Glucose Uptake and Metabolism in grr1/cat80 Mutants of Sacharomyces cerevisiae

Abstract

Glucose repression in the yeast Saccharomyces cerevisiae designates a global regulatory system controlling the expression of various sets of genes required for the utilization of alternate carbon sources. In a screen, designed for the selection of mutants with reduced glycolytic flux we obtained isolates which were shown by complementation of the cloned wild-type gene to be allelic to the glucose repression mutants grrllcat80lcot2 previously described. We demonstrate that the grrl lesion lead to a concentration-dependent decrease in glycolytic flux on glucose. It is very likely that this is caused by a significant decrease in the expression of various genes encoding hexose transporters (HXTI,3) leading to a reduced glucose-uptake rate. In contrast, expression of the maltose permease gene (MALll) and maltose utilization is normal. There is indirect evidence that grrl affects the uptake of amino acids, and others have shown that the sugar-induced transport of divalent cations is impaired. These effects are not glucose-specific. We suggest that Grrl, a putative cytoplasmic protein, has a central function in the sensing of nutritional conditions for a variety of unrelated substances, and that relief from glucose repression may be a corollary of this defect in sensing.In the yeast Saccharomyces cerevisiae glucose or other readily fermentable carbon sources act as global regulators of growth and metabolism. These regulatory effects are executed partly at the transcriptional level. Glucose repression and glucose induction designate mechanisms that exert specific control on glucose-sensitive genes. Glucose-repressible genes include SUC2 (invertase) and genes that are required for the utilization of alternate carbohydrates (the GAL, MAL and MEL regulons required for the utilization of galactose, maltose and melibiose, respectively; for review see [l]), mitochondrial genes [2] and nuclear genes for mitochondrial and gluconeogenic functions [3]. Analysis of glucose induction at the gene expression level is less advanced. Nevertheless, glucose or its metabolism exerts a plethora of activating effects on many cellular functions. It is not clear, in which way glucose repression and induction are functionally linked.Glucose repression mechanisms in S. cerevisiae have been revealed by means of appropriate mutants either defective in repression or in derepression (for review, see [4]). Analysis of individual mutants allowed in some cases the identification or localization of the regulatory defect. A number of such mutants lead to the identification of transcription factors or transcriptional regulators of glucose-sensitive genes. There are a number of mutants which apparently affect functions early in the signalling pathway for glucoseCorrespondence to

Rgt1p of Saccharomyces cerevisiae, a Key Regulator of Glucose-Induced Genes, Is both an Activator and a Repressor of Transcription

“…grr1⌬ mutants are defective in glucose repression of several genes, including GAL1. This is because grr1⌬ mutants fail to express glucose transporters (5,31,33,46), making them unable to sense glucose. Thus, grr1⌬ mutants growing on glucose express GAL1::HIS3 and GAL1::lacZ and are therefore His ϩ and form blue colonies on X-Gal (5-bromo-4-chloro-3-indolyl-␤-D-galactopyranoside) plates (14).…”

“…Glucose-stimulated inhibition of Rgt1p requires Grr1p, a leucine-rich repeat-containing protein, and Snf3p, a glucose transporter thought to be involved in sensing glucose and generating an intracellular signal (6,9,10,14,21,31,33). Thus, grr1⌬ and snf3⌬ mutants grow poorly on glucose and display severely impaired glucose transport because of reduced expression of glucose transporters (5,6,24,25,33,46).…”

“…In cells growing in the absence of glucose, Rgt1p represses HXT expression; addition of glucose causes Rgt1p function to be inhibited, leading to derepression of HXT expression. The different responses of HXT1 and HXT2 (and HXT4) to different levels of glucose are due to two additional regulatory mechanisms that act on these genes: HXT2 and HXT4 are also subject to glucose repression mediated by Mig1p, a repressor that is activated by high levels of glucose (32, 47), and HXT1 responds to an additional regulatory mechanism, whose components have not yet been identified, that requires high levels of glucose for function (31).Glucose-stimulated inhibition of Rgt1p requires Grr1p, a leucine-rich repeat-containing protein, and Snf3p, a glucose transporter thought to be involved in sensing glucose and generating an intracellular signal (6,9,10,14,21,31,33). Thus, grr1⌬ and snf3⌬ mutants grow poorly on glucose and display severely impaired glucose transport because of reduced expression of glucose transporters (5,6,24,25,33,46).…”

“…Thus, grr1⌬ and snf3⌬ mutants grow poorly on glucose and display severely impaired glucose transport because of reduced expression of glucose transporters (5,6,24,25,33,46). Mutations in RGT1 suppress the growth and glucose transport defects of grr1⌬ and snf3⌬ mutants by restoring HXT gene expression (13,25,31,46).…”

Amino Acid Signaling in Saccharomyces cerevisiae: a Permease-Like Sensor of External Amino Acids and F-Box Protein Grr1p Are Required for Transcriptional Induction of the AGP1 Gene, Which Encodes a Broad-Specificity Amino Acid Permease

“…Grr1p is in fact a component of a ubiquitin-protein ligase complex (SCF Grr1 ) (52) possibly involved in coupling nutrient availability to gene expression and cell cycle regulation (11,54). In addition to impaired glucose signaling, grr1 mutants display a number of other defects, including reduced uptake of aromatic amino acids (28) and leucine (69). These observations prompted us to test the role of Grr1p in Ssy1p-mediated induction of the AGP1 gene.…”