Analysis of the mitotic exit control system using locked levels of stable mitotic cyclin.
Cyclin-dependent kinase (Cdk) both promotes mitotic entry (spindle assembly and anaphase) and inhibits mitotic exit (spindle disassembly and cytokinesis), leading to an elegant quantitative hypothesis that a single cyclin oscillation can function as a ratchet to order these events. This ratchet is at the core of a published ODE model for the yeast cell cycle. However, the ratchet model requires appropriate cyclin dose-response thresholds. Here, we test the inhibition of mitotic exit in budding yeast using graded levels of stable mitotic cyclin (Clb2). In opposition to the ratchet model, stable levels of Clb2 introduced dose-dependent delays, rather than hard thresholds, that varied by mitotic exit event. The ensuing cell cycle was highly abnormal, suggesting a novel reason for cyclin degradation. Cdc14 phosphatase antagonizes Clb2-Cdk, and Cdc14 is released from inhibitory nucleolar sequestration independently of stable Clb2. Thus, Cdc14/Clb2 balance may be the appropriate variable for mitotic regulation. Although our results are inconsistent with the aforementioned ODE model, revision of the model to allow Cdc14/Clb2 balance to control mitotic exit corrects these discrepancies, providing theoretical support for our conclusions.
Drapkin
Benjamin J
BJ
Laboratory of Yeast Molecular Genetics, The Rockefeller University, New York, NY 10021, USA.
Lu
Ying
Y
Procko
Andrea L
AL
Timney
Benjamin L
BL
Cross
Frederick R
FR
D021581:Active Transport, Cell Nucleus / Q000187:drug effects; D000705:Anaphase / Q000187:drug effects; D015374:Biosensing Techniques; D002466:Cell Nucleolus / Q000187:drug effects / Q000378:metabolism; D019926:Cyclin B / Q000378:metabolism*; D016193:G1 Phase / Q000187:drug effects; D006657:Histones / Q000378:metabolism; D000072235:Mating Factor; D008938:Mitosis / Q000187:drug effects; D008957:Models, Genetic; D010455:Peptides / Q000494:pharmacology; D010749:Phosphoprotein Phosphatases / Q000378:metabolism; D011494:Protein Kinases / Q000378:metabolism; D011499:Protein Processing, Post-Translational / Q000187:drug effects; D012441:Saccharomyces cerevisiae / Q000166:cytology* / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins / Q000378:metabolism*; D008941:Spindle Apparatus / Q000187:drug effects / Q000378:metabolism
C093749:CLB2 protein, S cerevisiae; D019926:Cyclin B; D006657:Histones; D010455:Peptides; D029701:Saccharomyces cerevisiae Proteins; D000072235:Mating Factor; D011494:Protein Kinases; D010749:Phosphoprotein Phosphatases