STX6

STX6 (Syntaxin 6) is a SNARE protein that mediates retrograde vesicular transport within the endosomal-to-Golgi axis of apicomplexan parasites (PMID:23962112). In Toxoplasma gondii, TgStx6 localizes to endosomal-like compartments (ELCs) and the trans-Golgi network (TGN), and its overexpression expands ELCs, fragments the Golgi, and impairs inner membrane complex (IMC) maturation, defining its role in retrograde traffic between ELCs, the TGN, and the Golgi (PMID:23962112). This TGN-associated retrograde pathway is functionally distinct from the routes delivering proteins to micronemes, rhoptries, and the apicoplast, which are unaffected by TgStx6 overexpression (PMID:23962112). TgStx6 acts together with the SEC1/Munc18 factor TgVps45 and the syntaxin Stx16 during IMC (alveolar) biogenesis, a coordination conserved in both T. gondii and Plasmodium falciparum, placing STX6 in the endolysosomal/TGN trafficking pathway upstream of IMC formation (PMID:33082261). Beyond these findings, no further mechanistic detail on STX6 has been characterized in the available corpus.

1. 2013 Medium

   It was unknown where TgStx6 acts in the parasite secretory system; localizing it to ELCs and the TGN and showing that its overexpression disrupts organelle morphology established its role in retrograde transport between ELCs, the TGN, and the Golgi.

   Evidence Overexpression of TgStx6 with fluorescence microscopy of organelle morphology and IMC maturation in T. gondii

   PMID:23962112

   Open questions at the time

   • Phenotypes derive from overexpression rather than loss-of-function
   • Direct SNARE partners and cognate vesicle cargo not identified
   • Molecular basis of the IMC maturation defect not resolved
2. 2013 Medium

   Whether TgStx6 served general secretion or a specific pathway was unclear; showing that micronemes, rhoptries, and the apicoplast are unaffected established that the TgStx6-dependent retrograde route is distinct from apical organelle delivery pathways.

   Evidence Fluorescence microscopy of organelle markers following TgStx6 overexpression in T. gondii

   PMID:23962112

   Open questions at the time

   • Negative result from overexpression; a knockout might reveal contributions to these pathways
   • Does not define which cargos depend on TgStx6
3. 2020 Medium

   The trafficking machinery driving IMC biogenesis was undefined; genetic analysis showed TgStx6 acts with TgVps45 and Stx16 in alveolar/IMC formation in both T. gondii and P. falciparum, placing it in the endolysosomal/TGN pathway upstream of IMC assembly.

   Evidence Functional genetic analysis of SNARE/SEC1-Munc18 factors with IMC biogenesis readout in T. gondii and P. falciparum

   PMID:33082261

   Open questions at the time

   • Direct physical interaction among Stx6, Vps45, and Stx16 not demonstrated
   • Vesicle identity and cargo carried to the nascent IMC unresolved
   • Single study

• The cognate SNARE complex partners, vesicle cargo, and the mechanistic link between retrograde ELC/TGN transport and IMC membrane assembly remain undefined.
• No biochemical identification of the STX6-containing SNARE complex
• Cargo delivered via the TgStx6 retrograde pathway unknown
• No structural model of STX6 function