Protein disulfide isomerase of ER lumen required for formation of disulfide bonds in secretory and cell-surface proteins and which unscrambles non-native disulfide bonds. Forms a complex with MNL1 to process unfolded protein-bound Man8GlcNAc2 oligosaccharides to Man7GlcNAc2, promoting degradation in unfolded protein response. Catalyzes the rearrangement of -S-S- bonds in proteins. Interacts with EPS1, KAR2 and MNL1. The N-terminus is blocked. Belongs to the protein disulfide isomerase family.