Distinct mechanisms control the stability of the related S-phase cyclins Clb5 and Clb6.
The yeast S-phase cyclins Clb5 and Clb6 are closely related proteins that are synthesized late in G1. Although often grouped together with respect to function, Clb5 and Clb6 exhibit differences in their ability to promote S-phase progression. DNA replication is significantly slowed in clb5Delta mutants but not in clb6Delta mutants. We have examined the basis for the differential functions of Clb5 and Clb6 and determined that unlike Clb5, which is stable until mitosis, Clb6 is degraded rapidly at the G1/S border. N-terminal deletions of CLB6 were hyperstabilized, suggesting that the sequences responsible for directing the destruction of Clb6 reside in the N terminus. Clb6 lacks the destruction box motif responsible for the anaphase promoting complex-mediated destruction of Clb5 but contains putative Cdc4 degron motifs in the N terminus. Clb6 was hyperstabilized in cdc34-3 and cdc4-3 mutants at restrictive temperatures and when S/T-P phosphorylation sites in the N terminus were mutated to nonphosphorylatable residues. Efficient degradation of Clb6 requires the activities of both Cdc28 and Pho85. Finally, hyperstabilized Clb6 expressed from the CLB6 promoter rescued the slow S-phase defect exhibited by clb5Delta cells. Taken together, these findings suggest that the SCF(Cdc4) ubiquitin ligase complex regulates Clb6 turnover and that the functional differences exhibited by Clb5 and Clb6 arise from the distinct mechanisms controlling their stability.
Jackson
Leisa P
LP
DCMB Group, Department of Biology, Box 91000, LSRC Bldg., Research Dr., Durham, NC 27708, USA.
Reed
Steven I
SI
Haase
Steven B
SB
D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D064173:Anaphase-Promoting Complex-Cyclosome; D001483:Base Sequence; D017489:CDC28 Protein Kinase, S cerevisiae / Q000235:genetics / Q000378:metabolism; D018797:Cell Cycle Proteins / Q000235:genetics / Q000378:metabolism; D019926:Cyclin B / Q000235:genetics / Q000378:metabolism*; D018844:Cyclin-Dependent Kinases / Q000235:genetics / Q000378:metabolism; D044783:F-Box Proteins / Q000235:genetics / Q000378:metabolism; D015966:Gene Expression Regulation, Fungal; D008969:Molecular Sequence Data; D009154:Mutation; D010446:Peptide Fragments / Q000378:metabolism; D010766:Phosphorylation; D011401:Promoter Regions, Genetic; D016196:S Phase / Q000502:physiology*; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics / Q000378:metabolism*; D025801:Ubiquitin / Q000378:metabolism; D044763:Ubiquitin-Conjugating Enzymes; D043743:Ubiquitin-Protein Ligase Complexes / Q000235:genetics / Q000378:metabolism; D044767:Ubiquitin-Protein Ligases / Q000235:genetics / Q000378:metabolism
C076558:CLB5 protein, S cerevisiae; C095227:CLB6 protein, S cerevisiae; D018797:Cell Cycle Proteins; D019926:Cyclin B; D044783:F-Box Proteins; D010446:Peptide Fragments; D029701:Saccharomyces cerevisiae Proteins; D025801:Ubiquitin; C577449:CDC34 protein, S cerevisiae; D044763:Ubiquitin-Conjugating Enzymes; D043743:Ubiquitin-Protein Ligase Complexes; D064173:Anaphase-Promoting Complex-Cyclosome; D044767:Ubiquitin-Protein Ligases; D017489:CDC28 Protein Kinase, S cerevisiae; D018844:Cyclin-Dependent Kinases; C058605:PHO85 protein, S cerevisiae