Protein disulfide isomerase PDI8 is indispensable for parasite growth and associated with secretory protein processing in Toxoplasma gondii

Abstract

Protein disulfide isomerase, containing thioredoxin (Trx) domains, serves as a vital enzyme responsible for oxidative protein folding (the formation, reduction, and isomerization of disulfide bonds in newly synthesized proteins) in the endoplasmic reticulum (ER). However, the role of ER-localized PDI proteins in parasite growth and their interaction with secretory proteins remain poorly understood. In this study, we identified two ER-localized PDI proteins, TgPDI8 and TgPDI6, in Toxoplasma gondii. Conditional knockdown of TgPDI8 resulted in a significant reduction in intracellular proliferation and invasion abilities, leading to a complete block in plaque formation on human foreskin fibroblast monolayers, whereas parasites lacking TgPDI6 did not exhibit any apparent fitness defects. The complementation of TgPDI8 with mutant variants highlighted the critical role of the CXXC active site cysteines within its Trx domains for its enzymatic activity. By utilizing TurboID-based proximity labeling, we uncovered a close association between PDI proteins and canonical secretory proteins. Furthermore, parasites lacking TgPDI8 showed a significant reduction in the expression of secretory proteins, especially those from micronemes and dense granules. In summary, our study elucidates the roles of TgPDI8 and sets the stage for future drug discovery studies.