Friday, November 22, 2024
11/22/2024
SUMMARY Because glucose is the principal carbon and energy source for most cells, organisms have evolved numerous and sophisticated mechanisms for sensing glucose and responding to it appropriately. Impaired regulation of blood glucose levels, due to defects in glucose sensing, may cause severe metabolic disorders, such as diabetes, and many types of cancer cells depend on a high rate of glucose consumption to maintain their viability. Glucose sensing and signaling is of great significance to yeast, because it contributes to distinctive fermentative metabolism of yeast, a lifestyle it shares with many kinds of tumor cells. Our long-term goal is to understand how eukaryotic cells sense extracellular glucose and adapt their central metabolic pathways to better suit the availability of this crucial fuel using the yeast S. cerevisiae as a model system. Energy generation by fermentation of glucose is inefficient, requiring yeast cells to pump large amounts of glucose through glycolysis. They do this by enhancing the rate-limiting step of glucose metabolism, its transport. Yeast cells have learned how to sense the amount of extracellular glucose available and respond by expressing the most appropriate of its 17 glucose transporters. Expression of the glucose transporter genes (HXTs) is repressed by the Rgt1 transcription factor, which forms a repressor complex with its corepressors Mth1 and Std1 and the general corepressors Ssn6/Tup1 on the HXT promoters in the absence of glucose. Glucose induction of HXT expression is achieved through a signal transduction pathway that begins at the cell surface with the Rgt2 and Snf3 glucose sensing receptors (GSRs) and ends in the nucleus with Rgt1.The general outline of this pathway was in place at the beginning of the last funding period. It was clear that the GSRs generate an intracellular signal in response to glucose that alters the function of the Rgt1 repressor by causing proteasomal degradation of Mh1 and Std1, and that signal generation is receptor-mediated, because glucose metabolism is not necessary for its generation. What was less clearly understood was how the glucose signal is transduced from the GSRs to Mth1 and Std1. The Ycks (the yeast orthologs of mammalian CK1) were suggested to be coupled to the GSRs and act as downstream effectors of the GSRs, but genetic epistasis and biochemical analyses place the Ycks upstream of the GSRs. There are crucial gaps in our understanding of how the signal is generated and transduced from the cell surface to the cytosol. Over the next three years, our efforts will be devoted to filling the gaps and developing a new model for this glucose signal transduction pathway.
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