PDI8

PDI8 (PF3D7_0827900) is an endoplasmic reticulum-resident protein disulfide isomerase of P. falciparum that catalyzes disulfide-dependent oxidative protein folding (PMID:16806221, PMID:17166645). It carries two thioredoxin domains bearing CGHC active sites and a C-terminal SEEL ER-retrieval signal, and recombinant enzyme exhibits oxidase/isomerase, reductase, and chaperone-like activities in vitro, comparable to mammalian PDI (PMID:16806221, PMID:17166645). Its ER localization was confirmed by co-localization with the ER markers PfBIP and PfERC but not the Golgi marker PfERD2 (PMID:17166645). Functionally, PDI8 drives disulfide-dependent folding of parasite substrates, including the vaccine candidate EBA-175 (PMID:16806221). In intact parasites, PDI8 is trapped as a covalent mixed-disulfide intermediate with the ER chaperone PfJ2 by activity-based crosslinking, placing it within a PfJ2-containing oxidative folding network, and conditional knockdown establishes that PDI8 is required for asexual growth; its redox interactions are sensitive to small-molecule inhibition (PMID:33534803). PDI8 was originally purified through affinity for an antiplasmodial piperazine compound and separately identified as a heme (ferriprotoporphyrin IX)-binding protein, though the significance of heme binding has not been characterized in the available corpus (PMID:11078887, PMID:12748176).

1. 2000 Medium

   The question of whether P. falciparum encodes a druggable thiol:disulfide oxidoreductase was addressed by capturing the protein on an antiplasmodial small molecule, establishing PDI8 as a target-relevant enzyme.

   Evidence Affinity chromatography on a 1,4-bis(3-aminopropyl)piperazine antiplasmodial compound, microsequencing and cloning of the 453-aa single-copy gene

   PMID:11078887

   Open questions at the time

   • Did not demonstrate catalytic activity or in-cell function
   • Mode of compound binding and whether it inhibits catalysis not resolved
2. 2003 Low

   Whether PDI8 physically associates with heme was tested, adding a candidate ligand to its biochemistry.

   Evidence FP-agarose affinity pull-down from parasite lysate with mass spectrometry identification

   PMID:12748176

   Open questions at the time

   • Single pull-down/MS with no functional follow-up for PDI8
   • Functional consequence of heme binding unknown
   • Binding not validated with recombinant protein
3. 2005 Low

   The functional consequence of PDI8 activity on a disulfide-rich substrate was probed by testing its effect on folding of a heterologous transmission-blocking antigen.

   Evidence Heterologous overexpression of PDI8 in P. pastoris with mass spectrometry analysis of Pfs25 folding isoforms and O-glycosylation

   PMID:16274825

   Open questions at the time

   • Heterologous system, not native parasite ER
   • No direct enzymatic or interaction assay linking effect to PDI8 catalysis
   • Relevance to native substrates unestablished
4. 2006 Medium

   The domain architecture and catalytic identity of PDI8 were defined, and its ability to fold a real parasite substrate was demonstrated, establishing it as a bona fide oxidative-folding enzyme.

   Evidence Gene isolation, thioredoxin-domain/phylogenetic analysis, and in vitro oxido-reductase assay with EBA-175 as folding substrate

   PMID:16806221

   Open questions at the time

   • No active-site mutagenesis to confirm CGHC dependence
   • Single lab
   • In vivo relevance of EBA-175 folding not shown
5. 2006 Medium

   Where PDI8 acts in the cell and the breadth of its enzymatic repertoire were resolved by combining ER/Golgi co-localization with parallel activity assays.

   Evidence Immunofluorescence co-localization with PfBIP, PfERC (ER) and PfERD2 (Golgi); in vitro oxidase/isomerase, reductase, and chaperone assays on rhodanese

   PMID:17166645

   Open questions at the time

   • Single lab
   • Native substrate spectrum in the ER not defined
   • Chaperone role in vivo not tested
6. 2021 High

   Whether PDI8 acts within a defined ER folding network and is essential was established by trapping its covalent redox intermediates in living parasites and testing genetic requirement.

   Evidence Divinyl sulfone activity-based crosslinking with IP-MS trapping PfJ2–PDI8 mixed disulfides, DiCre conditional knockdown with asexual growth assays, and small-molecule inhibition assays

   PMID:33534803

   Open questions at the time

   • Full substrate set of the PfJ2–PDI8 network not enumerated
   • Stoichiometry and architecture of the complex undefined
   • Molecular target/selectivity of the inhibitors not resolved

• It remains unresolved what the complete native substrate repertoire of PDI8 is in the parasite ER and how its heme-binding and small-molecule sensitivity connect to catalysis and druggability.
• No comprehensive in-cell substrate map beyond EBA-175 and the PfJ2 interaction
• Functional role of heme binding unknown
• No structural model of PDI8 active-site chemistry from the corpus