Affinage · Toxoplasma gondii

RON4

rhoptry neck protein RON4 · UniProt S8F0B5

Annotated
2026-09-08
19 papers in source corpus 12 papers cited in narrative 12 extracted findings

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

RON4 is a rhoptry neck protein of Toxoplasma gondii that is discharged into the host cell during invasion and relocalizes to the moving junction (MJ), the circumferential ring through which the parasite penetrates the host cell (PMID:16309467). It functions as part of an exported rhoptry neck protein complex (RON2/RON4/RON5/RON8) whose RON4, RON5 and RON8 members associate with the cytosolic face of the host plasma membrane (PMID:19247437). Within this complex, trafficking and stability are mutually interdependent: RON5 is required for correct targeting of RON4 and for RON2 stability (PMID:24651769), while loss of RON4 in turn reduces the RON5 N-terminal fragment, indicating RON4 contributes to MJ complex integrity (PMID:28662729). From the cytosolic face of the MJ, RON4 anchors the invasion machinery to the host cell by binding host β-tubulin through defined reciprocal C-terminal interfaces (PMID:24217438) and host-surface heparan sulfate/glypican-1 via its C-terminal half (PMID:29081389), and it cooperates with RON2 and RON5 to recruit host CIN85, CD2AP, and the ESCRT-I components ALIX and TSG101 to the MJ, mimicking conserved host binding interfaces (PMID:28848228). Genetic ablation shows RON4 is not strictly essential—null parasites invade with only moderately reduced efficiency—but its loss and disruption of its host-recruitment function attenuate invasion and virulence (PMID:28662729, PMID:28848228). The MJ role is conserved across life stages and apicomplexans: RON4-containing junctions form in T. gondii sporozoites (PMID:32882286), and RON4 orthologues in Plasmodium associate with AMA1 and are required for invasion, though Plasmodium RON4 does not bind host β-tubulin (PMID:16835460, PMID:22177563, PMID:24217438).

Mechanistic history

Synthesis pass · year-by-year structured walk · 12 steps
  1. 2005 High

    Established RON4 as a rhoptry neck protein that redistributes to the moving junction during invasion, defining a previously unrecognized secretory contribution to the MJ distinct from micronemes and rhoptry bulb.

    Evidence Monoclonal antibody, affinity purification, mass spectrometry, and IFA in T. gondii tachyzoites

    PMID:16309467

    Open questions at the time
    • Did not define binding partners or the topology of RON4 at the junction
    • No genetic loss-of-function to test necessity
  2. 2006 Medium

    Extended the microneme-rhoptry MJ collaboration beyond Toxoplasma by showing the Plasmodium RON4 orthologue associates with AMA1, establishing conservation of the invasion junction across Apicomplexa.

    Evidence Co-immunoprecipitation and IFA in Plasmodium falciparum

    PMID:16835460

    Open questions at the time
    • Single Co-IP without reciprocal validation
    • Functional role of PfRON4 in invasion not tested
  3. 2008 Medium

    Characterized the Plasmodium yoelii RON4 orthologue and its partial AMA1 association, while showing its cysteine-rich domain is non-protective, providing comparative context for RON4 localization.

    Evidence cDNA screening, Co-IP, IFA, recombinant immunization/challenge in P. yoelii YM

    PMID:18710865

    Open questions at the time
    • Precise organellar assignment (rhoptry vs microneme) ambiguous
    • No invasion loss-of-function
  4. 2009 High

    Defined the exported RON2/RON4/RON5/RON8 complex and showed RON4, RON5, and RON8 reside on the cytosolic face of the host membrane, resolving the topology of RON4 at the junction.

    Evidence Differential permeabilization, metabolic labeling, Co-IP, antibody microinjection, IFA in T. gondii

    PMID:19247437

    Open questions at the time
    • Did not establish hierarchy of assembly or which member anchors RON4
    • Host-side receptors of RON4 unknown
  5. 2011 High

    Dissected the RON4-AMA1 relationship genetically, showing RON4 ring formation and MJ assembly proceed without AMA1 and that RON4 is required for Plasmodium sporozoite invasion independently of AMA1.

    Evidence Genetic knockdown/knockout, live imaging, invasion and epistasis assays in T. gondii tachyzoites and P. berghei sporozoites

    PMID:22177563

    Open questions at the time
    • Molecular basis of AMA1-independent MJ anchoring not defined
    • Did not identify the host receptor engaged by RON4
  6. 2013 Medium

    Identified host β-tubulin as a RON4 binding partner and mapped the reciprocal interfaces, providing a direct molecular link between the MJ and the host cytoskeleton at early invasion.

