RON3

RON3 is a rhoptry protein implicated in apicomplexan host-cell invasion, characterized mechanistically through its Plasmodium falciparum ortholog PfRON3. PfRON3 localizes to the rhoptry body/bulb rather than the rhoptry neck in merozoites, and reciprocal co-immunoprecipitation places it in a novel RON2/3/4 complex that does not include AMA1, distinguishing it from the AMA1-associated moving junction complex (PMID:21237287). Recombinant PfRON3 binds directly to erythrocytes, and anti-PfRON3 antibodies inhibit merozoite invasion, establishing a functional role in host-cell entry (PMID:25495792). In Toxoplasma gondii itself, only genomic variation and differential expression data are available: RON3 carries strain-specific SNPs and indels and is expressed more highly in the less virulent Wh6 strain than in the virulent Wh3 strain (PMID:26518334). No direct mechanistic experiments on TgRON3 protein are reported in the available corpus.

1. 2011 Medium

   Establishing that PfRON3 defines a rhoptry-body RON complex separate from the moving junction answered whether all RON proteins act through the AMA1-linked junction, revealing a distinct RON2/3/4 assembly.

   Evidence Immunofluorescence, immunoelectron microscopy, and reciprocal co-IP in P. falciparum merozoites

   PMID:21237287

   Open questions at the time

   • Functional consequence of the RON2/3/4 complex not tested
   • Findings in P. falciparum ortholog, not directly in T. gondii
   • Stoichiometry and assembly order of the complex undefined
2. 2014 Medium

   Direct erythrocyte binding by recombinant PfRON3 and antibody-mediated invasion inhibition addressed whether the protein participates functionally in host-cell entry, confirming a role in invasion.

   Evidence IEM, IFA, recombinant erythrocyte-binding assay, and invasion inhibition with specific antibodies in P. falciparum

   PMID:25495792

   Open questions at the time

   • Erythrocyte receptor for PfRON3 not identified
   • Mechanistic link between binding and junction formation unresolved
   • No orthologous validation in T. gondii
3. 2015 Low

   Cataloguing RON3 sequence variation and expression differences across virulence-divergent T. gondii strains raised the question of whether RON3 contributes to strain virulence, showing an inverse correlation between transcript level and virulence.

   Evidence Whole-genome sequencing and qRT-PCR of Chinese 1 T. gondii strains Wh3 and Wh6

   PMID:26518334

   Open questions at the time

   • Correlational only — no functional or mechanistic experiment on the RON3 protein
   • Causal contribution of the SNPs/indels to virulence untested
   • TgRON3 localization and interaction partners not experimentally determined

• Whether TgRON3 forms an equivalent RON2/3/4 complex, localizes to the rhoptry body, and functions in T. gondii host-cell invasion remains unestablished by direct experiment.
• No direct localization of TgRON3 protein reported
• No interaction partners defined in T. gondii
• No invasion or knockout phenotype for TgRON3