Disulfide transfer between two conserved cysteine pairs imparts selectivity to protein oxidation by Ero1

The membrane-associated flavoprotein Ero1p promotes disulfide bond formation in the endoplasmic reticulum (ER) by selectively oxidizing the soluble oxidoreductase protein disulfide isomerase (Pdi1p), which in turn can directly oxidize secretory proteins. Two redox-active disulfide bonds are essential for Ero1p oxidase activity: Cys100-Cys105 and Cys352-Cys355. Genetic and structural data indicate a disulfide bond is transferred from Cys100-Cys105 directly to Pdi1p, whereas a Cys352-Cys355 disulfide bond is used to reoxidize the reduced Cys100-Cys105 pair through an internal thiol-transfer reaction. Electron transfer from Cys352-Cys355 to molecular oxygen, by way of a flavin cofactor, maintains Cys352-Cys355 in an oxidized form. Herein, we identify a mixed disulfide species that confirms the Ero1p intercysteine thiol-transfer relay in vivo and identify Cys105 and Cys352 as the cysteines that mediate thiol-disulfide exchange. Moreover, we describe Ero1p mutants that have the surprising ability to oxidize substrates in the absence of Cys100-Cys105. We show the oxidase activity of these mutants results from structural changes in Ero1p that allow substrates increased access to Cys352-Cys355, which are normally buried beneath the protein surface. The altered activity of these Ero1p mutants toward selected substrates leads us to propose the catalytic mechanism involving transfer between cysteine pairs evolved to impart substrate specificity to Ero1p.