{"gene":"TG_233460","gene_symbol":"SAG1","run_date":"2026-09-10T21:09:19","timeline":{"discoveries":[{"year":1988,"finding":"SAG1 (P30) is encoded by a single-copy, intron-free gene and its primary translation product contains a hydrophobic signal sequence and a carboxy-terminal hydrophobic tail predictive of GPI anchor attachment via posttranslational cleavage and modification.","method":"cDNA cloning, sequence analysis of cDNA and cosmid clone, expression of beta-galactosidase fusion protein confirmed by anti-P30 monoclonal antibodies","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 1 / Strong — direct sequencing plus fusion-protein expression with immunological validation; foundational structural characterization replicated by subsequent studies","pmids":["3183382"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":"RH"}]},{"year":1993,"finding":"SAG1 protein is present on the tachyzoite surface but is absent from the parasitophorous vacuole network, indicating that the network is not enriched in GPI-anchored membrane proteins; dense-granule contents (GRA2, GRA3) are preferentially deposited into the vacuolar network.","method":"Immunofluorescence assay (IFA) and immunoelectron microscopy at multiple time-points post-invasion using SAG1-specific probe","journal":"Parasitology research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct localization by IFA and immunoEM, single lab, orthogonal methods","pmids":["8415546"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":1993,"finding":"Antibodies to SAG1 (P30), including Fab fragments, inhibit invasion of human fibroblasts and murine enterocytes by T. gondii tachyzoites; a P30-deficient mutant shows reduced inhibition by anti-P30 antisera, and a glucosamide neoglycoprotein competitively blocks infection of P30+ but not P30− parasites, suggesting SAG1 binds a glycosylated host cell receptor.","method":"In vitro neutralization/invasion assay with monoclonal and polyclonal antibodies, Fab fragments, and P30-deficient mutant parasites (PTgB); competitive inhibition with neoglycoprotein BSA-glucosamide","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — multiple orthogonal methods (Fab inhibition, mutant parasites, competitive ligand), consistent results across assay types","pmids":["7682587"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":"RH"},{"species":"Toxoplasma gondii","strain":"PTg"}]},{"year":1993,"finding":"SAG1 is a tachyzoite-specific surface marker; SAG1 mRNA/protein expression increases during reactivation of T. gondii in mouse brain and is reduced during bradyzoite stage, correlating with parasite stage identity.","method":"In vivo murine toxoplasmosis model; mRNA expression analysis correlating SAG1 with tachyzoite reactivation in anti-IFN-gamma and anti-TNF-alpha mAb-treated mice","journal":"Journal of immunology","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — mRNA detection in tissues across multiple treatment conditions, single lab but multiple experimental groups","pmids":["7690809"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":"ME-49"}]},{"year":1993,"finding":"During in vitro bradyzoite-to-tachyzoite interconversion, intermediate parasites co-express bradyzoite markers (P36, P34, P21, P18) and the tachyzoite marker SAG1 (P30), establishing SAG1 as a stage-specific surface antigen whose expression is acquired during differentiation.","method":"Stage-specific monoclonal antibody labeling of differentiating parasites in culture; kinetic immunofluorescence analysis","journal":"Experimental parasitology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct immunofluorescence with stage-specific antibodies, kinetic analysis, single lab","pmids":["7684705"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":"76K"},{"species":"Toxoplasma gondii","strain":"BQNC2"},{"species":"Toxoplasma gondii","strain":"Prugniaud"}]},{"year":1994,"finding":"SAG1 functions as a parasite ligand mediating attachment to host cells: polyclonal anti-SAG1 serum inhibited tachyzoite attachment to glutaraldehyde-fixed fibroblasts by 71%; a monoclonal antibody (6A8) and its Fab fragment inhibited attachment in a dose-dependent manner; SAG1-defective mutants (PTgA and PTgC) showed 26–39% reduced binding compared to wild-type.","method":"Radioisotope competitive binding assay with fixed fibroblasts and live tachyzoites; mAb and Fab inhibition assays; comparison of SAG1-defective mutants vs. wild-type","journal":"Experimental parasitology","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — quantitative binding assay, Fab inhibition, and genetic mutant controls, orthogonal methods converging on the same conclusion","pmids":["8050521"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":1994,"finding":"An antigen-specific CD8+ T cell clone (C3) specific for SAG1/p30 conferred ~100% protection against acute T. gondii infection upon adoptive transfer into naive mice; this protection was abrogated by anti-IFN-gamma treatment, establishing an IFN-gamma-dependent CD8+ T cell effector mechanism for SAG1-specific immunity.","method":"Adoptive transfer of T cell clones into naive mice; CD4+ T cell depletion; anti-IFN-gamma neutralization; mortality readout","journal":"Journal of immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic/immunological manipulation (depletion, neutralization) with defined in vivo readout, single lab","pmids":["7907106"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":1995,"finding":"A stretch of six repeated sequences in the region 35–190 bp upstream of the first transcription start site of the SAG1 gene is essential for efficient and accurate transcription initiation and acts as a selector of transcription start sites.","method":"Transfection of Toxoplasma with promoter deletion constructs; analysis of transcription initiation accuracy and efficiency","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — functional dissection of promoter elements by deletion analysis in the parasite; single lab but direct functional demonstration","pmids":["7799972"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":1996,"finding":"Anti-SAG1 monoclonal antibody TG05.54 causes a consistent and significant blockade of tachyzoite invasion into bovine kidney cells, and anti-SAG2 antibodies prevented parasite reorientation and immobilized zoites on the host cell membrane, indicating distinct functional roles for SAG1 in invasion.","method":"In vitro neutralization assay with monoclonal antibodies against five tachyzoite surface proteins in bovine kidney cell invasion assay","journal":"International journal for parasitology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vitro neutralization assay with multiple mAbs as controls, single lab","pmids":["8690540"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":1998,"finding":"SAG1 is a GPI-anchored surface protein; its N. caninum homolog NcSAG1 (Ncp29) also carries a GPI anchor (confirmed by PI-PLC treatment and cross-reacting determinant antibodies) and contains a tandemly duplicated motif and 12 conserved cysteine residues also found in T. gondii SAG1, suggesting a conserved overall topology and function.","method":"Surface biotinylation, PI-PLC treatment, immunoblotting with anti-CRD antibodies; sequence comparison","journal":"Infection and immunity","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — biochemical PI-PLC cleavage with CRD detection plus surface labeling, comparative analysis with Toxoplasma SAG1","pmids":["9784539"],"is_preprint":false,"attribution":[{"species":"Neospora caninum","strain":""}]},{"year":1998,"finding":"GFP fused to the P30/SAG1 major surface antigen is targeted to dense granules within living cells and is efficiently secreted into the parasitophorous vacuole, not remaining on the surface, demonstrating that the SAG1 ectodomain can redirect heterologous proteins to the secretory pathway when used as a fusion.","method":"Live-cell fluorescence microscopy of GFP-SAG1 fusion proteins in transfected Toxoplasma; FACS and immunoblotting","journal":"Molecular and biochemical parasitology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — live imaging plus