Serine/threonine protein kinase which activates checkpoint signaling upon genotoxic stresses such as ionizing radiation (IR), ultraviolet light (UV), or DNA replication stalling, thereby acting as a DNA damage sensor. Recognizes the substrate consensus sequence [ST]-Q. Phosphorylates histone H2A to form H2AS128ph (gamma-H2A) at sites of DNA damage, involved in the regulation of DNA damage response mechanism. Undirectly involved in the phosphorylation of rad32 which is necessary for its telomere function. Required for the control of telomere length and genome stability. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with nbs1. This interaction is required for phosphorylation of histone H2A. Localizes to nuclear DNA repair foci with other DNA repair proteins in response to DNA double strand breaks. Belongs to the PI3/PI4-kinase family. ATM subfamily.