Cdc28-Clb mitotic kinase negatively regulates bud site assembly in the budding yeast

In the budding yeast Saccharomyces cerevisiae, a prospective mother normall y commences the formation of a daughter (the bud) only in the G(1) phase of the cell division cycle. This suggests a strict temporal regulation of the processes that initiate the formation of a new bud. Using cortical localiz ation of bud site components Spa2 and Bni1 as an indicator of bud site asse mbly, we show that cells assemble a bud site following inactivation of the Cdc28-Clb mitotic kinase but prior to START. Interestingly, an untimely ina ctivation of the mitotic kinase is sufficient to drive cells to assemble a new bud site inappropriately in Gz or M phases. The induction of Cdc28/Clb kinase activity in G(1), on the other hand, dramatically reduces a cell's a bility to construct an incipient bud site. Our findings strongly suggest th at the Cdc28-Clb kinase plays a critical role in the mechanism that restric ts the timing of bud formation to the G(1) phase of the cell cycle.