GSK‐3 kinase Mck1 and calcineurin coordinately mediate Hsl1 down‐regulation by Ca2+ in budding yeast

The Ca2+‐activated pathways of Saccharomyces cerevisiae induce a delay in the onset of mitosis through the activation of Swe1, a negative regulatory kinase that inhibits the Cdc28–Clb complex. Calcineurin and Mpk1 activate Swe1 at the transcriptional and post‐translational level, respectively, and both pathways are essential for the cell cycle delay. Our genetic screening identified the MCK1 gene, which encodes a glycogen synthetase kinase‐3 family protein kinase, as a component of the Ca2+ signaling pathway. Genetic analyses indicated that Mck1 functions downstream of the Mpk1 pathway and down‐regulates Hsl1, an inhibitory kinase of Swe1. In medium with a high concentration of Ca2+, Hsl1 was delocalized from the bud neck and destabilized in a manner dependent on both calcineurin and Mck1. Calcineurin was required for the dephosphorylation of autophosphorylated Hsl1. The E3 ubiquitin ligase complex SCFCdc4, but not the anaphase‐promoting complex (APC), was essential for Hsl1 destabilization. The Ca2+‐activated pathway may play a role in the rapid inactivation of Hsl1 at the cell cycle stage(s) when APC activity is low.