.130. This article has 27 citations and is from a peer-reviewed journal.
(cho2014apoptosisinhibitor5overexpression pages 1-2): Hanbyoul Cho, Joon-Yong Chung, Kwon-Ho Song, Kyung Hee Noh, Bo Wook Kim, Eun Joo Chung, Kris Ylaya, Jin Hee Kim, Tae Woo Kim, Stephen M Hewitt, and Jae-Hoon Kim. Apoptosis inhibitor-5 overexpression is associated with tumor progression and poor prognosis in patients with cervical cancer. BMC Cancer, Jul 2014. URL: https://doi.org/10.1186/1471-2407-14-545, doi:10.1186/1471-2407-14-545. This article has 50 citations and is from a peer-reviewed journal.

Citations

rigou2009theantiapoptoticprotein pages 1-2
kuttanamkuzhi2023alteredexpressionof pages 1-2
pasquereaukotula2018theanticancerpeptide pages 1-2
bousquet2019highexpressionof pages 1-2
kim2018anovelfunction pages 1-3
sharma2021atrfacilitatesthe pages 1-4
sharma2021atrfacilitatesthe pages 31-33
https://doi.org/10.15252/embr.201643744
https://doi.org/10.1038/emboj.2009.106
https://doi.org/10.1371/journal.pone.0071443
https://doi.org/10.1038/s41598-021-95941-4
https://doi.org/10.1101/2021.08.08.455545
https://doi.org/10.3390/biom14010136
https://doi.org/10.1186/s12885-023-10866-7
https://doi.org/10.1038/s41586-022-05259-y
https://doi.org/10.1371/journal.pone.0201220
https://doi.org/10.18632/oncotarget.27312
https://doi.org/10.1128/JVI.00611-20
https://doi.org/10.1080/2162402X.2018.1472187
https://doi.org/10.1186/1471-2407-14-545
https://doi.org/10.1038/emm.2017.130.
https://doi.org/10.5483/bmbrep.2015.48.6.139.
https://doi.org/10.18632/oncotarget.17281
https://doi.org/10.1038/emm.2017.130
https://doi.org/10.3390/biom14010136,
https://doi.org/10.1038/s41598-021-95941-4,
https://doi.org/10.15252/embr.201643744,
https://doi.org/10.1038/emboj.2009.106,
https://doi.org/10.1371/journal.pone.0071443,
https://doi.org/10.1101/2021.08.08.455545,
https://doi.org/10.18632/oncotarget.17281,
https://doi.org/10.1186/s12885-023-10866-7,
https://doi.org/10.1038/s41586-022-05259-y,
https://doi.org/10.1371/journal.pone.0201220,
https://doi.org/10.18632/oncotarget.27312,
https://doi.org/10.1080/2162402x.2018.1472187,
https://doi.org/10.1038/emm.2017.130,
https://doi.org/10.1186/1471-2407-14-545,

