HAT1

PF3D7_0416400 (HAT1) is a GCN5-family histone acetyltransferase of Plasmodium falciparum that serves as a global chromatin regulator of asexual proliferation, sexual conversion, and stress adaptation (PMID:15075257, PMID:34403450). Recombinant enzyme preferentially acetylates histone H3 at Lys-8 and Lys-14, and its HAT domain is functionally interchangeable with that of yeast GCN5 (PMID:15075257); pharmacological inhibition confirms this specificity in parasites, producing selective hypoacetylation of H3K9/K14 without affecting H4 acetylation (PMID:17145789, PMID:18487348). It operates within a divergent multi-subunit HAT complex that includes PfADA2, two PHD-domain proteins (PfPHD1/PfPHD2), and the AP2 transcription factor PfAP2-LT (PMID:15246536, PMID:34403450). Through its bromodomain and HAT activity it is recruited to gene promoters and enhancers to deposit H3K9 acetylation, and its bromodomain is required for efficient merozoite invasion, restraint of sexual conversion, mutually exclusive var gene expression, and normal genome-wide H3K9ac/H3K4me3 patterns, with loss altering expression of the majority of genes (PMID:18692528, PMID:34403450). Beyond chromatin, it acetylates the metabolic enzyme PfGAPDH in vitro, indicating a non-histone substrate (PMID:31965989), and it binds stress- and artemisinin-response genes in a poised state such that its inhibition sensitizes artemisinin-resistant parasites (PMID:33441725). Uniquely, the protein undergoes essential proteolytic processing near the digestive vacuole—mediated by the food-vacuolar cysteine protease Falcipain 3—following trafficking through the ER-Golgi pathway, a step required for its promoter occupancy and HAT activity before nuclear import (PMID:31862795, PMID:38924147).

1. 2004 High

   Establishing that this parasite GCN5-family protein is a bona fide histone acetyltransferase answered whether it possesses catalytic activity and defined its histone substrate specificity.

   Evidence In vitro HAT assay with recombinant protein and substrate-specific antibodies, plus functional rescue of a yeast gcn5 deletion, and yeast two-hybrid/binding assays with PfADA2

   PMID:15075257

   Open questions at the time

   • Physiological histone targets and genome-wide sites not yet mapped
   • Composition of the full in vivo complex beyond PfADA2 unresolved
2. 2004 Medium

   Demonstrating an endogenous PfADA2–GCN5 association confirmed the enzyme functions as part of a complex in parasite erythrocytic stages rather than as an isolated catalytic subunit.

   Evidence Reciprocal co-immunoprecipitation from parasite lysate with a HAT activity readout

   PMID:15246536

   Open questions at the time

   • Full subunit inventory of the complex not defined
   • Functional consequence of the PfADA2 interaction unresolved
3. 2008 Medium

   Pharmacological inhibition established that the enzyme's H3K9/K14 acetylation drives transcriptional activation of specific target genes, linking its catalytic output to gene regulation.

   Evidence Curcumin and anacardic acid inhibition of recombinant enzyme and parasite nuclear extracts, with ChIP, microarray, and acetylation-specific antibodies

   PMID:17145789 PMID:18487348 PMID:18692528

   Open questions at the time

   • Direct genetic loss-of-function not yet performed
   • Inhibitor specificity for this HAT versus other parasite HATs incompletely excluded
4. 2020 Medium

   Discovery of trafficking-dependent proteolytic processing near the digestive vacuole answered how the nuclear enzyme is matured and showed this step is required for its chromatin function.

   Evidence Immunofluorescence, immuno-EM, cysteine protease inhibitor treatment, and ChIP

   PMID:31862795

   Open questions at the time

   • Identity of the responsible protease not established in this study
   • Molecular consequence of processing on activity not directly measured
5. 2020 Medium

   Identification of metabolic-enzyme interactors and in vitro acetylation of PfGAPDH extended the enzyme's substrate repertoire beyond histones, raising a potential nutrient-sensing role.

   Evidence IP-LC-MS/MS for interactors and in vitro acetylation assay for PfGAPDH

   PMID:31965989

   Open questions at the time

   • In vivo relevance of PfGAPDH acetylation not demonstrated
   • Functional effect of GAPDH acetylation on metabolism unknown
6. 2021 High

   Defining the divergent PHD/AP2-LT-containing complex and dissecting the bromodomain by genetic deletion answered which biological processes the enzyme governs, tying it to invasion, sexual conversion, and var gene control.

   Evidence CRISPR/Cas9 bromodomain deletion with invasion and sexual conversion assays, ChIP-seq, RNA-seq, and Co-IP

   PMID:34403450

   Open questions at the time

   • Separation of bromodomain reader function from catalytic HAT activity not fully resolved
   • Roles of individual PfPHD1/PfPHD2 subunits not dissected
7. 2021 Medium

   Mapping the enzyme to poised stress- and artemisinin-response genes showed it participates in transcriptional adaptation and modulates artemisinin sensitivity, implicating it in drug resistance.

   Evidence ChIP-seq, RNA-seq, and inhibitor growth assays in resistant parasites

   PMID:33441725

   Open questions at the time

   • Mechanism coupling drug exposure to expanded binding not defined
   • Direct causal contribution to clinical resistance not established
8. 2024 Medium

   Identifying Falcipain 3 as the processing protease answered the outstanding question of how and where the enzyme is proteolytically matured before nuclear entry.

   Evidence IP-LC-MS/MS, in vitro pull-down, co-IP, E64d inhibitor treatment, confocal and electron microscopy

   PMID:38924147

   Open questions at the time

   • Precise cleavage site(s) not mapped
   • Regulation coupling food-vacuolar processing to nuclear timing unresolved

• It remains unresolved how bromodomain-mediated reader activity, HAT catalysis, complex assembly, and Falcipain 3 processing are integrated to control the timing of specific transcriptional programs across the parasite life cycle.
• No structural model of the assembled complex
• Individual subunit contributions and processing-to-nuclear-import coupling undefined
• In vivo scope of non-histone acetylation unknown