DHHC7

TgDHHC7 is a rhoptry-localized palmitoyl acyltransferase (PAT) in Toxoplasma gondii that anchors the secretory rhoptry organelles at the parasite apex to enable host cell invasion (PMID:23408890). Its enzymatic function depends on an intact DHHC catalytic motif, as active-site point mutation abolishes rescue of apical rhoptry tethering (PMID:23408890). TgDHHC7 acts upstream of the palmitoylated armadillo-repeat protein TgARO: knockdown of the PAT mislocalizes TgARO, and independent TgARO depletion phenocopies the dispersed-rhoptry defect, placing the two in a single tethering pathway (PMID:23408890, PMID:23638681). Loss of TgDHHC7 activity disperses rhoptries and produces a complete block in invasion while leaving attachment and egress intact, establishing apical rhoptry positioning as specifically required for invasion (PMID:23408890). Rhoptry targeting of the enzyme depends on a short conserved C-terminal region rather than its divergent N-terminal domain (PMID:23408890), and rhoptry localization of DHHC7 is conserved across apicomplexans including Plasmodium falciparum (PMID:23638681).

1. 2013 High

   Whether a specific palmitoyl acyltransferase controls rhoptry organization was unknown; identifying TgDHHC7 as a rhoptry PAT whose loss disperses rhoptries and blocks invasion established apical rhoptry tethering as an invasion-specific requirement.

   Evidence TetO conditional knockdown with localization imaging plus invasion and egress assays in T. gondii

   PMID:23408890

   Open questions at the time

   • Direct enzymatic palmitoylation of a defined substrate not biochemically reconstituted
   • Mechanism by which tethering promotes invasion but not egress unresolved
2. 2013 High

   Whether the tethering role required catalytic activity versus a structural scaffold role was unresolved; DHHC-motif point mutation failing to rescue tethering established that enzymatic palmitoylation activity is required.

   Evidence Active-site point mutagenesis with complementation rescue in T. gondii

   PMID:23408890

   Open questions at the time

   • Palmitoylated residue on the substrate not mapped
   • In vitro acyltransferase activity not demonstrated
3. 2013 Medium

   The determinants of enzyme targeting were unknown; truncation mapping showed a short conserved C-terminal region directs rhoptry localization while the divergent N-terminus is dispensable.

   Evidence Domain deletion/truncation analysis with localization imaging in T. gondii

   PMID:23408890

   Open questions at the time

   • Trafficking machinery recognizing the C-terminal region not identified
   • Single-lab truncation mapping
4. 2013 High

   The relevant substrate pathway was undefined; epistasis showed TgDHHC7 acts upstream of the armadillo-repeat protein TgARO, with TgARO knockdown recapitulating the dispersed-rhoptry phenotype.

   Evidence Paired conditional knockdowns of TgDHHC7 and TgARO with matching phenotypic and localization readouts in T. gondii

   PMID:23408890 PMID:23638681

   Open questions at the time

   • Direct enzyme-substrate palmitoylation of TgARO not shown biochemically
   • Downstream effectors linking TgARO to rhoptry tethering unresolved
5. 2013 Medium

   Whether rhoptry targeting of this PAT is a general apicomplexan feature was untested; localization across multiple species showed DHHC7 targets rhoptries in Plasmodium falciparum and other apicomplexans.

   Evidence Fluorescence localization imaging in multiple apicomplexan species

   PMID:23638681

   Open questions at the time

   • No functional validation in P. falciparum
   • Whether the Plasmodium ortholog palmitoylates an ARO-equivalent substrate not established

• Direct biochemical demonstration of TgDHHC7-mediated palmitoylation of TgARO and the molecular link between rhoptry tethering and invasion remain open.
• No in vitro acyltransferase assay with defined substrate
• No structural model of the enzyme or its targeting region
• Mechanism connecting apical rhoptry position to invasion competence unknown