RON9

RON9 is a rhoptry neck protein of Toxoplasma gondii that forms a high molecular mass complex with RON10, and this heterotypic association is the central determinant of its trafficking: proper complex formation is required for exit from the ER and sorting to the rhoptries, such that disruption of either partner leaves the other retained in the ER and subsequently degraded (PMID:22427839). Unlike other RON proteins, the RON9/RON10 complex is excluded from the moving junction during host cell invasion and is not associated with the RON2/4/5/8 MJ complex, distinguishing it functionally from the invasion machinery (PMID:22427839). Consistent with this exclusion, loss of RON9/RON10 has no detectable effect on rhoptry morphology, invasion, intracellular growth in fibroblasts, or in vivo virulence under standard laboratory conditions, so its functional contribution beyond mutual stabilization and rhoptry sorting is uncharacterized in the available corpus (PMID:22427839).

1. 2012 Medium

   Establishing where RON9 resides and how it reaches its destination answered whether it is a bona fide rhoptry neck protein and revealed that its trafficking is co-dependent on a partner, showing complex assembly gates ER exit and rhoptry sorting.

   Evidence Reciprocal gene knockouts of RON9 and RON10 with localization and protein-stability readouts in Toxoplasma gondii

   PMID:22427839

   Open questions at the time

   • The molecular basis of complex formation and the sorting signal directing the complex to rhoptries are not defined
   • Whether additional partners participate in the high molecular mass complex is not resolved
2. 2012 Medium

   Testing whether RON9/RON10 behaves like canonical RON proteins answered whether it participates in the moving junction, and its absence there separated it functionally from the invasion-associated RON2/4/5/8 complex.

   Evidence Immunofluorescence localization during host cell invasion compared with known moving junction components in Toxoplasma gondii

   PMID:22427839

   Open questions at the time

   • The actual function served by a rhoptry neck complex excluded from the moving junction is unknown
   • No molecular interaction data excludes transient or context-specific MJ association
3. 2012 Medium

   Phenotyping the knockouts answered whether RON9/RON10 is required for the parasite's lytic cycle, and the robust negative result established that the complex is dispensable for invasion, growth, and virulence under standard conditions.

   Evidence Rhoptry morphology, invasion, growth in fibroblasts, and in vivo virulence assays on RON9/RON10 knockouts in Toxoplasma gondii

   PMID:22427839

   Open questions at the time

   • No non-standard host or environmental conditions were tested that might reveal a phenotype
   • The biological process the complex contributes to remains unidentified

• The molecular function of the RON9/RON10 complex remains unknown: no substrate, ligand, or process has been assigned beyond mutual stabilization and rhoptry sorting.
• No functional role identified despite clear rhoptry neck localization
• No conditions found under which RON9 loss produces a phenotype
• Structure and interaction basis of the complex undefined