Western blot and FACS, single lab, multiple detection methods","pmids":["9657336"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2000,"finding":"Two conserved cytoplasmic tail motifs of MIC2 (SYHYY and EIEYE) are necessary and sufficient for microneme targeting; replacing these motifs with the cytoplasmic tail of SAG1 redirected the chimeric protein away from micronemes, confirming that SAG1's own transmembrane/cytoplasmic domain lacks a microneme-targeting signal.","method":"Chimeric protein targeting assays: SAG1 cytoplasmic tail fused to MIC2 transmembrane domains in transfected Toxoplasma; immunofluorescence localization","journal":"Molecular and cellular biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — chimeric construct approach with subcellular localization readout, single lab","pmids":["10982850"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2001,"finding":"Soluble recombinant SAG3 binds heparin and cell-surface heparan sulfate proteoglycans (HSPGs), whereas recombinant SAG1 does not interact with heparin, establishing that SAG3 (not SAG1) is the GPI-anchored surface antigen responsible for HSPG-mediated attachment.","method":"Heparin-binding assay with recombinant proteins; CHO cell adhesion assay with HSPG-deficient mutants; competitive inhibition with soluble glycoconjugates; radiolabeled tachyzoite attachment blockade","journal":"Molecular and biochemical parasitology","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — in vitro binding assays with recombinant proteins, genetic HSPG-deficient cell mutants, and competitive inhibition; negative result for SAG1 is mechanistically informative","pmids":["11463464"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2002,"finding":"The crystal structure of SAG1 at 1.7 Å reveals a homodimeric configuration in which the dimeric interface is mediated by an extended beta-sheet forming a deep groove lined with positively charged amino acids; this basic groove is conserved among SRS proteins and is proposed to serve as a sulfated proteoglycan-binding site on target cell surfaces.","method":"X-ray crystallography at 1.7 Å resolution; structural analysis of homodimer interface","journal":"Nature structural biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — high-resolution crystal structure; structural groove consistent with known sulfated-proteoglycan binding biology of the SRS family","pmids":["12091874"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2002,"finding":"The rhomboid protease MPP1 (responsible for shedding microneme proteins) is constitutively active at the parasite surface; fusion of SAG1 with TgMIC2 or TgMIC6 transmembrane domains results in cleavage within the membrane-spanning domain, demonstrating that the ectodomain does not determine MPP1 cleavage site recognition.","method":"Site-specific mutagenesis of cleavage sites; SAG1 chimeras bearing TgMIC6 transmembrane domains expressed in Toxoplasma; cleavage assessed by biochemical assays","journal":"The EMBO journal","confidence":"Medium","confidence_rationale":"Tier 1-2 / Moderate — site-directed mutagenesis and chimeric constructs in parasite, single lab","pmids":["11927542"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2002,"finding":"Clostridium septicum alpha-toxin, which targets GPI-anchored proteins, binds to SAG1 (and SAG3) on the T. gondii tachyzoite surface, causing disruption of the plasma membrane–inner membrane complex connection, plasma membrane extension, parasite lysis, and ER swelling; binding profiles differed between virulent RH and avirulent P strains.","method":"Alpha-toxin treatment of tachyzoites; ultrastructural analysis by electron microscopy; immunoblotting to identify toxin-bound proteins","journal":"Infection and immunity","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — ultrastructural and biochemical characterization, single lab, two strain comparison","pmids":["12117945"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":"RH"},{"species":"Toxoplasma gondii","strain":"P"}]},{"year":2004,"finding":"SAG1 is expressed exclusively by tachyzoites and not by bradyzoites or enteric coccidian stages in vivo; stage conversion from tachyzoite to bradyzoite involves loss of SAG1 expression, while intermediate organisms co-express both SAG1 and bradyzoite marker BAG1.","method":"In vivo immunocytochemistry with stage-specific antibody panel (SAG1, BAG1, ENO isoforms, LDH isoforms, GRA proteins) in mouse and cat tissues; electron microscopy","journal":"International journal for parasitology","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal molecular markers with electron microscopy in vivo across multiple life-cycle stages and hosts","pmids":["15003495"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2005,"finding":"Constitutive expression of SAG1 during bradyzoite stage (SAG1c mutants) causes an early reduction in brain cyst loads followed by a substantial increase over time, accompanied by hyperproduction of IFN-gamma, TNF-alpha, and IL-10, and severe encephalitis; this demonstrates that stage-specific downregulation of SAG1 in bradyzoites is required for optimal parasite persistence.","method":"Transgenic Toxoplasma constitutively expressing tachyzoite SAG1 in bradyzoite stage; in vivo mouse infection model with cyst counting, cytokine measurement, and histopathology","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 / Strong — transgenic loss-of-stage-specificity approach with quantitative in vivo readouts (cyst burden, cytokines, pathology), epistasis between antigen expression pattern and immune response","pmids":["15944311"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2012,"finding":"A Δsag1 sag2A double mutant is significantly attenuated in murine acute virulence and shows upregulated SRS29C (formerly SRS2/SAG2) expression; transgenic overexpression of SRS29C in the virulent RH parent similarly attenuates virulence, establishing SRS29C as a negative regulator of acute virulence and implicating the SAG1/SRS29B surface coat in modulating virulence.","method":"Targeted gene deletion of SAG1 (sag1) and sag2A; transgenic overexpression of SRS29C; in vivo mouse acute virulence assay","journal":"mBio","confidence":"High","confidence_rationale":"Tier 2 / Strong — targeted gene deletion plus transgenic overexpression with in vivo virulence readout; complementary genetic approaches converging on same conclusion","pmids":["23149485"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":"RH"}]},{"year":2020,"finding":"Single-cell RNA-sequencing reveals that SAG1 expression among tachyzoites is highly heterogeneous and sporadic; AP2IX-1 is identified as a transcription factor that controls the switch from ubiquitous SAG1 expression to rare surface antigens not previously detected in tachyzoites.","method":"Single-cell RNA-sequencing (scRNA-seq) of >5,400 Toxoplasma tachyzoites and bradyzoites; transcription factor identification by computational analysis; three strains analyzed","journal":"eLife","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — scRNA-seq across multiple strains with TF identification; functional validation of AP2IX-1 role described but limited to correlation in this abstract","pmids":["32065584"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2025,"finding":"SAG1 mediates T. gondii fitness specifically in IFN-gamma-stimulated host cells; SAG1 knockout parasites show reduced attachment efficiency to host cells, an effect exacerbated in IFN-gamma-treated cells; host cell surface sialic acids are important for parasite attachment, especially under immune pressure.","method":"CRISPR/Cas9 knockout of SAG1; in vitro fitness assay in IFN-gamma-stimulated cells; attachment assay with SAG1-deficient parasites and sialic acid perturbation","journal":"Infection and immunity","confidence":"High","confidence_rationale":"Tier 1-2 / Moderate — CRISPR/Cas9 knockout with mechanistic follow-up (attachment assay, sialic acid perturbation), single lab, two orthogonal approaches","pmids":["40607979"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":""}]}],"current_model":"SAG1 (SRS29B/P30) is a GPI-anchored, homodimeric surface antigen expressed exclusively on tachyzoites that functions as an adhesin mediating parasite attachment to host cells via a positively charged groove that likely engages sulfated proteoglycans and sialic acids on host cell surfaces; its stage-specific expression (absent in bradyzoites and enteric stages) is controlled in part by upstream repeat promoter elements and the transcription factor AP2IX-1, and downregulation during bradyzoite differentiation is required for immune evasion and parasite persistence, while in IFN-gamma-stimulated environments SAG1 is specifically required for robust attachment and fitness."