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. Recruited in complex with protein LCD1 by the single-strand-binding protein complex RPA to DNA lesions in order to initiate the DNA repair by homologous recombination, after the MRX-complex and TEL1 are displaced. Phosphorylates LCD1 and RPA2, a subunit of RPA, involved in DNA replication, repair and recombination. Phosphorylates RAD9, CHK1 and RAD53, which leads to the activation of the CHK1 and RAD53 kinases involved in DNA damage repair cascade. Phosphorylates histone H2A to form H2AS128ph (gamma-H2A) at sites of DNA damage, also involved in the regulation of DNA damage response mechanism. Phosphorylates also SLX4 and RTT107 which are proteins involved in genome stability. Required for cell growth and meiotic recombination. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with LCD1, which is required for localization MEC1 to the RPA complex. Interacts directly with the RPA subunits RFA1 and RFA2. Localizes to nuclear DNA repair foci in response to DNA double strand breaks. The recruitment to DNA lesion sites requires its interaction with LCD1 and the presence of the RPA complex on DNA. Induced during meiosis. Belongs to the PI3/PI4-kinase family. ATM subfamily.