TGME49_239260

Toxoplasma gondii histone H4 is a core chromatin component whose post-translational modification state partitions the genome into active and silent domains (PMID:29101771, PMID:19120792). The MYST-family acetyltransferase TgMYST-A acetylates H4 in vitro preferentially at lysines 5, 8, 12, and 16 (PMID:16339723), and genome-wide, acetylated H4 (H4ac) marks the promoters of actively transcribed genes alongside H3K4me3 and H3K9ac (PMID:19120792). A distinct modification within the globular domain, H4K31, is bifunctionally regulated: H4K31 acetylation at promoters correlates with gene expression, whereas H4K31 monomethylation accumulates in the bodies of active genes yet inversely correlates with transcription (PMID:29101771). Silencing is imposed by methylation of H4K20: a Set8-related methyltransferase catalyzes mono-, di-, and trimethylation of H4K20 and is focally enriched at pericentric and telomeric heterochromatin (PMID:17562855), and H4K20me1 together with the histone variant H2A.X marks subtelomeric-like silenced heterochromatin (PMID:24417889). Global H4 acetylation levels are functionally coupled to the cell cycle, as HDAC inhibition raises nuclear H4 acetylation and impairs endodyogeny and daughter-cell budding (PMID:33360687). Beyond its chromatin role, recombinant TgH4 applied exogenously binds murine macrophages and modulates their inflammatory responses in vitro (PMID:29722109).

1. 2005 High

   Establishing which enzyme deposits activating H4 acetylation marks in Toxoplasma addressed how the parasite writes active chromatin; identifying TgMYST-A and its target lysines provided the writer for H4 acetylation.

   Evidence In vitro HAT assay with recombinant TgMYST-A plus immunofluorescence and stage-expression analysis

   PMID:16339723

   Open questions at the time

   • Genetic loss-of-function linking TgMYST-A to specific in vivo H4ac sites not established
   • Functional consequence of individual K5/K8/K12/K16 acetylation not resolved
2. 2007 High

   Whether apicomplexan parasites can generate higher-order H4K20 methylation states addressed the mechanism of heterochromatin silencing; showing Set8-related enzymes perform mono-, di-, and trimethylation (unlike monomethylase-restricted human Set8) defined the writer for silent-domain marks.

   Evidence In vitro histone methyltransferase assay with structural analysis of substrate channel residues and IF/ChIP localization, across Toxoplasma and Plasmodium

   PMID:17562855

   Open questions at the time

   • Direct genetic requirement of the enzyme for heterochromatin maintenance not tested
   • Cell-cycle coupling of methylation dynamics not mechanistically dissected
3. 2008 Medium

   Mapping where H4 acetylation resides genome-wide addressed whether H4ac correlates with transcription state; finding H4ac at active promoters alongside H3K4me3 and H3K9ac established H4ac as an active-promoter mark.

   Evidence Genome-wide ChIP analysis in Toxoplasma gondii

   PMID:19120792

   Open questions at the time

   • Summarized from other studies rather than direct primary perturbation
   • Causal role of H4ac in transcription not tested
4. 2014 Medium

   Determining the modification landscape of subtelomeric chromatin addressed which marks define silenced domains; enrichment of H4K20me1 and H2A.X identified these regions as heterochromatin.

   Evidence ChIP-qPCR with H4K20me1 and H2A.X antibodies in ME49

   PMID:24417889

   Open questions at the time

   • Single method, no functional perturbation
   • Enzyme responsible for local H4K20me1 not linked in this study
5. 2017 High

   Whether the globular-domain residue H4K31 carries regulatory information addressed a novel axis of chromatin control; showing H4K31ac marks active promoters while H4K31me1 populates active gene bodies but inversely tracks transcription established a bifunctional, position-dependent mark.

   Evidence ChIP-seq and mass spectrometry with genome-wide transcription correlation across Toxoplasma and Plasmodium

   PMID:29101771

   Open questions at the time

   • Writers/erasers for H4K31 modifications not identified
   • Mechanism reconciling gene-body H4K31me1 enrichment with transcriptional repression unresolved
6. 2018 Low

   Whether TgH4 has a role beyond chromatin addressed a potential host-interaction function; recombinant TgH4 binding and modulating macrophage responses raised an extracellular immunomodulatory activity.

   Evidence IFA binding assay plus flow cytometry and ELISA on Ana-1 macrophages exposed to recombinant TgH4

   PMID:29722109

   Open questions at the time

   • Recombinant protein applied exogenously with no genetic evidence of native extracellular secretion
   • Single method per readout, no in vivo confirmation
   • Receptor mediating macrophage binding not identified
7. 2020 Medium

   Whether global H4 acetylation is functionally linked to parasite replication addressed the physiological importance of the acetylation balance; HDAC inhibition raising H4ac and blocking endodyogeny connected the mark to cell-cycle progression.

   Evidence Western blot for H4 acetylation and fluorescence/electron microscopy of budding defects after Tubastatin A treatment in RH and ME49

   PMID:33360687

   Open questions at the time

   • Pharmacological perturbation lacks genetic specificity for H4 versus other HDAC substrates
   • Direct causal chain from H4ac to budding machinery not defined

• The writers and erasers for H4K31 modifications, the genetic requirement of TgMYST-A and Set8-related methyltransferases for defined H4 marks in vivo, and the mechanistic basis of any native extracellular H4 function remain unresolved.
• No enzyme assigned to H4K31 acetylation or monomethylation
• No genetic loss-of-function tying specific H4 marks to phenotypes
• Native secretion and receptor for extracellular TgH4 unknown