Kinases that control the cell cycle in response to DNA damage: Chk1, Chk2, and MK2

Abstract

In response to DNA damage eukaryotic cells activate cell cycle checkpoints -- complex kinase signaling networks that prevent further progression through the cell cycle. Parallel to implementing a cell cycle arrest, checkpoint signaling also mediates the recruitment of DNA repair pathways. If the extent of damage exceeds repair capacity, additional signaling cascades are activated to ensure elimination of these damaged cells. The DNA damage response has traditionally been divided into two major kinase branches. The ATM/Chk2 module is activated after DNA double strand breaks and the ATR/Chk1 pathway responds primarily to DNA single strand breaks or bulky lesions. Both pathways converge on Cdc25, a positive regulator of cell cycle progression, which is inhibited by Chk1-mediated or Chk2-mediated phosphorylation. Recently a third effector kinase complex consisting of p38MAPK and MK2 has emerged. This pathway is activated downstream of ATM and ATR in response to DNA damage. MK2 has been shown to share substrate homology with both Chk1 and Chk2. Here we will discuss recent advances in our understanding of the eukaryotic DNA damage response with emphasis on the Chk1, Chk2, and the newly emerged effector kinases p38MAPK and MK2.

Figure 1. Detection of the DSB and recruitment of DNA damage response mediators and effectors

A trimeric complex consisting of Mre11, Rad50 and Nbs1 (MRN) serves as a lesion sensor and recruits the DNA damage checkpoint kinase ATM via a direct interaction between ATM and Nbs1. ATM in turn phosphorylates histone H2AX (yielding so called γH2AX), creating a phosphoepitope that is engaged by the BRCT domain of MDC1. MDC1 itself is phosphorylated by CK2, which promotes a direct, phospho-dependent interaction between Nbs1 and MDC1. MDC1 is an ATM substrate and this phosphorylation creates a phospho-dependent binding site for the FHA domain of the E3 ubiquitin ligase RNF8. RNF8-mediated ubiquitination of γH2AX generates a binding site for direct interaction between the ubiquitin interacting motif (UIM) of Rap80 and ubiquitinated γH2AX. Rap80 serves as a scaffold protein to recruite a DNA repair complex consisting of Abraxas and BRCA1. The interaction between Abraxas and BRCA1 is phospho-dependent and requires the BRCA1 BRCT domains.

Figure 2. Activation of the DNA damage cell cycle checkpoint network results in rapid and persistent Cdk inhibition

Activation of the upstream activating PI3-Kinase-like kinases ATM and ATR triggers signaling through the cell cycle checkpoint signaling cascade. ATM activates the checkpoint effector kinase Chk2 by direct phosphorylation on Thr-68. Activation of ATR requires TopBP1 and ATR is recruited to damaged DNA via its regulatory subunit ATRIP. Efficient ATR-mediated activation of the downstream kinase Chk1 requires the presence of claspin. Both ATM and ATR appear to be required to activate the p38MAPK/MK2 effector kinase complex downstream of TAO kinases in response to DNA damage. The three effector kinases Chk1, Chk2 and MK2 are directly responsible for inhibitory phosphorylations on members of the Cdc25 family of phosphatases. Additionally, Chk1 phosphorylates and activates Wee1 kinase. Wee1 directly mediates inhibitory phosphorylation of Cdks, which are normally removed through the enzymatic activity of Cdc25 phosphatases to allow progression through the cell cycle.