    Evidence Co-IP in mammalian cells, deletion-construct domain mapping, IFA in T. gondii

    PMID:24217438

    Open questions at the time
    • Functional consequence of disrupting the RON4-β-tubulin interaction for invasion not tested
    • Interaction shown in a single lab
    • Plasmodium RON4 does not bind β-tubulin, so mechanism is Toxoplasma-specific
  7. 2014 High

    Placed RON4 in the assembly hierarchy of the MJ complex by showing RON5 controls RON4 targeting and RON2 stability, and that an intact complex is prerequisite for rhoptry bulb secretion and invasion.

    Evidence TET-conditional knockdown, IFA, invasion and evacuole assays, complementation in T. gondii

    PMID:24651769

    Open questions at the time
    • Did not test the reciprocal effect of RON4 loss on the complex
    • Structural basis of RON5-dependent RON4 trafficking unknown
  8. 2017 Medium

    Showed RON4 binds host-surface heparan sulfate and glypican-1 via its C-terminal half, identifying a host receptor engaged by RON4 at the cell surface.

    Evidence Binding assays on GAG-deficient CHO cells, heparitinase treatment, glycoarray, IP in T. gondii

    PMID:29081389

    Open questions at the time
    • No functional invasion rescue linking heparan sulfate binding to penetration
    • Single-lab binding assays
  9. 2017 High

    Demonstrated that RON4, with RON2 and RON5, actively recruits host CIN85, CD2AP, ALIX, and TSG101 to the MJ, revealing molecular mimicry of host interfaces that supports efficient invasion and virulence.

    Evidence Interaction mapping, Co-IP, parasite mutagenesis, invasion and mouse virulence assays in T. gondii

    PMID:28848228

    Open questions at the time
    • Individual contribution of RON4 versus RON2/RON5 to each host recruitment not fully separated
    • Downstream role of recruited ESCRT machinery in penetration unresolved
  10. 2017 Medium

    Defined RON4's necessity by CRISPR knockout, establishing it as a non-essential but stability-contributing complex member that supports the RON5 N-terminal fragment and invasion efficiency.

    Evidence CRISPR-Cas9 disruption, invasion assay, western blot for complex members in T. gondii RH

    PMID:28662729

    Open questions at the time
    • Mechanism by which RON4 stabilizes RON5 not defined
    • Residual invasion pathway compensating for RON4 loss unknown
  11. 2017 Medium

    Identified RON4L1 as a RON4-related MJ member trafficking independently of RON2/RON4/RON5, indicating the MJ contains parallel sub-complexes rather than a single obligatory assembly.

    Evidence Knockout, IFA, mouse virulence, epistasis in RON2/RON4/RON5 mutant backgrounds in T. gondii

    PMID:29263399

    Open questions at the time
    • Functional relationship between RON4 and RON4L1 sub-complexes unresolved
    • Whether RON4L1 substitutes for RON4 not tested
  12. 2020 Medium

    Extended the RON4-containing MJ to the sporozoite life stage, showing that invasion via the same junction structure is conserved beyond tachyzoites.

    Evidence Anti-RON4 IFA, TEM, differential permeabilization in T. gondii ME49 and VEG

    PMID:32882286

    Open questions at the time
    • No genetic test of RON4 requirement in sporozoites
    • Stage-specific partner differences not examined

Open questions

Synthesis pass · forward-looking unresolved questions
  • How RON4's multiple host contacts (β-tubulin, heparan sulfate/glypican-1, and the CIN85/CD2AP/ESCRT recruitment) are integrated to mechanically couple the parasite motor to host penetration remains unresolved.
  • No structure of RON4 or the assembled MJ complex
  • No experiment separating the invasion contributions of each RON4 host interaction
  • Basis of functional redundancy that renders RON4 non-essential unknown

Mechanism profile

Synthesis pass · controlled-vocabulary classification
Partners
AMA1 (VIA COMPLEX; RON2-AMA1 IN PLASMODIUM)HOST ALIX (ESCRT-I)HOST CD2APHOST CIN85HOST HEPARAN SULFATE / GLYPICAN-1HOST TSG101 (ESCRT-I)HOST Β-TUBULINRON2RON5RON8
Complex memberships
RON2/RON4/RON5/RON8 moving junction complexRON4L1-containing MJ sub-complex

