{"gene":"TG_306640","gene_symbol":"STX21","run_date":"2026-09-08T15:15:13","timeline":{"discoveries":[{"year":2023,"finding":"An unconventional SNARE complex comprising TgStx1, TgStx20, and TgStx21 mediates vesicular fusion at the plasma membrane in Toxoplasma gondii, and is essential for the exocytosis of surface proteins and vesicular fusion at the apical annuli.","method":"Genetic and biochemical analysis of SNARE complex composition; functional loss-of-function studies in T. gondii","journal":"PLoS pathogens","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — complex composition identified and functional role demonstrated by loss-of-function with specific phenotypic readouts (exocytosis defect), single lab study","pmids":["36972314"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2024,"finding":"SNARE proteins, including TgStx21, localize specifically to apical annuli — subapical annular structures embedded in the inner membrane complex — and these sites are the locations of dense granule vesicle fusion and exocytosis; perturbation of secretion at apical annuli strongly impairs parasite growth.","method":"Fluorescence localization, identification of integral membrane proteins at apical annuli, functional perturbation with growth phenotype readout","journal":"eLife","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct localization experiment tied to functional consequence (dense granule secretion defect, impaired growth), single lab with multiple orthogonal methods","pmids":["38270431"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":""}]}],"current_model":"TgStx21 is a component of an unconventional SNARE complex (with TgStx1 and TgStx20) that localizes to the apical annuli of Toxoplasma gondii, where it mediates vesicular fusion at the plasma membrane to drive exocytosis of surface proteins and post-invasion secretion of dense granule cargo, with loss of this function strongly impairing parasite growth."},"narrative":{"mechanistic_narrative":"TgStx21 is a syntaxin-family SNARE that functions as part of an unconventional SNARE complex with TgStx1 and TgStx20 to drive vesicular fusion at the plasma membrane of Toxoplasma gondii [PMID:36972314]. This complex localizes specifically to the apical annuli, subapical annular structures embedded in the inner membrane complex, which serve as sites of dense granule vesicle fusion and exocytosis [PMID:38270431]. Through this activity TgStx21 is required for exocytosis of surface proteins and post-invasion secretion of dense granule cargo, and perturbation of secretion at the apical annuli strongly impairs parasite growth [PMID:36972314, PMID:38270431]. Beyond its role in this fusion complex and its apical annuli localization, no further mechanistic detail on TgStx21 has been characterized in the available corpus.","teleology":[{"year":2023,"claim":"It was unknown how vesicular fusion at the Toxoplasma plasma membrane is executed; identifying an unconventional SNARE complex of TgStx1, TgStx20, and TgStx21 established the molecular machinery mediating fusion and its requirement for surface protein exocytosis.","evidence":"Genetic and biochemical SNARE complex composition analysis with loss-of-function functional studies in T. gondii","pmids":["36972314"],"confidence":"Medium","gaps":["Single lab study without independent replication","Stoichiometry and structural arrangement of the TgStx1/TgStx20/TgStx21 complex not resolved","The specific SNARE role of TgStx21 within the complex versus its partners not separated"],"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2024,"claim":"It was unclear where in the parasite this SNARE-mediated fusion occurs; localizing TgStx21 to the apical annuli and linking these sites to dense granule fusion established the apical annuli as functional exocytic hubs whose perturbation impairs growth.","evidence":"Fluorescence localization of SNARE proteins at apical annuli combined with functional perturbation and growth phenotype readout","pmids":["38270431"],"confidence":"Medium","gaps":["Molecular mechanism recruiting TgStx21 to the apical annuli unknown","Identity of the vesicular (R-)SNARE partner completing fusion not established","Full repertoire of cargo secreted at apical annuli not defined"],"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":null,"claim":"How TgStx21 is targeted to the apical annuli and how the complex is regulated to control the timing of dense granule and surface protein secretion remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No structural model of the SNARE complex","Regulatory factors controlling fusion timing unidentified","Cognate R-SNARE / regulatory proteins not identified"],"attribution":[{"species":"Toxoplasma gondii","strain":""}]}],"mechanism_profile":{"molecular_activity":[],"localization":[],"pathway":[],"complexes":["Unconventional SNARE complex (TgStx1 / TgStx20 / TgStx21)"],"partners":["TGSTX1","TGSTX20"],"other_free_text":["Mediates dense granule vesicle fusion and exocytosis at the apical annuli, subapical annular structures embedded in the inner membrane complex","Required for exocytosis of surface proteins; loss of apical annuli secretion strongly impairs parasite growth"]},"strain_conflicts":[]},"prefetch_data":{},"corpus_meta":[{"pmid":"36972314","id":"PMC_36972314","title":"An unconventional SNARE complex mediates exocytosis at the plasma membrane and vesicular fusion at the apical annuli in Toxoplasma gondii.","date":"2023","source":"PLoS pathogens","url":"https://pubmed.ncbi.nlm.nih.gov/36972314","citation_count":11,"is_preprint":false},{"pmid":"38270431","id":"PMC_38270431","title":"Apical annuli are specialised sites of post-invasion secretion of dense granules in Toxoplasma.","date":"2024","source":"eLife","url":"https://pubmed.ncbi.nlm.nih.gov/38270431","citation_count":10,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":3301,"output_tokens":599,"usd":0.009444,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":5312,"output_tokens":1720,"usd":0.03478,"stage2_stop_reason":"end_turn"},"total_usd":0.044224,"stage1_batch_id":"msgbatch_01S6GjoYQm8LTPUsdhASdqag","stage2_batch_id":"msgbatch_01VX63RTMdBJqsG6gQMqoCjz","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2023,\n      \"finding\": \"An unconventional SNARE complex comprising TgStx1, TgStx20, and TgStx21 mediates vesicular fusion at the plasma membrane in Toxoplasma gondii, and is essential for the exocytosis of surface proteins and vesicular fusion at the apical annuli.