Regulation of the yeast Ace2 transcription factor during the cell cycle

Previous studies have revealed many parallels in the cell cycle regulation of the Ace2 and Swi5 transcription factors. Although both proteins begin entry into the nucleus near the start of mitosis, here we show that Ace2 accumulates in the nucleus and binds DNA about 10 min later in the cell cycle than Swi5. We used chimeric fusions to identify the N-terminal region of Ace2 as responsible for the delay, and this same region of Ace2 was required for interaction with Cbk1, a kinase necessary for both transcriptional activation by Ace2 and asymmetric distribution of Ace2. Ace2 and Swi5 also showed differences in prevalence during the cell cycle. Swi5 is apparently degraded soon after nuclear entry, whereas constant Ace2 levels throughout the cell cycle suggest Ace2 is exported from the nucleus. Our work suggests that the precise timing of Ace2 accumulation in the nucleus involves both a nuclear export sequence and a nuclear localization signal, whose activities are regulated by phosphorylation.