NMT

TgNMT is the Toxoplasma gondii N-myristoyltransferase, an enzyme with demonstrated in vitro myristoyltransferase activity that is expressed in both intra- and extracellular parasites, interacts with predicted substrates, and whose activity is required for the parasite lytic cycle (PMID:31150653). It can be blocked by a small-molecule NMT inhibitor originally developed against Plasmodium, establishing functional conservation of the enzyme across apicomplexan parasites (PMID:32618271). Among its substrates is the microneme protein MIC7, which enters the secretory pathway unconventionally with its myristoylated N-terminus facing the microneme lumen; this modification is required for the initial steps of host cell invasion (PMID:32618271). Beyond this substrate set and its role in invasion, no further mechanistic detail has been characterized in the available corpus.

1. 2019 Medium

   Establishing that the T. gondii genome-encoded NMT is a bona fide active enzyme relevant to parasite biology answered whether myristoylation is enzymatically operative and functionally important in this parasite.

   Evidence In vitro enzymatic activity assay, co-immunoprecipitation with predicted substrates, expression profiling, and lytic cycle assay under NMT inhibition in T. gondii

   PMID:31150653

   Open questions at the time

   • Specific endogenous substrates were not yet defined
   • Mechanistic pathway placement validated in a single lab with limited orthogonal support
2. 2020 Medium

   Showing that a Plasmodium-directed NMT inhibitor also blocks TgNMT answered whether the enzyme is chemically tractable and conserved across apicomplexans, supporting cross-parasite drug targeting.

   Evidence Chemoproteomic profiling with small-molecule NMT inhibitor treatment in T. gondii

   PMID:32618271

   Open questions at the time

   • Inhibitor selectivity and off-target effects within the parasite not fully resolved
   • Structural basis of inhibitor binding not addressed
3. 2020 High

   Identifying MIC7 as a myristoylated substrate and linking its modification to invasion answered how NMT activity mechanistically contributes to the lytic cycle, connecting a single modification to a defined biological step.

   Evidence Chemoproteomic myristoylome profiling, MIC7 substrate identification, unconventional topology characterization, and myristoylation-site mutant host-penetration assays

   PMID:32618271

   Open questions at the time

   • The molecular mechanism by which luminal myristoylation of MIC7 aids penetration is unresolved
   • The full complement of TgNMT substrates beyond MIC7 is not defined
   • How MIC7 achieves unconventional secretory-pathway entry with a luminal myristoyl group is unexplained

• Whether other myristoylated TgNMT substrates contribute to distinct steps of the lytic cycle, and how myristoylation is mechanistically coupled to protein trafficking and invasion, remains open.
• No structural model of TgNMT
• Substrate repertoire beyond MIC7 uncharacterized
• Mechanism linking myristoylation to invasion function unknown