},"narrative":{"mechanistic_narrative":"SAG1 (P30/SRS29B) is a GPI-anchored, tachyzoite-specific surface antigen of Toxoplasma gondii that functions as an adhesin mediating parasite attachment to host cells [PMID:3183382, PMID:8050521]. Encoded by a single-copy, intron-free gene, its primary product carries an N-terminal signal sequence and a C-terminal hydrophobic tail directing GPI anchor attachment, confirmed biochemically by PI-PLC sensitivity and cross-reacting-determinant detection [PMID:3183382, PMID:9784539]. The mature protein forms a homodimer whose interface generates a deep groove lined with positively charged residues proposed to engage sulfated glycoconjugates on target cells [PMID:12091874]. Consistent with an adhesin role, anti-SAG1 antibodies and Fab fragments inhibit tachyzoite attachment and invasion, SAG1-deficient mutants bind host cells less efficiently, and a glucosamide neoglycoprotein competitively blocks infection of SAG1-positive parasites [PMID:7682587, PMID:8050521, PMID:8690540]. CRISPR knockout confirms that SAG1 is specifically required for attachment and fitness in IFN-gamma-stimulated host cells and implicates host-cell surface sialic acids as attachment determinants under immune pressure [PMID:40607979]; SAG1 itself does not bind heparin, distinguishing its ligand preference from the HSPG-binding SAG3 [PMID:11463464]. SAG1 is expressed exclusively by tachyzoites and lost during bradyzoite and enteric-stage differentiation [PMID:7690809, PMID:7684705, PMID:15003495]; this stage-specific downregulation is functionally essential, since forced SAG1 expression in bradyzoites drives excessive inflammatory cytokine responses and disrupts persistence [PMID:15944311]. Its promoter contains upstream repeat elements required for accurate transcription initiation [PMID:7799972], and its expression is heterogeneous among tachyzoites and controlled by the transcription factor AP2IX-1 [PMID:40607979]. At the surface-coat level, the SAG1/SAG2A surface antigens modulate acute virulence, with SRS29C acting as a negative regulator [PMID:23149485].","teleology":[{"year":1988,"claim":"Establishing the primary structure and predicted membrane topology of SAG1 defined it as a GPI-anchored surface protein, providing the molecular starting point for all subsequent functional work.","evidence":"cDNA/cosmid cloning and sequence analysis with beta-galactosidase fusion expression validated by anti-P30 monoclonal antibodies in RH","pmids":["3183382"],"confidence":"High","gaps":["Sequence-based GPI prediction not yet biochemically confirmed at this stage","No structural or ligand information"],"attribution":[{"species":"Toxoplasma gondii","strain":"RH"}]},{"year":1993,"claim":"Antibody neutralization and mutant-parasite experiments first assigned SAG1 a functional role in host-cell invasion and implicated a glycosylated host receptor, moving it beyond a mere serological marker.","evidence":"in vitro invasion/neutralization assays with mAbs, Fab fragments, a P30-deficient mutant, and competitive neoglycoprotein inhibition in RH and PTg","pmids":["7682587"],"confidence":"High","gaps":["Host receptor identity not defined","Attachment vs. later invasion steps not distinguished"],"attribution":[{"species":"Toxoplasma gondii","strain":"RH"},{"species":"Toxoplasma gondii","strain":"PTg"}]},{"year":1993,"claim":"Localization and stage-expression studies established SAG1 as a tachyzoite-restricted surface antigen absent from the vacuolar network, tying its expression to parasite stage identity.","evidence":"IFA/immunoEM localization and in vivo mRNA/protein expression during reactivation and differentiation across ME-49 and differentiating strains","pmids":["8415546","7690809","7684705"],"confidence":"Medium","gaps":["Transcriptional basis of stage restriction not yet defined","Functional consequence of stage-specific loss unknown"],"attribution":[{"species":"Toxoplasma gondii","strain":"ME-49"},{"species":"Toxoplasma gondii","strain":""}]},{"year":1994,"claim":"Quantitative binding assays resolved SAG1's role to the attachment step specifically, distinguishing adhesion from downstream invasion events.","evidence":"radioisotope competitive binding to fixed fibroblasts with mAb/Fab inhibition and SAG1-defective mutant comparison","pmids":["8050521"],"confidence":"High","gaps":["Residual attachment in mutants shows SAG1 is not the sole adhesin","Molecular ligand still unidentified"],"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":1994,"claim":"Adoptive transfer of a SAG1-specific CD8+ T cell clone established that SAG1 is an immunologically protective antigen acting through an IFN-gamma-dependent effector mechanism, linking the surface antigen to host immunity.","evidence":"adoptive transfer of a SAG1-specific CD8+ T cell clone with CD4 depletion and anti-IFN-gamma neutralization in mice","pmids":["7907106"],"confidence":"Medium","gaps":["Does not address SAG1's cell-biological function","Epitope and antigen-processing details not defined"],"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":1995,"claim":"Promoter dissection identified upstream repeat elements governing accurate and efficient SAG1 transcription initiation, opening the mechanism of its regulated expression.","evidence":"promoter deletion constructs transfected into Toxoplasma with transcription-initiation analysis","pmids":["7799972"],"confidence":"High","gaps":["Trans-acting factors not identified","Does not explain tachyzoite-specific restriction"],"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":1998,"claim":"Biochemical and comparative analyses confirmed the GPI anchor experimentally and revealed a conserved cysteine/domain topology shared with the Neospora homolog, supporting a conserved fold and function.","evidence":"surface biotinylation, PI-PLC cleavage, anti-CRD immunoblotting, and sequence comparison in N. caninum","pmids":["9784539"],"confidence":"Medium","gaps":["Functional conservation inferred from sequence, not tested","Cross-species work is on the Neospora homolog"],"attribution":[{"species":"Neospora caninum","strain":""}]},{"year":2002,"claim":"The 1.7 A crystal structure defined SAG1 as a homodimer forming a basic groove, providing a structural model for how it might engage negatively charged host-cell surface ligands.","evidence":"X-ray crystallography and homodimer interface analysis","pmids":["12091874"],"confidence":"High","gaps":["Ligand in the groove not co-crystallized","Groove specificity for a defined glycoconjugate not proven structurally"],"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2001,"claim":"Recombinant binding assays showed SAG1 does not bind heparin/HSPG, refining its ligand specificity and assigning HSPG-mediated attachment to the paralog SAG3 rather than SAG1.","evidence":"heparin-binding and CHO/HSPG-deficient adhesion assays with recombinant SAG1 and SAG3","pmids":["11463464"],"confidence":"High","gaps":["Positive ligand for SAG1 still undefined","Reconciliation with the basic-groove model deferred"],"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2002,"claim":"Chimera and toxin studies used SAG1's transmembrane/GPI features as tools, showing its cytoplasmic tail lacks microneme-targeting information and that its GPI anchor makes it a target of GPI-directed toxins.","evidence":"SAG1/MIC chimeric targeting and cleavage assays, and Clostridium septicum alpha-toxin binding with ultrastructural analysis in RH and P strains","pmids":["10982850","11927542","12117945","9657336"],"confidence":"Medium","gaps":["Trafficking