MYSTA

TgMYST-A (MYSTA) is a MYST-family lysine acetyltransferase of Toxoplasma gondii that acetylates histone H4 at lysines 5, 8, 12, and 16 in vitro, implicating it in chromatin-associated regulation (PMID:16339723). The protein localizes to both the nucleus and cytoplasm and is far more abundant in the rapidly replicating tachyzoite stage than in bradyzoites, linking its expression to replicative status (PMID:16339723). Beyond its in vitro acetyltransferase activity, dual localization, and stage-regulated abundance, no in vivo substrate, protein partner, or genetic loss-of-function phenotype has been characterized in the available corpus.

1. 2005 High

   Establishing whether TgMYST-A is a functional acetyltransferase and what it targets was needed to place it in Toxoplasma chromatin biology; the recombinant enzyme was shown to acetylate histone H4 at specific lysines, defining it as a bona fide MYST HAT.

   Evidence In vitro histone acetyltransferase assay with recombinant protein and site-specific substrate mapping

   PMID:16339723

   Open questions at the time

   • No in vivo substrate confirmed
   • Non-histone substrates not tested
   • Catalytic residues not mutated to confirm activity
2. 2005 Medium

   To connect enzymatic activity to a cellular compartment, antibody staining showed both isoforms distribute to nucleus and cytoplasm, indicating potential roles beyond chromatin.

   Evidence Immunofluorescence/immunolocalization with TgMYST-A-specific antibody

   PMID:16339723

   Open questions at the time

   • Cytoplasmic function unknown
   • No functional consequence linked to localization
   • Single lab, one antibody
3. 2005 Medium

   Whether TgMYST-A expression tracks the parasite life cycle was addressed by comparing stages, revealing predominant tachyzoite abundance and a link to replication status.

   Evidence Immunoblot/antibody-based detection comparing tachyzoite and bradyzoite stages

   PMID:16339723

   Open questions at the time

   • Mechanism of stage regulation unknown
   • No causal role in stage conversion tested
4. 2005 Low

   Domain architecture was defined to predict regulatory features, identifying a MYST catalytic domain, an atypical C2HC zinc finger, and a chromodomain across two isoforms.

   Evidence Genomic/transcriptomic analysis and bioinformatics domain identification

   PMID:16339723

   Open questions at the time

   • Domains established by bioinformatics without functional mutagenesis
   • Chromodomain binding target untested
   • Functional distinction between isoforms unknown

• Whether TgMYST-A is essential and what genes or non-histone substrates it regulates in vivo remains unresolved.
• No genetic knockout or conditional depletion
• No in vivo substrate or genomic target identified
• No interacting protein complex defined