Sequential Counteracting Kinases Restrict an Asymmetric Gene Expression Program to early G1

Abstract

Gene expression is restricted to specific times in cell division and differentiation through close control of both activation and inactivation of transcription. In budding yeast, strict spatiotemporal regulation of the transcription factor Ace2 ensures that it acts only once in a cell's lifetime: at the M-to-G1 transition in newborn daughter cells. The Ndr/LATS family kinase Cbk1, functioning in a system similar to metazoan hippo signaling pathways, activates Ace2 and drives its accumulation in daughter cell nuclei, but the mechanism of this transcription factor's inactivation is unknown. We found that Ace2's nuclear localization is maintained by continuous Cbk1 activity and that inhibition of the kinase leads to immediate loss of phosphorylation and export to the cytoplasm. Once exported, Ace2 cannot re-enter nuclei for the remainder of the cell cycle. Two separate mechanisms enforce Ace2's cytoplasmic sequestration: 1) phosphorylation of CDK consensus sites in Ace2 by the G1 CDKs Pho85 and Cdc28/CDK1 and 2) an unknown mechanism mediated by Pho85 that is independent of its kinase activity. Direct phosphorylation of CDK consensus sites is not necessary for Ace2's cytoplasmic retention, indicating that these mechanisms function redundantly. Overall, these findings show how sequential opposing kinases limit a daughter cell specific transcriptional program to a brief period during the cell cycle and suggest that CDKs may function as cytoplasmic sequestration factors.