An unconventional SNARE complex mediates exocytosis at the plasma membrane and vesicular fusion at the apical annuli in Toxoplasma gondii

Exocytosis is a key active process in cells by which proteins are released in bulk via the fusion of exocytic vesicles with the plasma membrane. Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein-mediated vesicle fusion with the plasma membrane is essential in most exocytotic pathways. In mammalian cells, the vesicular fusion step of exocytosis is normally mediated by Syntaxin-1 (Stx1) and SNAP25 family proteins (SNAP25 and SNAP23). However, in Toxoplasma gondii, a model organism of Apicomplexa, the only SNAP25 family protein, with a SNAP29-like molecular structure, is involved in vesicular fusion at the apicoplast. Here, we reveal that an unconventional SNARE complex comprising TgStx1, TgStx20, and TgStx21 mediates vesicular fusion at the plasma membrane. This complex is essential for the exocytosis of surface proteins and vesicular fusion at the apical annuli in T. gondii. Author summarySoluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins that mediate exocytosis in T. gondii have not been well identified. In this study, the functions of two unique SNAREs, TgStx20 and TgStx21, that share homology with the C- and N-terminal SNARE domains of SNAP23 were investigated through tagging an auxin-inducible degron system. The results indicate that both SNAREs are required for the lytic cycle of this parasite. Depletion of TgStx20 or TgStx21 caused a deficiency of exocytosis of surface proteins. In particular, the SNARE proteins located in T. gondii apical annuli were demonstrated to be required for the fusion of Rab11A-positive vesicles at these sites for the first time. We also found that a conserved Qa SNARE, TgStx1, forms a complex with TgStx20 and TgStx21. Depletion of TgStx1 in parasites caused similar phenotypes as those observed in TgStx20- or TgStx21-deficient parasites. These data reveal a unique SNARE complex that mediates vesicular fusion at the plasma membrane and provide further insight into the molecular mechanism of the vesicular trafficking pathway to the apical annuli.

Automated summary

Exocytosis is an important process in cells, and it is mediated by SNARE proteins. In Toxoplasma gondii, a unique SNARE complex is involved in vesicular fusion at the apical annuli, which is essential for the exocytosis of surface proteins.