Host cell KRT-18 as a non-redox dependent interactor of Cryptosporidium parvum PDI enables parasite infection.

The protein disulfide isomerase (PDI) is a conserved redox protein that plays an essential role during host cell adhesion and invasion in apicomplexan parasites. PDI is also expressed in Cryptosporidium parvum, but its role remains unknown. Here we show that C. parvum PDI (CpPDI, encoded by cgd6_120) contains two conserved thioredoxin domains with catalytic domains (CGHC), is highly expressed early in development in C. parvum, discharged from the invading sporozoites into host cells, and is mostly distributed in endoplasmic reticulum and the cytosol of free sporozoites. In addition, the PDI inhibitor bacitracin, DNTB, anti-CpPDI antibodies and recombinant CpPDI protein could effectively with interfere the invasion of sporozoites into host cells. LC-MS/MS and Co-immunoprecipitation analysis identified Keratin, type I cytoskeletal 18 protein (KRT-18), as a PDI interactor that is not redox-dependent and whose interaction is not related to the catalytic activity of disulfide bond formation. Downregulation in the expression of KRT-18 impairs C. parvum infection. Collectively, these results demonstrate that CpPDI is essential across invasive stages of the C. parvum parasite, and identify a non-redox dependent PDI interactor--KRT-18 in host cells, these data provide molecular insights into how Cryptosporidium interacts with its intestinal host.