Human ER Oxidoreductin-1α (Ero1α) Undergoes Dual Regulation through Complementary Redox Interactions with Protein-Disulfide Isomerase

In the mammalian endoplasmic reticulum, oxidoreductin-1 (Ero1) generates protein disulfide bonds and transfers them specifically to canonical protein-disulfide isomerase (PDI) to sustain oxidative protein folding. This oxidative process is coupled to the reduction of O-2 to H2O2 on the bound flavin adenine dinucleotide cofactor. Because excessive thiol oxidation and H2O2 generation cause cell death, Ero1 activity must be properly regulated. In addition to the four catalytic cysteines (Cys(94), Cys(99), Cys(104), and Cys(131)) that are located in the flexible active site region, the Cys(208)-Cys(241) pair located at the base of another flexible loop is necessary for Ero1 regulation, although the mechanistic basis is not fully understood. The present study revealed that the Cys(208)-Cys(241) disulfide was reduced by PDI and other PDI family members during PDI oxidation. Differential scanning calorimetry and small angle X-ray scattering showed that mutation of Cys(208) and Cys(241) did not grossly affect the thermal stability or overall shape of Ero1, suggesting that redox regulation of this cysteine pair serves a functional role. Moreover, the flexible loop flanked by Cys(208) and Cys(241) provides a platform for functional interaction with PDI, which in turn enhances the oxidative activity of Ero1 through reduction of the Cys(208)-Cys(241) disulfide. We propose a mechanism of dual Ero1 regulation by dynamic redox interactions between PDI and the two Ero1 flexible loops that harbor the regulatory cysteines.