📄 View Raw YAML

id: Q9BZZ5
gene_symbol: API5
product_type: PROTEIN
status: COMPLETE
taxon:
  id: NCBITaxon:9606
  label: Homo sapiens
description: >-
  API5 (Apoptosis Inhibitor 5, also known as AAC-11 and FIF) is a nuclear scaffold
  protein
  with HEAT/ARM-like helical repeat architecture that functions as a multi-mechanism
  anti-apoptotic factor. The protein lacks catalytic activity but mediates survival
  through
  protein-protein interactions via its modular helical repeat interfaces, leucine
  zipper (LZ)
  domain, and LxxLL motif. Core anti-apoptotic mechanisms include: (1) direct binding
  to the
  CARD domain of caspase-2, preventing its dimerization and activation; (2) binding
  to Acinus
  and protecting it from caspase-3 cleavage, thereby blocking apoptotic DNA fragmentation;
  (3) suppression of E2F1-dependent apoptosis while promoting E2F1-driven G1/S gene
  transcription. API5 is predominantly nuclear, with acetylation at K251 by p300 stabilizing
  the protein; HDAC1 reverses this mark. Upon DNA damage, ATR phosphorylation promotes
  cytoplasmic relocalization and proteasomal degradation via SCF-FBXW2.
existing_annotations:
  - term:
      id: GO:0005634
      label: nucleus
    evidence_type: IBA
    original_reference_id: GO_REF:0000033
    review:
      summary: >-
        API5 is predominantly nuclear as confirmed by multiple studies. The protein
        contains
        a nuclear localization signal (NLS) at residues 454-475 and localizes to the
        nucleus
        where it performs its anti-apoptotic scaffold functions [PMID:11075807].
      action: ACCEPT
      reason: >-
        Nuclear localization is well-established for API5. The IBA annotation is consistent
        with direct experimental evidence from PMID:11075807 showing nuclear localization,
        and UniProt confirms this with IDA evidence. The protein's functions in transcriptional
        regulation, caspase-2 inhibition, and Acinus binding all occur in the nucleus.
      supported_by:
        - reference_id: PMID:11075807
          supporting_text: "we have isolated a gene encoding a nuclear protein of
            55 kDa, FIF (FGF-2-interacting-factor), which interacts specifically with
            FGF-2"
  - term:
      id: GO:0003723
      label: RNA binding
    evidence_type: IBA
    original_reference_id: GO_REF:0000033
    review:
      summary: >-
        API5 has been implicated in mRNA export via TREX/eIF4E/CRM1 pathways and has
        been
        identified as part of the spliceosomal complex. High-throughput mRNA interactome
        studies have detected API5 as an mRNA-binding protein [file:human/API5/API5-deep-research-falcon.md].
      action: ACCEPT
      reason: >-
        The IBA annotation is supported by HDA evidence from mRNA interactome studies
        (PMID:22658674, PMID:22681889) and the protein's association with spliceosomal
        complexes. While not a primary catalytic function, RNA binding is consistent
        with
        API5's scaffold role in nuclear processes.
      supported_by:
        - reference_id: file:human/API5/API5-deep-research-falcon.md
          supporting_text: "API5 participates in nuclear FGF-2 complexes and has been
            implicated in mRNA export (TREX/eIF4E/CRM1 pathways)"
  - term:
      id: GO:0043066
      label: negative regulation of apoptotic process
    evidence_type: IBA
    original_reference_id: GO_REF:0000033
    review:
      summary: >-
        This is the core function of API5. The protein inhibits apoptosis through
        multiple
        specific mechanisms: (1) direct binding to caspase-2 CARD domain preventing
        activation,
        (2) binding and protecting Acinus from caspase-3 cleavage, and (3) suppressing
        E2F1-dependent apoptosis. The IBA annotation is strongly supported by extensive
        experimental literature [PMID:9307294, PMID:19387494].
      action: ACCEPT
      reason: >-
        Negative regulation of apoptosis is API5's defining function. This is supported
        by
        the original discovery paper [PMID:9307294] showing cells expressing AAC-11
        survive
        serum withdrawal, and the Acinus protection study [PMID:19387494].
      supported_by:
        - reference_id: PMID:9307294
          supporting_text: "BALB/c3T3 fibroblasts that were stably transfected with
            AAC-11 cDNA were viable in serum-free medium for up to 12 weeks"
        - reference_id: PMID:19387494
          supporting_text: "We report here that AAC-11, a survival protein whose expression
            prevents apoptosis that occurs on deprivation of growth factors, physiologically
            binds to Acinus and prevents Acinus-mediated DNA fragmentation"
  - term:
      id: GO:0005634
      label: nucleus
    evidence_type: IEA
    original_reference_id: GO_REF:0000120
    review:
      summary: >-
        IEA annotation for nuclear localization based on automated methods. Consistent
        with
        extensive experimental evidence.
      action: ACCEPT
      reason: >-
        Redundant with IBA and IDA annotations but correct. Nuclear localization is
        API5's primary location and is well-supported by experimental evidence.
      supported_by:
        - reference_id: PMID:11075807
