id: P13664 gene_symbol: SAG1 product_type: PROTEIN status: COMPLETE taxon: id: NCBITaxon:5811 label: Toxoplasma gondii description: >- SAG1 (P30, SRS29B) is the most abundant surface protein of the Toxoplasma gondii tachyzoite and the prototypic member of the SAG1-related sequence (SRS) superfamily of developmentally regulated surface antigens. It is encoded by a single-copy, intronless gene and is expressed only in the rapidly dividing, invasive tachyzoite stage. The precursor carries an N-terminal signal peptide and a C-terminal glycosylphosphatidylinositol (GPI) addition signal; the mature protein is held in the outer leaflet of the parasite plasma membrane by a GPI anchor that occurs as two glycoforms. It is built from two disulfide-rich SRS domains and crystallises as a homodimer whose interface forms a basic groove proposed to bind sulfated proteoglycans. SAG1 behaves as a surface adhesin: purified SAG1 binds host cells, partly through host-surface glucosamine-containing glycans; antibodies against it reduce attachment and invasion; and SAG1-null parasites attach less efficiently, a defect that is larger when host cells are stimulated with interferon-gamma or lack surface sialic acid. Loss of SAG1 leaves intracellular replication unchanged, so the protein is dispensable in unstimulated cultured cells. Part of the surface pool is shed during invasion, and SAG1 is also detected in vesicular extensions of the parasitophorous vacuole. SAG1 is immunodominant: it elicits strong antibody and T-cell responses, is sufficient to drive the lethal ileitis that follows oral infection of susceptible mice, and its stage-restricted expression shapes the balance between parasite clearance and persistence. Reported host-protein partners include S100A6, KRCC1 and RACK1. alternative_records: - id: A0A7J6K396 relationship: REFERENCE_PROTEOME_EQUIVALENT reference_proteome: UP000557509 reviewed_on: '2026-10-19' reason: Retain P13664 as the primary Swiss-Prot accession. The reference SRS29B sequence is exactly identical across all 336 residues to reviewed SAG1. The full-proteome comparison separates it from other SAG-related proteins. This supports an equivalent protein representation while preserving the experimentally curated SAG1 accession. evidence: - type: PROTEOME_SEARCH description: MMseqs2 searched 8,261 sequences across UP000557509 at E <= 1e-5 and sensitivity 7.5, returning 37 hits. Every returned sequence was globally aligned using BLOSUM62 with gap-open -10 and extension -0.5. Scores and sequences for all 37 returned candidates are retained, with 10 detailed alignment files. The heuristic search does not establish genomic absence or unique locus identity. supported_by: - reference_id: file:TOXGO/SAG1/SAG1-record-evidence.yaml supporting_text: 'target_sequence_count: 8261' - reference_id: file:TOXGO/SAG1/SAG1-record-evidence.yaml supporting_text: 'candidate_count: 37' - type: SEQUENCE_COMPARISON description: 'A0A7J6K396: 336 identities across 336 global-alignment columns (100.0% identity including gaps), 0 substitutions and 0 gap columns. Query coverage is 100.0%; target coverage is 100.0%. Sequence similarity is interpreted in the manual rationale. Identity over paired residues is 100.0000% (336/336); gap columns are excluded from this denominator.' supported_by: - reference_id: file:TOXGO/SAG1/SAG1-record-evidence.yaml supporting_text: |- query_length: 336 target_length: 336 alignment_columns: 336 identities: 336 mismatches: 0 gap_columns: 0 identity_percent: 100.0 query_coverage_percent: 100.0 target_coverage_percent: 100.0 - type: PROTEOME_MEMBERSHIP description: The primary accession is outside every searched exact-taxon reference proteome. Current UniProt records place A0A7J6K396 in UP000557509 and give it the same taxon as the primary. supported_by: - reference_id: file:TOXGO/SAG1/SAG1-record-evidence.yaml supporting_text: 'primary_in_any_reference_proteome: false' - reference_id: file:TOXGO/SAG1/SAG1-record-evidence.yaml supporting_text: 'alternative_in_reference_proteome: true' - reference_id: file:TOXGO/SAG1/SAG1-record-evidence.yaml supporting_text: 'same_taxon: true' - type: RECORD_HISTORY description: 'Sampled UniSave observations: 09-DEC-2015, entry version 84.: no Proteomes cross-reference; 02-DEC-2020, entry version 97.: no Proteomes cross-reference; 22-FEB-2023, entry