G₁/S and G₂/M Cyclin-Dependent Kinase Activities Commit Cells to Death in the Absence of the S-Phase Checkpoint

Abstract

The Mec1 and Rad53 protein kinases are essential for budding yeast cell viability and are also required to activate the S-phase checkpoint, which supports DNA replication under stress conditions. Whether these two functions are related to each other remains to be determined, and the nature of the replication stress-dependent lethality of mec1 and rad53 mutants is still unclear. We show here that a decrease in cyclin-dependent kinase 1 (Cdk1) activity alleviates the lethal effects of mec1 and rad53 mutations both in the absence and in the presence of replication stress, indicating that the execution of a certain Cdk1-mediated event(s) is detrimental in the absence of Mec1 and Rad53. This lethality involves Cdk1 functions in both G₁ and mitosis. In fact, delaying either the G₁/S transition or spindle elongation in mec1 and rad53 mutants allows their survival both after exposure to hydroxyurea and under unperturbed conditions. Altogether, our studies indicate that inappropriate entry into S phase and segregation of incompletely replicated chromosomes contribute to cell death when the S-phase checkpoint is not functional. Moreover, these findings suggest that the essential function of Mec1 and Rad53 is not necessarily separated from the function of these kinases in supporting DNA synthesis under stress conditions.

SUPPLEMENTAL MATERIAL

Supplemental material for this article may be found at http://dx.doi.org/10.1128/MCB.00956-12.

ACKNOWLEDGMENTS

We thank J. Diffley, R. Kolodner, S. Piatti, and R. Rothstein for providing yeast strains. We are grateful to M. Bazzi for preliminary data, L. D'Alfonso and M. Collini for valuable advice at the microscope, M. Lopes and M. Foiani for helpful discussions, and M. Clerici and S. Piatti for helpful suggestions and critical reading of the manuscript.

This study was supported by grants from the Associazione Italiana per la Ricerca sul Cancro (grant IG11407), Cofinanziamento 2008 MIUR/Università di Milano-Bicocca to M.P.L., and Cofinanziamento 2009 MIUR/Università di Milano-Bicocca to G.L. N.M. was supported by a fellowship from Fondazione Italiana per la Ricerca sul Cancro.