RAP2

PF3D7_0501600 (RAP2) is a rhoptry-resident protein of Plasmodium falciparum that participates in erythrocyte recognition and invasion by the merozoite (PMID:29352039, PMID:17624849). It assembles with RAP1 and RAP3 into the low-molecular-weight rhoptry protein complex, forming as early as or concurrent with rhoptry transport, and its correct targeting to the rhoptries is strictly dependent on RAP1: disruption of RAP1 abolishes the complex and mislocalizes RAP2 to an ER-like compartment, yet does not block efficient invasion, indicating the complex per se is not essential for invasion (PMID:10835342, PMID:9574915). During invasion RAP2 engages the host cell surface—synthetic RAP2 peptides bind erythrocyte receptors of 42, 62, and 77 kDa and inhibit invasion (PMID:14987794), and RAP2 tags uninfected erythrocytes in a RAP1-dependent manner (PMID:17624849). RAP2 interacts directly with the host erythrocyte protein CD147 at the parasitophorous-vacuole-formation stage, an interaction essential for invasion whose blockade abolishes parasitism in vitro and in a humanized mouse model (PMID:29352039). Beyond invasion, RAP2 discharged onto non-parasitized erythrocytes during abortive invasion marks those cells for complement- and monocyte-mediated destruction, implicating it in malarial anemia (PMID:19695213). In rodent malaria, the RAP2/3 ortholog localizes to rhoptries across invasive stages, is secreted into the parasitophorous vacuole during hepatocyte invasion in a host-CD81-dependent manner in P. yoelii (PMID:19428669, PMID:24798694), and contributes to virulence in P. berghei, where rap2/3 deletion attenuates infection and complementation partially restores virulence (PMID:29307564). RAP2 carries two defined disulfide bridges (Cys24–Cys88, Cys277–Cys376) that shape epitope accessibility (PMID:8946383).

1. 1996 Medium

   Defining the native disulfide architecture of RAP2 established its folded conformation and showed that a B-cell epitope is conformationally buried, framing how the native protein is presented to the immune system.

   Evidence Disulfide bond analysis of native protein with antibody epitope mapping using synthetic peptides

   PMID:8946383

   Open questions at the time

   • No full structural model of RAP2
   • Functional consequence of the disulfide bonds for invasion untested
2. 1998 Medium

   Showing that RAP2 (p39) co-fractionates with RAP1 processing products and RAP3 (p37), and that this persists under brefeldin A block, established that the low-molecular-weight rhoptry complex assembles at or before rhoptry transport.

   Evidence Radiolabeling with brefeldin A treatment, size fractionation, and immunoblotting in P. falciparum

   PMID:9574915

   Open questions at the time

   • Stoichiometry of the complex not defined
   • Direct binding interfaces between RAP2 and partners not mapped
3. 2000 High

   Whether RAP2 rhoptry targeting is autonomous or partner-dependent was resolved by RAP1 disruption, which mislocalized RAP2 to an ER-like compartment and abolished complex formation—establishing RAP1 as the trafficking chaperone and showing the complex is dispensable for efficient invasion.

   Evidence RAP1 gene disruption with immunoprecipitation and immunofluorescence/fractionation plus invasion assay in P. falciparum

   PMID:10835342

   Open questions at the time

   • Molecular determinants on RAP2 required for RAP1-dependent trafficking unknown
   • Why complex is dispensable for invasion despite RAP2 invasion roles not reconciled
4. 2002 Medium

   The functional relationship between RAP2 and its paralog RAP3 was addressed by RAP3 disruption, which left invasion intact, indicating RAP2 can compensate and that the RAP2/RAP3 duplication postdates Plasmodium species divergence.

   Evidence RAP3 gene disruption with invasion assay, phylogenetic and genomic analysis in P. falciparum

   PMID:12183575

   Open questions at the time

   • Direct test of RAP2 loss (rather than inference) not performed
   • Non-redundant functions of RAP2 vs RAP3 not delineated
5. 2004 Medium

   The question of whether RAP2 itself contacts the host cell was addressed by synthetic peptide binding, identifying candidate erythrocyte receptors (42, 62, 77 kDa) and showing peptide-mediated invasion inhibition—placing RAP2 in host-cell recognition.

   Evidence Synthetic peptide RBC binding, competition binding, SDS-PAGE receptor identification and invasion inhibition in P. falciparum

   PMID:14987794

   Open questions at the time

   • Molecular identity of the 42/62/77 kDa receptors not determined
   • Peptide binding not validated against native full-length RAP2
6. 2007 Medium

   Which complex member actually tags erythrocytes was resolved using RAP1- and RAP3-knockout parasites and surface iodination, identifying RAP2 (not RAP3) as the tagging component and confirming its RAP1-dependent surface presentation.