          supporting_text: "we have isolated a gene encoding a nuclear protein of
            55 kDa, FIF (FGF-2-interacting-factor)"
  - term:
      id: GO:0005737
      label: cytoplasm
    evidence_type: IEA
    original_reference_id: GO_REF:0000044
    review:
      summary: >-
        API5 is predominantly nuclear but can also be cytoplasmic. UniProt notes "Mainly
        nuclear. Can also be cytoplasmic." Cytoplasmic localization is associated
        with
        stress-induced degradation via the ATR-FBXW2 pathway [file:human/API5/API5-deep-research-falcon.md].
      action: ACCEPT
      reason: >-
        While nucleus is the primary location, cytoplasmic localization is a real
        physiological state, particularly during stress responses when API5 is
        targeted for proteasomal degradation. Isoform 3 is specifically cytoplasmic.
      supported_by:
        - reference_id: file:human/API5/API5-deep-research-falcon.md
          supporting_text: "DNA damage shifts API5 toward cytoplasmic degradation
            via ATR-FBXW2-proteasome, functionally coupling checkpoint activation
            to relief of API5-mediated apoptosis suppression"
  - term:
      id: GO:0006915
      label: apoptotic process
    evidence_type: IEA
    original_reference_id: GO_REF:0000043
    review:
      summary: >-
        This annotation from UniProtKB keyword mapping is too general. API5 specifically
        NEGATIVELY regulates apoptosis; it does not simply participate in the apoptotic
        process without direction.
      action: MODIFY
      reason: >-
        The term "apoptotic process" is ambiguous about direction. API5 is specifically
        an anti-apoptotic protein that inhibits multiple steps of the apoptotic cascade.
        The more accurate term is GO:0043066 (negative regulation of apoptotic process),
        which is already annotated with stronger evidence.
      proposed_replacement_terms:
        - id: GO:0043066
          label: negative regulation of apoptotic process
      supported_by:
        - reference_id: PMID:9307294
          supporting_text: "The corresponding native protein may function as a novel
            inhibitor of apoptosis"
  - term:
      id: GO:0043066
      label: negative regulation of apoptotic process
    evidence_type: IEA
    original_reference_id: GO_REF:0000117
    review:
      summary: >-
        IEA annotation from ARBA machine learning models for negative regulation of
        apoptotic process. Consistent with the core function of API5.
      action: ACCEPT
      reason: >-
        Correct annotation, redundant with IBA, TAS, ISS, NAS, and IMP annotations
        for the same term. This is API5's core function.
      supported_by:
        - reference_id: PMID:9307294
          supporting_text: "a cDNA whose expression supports cell viability after
            withdrawal of growth factors"
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:19387494
    review:
      summary: >-
        This annotation captures the interaction between API5 (AAC-11) and Acinus
        (ACIN1). The interaction is functionally significant as API5 binding protects
        Acinus from caspase-3 cleavage and prevents Acinus-mediated DNA fragmentation.
      action: MODIFY
      reason: >-
        While the protein binding annotation is technically correct, it is uninformative.
        The specific interaction with Acinus (a chromatin condensation/DNA fragmentation
        factor) is a core mechanistic function of API5. A more specific term describing
        this scaffold/protective binding function would be more informative, but GO
        may
        lack an appropriate term. The key point is this represents meaningful functional
        binding, not generic protein binding.
      proposed_replacement_terms:
        - id: GO:0043066
          label: negative regulation of apoptotic process
      additional_reference_ids:
        - PMID:19387494
      supported_by:
        - reference_id: PMID:19387494
          supporting_text: "AAC-11 was able to protect Acinus from caspase-3 cleavage
            in vivo and in vitro, thus interfering with its biological function"
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:30021884
    review:
      summary: >-
        This annotation is from a crosslinking mass spectrometry study detecting
        API5 interaction with DDX39B (a DEAD-box RNA helicase involved in mRNA export).
        This interaction may relate to API5's reported role in mRNA export pathways.
      action: KEEP_AS_NON_CORE
      reason: >-
        While the interaction with DDX39B is from high-quality crosslinking MS data,
        it represents a secondary function related to RNA biology rather than API5's
        core anti-apoptotic role. The "protein binding" term is uninformative.
      supported_by:
        - reference_id: file:human/API5/API5-deep-research-falcon.md
          supporting_text: "API5 participates in nuclear FGF-2 complexes and has been
            implicated in mRNA export (TREX/eIF4E/CRM1 pathways)"
        - reference_id: PMID:30021884
          supporting_text: Epub 2018 Jul 18. Histone Interaction Landscapes 
            Visualized by Crosslinking Mass Spectrometry in Intact Cell Nuclei.
