Rap80 Protein Recruitment to DNA Double-strand Breaks Requires Binding to Both Small Ubiquitin-like Modifier (SUMO) and Ubiquitin Conjugates

Ubiquitin (Ub) modifications at sites of DNA double-strand breaks (DSBs) play critical roles in the assembly of signaling and repair proteins. The Ub-interacting motif (UIM) domain of Rap80, which is a component of the BRCA1-A complex, interacts with Ub Lys-63 linkage conjugates and mediates the recruitment of BRCA1 to DSBs. Small ubiquitin-like modifier (SUMO) conjugation also occurs at DSBs and promotes Ub-dependent recruitment of BRCA1, but its molecular basis is not clear. In this study, we identified that Rap80 possesses a SUMO-interacting motif (SIM), capable of binding specifically to SUMO2/3 conjugates, and forms a tandem SIM-UIM-UIM motif at its N terminus. The SIM-UIM-UIM motif binds to both Ub Lys-63 linkage and SUMO2 conjugates. Both the SIM and UIM domains are required for efficient recruitment of Rap80 to DSBs immediately after damage and confer cellular resistance to ionizing radiation. These findings propose a model in which SUMO and Ub modification is coordinated to recruit Rap80 and BRCA1 to DNA damage sites.