Distinct spatiotemporal dynamics of mammalian checkpoint regulators induced by DNA damage.

Cell cycle checkpoints are signal transduction pathways activated after DNA damage to protect genomic integrity. Dynamic spatiotemporal coordination is a vital, but poorly understood aspect, of these checkpoints. Here, we provide evidence for a strikingly different behaviour of Chk2 versus Nbs1, key mediators of the ataxia-telangiecatesia-mutated (ATM)-controlled checkpoint pathways induced by DNA double-strand breaks (DSBs). In live human cells with DSBs restricted to small sub-nuclear areas, Nbs1 was rapidly recruited to the damaged regions and underwent a dynamic exchange in the close vicinity of the DSB sites. In contrast, Chk2 continued to rapidly move throughout the entire nucleus, irrespective of DNA damage and including the DSB-free areas. Although phosphorylation of Chk2 by ATM occurred exclusively at the DSB sites, forced immobilization of Chk2 to spatially restricted, DSB-containing nuclear areas impaired its stimulating effect on p53-dependent transcription. These results unravel a dynamic nature of Nbs1 interaction with DSB lesions and identify Chk2 as a candidate transmitter of the checkpoint signal, allowing for a coordinated pan-nuclear response to focal DNA damage.