The pH-sensing Rim101 pathway regulates cell size in budding yeast.

Although cell size regulation is crucial for cellular functions in a variety of organisms from bacteria to humans, the underlying mechanisms remain elusive. Here, we identify Rim21, a component of the pH-sensing Rim101 pathway, as a positive regulator of cell size through a flow cytometry-based genome-wide screen of Saccharomyces cerevisiae deletion mutants. We found that mutants defective in the Rim101 pathway were consistently smaller than wildtype cells in the log and stationary phases. We show that the expression of the active form of Rim101 increased the size of wildtype cells. Furthermore, the size of wildtype cells increased in response to external alkalization. Microscopic observation revealed that this cell size increase was associated with changes in both vacuolar and cytoplasmic volume. We also found that these volume changes were dependent on Rim21 and Rim101. In addition, a mutant lacking Vph1, a component of V-ATPase that is transcriptionally regulated by Rim101, was also smaller than wildtype cells, with no increase in size in response to alkalization. We demonstrate that the loss of Vph1 suppressed the Rim101-induced increase in cell size under physiological pH conditions. Taken together, our results suggest that the cell size of budding yeast is regulated by the Rim101-V-ATPase axis under physiological conditions as well as in response to alkaline stresses.