Transcriptional responses to glucose at different glycolytic rates in Saccharomyces cerevisiae.

The addition of glucose to Saccharomyces cerevisiae cells causes reprogramming of gene expression. Glucose is sensed by membrane receptors as well as (so far elusive) intracellular sensing mechanisms. The availability of four yeast strains that display different hexose uptake capacities allowed us to study glucose-induced effects at different glycolytic rates. Rapid glucose responses were observed in all strains able to take up glucose, consistent with intracellular sensing. The degree of long-term responses, however, clearly correlated with the glycolytic rate: glucose-stimulated expression of genes encoding enzymes of the lower part of glycolysis showed an almost linear correlation with the glycolytic rate, while expression levels of genes encoding gluconeogenic enzymes and invertase (SUC2) showed an inverse correlation. Glucose control of SUC2 expression is mediated by the Snf1-Mig1 pathway. Mig1 dephosphorylation upon glucose addition is known to lead to repression of target genes. Mig1 was initially dephosphorylated upon glucose addition in all strains able to take up glucose, but remained dephosphorylated only at high glycolytic rates. Remarkably, transient Mig1-dephosphorylation was accompanied by the repression of SUC2 expression at high glycolytic rates, but stimulated SUC2 expression at low glycolytic rates. This suggests that Mig1-mediated repression can be overruled by factors mediating induction via a low glucose signal. At low and moderate glycolytic rates, Mig1 was partly dephosphorylated both in the presence of phosphorylated, active Snf1, and unphosphorylated, inactive Snf1, indicating that Mig1 was actively phosphorylated and dephosphorylated simultaneously, suggesting independent control of both processes. Taken together, it appears that glucose addition affects the expression of SUC2 as well as Mig1 activity by both Snf1-dependent and -independent mechanisms that can now be dissected and resolved as early and late/sustained responses.     

Glucose repression in yeast.

The Snf1 protein kinase is a central component of the signaling pathway for glucose repression in yeast. Recent studies have addressed the regulation of Snf1 kinase activity and elucidated mechanisms by which Snf1 controls repression and activation of glucose-repressed genes. Important advances include evidence that Snf1 regulates the localization of the Mig1 repressor and that Snf1 functions at multiple points to control Cat8 and Sip4, the activators of gluconeogenic genes.     

Hxk2 regulates the phosphorylation state of Mig1 and therefore its nucleocytoplasmic distribution

Mig1 and Hxk2 are two major mediators of glucose repression in Saccharomyces cerevisiae. However, the mechanism by which Hxk2 participates in the glucose repression signaling pathway is not completely understood. Recently, it has been demonstrated that Hxk2 interacts with Mig1 to generate a repressor complex located in the nucleus of S. cerevisiae. However, the mechanism by which Mig1 favors the presence of Hxk2 in the nucleus is not clear, and the function of Hxk2 at the nuclear repressor complex level is still unknown. Here, we report that serine 311 of Mig1 is a critical residue for interaction with Hxk2 and that this interaction is regulated by glucose. Our findings suggest that Snf1 interacts constitutively with the Hxk2 component of the repressor complex at high and low glucose conditions. Furthermore, we show that Snf1 binds to Mig1 under low glucose conditions and that binding is largely abolished after a shift to high glucose medium. We found that phosphorylation of serine 311 of Mig1 by Snf1 kinase is essential for Mig1 protein nuclear export and derepression of the SUC2 gene in glucose-limited cells. These results allow postulating that the Hxk2 operates by interacting both with Mig1 and Snf1 to inhibit the Mig1 phosphorylation at serine 311 during high glucose grown.