MSP6

MSP6 (PF3D7_1035500) is a peripheral component of the Plasmodium falciparum merozoite surface protein coat that participates in erythrocyte invasion through its association with the MSP1 complex (PMID:11166390, PMID:26823464). It is expressed as a 371-amino acid precursor that is proteolytically processed to a 36 kDa fragment, MSP6(36), which associates non-covalently with the merozoite surface MSP1 complex and also exists as a soluble unbound form (PMID:11166390). This maturation is carried out by the subtilisin-like protease PfSUB1, which is discharged into the parasitophorous vacuole just before egress and processes MSP6 alongside MSP1 and MSP7; blocking PfSUB1 causes accumulation of unprocessed MSP6 and reduces erythrocyte invasion (PMID:19214190). Binding to MSP1 is direct and specific: the intact C-terminal region of MSP6 is required for association with the MSP1 p38 subunit and for multimerization of MSP6 into tetramers, and only the processed form of MSP1 is competent to bind (PMID:16940297, PMID:29511044). MSP6 engages MSP1 independently of MSP3, MSP7, MSPDBL1 and MSPDBL2, defining one of several distinct peripheral MSP1 complexes that themselves bind directly to human erythrocytes (PMID:26823464, PMID:29760216). MSP6 also binds erythrocyte surface receptors directly, and MSP6-derived peptides recognize discrete erythrocyte membrane receptors and inhibit merozoite invasion in vitro (PMID:16713025, PMID:29511044). As a dimorphic antigen with divergent 3D7-type and K1-type alleles, MSP6 is a target of invasion-inhibitory and monocyte-dependent (ADCI) antibody responses (PMID:15039357, PMID:15664972).

1. 2001 High

   Establishing the molecular identity of MSP6 answered whether this surface antigen is a processed protein physically linked to the MSP1 complex, defining it as a peripheral merozoite coat component.

   Evidence N-terminal sequencing, gene cloning, mass spectrometry, and co-immunoprecipitation from culture supernatant of P. falciparum

   PMID:11166390

   Open questions at the time

   • The MSP1 subunit responsible for binding was not yet mapped
   • The protease generating MSP6(36) was unknown
   • Functional consequence of complex membership for invasion untested
2. 2004 Medium

   Sequencing of multiple alleles revealed MSP6 is dimorphic and that allele-specific antibodies inhibit only homologous merozoites, defining a strain-specific determinant of the invasion-inhibitory response.

   Evidence Sequencing of eight MSP6 alleles, immunoblot, and allele-specific invasion inhibition assays in P. falciparum (3D7)

   PMID:15039357

   Open questions at the time

   • Structural basis of allele divergence not defined
   • Whether dimorphism alters MSP1 binding untested
   • Cross-strain immune escape not quantified in vivo
3. 2004 Low

   Comparative analysis of the MSP6(36) cleavage-site motif against those of other merozoite surface proteins raised the hypothesis of a shared processing protease, framing the search for the responsible enzyme.

   Evidence N-terminal sequencing of processed MSP3 with comparative sequence analysis of the published MSP6 cleavage site (no direct MSP6 experiment)

   PMID:15582517

   Open questions at the time

   • No direct experiment on MSP6 processing performed in this study
   • Identity of the protease not established here
   • Inference rests on sequence similarity only
4. 2005 Medium

   Functional ADCI assays established MSP6 as a target of protective antibody-dependent monocyte killing, extending its relevance from a structural coat component to an immune target.

   Evidence ADCI functional assay with affinity-purified antibodies against MSP6 C-terminal peptides in P. falciparum

   PMID:15664972

   Open questions at the time

   • Contribution of MSP3-cross-reactive versus MSP6-specific epitopes not separated in vivo
   • Protective relevance in natural infection untested
   • Mechanism of monocyte-mediated killing not resolved
5. 2006 High

   In vitro reconstitution mapped the MSP6-MSP1 interaction to the p38 subunit and the MSP6 C-terminus, and showed MSP6 tetramerizes, providing the molecular architecture of the association first seen by Co-IP.

   Evidence In vitro binding assays with recombinant proteins and analytical/biophysical characterization

   PMID:16940297

   Open questions at the time

   • Stoichiometry of MSP6 tetramer within the intact surface complex unresolved
   • No structural model of the p38-MSP6 interface
   • Role of tetramerization in invasion untested
6. 2006 Medium

   Erythrocyte-binding peptide mapping tested whether MSP6 contacts the host cell directly, identifying high-activity binding peptides that recognize defined erythrocyte receptors and inhibit invasion.

   Evidence Radiolabelled peptide erythrocyte binding assay and invasion inhibition assay in P. falciparum

   PMID:16713025

   Open questions at the time

   • Erythrocyte receptors identified only by molecular weight, not identity
   • Peptide-level data may not reflect full-length protein behavior
   • Modest inhibition (27-46%) leaves functional weight unclear
7. 2009 High

   Identifying PfSUB1 as the protease that processes MSP6 answered how and when MSP6(36) is generated and linked its maturation to efficient invasion.

   Evidence PfSUB1 inhibition with immunoblot of MSP processing and erythrocyte invasion assay in P. falciparum

   PMID:19214190

   Open questions at the time

   • Whether the invasion defect is specifically due to unprocessed MSP6 versus co-processed MSP1/MSP7 not isolated
   • Precise cleavage site not defined in this study
   • Whether processing is required for MSP1 binding untested
8. 2016 Medium

   Reconstitution with defined partners established that MSP6 binds MSP1 independently of other complex members, resolving the surface coat into multiple distinct MSP1 complexes each capable of erythrocyte binding.

   Evidence Recombinant protein binding assays, surface plasmon resonance, and erythrocyte binding assays in P. falciparum

   PMID:26823464

   Open questions at the time

   • Relative abundance of the MSP6-MSP1 complex versus other MSP1 complexes unknown
   • Whether complexes are mutually exclusive on the surface untested
   • Erythrocyte receptor for the complex not identified
9. 2018 Medium

   Two independent studies refined the MSP1 binding site to the p38/p42 region (not MSP1-19) and reciprocally confirmed MSP6 as a direct member of an MSP3-associated surface complex containing MSP1 and MSP7.

   Evidence ELISA-based interaction assays, surface plasmon resonance, erythrocyte binding, and anti-MSP3 immunoprecipitation with mass spectrometry in P. falciparum

   PMID:29511044 PMID:29760216

   Open questions at the time

   • Reconciliation of MSP6 as an independent MSP1 binder versus a member of the larger MSP3 complex not fully resolved
   • Quaternary organization of the multi-protein complex undefined
   • In vivo relevance of each interaction to invasion untested

• The identity of the human erythrocyte receptor(s) bound by MSP6 and the intact MSP6-MSP1 complex, and the quantitative contribution of MSP6 to invasion in vivo, remain unresolved.
• Erythrocyte receptors characterized only by apparent molecular weight
• No genetic knockout phenotype for MSP6 reported in the corpus
• No structure of MSP6 alone or in complex