VPS15

TgVPS15 is a protein kinase-domain-containing regulator of the single PI3-kinase activity in Toxoplasma gondii, driving generation of the phosphatidylinositol-3-phosphate (PI3P) pool that governs membrane-trafficking events in the parasite (PMID:36318587). Its kinase domain carries two active-site residues that diverge from conventional kinases yet are required for PI3P formation and parasite function, as demonstrated by active-site mutagenesis (PMID:36318587). The PI3P generated by TgVPS15 recruits the PI3P-binding protein TgATG18, which in turn controls trafficking and conjugation of TgATG8, thereby directing both apicoplast biogenesis/inheritance (PMID:36318587) and, under nutrient-limiting conditions, autophagosome formation (PMID:36318587). All available evidence derives from a single study in T. gondii; no structural, biochemical-reconstitution, or cross-species data are present in the corpus.

1. 2022 Medium

   Establishing whether TgVPS15 contributes to lipid signaling, this work showed it regulates the parasite's single PI3-kinase activity and promotes PI3P generation, defining it as a control point for PI3P-dependent membrane biology.

   Evidence Genetic/functional characterization with direct PI3P level measurements in T. gondii

   PMID:36318587

   Open questions at the time

   • No direct biochemical demonstration that TgVPS15 physically associates with or activates the PI3-kinase
   • Strain unspecified
   • No structural model of the enzyme complex
2. 2022 Medium

   To test whether the kinase domain is functionally required, active-site mutagenesis identified two non-canonical residues essential for PI3P formation and parasite functions, establishing a catalytic determinant distinct from conventional kinases.

   Evidence Active-site mutagenesis coupled to PI3P and functional assays in T. gondii

   PMID:36318587

   Open questions at the time

   • Whether these residues confer intrinsic kinase activity versus a scaffolding/regulatory role is not resolved
   • No in vitro reconstitution of the mutant enzyme
3. 2022 Medium

   To place TgVPS15 in organelle biology, loss-of-function studies showed it controls apicoplast biogenesis and inheritance by generating PI3P that recruits TgATG18 to direct TgATG8 trafficking/conjugation to the apicoplast, linking lipid signaling to organelle maintenance.

   Evidence Loss-of-function, TgATG8/TgATG18 localization, and organelle inheritance assays in T. gondii

   PMID:36318587

   Open questions at the time

   • Direct interaction between TgVPS15 and TgATG18/TgATG8 not biochemically demonstrated
   • Mechanism of PI3P-mediated TgATG18 recruitment to the apicoplast not resolved at molecular detail
4. 2022 Medium

   To extend the pathway to stress adaptation, the same system showed TgVPS15 promotes autophagosome formation under nutrient limitation via TgATG18 and TgATG8 conjugation, indicating a dual role of the PI3P pool in constitutive organelle biology and induced autophagy.

   Evidence Loss-of-function under nutrient-limiting conditions with autophagosome formation and TgATG8 localization assays in T. gondii

   PMID:36318587

   Open questions at the time

   • How the same PI3P pool is partitioned between apicoplast and autophagosome pathways is unknown
   • Upstream signals coupling nutrient status to TgVPS15 activity not identified

• Whether TgVPS15 possesses intrinsic catalytic activity, how it physically engages the PI3-kinase and the ATG machinery, and its structural organization remain uncharacterized in the available corpus.
• No structural or reconstituted-biochemistry evidence
• No direct binding partner map
• All evidence from a single study/lab