Structure of the human RAD17-RFC clamp loader and 9-1-1 checkpoint clamp bound to a dsDNA-ssDNA junction

The RAD9-RAD1-HUS1 (9-1-1) clamp forms one half of the DNA damage checkpoint system that signals the presence of substantial regions of single-stranded DNA arising from replication fork collapse or resection of DNA double strand breaks. Loaded at the 5 '-recessed end of a dsDNA-ssDNA junction by the RAD17-RFC clamp loader complex, the phosphorylated C-terminal tail of the RAD9 subunit of 9-1-1 engages with the mediator scaffold TOPBP1 which in turn activates the ATR kinase, localised through the interaction of its constitutive partner ATRIP with RPA-coated ssDNA. Using cryogenic electron microscopy (cryoEM) we have determined the structure of a complex of the human RAD17-RFC clamp loader bound to human 9-1-1, engaged with a dsDNA-ssDNA junction. The structure answers the key questions of how RAD17 confers specificity for 9-1-1 over PCNA, and how the clamp loader specifically recognises the recessed 5 ' DNA end and fixes the orientation of 9-1-1 on the ssDNA.

Automated summary

The RAD9-RAD1-HUS1 (9-1-1) clamp is part of the DNA damage checkpoint system that can detect large regions of single-stranded DNA caused by replication fork collapse or DNA double-strand breaks. A study using cryogenic electron microscopy reveals the structure of the human RAD17-RFC clamp loader complex bound to human 9-1-1 at a dsDNA-ssDNA junction, providing insight into how RAD17 confers specificity for 9-1-1 and how the clamp loader recognizes the recessed 5' DNA end and orients 9-1-1 on the ssDNA.