RAP2

RAP3 (PF3D7_0501500) is a rhoptry-resident protein of Plasmodium falciparum and the third component of the low-molecular-mass rhoptry complex it forms with RAP1 and RAP2 (PMID:12183575, PMID:10835342). It is the more recently duplicated paralog of RAP2, arising from a head-to-tail gene pair on chromosome 5, a duplication that postdates the divergence of Plasmodium species (PMID:12183575). Within the complex, RAP3 associates noncovalently with RAP1, and immunodepletion indicates that RAP3 and RAP2 bind one RAP1-class molecule in a mutually exclusive manner (PMID:1698657); this association forms early in the secretory pathway before rhoptry transport, and complex assembly requires intact RAP1 (PMID:10835342, PMID:9574915). RAP3 is dispensable for merozoite invasion and for rhoptry-mediated tagging of normal erythrocytes, functions that RAP2 can carry out in its absence (PMID:12183575, PMID:17624849). Conserved-region synthetic peptides of RAP3 directly bind red-blood-cell surface molecules of ~27–90 kDa and inhibit invasion in vitro, at binding sites shared or overlapping with those recognized by RAP1 and RAP2 peptides (PMID:20833215). In rodent malaria parasites, which encode a single RAP2/3 ortholog rather than distinct RAP2 and RAP3 genes, that ortholog localizes to rhoptries in merozoites and sporozoites, is secreted into the parasitophorous vacuole during invasion, and contributes to parasite virulence in vivo (PMID:19428669, PMID:24798694, PMID:29307564). No enzymatic activity or structural model for RAP3 has been characterized in the available corpus.

1. 1990 Medium

   Before its gene was cloned, biochemistry established that the p37 protein (RAP3) is a subunit of a defined rhoptry hetero-oligomer, framing RAP3 as a stoichiometric complex partner rather than a free protein.

   Evidence Sucrose gradient centrifugation, crosslinking, and immunodepletion of merozoite extracts

   PMID:1698657

   Open questions at the time

   • RAP3 identity was inferred from the p37 product before gene cloning
   • Mutual exclusivity with RAP2 inferred from immunodepletion, not direct structure
   • No stoichiometry beyond minimal one-RAP1-plus-one
2. 1998 Medium

   The question of when in trafficking RAP3 joins the complex was addressed by showing association forms early in the secretory pathway before rhoptry delivery.

   Evidence Radiolabeling under brefeldin A block, size fractionation, and immunoprecipitation

   PMID:9574915

   Open questions at the time

   • Does not define the trafficking signal within RAP3
   • Order of subunit assembly not resolved
3. 2000 High

   Genetic dissection defined the assembly hierarchy, showing that intact RAP1 is required to scaffold RAP2 and RAP3 into the complex.

   Evidence Immunoprecipitation in RAP1-disrupted parasites carrying truncated RAP1

   PMID:10835342

   Open questions at the time

   • Does not map the RAP1 region that binds RAP3
   • Does not test whether RAP3 has independent function outside the complex
4. 2002 High

   Targeted knockout answered whether RAP3 is required for invasion, showing it is dispensable and defining its evolutionary origin as a post-speciation duplicate of RAP2.

   Evidence Targeted gene disruption with invasion phenotyping, plus sequence and phylogenetic analysis

   PMID:12183575

   Open questions at the time

   • Redundancy with RAP2 inferred rather than directly shown by double knockout
   • No non-redundant function identified for RAP3
5. 2007 Medium

   A specific functional readout tested whether RAP3 mediates rhoptry tagging of erythrocytes, showing this function is carried by RAP1 and RAP2 and RAP3 is not needed.

   Evidence Surface iodination of RAP3-disrupted mutant parasites

   PMID:17624849

   Open questions at the time

   • Negative result does not exclude a RAP3-specific role masked by RAP2
   • Molecular mechanism of tagging not addressed for RAP3
6. 2010 Medium

   Peptide biochemistry probed whether RAP3 engages the host cell directly, showing conserved RAP3 regions bind RBC surface molecules and inhibit invasion at sites shared with RAP1/RAP2 peptides.

   Evidence Synthetic peptide RBC binding, competition, and in vitro invasion-inhibition assays

   PMID:20833215

   Open questions at the time

   • Peptide binding may not reflect native full-length protein behavior
   • The ~27–90 kDa RBC ligands were not identified
   • Reconciling direct RBC binding with dispensability for invasion is unresolved
7. 2018 Medium

   In rodent malaria, where a single RAP2/3 ortholog exists, knockout-plus-complementation tested its in vivo importance and showed it contributes to virulence.

   Evidence P. berghei rap2/3 knockout and complementation in the XAT strain with in vivo infection outcome

   PMID:29307564

   Open questions at the time

   • Attributable to the single P. berghei RAP2/3 ortholog, not P. falciparum RAP3 specifically
   • Mechanism linking RAP2/3 to virulence not defined
8. 2020 Low

   Interaction proteomics placed RAP3 in a broader rhoptry network beyond the minimal RAP complex, associating it with PfClag9, RhopH2, and RhopH3.

   Evidence Anti-PfClag9 immunoprecipitation followed by LC-MS/MS of merozoite extracts (and independent affinity-purification/MS confirmation of RAP1/RAP2/RAP3 complex in 2021)

   PMID:31712270 PMID:33537242

   Open questions at the time

   • Single Co-IP/MS with no functional follow-up specific to RAP3
   • Direct versus indirect association within the network not resolved

• The non-redundant molecular function of RAP3 within the RAP complex, and the biochemical activity that distinguishes it from its RAP2 paralog, remain undefined.
• No enzymatic or biochemical activity assigned to RAP3
• No structure of RAP3 or the RAP1–RAP3 interface
• Native RBC ligand and the biological consequence of RAP3-specific host binding unknown