RH2a

RH2a (PF3D7_1335400) is a >300 kDa apical merozoite protein of the reticulocyte binding-like family that participates in erythrocyte invasion by Plasmodium falciparum, contributing to the selection of alternative invasion pathways (PMID:12606570, PMID:30496294). It is expressed in late schizonts and merozoites and localizes to apical organelles and the apical merozoite surface, with antibodies against it inhibiting invasion (PMID:11160005, PMID:12228308). Unlike its paralog PfRh2b, disruption of RH2a does not by itself specify the sialic acid-independent, trypsin-resistant, chymotrypsin-sensitive invasion pathway, but both paralogs contribute to efficient sialic acid-independent invasion, and their variant expression at the PfRh2a/PfRh2b locus is the single major genetic determinant of invasion-pathway phenotype (PMID:12606570, PMID:19400777, PMID:30496294). The two paralogs share a conserved N-terminal ectodomain — mapped to a 15–40 kDa region — that mediates erythrocyte binding, while their functional divergence is conferred by their divergent cytoplasmic domains rather than the ectodomain or transmembrane region (PMID:20487292, PMID:21386888, PMID:21698217). RH2a is proteolytically processed both in the schizont and during invasion, including transmembrane cleavage consistent with the rhomboid protease PfROM4, yielding fragments with distinct erythrocyte binding specificities, and it translocates from the rhoptry neck to the moving junction during invasion independently of the pathway used (PMID:17040128, PMID:21698217, PMID:21628513). The RH2a cytoplasmic tail does not bind aldolase — in contrast to RH2b — explaining why RH2a is not directly coupled to the actin-myosin motor and is functionally distinct from RH2b (PMID:22991428). Relative to PfRH1 and PfRh2b, RH2a is a minor target of acquired human growth-inhibitory antibodies (PMID:40201183).

1. 2001 Medium

   Establishing where and when RH2a acts was the first requirement; its identification as a large apical schizont/merozoite protein whose antibodies block invasion placed it at the invasion interface.

   Evidence Protein expression analysis, immunofluorescence, erythrocyte binding and antibody invasion-inhibition assays in P. falciparum

   PMID:11160005

   Open questions at the time

   • No direct erythrocyte binding detected in this assay
   • Receptor and molecular function undefined
2. 2002 Medium

   Whether RH2a expression is uniform across parasites was addressed by showing variable transcription/translation across lines, establishing RH2a as subject to phenotypic variation at the protein level.

   Evidence Western blot, immunofluorescence and RT-PCR across multiple parasite lines (3D7, T996, FCB1) in P. falciparum

   PMID:12228308

   Open questions at the time

   • Mechanism controlling variant expression not defined
   • Consequence of expression variation for invasion untested here
3. 2003 High

   The question of whether RH2a and RH2b are functionally interchangeable was answered by disruption showing RH2a, unlike RH2b, does not mediate the sialic acid-independent invasion pathway, framing invasion-pathway variation as receptor exploitation.

   Evidence Targeted gene disruption with enzyme-treated RBC invasion assays in P. falciparum

   PMID:12606570

   Open questions at the time

   • Positive function of RH2a itself not defined
   • Molecular basis of paralog divergence unknown
4. 2006 Medium

   How RH2a is released from the merozoite surface was probed by identifying it as a substrate of the rhomboid protease PfROM4, establishing a candidate mechanism for transmembrane processing during invasion.

   Evidence Heterologous rhomboid cleavage assay with transmembrane-domain swaps in P. falciparum

   PMID:17040128

   Open questions at the time

   • Cleavage shown in heterologous system, not in native parasite
   • Functional consequence of cleavage untested here
5. 2009 High

   Reciprocal disruptions clarified that RH2a is not merely redundant, showing both paralogs contribute to efficient sialic acid-independent invasion and that selection favors RH2b over RH2a expression.

   Evidence Individual PfRh2a and PfRh2b knockouts with sialic acid-depleted erythrocyte invasion assays in W2mef P. falciparum

   PMID:19400777

   Open questions at the time

   • Quantitative contribution of RH2a alone not fully separable
   • Basis of selective pressure toward RH2b unexplained
6. 2010 High

   The domain responsible for functional divergence between the paralogs was mapped by chimeras to the cytoplasmic domain, localizing pathway specification to intracellular signaling rather than receptor binding.

