Monitoring ATM kinase activity in living cells.

DNA double strand breaks (DSB) in mammalian cells result in the activation of the ATM protein kinase. This leads to phosphorylation of numerous downstream transducer and effector proteins that coordinate a cellular response including DNA repair, cell cycle arrest or apoptosis. We have developed a reporter protein that allows the measurement of ATM kinase activity in single living cells. This CFP-YFP FRET-based biosensor uses an ATM phosphorylation site and an FHA phosphospecific binding domain to produce a phosphorylation-induced change in conformation, which alters the FRET efficiency between CFP and YFP. We show that the reporter provides a measurable output in response to DSBs and is specific for ATM over ATR or DNA-PK. We expect the description of the spatiotemporal dynamics of ATM activity in living cells that this reporter provides will be helpful in providing a more detailed understanding of the DNA damage response.