A novel soybean protein disulfide isomerase family protein possesses dithiol oxidation activity: Identification and characterization of GmPDIL6.

Secretory and membrane proteins synthesized in the endoplasmic reticulum (ER) are folded with intramolecular disulfide bonds, viz. oxidative folding, catalyzed by the protein disulfide isomerase (PDI) family proteins. Here, we identified a novel soybean PDI family protein, GmPDIL6. GmPDIL6 has a single thioredoxin-domain with a putative N-terminal signal peptide and an active center (CKHC). Recombinant GmPDIL6 forms various oligomers binding iron. Oligomers with or without iron binding, and monomers exhibited a dithiol oxidase activity level comparable to those of other soybean PDI family proteins. However, they displayed no disulfide reductase and extremely low oxidative refolding activity. Interestingly, GmPDIL6 was mainly expressed in the cotyledon during synthesis of seed storage proteins and GmPDIL6 mRNA was upregulated under ER stress. GmPDIL6 may play a role in the formation of disulfide bonds in nascent proteins for oxidative folding in the ER.