Abstract

Endoplasmic reticulum (ER)-resident protein disulfide isomerase (PDI) family members function not only as disulfide bond-catalysts but also as chaperones for the oxidative folding of client proteins. However, due to the scarcity of structural data, the client recognition mechanism remains poorly understood. We report the distinct recognition mechanisms for PDI/ERp46 proteins to recruit a reduced and denatured bovine pancreatic trypsin inhibitor (BPTI). NMR data demonstrated that PDI recognizes a broad region consisting of [~]30 amino acid residues in unfolded BPTI in order to function as a chaperone, while ERp46 interacts nonspecifically and weakly with unfolded BPTI to promiscuously and rapidly introduce disulfide bonds. These two distinct client recognition modes are likely important for the functional modulation of the disulfide-catalyst and chaperone against client proteins. Client recognition differences between PDI family proteins likely contribute to determining whether disulfide bonds are introduced into the client protein or aggregates are reduced to an unfolded state during the early folding step. Thus, the mammalian ER ensures concerted protein quality control through the differences in the client recognition modes of PDI family proteins, resulting in the production of large quantities of multiple-disulfide bonded proteins.