Whi5 phosphorylation embedded in the G(1)/S network dynamically controls critical cell size and cell fate

In budding yeast, overcoming of a critical size to enter S phase and the mitosis/mating switch—two central cell fate events—take place in the G(1) phase of the cell cycle. Here we present a mathematical model of the basic molecular mechanism controlling the G(1)/S transition, whose major regulatory feature is multisite phosphorylation of nuclear Whi5. Cln3–Cdk1, whose nuclear amount is proportional to cell size, and then Cln1,2–Cdk1, randomly phosphorylate both decoy and functional Whi5 sites. Full phosphorylation of functional sites releases Whi5 inhibitory activity, activating G(1)/S transcription. Simulation analysis shows that this mechanism ensures coherent release of Whi5 inhibitory action and accounts for many experimentally observed properties of mitotically growing or conjugating G(1) cells. Cell cycle progression and transcriptional analyses of a Whi5 phosphomimetic mutant verify the model prediction that coherent transcription of the G(1)/S regulon and ensuing G(1)/S transition requires full phosphorylation of Whi5 functional sites.