\",\n      \"method\": \"Genetic and biochemical analysis of SNARE complex composition; functional loss-of-function studies in T. gondii\",\n      \"journal\": \"PLoS pathogens\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — complex composition identified and functional role demonstrated by loss-of-function with specific phenotypic readouts (exocytosis defect), single lab study\",\n      \"pmids\": [\"36972314\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"SNARE proteins, including TgStx21, localize specifically to apical annuli — subapical annular structures embedded in the inner membrane complex — and these sites are the locations of dense granule vesicle fusion and exocytosis; perturbation of secretion at apical annuli strongly impairs parasite growth.\",\n      \"method\": \"Fluorescence localization, identification of integral membrane proteins at apical annuli, functional perturbation with growth phenotype readout\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct localization experiment tied to functional consequence (dense granule secretion defect, impaired growth), single lab with multiple orthogonal methods\",\n      \"pmids\": [\"38270431\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    }\n  ],\n  \"current_model\": \"TgStx21 is a component of an unconventional SNARE complex (with TgStx1 and TgStx20) that localizes to the apical annuli of Toxoplasma gondii, where it mediates vesicular fusion at the plasma membrane to drive exocytosis of surface proteins and post-invasion secretion of dense granule cargo, with loss of this function strongly impairing parasite growth.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"TgStx21 is a syntaxin-family SNARE that functions as part of an unconventional SNARE complex with TgStx1 and TgStx20 to drive vesicular fusion at the plasma membrane of Toxoplasma gondii [#0]. This complex localizes specifically to the apical annuli, subapical annular structures embedded in the inner membrane complex, which serve as sites of dense granule vesicle fusion and exocytosis [#1]. Through this activity TgStx21 is required for exocytosis of surface proteins and post-invasion secretion of dense granule cargo, and perturbation of secretion at the apical annuli strongly impairs parasite growth [#0, #1]. Beyond its role in this fusion complex and its apical annuli localization, no further mechanistic detail on TgStx21 has been characterized in the available corpus.\",\n  \"teleology\": [\n    {\n      \"year\": 2023,\n      \"claim\": \"It was unknown how vesicular fusion at the Toxoplasma plasma membrane is executed; identifying an unconventional SNARE complex of TgStx1, TgStx20, and TgStx21 established the molecular machinery mediating fusion and its requirement for surface protein exocytosis.\",\n      \"evidence\": \"Genetic and biochemical SNARE complex composition analysis with loss-of-function functional studies in T. gondii\",\n      \"pmids\": [\"36972314\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Single lab study without independent replication\",\n        \"Stoichiometry and structural arrangement of the TgStx1/TgStx20/TgStx21 complex not resolved\",\n        \"The specific SNARE role of TgStx21 within the complex versus its partners not separated\"\n      ],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"It was unclear where in the parasite this SNARE-mediated fusion occurs; localizing TgStx21 to the apical annuli and linking these sites to dense granule fusion established the apical annuli as functional exocytic hubs whose perturbation impairs growth.\",\n      \"evidence\": \"Fluorescence localization of SNARE proteins at apical annuli combined with functional perturbation and growth phenotype readout\",\n      \"pmids\": [\"38270431\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Molecular mechanism recruiting TgStx21 to the apical annuli unknown\",\n        \"Identity of the vesicular (R-)SNARE partner completing fusion not established\",\n        \"Full repertoire of cargo secreted at apical annuli not defined\"\n      ],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How TgStx21 is targeted to the apical annuli and how the complex is regulated to control the timing of dense granule and surface protein secretion remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"No structural model of the SNARE complex\",\n        \"Regulatory factors controlling fusion timing unidentified\",\n        \"Cognate R-SNARE / regulatory proteins not identified\"\n      ],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0000149\", \"supporting_discovery_ids\": [0], \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]},\n      {\"term_id\": \"GO:0006906\", \"supporting_discovery_ids\": [0, 1], \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0], \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]},\n      {\"term_id\": \"GO:0070258\", \"supporting_discovery_ids\": [1], \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0006887\", \"supporting_discovery_ids\": [0, 1], \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]}\n    ],\n    \"complexes\": [\"Unconventional SNARE complex (TgStx1 / TgStx20 / TgStx21)\"],\n    \"partners\": [\"TgStx1\", \"TgStx20\"],\n    \"other_free_text\": [\n      \"Mediates dense granule vesicle fusion and exocytosis at the apical annuli, subapical annular structures embedded in the inner membrane complex\",\n      \"Required for exocytosis of surface proteins; loss of apical annuli secretion strongly impairs parasite growth\"\n    ]\n  },\n  \"strain_conflicts\": []\n}\n```","audit_flag":null,"strains":[{"strain":"ME49","gene_id":"TGME49_306640","name":"STX21","product":"SNARE protein STX21","uniprot_acc":"A0A125YS09","chromosome":"TGME49_chrIX","start_pos":6234513},{"strain":"GT1","gene_id":"TGGT1_306640","name":null,"product":"hypothetical protein","uniprot_acc":null,"chromosome":"TGGT1_chrIX","start_pos":6015453},{"strain":"RH88","gene_id":"TGRH88_018090","name":null,"product":"unspecified product","uniprot_acc":null,"chromosome":"CM023090","start_pos":6357910},{"strain":"VEG","gene_id":"TGVEG_306640","name":null,"product":"snare region anchored in the vesicle membrane carboxy-terminal protein","uniprot_acc":"B6KRF3","chromosome":"TGVEG_chrIX","start_pos":6274186}]}