Abstract
Genotoxic stress inhibits transcription, generating RNA–DNA hybrid intermediates that promote toxic transcription–replication conflicts (TRCs). Here, we show that genotoxic stress triggers transient Importin-α/β1-dependent nuclear import of the mitochondrial exonuclease EXD2 to resolve these conflicts and maintain genome stability. Mechanistically, once imported into the nucleus, EXD2 is recruited to elongation-arrested RNA polymerase II (RNAPII) associated with RNA–DNA hybrids, and its exonuclease activity limits their accumulation, thereby suppressing TRCs. Mutation of a putative EXD2 nuclear localization signal phenocopies disruption of nuclear import, whereas forced nuclear targeting of EXD2 bypasses it. However, sustained nuclear localization of EXD2 leads to mitotic defects, underscoring the importance of its tight spatial regulation. Consistent with a direct role for EXD2 in RNA–DNA hybrid resolution, overexpression of the ribonuclease RNaseH1 compensates for its depletion following DNA damage. Together, our findings introduce the concept of a trafficking-dependent checkpoint that transiently licenses EXD2 nuclease activity in the nucleus in response to genotoxic stress to resolve RNA–DNA hybrid intermediates at stalled RNAPII complexes Teaser Stress-triggered nuclear import of EXD2 resolves harmful RNA–DNA hybrids to protect genome stability after DNA damage.
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Abstract
The DNA damage response (DDR) preserves genome integrity by transiently halting RNA polymerase II (RNAPII)-mediated transcription during lesion repair. Here, we identify stress-induced nuclear import as a key regulator of RNA synthesis recovery (RRS). Inhibition of IMPα/β1-dependent nuclear import sensitizes cells to UV irradiation by impairing RRS, without affecting DNA repair. We further show that a fraction of the RNA/DNA exonuclease EXD2 translocates from the mitochondria to the nucleus in response to UV-induced stress. In the nucleus, EXD2 interacts with RNAPII, promotes degradation of nascent mRNA, and facilitates transcription restart. Disruption of EXD2’s nuclear localization signal mimics import inhibition, abolishing its interaction with RNAPII, blocking RRS, and reducing cell survival after UV exposure. Remarkably, forced nuclear localization of EXD2 restores RRS and UV resistance, even when nuclear import is compromised. Strikingly, persistent nuclear localization of EXD2 in the absence of stress is detrimental, causing mitotic defects. Our findings establish stress-induced IMPα/β1-dependent nuclear import as a critical DDR checkpoint, identify EXD2 as its key effector, and underscore the importance of spatial and temporal control of nuclease activity in maintaining genome stability.
Competing Interest Statement
The authors have declared no competing interest.
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