Acts as a protein-folding catalyst that interacts with nascent polypeptides to catalyze the formation, isomerization, and reduction or oxidation of disulfide bonds. May play a role in storage protein biogenesis. Catalyzes the rearrangement of -S-S- bonds in proteins. Localizes on the surface of ER-derived type-I protein bodies in the endosperm. Belongs to the protein disulfide isomerase family.