The role of cell growth rate on accumulation of the mitotic cyclin Cdc13 in fission yeast.

Eukaryotic cells control their size by coordinating growth and division. Fission yeast divide at a reproducible cell size due to regulated activation of the cyclin-dependent kinase Cdk1. The nuclear concentration of mitotic cyclin Cdc13 increases in a time-dependent manner to promote Cdk1 activation as cells grow. Here, we show that interphase Cdc13 is stable against degradation and nuclear export, but is diluted by cell growth. Low glucose reduced cell growth rate but not time-dependent accumulation of Cdc13. Uncoupling the rates of cell growth and Cdc13 accumulation resulted in higher concentrations of nuclear Cdc13 despite reduced cell size. This change coincided with reduced activating phosphorylation of Cdk1-T167 and occurred dynamically during abrupt changes in glucose concentration. Mathematical modeling and experiments showed that cells maintain size homeostasis under these conditions. In contrast to low glucose, poor nitrogen reduced both cell growth rate and Cdc13 accumulation rate. Therefore, Cdc13 accumulation is independent of cell growth rate but can be altered by nutrient-specific mechanisms.