{"gene":"PF3D7_1335300","run_date":"2026-09-08T14:19:06","timeline":{"discoveries":[{"year":2003,"finding":"Targeted gene disruption of RH2b (PfRh2b) in P. falciparum demonstrated that it mediates a novel erythrocyte invasion pathway and functions independently of other related PfRh proteins. Phenotypic variation in RH2b expression allows the parasite to exploit different patterns of receptors on the erythrocyte surface.","method":"Targeted gene disruption; invasion pathway assays","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 2 / Strong — targeted gene disruption with defined invasion phenotype, replicated across multiple parasite lines","pmids":["12606570"],"is_preprint":false,"attribution":[{"species":"Plasmodium falciparum","strain":""}]},{"year":2001,"finding":"RH2b (encoded by one of the two PfRh2 genes) is expressed in late schizonts and localizes to the apical end of merozoites. Antibodies against these proteins inhibit merozoite invasion of red blood cells. No binding to mature RBCs or reticulocyte-enriched cell preparations was detected in the assay used.","method":"Immunofluorescence localization; antibody inhibition assay; binding assay (negative result for direct RBC binding)","journal":"Infection and immunity","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct localization experiment and functional antibody inhibition assay, single lab","pmids":["11160005"],"is_preprint":false,"attribution":[{"species":"Plasmodium falciparum","strain":""}]},{"year":2002,"finding":"RH2b (PfRH2b) protein is expressed in late schizonts and merozoites and is located in apical organelles and on the apical surface. In the FCB1 parasite line, PfRH2b is not transcribed and the protein is undetectable, demonstrating variant expression across parasite strains. A 585-bp deletion in the 3' end of RH2b was identified in strain T996.","method":"Western blot; immunofluorescence; RT-PCR; sequence analysis","journal":"Infection and immunity","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — direct localization and protein expression analysis across multiple strains, single lab, multiple orthogonal methods","pmids":["12228308"],"is_preprint":false,"attribution":[{"species":"Plasmodium falciparum","strain":"3D7"},{"species":"Plasmodium falciparum","strain":"T996"},{"species":"Plasmodium falciparum","strain":"FCB1"}]},{"year":2005,"finding":"Gene disruption of RH2b (PfRh2b) in 3D7 parasites resulted in invasion via a novel pathway. This novel pathway is not due to a molecular switch but is mediated by redeployment of invasion machinery already present in the parent parasite but masked by the dominant role of PfRh2b, indicating a hierarchy of invasion ligand interactions.","method":"Gene disruption; invasion pathway assays with enzyme-treated erythrocytes","journal":"PLoS pathogens","confidence":"High","confidence_rationale":"Tier 2 / Strong — targeted gene disruption with defined mechanistic interpretation, replicated in 3D7 background","pmids":["16362075"],"is_preprint":false,"attribution":[{"species":"Plasmodium falciparum","strain":"3D7"}]},{"year":2006,"finding":"PfROM4 (PFE0340c) efficiently cleaves RH2b (Rh2b) via its transmembrane domain in a heterologous cleavage assay, while PfROM1 was also able to cleave Rh2b. Cleavage specificity is determined by the adhesin transmembrane domain.","method":"Heterologous rhomboid protease cleavage assay; transmembrane domain swap experiments","journal":"PLoS pathogens","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro enzymatic assay with domain-swap mutagenesis establishing substrate specificity, single lab","pmids":["17040128"],"is_preprint":false,"attribution":[{"species":"Plasmodium falciparum","strain":""}]},{"year":2010,"finding":"Targeted gene disruption of RH2b (PfRh2b) in W2mef parasites showed that PfRh2b is functional in this parasite background. Both PfRh2b-null and PfRh2a-null lines could switch to a sialic acid-independent invasion pathway, but exhibited reduced efficiency for invasion into sialic acid-depleted cells, indicating cooperative roles of PfRh2b and PfRh2a in sialic acid-independent invasion. A strong selective pressure for reconstitution of PfRh2b expression at the expense of PfRh2a was observed.","method":"Targeted gene disruption; invasion assays with enzyme-treated erythrocytes","journal":"Molecular microbiology","confidence":"High","confidence_rationale":"Tier 2 / Moderate — targeted gene disruption with quantitative invasion phenotype, multiple mutant lines tested, single lab","pmids":["19400777"],"is_preprint":false,"attribution":[{"species":"Plasmodium falciparum","strain":"W2mef"}]},{"year":2010,"finding":"The functional diversification between