Cell size regulation in budding yeast does not depend on linear accumulation of Whi5

Abstract

Cells must couple cell cycle progress to their growth rate to restrict the spread of cell sizes present throughout a population. Linear, rather than exponential, accumulation of Whi5, was proposed to provide this coordination by causing a higher Whi5 concentration in cells born at smaller size. We tested this model using the inducible GAL1 promoter to make the Whi5 concentration independent of cell size. At an expression level that equalizes the mean cell size with that of wild-type cells, the size distributions of cells with galactose-induced Whi5 expression and wild-type cells are indistinguishable. Fluorescence microscopy confirms that the endogenous and GAL1 promoters produce different relationships between Whi5 concentration and cell volume without diminishing size control in the G1 phase. We also expressed Cln3 from the GAL1 promoter, finding that the spread in cell sizes for an asynchronous population is unaffected by this perturbation. Our findings contradict the previously proposed model for cell size control in budding yeast and demonstrate the need for a molecular mechanism that explains how cell size controls passage through Start. Author ContributionsFB performed the experiments, data analysis and simulations. All authors designed the experiments and wrote the manuscript. Significance StatementDespite decades of research, the question of how single cells regulate their size remains unclear. Here we demonstrate that a widely supported molecular model for the fundamental origin of size control in budding yeast is inconsistent with a set of experiments testing the models key prediction. We therefore conclude that the problem of cell size control in budding yeast remains unsolved. This work highlights the need for rigorous testing of future models of size control in order to make progress on this fundamental question.