Single-cell analysis of TORC1 and PKA signalling during the budding yeast cell cycle: towards a holistic view of growth and division

Cell growth, characterized by the accumulation of biomass, and the cell division process, are fundamental for cell survival and reproduction. Classically, it was thought that cell growth was independent from the cell cycle, but recent studies showed that the activity of several processes related to biomass accumulation are dependent on the cell cycle phase. How the activity of these processes is coordinated with cell cycle progression is still unknown. In budding yeast, the two main regulators of cell growth are the evolutionarily conserved Target of Rapamycin Complex 1 (TORC1) and Protein Kinase A (PKA) pathways. Given that TORC1 and PKA are also known to interact with cell cycle machinery components, it is possible that they mediate the coordination of cell growth with the cell cycle. In this thesis we investigate the connections of TORC1 and PKA with the budding yeast cell cycle.
By using single-cell analysis we revealed oscillations of TORC1 and PKA activity during the unperturbed budding yeast cell cycle, with a peak during G1 and minima at budding and in late mitosis. Furthermore, continuous inhibition of TORC1 or PKA decreases biomass accumulation and cell cycle progression both in G1 and during S/G2/M, and alters cell size both at budding and at the end of G2/M. Finally, changing the metabolic activity of the cell perturbs TORC1 and PKA cell-cycle dynamics and so do deletions of cell cycle machinery components showing that TORC1 and PKA mediate the interconnection of fundamental cellular processes such as metabolism, growth and cell cycle progression.