RH2b (PfRh2b) and its paralog PfRh2a — despite sharing 2700 N-terminal amino acids — is determined by their cytoplasmic domains, not the ectodomain or transmembrane regions. Chimeric parasite experiments showed the cytoplasmic domain of PfRh2b confers the ability to use a specific invasion pathway.","method":"Generation of chimeric parasites; invasion pathway assays","journal":"Molecular microbiology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — chimeric protein domain-swap in live parasites with defined invasion phenotype readout, single lab","pmids":["20487292"],"is_preprint":false,"attribution":[{"species":"Plasmodium falciparum","strain":""}]},{"year":2011,"finding":"Native RH2b (PfRH2a/2b) undergoes proteolytic processing near the N-terminus yielding a 220 kDa and an 80 kDa fragment with differential erythrocyte binding specificities. The 220 kDa fragment binds erythrocytes in a sialic acid-independent, trypsin-resistant, chymotrypsin-sensitive manner. The 80 kDa fragment (processed from the 220 kDa fragment) binds erythrocytes in a sialic acid-dependent, trypsin-resistant, chymotrypsin-sensitive manner. The erythrocyte-binding domain was mapped to a conserved 40 kDa N-terminal region common to PfRH2a and PfRH2b. Antibodies against this 40 kDa domain blocked invasion via a sialic acid-independent pathway.","method":"Native protein fractionation; erythrocyte binding assay; recombinant protein binding assay; antibody inhibition assay","journal":"PloS one","confidence":"High","confidence_rationale":"Tier 1 / Moderate — biochemical domain mapping, native protein characterization, and functional antibody inhibition, multiple orthogonal methods, single lab","pmids":["21386888"],"is_preprint":false,"attribution":[{"species":"Plasmodium falciparum","strain":""}]},{"year":2011,"finding":"RH2b (PfRh2a and PfRh2b) undergo a complex series of proteolytic cleavage events before and during merozoite invasion, including a cleavage event in the transmembrane region of PfRh2a during invasion consistent with PfROM4 activity, resulting in ectodomain release. The erythrocyte-binding domain of PfRh2a and PfRh2b was defined to a 15 kDa region at the N-terminus of each protein. Antibodies to this receptor-binding region block merozoite invasion.","method":"Biochemical analysis of proteolytic processing; erythrocyte binding domain mapping; antibody inhibition assay","journal":"PLoS pathogens","confidence":"High","confidence_rationale":"Tier 1 / Moderate — biochemical domain mapping combined with functional antibody inhibition, multiple orthogonal methods, single lab","pmids":["21698217"],"is_preprint":false,"attribution":[{"species":"Plasmodium falciparum","strain":""}]},{"year":2009,"finding":"A large sequence deletion (~0.58 kb) in the C-terminal region of RH2b (PfRh2b) in Senegalese field isolates is associated with preferential utilization of protease-resistant invasion pathways and reduced sialic acid-independent invasion (the latter only in blood group O patients).","method":"Sequence polymorphism analysis; invasion assays with enzyme-treated erythrocytes in clinical isolates","journal":"Parasitology","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — molecular epidemiology linking natural deletion polymorphism to invasion phenotype in field isolates, two methods, single lab","pmids":["19126266"],"is_preprint":false,"attribution":[{"species":"Plasmodium falciparum","strain":""}]},{"year":2013,"finding":"RH2b (PfRh2b/PfRH2b) binds heparin, as identified by heparin affinity chromatography and immunoblot. This suggests heparin may block invasion by masking RH2b at the apical surface of merozoites.","method":"Heparin affinity chromatography; immunoblot with ligand-specific antibodies; immunofluorescence localization","journal":"Scientific reports","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — direct biochemical binding assay confirmed by immunoblot, single lab","pmids":["24212193"],"is_preprint":false,"attribution":[{"species":"Plasmodium falciparum","strain":""}]},{"year":2025,"finding":"Loss of RH2b (PfRH2b) through gene disruption substantially impacts susceptibility of P. falciparum to human invasion-inhibitory antibodies from malaria-exposed individuals, identifying RH2b as a significant target of acquired growth-inhibitory antibodies and suggesting that variation in its expression contributes to immune evasion.","method":"Gene disruption; invasion inhibition assays with human sera from malaria-exposed donors","journal":"Frontiers in immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — knockout parasite lines tested with functional antibody inhibition assay, single lab","pmids":["40201183"],"is_preprint":false,"attribution":[{"species":"Plasmodium