The endoplasmic reticulum sulfhydryl oxidase Ero1β drives efficient oxidative protein folding with loose regulation

In eukaryotes, disulfide bonds are formed in the endoplasmic reticulum, facilitated by the Ero1 (endoplasmic reticulum oxidoreductin 1) oxidase/PDI (protein disulfide-isomerase) system. Mammals have two ERO1 genes, encoding Ero1 alpha and Ero1 beta proteins. Ero1 beta is constitutively expressed in professional secretory tissues and induced during the unfolded protein response. In the present work, we show that recombinant human Ero1 beta is twice as active as Ero1 alpha a in enzymatic assays. Ero1 beta oxidizes PDI more efficiently than other PDI family members and drives oxidative protein folding preferentially via the active site in the a' domain of PDI. Our results reveal that Ero1 beta oxidase activity is regulated by long-range disulfide bonds and that Cys(130) plays a critical role in feedback regulation. Compared with Ero1 alpha, however, Ero1 beta is loosely regulated, consistent with its role as a more active oxidase when massive oxidative power is required.