{"gene":"TG_292110","gene_symbol":"FNT2","run_date":"2026-09-08T14:08:30","timeline":{"discoveries":[{"year":2018,"finding":"TgFNT2 (along with TgFNT1 and TgFNT3) is localized to the plasma membrane of T. gondii tachyzoites and functions as a formate-nitrite transporter (FNT)-type monocarboxylate/proton symporter that transports both l-lactate and formate in a pH-dependent manner.","method":"Heterologous expression and transport assays; plasma membrane localization confirmed by cell fractionation/imaging; chemical biology approach with 2-hydroxy-chromanone inhibitors and active-site mutagenesis to identify substrate-selectivity residues","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro transport assays with active-site mutagenesis identifying critical residues, plus localization, in a single rigorous study","pmids":["30237165"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2018,"finding":"TgFNT2 is inhibited by 2-hydroxy-chromanone compounds, and critical residues in its substrate-selectivity region determine differential specificity and sensitivity toward both l-lactate/formate substrates and these inhibitors; pharmacological blockade of TgFNTs inhibits T. gondii growth.","method":"Active-site mutagenesis; growth inhibition assays with 2-hydroxy-chromanone compounds","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — mutagenesis of active-site residues combined with functional transport assays and parasite growth inhibition in a single study","pmids":["30237165"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2022,"finding":"Auxin-induced conditional knockdown and knockout of FNT2 (together with FNT1) reduced levels of lactate and pyruvate released by parasites and led to an abatement of parasitophorous vacuole (PV) acidification prior to and during egress.","method":"Auxin-inducible degron (AID) knockdown and genetic knockout; ratiometric pHluorin live-imaging to measure PV pH; lactate/pyruvate secretion measurements","journal":"PLoS pathogens","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic loss-of-function (KD and KO) with two orthogonal readouts (PV pH measurement and metabolite secretion), single lab","pmids":["35512005"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":"RH"}]},{"year":2026,"finding":"TgFNT2 (together with TgFNT1) acts as a principal plasma membrane monocarboxylate importer for host-derived lactate and pyruvate; disruption of TgFNT1 and TgFNT2 abolishes lactate- and pyruvate-driven mitochondrial respiratory responses, and combined loss of endogenous pyruvate production and exogenous uptake severely reduces mitochondrial basal respiratory capacity, membrane potential, ATP levels, extracellular survival, and virulence in mice.","method":"Genetic disruption of TgFNT1-3 (individual and combined knockouts); oxygen consumption assays; mitochondrial membrane potential and ATP measurements; hexokinase knockout (Δhk) to ablate endogenous pyruvate; mouse virulence assays","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal functional assays (respiration, membrane potential, ATP, virulence) in a single preprint lab study with clean genetic ablation","pmids":["42124602"],"is_preprint":true,"attribution":[{"species":"Toxoplasma gondii","strain":""}]}],"current_model":"TgFNT2 is a plasma membrane-localized formate-nitrite transporter-type monocarboxylate/proton symporter in T. gondii tachyzoites that exports l-lactate and formate (contributing to parasitophorous vacuole acidification and parasite egress) and imports host-derived lactate and pyruvate to sustain mitochondrial respiration, membrane potential, ATP production, extracellular survival, and acute virulence; its activity is inhibited by 2-hydroxy-chromanone compounds through critical residues in its substrate-selectivity region."},"narrative":{"mechanistic_narrative":"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].","teleology":[{"year":2018,"claim":"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","pmids":["30237165"],"confidence":"High","gaps":["Directionality (import vs export) under physiological conditions not resolved here","Contribution of TgFNT2 relative to TgFNT1/TgFNT3 to overall transport not dissected"],"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2018,"claim":"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","pmids":["30237165"],"confidence":"High","gaps":["Structural basis of inhibitor binding not resolved","In vivo efficacy of inhibitors not tested"],"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2022,"claim":"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","pmids":["35512005"],"confidence":"Medium","gaps":["Individual contribution of FNT2 versus FNT1 not separated","Mechanistic link between PV acidification and egress not fully defined"],"attribution":[{"species":"Toxoplasma gondii","strain":"RH"}]},{"year":2026,"claim":"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)","pmids":["42124602"],"confidence":"Medium","gaps":["Preprint, not yet peer-reviewed","FNT2-specific contribution distinct from FNT1 not fully isolated","Relative importance of import versus export functions in vivo unresolved"],"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":null,"claim":"The distinct, non-redundant contribution of TgFNT2 relative to TgFNT1/TgFNT3, and a structural model of substrate/inhibitor recognition, remain to be established.