determinants of native SAG1 not fully mapped","Strain differences in toxin binding not mechanistically explained"],"attribution":[{"species":"Toxoplasma gondii","strain":"RH"},{"species":"Toxoplasma gondii","strain":"P"},{"species":"Toxoplasma gondii","strain":""}]},{"year":2005,"claim":"Forced bradyzoite-stage expression showed that stage-specific SAG1 downregulation is functionally required for persistence, establishing a causal link between regulated antigen expression and immune-mediated control.","evidence":"transgenic constitutive SAG1 expression in bradyzoites with in vivo cyst counting, cytokine measurement, and histopathology","pmids":["15944311"],"confidence":"High","gaps":["Mechanism by which SAG1 provokes the cytokine response unclear","Direct target of the induced immune response not defined"],"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2004,"claim":"A comprehensive in vivo stage-marker survey confirmed SAG1 is confined to tachyzoites and lost during bradyzoite and enteric-stage conversion, with intermediate co-expression states.","evidence":"in vivo immunocytochemistry with a stage-specific marker panel and electron microscopy in mouse and cat tissues","pmids":["15003495"],"confidence":"High","gaps":["Regulatory logic of the switch not resolved here"],"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2012,"claim":"Genetic deletion and overexpression placed SAG1 within a surface-coat system that modulates acute virulence, identifying SRS29C as a negative regulator uncovered upon SAG1/SAG2A loss.","evidence":"targeted sag1/sag2A deletion, transgenic SRS29C overexpression, and mouse acute virulence assays in RH","pmids":["23149485"],"confidence":"High","gaps":["Whether virulence effect is due to SAG1 loss per se or coat remodeling not separated","Molecular mechanism of SRS29C-mediated attenuation unknown"],"attribution":[{"species":"Toxoplasma gondii","strain":"RH"}]},{"year":2020,"claim":"Single-cell transcriptomics revealed heterogeneous, sporadic SAG1 expression among tachyzoites and identified AP2IX-1 as a transcription factor controlling the switch between ubiquitous SAG1 and rare surface antigens.","evidence":"scRNA-seq of >5,400 tachyzoites/bradyzoites across three strains with computational TF identification","pmids":["32065584"],"confidence":"Medium","gaps":["AP2IX-1 regulation of SAG1 is largely correlative here","Functional significance of expression heterogeneity unresolved"],"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2025,"claim":"CRISPR knockout demonstrated a context-specific requirement for SAG1 in attachment and fitness under IFN-gamma stimulation and implicated host sialic acids as attachment determinants, resolving the ligand class SAG1 exploits under immune pressure.","evidence":"CRISPR/Cas9 SAG1 knockout with IFN-gamma fitness and attachment assays plus sialic acid perturbation","pmids":["40607979"],"confidence":"High","gaps":["Direct SAG1-sialic acid binding not structurally demonstrated","Why the requirement is exacerbated by IFN-gamma not mechanistically defined"],"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":null,"claim":"The precise host-cell ligand engaged by the SAG1 basic groove and the mechanism reconciling its non-heparin, sialic-acid-linked adhesion with the structural model remain undefined.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No co-structure of SAG1 with a host ligand","Mechanistic basis of IFN-gamma-specific fitness requirement unknown","Direct link between AP2IX-1 and SAG1 promoter elements not established"],"attribution":[{"species":"Toxoplasma gondii","strain":""}]}],"mechanism_profile":{"molecular_activity":[],"localization":[],"pathway":[],"complexes":["SAG1 homodimer (crystallographic; basic-groove interface) [#13]"],"partners":["HOST-CELL SURFACE SIALIC ACIDS (ATTACHMENT DETERMINANT) [#20]","GLYCOSYLATED/GLUCOSAMIDE-BEARING HOST RECEPTOR (COMPETITIVE INHIBITION) [#2]","CLOSTRIDIUM SEPTICUM ALPHA-TOXIN (GPI-DIRECTED BINDER OF SAG1) [#15]"],"other_free_text":["GPI-anchored, single-copy intron-free gene product with N-terminal signal sequence and C-terminal GPI signal [#0, #9]","Tachyzoite-specific; expression lost during bradyzoite and enteric-stage differentiation [#3, #4, #16]","Stage-specific downregulation required for parasite persistence; forced bradyzoite expression drives inflammatory cytokines and encephalitis [#17]","Promoter contains upstream repeat elements required for accurate transcription initiation [#7]; expression heterogeneous and linked to AP2IX-1 [#20]","Does not bind heparin/HSPG, unlike paralog SAG3 [#12]","SAG1/SAG2A surface coat modulates acute virulence; SRS29C is a negative regulator [#18]","Cytoplasmic tail lacks microneme-targeting signal [#11]; conserved topology shared with N. caninum NcSAG1 [#9]"]},"strain_conflicts":[]},"prefetch_data":{},"corpus_meta":[{"pmid":"3183382","id":"PMC_3183382","title":"Molecular analysis of the gene encoding the major surface antigen of Toxoplasma gondii.","date":"1988","source":"Journal of immunology (Baltimore, Md. : 1950)","url":"https://pubmed.ncbi.nlm.nih.gov/3183382","citation_count":363,"is_preprint":false},{"pmid":"11474426","id":"PMC_11474426","title":"Unusual abundance of atypical strains associated with human ocular toxoplasmosis.","date":"2001","source":"The Journal of infectious diseases","url":"https://pubmed.ncbi.nlm.nih.gov/11474426","citation_count":326,"is_preprint":false},{"pmid":"7690809","id":"PMC_7690809","title":"Acute cerebral toxoplasmosis is induced by in vivo neutralization of TNF-alpha and correlates with the down-regulated expression of inducible nitric oxide synthase and other markers of macrophage activation.","date":"1993","source":"Journal of immunology (Baltimore, Md. : 1950)","url":"https://pubmed.ncbi.nlm.nih.gov/7690809","citation_count":304,"is_preprint":false},{"pmid":"21320505","id":"PMC_21320505","title":"Genetic analyses of atypical Toxoplasma gondii strains reveal a fourth clonal lineage in North America.","date":"2011","source":"International journal for 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Parasites and wildlife","url":"https://pubmed.ncbi.nlm.nih.gov/42358338","citation_count":0,"is_preprint":false},{"pmid":"40216097","id":"PMC_40216097","title":"Designing a multi-epitope vaccine using Toxoplasma ROP5, ROP7, and SAG1 epitopes and immunogenicity evaluation against acute and chronic toxoplasmosis in BABL/c mice.","date":"2025","source":"Microbial pathogenesis","url":"https://pubmed.ncbi.nlm.nih.gov/40216097","citation_count":7,"is_preprint":false},{"pmid":"39894243","id":"PMC_39894243","title":"Immunization with plant-based vaccine expressing Toxoplasma gondii SAG1 fused to plant HSP90 elicits protective immune response in lambs.","date":"2025","source":"Acta tropica","url":"https://pubmed.ncbi.nlm.nih.gov/39894243","citation_count":6,"is_preprint":false},{"pmid":"40607979","id":"PMC_40607979","title":"Surface antigen SAG1 mediates Toxoplasma gondii fitness and host cell attachment in IFNγ-stimulated cells.","date":"2025","source":"Infection and immunity","url":"https://pubmed.ncbi.nlm.nih.gov/40607979","citation_count":3,"is_preprint":false},{"pmid":"40288708","id":"PMC_40288708","title":"SELEX-derived DNA aptamer utilized for sensitive electrochemical biosensing of Toxoplasma gondii surface antigen 1.","date":"2025","source":"International journal of biological macromolecules","url":"https://pubmed.ncbi.nlm.nih.gov/40288708","citation_count":3,"is_preprint":false},{"pmid":"40048863","id":"PMC_40048863","title":"Dolphins share Toxoplasma gondii Type II genotypes with terrestrial animals: Evidence of terrestrial T. gondii contamination in marine environments.","date":"2025","source":"Veterinary parasitology","url":"https://pubmed.ncbi.nlm.nih.gov/40048863","citation_count":3,"is_preprint":false},{"pmid":"39350555","id":"PMC_39350555","title":"Structural Prediction and Antigenic Analysis of ROP18, MIC4, and SAG1 Proteins to Improve Vaccine Design against Toxoplasma gondii: An In silico Approach.","date":"2025","source":"Infectious disorders drug targets","url":"https://pubmed.ncbi.nlm.nih.gov/39350555","citation_count":2,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":51600,"output_tokens":5740,"usd":0.12045,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":12947,"output_tokens":5859,"usd":0.105605,"stage2_stop_reason":"end_turn"},"total_usd":0.0,"stage1_batch_id":"msgbatch_01XFXJD6iZecXyWn9uU5Qom9","stage2_batch_id":"msgbatch_013GToKgs4AKSCRE4yvueviH","note":"batch pricing = 50% of standard","skipped":true,"corrective_rerun":"2026-09-10 wrong-gene rerun (round-4 rules)","corrective_reason":"paper list: removed papers about other genes"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1988,\n      \"finding\": \"SAG1 (P30) is encoded by a single-copy, intron-free gene and its primary translation product contains a hydrophobic signal sequence and a carboxy-terminal hydrophobic tail predictive of GPI anchor attachment via posttranslational cleavage and modification.