Regulation of glucose utilization in yeast

Functional identification of an Arabidopsis snf4 ortholog by screening for heterologous multicopy suppressors of snf4 deficiency in yeast

Yeast Snf4 is a prototype of activating gamma-subunits of conserved Snf1/AMPK-related protein kinases (SnRKs) controlling glucose and stress signaling in eukaryotes. The catalytic subunits of Arabidopsis SnRKs, AKIN10 and AKIN11, interact with Snf4 and suppress the snf1 and snf4 mutations in yeast. By expression of an Arabidopsis cDNA library in yeast, heterologous multicopy snf4 suppressors were isolated. In addition to AKIN10 and AKIN11, the deficiency of yeast snf4 mutant to grown on non-fermentable carbon source was suppressed by Arabidopsis Myb30, CAAT-binding factor Hap3b, casein kinase I, zinc-finger factors AZF2 and ZAT10, as well as orthologs of hexose/UDP-hexose transporters, calmodulin, SMC1-cohesin and Snf4. Here we describe the characterization of AtSNF4, a functional Arabidopsis Snf4 ortholog, that interacts with yeast Snf1 and specifically binds to the C-terminal regulatory domain of Arabidopsis SnRKs AKIN10 and AKIN11.

SUGAR-INDUCED SIGNAL TRANSDUCTION IN PLANTS

Sugars have important signaling functions throughout all stages of the plant's life cycle. This review presents our current understanding of the different mechanisms of sugar sensing and sugar-induced signal transduction, including the experimental approaches used. In plants separate sensing systems are present for hexose and sucrose. Hexokinase-dependent and -independent hexose sensing systems can further be distinguished. There has been progress in understanding the signal transduction cascade by analyzing the function of the SNF1 kinase complex and the regulatory PRL1 protein. The role of sugar signaling in seed development and in seed germination is discussed, especially with respect to the various mechanisms by which sugar signaling controls gene expression. Finally, recent literature on interacting signal transduction cascades is discussed, with particular emphasis on the ethylene and ABA signal transduction pathways.

Sucrose transporters in plants: update on function and structure

In plants, sucrose is the major transport form for photoassimilated carbon and is both a source of carbon skeletons and energy for plant organs unable to perform photosynthesis (sink organs). As a molecule translocated over distance, sucrose has to pass through a number of membranes. Membrane transport of sucrose has therefore been considered for a long time as a major determinant of plant productivity. After several decades of physiological and biochemical experiments measuring the activity of sucrose carriers, unequivocal evidence came from the first identification of a cDNA coding a sucrose carrier (SoSUT1, Riesmeier et al. (1992) EMBO J. 11, 4705-4713). At present 20 different cDNAs encoding sucrose carriers have been identified in different plant species, in both dicots and monocots (one case). The total number is increasing rapidly and most importantly, it can be guessed from the results obtained for Arabidopsis, that in each species, sucrose transporters represent a gene family. The sequences are highly conserved and those carriers display the typical 12 transmembrane alpha-helices of members of the Major Facilitator superfamily. Yeast expression of those carriers indicate that they are all influx carriers, all cotransport sucrose and proton and that their affinity for sucrose is surprisingly similar (0.2-2 mM). All their characteristics are in agreement with those demonstrated at the physiological level in plants. These characteristics are discussed in relation to the function in plants and the few data available on the structure of those transporters in relation to their function are presented.

Rapamycin-modulated transcription defines the subset of nutrient-sensitive signaling pathways directly controlled by the Tor proteins

The immunosuppressant rapamycin inhibits Tor1p and Tor2p (target of rapamycin proteins), ultimately resulting in cellular responses characteristic of nutrient deprivation through a mechanism involving translational arrest. We measured the immediate transcriptional response of yeast grown in rich media and treated with rapamycin to investigate the direct effects of Tor proteins on nutrient-sensitive signaling pathways. The results suggest that Tor proteins directly modulate the glucose activation and nitrogen discrimination pathways and the pathways that respond to the diauxic shift (including glycolysis and the citric acid cycle). Tor proteins do not directly modulate the general amino acid control, nitrogen starvation, or sporulation (in diploid cells) pathways. Poor nitrogen quality activates the nitrogen discrimination pathway, which is controlled by the complex of the transcriptional repressor Ure2p and activator Gln3p. Inhibiting Tor proteins with rapamycin increases the electrophoretic mobility of Ure2p. The work presented here illustrates the coordinated use of genome-based and biochemical approaches to delineate a cellular pathway modulated by the protein target of a small molecule.

