Biochemical basis of oxidative protein folding in the endoplasmic reticulum

The endoplasmic reticulum (ER) supports disulfide bond formation by a poorl y understood mechanism requiring protein disulfide isomerase (PDI) and ERO1 . In yeast, Ero1p-mediated oxidative folding was shown to depend on cellula r flavin adenine dinucleotide (FAD) levels but not on ubiquinone or heme an d Ero1p was shown to be a FAD-binding protein. We reconstituted efficient o xidative folding in vitro using FAD, PDI, and Ero1p. Disulfide formation pr oceeded by direct delivery of oxidizing equivalents from Ero1p to folding s ubstrates via PDI. This kinetic shuttling of oxidizing equivalents could al low the ER to support rapid disulfide formation while maintaining the abili ty to reduce and rearrange incorrect disulfide bonds.