The mechanism regulating proteasomal activity under proteotoxic stress conditions remains unclear. Here, we showed that arsenite-induced proteotoxic stress resulted in upregulation of Arabidopsis homologous PUB22 and PUB23 U-box E3 ubiquitin ligases and that pub22pub23 double mutants displayed arsenite-insensitive seed germination and root growth phenotypes. PUB22/PUB23 downregulated 26S proteasome activity by promoting the dissociation of the 19S regulatory particle from the holo-proteasome complex, resulting in intracellular accumulation of UbG76V-GFP, an artificial substrate of the proteasome complex, and insoluble poly-ubiquitinated proteins. These results suggest that PUB22/PUB23 play a critical role in arsenite-induced proteotoxic stress response via negative regulation of 26S proteasome integrity.

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