Human CTC1 primarily functions in telomere maintenance/protection and promotes CHK1 phosphorylation in response to global replication stress
CST (CTC1-STN1-TEN1) is a heterotrimeric, RPA-like protein that binds to single stranded DNA (ssDNA) and functions in the replication of telomeric and non-telomeric DNA. Previous studies have shown that deletion of CTC1 results in decreased cell proliferation and telomeric DNA damage signaling. However, a detailed analysis of the consequences of conditional CTC1 knockout (KO) have not been fully elucidated. Here, we investigated the effects of CTC1 KO on cell cycle progression, genome-wide replication and activation of the DNA damage response. We find that CTC1 KO results in p53-mediated G2 arrest and increased apoptosis, but not genome-wide replication defects or DNA damage. Instead, the G2 arrest is dependent on the accumulation of telomeric RPA following CTC1 KO, suggesting that the primary function of CST is in telomere end protection and maintenance not genome-wide replication. However, despite increased RPA-ssDNA, global CHK1 phosphorylation was not detected in CTC1 KO cells. Further analysis revealed that CTC1 KO significantly inhibits CHK1 phosphorylation following hydroxyurea-induced replication stress, due to decreased levels of the ATR activator TopBP1. Overall, our results identify that telomere not genome-wide DNA damaging signaling leads to decrease proliferation following CTC1 deletion and that CST promotes ATR-CHK1 signaling through the regulation of TopBP1.