ATM
Ataxia telangiectasia mutated (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 and H2AX are tumor suppressors.
The protein is named for the disorder Ataxia telangiectasia caused by mutations of ATM.[1]
Contents
1 Introduction
2 Structure
3 Function
4 Regulation
5 Role in cancer
6 Interactions
7 See also
8 References
9 Further reading
10 External links
Introduction[edit]
Throughout the cell cycle the DNA is monitored for damage. Damages result from errors during replication, by-products of metabolism, general toxic drugs or ionizing radiation. The cell cycle has different DNA damage checkpoints, which inhibit the next or maintain the current cell cycle step. There are two main checkpoints, the G1/S and the G2/M, during the cell cycle, which preserve correct progression. ATM plays a role in cell cycle delay after DNA damage, especially after double-strand breaks (DSBs).[2] ATM together with NBS1 act as primary DSB sensor proteins. Different mediators, such as Mre11 and MDC1, acquire post-translational modifications which are generated by the sensor proteins. These modified mediator proteins then amplify the DNA damage signal, and transduce the signals to downstream effectors such as CHK2 and p53.
Structure[edit]
The ATM gene codes for a 350 kDa protein consisting of 3056 amino acids.[3] ATM belongs to the superfamily of Phosphatidylinositol 3-kinase-related kinases (PIKKs). The PIKK superfamily comprises six Ser/Thr-protein kinases that show a sequence similarity to phosphatidylinositol 3-kinases (PI3Ks). This protein kinase family includes amongst others ATR (ATM- and RAD3-related), DNA-PKcs (DNA-dependent protein kinase catalytic subunit) and mTOR (mammalian target of rapamycin). Characteristic for ATM are five domains. These are from N-Terminus to C-Terminus the HEAT repeat domain, the FRAP-ATM-TRRAP (FAT) domain, the kinase domain (KD), the PIKK-regulatory domain (PRD) and the FAT-C-terminal (FATC) domain. The
HEAT repeats directly bind to the C-terminus of NBS1. The FAT domain interacts with ATM's kinase domain to stabilize the C-terminus region of ATM itself. The KD domain resumes kinase activity, while the PRD and the FATC domain regulate it. Although no structure for ATM has been solved, the overall shape of ATM is very similar to DNA-PKcs and is composed of a head and a long arm that is thought to wrap around double-stranded DNA after a conformational change. The entire N-terminal domain together with the FAT domain are predict ed to adopt an α-helical structure, which was found by sequence analysis. This α-helical structure is believed to form a tertiary structure, which has a curved, tubular shape present for example in the Huntingtin protein, which also contains HEAT repeats. FATC is the C-terminal domain with a length of about 30 amino acids. It is highly conserved and consists of an α-helix followed by a sharp turn, which is stabilized by a disulfide bond.[4]
Function[edit]
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.[5] 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 CDK2-Cyclin, resulting in cell-cycle arrest. If the DS
B can not be repaired during this rapid response, ATM additionally phosphorylates MDM2 and p53 at Ser15.[6] 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.[7]
ATM-mediated two-step response to DNA double strand breaks. In the rapid response activated ATM phosphorylates effector kinase CHK2 which phophphorylates 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.[7]
The protein kinase ATM may also be involved in mitochondrial homeostasis, as a regulator of mitochondrial autophagy (mitophagy) whereby old, dysfunctional mitochondria are removed.[8] Regulation[edit]
A functional MRN complex is required for ATM activation after double strand breaks (DSBs). The complex functions upstream of ATM in mammalian cells and induces conformational changes that facilitate an increase in the affinity of ATM towards its substrates, such as CHK2 and p53.[2] Inactive ATM is present in the cells without DSBs as dimers or multimers. Upon DNA damage, ATM