\",\n      \"method\": \"cDNA cloning, sequence analysis of cDNA and cosmid clone, expression of beta-galactosidase fusion protein confirmed by anti-P30 monoclonal antibodies\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — direct sequencing plus fusion-protein expression with immunological validation; foundational structural characterization replicated by subsequent studies\",\n      \"pmids\": [\"3183382\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"RH\"}]\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"SAG1 protein is present on the tachyzoite surface but is absent from the parasitophorous vacuole network, indicating that the network is not enriched in GPI-anchored membrane proteins; dense-granule contents (GRA2, GRA3) are preferentially deposited into the vacuolar network.\",\n      \"method\": \"Immunofluorescence assay (IFA) and immunoelectron microscopy at multiple time-points post-invasion using SAG1-specific probe\",\n      \"journal\": \"Parasitology research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct localization by IFA and immunoEM, single lab, orthogonal methods\",\n      \"pmids\": [\"8415546\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"Antibodies to SAG1 (P30), including Fab fragments, inhibit invasion of human fibroblasts and murine enterocytes by T. gondii tachyzoites; a P30-deficient mutant shows reduced inhibition by anti-P30 antisera, and a glucosamide neoglycoprotein competitively blocks infection of P30+ but not P30− parasites, suggesting SAG1 binds a glycosylated host cell receptor.\",\n      \"method\": \"In vitro neutralization/invasion assay with monoclonal and polyclonal antibodies, Fab fragments, and P30-deficient mutant parasites (PTgB); competitive inhibition with neoglycoprotein BSA-glucosamide\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — multiple orthogonal methods (Fab inhibition, mutant parasites, competitive ligand), consistent results across assay types\",\n      \"pmids\": [\"7682587\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"RH\"}, {\"species\": \"Toxoplasma gondii\", \"strain\": \"PTg\"}]\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"SAG1 is a tachyzoite-specific surface marker; SAG1 mRNA/protein expression increases during reactivation of T. gondii in mouse brain and is reduced during bradyzoite stage, correlating with parasite stage identity.\",\n      \"method\": \"In vivo murine toxoplasmosis model; mRNA expression analysis correlating SAG1 with tachyzoite reactivation in anti-IFN-gamma and anti-TNF-alpha mAb-treated mice\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — mRNA detection in tissues across multiple treatment conditions, single lab but multiple experimental groups\",\n      \"pmids\": [\"7690809\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"ME-49\"}]\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"During in vitro bradyzoite-to-tachyzoite interconversion, intermediate parasites co-express bradyzoite markers (P36, P34, P21, P18) and the tachyzoite marker SAG1 (P30), establishing SAG1 as a stage-specific surface antigen whose expression is acquired during differentiation.\",\n      \"method\": \"Stage-specific monoclonal antibody labeling of differentiating parasites in culture; kinetic immunofluorescence analysis\",\n      \"journal\": \"Experimental parasitology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct immunofluorescence with stage-specific antibodies, kinetic analysis, single lab\",\n      \"pmids\": [\"7684705\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"76K\"}, {\"species\": \"Toxoplasma gondii\", \"strain\": \"BQNC2\"}, {\"species\": \"Toxoplasma gondii\", \"strain\": \"Prugniaud\"}]\n    },\n    {\n      \"year\": 1994,\n      \"finding\": \"SAG1 functions as a parasite ligand mediating attachment to host cells: polyclonal anti-SAG1 serum inhibited tachyzoite attachment to glutaraldehyde-fixed fibroblasts by 71%; a monoclonal antibody (6A8) and its Fab fragment inhibited attachment in a dose-dependent manner; SAG1-defective mutants (PTgA and PTgC) showed 26–39% reduced binding compared to wild-type.\",\n      \"method\": \"Radioisotope competitive binding assay with fixed fibroblasts and live tachyzoites; mAb and Fab inhibition assays; comparison of SAG1-defective mutants vs. wild-type\",\n      \"journal\": \"Experimental parasitology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — quantitative binding assay, Fab inhibition, and genetic mutant controls, orthogonal methods converging on the same conclusion\",\n      \"pmids\": [\"8050521\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 1994,\n      \"finding\": \"An antigen-specific CD8+ T cell clone (C3) specific for SAG1/p30 conferred ~100% protection against acute T. gondii infection upon adoptive transfer into naive mice; this protection was abrogated by anti-IFN-gamma treatment, establishing an IFN-gamma-dependent CD8+ T cell effector mechanism for SAG1-specific immunity.\",\n      \"method\": \"Adoptive transfer of T cell clones into naive mice; CD4+ T cell depletion; anti-IFN-gamma neutralization; mortality readout\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic/immunological manipulation (depletion, neutralization) with defined in vivo readout, single lab\",\n      \"pmids\": [\"7907106\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"A stretch of six repeated sequences in the region 35–190 bp upstream of the first transcription start site of the SAG1 gene is essential for efficient and accurate transcription initiation and acts as a selector of transcription start sites.\",\n      \"method\": \"Transfection of Toxoplasma with promoter deletion constructs; analysis of transcription initiation accuracy and efficiency\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — functional dissection of promoter elements by deletion analysis in the parasite; single lab but direct functional demonstration\",\n      \"pmids\": [\"7799972\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"Anti-SAG1 monoclonal antibody TG05.54 causes a consistent and significant blockade of tachyzoite invasion into bovine kidney cells, and anti-SAG2 antibodies prevented parasite reorientation and immobilized zoites on the host cell membrane, indicating distinct functional roles for SAG1 in invasion.\",\n      \"method\": \"In vitro neutralization assay with monoclonal antibodies against five tachyzoite surface proteins in bovine kidney cell invasion assay\",\n      \"journal\": \"International journal for parasitology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vitro neutralization assay with multiple mAbs as controls, single lab\",\n      \"pmids\": [\"8690540\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"SAG1 is a GPI-anchored surface protein; its N. caninum homolog NcSAG1 (Ncp29) also carries a GPI anchor (confirmed by PI-PLC treatment and cross-reacting determinant antibodies) and contains a tandemly duplicated motif and 12 conserved cysteine residues also found in T. gondii SAG1, suggesting a conserved overall topology and function.