falciparum","strain":""}]},{"year":2025,"finding":"The EBA-181 merozoite ligand interacts with RH2b protein with a dissociation constant of approximately 3 µM as measured by Surface Plasmon Resonance, suggesting RH2b may act as a co-ligand cooperating with EBA-181 during erythrocyte invasion.","method":"Surface Plasmon Resonance (SPR) binding assay","journal":"Parasitology research","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single SPR binding measurement, single lab, no functional validation in parasite context","pmids":["41198968"],"is_preprint":false,"attribution":[{"species":"Plasmodium falciparum","strain":""}]}],"current_model":"RH2b (PF3D7_1335300) is a P. falciparum merozoite invasion ligand that localizes to the apical end of merozoites, undergoes proteolytic processing (including transmembrane cleavage by PfROM4) to yield fragments with differential sialic acid-dependent and -independent erythrocyte binding; its conserved 15–40 kDa N-terminal ectodomain mediates erythrocyte binding in a sialic acid-independent, trypsin-resistant, chymotrypsin-sensitive manner, and its C-terminal cytoplasmic domain (not the ectodomain or transmembrane region) determines functional diversification from its paralog PfRh2a, with RH2b acting as a dominant invasion ligand whose silencing hierarchically reveals alternative underlying invasion pathways and whose variant expression mediates immune evasion."},"narrative":{"mechanistic_narrative":"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].","teleology":[{"year":2001,"claim":"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","pmids":["11160005"],"confidence":"Medium","gaps":["No direct erythrocyte binding was detected in the assay used","Receptor identity unknown","Molecular mechanism of invasion contribution not defined"],"attribution":[{"species":"Plasmodium falciparum","strain":""}]},{"year":2002,"claim":"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","pmids":["12228308"],"confidence":"Medium","gaps":["Regulatory mechanism controlling variant expression not defined","Functional consequence of the T996 3' deletion not tested"],"attribution":[{"species":"Plasmodium falciparum","strain":"3D7"},{"species":"Plasmodium falciparum","strain":"T996"},{"species":"Plasmodium falciparum","strain":"FCB1"}]},{"year":2003,"claim":"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","pmids":["12606570"],"confidence":"High","gaps":["Receptor on the erythrocyte not identified","Biochemical basis of binding not resolved"],"attribution":[{"species":"Plasmodium falciparum","strain":""}]},{"year":2005,"claim":"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","pmids":["16362075"],"confidence":"High","gaps":["Molecular identity of the redeployed ligands unspecified here","Mechanism enforcing the hierarchy unknown"],"attribution":[{"species":"Plasmodium falciparum","strain":"3D7"}]},{"year":2006,"claim":"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","pmids":["17040128"],"confidence":"High","gaps":["Cleavage shown in heterologous system, not directly in native merozoites here","Timing relative to invasion not resolved in this assay"],"attribution":[{"species":"Plasmodium falciparum","strain":""}]},{"year":2009,"claim":"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","pmids":["19126266"],"confidence":"Medium","gaps":["Association is correlative in field isolates","Blood-group O-restricted effect on sialic acid-independent invasion not mechanistically explained"],"attribution":[{"species":"Plasmodium falciparum","strain":""}]},{"year":2010,"claim":"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","pmids":["20487292"],"confidence":"High","gaps":["Cytoplasmic-domain interacting partners not identified","Signaling or trafficking basis of the effect unknown"],"attribution":[{"species":"Plasmodium falciparum","strain":""}]},{"year":2010,"claim":"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","pmids":["19400777"],"confidence":"High","gaps":["Mechanism of cooperation between RH2b and PfRh2a not defined","Basis of selective pressure for RH2b reconstitution unknown"],"attribution":[{"species":"Plasmodium falciparum","strain":"W2mef"}]},{"year":2011,"claim":"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","pmids":["21386888","21698217"],"confidence":"High","gaps":["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"],"attribution":[{"species":"Plasmodium falciparum","strain":""}]},{"year":2013,"claim":"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","pmids":["24212193"],"confidence":"Medium","gaps":["Physiological relevance of heparin binding to native invasion unproven","Heparin-binding site not