Ero1α requires oxidizing and normoxic conditions to localize to the mitochondria-associated membrane (MAM)

Protein secretion from the endoplasmic reticulum (ER) requires the enzymatic activity of chaperones and oxidoreductases that fold polypeptides and form disulfide bonds within newly synthesized proteins. The best-characterized ER redox relay depends on the transfer of oxidizing equivalents from molecular oxygen through ER oxidoreductin 1 (Ero1) and protein disulfide isomerase to nascent polypeptides. The formation of disulfide bonds is, however, not the sole function of ER oxidoreductases, which are also important regulators of ER calcium homeostasis. Given the role of human Ero1 alpha in the regulation of the calcium release by inositol 1,4,5-trisphosphate receptors during the onset of apoptosis, we hypothesized that Ero1 alpha may have a redox-sensitive localization to specific domains of the ER. Our results show that within the ER, Ero1 alpha is almost exclusively found on the mitochondria-associated membrane (MAM). The localization of Ero1 alpha on the MAM is dependent on oxidizing conditions within the ER. Chemical reduction of the ER environment, but not ER stress in general leads to release of Ero1 alpha from the MAM. In addition, the correct localization of Ero1 alpha to the MAM also requires normoxic conditions, but not ongoing oxidative phosphorylation.