\",\n      \"method\": \"Surface biotinylation, PI-PLC treatment, immunoblotting with anti-CRD antibodies; sequence comparison\",\n      \"journal\": \"Infection and immunity\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — biochemical PI-PLC cleavage with CRD detection plus surface labeling, comparative analysis with Toxoplasma SAG1\",\n      \"pmids\": [\"9784539\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Neospora caninum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"GFP fused to the P30/SAG1 major surface antigen is targeted to dense granules within living cells and is efficiently secreted into the parasitophorous vacuole, not remaining on the surface, demonstrating that the SAG1 ectodomain can redirect heterologous proteins to the secretory pathway when used as a fusion.\",\n      \"method\": \"Live-cell fluorescence microscopy of GFP-SAG1 fusion proteins in transfected Toxoplasma; FACS and immunoblotting\",\n      \"journal\": \"Molecular and biochemical parasitology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — live imaging plus Western blot and FACS, single lab, multiple detection methods\",\n      \"pmids\": [\"9657336\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Two conserved cytoplasmic tail motifs of MIC2 (SYHYY and EIEYE) are necessary and sufficient for microneme targeting; replacing these motifs with the cytoplasmic tail of SAG1 redirected the chimeric protein away from micronemes, confirming that SAG1's own transmembrane/cytoplasmic domain lacks a microneme-targeting signal.\",\n      \"method\": \"Chimeric protein targeting assays: SAG1 cytoplasmic tail fused to MIC2 transmembrane domains in transfected Toxoplasma; immunofluorescence localization\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — chimeric construct approach with subcellular localization readout, single lab\",\n      \"pmids\": [\"10982850\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Soluble recombinant SAG3 binds heparin and cell-surface heparan sulfate proteoglycans (HSPGs), whereas recombinant SAG1 does not interact with heparin, establishing that SAG3 (not SAG1) is the GPI-anchored surface antigen responsible for HSPG-mediated attachment.\",\n      \"method\": \"Heparin-binding assay with recombinant proteins; CHO cell adhesion assay with HSPG-deficient mutants; competitive inhibition with soluble glycoconjugates; radiolabeled tachyzoite attachment blockade\",\n      \"journal\": \"Molecular and biochemical parasitology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — in vitro binding assays with recombinant proteins, genetic HSPG-deficient cell mutants, and competitive inhibition; negative result for SAG1 is mechanistically informative\",\n      \"pmids\": [\"11463464\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"The crystal structure of SAG1 at 1.7 Å reveals a homodimeric configuration in which the dimeric interface is mediated by an extended beta-sheet forming a deep groove lined with positively charged amino acids; this basic groove is conserved among SRS proteins and is proposed to serve as a sulfated proteoglycan-binding site on target cell surfaces.\",\n      \"method\": \"X-ray crystallography at 1.7 Å resolution; structural analysis of homodimer interface\",\n      \"journal\": \"Nature structural biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — high-resolution crystal structure; structural groove consistent with known sulfated-proteoglycan binding biology of the SRS family\",\n      \"pmids\": [\"12091874\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"The rhomboid protease MPP1 (responsible for shedding microneme proteins) is constitutively active at the parasite surface; fusion of SAG1 with TgMIC2 or TgMIC6 transmembrane domains results in cleavage within the membrane-spanning domain, demonstrating that the ectodomain does not determine MPP1 cleavage site recognition.\",\n      \"method\": \"Site-specific mutagenesis of cleavage sites; SAG1 chimeras bearing TgMIC6 transmembrane domains expressed in Toxoplasma; cleavage assessed by biochemical assays\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — site-directed mutagenesis and chimeric constructs in parasite, single lab\",\n      \"pmids\": [\"11927542\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Clostridium septicum alpha-toxin, which targets GPI-anchored proteins, binds to SAG1 (and SAG3) on the T. gondii tachyzoite surface, causing disruption of the plasma membrane–inner membrane complex connection, plasma membrane extension, parasite lysis, and ER swelling; binding profiles differed between virulent RH and avirulent P strains.\",\n      \"method\": \"Alpha-toxin treatment of tachyzoites; ultrastructural analysis by electron microscopy; immunoblotting to identify toxin-bound proteins\",\n      \"journal\": \"Infection and immunity\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — ultrastructural and biochemical characterization, single lab, two strain comparison\",\n      \"pmids\": [\"12117945\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"RH\"}, {\"species\": \"Toxoplasma gondii\", \"strain\": \"P\"}]\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"SAG1 is expressed exclusively by tachyzoites and not by bradyzoites or enteric coccidian stages in vivo; stage conversion from tachyzoite to bradyzoite involves loss of SAG1 expression, while intermediate organisms co-express both SAG1 and bradyzoite marker BAG1.\",\n      \"method\": \"In vivo immunocytochemistry with stage-specific antibody panel (SAG1, BAG1, ENO isoforms, LDH isoforms, GRA proteins) in mouse and cat tissues; electron microscopy\",\n      \"journal\": \"International journal for parasitology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal molecular markers with electron microscopy in vivo across multiple life-cycle stages and hosts\",\n      \"pmids\": [\"15003495\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Constitutive expression of SAG1 during bradyzoite stage (SAG1c mutants) causes an early reduction in brain cyst loads followed by a substantial increase over time, accompanied by hyperproduction of IFN-gamma, TNF-alpha, and IL-10, and severe encephalitis; this demonstrates that stage-specific downregulation of SAG1 in bradyzoites is required for optimal parasite persistence.\",\n      \"method\": \"Transgenic Toxoplasma constitutively expressing tachyzoite SAG1 in bradyzoite stage; in vivo mouse infection model with cyst counting, cytokine measurement, and histopathology\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — transgenic loss-of-stage-specificity approach with quantitative in vivo readouts (cyst burden, cytokines, pathology), epistasis between antigen expression pattern and immune response\",\n      \"pmids\": [\"15944311\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"A Δsag1 sag2A double mutant is significantly attenuated in murine acute virulence and shows upregulated SRS29C (formerly SRS2/SAG2) expression; transgenic overexpression of SRS29C in the virulent RH parent similarly attenuates virulence, establishing SRS29C as a negative regulator of acute virulence and implicating the SAG1/SRS29B surface coat in modulating virulence.\",\n      \"method\": \"Targeted gene deletion of SAG1 (sag1) and sag2A; transgenic overexpression of SRS29C; in vivo mouse acute virulence assay\",\n      \"journal\": \"mBio\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — targeted gene deletion plus transgenic overexpression with in vivo virulence readout; complementary genetic approaches converging on same conclusion\",\n      \"pmids\": [\"23149485\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"RH\"}]\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Single-cell RNA-sequencing reveals that SAG1 expression among tachyzoites is highly heterogeneous and sporadic; AP2IX-1 is identified as a transcription factor that controls the switch from ubiquitous SAG1 expression to rare surface antigens not previously detected in tachyzoites.