Balanced Ero1 activation and inactivation establishes ER redox homeostasis

The endoplasmic reticulum (ER) provides an environment optimized for oxidative protein folding through the action of Ero1 p, which generates disulfide bonds, and Pdi1p, which receives disulfide bonds from Ero1 p and transfers them to substrate proteins. Feedback regulation of Ero1 p through reduction and oxidation of regulatory bonds within Ero1 p is essential for maintaining the proper redox balance in the ER. In this paper, we show that Pdi1p is the key regulator of Ero1p activity. Reduced Pdi1p resulted in the activation of Ero1p by direct reduction of Ero1 p regulatory bonds. Conversely, upon depletion of thiol substrates and accumulation of oxidized Pdi 1 p, Ero1 p was inactivated by both autonomous oxidation and Pdi 1 mediated oxidation of Ero1 p regulatory bonds. Pdi1p responded to the availability of free thiols and the relative levels of reduced and oxidized glutathione in the ER to control Ero1 p activity and ensure that cells generate the minimum number of disulfide bonds needed for efficient oxidative protein folding.