title: Phosphorelay-regulated degradation of the yeast Ssk1p response regulator by the ubiquitin-proteasome system.
abstract: In Saccharomyces cerevisiae, a phosphorelay signal transduction pathway composed of Sln1p, Ypd1p, and Ssk1p, which are homologous to bacterial two-component signal transducers, is involved in the osmosensing mechanism. In response to high osmolarity, the phosphorelay system is inactivated and Ssk1p remains unphosphorylated. Unphosphorylated Ssk1p binds to and activates the Ssk2p mitogen-activated protein (MAP) kinase kinase kinase, which in turn activates the downstream components of the high-osmolarity glycerol response (HOG) MAP kinase cascade. Here, we report a novel inactivation mechanism for Ssk1p involving degradation by the ubiquitin-proteasome system. Degradation is regulated by the phosphotransfer from Ypd1p to Ssk1p, insofar as unphosphorylated Ssk1p is degraded more rapidly than phosphorylated Ssk1p. Ubc7p/Qri8p, an endoplasmic reticulum-associated ubiquitin-conjugating enzyme, is involved in the phosphorelay-regulated degradation of Ssk1p. In ubc7Delta cells in which the degradation is hampered, the dephosphorylation and/or inactivation process of the Hog1p MAP kinase is delayed compared with wild-type cells after the hyperosmotic treatment. Our results indicate that unphosphorylated Ssk1p is selectively degraded by the Ubc7p-dependent ubiquitin-proteasome system and that this mechanism downregulates the HOG pathway after the completion of the osmotic adaptation.
authors.affiliation: Institute of Molecular and Cellular Biosciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan.
authors.forename: Naoto
authors.initials: N
authors.lastname: Sato
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Kawahara
authors.forename: Akio
authors.initials: A
authors.lastname: Toh-e
authors.forename: Tatsuya
authors.initials: T
authors.lastname: Maeda
mesh_terms: D001692:Biological Transport; D003546:Cysteine Endopeptidases / Q000235:genetics / Q000378:metabolism*; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism; D015536:Down-Regulation; D047908:Intracellular Signaling Peptides and Proteins; D008025:Ligases / Q000235:genetics / Q000378:metabolism; D020930:MAP Kinase Kinase Kinases; D020935:MAP Kinase Signaling System; D009097:Multienzyme Complexes / Q000235:genetics / Q000378:metabolism*; D009154:Mutation; D009995:Osmosis; D010766:Phosphorylation; D046988:Proteasome Endopeptidase Complex; D011494:Protein Kinases; D017346:Protein Serine-Threonine Kinases / Q000235:genetics / Q000378:metabolism; D017434:Protein Structure, Tertiary; D012441:Saccharomyces cerevisiae / Q000235:genetics / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics / Q000378:metabolism*; D015398:Signal Transduction; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism; D025801:Ubiquitin / Q000378:metabolism*; D044763:Ubiquitin-Conjugating Enzymes
chemical_list: D004268:DNA-Binding Proteins; D047908:Intracellular Signaling Peptides and Proteins; D009097:Multienzyme Complexes; C083962:SKN7 protein, S cerevisiae; C087199:SSK1 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; D014157:Transcription Factors; D025801:Ubiquitin; C077072:UBC7 protein, S cerevisiae; D044763:Ubiquitin-Conjugating Enzymes; D011494:Protein Kinases; C102035:YPD1 protein, S cerevisiae; D017346:Protein Serine-Threonine Kinases; D020930:MAP Kinase Kinase Kinases; C472815:SSK2 protein, S cerevisiae; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex; D008025:Ligases