version 102.: no Proteomes cross-reference. These dated observations do not establish uninterrupted membership/absence, the exact change date, or UniProt''s motivation.' supported_by: - reference_id: file:TOXGO/SAG1/SAG1-record-evidence.yaml supporting_text: |- entry_date: 09-DEC-2015, entry version 84. proteomes: [] - reference_id: file:TOXGO/SAG1/SAG1-record-evidence.yaml supporting_text: |- entry_date: 02-DEC-2020, entry version 97. proteomes: [] - reference_id: file:TOXGO/SAG1/SAG1-record-evidence.yaml supporting_text: |- entry_date: 22-FEB-2023, entry version 102. proteomes: [] existing_annotations: - term: id: GO:0016020 label: membrane evidence_type: IEA original_reference_id: GO_REF:0000002 qualifier: located_in supporting_entities: - InterPro:IPR007226 review: summary: >- InterPro2GO mapping from the SRS domain signature. SAG1 is a membrane protein, but the evidence supports a far more specific location: GPI attachment in the outer leaflet of the tachyzoite plasma membrane. action: MODIFY reason: >- The term is right but too general. Metabolic labelling with palmitate and myo-inositol plus release by PI-PLC showed that P30/SAG1 carries a glycolipid anchor (PMID:2925621), and structural analysis of that anchor placed SAG1 in the parasite plasma membrane via GPI (PMID:11522346). A GPI-anchored protein displayed on the parasite surface lies on the external side of the plasma membrane, so GO:0005886 and GO:0009897, both descendants of membrane, are the supported replacements. The SRS domain occurs in GPI-anchored surface antigens throughout the family, so the InterPro mapping is imprecise rather than wrong. proposed_replacement_terms: - id: GO:0005886 label: plasma membrane - id: GO:0009897 label: external side of plasma membrane additional_reference_ids: - PMID:2925621 - PMID:11522346 supported_by: - reference_id: PMID:2925621 supporting_text: These results argue that P30 is anchored by a glycolipid. - reference_id: PMID:11522346 supporting_text: The protein is inserted into the parasite's plasma membrane by a glycosyl-phosphatidylinositol anchor, a modification that is present on all T. gondii surface proteins characterized so far. - reference_id: PMID:12091874 supporting_text: The surface of Toxoplasma is coated with a family of developmentally regulated glycosylphosphatidylinositol (GPI)-linked proteins (SRSs), of which SAG1 is the prototypic member. - term: id: GO:0020003 label: symbiont-containing vacuole evidence_type: IEA original_reference_id: GO_REF:0000044 qualifier: located_in supporting_entities: - UniProtKB-SubCell:SL-0194 review: summary: >- Mapped from the UniProt subcellular location "Parasitophorous vacuole". SAG1 is detected in vesicular extensions of the parasitophorous vacuole of intracellular tachyzoites, but that pool is surface material that has been shed or exported; the functional site is the parasite plasma membrane. action: KEEP_AS_NON_CORE reason: >- Immuno-electron microscopy places SAG1 in vacuoles throughout the infected cell, described either as extensions of the parasitophorous vacuole or as exported parasite surface material (PMID:8570580), and UniProt records SAG1 as a component of the vesicular network inside that vacuole. The parasitophorous vacuole is a genuine Toxoplasma compartment and the symbiont-containing vacuole term is used in the correct direction for a parasite protein, so this is not a mis-transfer. Whether SAG1 is on the intravacuolar network right after invasion was contested in the early literature (PMID:8570580), and the location reflects shedding rather than the adhesin activity carried out at the parasite surface, so it is retained as non-core. No more specific term for the intravacuolar network exists, so the term is as precise as the evidence allows. additional_reference_ids: - PMID:8570580 supported_by: - reference_id: PMID:8570580 supporting_text: SAG1 is present on the surface of daughter zoites and is found throughout the infected cell in distinct vacuoles; these vacuoles represent either direct extensions of the parasitophorous vacuole or true export of parasite surface material. - reference_id: file:TOXGO/SAG1/SAG1-uniprot.txt record_section: ["CC:SUBCELLULAR LOCATION"] supporting_text: vesicular network within the parasitophorous vacuole - term: id: GO:0046812 label: host cell surface binding evidence_type: