RON6

RON6 (PF3D7_0214900) is an essential rhoptry neck protein of Plasmodium falciparum that functions in host-cell invasion by the merozoite (PMID:19073187, PMID:37552754). It localizes to the rhoptry neck of merozoites and is discharged onto the newly formed parasitophorous vacuole membrane during red blood cell invasion (PMID:19073187). The protein carries a novel cysteine-rich domain conserved across Apicomplexa, and this domain is functionally required, as it cannot be truncated without loss of function (PMID:19073187). RON6 is refractory to gene disruption, and both conditional ER sequestration (knockER) and conditional genetic deletion independently confirm that it is essential for blood-stage invasion (PMID:19073187, PMID:37552754, PMID:38755636). RON6 physically associates with the sporozoite/merozoite protein Pf34, placing it within an invasion complex (PMID:38755636). Its role is broadened by its P. berghei ortholog PbRON6, which localizes to the sporozoite membrane with its C-terminal domain oriented extracellularly and is required for merozoite invasion of RBCs, sporozoite invasion of hepatocytes, and liver-stage development, with loss reducing infectivity and virulence in mice (PMID:40996030). Beyond these findings, the biochemical mechanism by which RON6 promotes invasion has not been characterized in the available corpus.

1. 2008 Medium

   Establishing where RON6 acts and whether it is essential was the first step in placing it within the invasion machinery.

   Evidence Immunofluorescence and electron microscopy localization plus attempted gene disruption in P. falciparum merozoites

   PMID:19073187

   Open questions at the time

   • The molecular function of RON6 at the PVM was not defined
   • Essentiality was inferred from disruption refractoriness rather than a conditional system
2. 2008 Low

   Identifying a conserved cysteine-rich domain established a structural feature required for RON6 function and linked it to the broader Apicomplexan invasion apparatus.

   Evidence Bioinformatic domain analysis combined with truncation showing loss of function in P. falciparum

   PMID:19073187

   Open questions at the time

   • Domain conservation was not biochemically reconstituted
   • The molecular activity of the cysteine-rich domain is unknown
3. 2023 Medium

   A conditional loss-of-function system was needed to move beyond disruption refractoriness and directly demonstrate that RON6 is required for blood-stage invasion.

   Evidence DiCre-mediated conditional ER-retrieval (knockER) sequestration in P. falciparum

   PMID:37552754

   Open questions at the time

   • Does not identify RON6 binding partners or its step in invasion
   • Phenotype reflects mislocalization rather than complete protein depletion
4. 2024 Medium

   Whether RON6 acts alone or within a complex was addressed by testing direct protein-protein interactions, identifying Pf34 as a partner.

   Evidence Plate-based recombinant ectodomain interaction screen plus conditional genetic deletion in P. falciparum

   PMID:38755636

   Open questions at the time

   • The RON6-Pf34 interaction was shown for recombinant ectodomains, not confirmed in native complexes
   • Functional consequence of the interaction for invasion was not tested
5. 2025 High

   The full breadth of RON6 function across life-cycle stages and its membrane topology were resolved using the P. berghei ortholog, extending its role beyond blood-stage invasion to sporozoite invasion and liver-stage development.

   Evidence GFP/epitope-tag localization, genetic knockout, mouse infection, and liver-stage culture in P. berghei

   PMID:40996030

   Open questions at the time

   • Findings are in P. berghei; the multi-stage role has not been demonstrated directly for PF3D7_0214900
   • The molecular mechanism by which RON6 enables invasion at each stage remains undefined
   • Whether the Pf34 interaction operates in the sporozoite/liver stage is unknown

• The biochemical mechanism by which RON6 and its cysteine-rich domain promote host-cell invasion, and the composition and function of its native invasion complex, remain unresolved.
• No molecular activity assigned to the cysteine-rich domain
• No native complex composition or structural model
• No mechanistic step in the invasion pathway defined for RON6