VPS45

VPS45 (TgVps45) is a Sec1/Munc18-like regulator of vesicle fusion that operates at the trans-Golgi network in apicomplexan parasites, where it acts through the SNARE machinery to support membrane trafficking and biogenesis of the parasite pellicle (PMID:31670849). It interacts with the SNARE proteins TgStx16 and TgVAMP4-1 to form a fusion complex, with conditional ablation of TgStx16 phenocopying TgVps45 deficiency and thereby placing the two in a common pathway (PMID:31670849). Conditional depletion of TgVps45 disrupts inner membrane complex (alveoli) formation, sub-pellicular microtubule nucleation, cell polarization, and the symmetrical assembly of daughter parasites during endodyogeny (PMID:31670849). A role in IMC biogenesis and in trafficking of endocytosed host proteins is conserved across Toxoplasma gondii and Plasmodium falciparum, where TgVps45 acts coordinately with Stx16 and Stx6 (PMID:33082261). Beyond these SNARE interactions and trafficking phenotypes, no further biochemical detail of the fusion mechanism has been characterized in the available corpus.

1. 2019 Medium

   Establishing the binding partners of TgVps45 was needed to place it in a defined membrane-fusion pathway; identifying its SNARE partners located its action at the trans-Golgi network.

   Evidence Co-immunoprecipitation identifying TgStx16 and TgVAMP4-1, supported by genetic epistasis via conditional TgStx16 ablation, in Toxoplasma gondii

   PMID:31670849

   Open questions at the time

   • Single Co-IP without reciprocal validation or reconstitution of the fusion complex
   • Direct SM-protein/SNARE binding mode and stoichiometry not defined
   • TGN localization inferred rather than mapped biochemically
2. 2019 Medium

   Whether TgVps45 has a cell-biological role beyond generic vesicle fusion was unknown; conditional depletion revealed it is required for pellicle and cytoskeletal morphogenesis during division.

   Evidence Auxin-inducible degron conditional knockout with microscopy readouts of IMC, microtubules, polarization, and endodyogeny in Toxoplasma gondii

   PMID:31670849

   Open questions at the time

   • Molecular link between vesicle fusion and IMC/microtubule assembly not resolved
   • Cargo delivered by the TgVps45-dependent route not identified
   • Single lab, single species
3. 2020 Medium

   Whether the TgVps45 trafficking function is conserved and what cargo it handles was open; comparative depletion showed a conserved role in alveoli biogenesis and in trafficking of endocytosed host proteins across two apicomplexans.

   Evidence Conditional knockdown/genetic depletion with organellar and trafficking phenotyping in Toxoplasma gondii and Plasmodium falciparum

   PMID:33082261

   Open questions at the time

   • Coordinated action with Stx16 and Stx6 stated as likely but not biochemically dissected
   • Mechanism connecting host-protein endocytosis to IMC biogenesis unresolved
   • Direct fusion assay for the endocytic route absent

• How TgVps45-mediated vesicle fusion is mechanistically coupled to inner membrane complex biogenesis, microtubule nucleation, and host-protein endocytosis remains unresolved.
• No structural model of the TgVps45-SNARE complex
• Full SNARE partner set and directionality of fusion not defined
• Cargo identity for the endocytic trafficking route not established