Cdc5L interacts with ATR and is required for the S‐phase cell‐cycle checkpoint

Cell division cycle 5‐like protein (Cdc5L) is a core component of the putative E3 ubiquitin ligase complex containing Prp19/Pso4, Plrg1 and Spf27. This complex has been shown to have a role in pre‐messenger RNA splicing from yeast to humans; however, more recent studies have described a function for this complex in the cellular response to DNA damage. Here, we show that Cdc5L interacts physically with the cell‐cycle checkpoint kinase ataxia‐telangiectasia and Rad3‐related (ATR). Depletion of Cdc5L by RNA‐mediated interference methods results in a defective S‐phase cell‐cycle checkpoint and cellular sensitivity in response to replication‐fork blocking agents. Furthermore, we show that Cdc5L is required for the activation of downstream effectors or mediators of ATR checkpoint function such as checkpoint kinase 1 (Chk1), cell cycle checkpoint protein Rad 17 (Rad17) and Fanconi anaemia complementation group D2 protein (FancD2). In addition, we have mapped the ATR‐binding region in Cdc5L and show that a deletion mutant that is unable to interact with ATR is defective in the rescue of the checkpoint deficiency in Cdc5L‐depleted cells. These findings show a new function for Cdc5L in the regulation of the ATR‐mediated cell‐cycle checkpoint in response to genotoxic agents.