Rtt107 BRCT domains act as a targeting module in the DNA damage response

Cells are constantly exposed to assaults that cause DNA damage, which must be detected and repaired to prevent genome instability. The DNA damage response is mediated by key kinases that activate various signaling pathways. In Saccharomyces cerevisiae, one of these kinases is Mec1, which phosphorylates numerous targets, including H2A and the DNA damage protein Rtt107. In addition to being phosphorylated, Rtt107 contains six BRCA1 C-terminal (BRCT) domains, which typically recognize phospho-peptides. Thus Rtt107 represented an opportunity to study complementary aspects of the phosphorylation cascades within one protein. Here we sought to describe the functional roles of the multiple BRCT domains in Rtt107. Rtt107 BRCT5/6 facilitated recruitment to sites of DNA lesions via its interaction with phosphorylated H2A. Rtt107 BRCT3/4 also contributed to Rtt107 recruitment, but BRCT3/4 was not sufficient for recruitment when BRCT5/6 was absent. Intriguingly, both mutations that affected Rtt107 recruitment also abrogated its phosphorylation. Pointing to its modular nature, replacing Rtt107 BRCT5/6 with the BRCT domains from the checkpoint protein Rad9 was able to sustain Rtt107 function. Although Rtt107 physically interacts with both the endonuclease Slx4 and the DNA replication and repair protein Dpb11, only Slx4 was dependent on Rtt107 for its recruitment to DNA lesions. Fusing Rtt107 BRCT5/6 to Slx4, which presumably allows artificial recruitment of Slx4 to DNA lesions, alleviated some phenotypes of rtt107Δ mutants, indicating the functional importance of Slx4 recruitment. Together this data revealed a key function of the Rtt107 BRCT domains for targeting of both itself and its interaction partners to DNA lesions.