RON5

RON5 (PF3D7_0817700) is a rhoptry neck protein that functions as a core component of the moving junction complex that Plasmodium parasites assemble at the host-cell interface during invasion (PMID:42379167, PMID:21185360, PMID:20228060). In P. falciparum, RON5 assembles in a 1:1:1:1 stoichiometric complex with AMA1, RON2, and RON4, and a cryo-EM structure of the endogenous complex shows that RON5, together with RON2 and RON4, forms a large, basic platform on the cytoplasmic (intracellular) face of the red blood cell that electrostatically engages the erythrocyte membrane and wedges amphipathic helices into the lipid bilayer, implicating RON5 in active remodeling of the host membrane during invasion (PMID:42379167). RON5 was originally identified as a ~110 kDa merozoite/schizont-stage protein that co-immunoprecipitates with AMA1 alongside RON2 and RON4 (PMID:21185360, PMID:20228060), and its incorporation into the AMA1–RON complex occurs after merozoite reorientation and actomyosin-motor engagement, preceding rhoptry release (PMID:20228060). Beyond its intracellular platform role, RON5-derived peptides bind human erythrocytes in a protease-sensitive, saturable manner and inhibit merozoite invasion, indicating a direct RON5–erythrocyte contact that contributes to invasion (PMID:23932940). The protein's role extends across invasive stages and species: in P. berghei sporozoites RON5 forms a RON2/RON4/RON5 complex, is trafficked to rhoptries independently of RON2 and RON4, and is required for sporozoite attachment to and invasion of mosquito salivary glands (PMID:32817376), and it relocates between the rhoptry neck (merozoites) and rhoptry body (sporozoites) in a stage-specific manner (PMID:31552198). Orthologs in P. vivax and P. yoelii show conserved rhoptry-neck localization, proteolytic processing, and host-cell (reticulocyte) binding preference (PMID:25888962, PMID:25102354).

1. 2010 Medium

   Establishing whether RON5 is part of the invasion machinery, this work identified RON5 as a stage-specific merozoite/schizont protein physically associated with the AMA1–RON moving junction complex.

   Evidence Anti-AMA1 co-immunoprecipitation, Western blot, and stage-specific expression analysis in P. falciparum FCB-2

   PMID:20228060 PMID:21185360

   Open questions at the time

   • Single Co-IP does not define whether RON5 binds AMA1 directly or via other RON subunits
   • No structural or stoichiometric information
   • Functional necessity of RON5 not tested
2. 2010 Medium

   To place complex assembly within the invasion sequence, R1-peptide competition showed AMA1–RON complex formation (and thus RON5's participation) occurs after reorientation and motor engagement but before rhoptry release.

   Evidence R1 inhibitory peptide competition and staged invasion-inhibition assays in P. falciparum

   PMID:20228060

   Open questions at the time

   • RON5's contribution was inferred as a complex member, not tested independently
   • Does not resolve RON5's specific molecular action at the junction
3. 2013 Medium

   Addressing whether RON5 directly engages the host cell, RON5-derived peptides were shown to bind erythrocytes in a saturable, protease-sensitive manner and block invasion, implicating a direct RON5–erythrocyte contact.

   Evidence Synthetic peptide erythrocyte-binding and invasion-inhibition assays in P. falciparum

   PMID:23932940

   Open questions at the time

   • Peptide-based approach may not reflect native full-length protein behavior
   • Erythrocyte receptor not identified
   • Not confirmed with recombinant full-length RON5 or genetic depletion
4. 2014 Medium

   Testing conservation and mode of complex association, PyRON5 was shown to localize to rhoptries, partition into membrane fractions despite lacking a predicted transmembrane domain, and associate with the MJ complex indirectly through other components.

   Evidence IFA, immuno-EM, Co-IP, and membrane fractionation in P. yoelii

   PMID:25102354

   Open questions at the time

   • Mechanism of membrane partitioning without a TM domain unresolved
   • Which subunit directly recruits RON5 not defined
   • No invasion phenotype from genetic depletion
5. 2015 Medium

   Extending to P. vivax, PvRON5 was shown to localize to the rhoptry neck with RON2/RON4, undergo proteolytic processing, and preferentially bind reticulocytes, linking RON5 to host-cell tropism.

   Evidence IFA, Western blot, and recombinant reticulocyte-binding assay in P. vivax VCG-1

   PMID:25888962

   Open questions at the time

   • Functional consequence of proteolytic processing unknown
   • Reticulocyte receptor unidentified
   • Binding preference not linked to invasion outcome genetically
6. 2019 Medium

   To explain stage-specific invasion behavior, immuno-EM revealed that RON5 occupies the rhoptry neck in merozoites but the rhoptry body in sporozoites, showing the same protein redistributes across invasive stages.

   Evidence Immuno-electron microscopy and rhoptry-gene transcriptional analysis in P. berghei

   PMID:31552198

   Open questions at the time

   • Functional significance of subcompartment switch unresolved
   • Trafficking signals directing the switch not defined
7. 2020 High

   Providing direct genetic evidence of function, sporozoite-stage knockdown of RON5 impaired attachment to and invasion of mosquito salivary glands, and RON5 was shown to traffic to rhoptries independently of RON2/RON4.

   Evidence Reciprocal Co-IP from sporozoite extracts plus stage-specific knockdown with attachment/invasion assays in P. berghei

   PMID:32817376

   Open questions at the time

   • Molecular basis of the attachment defect not defined
   • Trafficking signal enabling RON2/RON4-independent transport unknown
   • Blood-stage genetic requirement for RON5 not tested here
8. 2026 High

   To define RON5's molecular action at the junction, cryo-EM of the endogenous P. falciparum complex resolved a 1:1:1:1 AMA1–RON2–RON4–RON5 assembly in which RON5, RON2, and RON4 build a basic platform that electrostatically engages and wedges amphipathic helices into the RBC membrane from the cytoplasmic face.

   Evidence Cryo-EM of endogenous complex purified from invasion-stalled P. falciparum parasites

   PMID:42379167

   Open questions at the time

   • Which specific residues/helices of RON5 (versus RON2/RON4) insert into the membrane not individually dissected
   • Dynamics of membrane remodeling during active invasion not captured
   • Relationship between the intracellular platform and the surface-exposed RON5 peptide contacts not fully reconciled

• How RON5 crosses the erythrocyte membrane to establish an intracellular basic platform while also presenting surface-exposed peptides, and what specific molecular contribution RON5 makes distinct from RON2/RON4, remains unresolved.
• No host receptor for RON5 identified
• RON5-specific contribution to membrane remodeling not isolated from other subunits
• Mechanism of RON5 insertion/translocation across the RBC membrane unknown