RH2b

RH2b (PF3D7_1335300) is a Plasmodium falciparum merozoite ligand that mediates a distinct erythrocyte invasion pathway and functions independently of its related PfRh proteins (PMID:12606570). The protein is expressed in late schizonts and merozoites and localizes to apical organelles and the apical merozoite surface, and antibodies against it inhibit invasion (PMID:11160005, PMID:12228308). RH2b expression varies across parasite strains, and this phenotypic variation lets the parasite exploit different erythrocyte receptor patterns (PMID:12228308). Its erythrocyte-binding activity resides in a conserved 15–40 kDa N-terminal ectodomain region shared with the paralog PfRh2a, which binds erythrocytes in a sialic acid-independent, trypsin-resistant, chymotrypsin-sensitive manner; antibodies to this domain block sialic acid-independent invasion (PMID:21386888, PMID:21698217). RH2b undergoes a series of proteolytic cleavage events, including cleavage within its transmembrane domain by the rhomboid protease PfROM4 to release the ectodomain, generating fragments (a 220 kDa and a derived 80 kDa fragment) with differential sialic acid-dependent and -independent binding specificities (PMID:17040128, PMID:21386888, PMID:21698217). Functional diversification from PfRh2a—despite ~2700 shared N-terminal residues—is determined by the RH2b cytoplasmic domain rather than the ectodomain or transmembrane region (PMID:20487292). RH2b acts as a dominant invasion ligand: its disruption unmasks a pre-existing, hierarchically ordered alternative invasion pathway rather than triggering a molecular switch (PMID:16362075, PMID:19400777), and its variant expression additionally serves as a significant target of acquired human invasion-inhibitory antibodies, implicating it in immune evasion (PMID:40201183).

1. 2001 Medium

   Establishing where and when RH2b acts was the first step, defining it as an apical merozoite protein relevant to the invasion event rather than another life stage.

   Evidence Immunofluorescence localization, antibody invasion-inhibition, and RBC binding assays in P. falciparum

   PMID:11160005

   Open questions at the time

   • No direct erythrocyte binding was detected in the assay used
   • Receptor identity unknown
   • Molecular mechanism of invasion contribution not defined
2. 2002 Medium

   Comparing multiple parasite strains revealed that RH2b expression is variant, transitioning it from a fixed invasion factor to a switchable determinant of receptor usage.

   Evidence Western blot, immunofluorescence, RT-PCR, and sequence analysis across 3D7, T996, and FCB1 strains

   PMID:12228308

   Open questions at the time

   • Regulatory mechanism controlling variant expression not defined
   • Functional consequence of the T996 3' deletion not tested
3. 2003 High

   Genetic disruption directly proved RH2b defines a novel invasion pathway used independently of other PfRh proteins, establishing it as a functional invasion ligand.

   Evidence Targeted gene disruption with invasion pathway assays in P. falciparum

   PMID:12606570

   Open questions at the time

   • Receptor on the erythrocyte not identified
   • Biochemical basis of binding not resolved
4. 2005 High

   Dissecting the disruption phenotype showed the alternative pathway is not a molecular switch but redeployment of pre-existing machinery masked by RH2b, establishing a hierarchy of invasion ligands.

   Evidence Gene disruption with enzyme-treated erythrocyte invasion assays in 3D7 P. falciparum

   PMID:16362075

   Open questions at the time

   • Molecular identity of the redeployed ligands unspecified here
   • Mechanism enforcing the hierarchy unknown
5. 2006 High

   Identifying PfROM4 (and PfROM1) as proteases that cleave RH2b through its transmembrane domain defined how the ligand is processed and released during invasion.

   Evidence Heterologous rhomboid cleavage assay with transmembrane domain-swap mutagenesis

   PMID:17040128

   Open questions at the time

   • Cleavage shown in heterologous system, not directly in native merozoites here
   • Timing relative to invasion not resolved in this assay
6. 2009 Medium

   A natural C-terminal deletion in field isolates linked RH2b structural variation to altered invasion pathway preference, connecting the gene to clinical parasite diversity.

   Evidence Sequence polymorphism analysis with enzyme-treated erythrocyte invasion assays in Senegalese clinical isolates

   PMID:19126266

   Open questions at the time

   • Association is correlative in field isolates
   • Blood-group O-restricted effect on sialic acid-independent invasion not mechanistically explained
7. 2010 High

   Chimeric domain-swap experiments localized functional diversification between RH2b and PfRh2a to the cytoplasmic domain, explaining how two proteins with nearly identical ectodomains achieve distinct roles.

   Evidence Generation of chimeric parasites with invasion pathway readout in P. falciparum

   PMID:20487292

   Open questions at the time

   • Cytoplasmic-domain interacting partners not identified
   • Signaling or trafficking basis of the effect unknown
8. 2010 High

   Disruption in the W2mef background showed RH2b and PfRh2a cooperate in sialic acid-independent invasion and that there is selective pressure to reconstitute RH2b expression, reinforcing its dominant role.

   Evidence Targeted gene disruption with enzyme-treated erythrocyte invasion assays in W2mef P. falciparum

   PMID:19400777

   Open questions at the time

   • Mechanism of cooperation between RH2b and PfRh2a not defined
   • Basis of selective pressure for RH2b reconstitution unknown
9. 2011 High

   Biochemical mapping of processing and binding defined the erythrocyte-binding domain to a conserved N-terminal region (15–40 kDa) and linked distinct processed fragments to sialic acid-dependent versus -independent binding.

   Evidence Native protein fractionation, recombinant and native erythrocyte binding assays, domain mapping, and antibody inhibition in P. falciparum

   PMID:21386888 PMID:21698217

   Open questions at the time

   • Erythrocyte receptor(s) for each fragment not identified
   • Structure of the binding domain not determined
   • Mapping was shared across PfRh2a and PfRh2b, not RH2b-exclusive
10. 2013 Medium

    Identifying RH2b as a heparin-binding protein offered a biochemical basis for how sulfated glycans could block invasion by masking the ligand at the apical surface.

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

    PMID:24212193

    Open questions at the time

    • Physiological relevance of heparin binding to native invasion unproven
    • Heparin-binding site not mapped
11. 2025 Medium

    Disruption combined with human immune sera identified RH2b as a significant target of naturally acquired invasion-inhibitory antibodies, tying its variant expression to immune evasion.

    Evidence Gene disruption with invasion-inhibition assays using sera from malaria-exposed donors

    PMID:40201183

    Open questions at the time

    • Specific epitopes recognized not defined
    • Contribution relative to other invasion ligands not quantified
12. 2025 Low

    An in vitro interaction between EBA-181 and RH2b raised the possibility that RH2b acts as a co-ligand cooperating with other invasion ligands.

    Evidence Surface Plasmon Resonance binding assay (K_D ~3 µM)

    PMID:41198968

    Open questions at the time

    • Single SPR measurement without functional validation in parasites
    • Interaction not confirmed by an orthogonal method
    • Biological role of the co-ligand interaction not established

• The erythrocyte receptor(s) bound by the RH2b ectodomain and the structural/signaling basis by which its cytoplasmic domain directs a dominant invasion pathway remain unresolved.
• No erythrocyte receptor identified for RH2b
• No structural model of the binding domain
• Cytoplasmic-domain interaction partners unknown