A noncanonical cytoplasmic role for BUB1 in restraining DNA damage-induced dsRNA accumulation and viral mimicry within stress granules [RNA-seq]

BUB1 is a nuclear serine/threonine protein kinase and a vital spindle assembly checkpoint (SAC) complex member responsible for proper chromosome segregation before mitosis. Here we report that BUB1 plays an unexpected cytoplasmic role in restraining DNA damage-induced accumulation of cytoplasmic dsRNA and ensuing immune response. We showed that tumors deficient in BUB1 were sensitive to radiotherapy in a CD8+T-cell-dependent manner. Furthermore, we discovered increased immune cell infiltration accompanied by elevated type I interferon production from irradiated BUB1-deficient cells, caused by enhanced cytoplasmic dsRNA accumulation and activation of the MDA5/MAVS/IRF7 dsRNA-sensing pathway. Mechanistically, we discovered that after exposure to ionizing radiation, BUB1 underwent a robust nucleus-to-cytoplasm migration, where it bound and phosphorylated the polyA-binding protein PABPC1, which was degraded together with its associated mRNAs stored in the stress granules, thereby preventing dsRNA accumulation and activation of the innate immune response. Our study, therefore, reveals a novel biological role for BUB1 in suppressing DNA-damage-induced cytoplasmic dsRNA accumulation/viral mimicry and provides a rationale for inhibiting BUB1 to enhance cytotoxic cancer therapy immunologically.