SUB2

TgSUB2 is a subtilisin-like serine proteinase of Toxoplasma gondii that localizes to the rhoptry secretory organelles and functions as a candidate rhoptry protein maturase (PMID:12890015). As it traffics through the secretory pathway, TgSUB2 undergoes autocatalytic processing; site-directed mutagenesis of a motif homologous to the ROP1 cleavage site abolished this autoprocessing, establishing that the enzyme cleaves after Glu residues (PMID:12890015). Consistent with a maturase role, TgSUB2 associates with the rhoptry protein ROP1, a candidate substrate (PMID:12890015). Beyond this single body of work, no further mechanistic detail — including a validated in vitro substrate repertoire or structural model — has been characterized in the available corpus.

1. 2003 Medium

   Establishing where an uncharacterized subtilisin-like protease acts localized TgSUB2 to the rhoptry organelles, placing it within the invasion-associated secretory machinery.

   Evidence Direct subcellular localization experiment in T. gondii

   PMID:12890015

   Open questions at the time

   • Does not define the enzyme's substrates within the rhoptry
   • Trafficking route to the rhoptry not resolved
2. 2003 High

   Whether TgSUB2 is a functional protease and what specificity it holds was answered by showing it autoprocesses during secretory trafficking and that mutating the ROP1-homologous cleavage motif abolishes this, defining cleavage after Glu.

   Evidence Site-directed mutagenesis of the predicted cleavage motif plus in vivo processing assay

   PMID:12890015

   Open questions at the time

   • Cleavage specificity inferred from autoprocessing, not from a purified enzyme against defined peptide substrates
   • No structural basis for Glu specificity established
3. 2003 Medium

   The biological role of TgSUB2 as a rhoptry protein maturase was supported by its association with ROP1 as a candidate substrate.

   Evidence Co-association/pulldown with ROP1 combined with mutagenesis-validated cleavage-site homology

   PMID:12890015

   Open questions at the time

   • Single Co-IP without reciprocal validation
   • Direct in vitro cleavage of ROP1 by TgSUB2 not demonstrated
   • Broader rhoptry substrate set unknown
4. 2003 Low

   The requirement of TgSUB2 for parasite viability was inferred from an unsuccessful gene disruption, implying essentiality.

   Evidence Failed gene disruption attempt (negative result)

   PMID:12890015

   Open questions at the time

   • Essentiality inferred from failed knockout, not from a conditional/inducible knockout
   • Phenotype of depletion not characterized

• The full rhoptry substrate repertoire, the mechanism and timing of TgSUB2 activation, and a conditional-knockout demonstration of its essential function remain undefined.
• No validated substrate beyond candidate ROP1
• No structural model
• Essentiality not confirmed by conditional depletion