Saccharomyces cerevisiae must reach a carbon source-modulated critical cell size, protein content per cell at the onset of DNA replication (Ps), in order to enter S phase. Cells grown in glucose are larger than cells grown in ethanol. Here, we show that an increased level of the cyclin-dependent inhibitor Far1 increases cell size, whereas far1Δ cells start bud emergence and DNA replication at a smaller size than wild type. Cln3Δ, far1Δ, and strains overexpressing Far1 do not delay budding during an ethanol glucose shift-up as wild type does. Together, these findings indicate that Cln3 has to overcome Far1 to trigger Cln–Cdc28 activation, which then turns on SBF- and MBF-dependent transcription. We show that a second threshold is required together with the Cln3/Far1 threshold for carbon source modulation of Ps. A new molecular network accounting for the setting of Ps is proposed.

Saccharomyces cerevisiae cells grown in glucose have larger average size than cells grown in ethanol. Besides, yeast must reach a carbon source-modulated critical cell size in order to enter S phase at Start. This control is of outmost physiological relevance, since it allows us to coordinate cell growth with cell cycle progression and it is responsible for cell size homeostasis. The cell sizer mechanism requires the overcoming of two sequential thresholds, involving Cln3 and Far1, and Clb5,6 and Sic1, respectively. When both thresholds are non-functional, carbon source modulation of cell size at Start is completely abolished. Since inactivation of extracellular glucose sensing through deletion of either the GPR1 or the GPA2 gene causes a marked, but partial, reduction in the ability to modulate cell size and protein content at Start, it is proposed that both extracellular and intracellular glucose signalling is required for properly setting the cell sizer in glucose media.

Yeast cells undergoing a nutritional shift-up from a poor to a rich carbon source take several hours to adapt to the novel, richer carbon source. The budding index is a physiologically relevant "global" parameter that reflects the complex links between cell growth and division that are both coordinately and deeply affected by nutritional conditions. We used changes in budding index as a guide to choose appropriate, relevant time points during an ethanol to glucose nutritional shift-up for preparation of samples for the analysis of proteome by two-dimensional electrophoresis/mass spectrometry. About 600 spots were detected. 90 spots, mostly comprising proteins involved in intermediary metabolism, protein synthesis, and response to stress, showed differential expression after glucose addition. Among modulated proteins we identified a protein of previously unknown function, Gvp36, showing a transitory increase corresponding to the drop of the fraction of budded cells. A gvp36⌬ strain shares several phenotypes (including general growth defects, heat shock, and high salt sensitivity, defects in polarization of the actin cytoskeleton, in endocytosis and in vacuolar biogenesis, defects in entering stationary phase upon nutrient starvation) with secretory pathway mutants and with mutants in genes encoding the two previously known yeast BAR proteins (RSV161 and RSV167). We thus propose that Gvp36 represents a novel yeast BAR protein involved in vesicular traffic and in nutritional adaptation.

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