ATM's leucine-rich domain and adjacent sequences are essential for ATM to regulate the DNA damage response

The ATM protein kinase regulates the DNA damage response by phosphorylating proteins involved in cell cycle checkpoints and DNA repair. We report here on the function of the predicted leucine zipper (LZ) motif, and sequences adjacent to this, in regulating ATM activity. The predicted LZ sequence was deleted from ATM, generating ATMDeltaLZ, and expressed in an ATM-negative AT cell line. ATM increased cell survival following exposure to ionizing radiation, whereas expression of ATMDeltaLZ failed to increase cell survival. ATMDeltaLZ retained in vitro kinase activity, but was unable to phosphorylate p53 in vivo. Leucine zippers mediate homo- and heterodimerization of proteins. However, the predicted LZ of ATM did not mediate the formation of ATM dimers. We examined if the predicted LZ of ATM was a dominant-negative inhibitor of ATM function in SW480 cells. Expression of amino acids 769-1436 of ATM, including the predicted LZ, sensitized SW480 cells to ionizing radiation, but did not inhibit ATM's kinase activity or its ability to phosphorylate Brca1. Further, this dominant-negative activity was not dependent on the predicted LZ domain. The central region of the ATM protein therefore contains multiple sequences which regulate cell survival following DNA damage.