Glucose fuels life. Most cells prefer it as their carbon and energy source, and must rapidly and accurately sense it. This is particularly apparent for yeast (S. cerevisiae) because of the unusual way it uses glucose. Yeast cells must detect fluctuations in the glucose supply and rapidly adjust their metabolism to make maximum use of what is available. Yeasts control glucose utilization at its first, rate- limiting step - transport into the cell. We discovered two novel glucose sensors that initiate a signal transduction pathway that regulates expression of HXT genes encoding glucose transporters. These glucose sensors are the founding members of a novel class of nutrient receptor related to nutrient transporters;we would like to know they work. The signal transduction pathway initiated by the glucose sensors is now in focus;I believe we are poised to elevate it to a level of understanding on par with other signal transduction pathways. This glucose sensing pathway begins at the cell membrane with the Snf3 and Rgt2 glucose sensors, which are coupled to the Yck1 protein kinase. The glucose signal is transduced to the Rgt1 transcription factor, which represses HXT genes, and is inhibited by Mth1 and Std1. Glucose binding to the sensors causes Yck1 to phosphorylate Mth1 and Std1, thereby targeting them for ubiquitination and degradation. We want to know how the glucose signal is generated by the sensors, how the signal activates phosphorylation of Mth1 and Std1 by Yck1, how Mth1 and Std1 regulate Rgt1 function, and how the signaling pathway meshes with the metabolic network.
Our Specific Aims for the next four years are:
Aim 1 : Learn how the glucose signal is generated and transduced at the top of the pathway.
Aim1 A: Are the glucose sensors glucose receptors? Aim1B: What is the basis for functional differences between sensors and transporters? Aim1C: What is the role of Yck1 in the signaling pathway? Aim1D: Are we missing any components of the SRR pathway? Aim 2: Learn how the glucose signal regulates the Rgt1 repressor.
Aim2 A: How does the antirepressor region of Rgt1 inhibt its function?.
Aim2 B: What is the role of phosphorylated residues of Rgt1? Aim2C: How do Mth1 and Std1 inhibit the antirepressor region of Rgt1? Aim 3: Learn how the glucose signaling pathway is integrated with the metabolic network.
Aim 3 A: What is the role of Mth1 in adenine biosynthesis and one-carbon metabolism? Aim 3B: How is the signaling pathway deployed in other yeasts? How did it evolve?

Public Health Relevance

Glucose fuels life. Most cells prefer it as their food source;some cells require it. Cells have evolved numerous and sophisticated mechanisms for sensing glucose and responding to it appropriately. This is especially apparent in Bakers'yeast (S. cerevisiae), which has several highly evolved regulatory mechanisms for sensing and utilizing the widely varying amounts of glucose it encounters during its lifetime. These regulatory mechanisms determine the distinctive metabolism of yeast, a lifestyle it shares with many kinds of tumor cells. Our long-term goal is to understand how yeast cells sense and respond to glucose. We are studying a glucose sensing system whose key components are novel glucose receptors that sit in the cell membrane and detect glucose, sending a signal into the cell that affects gene expression and influences metabolism.
We aim to learn how these novel receptors work.

National Institute of Health (NIH)
National Institute of General Medical Sciences (NIGMS)
Research Project (R01)
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Cellular Signaling and Regulatory Systems Study Section (CSRS)
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Maas, Stefan
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University of Colorado Denver
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Snowdon, Chris; Johnston, Mark (2016) A novel role for yeast casein kinases in glucose sensing and signaling. Mol Biol Cell 27:3369-3375
Simpson-Lavy, Kobi J; Bronstein, Alex; Kupiec, Martin et al. (2015) Cross-Talk between Carbon Metabolism and the DNA Damage Response in S. cerevisiae. Cell Rep 12:1865-75
Simpson-Lavy, Kobi J; Johnston, Mark (2013) SUMOylation regulates the SNF1 protein kinase. Proc Natl Acad Sci U S A 110:17432-7
Libkind, Diego; Hittinger, Chris Todd; Valerio, Elisabete et al. (2011) Microbe domestication and the identification of the wild genetic stock of lager-brewing yeast. Proc Natl Acad Sci U S A 108:14539-44
Kuttykrishnan, Sooraj; Sabina, Jeffrey; Langton, Laura L et al. (2010) A quantitative model of glucose signaling in yeast reveals an incoherent feed forward loop leading to a specific, transient pulse of transcription. Proc Natl Acad Sci U S A 107:16743-8
Hittinger, Chris Todd; Gonçalves, Paula; Sampaio, José Paulo et al. (2010) Remarkably ancient balanced polymorphisms in a multi-locus gene network. Nature 464:54-8
Sabina, Jeffrey; Johnston, Mark (2009) Asymmetric signal transduction through paralogs that comprise a genetic switch for sugar sensing in Saccharomyces cerevisiae. J Biol Chem 284:29635-43
Brown, Victoria; Sabina, Jeffrey; Johnston, Mark (2009) Specialized sugar sensing in diverse fungi. Curr Biol 19:436-41
Kim, Jeong-Ho; Johnston, Mark (2006) Two glucose-sensing pathways converge on Rgt1 to regulate expression of glucose transporter genes in Saccharomyces cerevisiae. J Biol Chem 281:26144-9
Kim, Jeong-Ho; Brachet, Valerie; Moriya, Hisao et al. (2006) Integration of transcriptional and posttranslational regulation in a glucose signal transduction pathway in Saccharomyces cerevisiae. Eukaryot Cell 5:167-73

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