IDA original_reference_id: PMID:15151144 qualifier: enables review: summary: >- Proposed new annotation. A direct in vitro attachment assay showed that SAG1 itself binds MDBK host cells, and that this binding is competed by a glucosamine neoglycoprotein, pointing to glucosamine-containing glycans on the host surface. action: NEW reason: >- This is the most direct evidence for the activity underlying the adhesin role, because the isolated protein rather than the whole parasite was assayed. The host ligand is not settled (glucosamine-containing glycans, PMID:15151144; a sialic-acid dependence of SAG1-mediated attachment, PMID:40607979; a predicted sulfated-proteoglycan groove in the dimer, PMID:12091874), so a ligand-agnostic term is the right level rather than a carbohydrate- or proteoglycan-binding term. Inhibition of whole-parasite attachment by the 6A8 monoclonal and its Fab (PMID:8050521) corroborates that SAG1 engages the host cell. No molecular function term is present in GOA for this protein. supported_by: - reference_id: PMID:15151144 supporting_text: Soluble BSA-glucosamide blocked SAG1 attachment to MDBK cells in a dose-dependent manner, implying that SAGI binding is mediated, in part, via attachment to host cell surface glucosamine. - reference_id: PMID:8050521 supporting_text: Taken together, these observations suggest that SAG-1 is an important parasite ligand that binds to the host cell in the process of T. gondii invasion. - term: id: GO:0044650 label: adhesion of symbiont to host cell evidence_type: IMP original_reference_id: PMID:40607979 qualifier: involved_in review: summary: >- Proposed new annotation. A CRISPR SAG1 knockout attaches less efficiently to host cells; the defect is reversed by complementation and is larger in interferon-gamma-stimulated and in sialic-acid-deficient host cells. action: NEW reason: >- SAG1 carries out the adhesion step itself rather than being merely required for it: isolated SAG1 binds host cells (PMID:15151144), anti-SAG1 antibodies and Fab fragments inhibit attachment of tachyzoites to fixed host cells (PMID:8050521), and the knockout plus complementation experiment shows reduced attachment with no replication defect (PMID:40607979). The comparator check is consistent: other apicomplexan surface adhesins, such as P. falciparum CSP reviewed in this repository, carry this term. The term is the symbiont-side process term, correct for a parasite protein acting on a host cell, and is neither ancestor nor descendant of the binding term proposed above. GOA currently records no process annotation at all for SAG1. supported_by: - reference_id: PMID:40607979 supporting_text: the Δsag1 strain displayed a significantly reduced attachment, with only 81% of WT attachment levels, indicating the important role of SAG1 in mediating adhesion. - reference_id: PMID:40607979 supporting_text: SAG1-deficient parasites had reduced attachment efficiency, which was exacerbated in IFNγ-treated cells. - reference_id: PMID:8050521 supporting_text: Treatment of fresh extracellular T. gondii with rabbit polyclonal anti-SAG-1 serum inhibited parasite attachment to host cells by 71%. core_functions: - description: >- Tachyzoite surface adhesin. GPI-anchored SAG1, displayed on the outer face of the parasite plasma membrane, binds the host cell surface partly through host glycans and so contributes to the initial attachment of the tachyzoite that precedes microneme- and rhoptry-driven entry. Its contribution is partly redundant with other surface and micronemal adhesins in unstimulated cells, and becomes important when host cells are stimulated with interferon-gamma or lack surface sialic acid. supported_by: - reference_id: PMID:15151144 supporting_text: Soluble BSA-glucosamide blocked SAG1 attachment to MDBK cells in a dose-dependent manner, implying that SAGI binding is mediated, in part, via attachment to host cell surface glucosamine. - reference_id: PMID:40607979 supporting_text: These findings highlight the role of SAG1 in mediating robust parasite attachment, especially in the context of immune pressure. - reference_id: PMID:11522346 supporting_text: The protein is inserted into the parasite's plasma membrane by a glycosyl-phosphatidylinositol anchor, a modification that is present on all T. gondii surface proteins characterized so far. molecular_function: id: GO:0046812 label: host cell surface binding directly_involved_in: - id: GO:0044650 label: adhesion of symbiont to host cell locations: - id: GO:0009897 label: external side of plasma membrane - id: GO:0005886 label: plasma membrane knowledge_gaps: - gap_statement: >- The host molecule that SAG1 binds is unidentified. Separate studies implicate glucosamine-containing glycans, sialylated glycans and sulfated proteoglycans, and it is unknown which of these SAG1 contacts directly, or how interferon-gamma changes the host cell surface so that SAG1 becomes more important for attachment. boundary: >- That isolated SAG1 binds host cells, and that SAG1-null parasites attach less well, especially to interferon-gamma-stimulated or sialic-acid-deficient cells, is established; the basic groove of the crystallographic dimer is only a predicted proteoglycan site. gap_kind: - BIOLOGY dark_aspect: MF_DARK significance: >- Naming the ligand would convert the longest-standing descriptive claim about the most abundant Toxoplasma surface protein into a defined receptor-ligand pair, and would explain why the attachment defect of the knockout depends on host immune status. resolution: >- Glycan-array and host-cell-surface capture profiling with recombinant SAG1 dimer, combined with attachment assays on host cells carrying defined glycosylation knockouts and with structure-guided mutation of the basic groove. provenance: - reference_id: PMID:15151144 supporting_text: implying that SAGI binding is mediated, in part, via attachment to host cell surface glucosamine. - gap_statement: >- It is unknown whether the host proteins reported to interact with SAG1 - S100A6, KRCC1 and RACK1 - are contacted by SAG1 during infection and, if so, how a GPI-anchored protein on the parasite surface reaches intracellular host proteins. boundary: >- The interactions have been reported by yeast two-hybrid, isothermal titration calorimetry, proximity labelling and co-immunoprecipitation, each by a single group; the topological route and the in vivo relevance have not been established. gap_kind: - BIOLOGY dark_aspect: MF_DARK significance: >- If genuine, these interactions would give SAG1 a signalling role toward the host cell beyond adhesion, and would explain reported effects on host cytoskeletal organisation and cytokine output; if they are artefacts of the assays, the simpler adhesin model stands. resolution: >- Independent replication with endogenously tagged SAG1 and host alleles, with quantitative proximity labelling controlled against a GPI-anchored non-SAG1 surface protein, plus interface mutants that separate adhesion from the reported interaction. provenance: - reference_id: PMID:34950858 supporting_text: We utilized the biotin ligase -TurboID fusion with TgSAG1 to identify the host proteins interacting with TgSAG1, and identified that S100A6 was co-localized with TgSAG1 when T. gondii attached to the host cell. proposed_new_terms: [] suggested_questions: - question: >- Which host-cell-surface molecule does SAG1 bind directly, and does the basic groove of the SAG1 homodimer carry that binding site? experts: [] - question: >- Why does loss of SAG1 matter mainly in interferon-gamma-stimulated host cells - does interferon-gamma remove the redundant adhesion routes, or does it create the surface that SAG1 recognises? experts: [] - question: >- Are the reported interactions with host S100A6, KRCC1 and RACK1 reproducible in other laboratories, and can a surface-anchored protein plausibly reach them during infection? experts: [] - question: >- Is the immunodominance of SAG1 separable from its adhesive activity, given that the main conformational epitope does not overlap the predicted ligand pocket? experts: [] suggested_experiments: - description: >- Structure-guided mutagenesis of the basic groove in the SAG1 homodimer, expressed from the native locus in a SAG1 knockout background, followed by quantitative attachment assays on naive and interferon-gamma-stimulated host cells and on cells lacking sialic acid or heparan sulfate. hypothesis: >- The conserved basic groove formed at the SAG1 dimer interface is the host-ligand binding site, and its disruption reproduces the attachment defect of the knockout. - description: >- Glycan-array screening and host-cell-surface crosslinking with recombinant