title: Sli2 (Ypk1), a homologue of mammalian protein kinase SGK, is a downstream kinase in the sphingolipid-mediated signaling pathway of yeast.
abstract: ISP-1 is a new type of immunosuppressant, the structure of which is homologous to that of sphingosine. In a previous study, ISP-1 was found to inhibit mammalian serine palmitoyltransferase, the primary enzyme involved in sphingolipid biosynthesis, and to reduce the intracellular pool of sphingolipids. ISP-1 induces the apoptosis of cytotoxic T cells, which is triggered by decreases in the intracellular levels of sphingolipids. In this study, the inhibition of yeast (Saccharomyces cerevisiae) proliferation by ISP-1 was observed. This ISP-1-induced growth inhibition was also triggered by decreases in the intracellular levels of sphingolipids. In addition, DNA duplication without cytokinesis was detected in ISP-1-treated yeast cells on flow cytometry analysis. We have cloned multicopy suppressor genes of yeast which overcome the lethal sphingolipid depletion induced by ISP-1. One of these genes, SLI2, is synonymous with YPK1, which encodes a serine/threonine kinase. Kinase-dead mutants of YPK1 did not show any resistance to ISP-1, leading us to predict that the kinase activity of the Ypk1 protein should be essential for this resistance to ISP-1. Ypk1 protein overexpression had no effect on sphingolipid biosynthesis by the yeast. Furthermore, both the phosphorylation and intracellular localization of the Ypk1 protein were regulated by the intracellular sphingolipid levels. These data suggest that the Ypk1 protein is a downstream kinase in the sphingolipid-mediated signaling pathway of yeast. The Ypk1 protein was reported to be a functional homologue of the mammalian protein kinase SGK, which is a downstream kinase of 3-phosphoinositide-dependent kinase 1 (PDK1). PDK1 phosphotidylinositol (PI) is regulated by PI-3,4,5-triphosphate and PI-3,4-bisphosphate through the pleckstrin homology (PH) domain. Overexpression of mammalian SGK also overcomes the sphingolipid depletion in yeast. Taking both the inability to produce PI-3,4, 5-triphosphate and PI-3,4-bisphosphate and the lack of a PH domain in the yeast homologue of PDK1, the Pkh1 protein, into account, these findings further suggest that yeast may use sphingolipids instead of inositol phospholipids as lipid mediators.
authors.affiliation: Department of Biological Chemistry, Graduate School of Pharmaceutical Sciences, Graduate School of Biostudies, Kyoto University, Kyoto 606-8501, Japan.
authors.forename: Y
authors.initials: Y
authors.lastname: Sun
authors.forename: R
authors.initials: R
authors.lastname: Taniguchi
authors.forename: D
authors.initials: D
authors.lastname: Tanoue
authors.forename: T
authors.initials: T
authors.lastname: Yamaji
authors.forename: H
authors.initials: H
authors.lastname: Takematsu
authors.forename: K
authors.initials: K
authors.lastname: Mori
authors.forename: T
authors.initials: T
authors.lastname: Fujita
authors.forename: T
authors.initials: T
authors.lastname: Kawasaki
authors.forename: Y
authors.initials: Y
authors.lastname: Kozutsumi
mesh_terms: D005229:Fatty Acids, Monounsaturated / Q000494:pharmacology*; D005656:Fungal Proteins / Q000502:physiology*; D038362:Glycogen Synthase Kinase 3; D017874:Immediate-Early Proteins; D007166:Immunosuppressive Agents / Q000494:pharmacology*; D009687:Nuclear Proteins; D017346:Protein Serine-Threonine Kinases / Q000502:physiology; D011505:Protein-Tyrosine Kinases / Q000502:physiology*; D012441:Saccharomyces cerevisiae / Q000502:physiology*; D029701:Saccharomyces cerevisiae Proteins; D015398:Signal Transduction; D000099184:Serum-Glucocorticoid Regulated Kinases
chemical_list: D005229:Fatty Acids, Monounsaturated; D005656:Fungal Proteins; D038362:Glycogen Synthase Kinase 3; D017874:Immediate-Early Proteins; D007166:Immunosuppressive Agents; D009687:Nuclear Proteins; D017346:Protein Serine-Threonine Kinases; D011505:Protein-Tyrosine Kinases; D029701:Saccharomyces cerevisiae Proteins; D000099184:Serum-Glucocorticoid Regulated Kinases; C068124:MCK1 protein, S cerevisiae; C001996:thermozymocidin