ATR, a PI3K-like protein kinase, plays a key role in regulating DNA damage responses. Its nuclear checkpoint kinase function is well documented, but little is known about its function outside the nucleus. Here we report that ATR has an antiapoptotic activity at mitochondria in response to UV damage, and this activity is independent of its hallmark checkpoint/kinase activity and partner ATRIP. ATR contains a BH3-like domain that allows ATR-tBid interaction at mitochondria, suppressing cytochrome c release and apoptosis. This mitochondrial activity of ATR is downregulated by Pin1 that isomerizes ATR from cis-isomer to trans-isomer at the phosphorylated Ser428-Pro429 motif. However, UV inactivates Pin1 via DAPK1, stabilizing the pro-survival cis-isomeric ATR. In contrast, nuclear ATR remains in the trans-isoform disregarding UV. This cytoplasmic response of ATR may provide a mechanism for the observed antiapoptotic role of ATR in suppressing carcinogenesis and its inhibition in sensitizing anticancer agents for killing of cancer cells.
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