Chaperone Complexes From The Endoplasmic Reticulum (ER) And The Cytosol Inhibit wt-p53 By Activation The ER To Cytosol Signaling.

Abstract

The Endoplasmic Reticulum (ER) is an essential sensing organelle responsible for the folding and secretion of almost one-third of eukaryotic cells total proteins. The ER contains numerous enzymes and chaperones which assist in oxidative protein folding and other posttranslational modifications. However, environmental, chemical, and genetic insults often lead to protein misfolding in the ER, accumulating misfolded proteins, altering homeostasis, and causing ER stress. Recently, we reported a novel ER surveillance mechanism by which proteins from the secretory pathway are refluxed to the cytosol to relieve the ER of its content during stress. In cancer cells, the refluxed proteins gain new pro-survival functions, thereby increasing cancer cell fitness. We termed this phenomenon ER to CYtosol Signaling (or "ERCY"). In yeast, ERCYS is regulated by HLJ1 (an ER-resident tail-anchored HSP40 cochaperone). Here, we found that in mammalian cells, HLJ1 has five putative orthologs possessing J-domains facing the cytosol. Among those, DNAJB12 and DNAJB14 appear to be the most significant. Mechanistically, we found that DNAJB12 and DNAJB14 bind the cytosolic HSC70 and its cochaperone - SGTA - through their cytosolically localized J-domains to facilitate ER-protein reflux to the cytosol. Moreover, we found that DNAJB12 is necessary and sufficient to drive this phenomenon to increase AGR2 reflux and inhibit wt-p53 during ER stress. Thus, we concluded that targeting the DNAJB12/14-HSC70/SGTA axis is a promising strategy to inhibit ERCYS and impair cancer cell fitness.