WD40-repeat protein WDR18 collaborates with TopBP1 to facilitate DNA damage checkpoint signaling

► The Chk1 activation domain (CAD) of TopBP1 is critical in DNA damage response. ► WD40-repeat protein WDR18 associates with TopBP1 in vitro and in vivo. ► WDR18 and TopBP1 association is required for AT70-induced Chk1 phosphorylation. ► WDR18 is required for AT70-triggered Chk1 phosphorylation. ► WDR18 associates with Chk1 in vitro.