   Evidence RAP1- and RAP3-knockout parasites, surface iodination, and erythrocyte binding assay in P. falciparum

   PMID:17624849

   Open questions at the time

   • Receptor engaged during tagging not identified in this study
   • Single-lab result
7. 2009 Medium

   The consequence of RAP2 deposition on bystander erythrocytes was addressed by showing discharged RAP2 marks non-parasitized cells for complement- and monocyte-mediated clearance, linking RAP2 to malarial anemia.

   Evidence Immunofluorescence on non-infected erythrocytes plus complement lysis and monocyte phagocytosis assays in P. falciparum

   PMID:19695213

   Open questions at the time

   • In vivo contribution to anemia not quantified
   • Mechanism of RAP2 retention on erythrocyte surface unclear
8. 2009 Medium

   Whether the RAP2/3 protein functions beyond blood-stage merozoites was addressed in P. berghei, where GFP-tagged RAP2/3 localized to rhoptries in both merozoites and sporozoites, showing expression across invasive stages.

   Evidence GFP-fusion tagging and live/immunofluorescence imaging across life-cycle stages in P. berghei

   PMID:19428669

   Open questions at the time

   • Functional role in sporozoites not tested here
   • Ortholog is fused RAP2/3, not separable RAP2
9. 2014 Medium

   The behavior of RAP2/3 during productive invasion and its host-factor dependence was defined in P. yoelii, where the tagged protein is secreted into the parasitophorous vacuole during hepatocyte invasion in a CD81-dependent manner.

   Evidence Transgenic fluorescent RAP2/3 with live imaging during invasion of CD81-deficient host cells in P. yoelii

   PMID:24798694

   Open questions at the time

   • Direct RAP2/3–CD81 interaction not shown
   • Whether P. falciparum RAP2 behaves identically in hepatocyte invasion untested
10. 2018 High

    The direct host receptor for P. falciparum RAP2 was identified as CD147, an interaction essential for invasion at the parasitophorous-vacuole-formation stage and independent of prior invasion interactions, with antibody blockade abolishing invasion in vitro and in humanized mice.

    Evidence RAP2–CD147 binding assays, humanized anti-CD147 antibody blockade, in vitro invasion inhibition, and humanized mouse model in P. falciparum

    PMID:29352039

    Open questions at the time

    • Structural basis of the RAP2–CD147 interaction not resolved
    • Relationship of CD147 to the earlier 42/62/77 kDa receptors unclear
11. 2018 Medium

    The in vivo importance of RAP2/3 for pathogenesis was tested in P. berghei, where rap2/3 deletion attenuated infection and complementation of an attenuated natural-deletion strain partially restored virulence, establishing a virulence contribution.

    Evidence RAP2/3-deficient P. berghei, complementation of Pb XAT strain, and mouse infection outcome assays

    PMID:29307564

    Open questions at the time

    • Molecular mechanism linking RAP2/3 to virulence not defined
    • Fused RAP2/3 ortholog precludes assigning phenotype to RAP2 alone
12. 2020 Low

    Broader complex associations of RAP2 were probed by anti-PfClag9 pulldown, placing RAP2 in a PfClag9-associated rhoptry complex also containing RAP1, RAP3, RhopH2, and RhopH3.

    Evidence Anti-PfClag9 immunoprecipitation of merozoite lysate with mass spectrometry in P. falciparum

    PMID:31712270

    Open questions at the time

    • Single Co-IP/MS without reciprocal validation specific to RAP2
    • Direct RAP2–PfClag9 contact not demonstrated
13. 2021 Low

    Complex membership was reconfirmed by an independent proteomic approach, identifying RAP2 with RAP1 in the low-molecular-mass rhoptry complex.

    Evidence Monoclonal antibody immunoaffinity purification and LC-MS/MS from schizont-rich P. falciparum

    PMID:33537242

    Open questions at the time

    • No new mechanistic detail beyond confirming known membership
    • Single-lab confirmation

• How RAP2 engages CD147 at the structural level, whether the previously described erythrocyte receptors correspond to CD147, and the direct role of RAP2 in vacuole formation across species remain unresolved.
• No structure of RAP2 or the RAP2–CD147 interface
• Mechanistic link between rhoptry complex assembly and RAP2 host-receptor function unclear
• Separable P. falciparum RAP2 loss-of-function not directly tested in vivo