page_title: ATM serine/threonine kinase
section_title: Function
hierarchical_section_title: ATM serine/threonine kinase / Function
caption_attribution_description: English: ATM-mediated two-step response to DNA double strand breaks. In the rapid response activated ATM phosphorylates effector kinase CHK2 which phosphorylates CDC25A, targeting it for ubiquitination and degradation. Therefore phosphorylated CDK2-Cyclin accumulates and progression through the cell cycle is blocked. In the delayed response ATM phosphorylates the inhibitor of p53, MDM2, and p53 which is also phosphorylated by Chk2. The resulting activation and stabilization of p53 leads to an increased expression of Cdk inhibitor p21, which further helps to keep Cdk activity low and to maintain long-term cell cycle arrest.
context_page_description: ATM serine/threonine kinase, symbol ATM, is a serine/threonine protein kinase that is recruited and activated by DNA double-strand breaks. It phosphorylates several key proteins that initiate activation of the DNA damage checkpoint, leading to cell cycle arrest, DNA repair or apoptosis. Several of these targets, including p53, CHK2, BRCA1, NBS1 and H2AX are tumor suppressors.
  In 1995, the gene was discovered by Dr. Yosef Shiloh who named its product ATM since he found that its mutations are responsible for the disorder ataxia–telangiectasia. In 1998, the Shiloh and Kastan laboratories independently showed that ATM is a protein kinase whose activity is enhanced by DNA damage.
context_section_description: A complex of the three proteins MRE11, RAD50 and NBS1 (XRS2 in yeast), called the MRN complex in humans, recruits ATM to double strand breaks (DSBs) and holds the two ends together. ATM directly interacts with the NBS1 subunit and phosphorylates the histone variant H2AX on Ser139. This phosphorylation generates binding sites for adaptor proteins with a BRCT domain. These adaptor proteins then recruit different factors including the effector protein kinase CHK2 and the tumor suppressor p53. The ATM-mediated DNA damage response consists of a rapid and a delayed response. The effector kinase CHK2 is phosphorylated and thereby activated by ATM. Activated CHK2 phosphorylates phosphatase CDC25A, which is degraded thereupon and can no longer dephosphorylate CDK1-cyclin B, resulting in cell-cycle arrest. If the DSB can not be repaired during this rapid response, ATM additionally phosphorylates MDM2 and p53 at Ser15. p53 is also phosphorylated by the effector kinase CHK2. These phosphorylation events lead to stabilization and activation of p53 and subsequent transcription of numerous p53 target genes including CDK inhibitor p21 which lead to long-term cell-cycle arrest or even apoptosis.
  The protein kinase ATM may also be involved in mitochondrial homeostasis, as a regulator of mitochondrial autophagy (mitophagy) whereby old, dysfunctional mitochondria are removed. Increased ATM activity also occurs in viral infection where ATM is activated early during dengue virus infection as part of autophagy induction and ER stress response.
caption: ATM-mediated two-step response to DNA double strand breaks. In the rapid response activated ATM phosphorylates effector kinase CHK2 which phosphorylates CDC25A, targeting it for ubiquitination and degradation. Therefore, phosphorylated CDK2-Cyclin accumulates and progression through the cell cycle is blocked. In the delayed response ATM phosphorylates the inhibitor of p53, MDM2, and p53, which is also phosphorylated by Chk2. The resulting activation and stabilization of p53 leads to an increased expression of Cdk inhibitor p21, which further helps to keep Cdk activity low and to maintain long-term cell cycle arrest.[14]
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