Oxidative protein folding in eukaryotes: mechanisms and consequences

Abstract

The endoplasmic reticulum (ER) provides an environment that is highly optimized for oxidative protein folding. Rather than relying on small molecule oxidants like glutathione, it is now clear that disulfide formation is driven by a protein relay involving Ero1, a novel conserved FAD-dependent enzyme, and protein disulfide isomerase (PDI); Ero1 is oxidized by molecular oxygen and in turn acts as a specific oxidant of PDI, which then directly oxidizes disulfide bonds in folding proteins. While providing a robust driving force for disulfide formation, the use of molecular oxygen as the terminal electron acceptor can lead to oxidative stress through the production of reactive oxygen species and oxidized glutathione. How Ero1p distinguishes between the many different PDI-related proteins and how the cell minimizes the effects of oxidative damage from Ero1 remain important open questions.

Figure 1.

Schematic model of oxidative protein folding in the yeast ER. The formation of disulfide bonds in the ER is driven by Ero1p. FAD-bound Ero1p oxidizes PDI, which then subsequently oxidizes folding proteins directly. FAD-bound Ero1p then passes electrons to molecular oxygen, perhaps resulting in the production of ROS. FAD, which is synthesized in the cytosol, can readily enter the ER lumen and stimulate the activity of Ero1p. Disulfide isomerization and reduction may be performed by some of the four homologues of PDI, Eug1p, Mpd1p, Mpd2p, or Eps1p, in addition to PDI itself. Reduced glutathione (GSH) may also assist in disulfide reduction, resulting in the production of oxidized glutathione (GSSG).