PTEX88

PTEX88 (PF3D7_1105600) is an auxiliary component of the Plasmodium translocon of exported proteins (PTEX), the ATP-powered parasitophorous vacuole membrane complex that mediates export of parasite proteins into the host erythrocyte cytosol (PMID:19536257). Within this complex, PTEX88 associates most closely with the HSP101 core motor and more weakly with the other core constituents PTEX150 and EXP2, and it also engages other vacuole-resident chaperones and members of the exported protein-interacting complex (EPIC); its incorporation into the high molecular weight PTEX assembly is dynamic rather than constitutive (PMID:29637707). Beyond its diffuse intraparasitic distribution, PTEX88 localizes to motile tubular extensions of the parasitophorous vacuole that protrude from the parasite into the erythrocyte cytoplasm, most prominently during trophozoite growth (PMID:24076174, PMID:26219962). Functionally, PTEX88 is required for efficient blood-stage growth and for parasite sequestration in host tissue vasculature: its loss or knockdown abolishes brain sequestration and cerebral malaria in P. berghei and reduces CD36-dependent adhesion of P. falciparum-infected erythrocytes, yet does not impair general protein export, distinguishing its role from that of the core translocon components (PMID:24076174, PMID:25820521, PMID:26886275). In P. berghei the gene could not be deleted in some settings, consistent with an important blood-stage role (PMID:23869529).

1. 2009 High

   Establishing that a dedicated machine exports parasite proteins across the parasitophorous vacuole membrane required defining its components; identifying PTEX88 as part of PTEX placed it within the protein-export translocon.

   Evidence Biochemical complex purification and identification of PTEX components in P. falciparum

   PMID:19536257

   Open questions at the time

   • Does not define PTEX88's specific molecular role versus core components
   • No stoichiometry or structural placement within the complex
2. 2013 Medium

   To test whether PTEX88 is a bona fide, functionally important complex member, genetic tagging confirmed its PTEX membership and failed deletion attempts implied an important blood-stage role.

   Evidence Genetic tagging and attempted gene deletion in P. berghei

   PMID:23869529

   Open questions at the time

   • Essentiality inferred from failed knockout, not a direct functional assay
   • Species-specific: deletion outcome differs from later knockout reports
3. 2013 Medium

   Determining the functional consequence and subcellular distribution of PTEX88 showed a growth defect and revealed localization to extensions reaching into the erythrocyte cytoplasm.

   Evidence Null-mutant generation, intravital competition assay, and endogenous fluorescent tagging with live imaging in P. berghei

   PMID:24076174

   Open questions at the time

   • Molecular basis of the growth defect not resolved
   • Single lab; does not distinguish direct versus indirect effects on growth
4. 2015 Medium

   Refining the localization further, correlative imaging identified PTEX88 in motile tubular parasitophorous vacuole extensions and revealed distinct expression across sporogony and liver-stage development.

   Evidence Endogenous fluorescent tagging, live cell imaging, and correlative light and electron microscopy in P. berghei

   PMID:26219962

   Open questions at the time

   • Function of the tubular extensions not defined
   • Link between extension localization and export or sequestration not established
5. 2015 Medium

   To separate PTEX88's role from general protein export, knockout parasites were tested for sequestration and export, showing loss of brain and peripheral tissue sequestration and abolition of cerebral malaria while export remained intact.

   Evidence Null-mutant, mouse infection model, in vitro growth and sequestration assays in P. berghei

   PMID:25820521

   Open questions at the time

   • Mechanism linking PTEX88 to sequestration not identified
   • Single lab; molecular adhesion pathway not defined
6. 2016 High

   Extending the sequestration phenotype across species, inducible knockdown confirmed a specific role in adhesion, including reduced CD36 binding in P. falciparum, decoupling PTEX88 from universal protein trafficking.

   Evidence Inducible knockdown in P. falciparum and P. berghei, in vivo infection, protein export assays, and CD36 binding assay

   PMID:26886275

   Open questions at the time

   • Direct molecular mechanism connecting PTEX88 to CD36 adhesion not defined
   • Whether PTEX88 acts through PfEMP1 trafficking or another route unresolved
7. 2018 Medium

   Mapping the PTEX88 interactome defined its closest partner as HSP101 with weaker association to PTEX150 and EXP2, plus contacts with vacuolar chaperones and EPIC, and showed its PTEX incorporation is dynamic and stage-restricted.

   Evidence Co-immunoprecipitation, mass spectrometry interactome, and live imaging in P. falciparum

   PMID:29637707

   Open questions at the time

   • Functional consequence of the dynamic HSP101 interaction not resolved
   • How EPIC association relates to the sequestration phenotype not established

• The molecular mechanism by which PTEX88 promotes parasite sequestration and adhesion, and how this relates to its dynamic association with the PTEX core, remains unresolved.
• No defined substrate or adhesion-effector pathway for PTEX88
• No structural model of PTEX88 within PTEX
• Reason for dynamic/stage-restricted PTEX incorporation unknown