Cyclin regulation by the s phase checkpoint.
In eukaryotic cells a surveillance mechanism, the S phase checkpoint, detects and responds to DNA damage and replication stress, protecting DNA replication and arresting cell cycle progression. We show here that the S phase cyclins Clb5 and Clb6 are regulated in response to genotoxic stress in the budding yeast Saccharomyces cerevisiae. Clb5 and Clb6 are responsible for the activation of the specific Cdc28 cyclin-dependent kinase activity that drives the onset and progression of the S phase. Intriguingly, Clb5 and Clb6 are regulated by different mechanisms. Thus, the presence of Clb6, which is eliminated early in an unperturbed S phase, is stabilized when replication is compromised by replication stress or DNA damage. Such stabilization depends on the checkpoint kinases Mec1 and Rad53. The stabilization of Clb6 levels is a dynamic process that requires continued de novo protein synthesis, because the cyclin remains subject to degradation. It also requires the activity of the G(1) transcription factor Mlu1 cell cycle box-binding factor (MBF) in the S phase, whereas Dun1, the checkpoint kinase characteristically responsible for the transcriptional response to genotoxic stress, is dispensable in this case. On the other hand, two subpopulations of endogenous Clb5 can be distinguished according to turnover in an unperturbed S phase. In the presence of replication stress, the unstable Clb5 pool is stabilized, and such stabilization requires neither MBF transcriptional activity nor de novo protein synthesis.
Biophysics Unit, Department of Biochemistry and Molecular Biology, School of Medicine, and Center for Biophysic Studies, Universitat Autonoma de Barcelona, Bellaterra, Catalonia, Spain.
Gloria
G
Palou
Roger
R
Palou
Angel
A
Guerra-Moreno
Alba
A
Duch
Anna
A
Travesa
David G
DG
Quintana
D002453:Cell Cycle; D018797:Cell Cycle Proteins; D064447:Checkpoint Kinase 2; D019926:Cyclin B / Q000378:metabolism*; D004261:DNA Replication; D017346:Protein Serine-Threonine Kinases; D055550:Protein Stability; D016196:S Phase; D012441:Saccharomyces cerevisiae / Q000166:cytology; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics / Q000378:metabolism*; D013312:Stress, Physiological; D014157:Transcription Factors / Q000235:genetics
C076558:CLB5 protein, S cerevisiae; C095227:CLB6 protein, S cerevisiae; D018797:Cell Cycle Proteins; D019926:Cyclin B; C083209:MBP1 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; D014157:Transcription Factors; D064447:Checkpoint Kinase 2; D017346:Protein Serine-Threonine Kinases; C067022:RAD53 protein, S cerevisiae