   Evidence PfRh2a/2b domain-swap chimeric parasites with invasion pathway assays in P. falciparum

   PMID:20487292

   Open questions at the time

   • Cytoplasmic-domain binding partners not identified in this study
   • How the tail transduces pathway choice unknown
7. 2010 Medium

   Whether RH2 acts alone was tested by showing EBA-181 loss abolishes RH2a/b antibody inhibition, establishing functional cooperation between PfRh and EBL protein families.

   Evidence eba gene knockouts combined with antibody growth-inhibition assays in P. falciparum

   PMID:21149582

   Open questions at the time

   • Direct physical interaction not demonstrated
   • RH2a-specific vs RH2b-specific cooperation not separated
8. 2011 Medium

   Multiple studies defined how processing generates functionally distinct fragments and mapped the erythrocyte-binding activity to a conserved N-terminal ectodomain (15–40 kDa) shared with RH2b.

   Evidence Native protein processing western blots, enzyme-treated RBC binding assays, recombinant domain mapping and antibody inhibition in P. falciparum

   PMID:21386888 PMID:21698217

   Open questions at the time

   • Erythrocyte receptor for the binding domain not identified
   • Assays largely single-lab
9. 2011 Medium

   Tracking RH2a during live invasion revealed pathway-independent translocation from rhoptry neck to moving junction, implying a role beyond receptor binding.

   Evidence Live and fixed immunofluorescence with anti-PfRH2a monoclonal antibodies and binding assays of processed fragments in P. falciparum

   PMID:21628513

   Open questions at the time

   • The moving-junction function of RH2a is not mechanistically defined
   • Interactions at the junction not identified
10. 2012 High

    The mechanistic basis of RH2a/RH2b functional distinction was resolved biochemically: the RH2a cytoplasmic tail does not bind aldolase, unlike RH2b, explaining why RH2a is not coupled to the actin-myosin motor.

    Evidence Peptide binding, surface plasmon resonance and X-ray crystallography of aldolase with the RH2b tail as comparator in P. falciparum

    PMID:22991428

    Open questions at the time

    • Alternative binding partners of the RH2a tail not identified
    • Whether RH2a couples to the motor by another route unknown
11. 2013 Low

    A screen for heparin-binding surface proteins included RH2a among apical merozoite ligands, offering a possible route by which heparin masks invasion ligands.

    Evidence Heparin affinity chromatography, immunoblot and immunofluorescence in P. falciparum

    PMID:24212193

    Open questions at the time

    • Single pulldown without RH2a-specific functional follow-up
    • Physiological relevance of heparin binding for RH2a untested
12. 2018 High

    Genetics established the causal locus: forward linkage and CRISPR-Cas9 editing identified the PfRh2a/PfRh2b locus as the single major determinant of alternative invasion-pathway phenotype.

    Evidence Forward genetic cross linkage analysis and CRISPR-Cas9 editing with invasion phenotyping in P. falciparum

    PMID:30496294

    Open questions at the time

    • Precise contribution of RH2a versus RH2b expression at the locus not fully partitioned
    • Molecular switch controlling expression not defined
13. 2025 Medium

    The relative immunological importance of RH2a was assessed by showing its disruption has a smaller effect on susceptibility to human inhibitory antibodies than PfRH1 or RH2b loss, ranking RH2a as a minor immune-evasion contributor.

    Evidence pfrh2a knockout parasites challenged with human immune sera in invasion inhibition assays in P. falciparum

    PMID:40201183

    Open questions at the time

    • Epitopes and antibody targets on RH2a not mapped
    • Single study

• The intracellular signaling role of the RH2a cytoplasmic tail and its binding partners at the moving junction remain unresolved, leaving open how RH2a functions during invasion beyond ectodomain receptor binding.
• No RH2a cytoplasmic-tail interactor identified
• Erythrocyte receptor for the RH2a ectodomain unknown
• Function at the moving junction undefined