Activation of p53 transcriptional activity requires ATM's kinase domain and multiple N-terminal serine residues of p53

The ATM protein kinase regulates the cell's response to DNA damage by regul ating cell cycle checkpoints and DNA repair. ATM phosphorylates several pro teins involved in the DNA-damage response, including p53. We have examined the mechanism by which ATM regulates p53's transcriptional activity. Here, we demonstrate that reintroduction of ATM into AT cells restores the activa tion of p53 by the radio-mimetic agent bleomycin. Further, p53 activation i s lost when a kinase inactive ATM is used. or if the N-terminal of ATM is d eleted. In addition, AT cells stably expressing ATM showed decreased sensit ivity to Ionizing Radiation-induced cell killing, whereas cells expressing kinase inactive ATM or N-terminally deleted ATM were indistinguishable from AT cells. Finally, single point-mutations of serines 15, 20, 33 or 37 did not individually block the ATM-dependent activation of p53 transcriptional activity by bleomycin. However, double mutations of either serines 15 and 2 0 or serines 33 and 37 blocked the ability of ATM to activate p53. Our resu lts indicate that the N-terminal of ATM and ATM's kinase activity are requi red for activation of p53's transcriptionl activity and restoration of norm al sensitivity to DNA damage. In addition, activation of p53 by ATM require s multiple serine residues in p53's transactivation domain.