GST2

TgGST2 is a Golgi-endosomal membrane protein of Toxoplasma gondii that couples glutathione S-transferase activity to the machinery of post-Golgi secretory trafficking (PMID:33543805). It colocalizes with vesicle transport factors synaptobrevin, the putative sortilin VPS10, Rab5, and Rab6, and its own residence in the Golgi-endosomal system depends on intact post-Golgi transport (PMID:33543805). Loss of TgGST2 mislocalizes and reduces microneme, rhoptry, and dense granule proteins and disperses Rab5/Rab6 puncta, establishing it as required for accurate vesicle trafficking of secretory cargo; this trafficking role is functionally linked to the lytic cycle, as knockout parasites show reduced invasion and attenuated virulence in mice (PMID:33543805). TgGST2 also carries glutathione S-transferase enzymatic activity, and progesterone disrupts its Golgi localization with downstream mislocalization of VPS10 and micronemal MIC2/M2AP and impaired motility (PMID:36510329).

1. 2021 Medium

   Establishing where TgGST2 resides addressed whether a parasite glutathione S-transferase is linked to secretory trafficking rather than cytosolic detoxification, placing it in the Golgi-endosomal vesicle transport system.

   Evidence Immunofluorescence and colocalization with synaptobrevin, VPS10/sortilin, Rab5, and Rab6 in T. gondii

   PMID:33543805

   Open questions at the time

   • Colocalization does not demonstrate direct physical binding to these transport factors
   • Single lab, single study
   • Membrane topology and orientation not defined
2. 2021 Medium

   Genetic ablation tested whether TgGST2 is functionally required for trafficking, showing it is needed for correct delivery of secretory proteins and for maintaining Rab5/Rab6 organization.

   Evidence Knockout with IFA and Western blot showing mislocalization/reduced expression of microneme, rhoptry, and dense granule proteins and disrupted Rab distribution

   PMID:33543805

   Open questions at the time

   • Molecular mechanism by which a GST supports vesicle trafficking is unresolved
   • Whether the trafficking defect requires enzymatic activity is untested
3. 2021 Medium

   Phenotyping the knockout connected the trafficking defect to the parasite lytic cycle, showing TgGST2 loss reduces invasion and virulence.

   Evidence Invasion assay and mouse virulence model of TgGST2 knockout

   PMID:33543805

   Open questions at the time

   • Does not separate invasion defect from upstream secretory protein mislocalization
   • Contribution of each affected organelle class to virulence not dissected
4. 2021 Low

   Perturbing post-Golgi transport tested whether TgGST2 localization is a passive consequence of the trafficking system it supports, showing its distribution depends on intact post-Golgi transport.

   Evidence Subcellular localization analysis under vesicle transport perturbation

   PMID:33543805

   Open questions at the time

   • Localization dependency inferred without detailed mechanistic dissection
   • Single lab
5. 2022 Low

   Characterizing the enzyme addressed whether TgGST2 is an active glutathione S-transferase and whether steroid hormones engage it, showing GST activity that is inhibited by estradiol and progesterone.

   Evidence GST enzyme activity assay with E2/P4 inhibition plus molecular docking (computational)

   PMID:36510329

   Open questions at the time

   • Steroid binding rests on computational docking, awaiting direct biophysical binding measurement
   • Endogenous physiological substrate not identified
   • Single lab, single study
6. 2022 Low

   Hormone treatment tested whether steroid engagement has cellular consequences, showing progesterone relocalizes TgGST2, mislocalizes VPS10 and MIC2/M2AP, and impairs motility, while estradiol does not affect localization.

   Evidence IFA under progesterone and estradiol drug treatment in T. gondii

   PMID:36510329

   Open questions at the time

   • Pharmacological/indirect approach does not prove the effect is mediated through TgGST2 itself
   • Discordance between E2 enzyme inhibition and lack of localization effect unexplained

• How TgGST2 mechanistically links glutathione S-transferase chemistry to vesicle sorting, and whether its enzymatic activity is required for trafficking, remains unresolved.
• No physiological GST substrate identified
• Direct physical interaction with Rab/SNARE/sortilin machinery not demonstrated
• No structural model of the trafficking-active protein