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:35271311
    review:
      summary: >-
        This annotation is from the OpenCell project (endogenous tagging for cellular
        organization cartography) detecting API5 interaction with DDX39B.
      action: KEEP_AS_NON_CORE
      reason: >-
        Redundant with PMID:30021884 annotation for DDX39B interaction. High-throughput
        interaction data; the "protein binding" term is too general to be informative
        about API5's specific functions.
      supported_by:
        - reference_id: file:human/API5/API5-deep-research-falcon.md
          supporting_text: "API5 participates in nuclear FGF-2 complexes and has been
            implicated in mRNA export (TREX/eIF4E/CRM1 pathways)"
        - reference_id: PMID:35271311
          supporting_text: '2022 Mar 11. OpenCell: Endogenous tagging for the cartography
            of human cellular organization.'
  - term:
      id: GO:0005681
      label: spliceosomal complex
    evidence_type: IEA
    original_reference_id: GO_REF:0000107
    review:
      summary: >-
        IEA annotation based on transfer from mouse ortholog (O35841). Mouse Api5
        was
        found in spliceosomal complexes in the large-scale nuclear sub-compartment
        study.
      action: KEEP_AS_NON_CORE
      reason: >-
        While API5 may associate with splicing machinery, this is likely secondary
        to
        its core anti-apoptotic functions. The association may relate to API5's RNA
        binding capacity and nuclear scaffold role rather than direct splicing function.
      supported_by:
        - reference_id: PMID:11555636
          supporting_text: "The most common discrete localizations detected are at
            the nucleolus and the splicing speckles"
  - term:
      id: GO:0016607
      label: nuclear speck
    evidence_type: IDA
    original_reference_id: GO_REF:0000052
    review:
      summary: >-
        Nuclear speck localization from HPA immunofluorescence data. Nuclear speckles
        are enriched in splicing factors, consistent with API5's association with
        spliceosomal complexes and RNA binding.
      action: ACCEPT
      reason: >-
        IDA evidence from HPA immunofluorescence. Nuclear speck localization is
        consistent with API5's nuclear scaffold role and association with RNA
        processing machinery. This is a reasonable sub-nuclear localization annotation.
      supported_by:
        - reference_id: PMID:11555636
          supporting_text: "The most common discrete localizations detected are at
            the nucleolus and the splicing speckles"
  - term:
      id: GO:0016020
      label: membrane
    evidence_type: HDA
    original_reference_id: PMID:19946888
    review:
      summary: >-
        This annotation is from a study defining the membrane proteome of NK cells.
        API5 is not a transmembrane protein and is predominantly nuclear. This may
        represent non-specific association or a minor localization.
      action: MARK_AS_OVER_ANNOTATED
      reason: >-
        API5 is a nuclear scaffold protein without transmembrane domains. While it
        may
        have been detected in membrane fractions in the NK cell proteomics study,
        this
        is likely not a physiologically relevant localization for its core functions.
        The HDA evidence from a proteomics study may reflect contamination or transient
        association.
      supported_by:
        - reference_id: file:human/API5/API5-deep-research-falcon.md
          supporting_text: "Basal state: predominantly nuclear, chromatin-associated,
            with acetylation at K251 stabilizing the protein and supporting nuclear
            functions"
        - reference_id: PMID:19946888
          supporting_text: Defining the membrane proteome of NK cells.
  - term:
      id: GO:0003723
      label: RNA binding
    evidence_type: HDA
    original_reference_id: PMID:22658674
    review:
      summary: >-
        RNA binding detected in the mRNA-bound proteome atlas study by Castello et
        al.
        This supports API5's association with mRNA processing/export pathways.
      action: ACCEPT
      reason: >-
        HDA evidence from systematic mRNA interactome capture. Consistent with API5's
        reported roles in mRNA export and association with spliceosomal complexes.
        While not a core function, RNA binding is a validated molecular function.
      supported_by:
        - reference_id: file:human/API5/API5-deep-research-falcon.md
          supporting_text: "API5 participates in nuclear FGF-2 complexes and has been
            implicated in mRNA export (TREX/eIF4E/CRM1 pathways)"
        - reference_id: PMID:22658674
          supporting_text: May 31. Insights into RNA biology from an atlas of 
            mammalian mRNA-binding proteins.
  - term:
      id: GO:0003723
      label: RNA binding
    evidence_type: HDA
    original_reference_id: PMID:22681889
    review:
      summary: >-
        RNA binding detected in the Baltz et al. mRNA-bound proteome study. Redundant
        with PMID:22658674 annotation.
      action: ACCEPT
      reason: >-
        Independent HDA evidence supporting RNA binding function. Consistent with
        other annotations for this molecular function.