SAG1, comparing the candidate ligand classes implicated so far (glucosamine-containing glycans, sialylated glycans, sulfated proteoglycans) under naive and interferon-gamma-stimulated conditions. hypothesis: >- SAG1 binds a single class of host glycan whose abundance or accessibility is altered by interferon-gamma stimulation. - description: >- Independent re-examination of the reported SAG1 host interactors using endogenous tagging, proximity labelling controlled with an unrelated GPI-anchored surface protein, and reciprocal co-immunoprecipitation in infected cells. hypothesis: >- Only a subset of the reported host partners is enriched specifically by SAG1 over a generic surface-anchored control. references: - id: file:TOXGO/SAG1/SAG1-record-evidence.yaml title: Compact archived record and sequence evidence for P13664 - id: GO_REF:0000002 title: Gene Ontology annotation through association of InterPro records with GO terms findings: [] - id: GO_REF:0000044 title: Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location vocabulary mapping, accompanied by conservative changes to GO terms applied by UniProt findings: [] - id: PMID:3183382 title: Molecular analysis of the gene encoding the major surface antigen of Toxoplasma gondii. findings: - statement: >- The P30/SAG1 gene is single copy and intronless, and the primary translation product has a signal sequence and a C-terminal hydrophobic tail predicting replacement by a glycolipid anchor. supporting_text: carboxy-terminal hydrophobic tail which is predictive of a posttranslational cleavage and modification with a glycolipid anchor. reference_review: relevance: MEDIUM correctness: VERIFIED review_notes: >- Cited in the UniProt entry as the original sequence report; title and authors match the cached PubMed record. Background for gene structure and for the predicted GPI anchor later confirmed biochemically. - id: PMID:2925621 title: The major surface antigen, P30, of Toxoplasma gondii is anchored by a glycolipid. findings: - statement: >- P30/SAG1 incorporates palmitate and myo-inositol and is released from intact parasites by phosphatidylinositol-specific phospholipase C, establishing a glycolipid (GPI) membrane anchor. supporting_text: These results argue that P30 is anchored by a glycolipid. reference_review: relevance: HIGH correctness: VERIFIED review_notes: >- Located by PubMed search and cached here; title, authors and journal match the J Biol Chem 1989 record. Primary biochemical evidence for the membrane anchor that justifies replacing the generic membrane term. - id: PMID:11522346 title: Two glycoforms are present in the GPI-membrane anchor of the surface antigen 1 (P30) of Toxoplasma gondii. findings: - statement: >- SAG1 is inserted into the parasite plasma membrane by a GPI anchor, which exists as two glycoforms differing by a terminal hexose on the GalNAc branch. supporting_text: The protein is inserted into the parasite's plasma membrane by a glycosyl-phosphatidylinositol anchor, a modification that is present on all T. gondii surface proteins characterized so far. reference_review: relevance: HIGH correctness: VERIFIED review_notes: >- Structural characterisation of the SAG1 anchor; states explicitly that the anchor places SAG1 in the parasite plasma membrane, which is the basis for the MODIFY on GO:0016020. - id: PMID:12091874 title: Structure of the immunodominant surface antigen from the Toxoplasma gondii SRS superfamily. findings: - statement: >- The 1.7 A structure of SAG1 is a homodimer whose interface forms a deep, positively charged groove proposed to bind sulfated proteoglycans on target cells. supporting_text: This basic groove seems to be conserved among SRS proteins and potentially serves as a sulfated proteoglycan-binding site on target cell surfaces, thus rationalizing the promiscuous attachment properties of Toxoplasma to a broad range of host cell types. - statement: >- SAG1 is the prototypic member of a family of GPI-linked, developmentally regulated surface proteins that coat the parasite. supporting_text: The surface of Toxoplasma is coated with a family of developmentally regulated glycosylphosphatidylinositol (GPI)-linked proteins (SRSs), of which SAG1 is the prototypic member. reference_review: relevance: HIGH correctness: VERIFIED review_notes: >- Cited by UniProt for the crystal structure (PDB 1KZQ) and for the subunit description. The proteoglycan-binding groove is explicitly a structural proposal, not a measured binding activity, and is treated as such here. - id: PMID:16242717 title: Crystal structure of the complex between the monomeric form of Toxoplasma gondii surface antigen 1 (SAG1) and a monoclonal antibody that mimics the human immune response. findings: - statement: >- The immunodominant conformational epitope lies in the N-terminal domain and does not overlap the proposed ligand-binding pocket, so antibody recognition and ligand binding are structurally separable. supporting_text: this conformational epitope, located within the SAG1 N-terminal domain, does not overlap with the proposed ligand-binding pocket. This study provides the first structural description of the monomeric form of SAG1, and significant insights into its dual role of adhesin and immune target during parasite infection. reference_review: relevance: MEDIUM correctness: VERIFIED review_notes: >- Cited by UniProt for PDB 1YNT. Used here for the relationship between epitope and ligand site, which motivates one of the suggested questions. - id: PMID:1730266 title: 'Toxoplasma gondii: the role of a 30-kDa surface protein in host cell invasion.' findings: - statement: >- A monoclonal antibody against P30 consistently reduced invasion of bovine kidney cells to 37% of control values across ten replicate assays. supporting_text: In 10 replicate assays, the overall invasion was reduced to 37% of control values (P less than 0.0001). reference_review: relevance: MEDIUM correctness: VERIFIED review_notes: >- Early antibody-blockade evidence. Antibody inhibition can act by steric masking of the parasite surface rather than by blocking a SAG1-specific contact, so it is used as corroboration and not as the primary basis for any annotation. - id: PMID:8050521 title: Attachment of Toxoplasma gondii to host cells involves major surface protein, SAG-1 (P30). findings: - statement: >- Polyclonal anti-SAG-1 serum inhibited attachment of live tachyzoites to fixed human fibroblasts by 71%, and the 6A8 monoclonal and its Fab gave dose-dependent inhibition; parasite mutants with defective SAG-1 bound much less well. supporting_text: Treatment of fresh extracellular T. gondii with rabbit polyclonal anti-SAG-1 serum inhibited parasite attachment to host cells by 71%. - statement: >- The authors conclude that SAG-1 is a parasite ligand binding the host cell during invasion. supporting_text: Taken together, these observations suggest that SAG-1 is an important parasite ligand that binds to the host cell in the process of T. gondii invasion. reference_review: relevance: HIGH correctness: VERIFIED review_notes: >- Combines antibody blockade, Fab controls and SAG-1-defective mutants, so it is stronger than antibody inhibition alone. Abstract-only in the cache. - id: PMID:15151144 title: 'Toxoplasma gondii major surface antigen (SAG1): in vitro analysis of host cell binding.' findings: - statement: >- In an assay measuring binding of SAG1 itself rather than of whole tachyzoites, SAG1 attached to MDBK cells and was competed by soluble BSA-glucosamide, implicating host-surface glucosamine. supporting_text: Soluble BSA-glucosamide blocked SAG1 attachment to MDBK cells in a dose-dependent manner, implying that SAGI binding is mediated, in part, via attachment to host cell surface glucosamine. - statement: >- Polyclonal serum from chronically infected mice did not block SAG1 binding, arguing that anti-SAG1 antibodies do not act simply by masking the host-binding site. supporting_text: pre-incubation of SAG1 in polyclonal sera from chronically infected mice failed to block binding. reference_review: relevance: HIGH correctness: VERIFIED review_notes: >- Cited by UniProt for FUNCTION. The key study for the molecular-function call, because the protein rather than the parasite was the binding species. Only the abstract is cached, and the host cells are bovine kidney cells, so the ligand assignment is partial. - id: PMID:8570580 title: 'Toxoplasma gondii: redistribution of tachyzoite surface protein during host cell invasion and intracellular development.' findings: - statement: >- Immuno-electron microscopy showed SAG1 on daughter zoites and in vacuoles throughout the infected cell, interpreted as extensions of the