RON4L1

RON4L1 is a rhoptry neck protein of Toxoplasma gondii that participates in host cell invasion as a component of the moving junction (MJ) machinery (PMID:29263399). Upon secretion during invasion, it localizes to the cytosolic face of the host membrane and associates with the MJ complex (PMID:29263399). It shares sequence similarity with RON4 and undergoes proteolytic cleavage at its C-terminus during maturation before reaching the rhoptry neck (PMID:29263399). RON4L1 traffics to the MJ independently of the canonical MJ components RON2, RON4, and RON5, indicating that it belongs to a distinct MJ sub-complex and implying at least two independent MJ sub-complexes in tachyzoites (PMID:29263399). Although dispensable for the in vitro lytic cycle, RON4L1 contributes to full virulence, as knockout parasites kill mice less efficiently (PMID:29263399). Beyond these findings, the biochemical partners, protease responsible for cleavage, and precise molecular function of RON4L1 have not been characterized in the available corpus.

1. 2017 High

   Establishing whether an uncharacterized rhoptry protein belongs to the invasion machinery, this work identified RON4L1 as a new MJ complex member positioned at the cytosolic face of the host membrane during invasion.

   Evidence Direct imaging during invasion and biochemical association with the MJ complex in T. gondii

   PMID:29263399

   Open questions at the time

   • Direct binding partners within the MJ not identified
   • Molecular activity at the junction undefined
2. 2017 Medium

   To understand RON4L1 maturation, sequence and localization analysis showed it is related to RON4 and is proteolytically cleaved at its C-terminus before trafficking to the rhoptry neck.

   Evidence Sequence analysis and subcellular fractionation/localization in T. gondii

   PMID:29263399

   Open questions at the time

   • Protease responsible for cleavage unknown
   • Cleavage not reconstituted in vitro
   • Functional consequence of processing untested
3. 2017 High

   To place RON4L1 relative to the canonical MJ complex, epistasis in RON2/RON4/RON5 mutant backgrounds showed its trafficking is independent of these components, defining a separate MJ sub-complex.

   Evidence Genetic epistasis using RON2, RON4, and RON5 knock-out/depleted parasites with localization readout in T. gondii

   PMID:29263399

   Open questions at the time

   • The composition of the RON4L1-containing sub-complex is unresolved
   • Functional relationship between the two MJ sub-complexes unknown
4. 2017 High

   To test physiological importance, knockout parasites revealed RON4L1 is dispensable for the in vitro lytic cycle but required for full virulence, linking the protein to pathogenesis in vivo.

   Evidence Genetic knock-out with in vitro lytic cycle assay and mouse infection model in T. gondii

   PMID:29263399

   Open questions at the time

   • Mechanism connecting RON4L1 loss to attenuated virulence unknown
   • No invasion-efficiency defect defined at the molecular level

• The molecular activity of RON4L1 at the moving junction, its binding partners, the protease that processes it, and the basis of its virulence contribution remain unresolved.
• No direct interacting partners identified
• No structural model
• Protease and processing site uncharacterized