Glutathione limits Ero1-dependent oxidation in the endoplasmic reticulum

Many proteins of the secretory pathway contain disulfide
bonds that are essential for structure and function.
In the endoplasmic reticulum (ER), Ero1 and Ero1
oxidize protein disulfide isomerase (PDI), which in turn
transfers oxidative equivalents to newly synthesized
cargo proteins. However, oxidation must be limited, as
some reduced PDI is necessary for disulfide isomerization
and ER-associated degradation. Here we show that
in semipermeable cells, PDI is more oxidized, disulfide
bonds are formed faster, and high molecular mass covalent
protein aggregates accumulate in the absence of
cytosol. Addition of reduced glutathione (GSH) reduces
PDI and restores normal disulfide formation rates. A
higher GSH concentration is needed to balance oxidative
folding in semipermeable cells overexpressing
Ero1, indicating that cytosolic GSH and lumenal Ero1
play antagonistic roles in controlling the ER redox.
Moreover, the overexpression of Ero1 significantly increases
the GSH content in HeLa cells. Our data demonstrate
tight connections between ER and cytosol to
guarantee redox exchange across compartments: a reducing
cytosol is important to ensure disulfide isomerization
in secretory proteins.