parasitophorous vacuole or as exported parasite surface material. supporting_text: SAG1 is present on the surface of daughter zoites and is found throughout the infected cell in distinct vacuoles; these vacuoles represent either direct extensions of the parasitophorous vacuole or true export of parasite surface material. - statement: >- Whether SAG1 is present on the developing intravacuolar membrane network just after invasion was contested between laboratories. supporting_text: Conflicting reports exist concerning the presence of SAG1 on the developing intraphagosomal membrane (IPM) network immediately post-invasion (Sibley et al. 1986; Dubremetz et al. 1993). reference_review: relevance: MEDIUM correctness: VERIFIED review_notes: >- Supports the parasitophorous vacuole location recorded by UniProt and shows that it reflects shedding or export of surface material. Abstract-only in the cache. - id: PMID:40607979 title: Surface antigen SAG1 mediates Toxoplasma gondii fitness and host cell attachment in IFNγ-stimulated cells. findings: - statement: >- A CRISPR SAG1 knockout showed reduced attachment that was restored by complementation, with the defect amplified in interferon-gamma-stimulated cells and in host cells lacking sialic acid. supporting_text: the Δsag1 strain displayed a significantly reduced attachment, with only 81% of WT attachment levels, indicating the important role of SAG1 in mediating adhesion. - statement: >- Intracellular replication was unaffected by SAG1 deletion, so the fitness defect is attributable to attachment rather than growth. supporting_text: suggesting that parasite replication is not affected by SAG1 deletion. - statement: >- Heparan sulfate contributed little to SAG1-dependent adhesion, whereas sialic acid was required. supporting_text: suggesting that heparan sulfate plays a minimal role in SAG1-mediated adhesion. reference_review: relevance: HIGH correctness: VERIFIED review_notes: >- Full text available in the cache. The cleanest genetic evidence for SAG1 in attachment: knockout plus complementation, with host glycosylation mutants. Supports the proposed adhesion-of-symbiont-to-host-cell annotation. - id: PMID:34950858 title: Toxoplasma gondii SAG1 targeting host cell S100A6 for parasite invasion and host immunity. findings: - statement: >- Proximity labelling with a SAG1-TurboID fusion in infected human fibroblasts identified host S100A6 as a SAG1-proximal protein, and SAG1 was reported to reduce the host S100A6-vimentin interaction early after infection. supporting_text: We utilized the biotin ligase -TurboID fusion with TgSAG1 to identify the host proteins interacting with TgSAG1, and identified that S100A6 was co-localized with TgSAG1 when T. gondii attached to the host cell. - statement: >- Infection with wild-type parasites induced more TNF-alpha than infection with the SAG1 deletion strain, which the authors attribute to an S100A6-vimentin/PKC-theta/NF-kB route. supporting_text: As an immunogen, TgSAG1 could promote the secretion of tumor necrosis factor alpha (TNF-α) through S100A6-Vimentin/PKCθ-NF-κB signaling pathway. reference_review: relevance: MEDIUM correctness: UNVERIFIED review_notes: >- Full text cached and internally consistent, and cited by UniProt for FUNCTION and SUBUNIT. It is nonetheless a single-laboratory mechanism built on proximity labelling plus knockdown, and the proposed signalling chain involves intracellular host proteins that a GPI-anchored surface protein would not obviously reach. No annotation is proposed from it; it is recorded as a knowledge gap instead. - id: PMID:31165984 title: Identification of Host Proteins Interacting with Toxoplasma gondii SAG1 by Yeast Two-Hybrid Assay. findings: - statement: >- A yeast two-hybrid screen of a human cDNA library with SAG1 as bait recovered lysine-rich coiled-coil protein as the strongest interactor, supported by co-immunoprecipitation. supporting_text: The results indicated that a strong interaction existed between Homo sapiens lysine-rich coil-coiled and SAG1 protein, which could activate the expressions of the reporter genes in diploid yeast. Co-immunoprecipitation experiment result indicated the binding between this prey and SAG1 protein was significant (Mann-Whitney U test, Z = - 1.964, P = 0.05). reference_review: relevance: LOW correctness: UNVERIFIED review_notes: >- Cited by UniProt for the KRCC1 interaction. Yeast two-hybrid with a secreted, disulfide-rich, GPI-anchored bait is prone to artefacts, and no infection-based validation is reported; no annotation is derived from it. - id: PMID:29351065 title: Measurement of binding strength between prey proteins interacting with Toxoplasma gondii SAG1 and SAG2 using isothermal titration calorimetry (ITC). findings: - statement: >- Isothermal titration calorimetry with recombinant proteins produced a typical binding curve for SAG1 with its human prey protein. supporting_text: ITC results indicated there was a typical binding curve for interactions between SAG1 and HLY protein. reference_review: relevance: LOW correctness: UNVERIFIED review_notes: >- Follow-up biophysics on the yeast two-hybrid hit, using bacterially expressed SAG1, which is unlikely to be correctly disulfide-bonded. Supports a physical interaction in vitro only. - id: PMID:38157925 title: Toxoplasma gondii surface antigen 1 (SAG1) interacts in vitro with host cell receptor for activated C kinase 1 (RACK1). findings: - statement: >- RACK1 was identified as a SAG1 binding partner in host cells, and its depletion reduced host cell viability, leading the authors to propose that SAG1 preserves host cell viability through RACK1. supporting_text: the expression of RACK1 is manipulated by SAG1, and depletion of RACK1 negatively regulated host cell viability. reference_review: relevance: LOW correctness: UNVERIFIED review_notes: >- Cited by UniProt for FUNCTION and SUBUNIT, but the proposed partner is an intracellular host scaffold, and the paper's own title limits the claim to an in vitro interaction. Only the abstract is cached. No annotation is proposed. - id: PMID:15294991 title: The induction of acute ileitis by a single microbial antigen of Toxoplasma gondii. findings: - statement: >- Susceptible mice orally infected with a SAG1-deficient strain did not develop the lethal ileitis caused by wild-type parasites, although the mutant replicated normally in enterocytes and dendritic cells. supporting_text: C57BL/6 mice orally infected with Deltasag1 parasites failed to develop ileitis. - statement: >- Loss of SAG1 does not impair intracellular replication, so the immunopathology phenotype is not a growth defect. supporting_text: In vitro, the mutant parasites replicate in both enterocytes and dendritic cells. reference_review: relevance: MEDIUM correctness: VERIFIED review_notes: >- Strong in vivo immunology, but the phenotype is a host immune response to SAG1 as an antigen rather than an activity SAG1 performs, so no symbiont-side process term is proposed from it. Useful as independent evidence that SAG1 is dispensable for replication. - id: PMID:15944311 title: Stage-specific expression of surface antigens by Toxoplasma gondii as a mechanism to facilitate parasite persistence. findings: - statement: >- Forcing constitutive expression of the tachyzoite antigen SAG1 into the bradyzoite stage altered cyst burden and caused severe encephalitis, showing that the stage restriction of SRS antigens contributes to persistence. supporting_text: We conclude that stage-specific expression of SRS Ags is among the key mechanisms by which optimal parasite persistency is established and maintained. reference_review: relevance: MEDIUM correctness: VERIFIED review_notes: >- Supports the developmental-stage statement in the description. The phenotypes are properties of the host immune response to mis-timed antigen expression, not of a SAG1 molecular activity, so nothing is annotated from it. - id: PMID:11566296 title: 'Surface antigens of Toxoplasma gondii: variations on a theme.' findings: - statement: >- Review of the SAG1 and SAG2 families; notes that SAG3 null mutants show a clear attachment function, whereas rodent studies of a SAG1 knockout pointed to immune modulation or virulence attenuation. supporting_text: Analyses of a SAG1 knockout in rodents, however, indicate that this surface antigen may play a crucial role in immune modulation or virulence attenuation. reference_review: relevance: MEDIUM correctness: VERIFIED review_notes: >- Family-level context, used to keep the adhesion call proportionate: SAG1 is one of many partly redundant SRS adhesins, which is consistent with the mild knockout attachment phenotype seen later by CRISPR.