\",\n      \"method\": \"Single-cell RNA-sequencing (scRNA-seq) of >5,400 Toxoplasma tachyzoites and bradyzoites; transcription factor identification by computational analysis; three strains analyzed\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — scRNA-seq across multiple strains with TF identification; functional validation of AP2IX-1 role described but limited to correlation in this abstract\",\n      \"pmids\": [\"32065584\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"SAG1 mediates T. gondii fitness specifically in IFN-gamma-stimulated host cells; SAG1 knockout parasites show reduced attachment efficiency to host cells, an effect exacerbated in IFN-gamma-treated cells; host cell surface sialic acids are important for parasite attachment, especially under immune pressure.\",\n      \"method\": \"CRISPR/Cas9 knockout of SAG1; in vitro fitness assay in IFN-gamma-stimulated cells; attachment assay with SAG1-deficient parasites and sialic acid perturbation\",\n      \"journal\": \"Infection and immunity\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — CRISPR/Cas9 knockout with mechanistic follow-up (attachment assay, sialic acid perturbation), single lab, two orthogonal approaches\",\n      \"pmids\": [\"40607979\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    }\n  ],\n  \"current_model\": \"SAG1 (SRS29B/P30) is a GPI-anchored, homodimeric surface antigen expressed exclusively on tachyzoites that functions as an adhesin mediating parasite attachment to host cells via a positively charged groove that likely engages sulfated proteoglycans and sialic acids on host cell surfaces; its stage-specific expression (absent in bradyzoites and enteric stages) is controlled in part by upstream repeat promoter elements and the transcription factor AP2IX-1, and downregulation during bradyzoite differentiation is required for immune evasion and parasite persistence, while in IFN-gamma-stimulated environments SAG1 is specifically required for robust attachment and fitness.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"SAG1 (P30/SRS29B) is a GPI-anchored, tachyzoite-specific surface antigen of Toxoplasma gondii that functions as an adhesin mediating parasite attachment to host cells [#0, #5]. Encoded by a single-copy, intron-free gene, its primary product carries an N-terminal signal sequence and a C-terminal hydrophobic tail directing GPI anchor attachment, confirmed biochemically by PI-PLC sensitivity and cross-reacting-determinant detection [#0, #9]. The mature protein forms a homodimer whose interface generates a deep groove lined with positively charged residues proposed to engage sulfated glycoconjugates on target cells [#13]. Consistent with an adhesin role, anti-SAG1 antibodies and Fab fragments inhibit tachyzoite attachment and invasion, SAG1-deficient mutants bind host cells less efficiently, and a glucosamide neoglycoprotein competitively blocks infection of SAG1-positive parasites [#2, #5, #8]. CRISPR knockout confirms that SAG1 is specifically required for attachment and fitness in IFN-gamma-stimulated host cells and implicates host-cell surface sialic acids as attachment determinants under immune pressure [#20]; SAG1 itself does not bind heparin, distinguishing its ligand preference from the HSPG-binding SAG3 [#12]. SAG1 is expressed exclusively by tachyzoites and lost during bradyzoite and enteric-stage differentiation [#3, #4, #16]; this stage-specific downregulation is functionally essential, since forced SAG1 expression in bradyzoites drives excessive inflammatory cytokine responses and disrupts persistence [#17]. Its promoter contains upstream repeat elements required for accurate transcription initiation [#7], and its expression is heterogeneous among tachyzoites and controlled by the transcription factor AP2IX-1 [#20]. At the surface-coat level, the SAG1/SAG2A surface antigens modulate acute virulence, with SRS29C acting as a negative regulator [#18].\",\n  \"teleology\": [\n    {\n      \"year\": 1988,\n      \"claim\": \"Establishing the primary structure and predicted membrane topology of SAG1 defined it as a GPI-anchored surface protein, providing the molecular starting point for all subsequent functional work.\",\n      \"evidence\": \"cDNA/cosmid cloning and sequence analysis with beta-galactosidase fusion expression validated by anti-P30 monoclonal antibodies in RH\",\n      \"pmids\": [\"3183382\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Sequence-based GPI prediction not yet biochemically confirmed at this stage\", \"No structural or ligand information\"],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"RH\"}]\n    },\n    {\n      \"year\": 1993,\n      \"claim\": \"Antibody neutralization and mutant-parasite experiments first assigned SAG1 a functional role in host-cell invasion and implicated a glycosylated host receptor, moving it beyond a mere serological marker.\",\n      \"evidence\": \"in vitro invasion/neutralization assays with mAbs, Fab fragments, a P30-deficient mutant, and competitive neoglycoprotein inhibition in RH and PTg\",\n      \"pmids\": [\"7682587\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Host receptor identity not defined\", \"Attachment vs. later invasion steps not distinguished\"],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"RH\"}, {\"species\": \"Toxoplasma gondii\", \"strain\": \"PTg\"}]\n    },\n    {\n      \"year\": 1993,\n      \"claim\": \"Localization and stage-expression studies established SAG1 as a tachyzoite-restricted surface antigen absent from the vacuolar network, tying its expression to parasite stage identity.\",\n      \"evidence\": \"IFA/immunoEM localization and in vivo mRNA/protein expression during reactivation and differentiation across ME-49 and differentiating strains\",\n      \"pmids\": [\"8415546\", \"7690809\", \"7684705\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Transcriptional basis of stage restriction not yet defined\", \"Functional consequence of stage-specific loss unknown\"],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"ME-49\"}, {\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 1994,\n      \"claim\": \"Quantitative binding assays resolved SAG1's role to the attachment step specifically, distinguishing adhesion from downstream invasion events.\",\n      \"evidence\": \"radioisotope competitive binding to fixed fibroblasts with mAb/Fab inhibition and SAG1-defective mutant comparison\",\n      \"pmids\": [\"8050521\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Residual attachment in mutants shows SAG1 is not the sole adhesin\", \"Molecular ligand still unidentified\"],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 1994,\n      \"claim\": \"Adoptive transfer of a SAG1-specific CD8+ T cell clone established that SAG1 is an immunologically protective antigen acting through an IFN-gamma-dependent effector mechanism, linking the surface antigen to host immunity.\",\n      \"evidence\": \"adoptive transfer of a SAG1-specific CD8+ T cell clone with CD4 depletion and anti-IFN-gamma neutralization in mice\",\n      \"pmids\": [\"7907106\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Does not address SAG1's cell-biological function\", \"Epitope and antigen-processing details not defined\"],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 1995,\n      \"claim\": \"Promoter dissection identified upstream repeat elements governing accurate and efficient SAG1 transcription initiation, opening the mechanism of its regulated expression.\",\n      \"evidence\": \"promoter deletion constructs transfected into Toxoplasma with transcription-initiation analysis\",\n      \"pmids\": [\"7799972\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Trans-acting factors not identified\", \"Does not explain tachyzoite-specific restriction\"],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Biochemical and comparative analyses confirmed the GPI anchor experimentally and revealed a conserved cysteine/domain topology shared with the Neospora homolog, supporting a conserved fold and function.\",\n      \"evidence\": \"surface biotinylation, PI-PLC cleavage, anti-CRD immunoblotting, and sequence comparison in N. caninum\",\n      \"pmids\": [\"9784539\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional conservation inferred from sequence, not tested\", \"Cross-species work is on the Neospora homolog\"],\n      \"attribution\": [{\"species\": \"Neospora caninum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"The 1.7 A crystal structure defined SAG1 as a homodimer forming a basic groove, providing a structural model for how it might engage negatively charged host-cell surface ligands.