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Molecular and Cellular Biology, September 2003, p. 6662-6671, Vol. 23, No. 18
0270-7306/03/$08.00+0     DOI: 10.1128/MCB.23.18.6662-6671.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

Phosphorelay-Regulated Degradation of the Yeast Ssk1p Response Regulator by the Ubiquitin-Proteasome System

Naoto Sato,1,2 Hiroyuki Kawahara,3 Akio Toh-e,2 and Tatsuya Maeda1*

Institute of Molecular and Cellular Biosciences, The University of Tokyo, Bunkyo-ku, Tokyo 113-0032,1 Department of Biological Sciences, Graduate School of Science, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033,2 Department of Biochemistry, Graduate School of Pharmaceutical Sciences, Hokkaido University, Sapporo 060-0812, Japan3

Received 27 March 2003/ Returned for modification 2 June 2003/ Accepted 6 June 2003

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 ubc7{Delta} 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.

* Corresponding author. Mailing address: Institute of Molecular and Cellular Biosciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan. Phone: 81-3-5841-7820. Fax: 81-3-5841-7899. E-mail: maeda{at}ims.u-tokyo.ac.jp.

Molecular and Cellular Biology, September 2003, p. 6662-6671, Vol. 23, No. 18
0022-538X/03/$08.00+0     DOI: 10.1128/MCB.23.18.6662-6671.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.



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