Host: The Japan Radiation Research Society
ATM is required for the slow component of DNA double strand break (DSB) repair in mammalian cells. In previous studies, we have shown that this represents the repair of DSBs located within or close to regions of heterochromatin (HC). The damage response mediator proteins including H2AX, MDC1, the Mre11/Rad50/NBS1 complex, RNF8, RNF168 and 53BP1 are required for this component of DSB repair. We have shown previously that 53BP1 is required to tether ATM at DSBs and that this promotes dense phosphorylation of KAP1, generating pKAP-1, at HC-DSBs. Here we examine how pKAP1 enables the repair of HC-DSBs. Previous studies have shown that KAP1 is a sumo ligase and undergoes autosumoylation. Significantly, the sites undergoing sumoylation are close to the S824, the ATM-dependent phosphorylation site. Sumoylated KAP-1 interacts with the sumo-interacting motif (SIM) on the larger isoform of CHD3, a component of the NURD remodelling complex. Strikingly, we show that phosphorylation of KAP-1 at S824 does not impact upon sumoylation of KAP-1 but does impair the interaction between sumoylated KAP-1 and the SIM motif on CHD3. SUMO-SIM interactions are charge dependent and we suggest that the unstructured C-terminal region of KAP1, when phosphorylated, can out compete the SUMO-SIM interaction between KAP-1 and CHD3 via a charge-dependent reaction. Loss of CHD3 causes relaxation of the heterochromatic superstructure. siRNA of CHD3 overcomes the need for ATM for DSB repair, similar to the impact of siRNA KAP1. These findings represent a novel impact of phosphorylation and provide insight into the role of ATM in promoting the repair of DSBs. Interestingly, this mechanism for relaxing HC superstructure does not necessitate any changes to the epigenetic code.
