MDC1 mediates Pellino recruitment to sites of DNA double-strand breaks.

Ubiquitylation is critically implicated in the recognition and repair of DNA double-strand breaks. The adaptor protein MDC1 mediates the recruitment of the key DNA damage responsive E3 ubiquitin ligase RNF8 to the break sites. It does so by directly interacting with RNF8 in a phosphorylation-dependent manner that involves the RNF8 FHA domain, thus initiating targeted chromatin ubiquitylation at the break sites. Here, we report that MDC1 also directly binds to two additional E3 ubiquitin ligases, Pellino 1 and 2, which were recently implicated in the DNA damage response. Through a combination of biochemical, biophysical and X-ray crystallographic approaches, we reveal the molecular details of the MDC1-Pellino complexes. Furthermore, we show that in mammalian cells, MDC1 mediates Pellino recruitment to sites of DNA double-strand breaks by a direct phosphorylation-dependent interaction between the two proteins. Taken together, our findings provide new molecular insights into the ubiquitylation pathways that govern genome stability maintenance.