alphaSNAP

Toxoplasma gondii αSNAP is a component of the NSF/SNAP/SNARE/Rab vesicle-fusion machinery that drives dense granule secretion in the parasite (PMID:9891012). Reconstitution in streptolysin O-permeabilized parasites showed that recombinant αSNAP augments constitutive, calcium-independent dense granule release, that this release is N-ethylmaleimide-sensitive and Rab-GDI-inhibitable, and that mammalian αSNAP functionally substitutes for the parasite protein (PMID:9891012). αSNAP is phosphorylated, and this modification is required for its role in cis-SNARE complex disassembly and secretory traffic: conditional expression of a non-phosphorylatable form causes Golgi fragmentation, vesiculation of downstream secretory organelles (micronemes, rhoptries, dense granules), and disruption of apicoplast biogenesis (PMID:26566590). Beyond these findings, no further mechanistic detail has been characterized in the available corpus.

1. 1999 Medium

   Whether the NSF/SNAP/SNARE/Rab fusion machinery participates in parasite dense granule secretion was unknown; adding recombinant αSNAP established that αSNAP-dependent membrane fusion drives constitutive, calcium-independent dense granule release.

   Evidence Streptolysin O-permeabilized T. gondii secretion assay with recombinant bovine alpha-SNAP, N-ethylmaleimide inhibition, Rab-GDI inhibition, and GTPγS stimulation

   PMID:9891012

   Open questions at the time

   • Endogenous parasite αSNAP not directly assayed — activity inferred from mammalian ortholog substitution
   • Direct SNARE and NSF partners in the parasite not identified
   • Does not address the enzyme's role beyond dense granules
2. 2015 Medium

   How αSNAP function is regulated and whether it acts across the whole secretory pathway was unknown; a non-phosphorylatable mutant revealed that αSNAP phosphorylation controls cis-SNARE disassembly and is required for secretory organelle and apicoplast biogenesis.

   Evidence Conditional expression of a non-phosphorylatable αSNAP mutant with fluorescence microscopy of Golgi, secretory organelle, and apicoplast markers during intracellular development

   PMID:26566590

   Open questions at the time

   • The kinase responsible for αSNAP phosphorylation is not identified
   • The phosphorylated residue(s) and how phosphorylation alters SNARE disassembly biochemically are not defined
   • Whether apicoplast disruption is a direct or downstream consequence is unresolved

• The kinase, phosphosites, and direct SNARE/NSF partners of parasite αSNAP remain uncharacterized in the available corpus.
• No kinase assigned to αSNAP phosphorylation
• No structural or biochemical model of αSNAP-mediated cis-SNARE disassembly in the parasite
• Direct parasite SNARE/NSF binding partners not mapped