Interactions between Pho85 cyclin-dependent kinase complexes and the Swi5 transcription factor in budding yeast

Pho85 is a cyclin-dependent protein kinase (Cdk) in budding yeast with roles in cell metabolism and cell cycle progression. Activation of Pho85 occurs through association with Pho85 cyclins (Pcls), of which 10 are known. When complexed with the G1 cyclins, Pcl1 and Pcl2, Pho85 is required for cell cycle progression in the absence of the Cdc28-dependent cyclins, Cln1 and Cln2. To identify potential targets of Pcl2-Pho85, we performed a two-hybrid screen using the Pcl2 cyclin as bait and recovered the transcription factor Swi5 as a Pcl2-interacting protein. We performed both biochemical and genetic tests to discover the biological significance of the interaction between Pcl2 and Swi5 seen in the two-hybrid assay. We found that Swi5 interacts in vitro with Pho85 cyclins and is phosphorylated in vitro by the Pho80-Pho85 kinase. We discovered that a subset of genes that are controlled by Swi5 and a homologous transcription factor, Ace2, was misregulated in a pho85 deletion strain; expression of the ASH1 and CTS1 genes was reduced in an ace2 deletion strain, whereas expression of both genes was increased in an ace2Delta pho85Delta double mutant. We also found that overexpression of SWI5 caused cell lethality in a pho85 deletion strain. Our results are consistent with misregulation of Swi5 activity in vivo in the absence of Pho85 and implicate Swi5 as a potential substrate of Pho85 cyclin-dependent kinase complexes.