The replication stress response and the ubiquitin system: a new link in maintaining genomic integrity.

Maintenance of genomic integrity is important for cellular viability and proliferation. During DNA replication, cells respond to replication stress by activating checkpoint pathways that stabilize replication forks and prevent cell cycle progression. The Saccharomyces cerevisiae F-box protein Dia2 is a ubiquitin ligase component required for genomic stability and may help replication complexes negotiate damaged DNA or natural fragile sites. We recently implicated Dia2 in the replication stress response. We demonstrated that Dia2 is targeted for ubiquitin-mediated proteolysis and that activation of the S-phase checkpoint inhibits Dia2 protein turnover. S-phase checkpoint mutants fail to stabilize the Dia2 protein and checkpoint mutants that lack Dia2 exhibit increased sensitivity to replication stress. We also showed that Dia2 protein turnover is not the result of an autocatalytic mechanism. Instead, an N-terminal 20 amino acid motif that is also required for nuclear localization is necessary for Dia2 proteolysis. Dia2 mutants lacking this motif but modified with an exogenous strong nuclear localization signal are both nuclear and stable and disrupt cell cycle dynamics. In summary, our studies suggest that inhibition of Dia2 proteolysis is a novel target of the S-phase checkpoint. We think that this work will help to identify the mechanisms that function downstream of checkpoint activation and that intersect with cell cycle control pathways.