Probable protein disulfide isomerase that plays an essential role in the segregation of proglutelin and prolamin polypeptides within the ER lumen of endosperm. Required to retain proglutelin in the cisternal ER lumen until ER export and, thereby, indirectly prevents heterotypic interactions with prolamin polypeptides. Catalyzes the rearrangement of -S-S- bonds in proteins. Distributed asymmetrically within the cortical endoplasmic reticulum (ER) and largely restricted to the cisternal ER. Expressed in developing seeds from 3 to 9 days after anthesis. By dithiothreitol-induced endoplasmic reticulum (ER) stress response. Accumulation of large quantities of glutelin precursor (proglutelin) in the endosperm. Belongs to the protein disulfide isomerase family.