Modeling the START transition in the budding yeast cell cycle
Fig 4
Simulation of wildtype cells in glucose.
Each panel tracks the following proteins/components for ~3 cell cycles in glucose with mass doubling time 90 min and daughter cycle time ~107 min. Mass acts as a proxy for the growing and dividing cell; here we follow the daughter cells (~0.46 x total_size_at_division; see Equations in S2 Text). (A) Cyclins (Cln2, Clb5 (active), Clb2 (active)), (B) Cyclin antagonists (Cdh1, Sic1 (active), Cdc6 (active)), (C) Transcription factors (SBF, MBF, MCM1, Swi5), (D) Markers (CDK targets (BUD, ORI, SPN) used to indicate the occurrence of physiological events via concentration threshold (BUD = 1 (bud emergence), ORI = 1 (DNA synthesis initiation), SPN = 1 (spindle alignment in metaphase)), (E) Active SBF and MBF complexes. Each form shown (SBFa1-a5, MBFacln, MBFabck) is the activity of SBF or MBF contributed by a particular form. The complexes contributing to the different forms (all bound to promoter) are as follows: SBFa1 = unmodified SBF (SBFB in the model); SBFa2 = SBF phosphorylated on Swi6 (SBFB6P+SBFB6PQ); SBFa3 = SBF-Whi5 complex phosphorylated on Swi6 (WSB6P+WSB6PQ); SBFa4 = SBF-Whi5 complex phosphorylated on Whi5 (WSB5P); SBFa5 = Swi4dimers activated by Bck2 (Swi4B); MBFa = MBF activated by Clns or Bck2. Check and cross marks denote the presence or absence (due to mutation) of specific complexes. The absence of any sign denotes that the specified complex is absent in the simulation of that strain. The black curve in all panels denotes mass (corresponding to exponential cell growth and division). As shown in (E), for wildtype cells, the dominant form of SBF is SBFa2, the form of SBF phosphorylated on Swi6, whereas the dominant form of MBF is the Cln-activated MBF.