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No structure of TgFNT2 or its inhibitor complex reported","FNT2-specific loss-of-function phenotype not isolated from FNT1"],"attribution":[{"species":"Toxoplasma gondii","strain":""}]}],"mechanism_profile":{"molecular_activity":[],"localization":[],"pathway":[],"complexes":[],"partners":["TGFNT1","TGFNT3"],"other_free_text":["FNT-type monocarboxylate/proton symporter transporting l-lactate, pyruvate, and formate in a pH-dependent manner [#0, #3]","Mediates parasitophorous vacuole acidification via lactate/pyruvate export during egress [#2]","Imports host-derived lactate and pyruvate to sustain mitochondrial respiration, membrane potential, ATP, extracellular survival, and virulence [#3]","Inhibited by 2-hydroxy-chromanone compounds; substrate-selectivity residues determine specificity and inhibitor sensitivity [#0, #1]"]},"strain_conflicts":[]},"prefetch_data":{},"corpus_meta":[{"pmid":"30237165","id":"PMC_30237165","title":"The intracellular parasite Toxoplasma gondii harbors three druggable FNT-type formate and l-lactate transporters in the plasma membrane.","date":"2018","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/30237165","citation_count":29,"is_preprint":false},{"pmid":"35512005","id":"PMC_35512005","title":"Toxoplasma gondii excretion of glycolytic products is associated with acidification of the parasitophorous vacuole during parasite egress.","date":"2022","source":"PLoS pathogens","url":"https://pubmed.ncbi.nlm.nih.gov/35512005","citation_count":10,"is_preprint":false},{"pmid":"42124602","id":"PMC_42124602","title":"A two-membrane gateway for monocarboxylates couples host glycolysis to Toxoplasma mitochondrial fitness and virulence.","date":"2026","source":"bioRxiv : the preprint server for biology","url":"https://pubmed.ncbi.nlm.nih.gov/42124602","citation_count":0,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":4036,"output_tokens":1285,"usd":0.015692,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":6264,"output_tokens":2465,"usd":0.046472,"stage2_stop_reason":"end_turn"},"total_usd":0.062164,"stage1_batch_id":"msgbatch_015xmQKDRqnTh9anE4m7LiD6","stage2_batch_id":"msgbatch_01JLpkFTVhoQBNgqHCKFrVVU","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2018,\n      \"finding\": \"TgFNT2 (along with TgFNT1 and TgFNT3) is localized to the plasma membrane of T. gondii tachyzoites and functions as a formate-nitrite transporter (FNT)-type monocarboxylate/proton symporter that transports both l-lactate and formate in a pH-dependent manner.\",\n      \"method\": \"Heterologous expression and transport assays; plasma membrane localization confirmed by cell fractionation/imaging; chemical biology approach with 2-hydroxy-chromanone inhibitors and active-site mutagenesis to identify substrate-selectivity residues\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro transport assays with active-site mutagenesis identifying critical residues, plus localization, in a single rigorous study\",\n      \"pmids\": [\"30237165\"],\n      \"is_preprint\": false,\n      \"attribution\": [\n        {\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}\n      ]\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"TgFNT2 is inhibited by 2-hydroxy-chromanone compounds, and critical residues in its substrate-selectivity region determine differential specificity and sensitivity toward both l-lactate/formate substrates and these inhibitors; pharmacological blockade of TgFNTs inhibits T. gondii growth.\",\n      \"method\": \"Active-site mutagenesis; growth inhibition assays with 2-hydroxy-chromanone compounds\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — mutagenesis of active-site residues combined with functional transport assays and parasite growth inhibition in a single study\",\n      \"pmids\": [\"30237165\"],\n      \"is_preprint\": false,\n      \"attribution\": [\n        {\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}\n      ]\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Auxin-induced conditional knockdown and knockout of FNT2 (together with FNT1) reduced levels of lactate and pyruvate released by parasites and led to an abatement of parasitophorous vacuole (PV) acidification prior to and during egress.\",\n      \"method\": \"Auxin-inducible degron (AID) knockdown and genetic knockout; ratiometric pHluorin live-imaging to measure PV pH; lactate/pyruvate secretion measurements\",\n      \"journal\": \"PLoS pathogens\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic loss-of-function (KD and KO) with two orthogonal readouts (PV pH measurement and metabolite secretion), single lab\",\n      \"pmids\": [\"35512005\"],\n      \"is_preprint\": false,\n      \"attribution\": [\n        {\"species\": \"Toxoplasma gondii\", \"strain\": \"RH\"}\n      ]\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"TgFNT2 (together with TgFNT1) acts as a principal plasma membrane monocarboxylate importer for host-derived lactate and pyruvate; disruption of TgFNT1 and TgFNT2 abolishes lactate- and pyruvate-driven mitochondrial respiratory responses, and combined loss of endogenous pyruvate production and exogenous uptake severely reduces mitochondrial basal respiratory capacity, membrane potential, ATP levels, extracellular survival, and virulence in mice.\",\n      \"method\": \"Genetic disruption of TgFNT1-3 (individual and combined knockouts); oxygen consumption assays; mitochondrial membrane potential and ATP measurements; hexokinase knockout (Δhk) to ablate endogenous pyruvate; mouse virulence assays\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal functional assays (respiration, membrane potential, ATP, virulence) in a single preprint lab study with clean genetic ablation\",\n      \"pmids\": [\"42124602\"],\n      \"is_preprint\": true,\n      \"attribution\": [\n        {\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}\n      ]\n    }\n  ],\n  \"current_model\": \"TgFNT2 is a plasma membrane-localized formate-nitrite transporter-type monocarboxylate/proton symporter in T. gondii tachyzoites that exports l-lactate and formate (contributing to parasitophorous vacuole acidification and parasite egress) and imports host-derived lactate and pyruvate to sustain mitochondrial respiration, membrane potential, ATP production, extracellular survival, and acute virulence; its activity is inhibited by 2-hydroxy-chromanone compounds through critical residues in its substrate-selectivity region.