Zds2p: Negative Regulator of the Swe1p-Dependent G2/M Checkpoint

Deletion of the paralogs ZDS1 and ZDS2 in the budding yeast Saccharomyces cerevisiae causes a mis-regulation of polarized cell growth resulting in distinctive hyper-elongated buds. The cellular function of these proteins in the regulation of polar growth has been a source of mystery. Through molecular and cell biological approaches, I have characterized Zds1p and Zds2p as regulators of the Swe1p kinase-dependent G2/M checkpoint. I also show a conserved domain in the C-terminus of Zds2p, consisting of amino acids 813-912 (hereafter referred to as ZH4 for Zds Homology 4), is required for regulation of Swe1p-dependent polarized bud growth. The ZH4 domain is shown by protein affinity assays to be necessary and sufficient for interaction with Cdc55p, a regulatory subunit of Protein Phosphatase 2A (PP2A). The similarity of the elongated growth phenotype to that induced by hyper-activation of the Swe1p checkpoint led me to hypothesize that the Zds proteins are in a pathway that negatively regulates the Swe1pdependent G2/M checkpoint via Cdc55p. Supporting this model, I found that deletion of CDC55 rescues the aberrant bud morphology of a zds1zds2 strain. I also show that expression of Zds1p or Zds2p from a strong galactose-inducible promoter induces mitosis even when the Swe1p-dependent G2/M checkpoint is activated by mis-organization of the actin cytoskeleton. Together these data indicate that the Cdc55p/Zds2p module regulates the Swe1p-dependent G2/M checkpoint.

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