mapped"],"attribution":[{"species":"Plasmodium falciparum","strain":""}]},{"year":2025,"claim":"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","pmids":["40201183"],"confidence":"Medium","gaps":["Specific epitopes recognized not defined","Contribution relative to other invasion ligands not quantified"],"attribution":[{"species":"Plasmodium falciparum","strain":""}]},{"year":2025,"claim":"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)","pmids":["41198968"],"confidence":"Low","gaps":["Single SPR measurement without functional validation in parasites","Interaction not confirmed by an orthogonal method","Biological role of the co-ligand interaction not established"],"attribution":[{"species":"Plasmodium falciparum","strain":""}]},{"year":null,"claim":"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.","evidence":"","pmids":[],"confidence":"Low","gaps":["No erythrocyte receptor identified for RH2b","No structural model of the binding domain","Cytoplasmic-domain interaction partners unknown"],"attribution":[{"species":"Plasmodium falciparum","strain":""}]}],"mechanism_profile":{"molecular_activity":[],"localization":[],"pathway":[],"complexes":[],"partners":["PFROM4 (PFE0340C) RHOMBOID PROTEASE — CLEAVES RH2B TRANSMEMBRANE DOMAIN [#4]","PFROM1 — ALSO CLEAVES RH2B [#4]","PFRH2A — PARALOG COOPERATING IN SIALIC ACID-INDEPENDENT INVASION [#5, #6]","EBA-181 — IN VITRO BINDING PARTNER (K_D ~3 ΜM) [#12]","HEPARIN — BINDS RH2B [#10]"],"other_free_text":["RH2b acts as a dominant invasion ligand; its disruption unmasks a pre-existing hierarchical alternative invasion pathway rather than a molecular switch [#3, #5].","Functional diversification from PfRh2a is governed by the RH2b cytoplasmic domain, not the ectodomain or transmembrane region [#6].","Erythrocyte-binding activity maps to a conserved 15–40 kDa N-terminal ectodomain, active in a sialic acid-independent, trypsin-resistant, chymotrypsin-sensitive manner [#7, #8].","RH2b expression is variant across strains and is a target of naturally acquired invasion-inhibitory antibodies, linking it to immune evasion [#2, #11].","Proteolytic processing yields a 220 kDa (sialic acid-independent) and derived 80 kDa (sialic acid-dependent) erythrocyte-binding fragment [#7]."]}},"prefetch_data":{},"corpus_meta":[{"pmid":"12606570","id":"PMC_12606570","title":"Phenotypic variation of Plasmodium falciparum merozoite proteins directs receptor targeting for invasion of human erythrocytes.","date":"2003","source":"The EMBO journal","url":"https://pubmed.ncbi.nlm.nih.gov/12606570","citation_count":224,"is_preprint":false,"source_species":["Pf"],"role":"primary","match_terms":["PfRH2b"],"competing_organism_flag":false,"competing_organism_match":"","competing_organism_only":false},{"pmid":"17040128","id":"PMC_17040128","title":"Two Plasmodium rhomboid proteases preferentially cleave different adhesins implicated in all invasive stages of malaria.","date":"2006","source":"PLoS 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independently of other related PfRh proteins. Phenotypic variation in RH2b expression allows the parasite to exploit different patterns of receptors on the erythrocyte surface.\",\n      \"method\": \"Targeted gene disruption; invasion pathway assays\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — targeted gene disruption with defined invasion phenotype, replicated across multiple parasite lines\",\n      \"pmids\": [\"12606570\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"RH2b (encoded by one of the two PfRh2 genes) is expressed in late schizonts and localizes to the apical end of merozoites. Antibodies against these proteins inhibit merozoite invasion of red blood cells. No binding to mature RBCs or reticulocyte-enriched cell preparations was detected in the assay used.\",\n      \"method\": \"Immunofluorescence localization; antibody inhibition assay; binding assay (negative result for direct RBC binding)\",\n      \"journal\": \"Infection and immunity\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct localization experiment and functional antibody inhibition assay, single lab\",\n      \"pmids\": [\"11160005\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"RH2b (PfRH2b) protein is expressed in late schizonts and merozoites and is located in apical organelles and on the apical surface. In the FCB1 parasite line, PfRH2b is not transcribed and the protein is undetectable, demonstrating variant expression across parasite strains. A 585-bp deletion in the 3' end of RH2b was identified in strain T996.