Cytoplasmic proteotoxicity regulates HRI-dependent phosphorylation of eIF2 via the Hsp70-Bag3 module

Themajor heat shock protein Hsp70 forms a complexwith a scaffold protein Bag3 that links it to components of signaling pathways. Via these interactions, the Hsp70-Bag3 module functions as a proteotoxicity sensor that controls cell signaling. Here, to search for pathways regulated by the complex, we utilized JG- 98, an allosteric inhibitor of Hsp70 that blocks its interaction with Bag3. RNAseq followed by the pathway analysis indicated that several signaling path-ways including UPR were activated by JG-98. Surprisingly, only the eIF2a-associated branch of the UPR was activated, while other UPR branches were not induced, suggesting that the response was unrelated to the ER proteotoxicity and ER-associated kinase PERK1. Indeed, induction of the UPR genes under these conditions was driven by a distinct eIF2 alpha kinase HRI. Hsp70-Bag3 directly interacted with HRI and regulated eIF2 alpha phosphorylation upon cytoplasmic proteotoxicity. Therefore, cytosolic proteotoxicity can activate certain UPR genes via Hsp70-Bag3-HRI-eIF2 alpha axis.