Protein disulfide isomerase; multifunctional oxidoreductase of the ER lumen, essential for disulfide bond formation in secretory and cell-surface proteins, and processing non-native disulfide bonds; complexes with exomannosidase, Mnl1p, recognizing misfolded glycoproteins, and reducing disulfide bonds of demannosylated substrates to generate an unfolded polypeptide that can be retrotranslocated across the ER membrane, as part of ERAD; maintains Mnl1p solubility in the ER lumen; Ero1p activator