      supported_by:
        - reference_id: file:human/API5/API5-deep-research-falcon.md
          supporting_text: "API5 participates in nuclear FGF-2 complexes and has been
            implicated in mRNA export (TREX/eIF4E/CRM1 pathways)"
        - reference_id: PMID:22681889
          supporting_text: The mRNA-bound proteome and its global occupancy 
            profile on protein-coding transcripts.
  - term:
      id: GO:0005634
      label: nucleus
    evidence_type: IDA
    original_reference_id: PMID:11075807
    review:
      summary: >-
        Direct experimental evidence for nuclear localization from the FIF discovery
        paper. This is the primary localization study for API5.
      action: ACCEPT
      reason: >-
        IDA evidence from the study that characterized FIF (API5) as an FGF-2
        interacting factor. Nuclear localization is fundamental to API5's functions
        in apoptosis regulation and transcriptional control.
      supported_by:
        - reference_id: PMID:11075807
          supporting_text: "we have isolated a gene encoding a nuclear protein of
            55 kDa, FIF (FGF-2-interacting-factor), which interacts specifically with
            FGF-2"
  - term:
      id: GO:0043066
      label: negative regulation of apoptotic process
    evidence_type: TAS
    original_reference_id: PMID:9307294
    review:
      summary: >-
        TAS annotation based on the original AAC-11 discovery paper by Tewari et al.
        This seminal paper identified API5 through functional expression cloning as
        a gene that prevents apoptosis after growth factor withdrawal.
      action: ACCEPT
      reason: >-
        The original discovery paper provides strong evidence for anti-apoptotic
        function. Cells expressing AAC-11 survived serum-free conditions for up to
        12 weeks. The leucine zipper domain was shown to be essential for this
        protective function.
      supported_by:
        - reference_id: PMID:9307294
          supporting_text: "BALB/c3T3 fibroblasts that were stably transfected with
            AAC-11 cDNA were viable in serum-free medium for up to 12 weeks"
  - term:
      id: GO:0043066
      label: negative regulation of apoptotic process
    evidence_type: ISS
    original_reference_id: PMID:9307294
    review:
      summary: >-
        ISS annotation based on sequence similarity to mouse Api5, with reference
        to
        the original AAC-11 paper showing conservation of anti-apoptotic function.
      action: ACCEPT
      reason: >-
        API5 family is conserved across species and the anti-apoptotic function is
        preserved. The original paper noted "strong species conservation" of the
        AAC-11 gene.
      supported_by:
        - reference_id: PMID:9307294
          supporting_text: "The AAC-11 gene shows strong species conservation and
            is ubiquitously expressed in embryonic and adult tissues"
  - term:
      id: GO:0005681
      label: spliceosomal complex
    evidence_type: ISS
    original_reference_id: PMID:11555636
    review:
      summary: >-
        ISS annotation based on mouse Api5 localization to splicing speckles in the
        large-scale nuclear sub-compartment identification study.
      action: KEEP_AS_NON_CORE
      reason: >-
        Association with spliceosomal complex/splicing speckles is supported by
        independent localization data but represents a secondary function or
        localization rather than API5's core anti-apoptotic role.
      supported_by:
        - reference_id: PMID:11555636
          supporting_text: "The most common discrete localizations detected are at
            the nucleolus and the splicing speckles"
  - term:
      id: GO:0005737
      label: cytoplasm
    evidence_type: NAS
    original_reference_id: PMID:11075807
    review:
      summary: >-
        NAS annotation for cytoplasmic localization. While API5 is predominantly
        nuclear, cytoplasmic localization exists, especially for isoform 3 and
        during stress-induced degradation.
      action: ACCEPT
      reason: >-
        The FIF paper identified multiple isoforms with different localizations.
        While isoform 3 is specifically cytoplasmic, the full-length protein can
        shuttle to cytoplasm under certain conditions.
      supported_by:
        - reference_id: PMID:11075807
          supporting_text: "In contrast to the full-length FIF protein, several deletion
            constructs were able to transactivate a lac-Z reporter gene"
  - term:
      id: GO:0017134
      label: fibroblast growth factor binding
    evidence_type: IPI
    original_reference_id: PMID:11075807
    review:
      summary: >-
        IPI annotation for FGF2 binding. The FIF (API5) protein was originally
        identified as an FGF-2 interacting factor through yeast two-hybrid screening.
        The interaction is specific for FGF-2 over FGF-1, FGF-3, and FGF-6.
      action: ACCEPT
      reason: >-
        FGF2 binding is well-documented and specific. Two regions of API5 (N-terminal
        aa 96-107 and C-terminal aa 274-311) are required for FGF2 binding. This
        interaction may contribute to API5's role in growth factor signaling and
        survival pathways.
      supported_by:
        - reference_id: PMID:11075807
          supporting_text: "The region spanning these repeats is, together with a
            region located in the N-terminal part of the FIF protein, implicated in
            the binding to FGF-2"
  - term:
      id: GO:0043066
      label: negative regulation of apoptotic process
    evidence_type: NAS
    original_reference_id: PMID:10780674
    review:
      summary: >-
        NAS annotation based on cervical cancer study showing AAC-11 overexpression
        protects cells from apoptosis. Cells survived serum-free conditions longer
        when expressing AAC-11.
      action: ACCEPT
      reason: >-
        The study provides additional evidence for anti-apoptotic function in
        cancer cell context. AAC-11 transfected cells showed extended survival
        under serum-free conditions.
      supported_by:
        - reference_id: PMID:10780674
          supporting_text: "Approximately 50% of the AAC-11 transfected cells in serum-free
            medium died after 2 weeks, compared to 1 week for vector alone or wild-type
            cells"
  - term:
      id: GO:0043066
      label: negative regulation of apoptotic process
    evidence_type: IMP
    original_reference_id: PMID:11075807
    review:
      summary: >-
        IMP annotation from the FIF paper showing anti-apoptotic function through
        mutant phenotype analysis. The C-terminal portion of FIF was shown to have
        antiapoptotic properties.
      action: ACCEPT
      reason: >-
        IMP evidence from functional studies. The paper discusses that the C-terminal
        part exhibits antiapoptotic properties, supporting the anti-apoptotic function
        of API5.
      supported_by:
        - reference_id: PMID:11075807
          supporting_text: "the COOH-terminal part, but not the full-length FIF protein,
            has previously been shown to exhibit antiapoptotic properties"
# Suggest caspase-2 inhibition as new annotation - core mechanism from deep research
  - term:
      id: GO:0043027
      label: cysteine-type endopeptidase inhibitor activity involved in 
        apoptotic process
    evidence_type: IDA
    original_reference_id: PMID:28336776
    review:
      summary: >-
        API5 directly binds the CARD domain of caspase-2, preventing its dimerization
        and activation. This is a key mechanism of API5's anti-apoptotic function
        distinct from Acinus protection [file:human/API5/API5-deep-research-falcon.md].
      action: NEW
      reason: >-
        The deep research review identifies direct caspase-2 inhibition as a major
        mechanism of API5 anti-apoptotic function (Imre et al., EMBO Reports 2017,
        PMID:28336776). This specific molecular function is not captured by the general
        "negative regulation of apoptotic process" annotations.
      supported_by:
        - reference_id: file:human/API5/API5-deep-research-falcon.md
          supporting_text: "API5 directly binds the CARD of caspase-2, preventing
            its dimerization/activation; recombinant API5 inhibits full-length but
            not processed caspase-2"
        - reference_id: PMID:28336776
          supporting_text: Apoptosis inhibitor 5 is an endogenous inhibitor of 
            caspase-2.
references:
  - id: GO_REF:0000033
    title: Annotation inferences using phylogenetic trees
    findings:
      - statement: IBA annotations for nucleus, RNA binding, and negative 
          regulation of apoptotic process
  - id: GO_REF:0000043
    title: Gene Ontology annotation based on UniProtKB/Swiss-Prot keyword 
      mapping
    findings:
      - statement: IEA annotation for apoptotic process (should be more 
          specific)
  - id: GO_REF:0000044
    title: Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular 
      Location vocabulary mapping
    findings:
      - statement: Cytoplasm localization annotation
  - id: GO_REF:0000052
    title: Gene Ontology annotation based on curation of immunofluorescence data
    findings:
      - statement: Nuclear speck localization from HPA
  - id: GO_REF:0000107
    title: Automatic transfer of experimentally verified manual GO annotation 
      data to orthologs using Ensembl Compara
    findings:
      - statement: Spliceosomal complex annotation transferred from mouse
  - id: GO_REF:0000117
    title: Electronic Gene Ontology annotations created by ARBA machine learning
      models
    findings:
      - statement: Negative regulation of apoptotic process annotation
  - id: GO_REF:0000120
    title: Combined Automated Annotation using Multiple IEA Methods
    findings:
      - statement: Nucleus localization annotation
  - id: PMID:9307294
    title: AAC-11, a novel cDNA that inhibits apoptosis after growth factor 
      withdrawal.
    findings:
      - statement: Original discovery of API5/AAC-11 as anti-apoptotic factor
        supporting_text: "a cDNA whose expression supports cell viability after withdrawal
          of growth factors"
      - statement: Leucine zipper domain essential for protective function
        supporting_text: "The protective action of AAC-11 was abolished by mutation
          of leucines to arginines within the leucine zipper domain"
      - statement: Strong species conservation
