Protein Disulfide Isomerase (PDIa) family, redox active TRX domains; composed of eukaryotic proteins involved in oxidative protein folding in the endoplasmic reticulum (ER) by acting as catalysts and folding assistants. Members of this family include PDI and PDI-related proteins like ERp72, ERp57 (or ERp60), ERp44, P5, PDIR, ERp46 and the transmembrane PDIs. PDI, ERp57, ERp72, P5, PDIR and ERp46 are all oxidases, catalyzing the formation of disulfide bonds of newly synthesized polypeptides in the ER. They also exhibit reductase activity in acting as isomerases to correct any non-native disulfide bonds, as well as chaperone activity to prevent protein aggregation and facilitate the folding of newly synthesized proteins. These proteins usually contain multiple copies of a redox active TRX (a) domain containing a CXXC motif, and may also contain one or more redox inactive TRX-like (b) domains. Only one a domain is required for the oxidase function but multiple copies are necessary for the isomerase function. The different types of PDIs may show different substrate specificities and tissue-specific expression, or may be induced by stress. PDIs are in their reduced form at steady state and are oxidized to the active form by Ero1, which is localized in the ER through ERp44. Some members of this family also contain a DnaJ domain in addition to the redox active a domains; examples are ERdj5 and Pfj2. Also included in the family is the redox inactive N-terminal TRX-like domain of ERp29.
Comment:The N-terminal cysteine in the CXXC motif is essential for both oxidase and isomerase activities.
Comment:The C-terminal cysteine in the CXXC motif is essential for oxidase activity. It is important, although not required, for isomerase activity. It provides PDI with an escape mechanism from mixed disulfide intermediates that isomerize slowly.
Jiyao W et al.(2023). "The conserved domain database in 2023.",