A Shld1-Controlled POT1a Provides Support for Repression of ATR Signaling at Telomeres through RPA Exclusion

We previously proposed that POT1 prevents ATR signaling at telomeres by excluding RPA from the single-stranded TTAGGG repeats. Here, we use a Shld1-stabilized degron-POT1 a fusion (DD-POT1a) to study the telomeric AIR kinase response. In the absence of Shld1, DD-POT1 a degradation resulted in rapid and reversible activation of the ATR pathway in G1 and S/G2. AIR signaling was abrogated by shRNAs to AIR and TopBP1, but shRNAs to the ATM kinase or DNA-PKcs did not affect the telomere damage response. Importantly, ATR signaling in G1 and S/G2 was reduced by shRNAs to RPA. In S/G2, RPA was readily detectable at dysfunctional telomeres, and both POT1 a and POT1b were required to exclude RPA and prevent ATR activation. In G1, the accumulation of RPA at dysfunctional telomeres was strikingly less, and POT1 a was sufficient to repress AIR signaling. These results support an RPA exclusion model for the repression of ATR signaling at telomeres.