A size-invariant bud-duration timer enables robustness in yeast cell size control

SUMMARY Cell size drives key aspects of cell physiology, including organelle abundance [1, 2] and DNA ploidy [3]. While cells employ diverse strategies to regulate size [4–11], it is unclear how they are integrated to provide robust, systems-level control. In budding yeast, a molecular size sensor restricts passage of small cells through G1, enabling them to gain proportionally more volume than larger cells before progressing to Start [7, 12, 13]. Size control post-Start is less clear. S/G2/M duration in wildtype cells shows only a weak dependence on cell size; and since yeast exhibit exponential growth, larger cells would be expected to add more volume than smaller ones [7, 14–17]. However, even large mother cells produce smaller daughters, suggesting that additional regulation may occur during S/G2/M [7]. To gain further insight into post-Start size control, we prepared ‘giant’ yeast (>10-fold larger than typical volume) using two approaches to reversibly block cell cycle progression but not growth: optogenetic disruption of the cell polarity factor Bem1 [18, 19] and a temperature-sensitive cdk1 allele [20]. We reasoned that giant yeast would satisfy pre-Start size control while enabling us to uncover post-Start size-limiting mechanisms though the identification of invariant growth parameters. Upon release from their block, giant mothers reenter the cell cycle and their progeny rapidly return to the original unperturbed size. This behavior is consistent with a size-invariant ‘timer’ specifying the duration of S/G2/M and indicates that yeast use at least two distinct mechanisms at different cell cycle phases to ensure size homeostasis.