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"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 [#0]. Acting redundantly with TgFNT1, it exports parasite-derived lactate and pyruvate to drive acidification of the parasitophorous vacuole before and during egress [#2], and imports host-derived lactate and pyruvate to sustain mitochondrial respiration, membrane potential, ATP production, extracellular survival, and acute virulence in mice [#3]. 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 [#0, #1].\",\n  \"teleology\": [\n    {\n      \"year\": 2018,\n      \"claim\": \"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.\",\n      \"evidence\": \"Heterologous expression and transport assays, cell fractionation/imaging for localization, and active-site mutagenesis with 2-hydroxy-chromanone inhibitors in T. gondii\",\n      \"pmids\": [\"30237165\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Directionality (import vs export) under physiological conditions not resolved here\",\n        \"Contribution of TgFNT2 relative to TgFNT1/TgFNT3 to overall transport not dissected\"\n      ],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"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.\",\n      \"evidence\": \"Active-site mutagenesis and growth inhibition assays with 2-hydroxy-chromanone compounds\",\n      \"pmids\": [\"30237165\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Structural basis of inhibitor binding not resolved\",\n        \"In vivo efficacy of inhibitors not tested\"\n      ],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"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.\",\n      \"evidence\": \"Auxin-inducible degron knockdown and knockout, ratiometric pHluorin PV pH imaging, and metabolite secretion measurements in the RH strain\",\n      \"pmids\": [\"35512005\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Individual contribution of FNT2 versus FNT1 not separated\",\n        \"Mechanistic link between PV acidification and egress not fully defined\"\n      ],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"RH\"}]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"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.\",\n      \"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)\",\n      \"pmids\": [\"42124602\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Preprint, not yet peer-reviewed\",\n        \"FNT2-specific contribution distinct from FNT1 not fully isolated\",\n        \"Relative importance of import versus export functions in vivo unresolved\"\n      ],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The distinct, non-redundant contribution of TgFNT2 relative to TgFNT1/TgFNT3, and a structural model of substrate/inhibitor recognition, remain to be established.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"No structure of TgFNT2 or its inhibitor complex reported\",\n        \"FNT2-specific loss-of-function phenotype not isolated from FNT1\"\n      ],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0015355\", \"supporting_discovery_ids\": [0], \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]},\n      {\"term_id\": \"GO:0015132\", \"supporting_discovery_ids\": [0], \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]},\n      {\"term_id\": \"GO:0050833\", \"supporting_discovery_ids\": [2, 3], \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0], \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0006090\", \"supporting_discovery_ids\": [3], \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]},\n      {\"term_id\": \"GO:0045333\", \"supporting_discovery_ids\": [3], \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"TgFNT1\", \"TgFNT3\"],\n    \"other_free_text\": [\n      \"FNT-type monocarboxylate/proton symporter transporting l-lactate, pyruvate, and formate in a pH-dependent manner [#0, #3]\",\n      \"Mediates parasitophorous vacuole acidification via lactate/pyruvate export during egress [#2]\",\n      \"Imports host-derived lactate and pyruvate to sustain mitochondrial respiration, membrane potential, ATP, extracellular survival, and virulence [#3]\",\n      \"Inhibited by 2-hydroxy-chromanone compounds; substrate-selectivity residues determine specificity and inhibitor sensitivity [#0, #1]\"\n    ]\n  },\n  \"strain_conflicts\": []\n}\n```","audit_flag":null,"strains":[{"strain":"ME49","gene_id":"TGME49_292110","name":"FNT2","product":"formate/nitrite transporter FNT2","uniprot_acc":"S8EXD2","chromosome":"TGME49_chrIX","start_pos":4556579},{"strain":"GT1","gene_id":"TGGT1_292110","name":null,"product":"formate/nitrite transporter protein","uniprot_acc":null,"chromosome":"TGGT1_chrIX","start_pos":4363604},{"strain":"RH88","gene_id":"TGRH88_015950","name":null,"product":"formate/nitrite transporter protein","uniprot_acc":null,"chromosome":"CM023090","start_pos":4622438},{"strain":"VEG","gene_id":"TGVEG_292110","name":null,"product":"formate/nitrite transporter protein","uniprot_acc":"A0A125YKY1","chromosome":"TGVEG_chrIX","start_pos":4592903}]}