Casein kinase II phosphorylates the C-terminal region of Lif1 to promote the Lif1-Xrs2 interaction needed for non-homologous end joining.

A DNA double strand break (DSB) is one of the most cytotoxic DNA lesions, but it can be repaired by non-homologous end joining (NHEJ) or by homologous recombination. The choice between these two repair pathways depends on the cell cycle stage. Although NHEJ constitutes a simple re-ligation reaction, the regulatory mechanism(s) controlling its activity has not been fully characterized. Lif1 is a regulatory subunit of the NHEJ-specific DNA ligase IV and interacts with Xrs2 of the MRX complex which is a key factor in DSB repair. Specifically, the C-terminal region of Lif1, which contains a CK2-specific phosphorylation motif, interacts with the FHA domain of Xrs2 during canonical- NHEJ (C-NHEJ). Herein, we show that Lif1 and Cka2, a catalytic subunit of yeast CK2, interact and that the C-terminal phosphorylation consensus motif in Lif1 is phosphorylated by recombinant CK2. These observations suggest that phosphorylation of Lif1 by CK2 at a DSB site promotes the Lif1-Xrs2 interaction and facilitates C-NHEJ.