UC Santa Cruz

Cell size and shape are essential for the survival and function of all cells. Cell size checkpoints coordinate cell growth and cell division by ensuring passage through G1 and mitosis only occurs when cells reach a critical size; however, the mechanisms that control these checkpoints are largely unknown.

PP2A associated with the Rts1 regulatory subunit (PP2ARts1) is required for cell size control in budding yeast, but the relevant targets are unknown. Here, we used quantitative proteome-wide mass spectrometry to identify proteins controlled by PP2ARts1. This revealed that PP2ARts1 controls the two key checkpoint pathways thought to regulate the cell cycle in response to cell growth. To investigate the role of PP2ARts1 in these pathways, we focused on the Ace2 transcription factor, which is thought to delay cell cycle entry by repressing transcription of the G1 cyclin CLN3. Diverse experiments suggest that PP2ARts1 promotes cell cycle entry by inhibiting the repressor functions of Ace2.

Furthermore, we found that Rts1 phosphorylation is dependent on the budding yeast homologs of the vertebrate phosphoinositide-dependent kinase 1 (PDK1) and protein kinase C (PKC). We also found that hyperphosphorylation of Rts1 in response to poor nutrients is dependent on PDK1, PKC and casein kinase 1.

Together these results support a model in which Rts1 links nutrient availability and cell size to entry into the cell cycle.