Unwind and slow down: checkpoint activation by helicase and polymerase uncoupling
- Department of Biochemistry, Vanderbilt University, Nashville, Tennessee 37232, USA
This extract was created in the absence of an abstract.
Phosphoinositide 3-kinase related protein kinases (PIKK) including ataxia-telangiectasia mutated (ATM), DNA-dependent protein kinase (DNA-PK), and ATM and Rad3-related (ATR) coordinate cellular responses to DNA damage. DNA-PK and ATM are primarily activated by double-strand breaks. The ATR kinase, in contrast, responds to numerous forms of genotoxic stress including intrastrand cross-links, oxidative damage, and polymerase toxins. At first glance, the disparate DNA structures that activate ATR impose a difficult biochemical challenge for damage sensing. However, recent data indicate that one DNA structure—single-stranded DNA (ssDNA) coated with a single-stranded DNA-binding protein (RPA)—is a common intermediate to activate ATR signaling in response to all of these genotoxic lesions. In this issue, Cimprich and colleagues (Byun et al. 2005) define how DNA damage and polymerase toxins use a common mechanism to generate the RPA–ssDNA needed to activate ATR. The method is the decoupling of helicase and polymerase activities at a replication fork.
Linking RPA-coated ssDNA to checkpoint kinase activation
The ATM and ATR protein kinases share many biochemical and functional similarities. Both are large protein kinases with significant sequence homology and a strong preference to phosphorylate serine or threonine residues followed by glutamine. Both target an overlapping set of substrates that promote cell cycle arrest. However, ATR is essential for the viability of replicating human and mouse cells while ATM is not (Brown and Baltimore 2000; Cortez et al. 2001). ATM functions in response to rare occurrences of double-strand breaks. ATR is activated during every S phase to regulate the spatial and temporal firing of replication origins, the repair of damaged replication forks, and to prevent the premature onset of mitosis (Nyberg et al. 2002; Shechter et al. 2004b). ATM mutations predispose carriers to develop cancer and are found in ∼1% of people. ATR mutations are rare and probably only compatible with viability when heterozygous or hypomorphic. While the only …
