SAG1 (P13664, Toxoplasma gondii) - curation notes

Automated deep research was unavailable for this gene (Falcon authentication and
OpenAI key both failed in this environment). These notes are a manual literature
review built from the UniProt record (SAG1-uniprot.txt), the cached PubMed
records in publications/, and PubMed searches (E-utilities) used to locate
classic primary papers, which were then cached with just fetch-pmid.
Most cached papers are abstract-only; PMID:34950858 and PMID:40607979 are full text.

Identity

• SAG1 = P30 = SRS29B, the prototypic member of the SAG1-related sequence (SRS)
  superfamily; locus TGME49_233460 / TGGT1_233460. 336 aa precursor with N-terminal
  signal peptide (1-47) and a C-terminal hydrophobic GPI-addition signal; two SRS
  (Pfam PF04092) domains, six disulfides; structures 1KZQ and 1YNT.
• Single-copy, intronless gene; mRNA extremely abundant
  PMID:3183382.
• The C terminus predicted a glycolipid anchor
  PMID:3183382.
• Tachyzoite-specific: [file:TOXGO/SAG1/SAG1-uniprot.txt "Detected only in rapidly dividing and invasive"].

Localization

• GPI-anchored in the parasite plasma membrane, shown biochemically:
  PMID:2925621;
  PMID:11522346.
  The GPI anchor carries two glycoforms (GalNAc side branch +/- terminal Glc) PMID:11522346.
• Note: the UniProt record does not carry a GPI-anchor keyword or a "Cell membrane"
  subcellular location; it records only "Parasitophorous vacuole" with the note that
  SAG1 is on the surface of tachyzoites and in the vesicular network of the PV.
• Surface protein is shed on invasion; some is found in vacuolar extensions of the PV
  PMID:8570580;
  presence on the intravacuolar network immediately post-invasion was contested
  PMID:8570580.
  So the PV (symbiont-containing vacuole) location is real but secondary to the
  parasite-surface location.

Structure

• 1.7 A structure: homodimer with a basic groove proposed to bind sulfated
  proteoglycans PMID:12091874.
• Monomeric SAG1-Fab structure: immunodominant epitope does not overlap the proposed
  ligand pocket PMID:16242717.

Function: adhesion to host cells

• Antibody blocking: PMID:1730266;
  PMID:8050521;
  SAG1-defective mutants bind less PMID:8050521.
• Recombinant/purified SAG1 binds host cells directly, partly via glucosamine
  PMID:15151144.
  This is the best evidence that SAG1 itself performs the binding (MF host cell surface binding).
• Clean genetics (CRISPR KO + complement, 2025): fitness defect only in IFNg-treated
  cells; attachment reduced, more so with IFNg; sialic acid dependence
  PMID:40607979;
  PMID:40607979;
  replication unaffected PMID:40607979.
• Rachinel et al. Deltasag1 still replicates in vitro PMID:15294991.
• Conclusion: SAG1 is a non-essential, partially redundant surface adhesin; its
  participation in attachment is direct (it is the binding molecule), so
  "adhesion of symbiont to host cell" (GO:0044650) and "host cell surface binding"
  (GO:0046812) are appropriate and pass the participation test (SAG1 does the binding;
  it is not a substrate). Comparator: P. falciparum CSP in this repo carries both.
• Receptor identity unresolved: sulfated proteoglycans (structure prediction),
  glucosamine (PMID:15151144), sialic acid (PMID:40607979; heparan sulfate minimal role).

Host interactions / immune modulation

• Immunodominant antigen; SAG1 alone drives lethal ileitis in C57BL/6
  PMID:15294991.
• Stage-restricted expression matters for persistence: constitutive SAG1 gives
  encephalitis and high cyst burden later PMID:15944311.
• Lekutis review: KO suggests immune modulation / virulence attenuation
  PMID:11566296.
• Host interactors from single labs, mostly abstract-only: KRCC1 (Y2H, ITC;
  PMID:31165984, PMID:29351065), S100A6 (TurboID proximity, co-IP; PMID:34950858),
  RACK1 (PMID:38157925; intracellular host protein - topologically odd for a
  GPI-anchored surface protein, needs caution).
• S100A6 study (full text): TgSAG1 inhibits S100A6-vimentin interaction and promotes
  TNF via S100A6-vimentin/PKCtheta-NF-kB PMID:34950858.
  Treated as preliminary; not annotated as a new process.

GOA annotations (2 rows, both IEA CC)

1. GO:0016020 membrane (InterPro IPR007226 SRS domain) - true but general; SAG1 is a
   GPI-anchored protein on the outer leaflet of the parasite plasma membrane -> MODIFY to
   plasma membrane (GO:0005886) and external side of plasma membrane (GO:0009897).
2. GO:0020003 symbiont-containing vacuole (UniProt SubCell PV) - supported by EM
   (PMID:8570580) and UniProt note; secondary location of shed/exported surface material ->
   KEEP_AS_NON_CORE. Organelle check: PV is a genuine Toxoplasma compartment; term is
   used correctly for a symbiont protein.

Proposed NEW

• GO:0044650 adhesion of symbiont to host cell (IMP, PMID:40607979; also PMID:8050521).
• GO:0046812 host cell surface binding (IDA, PMID:15151144).
• Not proposed: immune-modulation/TNF process terms (single-lab, mechanism unclear);
  "protein homodimerization" (crystal-only, no functional relevance shown).