Affinage · Toxoplasma gondii

GCN5B

histone lysine acetyltransferase GCN5-B · UniProt S8F3V2

Annotated
2026-09-10
14 papers in source corpus 8 papers cited in narrative 7 extracted findings

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

TgGCN5b is an essential nuclear lysine acetyltransferase (KAT) of Toxoplasma gondii that drives transcriptional gene regulation by acetylating histone H3 tail lysines and occupying protein-coding gene loci genome-wide (PMID:16400169, PMID:24391497, PMID:40879678, PMID:40152591). Unlike its paralogue GCN5-A, which shows a narrow bias for H3K18 and localizes to telomeres, GCN5b acetylates multiple H3 tail lysines and its chromatin occupancy maps to coding regions of the genome (PMID:16400169, PMID:40879678, PMID:40152591). Its catalytic activity is required for tachyzoite viability: a catalytically dead point mutant (E703G) arrests replication, lowers H3 acetylation, and downregulates GCN5b target genes, and small-molecule inhibition of its KAT activity by garcinol reproduces this transcriptional dysregulation and growth block (PMID:24391497, PMID:26810649). GCN5b assembles into at least two distinct multi-protein complexes distinguished by different pairs of AP2-domain transcription factors (AP2IX-7 and AP2XII-4) together with the co-activator ADA2-A, TFIID subunits, LEO1/Paf1, and RRM proteins, and this composition shifts during bradyzoite induction (PMID:24391497, PMID:31381949). Beyond its catalytic KAT domain, its bromodomain—which reads acetylated histone residues—is independently required for viability (PMID:32119930). Nuclear entry is mediated by a novel NLS (RPAENKKRGR) embedded in an intrinsically disordered N-terminal region (PMID:21055425). GCN5b also interacts with the essential kinase TgGSK, whose protein stability depends on GCN5b acetyltransferase activity (PMID:40879678, PMID:40152591).

Mechanistic history

Synthesis pass · year-by-year structured walk · 7 steps
  1. 2006 High

    Distinguishing GCN5b from GCN5-A—broader H3 tail acetylation, a divergent nuclear-targeting N-terminus, and a distinct ADA2 partner—defined GCN5b as a functionally separate enzyme rather than a redundant paralogue.

    Evidence In vitro HAT assays, co-IP of ADA2 homologues, domain-deletion localization experiments

    PMID:16400169

    Open questions at the time
    • Genome-wide targets not yet mapped
    • Essentiality not tested
    • Full complex composition unknown
  2. 2010 Medium

    Localizing GCN5b's NLS to an intrinsically disordered N-terminal region, distinct from the GCN5-A basic NLS, explained how the two paralogues use different import determinants.

    Evidence Computational disorder prediction with GFP-fusion localization validation

    PMID:21055425

    Open questions at the time
    • Import receptor for this NLS not identified
    • Functional consequence of disorder region beyond import unknown
  3. 2014 High

    Demonstrating GCN5b essentiality and mapping its interactome and target genes converted it from a biochemical curiosity into a validated regulator: its catalysis is required for viability, target-gene expression, and H3 acetylation, and it works within AP2/ADA2-A/TFIID/LEO1/RRM complexes.

    Evidence Inducible dominant-negative catalytic mutant (E703G), ChIP-chip target mapping, MS interactome, global acetylation assessment

    PMID:24391497

    Open questions at the time
    • Which acetylation events are direct vs indirect not fully resolved
    • Roles of individual complex members unclear
  4. 2016 High

    Chemical inhibition by garcinol confirmed that GCN5b KAT activity is pharmacologically druggable and that blocking it phenocopies genetic disruption, reinforcing GCN5b as an antiparasitic target.

    Evidence In vitro KAT inhibition assay, global H3 acetylation western blot, RNA-seq, overlap with prior ChIP-chip targets

    PMID:26810649

    Open questions at the time
    • Garcinol specificity for GCN5b vs other KATs not fully delineated
    • Self-acetylation function not defined
  5. 2019 Medium

    Resolving that GCN5b partitions into at least two complexes defined by distinct AP2 factor pairs, with composition shifting during bradyzoite induction, established a mechanism for stage-specific transcriptional control.

