We describe a conserved yeast gene, ERO1, that is induced by the unfolded protein response and encodes a novel glycoprotein required for oxidative protein folding in the ER. In a temperature-sensitive ero1-1 mutant, newly synthesized carboxypeptidase Y is retained in the ER and lacks disulfide bonds, as shown by thiol modification with AMS. ERO1 apparently determines cellular oxidizing capacity since mutation of ERO1 causes hypersensitivity to the reductant DTT, whereas overexpression of ERO1 confers resistance to DTT. Moreover, the oxidant diamide can restore growth and secretion in ero1 mutants. Genetic tests distinguish the essential function of ERO1 from that of PDI1. We show that glutathione is not required for CPY folding and conclude that Ero1p functions in a novel mechanism that sustains the ER oxidizing potential, supporting net formation of protein disulfide bonds.
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