Purification and DNA binding properties of the ataxia-telangiectasia gene product ATM

The human neurodegenerative and cancer predisposition condition ataxia-tela ngiectasia is characterized at the cellular level by radiosensitivity, chro mosomal instability, and impaired induction of ionizing radiation-induced c ell cycle checkpoint controls. Recent work has revealed that the gene defec tive in ataxia-telangiectasia, termed ATM, encodes an approximate to 350-kD a polypeptide, ATM, that is a member of the phosphatidylinositol 3-kinase f amily. We show that ATM binds DNA and exploit this to purify ATM to near ho mogeneity. Atomic force microscopy reveals that ATM exists in two populatio ns, with sizes consistent with monomeric and tetrameric states. Atomic forc e microscopy analyses also show that ATR I binds preferentially to DNA ends . This property is similar to that displayed by the DNA-dependent protein k inase catalytic subunit, a phosphatidylinositol 3-kinase family member that functions in DNA damage detection in conjunction with the DNA end binding protein Ku. Furthermore, purified ATM contains a kinase activity that phosp horylates serine-15 of p53 in a DNA-stimulated manner. These results provid e a biochemical assay system for ATM, support genetic data indicating disti nct roles for DNA-dependent protein kinase and ATM, and suggest how ATM may signal the presence of DNA damage to p53 and other downstream effectors.