Sit4 phosphatase is functionally linked to the ubiquitin-proteasome system.

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.
Mesh Terms:
Amino Acid Sequence, CDC2 Protein Kinase, Chromosomes, Fungal, Conserved Sequence, Cysteine Endopeptidases, Ethyl Methanesulfonate, Gene Expression Regulation, Fungal, Genes, Lethal, Molecular Sequence Data, Multienzyme Complexes, Mutagens, Mutation, Phosphoprotein Phosphatases, Proteasome Endopeptidase Complex, Protein Phosphatase 2, Saccharomyces cerevisiae, Saccharomyces cerevisiae Proteins, Sequence Homology, Amino Acid, Ubiquitin-Conjugating Enzymes, Ubiquitins
Genetics
Date: Aug. 01, 2003
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