GLUCOSE SENSING AND SIGNALING IN YEAST

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Glucose is the preferred carbon source for yeasts, as it is for most mammalian cells. Both types of organisms have evolved sophisticated regulatory mechanisms to optimize its utilization. In yeasts, glucose represses expression of many genes that are dispensables for cells growing on glucose (e.g., GAL genes), and induces expression of several genes necessary for efficient utilization (e.g., HXT genes, encoding glucose transporters); mammals have devised multiple mechanisms for maintaining glucose homeostasis. We believe that understanding glucose sensing and signalling mechanisms in the simple eukaroyte bakers' yeast (Saccharomyces cerevisiae) will inform the analysis the glucose signaling in larger eukaryotic cells. Both glucose repression and glucose induction of gene expression in yeast can be viewed as signal transduction pathways: extracellular glucose is sensed by receptors (possibly the glucose transporters), generating a signal that ultimately affects the function of gene regulatory proteins. Gene regulatory proteins for both pathways have been identified: the Mig1p repressor is a central component of the glucose repression pathway, and several proteins that affect its function have been identified; we have recently identified the Rgt1p repressor and its regulator, Grr1p, as a key components of the glucose induction pathway. Our long-term goals are to understand how glucose is sensed, to learn what the signal is and how it is generated, and to determine how it is transduced to the gene regulatory proteins to alter gene expression: The specific aims are: . To understand the mechanism of glucose repression: We will focus on two central issues of glucose repression: 1) how is Mig1p function regulated; 2) how do Ga183p and Reg1p transmit the glucose signal to the Snf1p protein kinase, an inhibitor of Mig1p function)? . To understand the glucose induction mechanism: We intend 1) determine how glucose inhibits function of the Rgt1p repressor, 2) learn the role of Grr1p in this process, and 3) identify other components of the glucose-induction pathway that almost certainly exist. . To understand the glucose signal-generation mechanism: Three main issues are: what is the glucose sensor(s), how does it generate the glucose signal(s), and what is the nature of the signal. To address these issues, we propose to 1) test the role of glucose transporters as glucose sensors, 2) analyze the Snf3p glucose transporter, which appears to be a sensor of low levels of glucose, and 3) test glucose analogs for their ability to generate a glucose induction signal. . To identify activators of the GAL4 promoter, and understand why it is so weak: This promoter, which is among the weakest in yeasts, appears to contain novel functional elements. To understand the basis of promoter strength, and identify proteins that regulate expression of transcriptional activators, we propose to 1) analyze the novel TATA-like UES/GAL4 element, and identify proteins that act through it, and 2) characterize proteins we have identified that appear to act through the UAS/GAL4 element.