\",\n      \"method\": \"Western blot; immunofluorescence; RT-PCR; sequence analysis\",\n      \"journal\": \"Infection and immunity\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — direct localization and protein expression analysis across multiple strains, single lab, multiple orthogonal methods\",\n      \"pmids\": [\"12228308\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"3D7\"}, {\"species\": \"Plasmodium falciparum\", \"strain\": \"T996\"}, {\"species\": \"Plasmodium falciparum\", \"strain\": \"FCB1\"}]\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Gene disruption of RH2b (PfRh2b) in 3D7 parasites resulted in invasion via a novel pathway. This novel pathway is not due to a molecular switch but is mediated by redeployment of invasion machinery already present in the parent parasite but masked by the dominant role of PfRh2b, indicating a hierarchy of invasion ligand interactions.\",\n      \"method\": \"Gene disruption; invasion pathway assays with enzyme-treated erythrocytes\",\n      \"journal\": \"PLoS pathogens\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — targeted gene disruption with defined mechanistic interpretation, replicated in 3D7 background\",\n      \"pmids\": [\"16362075\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"3D7\"}]\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"PfROM4 (PFE0340c) efficiently cleaves RH2b (Rh2b) via its transmembrane domain in a heterologous cleavage assay, while PfROM1 was also able to cleave Rh2b. Cleavage specificity is determined by the adhesin transmembrane domain.\",\n      \"method\": \"Heterologous rhomboid protease cleavage assay; transmembrane domain swap experiments\",\n      \"journal\": \"PLoS pathogens\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro enzymatic assay with domain-swap mutagenesis establishing substrate specificity, single lab\",\n      \"pmids\": [\"17040128\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Targeted gene disruption of RH2b (PfRh2b) in W2mef parasites showed that PfRh2b is functional in this parasite background. Both PfRh2b-null and PfRh2a-null lines could switch to a sialic acid-independent invasion pathway, but exhibited reduced efficiency for invasion into sialic acid-depleted cells, indicating cooperative roles of PfRh2b and PfRh2a in sialic acid-independent invasion. A strong selective pressure for reconstitution of PfRh2b expression at the expense of PfRh2a was observed.\",\n      \"method\": \"Targeted gene disruption; invasion assays with enzyme-treated erythrocytes\",\n      \"journal\": \"Molecular microbiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — targeted gene disruption with quantitative invasion phenotype, multiple mutant lines tested, single lab\",\n      \"pmids\": [\"19400777\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"W2mef\"}]\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"The functional diversification between RH2b (PfRh2b) and its paralog PfRh2a — despite sharing 2700 N-terminal amino acids — is determined by their cytoplasmic domains, not the ectodomain or transmembrane regions. Chimeric parasite experiments showed the cytoplasmic domain of PfRh2b confers the ability to use a specific invasion pathway.\",\n      \"method\": \"Generation of chimeric parasites; invasion pathway assays\",\n      \"journal\": \"Molecular microbiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — chimeric protein domain-swap in live parasites with defined invasion phenotype readout, single lab\",\n      \"pmids\": [\"20487292\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Native RH2b (PfRH2a/2b) undergoes proteolytic processing near the N-terminus yielding a 220 kDa and an 80 kDa fragment with differential erythrocyte binding specificities. The 220 kDa fragment binds erythrocytes in a sialic acid-independent, trypsin-resistant, chymotrypsin-sensitive manner. The 80 kDa fragment (processed from the 220 kDa fragment) binds erythrocytes in a sialic acid-dependent, trypsin-resistant, chymotrypsin-sensitive manner. The erythrocyte-binding domain was mapped to a conserved 40 kDa N-terminal region common to PfRH2a and PfRH2b. Antibodies against this 40 kDa domain blocked invasion via a sialic acid-independent pathway.\",\n      \"method\": \"Native protein fractionation; erythrocyte binding assay; recombinant protein binding assay; antibody inhibition assay\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — biochemical domain mapping, native protein characterization, and functional antibody inhibition, multiple orthogonal methods, single lab\",\n      \"pmids\": [\"21386888\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"RH2b (PfRh2a and PfRh2b) undergo a complex series of proteolytic cleavage events before and during merozoite invasion, including a cleavage event in the transmembrane region of PfRh2a during invasion consistent with PfROM4 activity, resulting in ectodomain release. The erythrocyte-binding domain of PfRh2a and PfRh2b was defined to a 15 kDa region at the N-terminus of each protein. Antibodies to this receptor-binding region block merozoite invasion.