CaMKII suppresses proteotoxicity by phosphorylating BAG3 in response to proteasomal dysfunction

Protein quality control serves as the primary defense mechanism for cells against proteotoxicity induced by proteasome dysfunction. While cells can limit the build-up of ubiquitinated misfolded proteins during proteasome inhibition, the precise mechanism is unclear. Here, we find that protein kinase Ca 2+ /Calmodulin (CaM)-dependent protein kinase II (CaMKII) maintains proteostasis during proteasome inhibition. We show that proteasome inhibition activates CaMKII, which phosphorylates B-cell lymphoma 2 (Bcl-2)-associated athanogene 3 (BAG3) at residues S173, S377, and S386. Phosphorylated BAG3 activates the heme-regulated inhibitor (HRI)- eukaryotic initiation factor-2α (eIF2α) signaling pathway, suppressing protein synthesis and the production of aggregated ubiquitinated misfolded proteins, ultimately mitigating the proteotoxic crisis. Inhibition of CaMKII exacerbates the accumulation of aggregated misfolded proteins and paraptosis induced by proteasome inhibitors. Based on these findings, we validate that combined targeting of proteasome and CaMKII accelerates tumor cell death and enhances the efficacy of proteasome inhibitors in tumor treatment. Our data unveil a new proteasomal inhibition-induced misfolded protein quality control mechanism and propose a novel therapeutic intervention for proteasome inhibitor-mediated tumor treatment. Synopsis CaMKII activates the HRI/eIF2α signaling pathway and inhibits the proteotoxicity induced by proteasome inhibition through phosphorylating BAG3. CaMKII suppresses proteotoxicity and paraptosis during proteasome inhibition. Proteasome inhibition induces CaMKII to phosphorylate BAG3 at S173/377/386. Phosphorylated BAG3 activates the HRI/eIF2α signaling pathway and inhibits the production of aggregated and misfolded proteins. CamKII inhibition enhances the anti-tumor activity of the proteasome inhibitor Bortezimib. CaMKII activates the HRI/eIF2α signaling pathway and inhibits the proteotoxicity induced by proteasome inhibition through phosphorylating BAG3.