FNT2

TgFNT2 is a plasma membrane-localized formate-nitrite transporter (FNT)-type monocarboxylate/proton symporter of Toxoplasma gondii tachyzoites that couples parasite carbon metabolism to its host environment by transporting l-lactate, pyruvate, and formate in a pH-dependent manner (PMID:30237165). Acting redundantly with TgFNT1, it exports parasite-derived lactate and pyruvate to drive acidification of the parasitophorous vacuole before and during egress (PMID:35512005), and imports host-derived lactate and pyruvate to sustain mitochondrial respiration, membrane potential, ATP production, extracellular survival, and acute virulence in mice (PMID:42124602). Substrate selectivity and inhibitor sensitivity are governed by critical residues in its substrate-selectivity region, and pharmacological blockade of TgFNTs with 2-hydroxy-chromanone compounds inhibits parasite growth (PMID:30237165).

1. 2018 High

   Establishing what molecular activity TgFNT2 carries out, this work identified it as a plasma membrane FNT-type monocarboxylate/proton symporter transporting l-lactate and formate, defining its biochemical identity and druggable active site.

   Evidence Heterologous expression and transport assays, cell fractionation/imaging for localization, and active-site mutagenesis with 2-hydroxy-chromanone inhibitors in T. gondii

   PMID:30237165

   Open questions at the time

   • Directionality (import vs export) under physiological conditions not resolved here
   • Contribution of TgFNT2 relative to TgFNT1/TgFNT3 to overall transport not dissected
2. 2018 High

   To test whether the transporter is a viable drug target, mutagenesis defined which substrate-selectivity residues confer specificity and inhibitor sensitivity, and showed that chemical blockade impairs parasite growth.

   Evidence Active-site mutagenesis and growth inhibition assays with 2-hydroxy-chromanone compounds

   PMID:30237165

   Open questions at the time

   • Structural basis of inhibitor binding not resolved
   • In vivo efficacy of inhibitors not tested
3. 2022 Medium

   To determine the physiological role of the transporter in the lytic cycle, conditional loss-of-function showed FNT2 (with FNT1) mediates lactate/pyruvate release driving parasitophorous vacuole acidification during egress.

   Evidence Auxin-inducible degron knockdown and knockout, ratiometric pHluorin PV pH imaging, and metabolite secretion measurements in the RH strain

   PMID:35512005

   Open questions at the time

   • Individual contribution of FNT2 versus FNT1 not separated
   • Mechanistic link between PV acidification and egress not fully defined
4. 2026 Medium

   To resolve the transporter's role in host-derived carbon uptake and bioenergetics, combined genetic ablation demonstrated FNT1/FNT2 import host lactate and pyruvate to fuel mitochondrial respiration, ATP production, survival, and virulence.

   Evidence Individual and combined knockouts of TgFNT1-3, hexokinase knockout to ablate endogenous pyruvate, oxygen consumption/membrane potential/ATP assays, and mouse virulence assays (preprint)

   PMID:42124602

   Open questions at the time

   • Preprint, not yet peer-reviewed
   • FNT2-specific contribution distinct from FNT1 not fully isolated
   • Relative importance of import versus export functions in vivo unresolved

• The distinct, non-redundant contribution of TgFNT2 relative to TgFNT1/TgFNT3, and a structural model of substrate/inhibitor recognition, remain to be established.
• No structure of TgFNT2 or its inhibitor complex reported
• FNT2-specific loss-of-function phenotype not isolated from FNT1