\",\n      \"method\": \"Biochemical analysis of proteolytic processing; erythrocyte binding domain mapping; antibody inhibition assay\",\n      \"journal\": \"PLoS pathogens\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — biochemical domain mapping combined with functional antibody inhibition, multiple orthogonal methods, single lab\",\n      \"pmids\": [\"21698217\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"A large sequence deletion (~0.58 kb) in the C-terminal region of RH2b (PfRh2b) in Senegalese field isolates is associated with preferential utilization of protease-resistant invasion pathways and reduced sialic acid-independent invasion (the latter only in blood group O patients).\",\n      \"method\": \"Sequence polymorphism analysis; invasion assays with enzyme-treated erythrocytes in clinical isolates\",\n      \"journal\": \"Parasitology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — molecular epidemiology linking natural deletion polymorphism to invasion phenotype in field isolates, two methods, single lab\",\n      \"pmids\": [\"19126266\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"RH2b (PfRh2b/PfRH2b) binds heparin, as identified by heparin affinity chromatography and immunoblot. This suggests heparin may block invasion by masking RH2b at the apical surface of merozoites.\",\n      \"method\": \"Heparin affinity chromatography; immunoblot with ligand-specific antibodies; immunofluorescence localization\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — direct biochemical binding assay confirmed by immunoblot, single lab\",\n      \"pmids\": [\"24212193\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Loss of RH2b (PfRH2b) through gene disruption substantially impacts susceptibility of P. falciparum to human invasion-inhibitory antibodies from malaria-exposed individuals, identifying RH2b as a significant target of acquired growth-inhibitory antibodies and suggesting that variation in its expression contributes to immune evasion.\",\n      \"method\": \"Gene disruption; invasion inhibition assays with human sera from malaria-exposed donors\",\n      \"journal\": \"Frontiers in immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — knockout parasite lines tested with functional antibody inhibition assay, single lab\",\n      \"pmids\": [\"40201183\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"The EBA-181 merozoite ligand interacts with RH2b protein with a dissociation constant of approximately 3 µM as measured by Surface Plasmon Resonance, suggesting RH2b may act as a co-ligand cooperating with EBA-181 during erythrocyte invasion.\",\n      \"method\": \"Surface Plasmon Resonance (SPR) binding assay\",\n      \"journal\": \"Parasitology research\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single SPR binding measurement, single lab, no functional validation in parasite context\",\n      \"pmids\": [\"41198968\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    }\n  ],\n  \"current_model\": \"RH2b (PF3D7_1335300) is a P. falciparum merozoite invasion ligand that localizes to the apical end of merozoites, undergoes proteolytic processing (including transmembrane cleavage by PfROM4) to yield fragments with differential sialic acid-dependent and -independent erythrocyte binding; its conserved 15–40 kDa N-terminal ectodomain mediates erythrocyte binding in a sialic acid-independent, trypsin-resistant, chymotrypsin-sensitive manner, and its C-terminal cytoplasmic domain (not the ectodomain or transmembrane region) determines functional diversification from its paralog PfRh2a, with RH2b acting as a dominant invasion ligand whose silencing hierarchically reveals alternative underlying invasion pathways and whose variant expression mediates immune evasion.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"RH2b (PF3D7_1335300) is a Plasmodium falciparum merozoite ligand that mediates a distinct erythrocyte invasion pathway and functions independently of its related PfRh proteins [#0]. 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 [#1, #2]. RH2b expression varies across parasite strains, and this phenotypic variation lets the parasite exploit different erythrocyte receptor patterns [#2]. Its erythrocyte-binding activity resides in a conserved 15\\u201340 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 [#7, #8]. 