Dissecting Ubiquitylation and DNA Damage Response Pathways in the Yeast Saccharomyces cerevisiae Using a Proteome-Wide Approach.

In response to genotoxic stress, cells evolved with a complex signaling network referred to as the DNA damage response (DDR). It is now well established that the DDR depends upon various posttranslational modifications; among them, ubiquitylation plays a key regulatory role. Here, we profiled ubiquitylation in response to the DNA alkylating agent methyl methanesulfonate (MMS) in the budding yeast Saccharomyces cerevisiae using quantitative proteomics. To discover new proteins ubiquitylated upon DNA replication stress, we used stable isotope labeling by amino acids in cell culture, followed by an enrichment of ubiquitylated peptides and LC-MS/MS. In total, we identified 1853 ubiquitylated proteins, including 473 proteins that appeared upregulated more than 2-fold in response to MMS treatment. This enabled us to localize 519 ubiquitylation sites potentially regulated upon MMS in 435 proteins. We demonstrated that the overexpression of some of these proteins renders the cells sensitive to MMS. We also assayed the abundance change upon MMS treatment of a selection of yeast nuclear proteins. Several of them were differentially regulated upon MMS treatment. These findings corroborate the important role of ubiquitin-proteasome-mediated degradation in regulating the DDR.