Evidence

Reading pass · 12 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2005 RON4 is a rhoptry neck protein that re-localizes to the moving junction (MJ) during Toxoplasma gondii tachyzoite invasion of host cells, as demonstrated by a monoclonal antibody that recognizes the MJ in a pattern distinct from other microneme and rhoptry proteins; the protein was affinity purified and identified by mass spectrometry. Monoclonal antibody generation, affinity purification, mass spectrometry, immunofluorescence assay Cellular microbiology High 16309467
2006 Plasmodium falciparum AMA1 (PfAMA1) co-immunoprecipitates with PfRON4, the Plasmodium homologue of TgRON4, and PfRON4 originates in the rhoptry necks, indicating that the microneme–rhoptry collaboration at the moving junction is conserved across Apicomplexa. Co-immunoprecipitation, immunofluorescence assay Eukaryotic cell Medium 16835460
2009 RON4 is part of a rhoptry neck protein complex (RON2/RON4/RON5/RON8) that is exported to the host cell membrane during invasion, where RON4, RON5 and RON8 remain associated with the cytoplasmic face of the host membrane. RON2 and AMA1 can interact in the absence of the other complex members. All MJ proteins undergo proteolytic maturation during trafficking and can associate as non-mature forms in vitro. Biochemical fractionation, differential membrane permeabilization, metabolic labeling, co-immunoprecipitation, antibody loading into host cells, immunofluorescence PLoS pathogens High 19247437
2008 The Plasmodium yoelii orthologue of TgRON4 (PyP140/RON4) is located within rhoptries or micronemes of merozoites and may associate in part with AMA1, but the conserved cysteine-rich domain does not elicit inhibitory antibodies and fails to protect against lethal infection. cDNA library screening, co-immunoprecipitation, immunofluorescence assay, recombinant protein production, mouse immunization challenge Infection and immunity Medium 18710865
2011 In Toxoplasma gondii tachyzoites, AMA1 is dispensable for normal RON4 ring formation and functional tight-junction (MJ) assembly, but AMA1 enhances tachyzoite apposition to the host cell and internalization frequency. In Plasmodium berghei sporozoites, RON4 is required for hepatocyte invasion independently of AMA1, establishing that RON4 can act independently of AMA1. Genetic knockdown/knockout, live imaging, invasion assays, epistasis analysis Cell host & microbe High 22177563
2013 Host cellular β-tubulin is a binding partner of TgRON4 during invasion. The C-terminal 15-kDa region of β-tubulin is sufficient for binding TgRON4, and a 17-kDa region in the proximal C-terminus of TgRON4 is required for binding. This interaction occurs at the early invasion step, linking RON4 to the host cytoskeleton. Plasmodium RON4 does not bind host β-tubulin. Co-immunoprecipitation in mammalian cells, domain mapping with deletion constructs, immunofluorescence assay Scientific reports Medium 24217438
2014 RON5 is required for stability of RON2 and correct targeting of RON4 within the parasite secretory pathway: conditional knockdown of RON5 causes complete degradation of RON2 and mistargeting of RON4, while RON8 is unaffected. Parasites lacking RON5 cannot invade host cells and fail to form evacuoles, demonstrating that an intact MJ RON complex is prerequisite for rhoptry bulb secretion. Proteolytic cleavage of RON5 into N- and C-terminal fragments is dispensable, but a portion of the C-terminal domain is critical for RON2 stability and invasion function. Conditional knockdown (TET-regulatable), immunofluorescence, invasion assays, evacuale formation assay, complementation PLoS pathogens High 24651769
2017 TgRON4 binds heparan sulfate on the host cell surface; binding is abolished in glycosaminoglycan- and heparan sulfate-deficient CHO cells and after heparitinase treatment. The C-terminal half of TgRON4 mediates binding to heparin agarose, and TgRON4 interacts with glypican-1 during Toxoplasma invasion as shown by immunoprecipitation. Binding assays on glycosaminoglycan-deficient cells, heparitinase treatment, glycoarray assay, immunoprecipitation Parasitology international Medium 29081389
2017 RON4 (together with RON2 and RON5) cooperates to actively recruit host CIN85, CD2AP, and ESCRT-I components ALIX and TSG101 to the moving junction during invasion. Parasite mutants unable to recruit these host proteins show inefficient host cell invasion in culture and attenuated virulence in mice, demonstrating that the parasite mimics conserved host binding interfaces via RON proteins. Interaction mapping, co-immunoprecipitation, parasite mutagenesis, invasion assays, mouse virulence assays Nature microbiology High 28848228
2017 CRISPR-Cas9-mediated null mutation of TgRON4 in RH strain moderately decreases invasion ability relative to wild-type, and the endogenous N-terminal fragment of RON5 is reduced in expression in the RON4-null mutant, indicating RON4 contributes to MJ complex stability (specifically stabilizing RON5) but is not strictly essential for invasion. CRISPR-Cas9 gene disruption, invasion assay, western blot for complex member expression Parasitology Medium 28662729
2017 RON4L1, a protein with sequence similarity to RON4, is an additional member of the MJ complex in Toxoplasma. It is cleaved at its C-terminal end before reaching the rhoptry neck and is targeted to the cytosolic face of the host membrane during invasion. RON4L1 knockout parasites are not essential for the lytic cycle in vitro but show reduced virulence in mice. RON4L1 traffic to the MJ is not affected by absence of RON2, RON4, or RON5, suggesting independent MJ sub-complexes exist. Knockout cell line generation, immunofluorescence, mouse virulence assay, epistasis analysis in RON2/RON4/RON5 mutant backgrounds Scientific reports Medium 29263399
2020 Using anti-RON4 antibodies, the moving junction was visualized in T. gondii sporozoites during invasion and shortly after its completion, demonstrating that sporozoites utilize the same RON4-containing MJ structure during host cell invasion as tachyzoites. Immunofluorescence assay with anti-RON4 antibody, transmission electron microscopy, differential permeabilization International journal for parasitology Medium 32882286

