Chaperone-Mediated Reflux of Secretory Proteins to the Cytosol During Endoplasmic Reticulum Stress

Abstract

Diverse perturbations to endoplasmic reticulum (ER) functions compromise the proper folding and structural maturation of secretory proteins. To study secretory pathway physiology during such \"ER stress\", we employed an ER-targeted, redox-responsive, green fluorescent protein--eroGFP--that reports on ambient changes in oxidizing potential. Here we find that diverse ER stress agents cause properly folded, ER-resident eroGFP (and other ER luminal proteins) to \"reflux\" back to the reducing environment of the cytosol as intact, folded proteins. By utilizing eroGFP in a comprehensive genetic screen in S. cerevisiae, we show that ER protein reflux during ER stress requires specific chaperones and co-chaperones residing in both the ER and the cytosol. Chaperone-mediated ER protein reflux does not require E3 ligase activity, and proceeds even more vigorously when these ER-associated degradation (ERAD) factors are crippled, suggesting that reflux may work in parallel with ERAD. In summary, chaperone-mediated ER-protein reflux may be a conserved protein quality control process that evolved to maintain secretory pathway homeostasis during ER protein-folding stress.\n\nSIGNIFICANCEApproximately one third of eukaryotic proteins are synthesized on ribosomes attached to the endoplasmic reticulum (ER) membrane. Many of these polypeptides co- or post-translationally translocate into the ER, wherein they fold and mature. An ER quality-control system proofreads these proteins by facilitating their folding and modification, while eliminating misfolded proteins through ER-associated degradation (ERAD). Yet, the fate of many secretory proteins during ER stress is not completely understood. Here, we uncovered an ER-stress induced \"protein reflux\" system that delivers intact, folded ER luminal proteins back to the cytosol without degrading them. We found that ER protein reflux works in parallel to ERAD and requires distinct ER-resident and cytosolic chaperones and co-chaperones.