    Evidence Co-IP under tachyzoite and bradyzoite-inducing conditions, MS, reciprocal co-IP with AP2IX-7 and AP2XII-4

    PMID:31381949

    Open questions at the time
    • How AP2 factors direct GCN5b to specific loci not shown
    • Functional consequence of the composition shift on differentiation not established
  6. 2020 Medium

    Showing the bromodomain is independently essential and druggable established that GCN5b requires both writing (KAT) and reading (bromodomain) of acetyl marks for function.

    Evidence Inducible dominant-negative bromodomain disruption, in vitro acetyl-histone peptide binding assay, GCN5b over-expression rescue of L-Moses toxicity

    PMID:32119930

    Open questions at the time
    • In vivo acetyl-lysine targets read by the bromodomain not mapped
    • Whether reading and writing are coupled at chromatin unresolved
  7. 2025 Medium

    Genome-wide CUT&Tag mapping placed GCN5b at protein-coding loci (versus GCN5a at telomeres) and linked its catalytic activity to stabilization of the essential kinase TgGSK, extending GCN5b's role from histone acetylation to control of a signaling kinase.

    Evidence CUT&Tag occupancy mapping, co-IP of GCN5b-TgGSK, chemical inhibition with TgGSK protein-level readout

    PMID:40879678 PMID:40152591

    Open questions at the time
    • Whether TgGSK is a direct acetylation substrate of GCN5b not demonstrated
    • Mechanism coupling GCN5b activity to TgGSK stability unresolved
    • Single strain (Pru)

Mechanism profile

Synthesis pass · controlled-vocabulary classification
Partners
ADA2-A (CO-ACTIVATOR)AP2IX-7 (AP2-DOMAIN TRANSCRIPTION FACTOR)AP2XII-4 (AP2-DOMAIN TRANSCRIPTION FACTOR)LEO1 / PAF1 SUBUNITRRM PROTEINSTFIID SUBUNITSTGGSK (ESSENTIAL KINASE)
Complex memberships
GCN5b–AP2IX-7 complex (with ADA2-A, TFIID subunits, LEO1/Paf1, RRM proteins)GCN5b–AP2XII-4 complex (bradyzoite-associated composition)

Evidence

Reading pass · 7 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2006 TgGCN5-B acetylates multiple lysine residues in the H3 tail (broader substrate specificity than TgGCN5-A, which exclusively targets H3K18), and harbors a divergent N-terminal domain required for nuclear localization. TgGCN5-B also interacts with a distinct ADA2 homologue compared to TgGCN5-A. In vitro histone acetyltransferase assays with histone substrates; co-immunoprecipitation of ADA2 homologues; domain deletion/nuclear localization experiments Eukaryotic cell High 16400169
2014 GCN5b is essential for Toxoplasma tachyzoite viability; expression of a dominant-negative catalytically dead point mutant (E703G) causes rapid arrest in parasite replication, decreased H3 acetylation at specific lysine residues, and reduced expression of GCN5b target genes. GCN5b forms a complex with AP2-domain transcription factors, the co-activator ADA2-A, TFIID subunits, LEO1 (Paf1 subunit), and RRM proteins. Dominant-negative inducible expression (destabilization domain strategy); ChIP-chip to identify target genes; mass spectrometry proteomics (Co-IP) to identify complex members; global histone acetylation assessment PLoS pathogens High 24391497
2010 TgGCN5-B contains a novel nuclear localization signal (RPAENKKRGR) located within an intrinsically disordered region of its N-terminus, distinct from the NLS of TgGCN5-A; functionality of this NLS was experimentally validated. Computational prediction of intrinsically disordered regions combined with experimental validation of NLS activity (GFP-fusion localization assays) Molecular and biochemical parasitology Medium 21055425
2016 Garcinol inhibits TgGCN5b lysine acetyltransferase activity in vitro and in vivo, reducing global H3 acetylation and acetylation of GCN5b itself, and dysregulating GCN5b target genes to block tachyzoite replication. Genes most affected by garcinol treatment overlap with GCN5b ChIP-chip targets. In vitro KAT inhibition assay; global histone acetylation western blot; RNA-seq transcriptome analysis; comparison with prior ChIP-chip data Antimicrobial agents and chemotherapy High 26810649
2019 GCN5b associates with at least two distinct complexes characterized by different pairs of AP2 transcription factors (AP2IX-7 and AP2XII-4) under normal and bradyzoite-induction conditions. The composition of the GCN5b complex shifts during bradyzoite induction, and reciprocal co-IPs with AP2IX-7 and AP2XII-4 delineate their respective interactomes. Co-immunoprecipitation under tachyzoite and bradyzoite-inducing conditions; mass spectrometry; reciprocal co-IP with AP2IX-7 and AP2XII-4 Molecular and biochemical parasitology Medium 31381949
2020 The bromodomain of GCN5b is required for Toxoplasma viability; dominant-negative bromodomain disruption arrests parasite growth. The GCN5-family bromodomain inhibitor L-Moses blocks GCN5b bromodomain binding to acetylated histone residues in vitro, and kills tachyzoites in a GCN5b-dependent manner (over-expression of GCN5b overcomes L-Moses toxicity). Inducible dominant-negative bromodomain strategy; in vitro binding assay of bromodomain to acetylated histone peptides; parasite growth assay with GCN5b over-expression rescue Experimental parasitology Medium 32119930
2025 GCN5b localizes to coding regions of the genome (not telomeres), as determined by CUT&Tag chromatin occupancy mapping, distinguishing its genomic targets from those of GCN5a (which localizes exclusively to telomeres). GCN5b interacts with the essential kinase TgGSK, and chemical inhibition of GCN5b leads to specific degradation of TgGSK protein. CUT&Tag chromatin occupancy mapping; co-immunoprecipitation (GCN5b–TgGSK interaction); chemical inhibition of GCN5b with monitoring of TgGSK protein levels mSphere Medium 40152591 40879678

