Biochemical evidence that regulation of Ero1β activity in human cells does not involve the isoform-specific cysteine 262

In the ER (endoplasmic reticulum) of human cells, disulfide bonds are predominantly generated by the two isoforms of Ero1 (ER oxidoreductin-1): Ero1α and Ero1β. The activity of Ero1α is tightly regulated through the formation of intramolecular disulfide bonds to help ensure balanced ER redox conditions. Ero1β is less tightly regulated, but the molecular details underlying control of activity are not as well characterized as for Ero1α. Ero1β contains an additional cysteine residue (Cys(262)), which has been suggested to engage in an isoform-specific regulatory disulfide bond with Cys(100). However, we show that the two regulatory disulfide bonds in Ero1α are likely conserved in Ero1β (Cys(90)–Cys(130) and Cys(95)–Cys(100)). Molecular modelling of the Ero1β structure predicted that the side chain of Cys(262) is completely buried. Indeed, we found this cysteine to be reduced and partially protected from alkylation in the ER of living cells. Furthermore, mutation of Cys(100)–but not of Cys(262)–rendered Ero1β hyperactive in cells, as did mutation of Cys(130). Ero1β hyperactivity induced the UPR (unfolded protein response) and resulted in oxidative perturbation of the ER redox state. We propose that features other than a distinct pattern of regulatory disulfide bonds determine the loose redox regulation of Ero1β relative to Ero1α.