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The yeast Xrs2 complex functions in S phase checkpoint regulation
1Wellcome Trust and Cancer Research Campaign Institute of Cancer and Developmental Biology, and Department of Zoology, University of Cambridge, CB2 1QR Cambridge, UK.
Abstract:
The Nbs1 complex is an evolutionarily conserved multisubunit nuclease composed of the Mre11, Rad50, and Nbs1 proteins. Hypomorphic mutations in the NBS1 or MRE11 genes in humans result in conditions characterized by DNA damage sensitivity, cell cycle checkpoint deficiency, and high cancer incidence. The equivalent complex in the yeast Saccharomyces cerevisiae (Xrs2p complex) has been implicated in DNA double-strand break repair and in telomere length regulation. Here, we find that xrs2Delta, mre11Delta, and rad50Delta mutants are markedly defective in the initiation of the intra-S phase checkpoint in response to DNA damage. Furthermore, the absence of a functional Xrs2p complex leads to sensitivity to deoxynucleotide depletion and to an inability to efficiently slow down cell cycle progression in response to hydroxyurea. The checkpoint appears to require the nuclease activity of Mre11p and its defect is associated with the abrogation of the Tel1p/Mec1p signaling pathway. Notably, DNA damage induces phosphorylation of both Xrs2p and Mre11p in a Tel1p-dependent manner. These results indicate that the Tel1p/ATM signaling pathway is conserved from yeast to humans and suggest that the Xrs2p/Nbs1 complexes act as signal modifiers.
Insights
The Nbs1/Xrs2 complex is crucial for DNA damage response and cell cycle control. Its nuclease activity and interaction with Tel1p/ATM signaling are vital for checkpoint activation in yeast and humans.
Area of Science:
- Molecular Biology
- Cell Biology
- Genetics
Background:
- The Nbs1 complex (Mre11, Rad50, Nbs1 proteins) is essential for DNA repair and genome stability.
- Mutations in NBS1 or MRE11 cause DNA damage sensitivity, checkpoint defects, and cancer in humans.
- The yeast homolog, Xrs2p complex, is involved in DNA double-strand break repair and telomere length regulation.
Purpose of the Study:
- To investigate the role of the Xrs2p complex in the intra-S phase DNA damage checkpoint in Saccharomyces cerevisiae.
- To elucidate the functional relationship between the Xrs2p complex, Mre11p nuclease activity, and the Tel1p/Mec1p signaling pathway.
Main Methods:
- Analysis of xrs2Δ, mre11Δ, and rad50Δ yeast mutants.
- Assessment of checkpoint activation and cell cycle progression in response to DNA damaging agents (e.g., hydroxyurea).
- Investigation of protein phosphorylation and signaling pathway involvement.
Main Results:
- Xrs2p complex mutants exhibit severe defects in initiating the intra-S phase checkpoint upon DNA damage.
- Absence of functional Xrs2p complex leads to sensitivity to deoxynucleotide depletion and impaired hydroxyurea response.
- Checkpoint function requires Mre11p nuclease activity and is linked to Tel1p/Mec1p pathway abrogation.
- DNA damage induces Tel1p-dependent phosphorylation of Xrs2p and Mre11p.
Conclusions:
- The Xrs2p/Nbs1 complex is critical for DNA damage-induced intra-S phase checkpoint activation.
- The Tel1p/ATM signaling pathway is conserved and functions through the Xrs2p/Nbs1 complex in yeast.
- The Xrs2p/Nbs1 complex acts as a signal modifier within the DNA damage response pathway.