The nuclear exportin Msn5 is required for nuclear export of the Mig1 glucose repressor of Saccharomyces cerevisiae

BACKGROUND

Mig1 is a transcriptional repressor responsible for glucose repression of many genes in the budding yeast Saccharomyces cerevisiae. Glucose regulates Mig1 function by affecting its phosphorylation, which is catalyzed by the Snf1 protein kinase. Phosphorylation alters the subcellular localization of Mig1, causing it to be nuclear when glucose is present, and cytoplasmic when glucose is absent.

RESULTS

Here, we report that Msn5, a member of the importin beta family of nuclear transport receptors, is required to export Mig1 from the nucleus when glucose is removed. Mig1 and Msn5 interacted in a yeast two-hybrid assay. Within the portion of Mig1 that regulates its nuclear transport, we found a region that directed its nuclear export. Within this region, two leucine-rich sequences similar to known nuclear export signals were not required for Mig1 export. The corresponding domain of the yeast Kluyveromyces lactis Mig1 conferred glucose-regulated Msn5-dependent protein export from the nucleus in S. cerevisiae. Sequence comparison with S. cerevisiae Mig1 revealed short patches of homology in K. lactis and K. marxianus Mig1 that might be Msn5-interaction domains. These regions overlapped with the serine residues predicted to be Snf1 phosphorylation sites, suggesting that Msn5 and Snf1 recognize similar sequences in Mig1. Altering these serines abolished glucose-dependent phosphorylation of Mig1 and caused it to be a constitutive repressor that was retained in the nucleus.

CONCLUSIONS

Mig1 contains a new nuclear export signal that is phosphorylated by Snf1 upon glucose removal, causing it to be recognized by the nuclear exportin Msn5 and carried out of the nucleus into the cytoplasm where it contributes to derepression of glucose-repressed genes.

Deregulation of gluconeogenic structural genes by variants of the transcriptional activator Cat8p of the yeast Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, growth with a non-fermentable carbon source requires co-ordinate transcriptional activation of gluconeogenic structural genes by an upstream activation site (UAS) element, designated CSRE (carbon source-responsive element). The zinc cluster protein encoded by CAT8 is necessary for transcriptional derepression mediated by a CSRE. Expression of CAT8 as well as transcriptional activation by Cat8p is regulated by the carbon source, requiring a functional Cat1p (= Snf1p) protein kinase. The importance of both regulatory levels was investigated by construction of CAT8 variants with a constitutive transcriptional activation domain (INO2TAD) and/or a carbon source-independent promoter (MET25 ). Whereas a reporter gene driven by a CSRE-dependent synthetic minimal promoter showed a 40-fold derepression with wild-type CAT8, an almost constitutive expression was found with a MET25-CAT8-INO2TAD fusion construct due to a dramatically increased gene activation under conditions of glucose repression. Similar results were obtained with the mRNA of the isocitrate lyase gene ICL1 and at the level of ICL enzyme activity. Taking advantage of a Cat8p size variant, we demonstrate its binding to the CSRE. Our data show that carbon source-dependent transcriptional activation by Cat8p is the most important mechanism affecting the regulated expression of gluconeogenic structural genes.

Function and regulation of yeast hexose transporters

Glucose, the most abundant monosaccharide in nature, is the principal carbon and energy source for nearly all cells. The first, and rate-limiting, step of glucose metabolism is its transport across the plasma membrane. In cells of many organisms glucose ensures its own efficient metabolism by serving as an environmental stimulus that regulates the quantity, types, and activity of glucose transporters, both at the transcriptional and posttranslational levels. This is most apparent in the baker's yeast Saccharomyces cerevisiae, which has 20 genes encoding known or likely glucose transporters, each of which is known or likely to have a different affinity for glucose. The expression and function of most of these HXT genes is regulated by different levels of glucose. This review focuses on the mechanisms S. cerevisiae and a few other fungal species utilize for sensing the level of glucose and transmitting this information to the nucleus to alter HXT gene expression. One mechanism represses transcription of some HXT genes when glucose levels are high and works through the Mig1 transcriptional repressor, whose function is regulated by the Snf1-Snf4 protein kinase and Reg1-Glc7 protein phosphatase. Another pathway induces HXT expression in response to glucose and employs the Rgt1 transcriptional repressor, a ubiquitin ligase protein complex (SCF(Grr1)) that regulates Rgt1 function, and two glucose sensors in the membrane (Snf3 and Rgt2) that bind glucose and generate the intracellular signal to which Rgt1 responds. These two regulatory pathways collaborate with other, less well-understood, pathways to ensure that yeast cells express the glucose transporters best suited for the amount of glucose available.