        supporting_text: "The AAC-11 gene shows strong species conservation"
      - statement: Cells survive serum withdrawal for up to 12 weeks when 
          expressing AAC-11
        supporting_text: "BALB/c3T3 fibroblasts that were stably transfected with
          AAC-11 cDNA were viable in serum-free medium for up to 12 weeks"
  - id: PMID:10780674
    title: AAC-11 overexpression induces invasion and protects cervical cancer 
      cells from apoptosis.
    findings:
      - statement: AAC-11 overexpression protects cervical cancer cells from 
          apoptosis
        supporting_text: "The effect of AAC-11 on cultured cervical cancer cells was
          associated with antiapoptotic process"
      - statement: Extended survival under serum-free conditions
        supporting_text: "Approximately 50% of the AAC-11 transfected cells in serum-free
          medium died after 2 weeks"
      - statement: Also promotes invasion (secondary function)
        supporting_text: "Invasion of matrigel and adhesion to laminin by AAC-11 transfected
          CUMC-6 cells were increased"
  - id: PMID:11075807
    title: FIF [fibroblast growth factor-2 (FGF-2)-interacting-factor], a 
      nuclear putatively antiapoptotic factor, interacts specifically with 
      FGF-2.
    findings:
      - statement: Identified FIF (API5) as nuclear 55 kDa protein
        supporting_text: "we have isolated a gene encoding a nuclear protein of 55
          kDa, FIF (FGF-2-interacting-factor)"
      - statement: Specific interaction with FGF-2 but not FGF-1, FGF-3, or 
          FGF-6
        supporting_text: "which interacts specifically with FGF-2 but not with FGF-1,
          FGF-3, or FGF-6"
      - statement: C-terminal part has antiapoptotic properties
        supporting_text: "the COOH-terminal part, but not the full-length FIF protein,
          has previously been shown to exhibit antiapoptotic properties"
      - statement: Multiple isoforms with different localizations
        supporting_text: "In contrast to the full-length FIF protein, several deletion
          constructs were able to transactivate a lac-Z reporter gene"
  - id: PMID:11555636
    title: Large-scale identification of mammalian proteins localized to nuclear
      sub-compartments.
    findings:
      - statement: Mouse Api5 localizes to splicing speckles
        supporting_text: "The most common discrete localizations detected are at the
          nucleolus and the splicing speckles"
  - id: PMID:19387494
    title: The antiapoptotic protein AAC-11 interacts with and regulates 
      Acinus-mediated DNA fragmentation.
    findings:
      - statement: AAC-11 binds Acinus and prevents DNA fragmentation
        supporting_text: "AAC-11, a survival protein whose expression prevents apoptosis
          that occurs on deprivation of growth factors, physiologically binds to Acinus
          and prevents Acinus-mediated DNA fragmentation"
      - statement: Protects Acinus from caspase-3 cleavage
        supporting_text: "AAC-11 was able to protect Acinus from caspase-3 cleavage
          in vivo and in vitro"
      - statement: Leucine zipper domain required for Acinus binding
        supporting_text: "AAC-11 possesses a leucine-zipper domain that dictates,
          upon oligomerization, its interaction with Acinus"
      - statement: AAC-11 depletion sensitizes cells to chemotherapy
        supporting_text: "AAC-11 depletion markedly increased cellular sensitivity
          to anticancer drugs"
  - id: PMID:19946888
    title: Defining the membrane proteome of NK cells.
    findings:
      - statement: High-throughput proteomics detected API5 in membrane fraction
        supporting_text: "Mass spectrometric analysis identified 1843 proteins with
          high confidence scores"
  - id: PMID:22658674
    title: Insights into RNA biology from an atlas of mammalian mRNA-binding 
      proteins.
    findings:
      - statement: API5 detected as mRNA-binding protein
        supporting_text: "We identify 860 proteins that qualify as RBPs by biochemical
          and statistical criteria, adding more than 300 RBPs to those previously
          known"
  - id: PMID:22681889
    title: The mRNA-bound proteome and its global occupancy profile on 
      protein-coding transcripts.
    findings:
      - statement: API5 detected in mRNA-bound proteome
        supporting_text: "Application to a human embryonic kidney cell line identified
          close to 800 proteins"
  - id: PMID:28336776
    title: Apoptosis inhibitor 5 is an endogenous inhibitor of caspase-2.
    findings:
      - statement: API5 directly binds caspase-2 CARD domain
        supporting_text: "API5 protein directly binds to the caspase recruitment domain
          (CARD) of caspase-2"
      - statement: Prevents caspase-2 dimerization and activation
        supporting_text: "impedes dimerization and activation of caspase-2"
      - statement: Key mechanism of anti-apoptotic function
        supporting_text: "recombinant API5 directly inhibits full length but not processed
          caspase-2"
  - id: PMID:30021884
    title: Histone Interaction Landscapes Visualized by Crosslinking Mass 
      Spectrometry in Intact Cell Nuclei.
    findings:
      - statement: API5-DDX39B interaction detected
