Tousled homolog, TLK1, binds and phosphorylates Rad9; TLK1 acts as a molecular chaperone in DNA repair

Abstract

The Tousled-like kinases are involved in chromatin assembly, DNA repair, transcription, and chromosome segregation. In this work, we show that overexpression of TLK1B hastens repair of double strand breaks (DSBs) in mouse cells. We have identified Rad9 as a protein interacting tightly with TLK1B. TLK1B phosphorylates hRad9 at S328, and the significance of this phosphorylation was addressed by expressing wild-type (WT) or mutant (S328A) hRad9 in mouse Rad9-null cells. Complementation with WT or mutant Rad9 restored the ability to survive ionizing radiation and doxorubicin, but the effect was greater for the WT protein. The S328A mutation had little effect on the level of 9-1-1 complex, but evidence shows that TLK1/1B functions to modulate the amount of 9-1-1 at DSBs. Rad9 competed with the chromatin assembly factor Asf1 for binding to TLK1B. TLK1B hastened the reassembly of nucleosomes adjacent to a DSB introduced with HO nuclease. This effect was also seen in cells expressing a kinase-dead TLK1B (KD), implicating that the kinase activity is dispensable for stimulation of chromatin remodeling at DSBs. Likewise, chromatin assembly on a plasmid was stimulated by addition of either TLK1B or KD. After the induction of DSB, occupancy of Rad9 adjacent to the break increased during repair while that of Asf1 decreased, and the effect was more pronounced in TLK1B-overexpressing cells. We propose that following genotoxic stress, TLK1/1B is first recruited to the DSB in a complex with Rad9. It then exchanges with Asf1 to promote nucleosomes eviction at the DSB and access of the repair machinery to unencumbered DNA.