Functional analysis of the Zn(2)Cys(6) transcription factors Oaf1p and Pip2p. Different roles in fatty acid induction of beta-oxidation in Saccharomyces cerevisiae

Fatty acid induction of the peroxisomal beta-oxidation machinery in Saccharomyces cerevisiae involves transcriptional control of genes regulated by the oleate response element (ORE). Glucose as the preferred carbon source antagonizes this effect. Induction is dependent on the Zn(2)Cys(6) family members Oaf1p and Pip2p, which bind to this element as a heterodimer. We show here by ectopically expressing both components and LexA fusion derivatives that this transcription factor complex is only active in the presence of oleate. In contrast to Pip2p, Oaf1p is responsive to oleate activation in the absence of the other component of the heterodimer. Therefore, it is the exclusive receptor of the oleate signal. Pip2p is active also under noninducing conditions but is effectively inhibited when complexed with Oaf1p in the absence of inducer. It contributes to the trans-activation properties of the Oaf1p-Pip2p heterodimer and is required for efficient binding of Oaf1p to OREs in vivo. Repression of ORE-dependent transcription by glucose occurs via both Oaf1p and Pip2p. By dissecting functional domains of both proteins, we identified a region required for regulated activity of the C-terminal activation domain. These findings allow us to postulate a model for carbon source-regulated transcription of peroxisomal protein genes.

Growth and glucose repression are controlled by glucose transport in Saccharomyces cerevisiae cells containing only one glucose transporter

A set of Saccharomyces cerevisiae strains with variable expression of only the high-affinity Hxt7 glucose transporter was constructed by partial deletion of the HXT7 promoter in vitro and integration of the gene at various copy numbers into the genome of an hxt1-7 gal2 deletion strain. The glucose transport capacity increased in strains with higher levels of HXT7 expression. The consequences for various physiological properties of varying the glucose transport capacity were examined. The control coefficient of glucose transport with respect to growth rate was 0.54. At high extracellular glucose concentrations, both invertase activity and the rate of oxidative glucose metabolism increased manyfold with decreasing glucose transport capacity, which is indicative of release from glucose repression. These results suggest that the intracellular glucose concentration produces the signal for glucose repression.

Efficient export of the glucose transporter Hxt1p from the endoplasmic reticulum requires Gsf2p

Mutations in the GSF2 gene cause glucose starvation phenotypes in Saccharomyces cerevisiae. We have isolated the HXT1 gene, which encodes a low-affinity, high-capacity glucose transporter, as a multicopy suppressor of a gsf2 mutation. We show that gsf2 mutants accumulate Hxt1p in the endoplasmic reticulum (ER) and that Gsf2p is a 46-kDa integral membrane protein localized to the ER. gsf2 mutants also display a galactose growth defect and abnormal localization of the galactose transporter Gal2p but are not defective in function or localization of the high-affinity glucose transporter Hxt2p. These findings suggest that Gsf2p functions in the ER to promote the secretion of certain hexose transporters.

Regulatory interaction of PRL1 WD protein with Arabidopsis SNF1-like protein kinases

Mutation of the PRL1 gene, encoding a regulatory WD protein, results in glucose hypersensitivity and derepression of glucose-regulated genes in Arabidopsis. The yeast SNF1 protein kinase, a key regulator of glucose signaling, and Arabidopsis SNF1 homologs AKIN10 and AKIN11, which can complement the Deltasnf1 mutation, were found to interact with an N-terminal domain of the PRL1 protein in the two-hybrid system and in vitro. AKIN10 and AKIN11 suppress the yeast Deltasnf4 mutation and interact with the SNF4p-activating subunit of SNF1. PRL1 and SNF4 bind independently to adjacent C-terminal domains of AKIN10 and AKIN11, and these protein interactions are negatively regulated by glucose in yeast. AKIN10 and AKIN11, purified in fusion with glutathione S-transferase, undergo autophosphorylation and phosphorylate a peptide of sucrose phosphate synthase in vitro. The sucrose phosphate synthase-peptide kinase activity of AKIN complexes detected by immunoprecipitation is stimulated by sucrose in light-grown Arabidopsis plants. In comparison with wild type, the activation level of AKIN immunocomplexes is higher in the prl1 mutant, suggesting that PRL1 is a negative regulator of Arabidopsis SNF1 homologs. This conclusion is supported by the observation that PRL1 is an inhibitor of AKIN10 and AKIN11 in vitro.

Hyperosmotic stress represses the transcription of HXT2 and HXT4 genes in Saccharomyces cerevisiae

Effects of hyperosmotic stress on the transcriptional regulation of the HXT2 and HXT4 genes of Saccharomyces cerevisiae were investigated under glucose-repressed and -depressed growth conditions. Hyperosmotic stress repressed the transcription of these HXT genes up to 81% depending on growth conditions. Preconditioning of yeast cells for the hyperosmotic stress resulted in a much stronger repression of both HXT genes. The negative effect of hyperosmotic stress was much higher for HXT4 than HXT2. These results also show that hyperosmotic stress interferes with the glucose-dependent transcriptional activation or derepression of HXT2 and HXT4 genes transcription in S. cerevisiae.