Source papers

Stage 0 corpus · 19 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2009 Export of a Toxoplasma gondii rhoptry neck protein complex at the host cell membrane to form the moving junction during invasion. PLoS pathogens 238 19247437
2005 The rhoptry neck protein RON4 re-localizes at the moving junction during Toxoplasma gondii invasion. Cellular microbiology 172 16309467
2006 Plasmodium falciparum AMA1 binds a rhoptry neck protein homologous to TgRON4, a component of the moving junction in Toxoplasma gondii. Eukaryotic cell 127 16835460
2011 Independent roles of apical membrane antigen 1 and rhoptry neck proteins during host cell invasion by apicomplexa. Cell host & microbe 100 22177563
2017 Efficient invasion by Toxoplasma depends on the subversion of host protein networks. Nature microbiology 50 28848228
2017 Toxoplasma gondii induces FAK-Src-STAT3 signaling during infection of host cells that prevents parasite targeting by autophagy. PLoS pathogens 50 29036202
2014 RON5 is critical for organization and function of the Toxoplasma moving junction complex. PLoS pathogens 47 24651769
2008 Identification and characterization of the Plasmodium yoelii PyP140/RON4 protein, an orthologue of Toxoplasma gondii RON4, whose cysteine-rich domain does not protect against lethal parasite challenge infection. Infection and immunity 32 18710865
2015 Protective immunity induced by peptides of AMA1, RON2 and RON4 containing T-and B-cell epitopes via an intranasal route against toxoplasmosis in mice. Parasites & vectors 28 25582167
2011 Immunological responses induced by a DNA vaccine expressing RON4 and by immunogenic recombinant protein RON4 failed to protect mice against chronic toxoplasmosis. Vaccine 25 21983362
2013 Characterization of the interaction between Toxoplasma gondii rhoptry neck protein 4 and host cellular β-tubulin. Scientific reports 22 24217438
2017 RON4L1 is a new member of the moving junction complex in Toxoplasma gondii. Scientific reports 16 29263399
2017 Toxoplasma gondii RON4 binds to heparan sulfate on the host cell surface. Parasitology international 10 29081389
2020 During host cell traversal and cell-to-cell passage, Toxoplasma gondii sporozoites inhabit the parasitophorous vacuole and posteriorly release dense granule protein-associated membranous trails. International journal for parasitology 9 32882286
2017 Design of inhibitory peptide targeting Toxoplasma gondii RON4-human β-tubulin interactions by implementing structural bioinformatics methods. Journal of cellular biochemistry 8 29091310
2017 The moving junction protein RON4, although not critical, facilitates host cell invasion and stabilizes MJ members. Parasitology 6 28662729
2018 Deciphering ophthalmic adaptive inhibitors targeting RON4 of Toxoplasma gondii: An integrative in silico approach. Life sciences 5 30326220
2025 Recombinant vaccinia virus expressing MIC8, AMA1, or RON4 induce protection against Toxoplasma gondii ME49 strain infection. Acta tropica 3 40886931
2025 In silico evaluation of Toxoplasma gondii rhoptry neck proteins (TgRONs) for potential immunogenic epitopes. EXCLI journal 1 40881926