Modeling the Budding Yeast Cell Cycle

sic1∆ cdc6∆2-49 cdh1∆

Simulation:

Change of parameters: ksc1'=ksc1"=0, ksf6'=ksf6"=ksf6"'=0, kscdh=0, init CDH1T=CDH1=0.
Arrest: Inviable, telophase arrest.

Experiments:

Archambault, V., Li, C.X., Tackett, A.J., Wasch, R., Chait, B.T., Rout, M.P. and Cross, F.R. (2003). Genetic and biochemical evaluation of the importance of Cdc6 in regulating mitotic exit. Mol. Biol. Cell 14:4592-4604.
[Abstract] [Article]
Experimental results: These mutant cells ("triple-antagonist", for short) are inviable, but they are not telophase arrested. When transferred the triple-antagonist strain carrying an integrated copy of GAL-SIC1 from galactose to glucose medium, the mutant cells reproducibly accumulated as "4-cell body" objects that were resistant to sonication. They were able to undergo DNA synthesis and nuclear division, but not cell division and arrested as binucleate cells with 4C DNA content. The phenotypes of the triple-antagonist strain and that of the double mutant sic1∆ cdh1∆ strain are similar. The additional removal of the n-terminus of Cdc6 confers a cytokinesis defect to the triple mutant.
Comments: Problem for the model. In our simulation, the triple-antagonist would be unbudded, arrest in telophase with 2C DNA content.

There are 3 problems with this mutant. First of all, how can it exit from mitosis? There is nothing (except the degradation by Cdc20/APC which should not be enough) to bring Clb2 activity down to below the threshold for mitotic exit. The phenotype of the triple-antagonist suggests that our requirement for nuclear division is incorrect. Perhaps it is the rise in Cdc14 activity, rather than (or in addition to) the fall of Clb2-kinase activity, that triggers nuclear division. That is, there is a TARGET protein, when its phosphorylation state drops below a critical value then nuclear division is triggered; and the phosphorylation state is controlled by Clb2-dependent kinase and by Cdc14 phosphatase. In this case, the triple-antagonist cells which have high Cdc14 activity could exit from mitosis.

Even though they could execute nuclear division, they would have trouble in the next cell cycle because of the budding problem. The persistent high Clb2 kinase activity after nuclear division would prevent SBF activation and Cln2 accumulation, hence [Bud]max=0.25 in simulation (it never reaches 1). However, triple-antagonist cells do have buds. It may be that Clb2 kinases are able to trigger bud formation, albeit at very low efficiencies compared to Cln2 kinases.

Even though the first two problems could be avoided, the triple-antagonist cells would have a third problem to deal with. The high Clb2 kinase activity after the first nuclear division, not only interferes with bud formation, it should also prevent reactivating the licensing factors. Surprising, the triple-antagonist cells are able to replicate DNA but block in G2/M for some unknown reason. Maybe, high Clb activity are not able to block origin re-licensing totally, such that these mutants enter S phase with few licensed origins, but somehow S phase cannot be properly completed, causing them to arrest in a G2/M state with seemingly replicated DNA.

See the Targe model for alternative possibility.