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.
The preferred accession for this review is P13664. These relationships are manually assessed.
REFERENCE PROTEOME EQUIVALENT Β· Reviewed 2026-10-19
Reference proteome: UP000557509
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.
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.
file:TOXGO/SAG1/SAG1-record-evidence.yaml
target_sequence_count: 8261
file:TOXGO/SAG1/SAG1-record-evidence.yaml
candidate_count: 37
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.
file:TOXGO/SAG1/SAG1-record-evidence.yaml
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
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.
file:TOXGO/SAG1/SAG1-record-evidence.yaml
primary_in_any_reference_proteome: false
file:TOXGO/SAG1/SAG1-record-evidence.yaml
alternative_in_reference_proteome: true
file:TOXGO/SAG1/SAG1-record-evidence.yaml
same_taxon: true
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.
file:TOXGO/SAG1/SAG1-record-evidence.yaml
entry_date: 09-DEC-2015, entry version 84. proteomes: []
file:TOXGO/SAG1/SAG1-record-evidence.yaml
entry_date: 02-DEC-2020, entry version 97. proteomes: []
file:TOXGO/SAG1/SAG1-record-evidence.yaml
entry_date: 22-FEB-2023, entry version 102. proteomes: []
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0016020 membrane | IEA GO_REF:0000002 | MODIFY | 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. 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 replacements: plasma membrane external side of plasma membrane Supporting Evidence: PMID:2925621 These results argue that P30 is anchored by a glycolipid. PMID:11522346 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. PMID:12091874 The surface of Toxoplasma is coated with a family of developmentally regulated glycosylphosphatidylinositol (GPI)-linked proteins (SRSs), of which SAG1 is the prototypic member. |
| GO:0020003 symbiont-containing vacuole | IEA GO_REF:0000044 | KEEP AS NON CORE | 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. 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. Supporting Evidence: PMID:8570580 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. file:TOXGO/SAG1/SAG1-uniprot.txt vesicular network within the parasitophorous vacuole |
| GO:0046812 host cell surface binding | IDA PMID:15151144 Toxoplasma gondii major surface antigen (SAG1): in vitro ana... | NEW | 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. 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. Supporting Evidence: PMID:15151144 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. PMID:8050521 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. |
| GO:0044650 adhesion of symbiont to host cell | IMP PMID:40607979 Surface antigen SAG1 mediates Toxoplasma gondii fitness and ... | NEW | 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. 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. Supporting Evidence: PMID:40607979 the Ξsag1 strain displayed a significantly reduced attachment, with only 81% of WT attachment levels, indicating the important role of SAG1 in mediating adhesion. PMID:40607979 SAG1-deficient parasites had reduced attachment efficiency, which was exacerbated in IFNΞ³-treated cells. PMID:8050521 Treatment of fresh extracellular T. gondii with rabbit polyclonal anti-SAG-1 serum inhibited parasite attachment to host cells by 71%. |
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Download this section (compressed HTML)Q: Which host-cell-surface molecule does SAG1 bind directly, and does the basic groove of the SAG1 homodimer carry that binding site?
Q: 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?
Q: 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?
Q: Is the immunodominance of SAG1 separable from its adhesive activity, given that the main conformational epitope does not overlap the predicted ligand pocket?
Experiment: 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.
Experiment: 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.
Experiment: 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.
What is not known β curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: 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.
BIOLOGY MF_DARK
What is known: 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.
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.
What would resolve it: 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 (the field's own admissions):
Gap: 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.
BIOLOGY MF_DARK
What is known: 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.
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.
What would resolve it: 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 (the field's own admissions):
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