Different Interaction Modes for Protein-disulfide Isomerase (PDI) as an Efficient Regulator and a Specific Substrate of Endoplasmic Reticulum Oxidoreductin-1α (Ero1α)

Protein-disulfide isomerase (PDI) and sulfhydryl oxidase endo-plasmic reticulum oxidoreductin- 1 alpha(Ero1 alpha) constitute the pivotal pathway for oxidative protein folding in the mammalian endoplasmic reticulum (ER). Ero1 alpha oxidizes PDI to introduce disulfides into substrates, and PDI can feedback-regulate Ero1 alpha activity. Here, we show the regulatory disulfide of Ero1 alpha responds to the redox fluctuation in ER very sensitively, relying on the availability of redox active PDI. The regulation of Ero1 alpha is rapidly facilitated by either a or a' catalytic domain of PDI, independent of the substrate binding domain. On the other hand, activated Ero1 alpha specifically binds to PDI via hydrophobic interactions and preferentially catalyzes the oxidation of domain a'. This asymmetry ensures PDI to function simultaneously as an oxidoreductase and an isomerase. In addition, several PDI family members are also characterized to be potent regulators of Ero1 alpha. The novel modes for PDI as a competent regulator and a specific substrate of Ero1 alpha govern efficient and faithful oxidative protein folding and maintain the ER redox homeostasis.