PF3D7_0604800

PF3D7_0604800 (RAP-1) is a rhoptry protein that serves as the organizing core of low-molecular-weight rhoptry complexes required for post-invasion intra-erythrocytic development in Plasmodium falciparum (PMID:17553576, PMID:28205319). It is synthesized as an 86-kDa precursor (Pr86) bearing a cleavable N-terminal signal peptide and is transported to the rhoptries through the secretory pathway, as its precursor-to-mature processing is reversibly blocked by brefeldin A and low temperature and irreversibly blocked by N-ethylmaleimide (PMID:8719244). Within the rhoptries Pr86 undergoes sequential proteolytic maturation, first rapidly to p82 and then more slowly to p67, with these mature forms — and not the surface — being the species retained in released merozoites (PMID:2200961, PMID:9574915). RAP-1 nucleates two distinct complexes with RAP-2 and RAP-3 respectively, which partition into detergent-resistant membranes (PMID:9574915, PMID:17553576), and in P. berghei RAP-1 is required to target RAP-2 to the rhoptries (PMID:29307564). RAP-1 can bind the surface of normal erythrocytes (PMID:17624849), yet the RAP complex is dispensable for rhoptry morphology and erythrocyte invasion; instead, conditional knockdown shows it is required after invasion for normal parasitophorous vacuolar membrane structure, efficient intra-erythrocytic growth, and virulence in mice (PMID:28205319, PMID:29307564).

1. 1990 Medium

   Establishing that the multiple RAP-1 antigen bands are one protein resolved the question of whether the immunodominant rhoptry species were distinct antigens or products of a single precursor.

   Evidence Immunoaffinity purification, SDS-PAGE, monoclonal antibody epitope analysis and autocatalytic processing assays in P. falciparum

   PMID:2200961

   Open questions at the time

   • Protease responsible for processing not identified
   • Functional role of the lipophilic/disulfide-bonded structure not defined
2. 1992 Medium

   Mapping inhibitory antibody epitopes near the cleavage site and showing p65 is absent from merozoites tied RAP-1 processing to merozoite release rather than to invasion itself.

   Evidence Fusion-protein and synthetic-peptide epitope mapping with in vitro growth-inhibition assays and purified merozoite immunoblots in P. falciparum

   PMID:1279419

   Open questions at the time

   • Mechanism by which epitope-directed antibodies inhibit growth not established
   • Timing of cleavage relative to release not directly imaged
3. 1995 High

   Pharmacological dissection showed RAP-1 reaches the rhoptries via the classical secretory pathway, placing its maturation within a defined trafficking route.

   Evidence Brefeldin A, low-temperature and N-ethylmaleimide blocks plus in vitro translation and metabolic labeling in P. falciparum

   PMID:8719244

   Open questions at the time

   • Nature of the modification distinguishing the 84-kDa primary product from Pr86 not identified
   • Rhoptry-targeting signal within RAP-1 not mapped
4. 1998 High

   Resolving the ordered Pr86→p82→p67 cleavage and showing complex assembly precedes processing defined the biogenesis sequence of RAP-1 and its partners.

   Evidence Epitope-mapped immunoblotting, BFA-block size fractionation, immunofluorescence and purified merozoite analysis in P. falciparum

   PMID:9574915

   Open questions at the time

   • Identity of the processing protease unknown
   • Functional consequence of the delayed p82→p67 cleavage not defined
5. 2007 Medium

   Biochemical fractionation defined RAP-1 as the shared core of two membrane-associated complexes, refining its role from single antigen to a complex-organizing subunit.

   Evidence Detergent-resistant membrane purification, blue native PAGE and mass spectrometry in P. falciparum schizonts

   PMID:17553576

   Open questions at the time

   • Stoichiometry and composition of the high-molecular-weight species not resolved
   • Functional significance of DRM partitioning not tested
6. 2007 Medium

   Gene disruption showed RAP-1 mediates binding of the parasite to the surface of normal erythrocytes, assigning it a direct erythrocyte-interaction activity.

   Evidence RAP-1 gene-disruption mutants with surface iodination and antibody staining of normal erythrocytes in P. falciparum

   PMID:17624849

   Open questions at the time

   • Erythrocyte receptor for RAP-1 not identified
   • Physiological role of surface binding relative to invasion not defined
7. 2017 High

   Conditional knockdown reassigned the RAP complex from a presumed invasion factor to a post-invasion effector needed for PVM integrity and intra-erythrocytic growth.

   Evidence Conditional knockdown, PVM electron microscopy, growth assays and mouse infection in P. falciparum and P. berghei

   PMID:28205319

   Open questions at the time

   • Molecular mechanism linking RAP complex to PVM structure unknown
   • RAP-1-specific contribution versus other complex members not isolated
8. 2018 Medium

   Knockout and complementation in P. berghei established RAP-1 as required for rhoptry targeting of RAP-2 and as a contributor to virulence.

   Evidence RAP1- and RAP2/3-deficient P. berghei mutants with complementation and mouse infection

   PMID:29307564

   Open questions at the time

   • Mechanism of RAP-2 targeting by RAP-1 not defined
   • Basis of the differing parasitemia/survival phenotypes across mutants unresolved

• The molecular activity by which RAP-1 and its complex shape the parasitophorous vacuolar membrane, and the identity of its processing protease and erythrocyte receptor, remain unknown.
• No enzymatic or structural mechanism for the RAP complex's post-invasion function
• Processing protease unidentified
• Erythrocyte surface receptor unidentified