Cytoplasmic proteotoxicity regulates HRI-dependent phosphorylation of eIF2α via the Hsp70-Bag3 module

The major heat shock protein Hsp70 forms a complex with a scaffold protein Bag3 that links it to components of multiple signaling pathways. Via these interactions, the Hsp70-Bag3 module functions as a proteotoxicity sensor that controls cell signaling. Here, in a search for signaling pathways regulated by the complex as a tool we utilized JG-98, an allosteric inhibitor of Hsp70 that blocks its interaction with Bag3. Gene expression profiling followed by the pathway analysis indicated that a set of signaling pathways including UPR was activated by JG-98. Surprisingly, only the eIF2a-associated branch of the UPR was activated under these conditions, while other UPR branches including induction of ER chaperones were not induced, suggesting that the response was not related to the ER proteotoxicity and ER-associated kinase PERK1. Indeed, induction of the UPR genes under these conditions was dependent on activation of a distinct cytoplasmic eIF2a kinase HRI. Hsp70-Bag3 complex directly interacted with HRI and regulated phosphorylation of eIF2a upon cytoplasmic proteotoxicity. Therefore, activation of certain UPR genes can be triggered by cytosolic proteotoxicity via Hsp70-Bag3-HRI-eIF2a axis.