title: Phosphorylation by Pho85 cyclin-dependent kinase acts as a signal for the down-regulation of the yeast sphingoid long-chain base kinase Lcb4 during the stationary phase.
abstract: Sphingoid long-chain base 1-phosphates (LCBPs) act as bioactive lipid molecules in eukaryotic cells. In yeast, LCBPs are synthesized mainly by the long-chain base kinase Lcb4p. Until now, the regulatory mechanism for Lcb4p has been unclear. In the present study, we found that Lcb4p is post-translationally modified by phosphorylation. Using a protein kinase mutant yeast collection, we further demonstrated that the cyclin-dependent kinase Pho85p is involved in this phosphorylation. Pho85p functions in a number of cellular processes, especially in response to environmental changes. Two of 10 Pho85p cyclins, Pcl1p and Pcl2p had overlapping functions in the phosphorylation of Lcb4p. Site-directed mutagenesis identified the phosphorylation sites in Lcb4p as Ser(451) and Ser(455). Additionally, pulse-chase experiments revealed that Lcb4p is degraded via the ubiquitin-dependent pathway. The protein was stabilized in Deltapho85 cells, suggesting that phosphorylation acts as a signal for the degradation. Lcb4p is down-regulated in the stationary phase of cell growth, and both phosphorylation and ubiquitination appear to be important for this process. Moreover, we demonstrated that Lcb4p is delivered to the vacuole for degradation via the multivesicular body. Since forced accumulation of LCBPs results in prolonged growth during the stationary phase, down-regulation of Lcb4p may be physiologically important for proper cellular responses to nutrient deprivation.
authors.affiliation: Department of Biomembrane and Biofunctional Chemistry, Graduate School of Pharmaceutical Sciences, Hokkaido University, Kita 12-jo, Nishi 6-choume, Kita-ku, Sapporo 060-0812, Japan.
authors.forename: Soichiro
authors.initials: S
authors.lastname: Iwaki
authors.forename: Akio
authors.initials: A
authors.lastname: Kihara
authors.forename: Takamitsu
authors.initials: T
authors.lastname: Sano
authors.forename: Yasuyuki
authors.initials: Y
authors.lastname: Igarashi
mesh_terms: D001665:Binding Sites; D018844:Cyclin-Dependent Kinases / Q000378:metabolism*; D015536:Down-Regulation; D010766:Phosphorylation; D017853:Phosphotransferases (Alcohol Group Acceptor) / Q000235:genetics* / Q000378:metabolism; D011499:Protein Processing, Post-Translational; D012097:Repressor Proteins / Q000378:metabolism*; D016192:Resting Phase, Cell Cycle; D029701:Saccharomyces cerevisiae Proteins / Q000378:metabolism*; D025801:Ubiquitin / Q000378:metabolism; D014617:Vacuoles / Q000378:metabolism
chemical_list: D012097:Repressor Proteins; D029701:Saccharomyces cerevisiae Proteins; D025801:Ubiquitin; D017853:Phosphotransferases (Alcohol Group Acceptor); C426837:sphingolipid long-chain base kinase; D018844:Cyclin-Dependent Kinases; C058605:PHO85 protein, S cerevisiae