The Human RNA Surveillance Factor UPF1 Is Required for S Phase Progression and Genome Stability

SummaryThe eukaryotic nonsense-mediated mRNA decay (NMD) pathway degrades mRNAs carrying premature stop codons (PTC). In humans, NMD depends on the RNA- and DNA-dependent 5′-3′ helicase UPF1 and six other gene products referred to as SMG1, UPF2, UPF3, EST1A/SMG6, EST1B/SMG5, and EST1C/SMG7. The NMD machinery is also thought to coordinate mRNA nuclear export and translation and to regulate the levels of several physiologic transcripts 1, 2, 3, 4, 5, 6 and 7. Furthermore, in a process named SMD, UPF1 promotes degradation of mRNAs that are bound by Staufen 1 [8]. Intriguingly, SMG1 and EST1A/SMG6 function also in DNA repair and telomere maintenance, respectively 9, 10 and 11. Here, we show that UPF1 is also required for genome stability. shRNA-mediated depletion of UPF1 causes human cells to arrest early in S phase, inducing an ATR-dependent DNA-damage response. A fraction of hyperphosphorylated UPF1 associates with chromatin of unperturbed cells, and chromatin association increases in S phase and upon γ irradiation. ATR phosphorylates UPF1 both in vitro and in vivo, and shRNA-mediated downregulation of ATR diminished the association of UPF1 with chromatin, although it did not affect NMD. Physical interaction of UPF1 with DNA polymerase δ suggests a role for human UPF1 in DNA synthesis during replication or repair.