The yeast Xrs2 complex functions in S phase checkpoint regulation.

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.