PDI

In apicomplexan parasites, protein disulfide isomerase (PDI) activity supports both intracellular oxidative folding of secretory-pathway proteins and, in a surface-associated pool, direct engagement of host cells during invasion (PMID:35913173, PMID:39162526). Two ER-resident PDIs drive the lytic cycle: TgPDI8, whose thioredoxin-domain CXXC active-site cysteines are required for enzymatic function, is essential for parasite growth, invasion, and intracellular proliferation, associates with canonical secretory proteins by proximity labeling, and is needed for normal expression of microneme and dense-granule proteins (PMID:39162526); TgPDIA3 couples ER redox homeostasis to ER calcium sequestration, and its loss disrupts both, impairing microneme protein maturation and secretion, invasion, and replication and thereby linking ER redox control to calcium-dependent lytic-cycle progression (PMID:41919918). A distinct surface-exposed TgPDI (the originally described 49–52 kDa antigen) acts as a parasite ligand for the host receptor basigin: recombinant TgPDI binds basigin directly, active-site cysteine mutagenesis abolishes this binding, and PDI inhibitors reduce parasite attachment and invasion, establishing an active-site-dependent adhesion function (PMID:26896642, PMID:35913173). This surface PDI is also a target of human mucosal IgA, whose epitopes map to the catalytic thioredoxin domain (PMID:12095627, PMID:12407020).

1. 2002 Medium

   Establishing the molecular identity of a prominent T. gondii tachyzoite antigen defined PDI as a parasite protein recognized by the host mucosal immune response.

   Evidence Mass-spectrometric identification and recombinant expression of a 49 kDa tachyzoite antigen, immunoblotted against human tear IgA

   PMID:12095627 PMID:12407020

   Open questions at the time

   • Did not test a catalytic or adhesion function for the protein
   • Epitope mapping localized IgA binding to the thioredoxin domain but did not test whether antibody binding neutralizes enzymatic activity
2. 2005 Medium

   Localizing the ortholog to the parasite surface and showing that sulfhydryl blockers and inhibitors reduce adhesion linked surface-associated PDI to host-cell interaction.

   Evidence Immunogold-EM and immunofluorescence localization plus adhesion-inhibition assays with DTNB, pCMBA, bacitracin, and blocking antibodies in Neospora caninum tachyzoites

   PMID:16129440

   Open questions at the time

   • Shown in the Neospora caninum ortholog, not T. gondii
   • Inhibitors are not PDI-specific and no host receptor was identified
3. 2016 Medium

   Functional inhibition and recombinant-protein interference tests established that TgPDI participates in the adhesion and invasion steps of infection.

   Evidence Invasion-inhibition assays with recombinant TgPDI and PDI inhibitors (bacitracin, nitroblue tetrazolium) plus infection-stage gene expression analysis in human epithelial and retinal cells

   PMID:26896642

   Open questions at the time

   • No host binding partner identified
   • Relied on non-specific inhibitors without genetic or active-site validation
4. 2022 High

   Identifying host basigin as the receptor and showing active-site-dependent binding defined the molecular mechanism of surface TgPDI-mediated attachment.

   Evidence Proteomic screen for basigin ligands, direct recombinant TgPDI–basigin binding assay, active-site cysteine mutagenesis abolishing binding, and inhibitor-pretreatment attachment assay

   PMID:35913173

   Open questions at the time

   • Did not resolve how an ER-type PDI reaches the parasite surface
   • Downstream signaling or invasion consequences of the PDI–basigin interaction not defined
5. 2024 High

   Genetic dissection distinguished an essential ER PDI (TgPDI8) from a dispensable one (TgPDI6) and tied its function to oxidative folding of secretory proteins.

   Evidence Conditional knockdown with plaque and invasion assays, active-site cysteine mutagenesis with complementation, and TurboID proximity proteomics in T. gondii

   PMID:39162526

   Open questions at the time

   • Specific folding substrates responsible for the growth defect not individually validated
   • Relationship of TgPDI8 to the surface basigin-binding PDI not addressed
6. 2026 High

   Characterizing TgPDIA3 revealed that a second ER PDI couples ER redox state to calcium sequestration, connecting redox homeostasis to calcium-dependent lytic-cycle progression.

   Evidence Localization, conditional loss-of-function, ER redox and calcium measurements, microneme secretion and invasion assays, and substrate identification in T. gondii (peer-reviewed; preprint version also in the timeline)

   PMID:41919918 PMID:40463016

   Open questions at the time

   • Mechanism by which redox activity controls calcium sequestration not fully resolved
   • Functional overlap or division of labor with TgPDI8 not delineated

• How the ER-resident PDI machinery relates to the surface-exposed basigin-binding PDI—including trafficking to the cell surface and whether distinct or shared gene products underlie the intracellular folding and adhesion functions—remains unresolved.
• No study reconciles surface localization with ER residency
• Division of function among the multiple T. gondii PDIs not fully mapped