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Phosphorylation of Mcm2 modulates Mcm2-7 activity and affects the cell's response to DNA damage.

ABSTRACT: The S-phase kinase, DDK controls DNA replication through phosphorylation of the replicative helicase, Mcm2-7. We show that phosphorylation of Mcm2 at S164 and S170 is not essential for viability. However, the relevance of Mcm2 phosphorylation is demonstrated by the sensitivity of a strain containing alanine at these positions (mcm2(AA)) to methyl methanesulfonate (MMS) and caffeine. Consistent with a role for Mcm2 phosphorylation in response to DNA damage, the mcm2(AA) strain accumulates more RPA foci than wild type. An allele with the phosphomimetic mutations S164E and S170E (mcm2(EE)) suppresses the MMS and caffeine sensitivity caused by deficiencies in DDK function. In vitro, phosphorylation of Mcm2 or Mcm2(EE) reduces the helicase activity of Mcm2-7 while increasing DNA binding. The reduced helicase activity likely results from the increased DNA binding since relaxing DNA binding with salt restores helicase activity. The finding that the ATP site mutant mcm2(K549R) has higher DNA binding and less ATPase than mcm2(EE), but like mcm2(AA) results in drug sensitivity, supports a model whereby a specific range of Mcm2-7 activity is required in response to MMS and caffeine. We propose that phosphorylation of Mcm2 fine-tunes the activity of Mcm2-7, which in turn modulates DNA replication in response to DNA damage.

SUBMITTER: Stead BE

PROVIDER: S-EPMC3167627 | biostudies-literature | 2011 Sep

REPOSITORIES: biostudies-literature

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Phosphorylation of Mcm2 modulates Mcm2-7 activity and affects the cell's response to DNA damage.

Stead Brent E BE Brandl Christopher J CJ Davey Megan J MJ

Nucleic acids research 20110519 16

The S-phase kinase, DDK controls DNA replication through phosphorylation of the replicative helicase, Mcm2-7. We show that phosphorylation of Mcm2 at S164 and S170 is not essential for viability. However, the relevance of Mcm2 phosphorylation is demonstrated by the sensitivity of a strain containing alanine at these positions (mcm2(AA)) to methyl methanesulfonate (MMS) and caffeine. Consistent with a role for Mcm2 phosphorylation in response to DNA damage, the mcm2(AA) strain accumulates more RP ...[more]

PMID: 21596784

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Project description:Minichromosome maintenance proteins (Mcm's) are components of the DNA replication licensing complex. In vivo, reduced expression or activity of Mcm's has been shown to result in highly penetrant early onset cancers (Shima et al., 2007; Pruitt et al., 2007) and stem cell deficiencies (Pruitt et al., 2007). Here we use mouse embryonic fibroblasts from an Mcm2-deficient strain of mice to show by DNA fiber analysis that origin usage is decreased in Mcm2-deficient cells under conditions of hydroxyurea (HU)-mediated replication stress. DNA damage responses (DDRs) resulting from HU and additional replication-dependent and replication-independent genotoxic agents were also examined and shown to function at wild-type (wt) levels. Further, basal levels of many components of the DDR were expressed at wt levels, showing that there is no acute replicative stress under normal growth conditions. Only very modest, 1.5- to 2-fold increases in the basal levels of gamma-H2AX, p21(cip1) and 53bp foci were found, consistent with a slight chronic elevation in DDR pathways. The one condition in which a larger difference between wt- and Mcm2-deficient cells was found occurred after ultraviolet irradiation and may reflect the role of Chk1-mediated suppression of dormant origins. In vivo, abrogating p53-mediated DDR in Mcm2-deficient mice results in increased embryonic lethality and accelerated cancer formation in surviving mice. Further, p53 mutation rescues the negative effect of Mcm2 deficiency on the survival of neural stem cells in vitro; however, the enhanced survival correlates with increased genetic damage relative to Mcm2 wt cells carrying the p53 mutation. Together these results show that even relatively minor perturbations to primary or dormant replication origin usage contribute to accelerated genetic damage in vivo. In addition, these studies show that tumor types resulting from Mcm2 deficiency are strongly affected by interaction with both genetic background and p53.

Project description:BackgroundThe replicative helicase in eukaryotic cells is comprised of minichromosome maintenance (Mcm) proteins 2 through 7 (Mcm2-7) and is a key target for regulation of cell proliferation. In addition, it is regulated in response to replicative stress. One of the protein kinases that targets Mcm2-7 is the Dbf4-dependent kinase Cdc7 (DDK). In a previous study, we showed that alanine mutations of the DDK phosphorylation sites at S164 and S170 in Saccharomyces cerevisiae Mcm2 result in sensitivity to caffeine and methyl methanesulfonate (MMS) leading us to suggest that DDK phosphorylation of Mcm2 is required in response to replicative stress.ResultsWe show here that a strain with the mcm2 allele lacking DDK phosphorylation sites (mcm2AA) is also sensitive to the ribonucleotide reductase inhibitor, hydroxyurea (HU) and to the base analogue 5-fluorouracil (5-FU) but not the radiomimetic drug, phleomycin. We screened the budding yeast non-essential deletion collection for synthetic lethal interactions with mcm2AA and isolated deletions that include genes involved in the control of genome integrity and oxidative stress. In addition, the spontaneous mutation rate, as measured by mutations in CAN1, was increased in the mcm2AA strain compared to wild type, whereas with a phosphomimetic allele (mcm2EE) the mutation rate was decreased. These results led to the idea that the mcm2AA strain is unable to respond properly to DNA damage. We examined this by screening the deletion collection for suppressors of the caffeine sensitivity of mcm2AA. Deletions that decrease spontaneous DNA damage, increase homologous recombination or slow replication forks were isolated. Many of the suppressors of caffeine sensitivity suppressed other phenotypes of mcm2AA including sensitivity to genotoxic drugs, the increased frequency of cells with RPA foci and the increased mutation rate.ConclusionsTogether these observations point to a role for DDK-mediated phosphorylation of Mcm2 in the response to replicative stress, including some forms of DNA damage. We suggest that phosphorylation of Mcm2 modulates Mcm2-7 activity resulting in the stabilization of replication forks in response to replicative stress.

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Phosphorylation of Mcm2 modulates Mcm2-7 activity and affects the cell's response to DNA damage.

Publications

Phosphorylation of Mcm2 modulates Mcm2-7 activity and affects the cell's response to DNA damage.

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