G1-Cdk modulation by time and stress

Resumen

The cell cycle is regulated at different layers of control in either homeostatic or non-homeostatic conditions. Hence, in this work we analyse how time and stress modulate the cell cycle through the activity of the G1 Cdk. Cell growth and division are coordinated processes that ensure life perpetuation with optimal cell fitness. One fundamental trait that emerges from this interplay is cell size. Despite being a seemingly basic attribute, little is known about the molecular machinery that ensures a homeostatic cell size control. In budding yeast, cell size control is mostly established in G1 and relies on the machinery that regulates the Start checkpoint transition. It is at this point where the G1 Cdk, formed by the Cdc28 Cdk and cyclin Cln3, promotes the inhibitory phosphorylation of the Whi5 repressor at the same time that activates the function of RNA polymerase II at specific SBF/MBF loci. Three models have been proposed to orchestrate these factors in size control: sizers, timers, and adders. So far, cell size control has been reported to be exerted through the dilution of Whi5 repressor acting as sizer. Even so, cell size still depicts high single-cell variability and thus warms the debate about a stochastic or deterministic size control. Favouring the latter, our work adds a time-dependent control layer executed by the interplay of APCCdh1, the mitotic-exit protein-degradation machine, Mad3, a centromeric-signalling protein, and Cln3, the G1 cyclin. APCCdh1 is activated during exit from the previous mitosis and degrades Mad3 decreasing its protein levels as cells progress through G1. At the same time, since Mad3 actively participates in the degradation of Cln3 by SCF, levels of active G1 Cdk increase in the nucleus throughout G1 until Start, where cells become committed to enter the next cell cycle. Our model bestows a timer control in G1 emerging from the intertwined coordination of APC and SCF degradation machineries. Aside of its role in the cell cycle, here we show that the G1 Cdk is also an active regulator of cell fitness under stress conditions. Many strategies conform the so-called stress response and its adaptation to a wide range of stresses and intensities. Among them, the rapid formation of stress granules (SGs) upholds cell survival and growth recovery to optimal levels upon stress relief. Stress granules are membraneless condensates rich in translationally-inhibited mRNAs and RNA-binding proteins. Their internal multivalent interactions settle a proper thermodynamic environment that generates the formation of protein-RNA assemblies by liquid-liquid phase separation. Among the diverse transcriptome and proteome composition of SGs, we have found Whi8 as a key element that connects the stress response to the cell cycle control machinery. Whi8 modulates the budding volume and interacts with both the CLN3 mRNA and the Cdc28 kinase. Interestingly, Whi8 is also a bona fide component of SGs that recruits Cdc28 and the CLN3 mRNA into these condensates. The consequence of this recruitment directly influences on both the kinetics of SG dissolution and the inactivation of the yeast Cdk. Our results designate Caprin1 as the mammalian functional homolog of Whi8 that interacts with Cdk4 and the mRNA of cyclin D1. In summary, the G1 Cdk favours disassembly of stress granules which, in turn, inhibit the activity of the Cdk. This mutual inhibition relationship generates a bistable system that ensures a rapid and abrupt SG formation only when stress exceeds a level that would otherwise endanger cell viability.