SRS1

SRS1 (SAG1-related sequence 1) is a GPI-anchored surface protein of Toxoplasma gondii tachyzoites, encoded by a gene positioned upstream of and co-directionally transcribed with SAG1 (PMID:9364971). It is one of the abundant GPI-anchored antigens dominating the tachyzoite surface, migrating as a ~46 kDa band and sharing the tandemly duplicated motif and 12 conserved cysteines characteristic of the SAG/SRS family, features consistent with a conserved fold shared with SAG1 (PMID:9364971, PMID:9573113). SRS1 carries strain-polymorphic epitopes and is immunogenic in vivo: recombinant SRS1 elicits partial protective immunity against lethal challenge in mice (PMID:11346179). Beyond its surface localization and immunogenicity, the molecular function of SRS1 in host-cell interaction or parasite biology has not been characterized in the available corpus.

1. 1997 Medium

   Establishing that the SAG1-adjacent locus encodes a bona fide surface protein answered whether SRS1 is expressed and membrane-associated, defining it as a GPI-anchored tachyzoite surface antigen.

   Evidence Genomic/cDNA cloning, recombinant expression in E. coli, Western blot, and GPI-anchor analysis in T. gondii

   PMID:9364971

   Open questions at the time

   • No molecular function or binding partner identified
   • Role in host-cell attachment or invasion untested
   • Regulation and stage-specificity of expression not resolved
2. 1998 Medium

   Placing SRS1 within the SAG/SRS family clarified that it shares conserved structural motifs with SAG1, implying a common fold and potentially a conserved function among dominant surface antigens.

   Evidence EST database search, sequence analysis, and immunofluorescence confirming surface expression in T. gondii

   PMID:9573113

   Open questions at the time

   • Shared structure does not establish a shared molecular activity
   • No structural model of SRS1 itself
   • Functional consequence of the conserved cysteines untested
3. 2001 Low

   Immunization experiments addressed whether SRS1 is a protective antigen, showing it can elicit partial immunity but leaving its native role undefined.

   Evidence Recombinant protein immunization with lethal challenge and brain cyst quantification in mice (RH strain)

   PMID:11346179

   Open questions at the time

   • Protection is partial (up to 25% survival) and mechanism of immunity undefined
   • Does not reveal the parasite-intrinsic function of SRS1
   • Single lab, single method set
4. 2035 Low

   Serological profiling established that SRS1 harbors strain-polymorphic epitopes distinguishable by host antibody responses, enabling clonal-type discrimination.

   Evidence Synthetic peptide microarray and serology in experimentally infected chickens and turkeys with ROC analysis (RH, ME49, NED strains)

   PMID:29969590

   Open questions at the time

   • Indirect inference from a diagnostic tool; no direct mechanistic experiment on SRS1 protein function
   • Functional significance of the polymorphisms unknown
   • Anomalous timeline year

• The molecular function of SRS1 — whether it mediates host-cell adhesion, invasion, or immune modulation like other SAG/SRS members — remains unresolved.
• No ligand or host receptor identified
• No knockout/knockdown phenotype reported
• No direct role in attachment or invasion demonstrated