The DNA damage response encompasses a complex series of signaling pathways that 
function to regulate and facilitate the repair of damaged DNA. Recent studies 
have shown that the repair of transcriptionally inactive chromatin, named 
heterochromatin, is dependent upon the phosphorylation of the co-repressor, 
Krüppel-associated box (KRAB) domain-associated protein (KAP-1), by the ataxia 
telangiectasia-mutated (ATM) kinase. Co-repressors, such as KAP-1, function to 
regulate the rigid structure of heterochromatin by recruiting histone-modifying 
enzymes, such HDAC1/2, SETDB1, and nucleosome-remodeling complexes such as CHD3. 
Here, we have characterized a phosphorylation site in the HP1-binding domain of 
KAP-1, Ser-473, which is phosphorylated by the cell cycle checkpoint kinase 
Chk2. Expression of a nonphosphorylatable S473A mutant conferred cellular 
sensitivity to DNA-damaging agents and led to defective repair of DNA 
double-strand breaks in heterochromatin. In addition, cells expressing S473A 
also displayed defective mobilization of the HP1-β chromodomain protein. The DNA 
repair defect observed in cells expressing S473A was alleviated by depletion of 
HP1-β, suggesting that phosphorylation of KAP-1 on Ser-473 promotes the 
mobilization of HP1-β from heterochromatin and subsequent DNA repair. These 
results suggest a novel mechanism of KAP-1-mediated chromatin restructuring via 
Chk2-regulated HP1-β exchange from heterochromatin, promoting DNA repair.