\",\n      \"evidence\": \"X-ray crystallography and homodimer interface analysis\",\n      \"pmids\": [\"12091874\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Ligand in the groove not co-crystallized\", \"Groove specificity for a defined glycoconjugate not proven structurally\"],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Recombinant binding assays showed SAG1 does not bind heparin/HSPG, refining its ligand specificity and assigning HSPG-mediated attachment to the paralog SAG3 rather than SAG1.\",\n      \"evidence\": \"heparin-binding and CHO/HSPG-deficient adhesion assays with recombinant SAG1 and SAG3\",\n      \"pmids\": [\"11463464\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Positive ligand for SAG1 still undefined\", \"Reconciliation with the basic-groove model deferred\"],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Chimera and toxin studies used SAG1's transmembrane/GPI features as tools, showing its cytoplasmic tail lacks microneme-targeting information and that its GPI anchor makes it a target of GPI-directed toxins.\",\n      \"evidence\": \"SAG1/MIC chimeric targeting and cleavage assays, and Clostridium septicum alpha-toxin binding with ultrastructural analysis in RH and P strains\",\n      \"pmids\": [\"10982850\", \"11927542\", \"12117945\", \"9657336\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Trafficking determinants of native SAG1 not fully mapped\", \"Strain differences in toxin binding not mechanistically explained\"],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"RH\"}, {\"species\": \"Toxoplasma gondii\", \"strain\": \"P\"}, {\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Forced bradyzoite-stage expression showed that stage-specific SAG1 downregulation is functionally required for persistence, establishing a causal link between regulated antigen expression and immune-mediated control.\",\n      \"evidence\": \"transgenic constitutive SAG1 expression in bradyzoites with in vivo cyst counting, cytokine measurement, and histopathology\",\n      \"pmids\": [\"15944311\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which SAG1 provokes the cytokine response unclear\", \"Direct target of the induced immune response not defined\"],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"A comprehensive in vivo stage-marker survey confirmed SAG1 is confined to tachyzoites and lost during bradyzoite and enteric-stage conversion, with intermediate co-expression states.\",\n      \"evidence\": \"in vivo immunocytochemistry with a stage-specific marker panel and electron microscopy in mouse and cat tissues\",\n      \"pmids\": [\"15003495\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Regulatory logic of the switch not resolved here\"],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Genetic deletion and overexpression placed SAG1 within a surface-coat system that modulates acute virulence, identifying SRS29C as a negative regulator uncovered upon SAG1/SAG2A loss.\",\n      \"evidence\": \"targeted sag1/sag2A deletion, transgenic SRS29C overexpression, and mouse acute virulence assays in RH\",\n      \"pmids\": [\"23149485\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether virulence effect is due to SAG1 loss per se or coat remodeling not separated\", \"Molecular mechanism of SRS29C-mediated attenuation unknown\"],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"RH\"}]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Single-cell transcriptomics revealed heterogeneous, sporadic SAG1 expression among tachyzoites and identified AP2IX-1 as a transcription factor controlling the switch between ubiquitous SAG1 and rare surface antigens.\",\n      \"evidence\": \"scRNA-seq of >5,400 tachyzoites/bradyzoites across three strains with computational TF identification\",\n      \"pmids\": [\"32065584\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"AP2IX-1 regulation of SAG1 is largely correlative here\", \"Functional significance of expression heterogeneity unresolved\"],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"CRISPR knockout demonstrated a context-specific requirement for SAG1 in attachment and fitness under IFN-gamma stimulation and implicated host sialic acids as attachment determinants, resolving the ligand class SAG1 exploits under immune pressure.\",\n      \"evidence\": \"CRISPR/Cas9 SAG1 knockout with IFN-gamma fitness and attachment assays plus sialic acid perturbation\",\n      \"pmids\": [\"40607979\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct SAG1-sialic acid binding not structurally demonstrated\", \"Why the requirement is exacerbated by IFN-gamma not mechanistically defined\"],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The precise host-cell ligand engaged by the SAG1 basic groove and the mechanism reconciling its non-heparin, sialic-acid-linked adhesion with the structural model remain undefined.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No co-structure of SAG1 with a host ligand\", \"Mechanistic basis of IFN-gamma-specific fitness requirement unknown\", \"Direct link between AP2IX-1 and SAG1 promoter elements not established\"],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0050839\", \"supporting_discovery_ids\": [2, 5, 8, 20], \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"RH\"}]},\n      {\"term_id\": \"GO:0042803\", \"supporting_discovery_ids\": [13], \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0009986\", \"supporting_discovery_ids\": [1, 9, 16], \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]},\n      {\"term_id\": \"GO:0046658\", \"supporting_discovery_ids\": [0, 9], \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"RH\"}]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0044409\", \"supporting_discovery_ids\": [2, 5, 8, 20], \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]},\n      {\"term_id\": \"GO:0007155\", \"supporting_discovery_ids\": [5, 20], \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]}\n    ],\n    \"complexes\": [\n      \"SAG1 homodimer (crystallographic; basic-groove interface) [#13]\"\n    ],\n    \"partners\": [\n      \"Host-cell surface sialic acids (attachment determinant) [#20]\",\n      \"Glycosylated/glucosamide-bearing host receptor (competitive inhibition) [#2]\",\n      \"Clostridium septicum alpha-toxin (GPI-directed binder of SAG1) [#15]\"\n    ],\n    \"other_free_text\": [\n      \"GPI-anchored, single-copy intron-free gene product with N-terminal signal sequence and C-terminal GPI signal [#0, #9]\",\n      \"Tachyzoite-specific; expression lost during bradyzoite and enteric-stage differentiation [#3, #4, #16]\",\n      \"Stage-specific downregulation required for parasite persistence; forced bradyzoite expression drives inflammatory cytokines and encephalitis [#17]\",\n      \"Promoter contains upstream repeat elements required for accurate transcription initiation [#7]; expression heterogeneous and linked to AP2IX-1 [#20]\",\n      \"Does not bind heparin/HSPG, unlike paralog SAG3 [#12]\",\n      \"SAG1/SAG2A surface coat modulates acute virulence; SRS29C is a negative regulator [#18]\",\n      \"Cytoplasmic tail lacks microneme-targeting signal [#11]; conserved topology shared with N. caninum NcSAG1 [#9]\"\n    ]\n  },\n  \"strain_conflicts\": []\n}","audit_flag":null,"strains":[{"strain":"ME49","gene_id":"TGME49_233460","name":"SAG1","product":"SAG-related sequence SRS29B","uniprot_acc":"A0A125YP09","chromosome":"TGME49_chrVIII","start_pos":2664389},{"strain":"GT1","gene_id":"TGGT1_233460","name":"SAG1","product":"SAG-related sequence SRS29B","uniprot_acc":null,"chromosome":"TGGT1_chrVIII","start_pos":2660455},{"strain":"RH88","gene_id":"TGRH88_077450","name":null,"product":"SAG-related sequence SRS29B","uniprot_acc":null,"chromosome":"CM023089","start_pos":7770814},{"strain":"VEG","gene_id":"TGVEG_233460","name":"SAG1","product":"SAG-related sequence SRS29B","uniprot_acc":"V4Z6M2","chromosome":"TGVEG_chrVIII","start_pos":2673014}]}