Molecular insights into human Shieldin complex assembly and recruitment to DSBs

The Shieldin complex represses end resection at DNA double-strand breaks (DSBs) and thereby serves as a pro-non homologous end joining (NHEJ) factor in the G1 phase of the cell cycle. Its components SHLD1, SHLD2, SHLD3 and REV7 are recruited in a hierarchical fashion. SHLD3 and REV7 localize first to DSBs, while the subsequently recruited SHLD2 is the only known DNA binding protein in the complex. The molecular details of the initial recruitment of SHLD3 and REV7, and the subsequent assembly of Shieldin on DSBs are unclear. Here, we report the identification of a promiscuous DNA binding domain in the C-terminal half of SHLD3. At the N-terminus, SHLD3 interacts with a dimer of REV7 molecules. We show that the interaction between SHLD3 and the first REV7 is remarkably slow, which is likely due to the substantial activation energy required to remodel mobile structural elements within the REV7 molecule to allow for binding to SHLD3. In contrast, the interaction between SHLD3 and SHLD2 with a second REV7 molecule is fast and does not require structural remodelling. Overall, these results provide insights into the rate-limiting step of the molecular assembly and recruitment of Shieldin complex at DNA DSBs.