Relationship between protein kinase C and derepression of different enzymes

Phosphoproteomic responses of TORC1 target kinases reveal discrete and convergent mechanisms that orchestrate the quiescence program in yeast

The eukaryotic TORC1 kinase assimilates diverse environmental cues, including growth factors and nutrients, to control growth by tuning anabolic and catabolic processes. In yeast, TORC1 stimulates protein synthesis in response to abundant nutrients primarily through its proximal effector kinase Sch9. Conversely, TORC1 inhibition following nutrient limitation unlocks various distally controlled kinases (e.g., Atg1, Gcn2, Npr1, Rim15, Slt2/Mpk1, and Yak1), which cooperate through poorly defined circuits to orchestrate the quiescence program. To better define the signaling landscape of the latter kinases, we use in vivo quantitative phosphoproteomics. Through pinpointing known and uncharted Npr1, Rim15, Slt2/Mpk1, and Yak1 effectors, our study examines the architecture of the distally controlled TORC1 kinase network. Accordingly, this is built on a combination of discrete, convergent, and multilayered feedback regulatory mechanisms, which likely ensure homeostatic control of and/or robust responses by TORC1 and its effector kinases under fluctuating nutritional conditions.

The early steps of glucose signalling in yeast

In the presence of glucose, yeast undergoes an important remodelling of its metabolism. There are changes in the concentration of intracellular metabolites and in the stability of proteins and mRNAs; modifications occur in the activity of enzymes as well as in the rate of transcription of a large number of genes, some of the genes being induced while others are repressed. Diverse combinations of input signals are required for glucose regulation of gene expression and of other cellular processes. This review focuses on the early elements in glucose signalling and discusses their relevance for the regulation of specific processes. Glucose sensing involves the plasma membrane proteins Snf3, Rgt2 and Gpr1 and the glucose-phosphorylating enzyme Hxk2, as well as other regulatory elements whose functions are still incompletely understood. The similarities and differences in the way in which yeasts and mammalian cells respond to glucose are also examined. It is shown that in Saccharomyces cerevisiae, sensing systems for other nutrients share some of the characteristics of the glucose-sensing pathways.

Thermosensitivity of the Saccharomyces cerevisiae gpp1gpp2 double deletion strain can be reduced by overexpression of genes involved in cell wall maintenance

A Saccharomyces cerevisiae strain in which the GPP1 and GPP2 genes, both encoding glycerol-3-phosphate phosphatase isoforms, are deleted, displays both osmo- and thermosensitive (ts) phenotypes. We isolated genes involved in cell wall maintenance as multicopy suppressors of the gpp1gpp2 ts phenotype. We found that the gpp1gpp2 strain is hypersensitive to cell wall stress such as treatment with beta-1,3-glucanase containing cocktail Zymolyase and chitin-binding dye Calcofluor-white (CFW). Sensitivity to Zymolyase was rescued by overexpression of SSD1, while CFW sensitivity was rescued by SSD1, FLO8 and WSC3-genes isolated as multicopy suppressors of the gpp1gpp2 ts phenotype. Some of the isolated suppressor genes (SSD1, FLO8) also rescued the lytic phenotype of slt2 deletion strain. Additionally, the sensitivity to CFW was reduced when the cells were supplied with glycerol. Both growth on glycerol-based medium and overexpression of SSD1, FLO8 or WSC3 had additive suppressing effect on CFW sensitivity of the gpp1gpp2 mutant strain. We also confirmed that the internal glycerol level changed in cells exposed to cell wall perturbation.

Effect of osmotic stress on the derepression of invertase synthesis in nonconventional yeasts

AIMS

The aim of this study was to analyse the effect of osmotic stress on the biosynthesis of invertase enzyme in nonconventional yeasts.

METHODS AND RESULTS

Invertase activities of the nonconventional yeast species belonging to Kluyveromyces, Schwanniomyces and Pichia genus were measured either in the presence or in the absence of various amounts of NaCl. The effect of hyperosmotic stress on the glucose consumption of Saccharomyces cerevisiae and Pichia anomala were also compared. Like S. cerevisiae, derepression of invertase synthesis in Kluyveromyces lactis, Schwanniomyces occidentalis and Pichia jadinii is inhibited by hyperosmotic stress. However, derepression of invertase synthesis in P. anomala is not affected by hyperosmotic stress. In addition, low levels of osmotic stress activated invertase synthesis three- to fourfold in P. anomala and K. lactis.

CONCLUSIONS

This study shows that low levels of osmotic stress induces the invertase synthesis at very high levels in P. anomala and K. lactis. Glucose consumption was not influenced at significant levels by the hyperosmotic stress in P. anomala.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study shows the activation of invertase synthesis by low levels of osmotic stress in P. anomala and K. lactis.

Deficiency of Pkc1 activity affects glycerol metabolism in Saccharomyces cerevisiae

Protein kinase C is apparently involved in the control of many cellular systems: the cell wall integrity pathway, the synthesis of ribosomes, the appropriated reallocation of transcription factors under specific stress conditions and also the regulation of N-glycosylation activity. All these observations suggest the existence of additional targets not yet identified. In the context of the control of carbon metabolism, previous data had demonstrated that Pkc1p might play a central role in the control of cellular growth and metabolism in yeast. In particular, it has been suggested that it might be involved in the derepression of genes under glucose-repression by driving an appropriated subcellular localization of transcriptional factors, such as Mig1p. In this work, we show that a pkc1Delta mutant is unable to grow on glycerol because it cannot perform the derepression of the GUT1 gene that encodes glycerol kinase. Additionally, active transport is also partially affected. Using this phenotype, we were able to isolate a new pkc1Delta revertant. We also isolated two transformants identified as the nuclear exportin Msn5 and the histone deacetylase Hos2 extragenic suppressors of this mutation. Based on these results, we postulate that Pkc1p may be involved in the control of the cellular localization and/or regulation of the activity of nuclear proteins implicated in gene expression.

A Short-range Gradient of Histone H3 Acetylation and Tup1p Redistribution at the Promoter of the Saccharomyces cerevisiae SUC2 Gene

Chromatin immunoprecipitation assays are used to map H3 and H4 acetylation over the promoter nucleosomes and the coding region of the Saccharomyces cerevisiae SUC2 gene, under repressed and derepressed conditions, using wild type and mutant strains. In wild type cells, a high level of H3 acetylation at the distal end of the promoter drops sharply toward the proximal nucleosome that covers the TATA box, a gradient that become even steeper on derepression. In contrast, substantial H4 acetylation shows no such gradient and extends into the coding region. Overall levels of both H3 and H4 acetylation rise on derepression. Mutation of GCN5 or SNF2 lead to substantially reduced SUC2 expression; in gnc5 there is no reduction in basal H3 acetylation, but large reductions occur on derepression. SNF2 mutation has little effect on H3 acetylation, so SAGA and SWI/SNF recruitment seem to be independent events. H4 acetylation is little affected by either GCN5 or SNF2 mutation. In a double snf2/gcn5 mutant (very low SUC2 expression), H3 acetylation is at the minimal level, but H4 acetylation remains largely unaffected. Transcription is thus linked to H3 but not H4 acetylation. Chromatin immunoprecipitation assays show that Tup1p is evenly distributed over the four promoter nucleosomes in repressed wild type cells but redistributes upstream on derepression, a movement probably linked to its conversion from a repressor to an activator.