Phosphorylation of Yeast Hexokinase 2 Regulates Its Nucleocytoplasmic Shuttling

Nucleocytoplasmic shuttling of Hxk2 induced by glucose levels has been reported recently. Here we present evidence that indicates that Hxk2 nucleocytoplasmic traffic is regulated by phosphorylation and dephosphorylation at serine 14. Moreover, we identified the protein kinase Snf1 and the protein phosphatase Glc7-Reg1 as novel regulatory partners for the nucleocytoplasmic shuttling of Hxk2. Functional studies revealed that, in contrast to the wild-type protein, the dephosphorylation-mimicking mutant of Hxk2 retains its nuclear localization in low glucose conditions, and the phosphomimetic mutant of Hxk2 retains its cytoplasmic localization in high glucose conditions. Interaction experiments of Hxk2 with Kap60 and Xpo1 indicated that nuclear import of the S14D mutant of Hxk2 is severely decreased but that the export is significantly enhanced. Conversely, nuclear import of the S14A mutant of Hxk2 was significantly enhanced, although the export was severely decreased. The interaction of Hxk2 with Kap60 and Xpo1 was found to occur in the dephosphorylated and phosphorylated states of the protein, respectively. In addition, we found that Hxk2 is a substrate for Snf1. Mutational analysis indicated that serine 14 is a major in vitro and in vivo phosphorylation site for Snf1. We also provide evidence that dephosphorylation of Hxk2 at serine 14 is a protein phosphatase Glc7-Reg1-dependent process. Taken together, this study establishes a functional link between Hxk2, Reg1, and Snf1 signaling, which involves the regulation of Hxk2 nucleocytoplasmic shuttling by phosphorylation-dephosphorylation of serine 14.     

Regulatory interactions between the Reg1-Glc7 protein phosphatase and the Snf1 protein kinase

Protein phosphatase 1, comprising the regulatory subunit Reg1 and the catalytic subunit Glc7, has a role in glucose repression in Saccharomyces cerevisiae. Previous studies showed that Reg1 regulates the Snf1 protein kinase in response to glucose. Here, we explore the functional relationships between Reg1, Glc7, and Snf1. We show that different sequences of Reg1 interact with Glc7 and Snf1. We use a mutant Reg1 altered in the Glc7-binding motif to demonstrate that Reg1 facilitates the return of the activated Snf1 kinase complex to the autoinhibited state by targeting Glc7 to the complex. Genetic evidence indicated that the catalytic activity of Snf1 negatively regulates its interaction with Reg1. We show that Reg1 is phosphorylated in response to glucose limitation and that this phosphorylation requires Snf1; moreover, Reg1 is dephosphorylated by Glc7 when glucose is added. Finally, we show that hexokinase PII (Hxk2) has a role in regulating the phosphorylation state of Reg1, which may account for the effect of Hxk2 on Snf1 function. These findings suggest that the phosphorylation of Reg1 by Snf1 is required for the release of Reg1-Glc7 from the kinase complex and also stimulates the activity of Glc7 in promoting closure of the complex.     

The Snf1 kinase of the filamentous fungus Hypocrea jecorina phosphorylates regulation-relevant serine residues in the yeast carbon catabolite repressor Mig1 but not in the filamentous fungal counterpart Cre1

In Saccharomyces cerevisiae, the SNF1 gene product phosphorylates the carbon catabolite repressor protein Mig1 under conditions when glucose is limiting, thereby relieving the fungus from catabolite repression. We have investigated whether the corresponding counterpart of filamentous fungi-the Cre1 protein-is also phosphorylated by Snf1. To this end, snf1, an ortholog of SNF1, was isolated from the ascomycete Hypocrea jecorina. The gene encodes a protein with high similarity to Snf1 kinases from other eukaryotes in its N-terminal catalytic domain, but little similarity in the C-terminal half of the protein, albeit some short aa-areas were detected, however, which are conserved in filamentous fungi and in yeast. Expression of snf1 is independent of the carbon source. An overexpressed catalytic domain of H. jecorina Snf1 readily phosphorylated yeast Mig1, but not a Mig1 mutant form, in which all four identified Snf1 phosphorylation sites (Phi XRXXSXXX Phi) had been mutated. The enzyme did neither phosphorylate H. jecorina Cre1 nor histone H3, another substrate of Snf1 kinase in yeast. H. jecorina Snf1 also phosphorylated peptides comprising the strict Snf1 consensus, but notably did not phosphorylate peptides containing the regulatory serine residue in Cre1 (=Ser(241) in H. jecorina Cre1 and Ser(266) in Sclerotinia sclerotiorum CRE1). The use of cell-free extracts of H. jecorina as protein source for Snf1 showed phosphorylation of an unknown 36 kDa protein, which was present only in extracts from glucose-grown mycelia. We conclude that the Snf1 kinase from H. jecorina is not involved in the phosphorylation of Cre1.     

Snf1 Protein Kinase Regulates Phosphorylation of the Mig1 Repressor in Saccharomyces cerevisiae

In glucose-grown cells, the Mig1 DNA-binding protein recruits the Ssn6-Tup1 corepressor to glucose-repressed promoters in the yeast Saccharomyces cerevisiae. Previous work showed that Mig1 is differentially phosphorylated in response to glucose. Here we examine the role of Mig1 in regulating repression and the role of the Snf1 protein kinase in regulating Mig1 function. Immunoblot analysis of Mig1 protein from a snf1 mutant showed that Snf1 is required for the phosphorylation of Mig1; moreover, hxk2 and reg1 mutations, which relieve glucose inhibition of Snf1, correspondingly affect phosphorylation of Mig1. We show that Snf1 and Mig1 interact in the two-hybrid system and also coimmunoprecipitate from cell extracts, indicating that the two proteins interact in vivo. In immune complex assays of Snf1, coprecipitating Mig1 is phosphorylated in a Snf1-dependent reaction. Mutation of four putative Snf1 recognition sites in Mig1 eliminated most of the differential phosphorylation of Mig1 in response to glucose in vivo and improved the two-hybrid interaction with Snf1. These studies, together with previous genetic findings, indicate that the Snf1 protein kinase regulates phosphorylation of Mig1 in response to glucose.     

Glucose controls multiple processes in Saccharomyces cerevisiae through diverse combinations of signaling pathways

We have studied how the lack of glucose sensors in the plasma membrane, or of the enzymes Hxk1, Hxk2, Glk1, which catalyze the first intracellular step in glucose metabolism, affect the different responses of Saccharomyces cerevisiae to glucose. Lack of the G-protein-coupled receptor Gpr1 or of Snf3/Rgt2 did not affect glucose repression of different genes or activation by glucose of plasma membrane ATPase, whereas lack of Gpr1 decreased, in an additive manner with lack of Mth1, the degradation of fructose 1,6-bisphosphatase that takes place in the presence of glucose. In an hxk1 hxk2 glk1 strain, unable to phosphorylate glucose, all of these responses to the sugar were suppressed or strongly reduced. In the absence of Hxk2 (or Hxk1 and Hxk2), glucose repression of SUC2, GAL1 and GDH2 was relieved, but that of FBP1 and ICL1 was maintained. Hxk1 or Hxk2 were needed for activation of plasma membrane ATPase but not for degradation of FbPase.     

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.