P36

P36 (PF3D7_0404400) is a 6-cysteine (s48/45-domain) sporozoite protein that is essential for productive hepatocyte infection, where it enables formation of the parasitophorous vacuole (PV) in which the parasite develops as a liver stage (PMID:17517871, PMID:19625622). In P. yoelii, P36 localizes to micronemal secretory organelles and is released as a secreted protein upon sporozoite stimulation, forming a complex with GPI-anchored P52; P52 is not itself secreted, consistent with a model in which P52 scaffolds P36 for engagement of the host cell, and single deletion of P36 alone phenocopies the PV-loss defect of the double deletion (PMID:30547015). Genetically, P36 acts as the parasite determinant of host-cell receptor usage during invasion: P. falciparum P36 directs use of CD81, P. vivax P36 directs use of SR-BI, and P. berghei P36 is compatible with both receptors, while loss of P36 phenocopies loss of the hepatocyte EphA2 receptor (PMID:26612952, PMID:28506360). Structurally, integrative modeling in P. berghei/P. falciparum shows P36 and P52 assemble as a head-to-tail heterodimer with a conserved interface, and antibodies targeting membrane-distal epitopes on this heterodimer block sporozoite invasion, marking P36 surface sites as neutralization-vulnerable (PMID:42418521).

1. 2007 High

   Established that P36 is required for the parasite to build the parasitophorous vacuole it needs to survive inside hepatocytes, defining its role at the invasion-to-liver-stage transition.

   Evidence Double knockout of P36/P52 in P. yoelii with IFA, EM, and in vitro/in vivo liver-stage assays

   PMID:17517871

   Open questions at the time

   • Deletion was combined with P52, so P36's individual contribution was not isolated here
   • No molecular mechanism for how P36 promotes PV formation
   • Host receptor engagement not addressed
2. 2009 High

   Extended the requirement to the human parasite and showed P36 acts specifically at hepatocyte infection without affecting other life-cycle stages, confirming a liver-stage-restricted function.

   Evidence Double-crossover deletion of P36/P52 in P. falciparum NF54 with in vitro liver assays and humanized mouse model

   PMID:19625622

   Open questions at the time

   • Single-deletion phenotype less severe than double, leaving P36/P52 functional redundancy unresolved
   • No biochemical mechanism defined
3. 2015 Medium

   Linked P36 to a specific host surface receptor, EphA2, providing the first host-side molecular partner for P36-dependent invasion.

   Evidence Genetic deletion of P36 and EphA2, hepatocyte infection assays, and EphA2 expression correlation in P. yoelii/P. falciparum

   PMID:26612952

   Open questions at the time

   • Direct P36–EphA2 binding not biochemically demonstrated
   • Relationship between EphA2 and CD81/SR-BI usage not resolved
4. 2017 High

   Showed P36 is the parasite determinant selecting which host receptor is engaged, explaining species differences in hepatocyte tropism.

   Evidence Cross-species P36 swapping/deletion with CD81- and SR-BI-deficient hepatocytes across P. falciparum, P. vivax, and P. berghei

   PMID:28506360

   Open questions at the time

   • Molecular basis of P36-receptor specificity not defined
   • Direct binding to CD81/SR-BI not shown biochemically
5. 2018 High

   Defined P36's subcellular behavior and its physical partnership with P52, establishing a secretion/scaffolding model for host-cell engagement.

   Evidence Live imaging of P36-mCherry, in vivo co-immunoprecipitation, secretion assay, and single-KO phenotyping in P. yoelii

   PMID:30547015

   Open questions at the time

   • Direct interaction with a host receptor not captured in the Co-IP
   • Mechanism converting secretion into PV formation unknown
6. 2012 Medium

   Provided a structural rationale for P36 adhesion function via the s48/45 fold and its homology to ephrins, though shown for the paralog Pf12 rather than P36 itself.

   Evidence NMR structure of the Pf12 s48/45 domain with structural comparison to SRS/SAG folds and ephrins

   PMID:22493233

   Open questions at the time

   • Structure solved on Pf12, not P36; applied to P36 only by family membership
   • No experimental structure of P36 at this stage
7. 2012 Medium

   Characterized the fate of parasites that bypass P36/P52 requirement, showing PV-independent development can occur but only in the cytosol, not the nucleus.

   Evidence IFA, EM, and blood-stage infectivity readouts of Δp52&p36 P. berghei in Huh-7 cells

   PMID:23227206

   Open questions at the time

   • P36-specific contribution not separated from P52
   • Why some parasites escape the PV requirement is unexplained
8. 2023 Low

   Placed P36 in a larger surface invasion assembly by showing it can associate with a third 6-cys member, B9.

   Evidence Heterologous co-expression/pulldown interaction assay in P. falciparum

   PMID:36761022

   Open questions at the time

   • Single pulldown in a heterologous system, not confirmed in native parasites
   • Functional consequence of B9–P36 association untested
   • Stoichiometry and architecture of the putative complex unknown
9. 2026 High

   Resolved the P36–P52 assembly as a conserved head-to-tail heterodimer and mapped neutralization-vulnerable surface sites on P36, connecting structure to vaccine-relevant function.

   Evidence AlphaFold modeling, EM, SAXS, and epitope-tagged neutralization assays in P. berghei/P. falciparum

   PMID:42418521

   Open questions at the time

   • No co-structure of the heterodimer bound to a host receptor
   • How antibody binding mechanistically blocks invasion not defined

• How P36 physically bridges its receptor-specificity function (CD81/SR-BI/EphA2) to PV membrane formation remains undefined at the biochemical level.
• No demonstrated direct binding of P36 to CD81, SR-BI, or EphA2
• Molecular link between receptor engagement and PV biogenesis unknown
• No experimental P36 structure in a receptor-bound state