Compact archived record and sequence evidence for P13664
Gene Ontology annotation through association of InterPro records with GO terms
Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location vocabulary mapping, accompanied by conservative changes to GO terms applied by UniProt
Molecular analysis of the gene encoding the major surface antigen of Toxoplasma gondii.
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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.
"carboxy-terminal hydrophobic tail which is predictive of a posttranslational cleavage and modification with a glycolipid anchor."
The major surface antigen, P30, of Toxoplasma gondii is anchored by a glycolipid.
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P30/SAG1 incorporates palmitate and myo-inositol and is released from intact parasites by phosphatidylinositol-specific phospholipase C, establishing a glycolipid (GPI) membrane anchor.
"These results argue that P30 is anchored by a glycolipid."
Two glycoforms are present in the GPI-membrane anchor of the surface antigen 1 (P30) of Toxoplasma gondii.
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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.
"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."
Structure of the immunodominant surface antigen from the Toxoplasma gondii SRS superfamily.
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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.
"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."
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SAG1 is the prototypic member of a family of GPI-linked, developmentally regulated surface proteins that coat the parasite.
"The surface of Toxoplasma is coated with a family of developmentally regulated glycosylphosphatidylinositol (GPI)-linked proteins (SRSs), of which SAG1 is the prototypic member."
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.
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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.
"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."
Toxoplasma gondii: the role of a 30-kDa surface protein in host cell invasion.
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A monoclonal antibody against P30 consistently reduced invasion of bovine kidney cells to 37% of control values across ten replicate assays.
"In 10 replicate assays, the overall invasion was reduced to 37% of control values (P less than 0.0001)."
Attachment of Toxoplasma gondii to host cells involves major surface protein, SAG-1 (P30).
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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.
"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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The authors conclude that SAG-1 is a parasite ligand binding the host cell during invasion.
"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."
Toxoplasma gondii major surface antigen (SAG1): in vitro analysis of host cell binding.
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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.
"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."
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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.
"pre-incubation of SAG1 in polyclonal sera from chronically infected mice failed to block binding."
Toxoplasma gondii: redistribution of tachyzoite surface protein during host cell invasion and intracellular development.
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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.
"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."
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Whether SAG1 is present on the developing intravacuolar membrane network just after invasion was contested between laboratories.
"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)."
Surface antigen SAG1 mediates Toxoplasma gondii fitness and host cell attachment in IFNγ-stimulated cells.
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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.
"the Δsag1 strain displayed a significantly reduced attachment, with only 81% of WT attachment levels, indicating the important role of SAG1 in mediating adhesion."
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Intracellular replication was unaffected by SAG1 deletion, so the fitness defect is attributable to attachment rather than growth.
"suggesting that parasite replication is not affected by SAG1 deletion."
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Heparan sulfate contributed little to SAG1-dependent adhesion, whereas sialic acid was required.
"suggesting that heparan sulfate plays a minimal role in SAG1-mediated adhesion."
Toxoplasma gondii SAG1 targeting host cell S100A6 for parasite invasion and host immunity.
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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.
"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."
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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.
"As an immunogen, TgSAG1 could promote the secretion of tumor necrosis factor alpha (TNF-α) through S100A6-Vimentin/PKCθ-NF-κB signaling pathway."
Identification of Host Proteins Interacting with Toxoplasma gondii SAG1 by Yeast Two-Hybrid Assay.
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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.
"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)."
Measurement of binding strength between prey proteins interacting with Toxoplasma gondii SAG1 and SAG2 using isothermal titration calorimetry (ITC).
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Isothermal titration calorimetry with recombinant proteins produced a typical binding curve for SAG1 with its human prey protein.
"ITC results indicated there was a typical binding curve for interactions between SAG1 and HLY protein."
Toxoplasma gondii surface antigen 1 (SAG1) interacts in vitro with host cell receptor for activated C kinase 1 (RACK1).
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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.
"the expression of RACK1 is manipulated by SAG1, and depletion of RACK1 negatively regulated host cell viability."
The induction of acute ileitis by a single microbial antigen of Toxoplasma gondii.
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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.
"C57BL/6 mice orally infected with Deltasag1 parasites failed to develop ileitis."
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Loss of SAG1 does not impair intracellular replication, so the immunopathology phenotype is not a growth defect.
"In vitro, the mutant parasites replicate in both enterocytes and dendritic cells."
Stage-specific expression of surface antigens by Toxoplasma gondii as a mechanism to facilitate parasite persistence.
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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.
"We conclude that stage-specific expression of SRS Ags is among the key mechanisms by which optimal parasite persistency is established and maintained."
Surface antigens of Toxoplasma gondii: variations on a theme.
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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.
"Analyses of a SAG1 knockout in rodents, however, indicate that this surface antigen may play a crucial role in immune modulation or virulence attenuation."