For decades, the growth rate of cells was thought to be fairly constant throughout the cell cycle. Recent studies in Saccharomyces cerevisiae (budding yeast) have challenged this notion, showing that the growth rate peaks during the G1 and G2/M phases. Still, the regulatory mechanisms mediating these changes are largely unknown. In this study, Andreas Milias-Argeitis and colleagues (Guerra et al., 2022) investigate the role of target of rapamycin complex 1 (TORC1) and protein kinase A (PKA) pathways in this process. Using single-cell time-lapse microscopy with fluorescent reporters of TORC1 and PKA activity, the authors demonstrate that the activity of these pathways shows a maximum during G1 and minima at budding (early S phase) and in late M/early G1. Nutrient-sensing mechanisms upstream of TORC1 and PKA seem to be involved in the regulation of TORC1 and PKA, suggesting that these signalling pathways respond to oscillatory internal metabolic signals during the cell cycle. Finally, the authors show that ribosome synthesis rates, which are regulated by TORC1 and PKA, display oscillations that reflect the observed TORC1 and PKA activity patterns. Taken together, these findings suggest that TORC1 and PKA activity oscillations are central in coupling cell growth to the cell cycle and reveal a new aspect of TORC1 and PKA signalling.