        supporting_text: "we use crosslinking mass spectrometry (XL-MS) to chart the
          protein-protein interactions in intact human nuclei"
  - id: PMID:35271311
    title: 'OpenCell: Endogenous tagging for the cartography of human cellular organization.'
    findings:
      - statement: API5-DDX39B interaction confirmed
        supporting_text: "We combined genome engineering, confocal live-cell imaging,
          mass spectrometry, and data science to systematically map the localization
          and interactions of human proteins"
  - id: file:human/API5/API5-deep-research-falcon.md
    title: Deep research summary for API5
    findings:
      - statement: Comprehensive review of API5 anti-apoptotic mechanisms
        supporting_text: "API5 is a nuclear scaffold protein that inhibits apoptosis
          through multiple mechanisms"
core_functions:
  - description: >-
      API5 inhibits apoptosis through multiple specific mechanisms including direct
      binding
      to caspase-2 CARD domain preventing caspase-2 dimerization and activation;
      binding to Acinus to protect it from caspase-3 cleavage, blocking DNA
      fragmentation; and suppression of E2F1-dependent apoptosis while promoting
      E2F1-driven G1/S gene transcription.
    molecular_function:
      id: GO:0043027
      label: cysteine-type endopeptidase inhibitor activity involved in 
        apoptotic process
    directly_involved_in:
      - id: GO:0043066
        label: negative regulation of apoptotic process
    locations:
      - id: GO:0005634
        label: nucleus
    supported_by:
      - reference_id: PMID:9307294
        supporting_text: "BALB/c3T3 fibroblasts that were stably transfected with
          AAC-11 cDNA were viable in serum-free medium for up to 12 weeks"
      - reference_id: PMID:19387494
        supporting_text: "AAC-11 was able to protect Acinus from caspase-3 cleavage
          in vivo and in vitro"
  - description: >-
      API5/FIF specifically binds FGF2 through two regions (N-terminal aa 96-107
      and C-terminal aa 274-311). This interaction may contribute to growth factor
      signaling and survival pathway activation.
    molecular_function:
      id: GO:0017134
      label: fibroblast growth factor binding
    locations:
      - id: GO:0005634
        label: nucleus
    supported_by:
      - reference_id: PMID:11075807
        supporting_text: "which interacts specifically with FGF-2 but not with FGF-1,
          FGF-3, or FGF-6"
  - description: >-
      API5 localizes to nucleus via NLS (aa 454-475) where it performs scaffold
      functions through HEAT/ARM-like helical repeats, leucine zipper domain, and
      LxxLL motif. Associates with nuclear speckles and spliceosomal complexes.
    molecular_function:
      id: GO:0003723
      label: RNA binding
    locations:
      - id: GO:0005634
        label: nucleus
      - id: GO:0016607
        label: nuclear speck
    supported_by:
      - reference_id: PMID:11075807
        supporting_text: "we have isolated a gene encoding a nuclear protein of 55
          kDa, FIF (FGF-2-interacting-factor)"
proposed_new_terms: []
suggested_questions:
  - question: >-
      What is the relative contribution of caspase-2 inhibition vs. Acinus protection
      to API5's anti-apoptotic function in different cellular contexts?
  - question: >-
      Does API5's FGF2 binding contribute significantly to its anti-apoptotic function
      or is this a separate regulatory interaction?
  - question: >-
      What is the functional significance of API5's RNA binding and spliceosomal
      association - is this related to apoptosis regulation or a separate function?
suggested_experiments:
  - description: >-
      Structure-function analysis of API5 domains to dissect contributions of
      caspase-2 binding, Acinus binding, and FGF2 binding to overall anti-apoptotic
      function.
  - description: >-
      Systematic analysis of API5 interactome during apoptotic stress vs. normal
      conditions to identify context-dependent interaction partners.

API5

Gene Description

Existing Annotations Review

Core Functions

API5/FIF specifically binds FGF2 through two regions (N-terminal aa 96-107 and C-terminal aa 274-311). This interaction may contribute to growth factor signaling and survival pathway activation.

API5 localizes to nucleus via NLS (aa 454-475) where it performs scaffold functions through HEAT/ARM-like helical repeats, leucine zipper domain, and LxxLL motif. Associates with nuclear speckles and spliceosomal complexes.

References

Suggested Questions for Experts

Suggested Experiments

📚 Additional Documentation

Deep Research Falcon

Question

Gene Research for Functional Annotation

⚠️ CRITICAL: Gene/Protein Identification Context

Target Gene/Protein Identity (from UniProt):

MANDATORY VERIFICATION STEPS:

If Gene Symbol is Ambiguous or You Cannot Find Relevant Literature:

Research Target:

Output

Gene Research for Functional Annotation

⚠️ CRITICAL: Gene/Protein Identification Context

Target Gene/Protein Identity (from UniProt):

MANDATORY VERIFICATION STEPS:

If Gene Symbol is Ambiguous or You Cannot Find Relevant Literature:

Research Target:

Citations

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