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

Abstract

In their manuscript, Litsios et al. 1 report a new model for how cell growth and biosynthetic activity control the G1/S transition in budding yeast. In essence, Litsios et al. claim that Start is driven by an increasing concentration of the G1 cyclin Cln3 due to a dramatic acceleration of protein synthesis in pre-Start G1 and not by the dilution of the cell cycle inhibitor Whi5. While we previously reported that Start was in part driven by cell growth during G1 diluting out the Start inhibitor Whi5 2 , Litsios et al. report that Whi5 remains at constant concentration during G1, and changes in Whi5 concentration therefore do not contribute to Start.Since Litsios et al. directly contradict several key points of our own model of how cell growth triggers Start, we decided to investigate their claims and data. More specifically, we decided to investigate Litsios et al.'s three major claims:1. Whi5 concentration remains constant during G1 2. Cln3 concentration strongly increases prior to Start 3. Global protein synthesis rates increase by 2-3 fold prior to StartWe investigated each of these three claims and found that the evidence presented by Litsios et al. does not support their claims due to inadequate analysis methods and flaws in their experiments.

Subscaling of a cytosolic RNA binding protein governs cell size homeostasis in the multiple fission alga Chlamydomonas

“…Evidence for cell size checkpoints based on some form of protein subscaling has been found in different eukaryotic taxa, including fungi, animal cells and plant meristems (Figure S6) (7,8,10,29). An appealing property of subscaled proteins is that their absolute abundance in a cell can act as a denominator for perceiving changes in cell size whose proxy is a protein or other molecule whose cytoplasmic concentration is constant.…”

“…In budding yeast, nuclear Whi5 protein binds to and inhibits the DNA bound transcription factor SBF, a key activator of S phase transcription. While some regulation of Whi5 abundance may occur based on synthesis of Whi5, it is also limited by chromatin binding (8, 29, 30). Similar findings were made for the RB protein in mammalian cells which is a functional analog of Whi5 for S phase transcription (10).…”

“…Chromatin or nuclear DNA content is a naturally subscaled component of cells that has been exploited in Arabidopsis as a way of ensuring that the absolute amount of the inhibitor protein KRP4 apportioned to daughters is independent of birth size (7). In yeast and mammalian cells, chromatin-bound cell cycle inhibitor proteins, Whi5 and Rb respectively, are also subscaled and act as limiting inhibitors of cell cycle progression (8)(9)(10).…”