RH1

RH1 (PF3D7_0402300) is a P. falciparum merozoite apical protein that directs the sialic acid-dependent, trypsin-resistant pathway of erythrocyte invasion by binding a distinct erythrocyte surface receptor (PMID:11733572, PMID:15612925). It is expressed in late schizonts and merozoites and localizes to apical organelles and the apical surface, with the erythrocyte-binding activity residing in an N-terminal region whose region-specific antibodies inhibit invasion (PMID:12228308, PMID:18617995). Beyond receptor engagement, RH1 acts upstream as a sensor and signal transducer: antibody blockade of RH1 inhibits parasite calcium signalling, preventing EBA175 discharge and tight-junction formation (PMID:24280897). During invasion RH1 is proteolytically processed, releasing ectodomain fragments in a manner consistent with transmembrane-domain cleavage by rhomboid proteases (PfROM4 and PfROM1 can cleave RH1 in vitro), and a subset of processing products associates closely with AMA1 at the moving junction (PMID:17040128, PMID:19614665, PMID:32452132). Its cytoplasmic tail domain independently promotes actin polymerization in vitro, linking RH1 to the invasion motor (PMID:25261592). RH1 is also a target of naturally acquired invasion-inhibitory antibodies, and variation in its expression contributes to immune evasion (PMID:40201183).

1. 2001 High

   Established that RH1 engages a previously unrecognized erythrocyte receptor, defining a distinct sialic acid-dependent, trypsin-resistant entry route.

   Evidence Erythrocyte binding assays, antibody invasion inhibition, and analysis of truncated-RH1 strains on enzyme-treated cells

   PMID:11733572

   Open questions at the time

   • The molecular identity of the erythrocyte receptor was not defined
   • No structural characterization of the binding interaction
2. 2002 Medium

   Placed RH1 protein temporally and spatially in the merozoite, showing apical organelle localization with strain-dependent trafficking differences.

   Evidence Immunofluorescence and expression analysis of late schizonts/merozoites across 3D7, T996, and FCB1 lines

   PMID:12228308

   Open questions at the time

   • Functional consequence of mistargeting in 3D7/T996 not established
   • Single lab, no complementation of trafficking defect
3. 2005 High

   Demonstrated genetically that RH1 is required for sialic acid-dependent invasion and dictates receptor-usage pathway selection.

   Evidence Targeted gene disruption and invasion assays with neuraminidase-treated erythrocytes

   PMID:15612925

   Open questions at the time

   • Does not identify the cognate receptor
   • Redundancy with sibling family members not fully resolved
4. 2006 Medium

   Identified candidate proteases for RH1 shedding, showing PfROM4 and PfROM1 cleave RH1 in a transmembrane-domain-dependent manner.

   Evidence Heterologous rhomboid protease cleavage assay with transmembrane-domain swaps

   PMID:17040128

   Open questions at the time

   • Cleavage shown in a heterologous system, not in the parasite
   • In vivo protease responsible for RH1 shedding not confirmed
5. 2008 Medium

   Mapped the erythrocyte-binding activity to the N-terminal region and validated it as a functional, antibody-accessible invasion target.

   Evidence Erythrocyte binding assays with RH1 fragments and invasion inhibition with region-specific antibodies

   PMID:18617995

   Open questions at the time

   • No high-resolution structure of the binding domain
   • Receptor contact residues not defined
6. 2009 Medium

   Showed RH1 is proteolytically processed with ectodomain release before/during invasion and is a component of the tight junction linking erythrocyte to the actin-myosin motor.

   Evidence Immunoblot processing analysis, supernatant release assays, functional invasion assays

   PMID:19614665

   Open questions at the time

   • Cleavage sites not defined by mutagenesis
   • Direct protease identity in the parasite not established
7. 2013 High

   Positioned RH1 upstream in the invasion signalling cascade as a sensor that triggers calcium signalling to drive EBA175 discharge and junction formation.

   Evidence Anti-RH1 monoclonal antibody invasion inhibition, calcium signalling measurements, EBA175 discharge assays

   PMID:24280897

   Open questions at the time

   • The intracellular transduction pathway connecting RH1 engagement to calcium release is undefined
   • How antibody binding blocks signalling mechanistically is unresolved
8. 2013 Low

   Reported that RH1 binds heparin, raising the possibility that heparin masks the apical surface to block RH1-erythrocyte interaction.

   Evidence Heparin affinity chromatography fractionation and ligand-specific immunoblot

   PMID:24212193

   Open questions at the time

   • Single pulldown method with no functional mutagenesis of the binding site
   • Physiological relevance of heparin binding not established
   • Shared across the family, not RH1-specific
9. 2014 Medium

   Revealed a motor-linked function for the RH1 cytoplasmic tail, which promotes actin polymerization independently of aldolase.

   Evidence Biolayer interferometry, co-sedimentation, and in vitro actin polymerization assays with recombinant RH1 CTD

   PMID:25261592

   Open questions at the time

   • Activity shown in vitro only, not in the parasite
   • Direct binding partners of the CTD not identified
10. 2020 Medium

    Linked RH1 processing products to the moving junction by demonstrating a close spatial association with AMA1 during active invasion.

    Evidence Region-specific monoclonal antibodies with in-situ Proximity Ligation Assay, FRET, and immunofluorescence

    PMID:32452132

    Open questions at the time

    • Whether the RH1-AMA1 proximity reflects a direct interaction is not established
    • Functional requirement of the association for junction formation not tested
11. 2025 Medium

    Established RH1 as a target of naturally acquired invasion-inhibitory antibodies and a contributor to immune evasion via expression variation.

    Evidence pfrh1 gene disruption and invasion inhibition assays with human immune sera from Kenyan children and adults

    PMID:40201183

    Open questions at the time

    • Specific epitopes recognized by natural antibodies not mapped
    • Single study, contribution relative to other invasion antigens not quantified

• The molecular identity of the RH1 erythrocyte receptor and the intracellular machinery linking RH1 engagement to calcium signalling remain unknown.
• Cognate erythrocyte receptor unidentified
• Signal transduction components downstream of RH1 undefined
• No parasite-confirmed protease or cleavage-site mapping