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Molecular and Cellular Biology, January 2008, p. 551-563, Vol. 28, No. 2
0270-7306/08/$08.00+0     doi:10.1128/MCB.00929-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Activation of the SPS Amino Acid-Sensing Pathway in Saccharomyces cerevisiae Correlates with the Phosphorylation State of a Sensor Component, Ptr3{triangledown} ,{dagger} ,{ddagger}

Zhengchang Liu,* Janet Thornton, Mário Spírek,§ and Ronald A. Butow

Department of Molecular Biology, University of Texas Southwestern Medical Center, 5323 Harry Hines Blvd., Dallas, Texas 75390-9148

Received 25 May 2007/ Returned for modification 10 July 2007/ Accepted 23 October 2007

Cells of the budding yeast Saccharomyces cerevisiae sense extracellular amino acids and activate expression of amino acid permeases through the SPS-sensing pathway, which consists of Ssy1, an amino acid sensor on the plasma membrane, and two downstream factors, Ptr3 and Ssy5. Upon activation of SPS signaling, two transcription factors, Stp1 and Stp2, undergo Ssy5-dependent proteolytic processing that enables their nuclear translocation. Here we show that Ptr3 is a phosphoprotein whose hyperphosphorylation is increased by external amino acids and is dependent on Ssy1 but not on Ssy5. A deletion mutation in GRR1, encoding a component of the SCFGrr1 E3 ubiquitin ligase, blocks amino acid-induced hyperphosphorylation of Ptr3. We found that two casein kinase I (CKI) proteins, Yck1 and Yck2, previously identified as positive regulators of SPS signaling, are required for hyperphosphorylation of Ptr3. Loss- and gain-of-function mutations in PTR3 result in decreased and increased Ptr3 hyperphosporylation, respectively. We found that a defect in PP2A phosphatase activity leads to the hyperphosphorylation of Ptr3 and constitutive activation of SPS signaling. Two-hybrid analysis revealed interactions between the N-terminal signal transduction domain of Ssy1 with Ptr3 and Yck1. Our findings reveal that CKI and PP2A phosphatase play antagonistic roles in SPS sensing by regulating Ptr3 phosphorylation.

* Corresponding author. Present address: Department of Biological Sciences, University of New Orleans, New Orleans, LA 70148. Phone: (504) 280-6314. Fax: (504) 280-6121. E-mail: zliu5{at}uno.edu

{triangledown} Published ahead of print on 5 November 2007.

{dagger} Supplemental material for this article may be found at http://mcb.asm.org/.

{ddagger} We dedicate this paper to the memory of Ronald A. Butow (1936-2007).

§ Present address: Vienna Biocenter II, Max Perutz Laboratories, Department of Chromosome Biology, Vienna, Austria.

Molecular and Cellular Biology, January 2008, p. 551-563, Vol. 28, No. 2
0270-7306/08/$08.00+0     doi:10.1128/MCB.00929-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.



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