Source papers

Stage 0 corpus · 14 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2005 Histone-modifying complexes regulate gene expression pertinent to the differentiation of the protozoan parasite Toxoplasma gondii. Molecular and cellular biology 160 16287846
2014 Lysine acetyltransferase GCN5b interacts with AP2 factors and is required for Toxoplasma gondii proliferation. PLoS pathogens 63 24391497
2006 Pair of unusual GCN5 histone acetyltransferases and ADA2 homologues in the protozoan parasite Toxoplasma gondii. Eukaryotic cell 44 16400169
2004 Histone acetylase GCN5 enters the nucleus via importin-alpha in protozoan parasite Toxoplasma gondii. The Journal of biological chemistry 37 15591057
2016 Garcinol Inhibits GCN5-Mediated Lysine Acetyltransferase Activity and Prevents Replication of the Parasite Toxoplasma gondii. Antimicrobial agents and chemotherapy 36 26810649
1999 Cloning and analysis of a Toxoplasma gondii histone acetyltransferase: a novel chromatin remodelling factor in Apicomplexan parasites. Nucleic acids research 25 10536141
2006 Quinoline derivative MC1626, a putative GCN5 histone acetyltransferase (HAT) inhibitor, exhibits HAT-independent activity against Toxoplasma gondii. Antimicrobial agents and chemotherapy 24 17178801
2000 Cloning and characterization of a novel histone acetyltransferase homologue from the protozoan parasite Toxoplasma gondii reveals a distinct GCN5 family member. Gene 23 10721712
2019 A novel GCN5b lysine acetyltransferase complex associates with distinct transcription factors in the protozoan parasite Toxoplasma gondii. Molecular and biochemical parasitology 20 31381949
2010 Regions of intrinsic disorder help identify a novel nuclear localization signal in Toxoplasma gondii histone acetyltransferase TgGCN5-B. Molecular and biochemical parasitology 15 21055425
2020 Evaluating the GCN5b bromodomain as a novel therapeutic target against the parasite Toxoplasma gondii. Experimental parasitology 13 32119930
2025 The essential kinase TgGSK regulates centrosome segregation and endodyogeny in Toxoplasma gondii. mSphere 4 40152591
2024 The essential kinase TgGSK regulates centrosome division and endodyogeny in Toxoplasma gondii. bioRxiv : the preprint server for biology 2 39386585
2025 GCN5a is a telomeric lysine acetyltransferase whose loss primes Toxoplasma gondii for latency. mSphere 1 40879678