TBC9

TgTBC9 is an ER-localized, TBC-domain GTPase-activating protein (GAP) that serves as an essential regulator of vesicular trafficking and organelle biogenesis in the apicomplexan parasite Toxoplasma gondii (PMID:38713739, PMID:38762629). Its GAP activity depends on a conserved dual-finger active site in the TBC domain, and it acts preferentially on the Rab2 GTPase, defining a TBC9–Rab2 axis that governs ER morphology and ER-to-Golgi vesicular transport (PMID:38713739, PMID:38762629). TgTBC9 also engages Rab11A and Rab11B, linking it additionally to early vesicular transport and to the formation of daughter inner membrane complexes during parasite division (PMID:38762629). Depletion of TgTBC9 arrests growth and produces pleiotropic organelle defects—disrupted ER and mitochondrial morphology, aberrant lipid droplets and multi-membranous structures, mislocalized ER/Golgi cargo, and impaired daughter IMC assembly—establishing the protein as essential for parasite survival (PMID:38713739, PMID:38762629). Functional conservation across apicomplexans is demonstrated by rescue of the lethal knockdown with the Plasmodium falciparum orthologue (PMID:38713739).

1. 2024 High

   Systematic subcellular mapping was needed to place T. gondii TBC-domain proteins into a functional context; localizing TgTBC9 to the ER positioned it as a candidate regulator of early secretory trafficking.

   Evidence Fluorescence microscopy of tagged proteins in a systematic localization of all 18 TBC-domain proteins in tachyzoites

   PMID:38713739 PMID:38762629

   Open questions at the time

   • Does not identify the GAP substrate or catalytic requirement
   • Sub-ER compartmentalization not resolved
2. 2024 High

   Whether TgTBC9 is dispensable was unknown; conditional depletion established it as essential and revealed a pleiotropic organelle-biogenesis role rather than a single trafficking step.

   Evidence Auxin-inducible degron knockdown with plaque/growth assays and electron/fluorescence microscopy of ER, mitochondria, and lipid droplets

   PMID:38713739 PMID:38762629

   Open questions at the time

   • Does not distinguish direct from downstream secondary organelle defects
   • Molecular basis of lipid droplet and mitochondrial phenotypes unresolved
3. 2024 High

   It was unclear whether TgTBC9 acts catalytically; mutagenesis of the conserved dual-finger motif showed its GAP activity is required for function, confirming a bona fide catalytic TBC-GAP mechanism.

   Evidence Active-site mutagenesis of the TBC-domain dual-finger motif with functional readout

   PMID:38713739

   Open questions at the time

   • No direct in vitro GTP-hydrolysis kinetics reported
   • Single lab
4. 2024 High

   The physiological Rab target was undefined; identifying preferential Rab2 binding established TBC9–Rab2 as the GAP–substrate pair coupling GAP activity to ER morphology and vesicular trafficking.

   Evidence Reciprocal co-immunoprecipitation and yeast two-hybrid; interaction independently corroborated by pull-down

   PMID:38713739 PMID:38762629

   Open questions at the time

   • Direct demonstration of Rab2-GTP hydrolysis by TBC9 not shown biochemically
   • Whether Rab2 is the sole physiologically relevant substrate unclear
5. 2024 Medium

   Beyond Rab2, additional Rab partners were tested; interaction with Rab11A/B links TgTBC9 to daughter IMC formation and early vesicular transport, broadening its regulatory scope.

   Evidence Pull-down assay identifying Rab11A and Rab11B binding

   PMID:38762629

   Open questions at the time

   • Pull-down without reciprocal or in vitro GAP validation for Rab11A/B
   • Whether TBC9 exerts GAP activity toward Rab11 or only binds is untested
6. 2024 Medium

   Whether TBC9 function is conserved across apicomplexans was unknown; rescue of the lethal knockdown by the P. falciparum orthologue demonstrated cross-species functional conservation.

   Evidence Genetic complementation with the P. falciparum orthologue in TgTBC9-AID knockdown parasites

   PMID:38713739

   Open questions at the time

   • Single lab
   • Does not establish whether the orthologue shares the same Rab specificity

• Direct enzymatic characterization of TgTBC9 GAP activity toward Rab2 and Rab11 in a reconstituted system, and the mechanistic link between individual Rab interactions and the distinct organelle phenotypes, remain to be established.
• No in vitro GTP-hydrolysis assay reported
• Causal attribution of each organelle defect to a specific Rab unresolved
• No structural model of the TBC9–Rab complex