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 [#4, #7, #8]. Functional diversification from PfRh2a\\u2014despite ~2700 shared N-terminal residues\\u2014is determined by the RH2b cytoplasmic domain rather than the ectodomain or transmembrane region [#6]. RH2b acts as a dominant invasion ligand: its disruption unmasks a pre-existing, hierarchically ordered alternative invasion pathway rather than triggering a molecular switch [#3, #5], and its variant expression additionally serves as a significant target of acquired human invasion-inhibitory antibodies, implicating it in immune evasion [#11].\",\n  \"teleology\": [\n    {\n      \"year\": 2001,\n      \"claim\": \"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.\",\n      \"evidence\": \"Immunofluorescence localization, antibody invasion-inhibition, and RBC binding assays in P. falciparum\",\n      \"pmids\": [\"11160005\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No direct erythrocyte binding was detected in the assay used\", \"Receptor identity unknown\", \"Molecular mechanism of invasion contribution not defined\"],\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Comparing multiple parasite strains revealed that RH2b expression is variant, transitioning it from a fixed invasion factor to a switchable determinant of receptor usage.\",\n      \"evidence\": \"Western blot, immunofluorescence, RT-PCR, and sequence analysis across 3D7, T996, and FCB1 strains\",\n      \"pmids\": [\"12228308\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Regulatory mechanism controlling variant expression not defined\", \"Functional consequence of the T996 3' deletion not tested\"],\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"3D7\"}, {\"species\": \"Plasmodium falciparum\", \"strain\": \"T996\"}, {\"species\": \"Plasmodium falciparum\", \"strain\": \"FCB1\"}]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Genetic disruption directly proved RH2b defines a novel invasion pathway used independently of other PfRh proteins, establishing it as a functional invasion ligand.\",\n      \"evidence\": \"Targeted gene disruption with invasion pathway assays in P. falciparum\",\n      \"pmids\": [\"12606570\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Receptor on the erythrocyte not identified\", \"Biochemical basis of binding not resolved\"],\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"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.\",\n      \"evidence\": \"Gene disruption with enzyme-treated erythrocyte invasion assays in 3D7 P. falciparum\",\n      \"pmids\": [\"16362075\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular identity of the redeployed ligands unspecified here\", \"Mechanism enforcing the hierarchy unknown\"],\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"3D7\"}]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Identifying PfROM4 (and PfROM1) as proteases that cleave RH2b through its transmembrane domain defined how the ligand is processed and released during invasion.\",\n      \"evidence\": \"Heterologous rhomboid cleavage assay with transmembrane domain-swap mutagenesis\",\n      \"pmids\": [\"17040128\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Cleavage shown in heterologous system, not directly in native merozoites here\", \"Timing relative to invasion not resolved in this assay\"],\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"A natural C-terminal deletion in field isolates linked RH2b structural variation to altered invasion pathway preference, connecting the gene to clinical parasite diversity.\",\n      \"evidence\": \"Sequence polymorphism analysis with enzyme-treated erythrocyte invasion assays in Senegalese clinical isolates\",\n      \"pmids\": [\"19126266\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Association is correlative in field isolates\", \"Blood-group O-restricted effect on sialic acid-independent invasion not mechanistically explained\"],\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"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.\",\n      \"evidence\": \"Generation of chimeric parasites with invasion pathway readout in P. falciparum\",\n      \"pmids\": [\"20487292\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Cytoplasmic-domain interacting partners not identified\", \"Signaling or trafficking basis of the effect unknown\"],\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"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.