Nuclear retention of ATM at sites of DNA double strand breaks

The ATM protein kinase mediates a rapid induction of cellular responses to DNA double strand breaks (DSBs). ATM kinase activity is enhanced immediatel y after exposure of cells to DSB-inducing agents, but no changes in its amo unt or subcellular location following that activation have been reported. W e speculated that some of the ATM molecules associate with sites of DSBs, w hile the rest of the nuclear ATM pool remains in the nucleoplasm, masking d etection of the damage-associated ATM fraction. Using detergent extraction to remove nucleoplasmic proteins, we show here that immediately following i nduction of DSBs, a fraction of the ATM pool becomes resistant to extractio n and is detected in nuclear aggregates. Colocalization of the retained ATM with the phosphorylated form of histone H2AX (gamma -H2AX) and with foci o f the Nbs1 protein suggests that ATM associates with sites of DSBs. The str iking correlation between the appearance of retained ATM and of gamma -H2AX , and the rapid association of a fraction of ATM with gamma -H2AX foci, are consistent with a major role for ATM in the early detection of DSBs and su bsequent induction of cellular responses.