MDC1 is ubiquitylated on its tandem BRCT domain and directly binds RAP80 in a UBC13-dependent manner

The cellular response to DNA damage is essential for maintenance of genomic stability. MDC1 is a key member of the DNA damage response. It is an adaptor protein that binds and recruits proteins to sites of DNA damage, a crucial step for a proper response. MDC1 contains several protein-protein interacting modules, including a tandem BRCT domain that mediates various interactions involving MDC1. Here we demonstrate that MDC1 binds directly to RAP80, which is a DNA damage response protein that recruits BRCA1 to sites of damage. The interaction between MDC1 and RAP80 requires the tandem BRCT domain of MDC1 and the ubiquitin-interacting motifs of RAP80. Moreover, the interaction depends on UBC13, an E2 ubiquitin ligase that catalyzes K63-linked poly-ubiquitin chain formation. The results highly propose that the interaction between MDC1 and RAP80 depends on a ubiquitylation event, which we found to take place on K-1977 of MDC1. This study provides the first evidence that interactions involving MDC1 can be regulated by ubiquitylation.