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 t he onset of mitosis through the activation of Swe1, a negative regulatory k inase that inhibits the Cdc28-Clb complex. Calcineurin and Mpk1 activate Sw e1 at the transcriptional and post-translational level, respectively, and b oth pathways are essential for the cell cycle delay. Our genetic screening identified the MCK1 gene, which encodes a glycogen synthetase kinase-3 fami ly protein kinase, as a component of the Ca2+ signaling pathway. Genetic an alyses indicated that Mck1 functions downstream of the Mpk1 pathway and dow n-regulates Hsl1, an inhibitory kinase of Swe1, In medium with a high conce ntration of Ca2+, Hsl1 was delocalized from the bud neck and destabilized i n a manner dependent on both calcineurin and Mck1, Calcineurin was required for the dephosphorylation of autophosphorylated Hsl1. The E3 ubiquitin lig ase complex SCFCdc4, but not the anaphase-promoting complex (APC), was esse ntial 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 activ ity is low.