A MAP kinase cascade modulates expression of late G1 phase cyclins in budding yeast.

Expression of late G1 phase cyclins is the critical molecular event that marks commitment to enter the cell cycle. In budding yeast, late G1 phase cyclins initiate and sustain growth of a new daughter bud, so their expression also marks the start of a new growth phase during the cell cycle. Expression of late G1 phase cyclins is influenced by nutrient availability - cells growing in poor nutrients progress through late G1 phase with lower levels of late G1 phase cyclins. However, little is known about how or why nutrients modulate expression of late G1 phase cyclins. Here, we investigated the signals that control expression of the late G1 phase cyclin Cln2. We discovered that nutrients modulate expression of Cln2 primarily via post-transcriptional mechanisms that influence Cln2 phosphorylation and turnover. Nutrient modulation of Cln2 protein expression requires a TORC2-Pkc1-MAP kinase signaling axis. Furthermore, expression of Cln2 is closely correlated with bud growth and required for bud growth. A model that could explain the data is that nutrients modulate Cln2 expression to ensure that the rate of bud growth is matched to the availability of nutrients that support bud growth.