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

By Aeid Igbaria, Philip I Merksamer, Ala Trusina, Firehiwot Tilahun, Jefferey R Johnson, Onn Brandman, Nevan J. Krogan, Jonathan S. Weissman, Feroz R. Papa

Posted 12 Mar 2019
bioRxiv DOI: 10.1101/562306 (published DOI: 10.1073/pnas.1904516116)

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.

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