Redox State of the Endoplasmic Reticulum Is Controlled by Ero1L-alpha and Intraluminal Calcium

Formation of intra-and intermolecular disulfide bonds is an essential step in the synthesis of secretory proteins. In eukaryotic cells, this process occurs in the endoplasmic reticulum (ER) and requires an oxidative environment with the action of several chaperones and folding catalysts. During protein folding, Ero1p oxidizes protein disulfide isomerase (PDI), which then directly catalyzes the formation of disulfide bonds in folding proteins. Recent cell-free studies suggest that the terminal electron acceptor in the pathway is molecular oxygen, with the resulting formation of hydrogen peroxide (H2O2). We report for the first time the measurement of ER H2O2 level in live cells. By targeting a fluorescent protein-based H2O2 sensor to various intracellular compartments, we show that the ER has the highest level of H2O2, and this high concentration is well confined to the lumen of the organelle. Manipulation of the Ero1-La level-either by overexpression or by siRNA-mediated inhibition-caused parallel changes in luminal H2O2, proving that the activity of Ero1-La results in H2O2 formation in the ER. We also found that calcium mobilization from intracellular stores induces a decrease in ER H2O2 level, suggesting a complex interplay between redox and calcium signaling in the mammalian ER. Antioxid. Redox Signal. 13, 721-729.