Most organisms must rapidly and accurately detect nutrients and respond appropriately to scarcity and surfeit. Sensing of glucose is especially important because it is the main carbon and energy source for most cells. Accurate glucose sensing and response is crucial for humans, because even minor defects in the process eventually leads to diabetes, one of our most serious and prevalent diseases. The importance of glucose sensing is particularly apparent in Bakers' yeast (S. cerevisiae) because of its unorthodox manner of metabolizing glucose: yeasts prefer to ferment glucose rather than use the more efficient oxidative pathway (a trait they share with cancer cells). Since fermentation demands a high metabolic flux of glucose, yeast (and cancer) cells have evolved sophisticated systems for sensing when glucose levels are sufficient to support fermentation and developed efficient ways to acquire glucose. The (rate-limiting) first step of glucose utilization - its tranport into the cell - is a key point of glucose sensing and acquisition; both yeast and cancer cells regulate it closely. We previously identified in yeast the founding members of a family of nutrient sensors related to transporters for the nutrient: Snf3 and Rgt2 are glucose sensors that obviously evolved from glucose transporters. They sense extracellular glucose and generate an intracellular signal that results in expression of several HXT genes encoding bona fide glucose transporters. This is a novel signal transduction pathway, and we wish to understand how it works, from how glucose is sensed by these novel receptors at the top of the pathway through to the unusual mode of regulation of the transcription factor at the bottom of the pathway. The central questions that need to be answered are at the top and bottom of the signaling pathway. We need to understand how the glucose signal generated by the sensors stimulates the key event in the pathway: activation of Casein kinase (Yck) to phosphorylate the transcription factor regulators Mth1 and Std1. Our realization that the Snf1 protein kinase may regulate Yck function, along with our recent discovery of three other components of the signal transduction pathway - two protein kinases and their potential scaffold - have revealed a signaling pathway more complex than we had realized. Learning where these new components act in the pathway and what they do is a major goal (Aim 1). We must learn how the Mth1/Std1 co-repressors regulate function of the Rgt1 transcription factor at the bottom of the pathway. We have a crude model based on genetic evidence; testing and refining it (Aim 2) promises insight into a novel mechanism of transcription factor regulation. Transporter-like nutrient sensors have only been identified in fungi. We know enough about the pathway to be able to use yeast to search for them in other organisms (Aim 3). Identifying such a glucose sensor in a metazoan would be a significant finding; characterizing their evolutionary breadth in fungi would be a useful and valuable contribution.
Most organisms must rapidly and accurately detect the nutrient glucose, their main carbon and energy source, and respond appropriately to scarcity and surfeit. Accurate glucose sensing and response is crucial for humans, because even minor defects in the process eventually lead to diabetes, one of our most serious and prevalent diseases. We aim to elucidate the mechanism of a novel glucose sensing pathway in Bakers' yeast (a model for human biology) that is anchored by unique glucose receptors, and to search for this novel type of glucose receptor in other organisms (including humans).
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