Endoplasmic reticulum stress compromises the ubiquitin–proteasome system

The presence of endoplasmic reticulum (ER) stress and impaired ubiquitin–proteasome system (UPS) activity has been independently implicated in the pathophysiology of conformational diseases. Here, we reveal a link between ER stress and the functionality of the UPS. Treatment of cells with different ER stressors delayed the degradation of an ER reporter substrate and caused a subtle but consistent accumulation of three independent nuclear/cytosolic UPS reporter substrates. A similar signature increase was observed upon induction of ER stress in transgenic mice expressing a reporter substrate. Cells undergoing ER stress failed to clear efficiently UBB+1, an aberrant ubiquitin found in conformational diseases, which in turn caused general impairment of the UPS. We conclude that ER stress has a general inhibitory effect on the UPS. The compromised UPS during ER stress may explain the long-term gradual accumulation of misfolded proteins as well as the selective vulnerability of particular cell populations in conformational diseases.