\",\n      \"evidence\": \"Targeted gene disruption with enzyme-treated erythrocyte invasion assays in W2mef P. falciparum\",\n      \"pmids\": [\"19400777\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of cooperation between RH2b and PfRh2a not defined\", \"Basis of selective pressure for RH2b reconstitution unknown\"],\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"W2mef\"}]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Biochemical mapping of processing and binding defined the erythrocyte-binding domain to a conserved N-terminal region (15\\u201340 kDa) and linked distinct processed fragments to sialic acid-dependent versus -independent binding.\",\n      \"evidence\": \"Native protein fractionation, recombinant and native erythrocyte binding assays, domain mapping, and antibody inhibition in P. falciparum\",\n      \"pmids\": [\"21386888\", \"21698217\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"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\"],\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"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.\",\n      \"evidence\": \"Heparin affinity chromatography, immunoblot, and immunofluorescence in P. falciparum\",\n      \"pmids\": [\"24212193\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Physiological relevance of heparin binding to native invasion unproven\", \"Heparin-binding site not mapped\"],\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"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.\",\n      \"evidence\": \"Gene disruption with invasion-inhibition assays using sera from malaria-exposed donors\",\n      \"pmids\": [\"40201183\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Specific epitopes recognized not defined\", \"Contribution relative to other invasion ligands not quantified\"],\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"An in vitro interaction between EBA-181 and RH2b raised the possibility that RH2b acts as a co-ligand cooperating with other invasion ligands.\",\n      \"evidence\": \"Surface Plasmon Resonance binding assay (K_D ~3 \\u00b5M)\",\n      \"pmids\": [\"41198968\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"Single SPR measurement without functional validation in parasites\", \"Interaction not confirmed by an orthogonal method\", \"Biological role of the co-ligand interaction not established\"],\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"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.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No erythrocyte receptor identified for RH2b\", \"No structural model of the binding domain\", \"Cytoplasmic-domain interaction partners unknown\"],\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0030246\", \"supporting_discovery_ids\": [7, 8, 10]},\n      {\"term_id\": \"GO:0008201\", \"supporting_discovery_ids\": [10]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0020009\", \"supporting_discovery_ids\": [1, 2]},\n      {\"term_id\": \"GO:0009986\", \"supporting_discovery_ids\": [2, 10]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0044409\", \"supporting_discovery_ids\": [0, 3, 5]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"PfROM4 (PFE0340c) rhomboid protease \\u2014 cleaves RH2b transmembrane domain [#4]\",\n      \"PfROM1 \\u2014 also cleaves RH2b [#4]\",\n      \"PfRh2a \\u2014 paralog cooperating in sialic acid-independent invasion [#5, #6]\",\n      \"EBA-181 \\u2014 in vitro binding partner (K_D ~3 \\u00b5M) [#12]\",\n      \"Heparin \\u2014 binds RH2b [#10]\"\n    ],\n    \"other_free_text\": [\n      \"RH2b acts as a dominant invasion ligand; its disruption unmasks a pre-existing hierarchical alternative invasion pathway rather than a molecular switch [#3, #5].\",\n      \"Functional diversification from PfRh2a is governed by the RH2b cytoplasmic domain, not the ectodomain or transmembrane region [#6].\",\n      \"Erythrocyte-binding activity maps to a conserved 15\\u201340 kDa N-terminal ectodomain, active in a sialic acid-independent, trypsin-resistant, chymotrypsin-sensitive manner [#7, #8].\",\n      \"RH2b expression is variant across strains and is a target of naturally acquired invasion-inhibitory antibodies, linking it to immune evasion [#2, #11].\",\n      \"Proteolytic processing yields a 220 kDa (sialic acid-independent) and derived 80 kDa (sialic acid-dependent) erythrocyte-binding fragment [#7].\"\n    ]\n  }\n}","audit_flag":null,"strains":[{"strain":"Pf3D7","gene_id":"PF3D7_1335300","name":null,"product":"reticulocyte binding protein 2 homologue b","uniprot_acc":null,"chromosome":null,"start_pos":null},{"strain":"PbANKA","gene_id":"PBANKA_1000011","name":null,"product":"reticulocyte binding protein, putative","uniprot_acc":null,"chromosome":null,"start_pos":null},{"strain":"PkH","gene_id":"PKNH_1472300","name":null,"product":"reticulocyte binding protein","uniprot_acc":null,"chromosome":null,"start_pos":null}],"gene_symbol":"RH2b","evaluation":null}