SNAP29

SNAP29 (TgSNAP29), the sole SNAP25-family SNARE protein in Toxoplasma gondii, adopts a SNAP29-like structure and functions as a Qbc SNARE mediating SNARE-dependent vesicular fusion at the apicoplast, where it is required for apicoplast biogenesis and maintenance (PMID:36095096, PMID:36972314). Its depletion disrupts localization of TgATG8 at the apicoplast, indicating that TgATG8 is delivered to the organelle via vesicles in a SNARE-dependent manner (PMID:36095096). TgSNAP29 acts specifically at the apicoplast and does not mediate vesicular fusion at the plasma membrane, a role instead fulfilled by an unconventional SNARE complex of TgStx1, TgStx20, and TgStx21 (PMID:36972314). Beyond this apicoplast fusion role, no further mechanistic detail on TgSNAP29 has been characterized in the available corpus.

1. 2022 Medium

   Whether apicoplast maintenance and TgATG8 delivery depend on membrane fusion machinery was unknown; showing that depleting a Qbc SNARE disrupts both established a SNARE-dependent vesicular route for ATG8 translocation to the apicoplast.

   Evidence Auxin-inducible degron depletion of TgSNAP29 in T. gondii with monitoring of apicoplast maintenance and TgATG8 localization

   PMID:36095096

   Open questions at the time

   • The direct SNARE partners of TgSNAP29 at the apicoplast are not identified
   • The vesicular source and cargo composition delivering TgATG8 are not defined
   • Whether TgATG8 is a direct fusion cargo or an indirect consequence of disrupted fusion is unresolved
2. 2023 Medium

   Whether the single SNAP25-family protein acts generally in secretion or at a specific compartment was unknown; functional characterization assigned it to apicoplast vesicular fusion and excluded a role at the plasma membrane, which is instead served by a distinct syntaxin complex.

   Evidence Genetic and cell biological characterization of SNARE components including plasma membrane exocytosis assays and localization studies in T. gondii

   PMID:36972314

   Open questions at the time

   • The molecular structure of TgSNAP29 is inferred as SNAP29-like rather than experimentally solved
   • The full apicoplast SNARE complex composition is not defined
   • How TgSNAP29 is targeted specifically to the apicoplast and excluded from the plasma membrane is unknown

• The direct SNARE partners, structural basis, and vesicle-targeting mechanism of TgSNAP29 at the apicoplast remain undefined.
• No apicoplast SNARE complex partners have been identified for TgSNAP29
• No experimental structure exists
• The mechanism restricting TgSNAP29 function to the apicoplast is uncharacterized