VPS9

TgVps9 is a guanine nucleotide exchange factor (GEF) that governs vesicular trafficking to the secretory organelles of Toxoplasma gondii, thereby supporting the parasite's ability to invade host cells (PMID:27966671). Biochemically, it activates Rab5a by catalyzing nucleotide exchange, coupling this GTPase to trafficking toward the secretory organelles (PMID:27966671). Functionally, loss of TgVps9 disrupts protein maturation and biogenesis of rhoptries, micronemes, and dense granules, producing defects in the correct sorting of proteins to these organelles (PMID:27966671) and impairing secretion required for host cell infection (PMID:27966671). Beyond this Rab5a-dependent trafficking role in T. gondii, no further mechanistic detail has been characterized in the available corpus.

1. 2016 Medium

   Establishing the biochemical activity of TgVps9 answered whether it is a catalytic regulator of Rab GTPase signaling: it was shown to be a GEF that activates Rab5a, defining the molecular reaction it drives in secretory trafficking.

   Evidence in vitro GEF activity assay measuring nucleotide exchange toward Rab5a in Toxoplasma gondii

   PMID:27966671

   Open questions at the time

   • Structural basis of the TgVps9-Rab5a interaction not resolved
   • Whether TgVps9 acts on Rab GTPases other than Rab5a not tested
   • Kinetics and specificity of exchange not quantified against other candidate substrates
2. 2016 Medium

   Loss-of-function analysis connected TgVps9 GEF activity to organelle biology, showing it is required for correct protein sorting to and biogenesis of rhoptries, micronemes, and dense granules.

   Evidence genetic loss-of-function with phenotypic readout of secretory organelle biogenesis and protein maturation in T. gondii

   PMID:27966671

   Open questions at the time

   • Which cargo proteins depend directly on TgVps9-mediated trafficking not delineated
   • Whether the sorting defect is a direct consequence of impaired Rab5a activation not dissected
   • Subcellular site of TgVps9 action within the endosomal/secretory pathway not mapped
3. 2016 Medium

   Host infection assays established the physiological consequence of TgVps9 function, showing it is a critical trafficking facilitator required for secretion and host cell invasion.

   Evidence loss-of-function genetic analysis coupled to host infection assay in T. gondii

   PMID:27966671

   Open questions at the time

   • Contribution of individual organelle defects to the invasion phenotype not separated
   • In vivo relevance beyond cultured host cell infection not assessed

• Whether TgVps9 has additional Rab substrates, defined domain determinants of its GEF activity, or a resolved localization within the endosomal-secretory pathway remains unaddressed in the available corpus.
• No structural or domain-level characterization of TgVps9
• No interactome beyond Rab5a
• No independent-lab replication of the GEF and phenotypic findings