Threonine 68 phosphorylation by ataxia telangiectasia mutated is required for efficient activation of Chk2 in response to ionizing radiation

Eukaryotic cells activate an evolutionarily conserved set of proteins that rapidly induce cell cycle arrest to prevent replication or segregation of d amaged DNA before repair is completed. In response to ionizing radiation (I R), the cell cycle checkpoint kinase, Chk2 (Cds1), is phosphorglated and ac tivated in an ataxia telangiectasia mutated (ATM)dependent manner. sere we show that the ATM protein kinase directly phosphorylates T68 within the SQ/ TQ-rich domain of Chk2 in vitro and that T68 is phosphorylated irt vivo in response to IR in an ATM-dependent manner. Furthermore, phosphorylation of T68 was required for full activation of Chk2 after IR. Together, these data are consistent with the model that ATM directly phosphorylates Chk2 in viv o and that this event contributes to the activation of Chk2 in irradiated c ells.