In response to DNA damage, the kinase ATR is recruited to single-stranded DNA, where it activates the cell-cycle-checkpoint kinase Chk1 – in turn, Chk1 regulates cell-cycle arrest and other aspects of the DNA damage response. Along with ATR, the protein complexes Rad17-RFC and 9-1-1 are both known to be important for Chk1 phosphorylation – Rad17-RFC recruits 9-1-1 to single-stranded DNA and is phosphorylated by ATR, whereas 9-1-1 recruits TopBP1, a protein that phosphorylates ATR. Less is known, however, about how the DNA damage response is propagated and maintained, and this issue is now addressed by Nick Lakin, Veronique Smits and colleagues (). In cultured cells, the authors report, genotoxic stress results in the immobilisation of Rad9 (a 9-1-1 component) in nuclear foci; this localisation is Rad17-dependent. Cells expressing a mutated Rad17 (Rad17AA) that lacks both ATR phosphorylation sites form fewer Rad9 foci in response to stress, as do cells in which ATR is knocked down. By using FLIP and FRAP to test the retention time of Rad9 at nuclear foci, the authors show that Rad9 is more dynamic when ATR is depleted or Rad17AA is expressed. Thus, ATR and Rad17 might act together to stabilise the DNA damage response, modulating the retention – rather than the recruitment – of Rad9 at sites of DNA damage.
