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. ClassificationBiological Sciences, Cell Biology. KeywordsWhi5, Start, cell size control, budding yeast, single-cell time-lapse microscopy. 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 model's 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.

Whi5 is diluted and protein synthesis does not dramatically increase in pre-StartG1

“…Simultaneous estimation of cell volume based on cell segmentations obtained from phase contrast images can then be used to calculate the relative change of Whi5 concentration over time. Since our initial report of Whi5 dilution, several groups have independently reproduced many aspects of our findings in published [8][9][10] and in new data reported here that was previously unpublished ( Fig. S2-4).…”

Strongly oversized fission yeast cells lack any size control and tend to grow linearly rather than bilinearly

“…During the last couple of years, there have been many, many new results published on both individual cellular growth and population‐level size homeostasis and the connections between these two aspects (Allard et al, 2018; Barber, Amir, & Murray, 2020; Cantwell & Nurse, 2019; Facchetti, Knapp, Chang, & Howard, 2019; Facchetti et al, 2019; Hannebelle et al, 2020; Nordholt, van Heerden, & Bruggeman, 2020; Opalko et al, 2019; Patterson et al, 2019; Pickering et al, 2019; Si et al, 2019; Vuaridel‐Thurre, Vuaridel, Dhar, & McKinney, 2020; Xie & Skotheim, 2020; Zatulovskiy & Skotheim, 2020; Zatulovskiy, Zhang, Berenson, Topacio, & Skotheim, 2020). The model organisms studied vary from unicellular prokaryotes via yeasts to animal cells; moreover, modern techniques have even made possible to examine mammalian cells in tissues in vivo.…”

“…In that case, it is not surprising that several proteins should have very important (even essential) roles in setting the right cell size in G2 in fission yeast. In budding yeast, probably inhibitor dilution and activator accumulation both matter in setting the proper division size, because the transcription rates of some proteins do not always scale with cell size, which may lead either to dilution of an inhibitor or to accumulation of an activator (or to both) as the cells grow during their cycle (Barber et al, 2020; Chen, Zhao, Zahumensky, Honey, & Futcher, 2020).…”

Cell size: a key determinant of meristematic potential in plant protoplasts

“…In sizer experiments, a population with different cell sizes, raised in varying growth conditions, either created transitory blockades in cell cycle progression or engaged in natural asymmetric divisions (Barber et al 2020). Size control occurs at one or more checkpointsone that precedes the initiation of DNA synthesis (G1/S transition) and the other before nuclear division (G2/M transition).…”