Keeping it together in times of stress: checkpoint function at stalled replication forks.
ABSTRACT In this issue of Molecular Cell, De Piccoli et al. (2012) show that, contrary to current models of DNA replication checkpoint function, replication proteins remain associated with each other and with replicating DNA when replication is stressed in checkpoint-deficient cells.
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ABSTRACT: The replication of eukaryote chromosomes slows down when DNA is damaged and the proteins that work at the fork (the replisome) are known targets for the signaling pathways that mediate such responses critical for accurate genomic inheritance. However, the molecular mechanisms and details of how this response is mediated are poorly understood. In this report we show that the activity of replisome helicase, the Cdc45/MCM2-7/GINS (CMG) complex, can be inhibited by protein phosphorylation. Recombinant Drosophila melanogaster CMG can be stimulated by treatment with phosphatase whereas Chk2 but not Chk1 interferes with the helicase activity in vitro. The targets for Chk2 phosphorylation have been identified and reside in MCM subunits 3 and 4 and in the GINS protein Psf2. Interference requires a combination of modifications and we suggest that the formation of negative charges might create a surface on the helicase to allosterically affect its function. The treatment of developing fly embryos with ionizing radiation leads to hyperphosphorylation of Psf2 subunit in the active helicase complex. Taken together these data suggest that the direct modification of the CMG helicase by Chk2 is an important nexus for response to DNA damage.Proceedings of the National Academy of Sciences 08/2012; 109(33):13163-70. DOI:10.1073/pnas.1211525109 · 9.81 Impact Factor
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ABSTRACT: Homologous recombination (HR) is essential for genome integrity. Recombination proteins participate in tolerating DNA lesions that interfere with DNA replication, but can also generate toxic recombination intermediates and genetic instability when they are not properly regulated. Here, we have studied the role of the recombination proteins Rad51 and Rad52 at replication forks and replicative DNA lesions. We show that Rad52 loads Rad51 onto unperturbed replication forks, where they facilitate replication of alkylated DNA by non-repair functions. The recruitment of Rad52 and Rad51 to chromatin during DNA replication is a prerequisite for the repair of the non-DSB DNA lesions, presumably single-stranded DNA gaps, which are generated during the replication of alkylated DNA. We also show that the repair of these lesions requires CDK1 and is not coupled to the fork but rather restricted to G2/M by the replicative checkpoint. We propose a new scenario for HR where Rad52 and Rad51 are recruited to the fork to promote DNA damage tolerance by distinct and cell cycle-regulated replicative and repair functions.The EMBO Journal 04/2013; DOI:10.1038/emboj.2013.73 · 10.75 Impact Factor
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ABSTRACT: Although the DNA double-strand break (DSB) is defined as a rupture in the double-stranded DNA molecule that can occur without chemical modification in any of the constituent building blocks, it is recognized that this form is restricted to enzyme-induced DSBs. DSBs generated by physical or chemical agents can include at the break site a spectrum of base alterations (lesions). The nature and number of such chemical alterations define the complexity of the DSB and are considered putative determinants for repair pathway choice and the probability that errors will occur during this processing. As the pathways engaged in DSB processing show distinct and frequently inherent propensities for errors, pathway choice also defines the error-levels cells opt to accept. Here, we present a classification of DSBs on the basis of increasing complexity and discuss how complexity may affect processing, as well as how it may cause lethal or carcinogenic processing errors. By critically analyzing the characteristics of DSB repair pathways, we suggest that all repair pathways can in principle remove lesions clustering at the DSB but are likely to fail when they encounter clusters of DSBs that cause a local form of chromothripsis. In the same framework, we also analyze the rational of DSB repair pathway choice.Nucleic Acids Research 06/2013; 41(16). DOI:10.1093/nar/gkt556 · 9.11 Impact Factor