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. 2003 Aug;164(4):1305–1321. doi: 10.1093/genetics/164.4.1305

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

Thorsten Singer 1, Stefan Haefner 1, Michael Hoffmann 1, Michael Fischer 1, Julia Ilyina 1, Wolfgang Hilt 1
PMCID: PMC1462641  PMID: 12930741

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.

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Selected References

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