title: Sit4 phosphatase is functionally linked to the ubiquitin-proteasome system.
abstract: Using a synthetic lethality screen we found that the Sit4 phosphatase is functionally linked to the ubiquitin-proteasome system. Yeast cells harboring sit4 mutations and an impaired proteasome (due to pre1-1 pre4-1 mutations) exhibited defective growth on minimal medium. Nearly identical synthetic effects were found when sit4 mutations were combined with defects of the Rad6/Ubc2- and Cdc34/Ubc3-dependent ubiquitination pathways. Under synthetic lethal conditions, sit4 pre or sit4 ubc mutants formed strongly enlarged unbudded cells with a DNA content of 1N, indicating a defect in the maintenance of cell integrity during starvation-induced G(1) arrest. Sit4-related synthetic effects could be cured by high osmotic pressure or by the addition of certain amino acids to the growth medium. These results suggest a concerted function of the Sit4 phosphatase and the ubiquitin-proteasome system in osmoregulation and in the sensing of nutrients. Further analysis showed that Sit4 is not a target of proteasome-dependent protein degradation. We could also show that Sit4 does not contribute to regulation of proteasome activity. These data suggest that both Sit4 phosphatase and the proteasome act on a common target protein.
authors.affiliation: Institut für Biochemie, Universität Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany.
authors.forename: Thorsten
authors.initials: T
authors.lastname: Singer
authors.forename: Stefan
authors.initials: S
authors.lastname: Haefner
authors.forename: Michael
authors.initials: M
authors.lastname: Hoffmann
authors.forename: Michael
authors.initials: M
authors.lastname: Fischer
authors.forename: Julia
authors.initials: J
authors.lastname: Ilyina
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Hilt
mesh_terms: D000595:Amino Acid Sequence; D016203:CDC2 Protein Kinase / Q000378:metabolism; D015825:Chromosomes, Fungal; D017124:Conserved Sequence; D003546:Cysteine Endopeptidases / Q000235:genetics / Q000378:metabolism*; D005020:Ethyl Methanesulfonate / Q000494:pharmacology; D015966:Gene Expression Regulation, Fungal; D005804:Genes, Lethal; D008969:Molecular Sequence Data; D009097:Multienzyme Complexes / Q000235:genetics / Q000378:metabolism*; D009153:Mutagens / Q000494:pharmacology; D009154:Mutation; D010749:Phosphoprotein Phosphatases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D046988:Proteasome Endopeptidase Complex; D054648:Protein Phosphatase 2; D012441:Saccharomyces cerevisiae / Q000166:cytology / Q000235:genetics / Q000254:growth & development; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D044763:Ubiquitin-Conjugating Enzymes / Q000378:metabolism; D014452:Ubiquitins / Q000378:metabolism*
chemical_list: D009097:Multienzyme Complexes; D009153:Mutagens; D029701:Saccharomyces cerevisiae Proteins; D014452:Ubiquitins; D005020:Ethyl Methanesulfonate; C471606:RAD6 protein, S cerevisiae; D044763:Ubiquitin-Conjugating Enzymes; D016203:CDC2 Protein Kinase; D010749:Phosphoprotein Phosphatases; D054648:Protein Phosphatase 2; C104343:SIT4 protein, S cerevisiae; D003546:Cysteine Endopeptidases; C083706:PRE1 protein, S cerevisiae; D046988:Proteasome Endopeptidase Complex