An evaluation of D-glucosamine as a gratuitous catabolite repressor of Saccharomyces carlsbergensis

Slt2 Is Required to Activate ER-Stress-Protective Mechanisms through TORC1 Inhibition and Hexosamine Pathway Activation

Slt2, the MAPK of the cell wall integrity (CWI) pathway, connects different signaling pathways and performs different functions in the protective response of S. cerevisiae to stress. Previous work has evidenced the relation of the CWI pathway and the unfolded protein response (UPR), a transcriptional program activated upon endoplasmic reticulum (ER) stress. However, the mechanisms of crosstalk between these pathways and the targets regulated by Slt2 under ER stress remain unclear. Here, we demonstrated that ectopic expression of GFA1, the gene encoding the first enzyme in the synthesis of UDP-GlcNAc by the hexosamine biosynthetic pathway (HBP) or supplementation of the growth medium with glucosamine (GlcN), increases the tolerance of slt2 mutant cells to different ER-stress inducers. Remarkably, GlcN also alleviates the sensitivity phenotype of cells lacking IRE1 or HAC1, the main actors in controlling the UPR. The exogenous addition of GlcN reduced the abundance of glycosylated proteins and triggered autophagy. We also found that TORC1, the central stress and growth controller, is inhibited by tunicamycin exposure in cells of the wild-type strain but not in those lacking Slt2. Consistent with this, the tunicamycin-induced activation of autophagy and the increased synthesis of ATP in response to ER stress were absent by knock-out of SLT2. Altogether, our data placed Slt2 as an essential actor of the ER stress response by regulating the HBP activity and the TORC1-dependent signaling.

Secreted Acb1 Contributes to the Yeast-to-Hypha Transition in Cryptococcus neoformans

Adaptation to stress by eukaryotic pathogens is often accompanied by a transition in cellular morphology. The human fungal pathogen Cryptococcus neoformans is known to switch between the yeast and the filamentous form in response to amoebic predation or during mating. As in the classic dimorphic fungal pathogens, the morphotype is associated with the ability of cryptococci to infect various hosts. Many cryptococcal factors and environmental stimuli, including pheromones (small peptides) and nutrient limitation, are known to induce the yeast-to-hypha transition. We recently discovered that secreted matricellular proteins could also act as intercellular signals to promote the yeast-to-hypha transition. Here we show that the secreted acyl coenzyme A (acyl-CoA)-binding protein Acb1 plays an important role in enhancing this morphotype transition. Acb1 does not possess a signal peptide. Its extracellular secretion and, consequently, its function in filamentation are dependent on an unconventional GRASP (Golgi reassembly stacking protein)-dependent secretion pathway. Surprisingly, intracellular recruitment of Acb1 to the secretory vesicles is independent of Grasp. In addition to Acb1, Grasp possibly controls the secretion of other cargos, because the graspΔ mutant, but not the acb1Δ mutant, is defective in capsule production and macrophage phagocytosis. Nonetheless, Acb1 is likely the major or the sole effector of Grasp in terms of filamentation. Furthermore, we found that the key residue of Acb1 for acyl binding, Y80, is critical for the proper subcellular localization and secretion of Acb1 and for cryptococcal morphogenesis.

, the co-repressor of histone gene transcription of , acts as a multicopy suppressor of the apoptotic phenotypes of the mRNA degradation mutant

We previously have reported that Saccharomyces cerevisiae mutants expressing Kllsm4Delta1, a truncated form of the KlLSM4 gene, as well as mutants in genes of the mRNA-decapping pathway, show phenotypic markers of apoptosis, increased temperature sensitivity and reduced growth in the presence of different drugs and oxidative stressing agents, such as acetic acid and H(2)O(2). To isolate multicopy extra-genic suppressors of these defects, we transformed the Kllsm4Delta1 mutant with a yeast DNA library and we selected a series of clones showing resistance to acetic acid. One of these clones carried a DNA fragment containing the HIR1 gene that encodes a transcriptional co-repressor of histone genes. The over-expression of HIR1 in the Kllsm4Delta1 mutant prevented rapid cell death during chronological aging, reduced nuclei fragmentation and increased resistance to H(2)O(2). Transcription analysis revealed that the expression of histone genes was lowered in the mutant over-expressing HIR1, indicating a relationship between the latter gene and apoptosis.

Galactose induces in Saccharomyces cerevisiae sensitivity of the utilization of hexoses to inhibition by D-glucosamine

Inhibition by glucosamine of the utilization of hexoses by Saccharomyces cerevisiae is induced by growing the cells in media with galactose as carbon source. The intensity of inhibition parallels the induction of the galactose pathway. These findings contrast with the fact that glucosamine is a substrate of the constitutive glucose but not of the inducible. galactose transport and phosphorylation systems. The inhibition by glucosamine is pH dependent; the extent seems to be related with phosphorylation of the hexosamine, as shown by its greater effect with substrates or with conditions that less interfere with the phosphorylation of the inhibitor. Inhibition is not a consequence of ATP depletion of the cell. Intracellular accumulated glucosamine derivatives impair the transport of glucose and mannose in yeast cells grown in galactose-supplemented media but not those grown with glucose or ethanol supplements (i.e., under conditions in which the utilization of these sugars is inhibited). However, impairment of the transport is not enough to explain the characteristics of the observed inhibition. The changes induced by growing the yeast in galactose that render the cells sensitive to glucosamine are under the control of the gal80 and gal4 genes.

The glucose-6-phosphate-isomerase reaction is essential for normal glucose repression in Saccharomyces cerevisiae

Wild-type Saccharomyces cerevisiae and a strain carrying a deletion in the glucose-6-phosphate-isomerase gene (pgi1) were grown in carbon-limited continuous cultures on a mixture of fructose and galactose. Pulses of glucose, fructose and galactose were given to these cultures to investigate whether the pgi1 strain was capable of normal glucose repression. Glucose and galactose pulses inhibited fructose consumption and thus glycolysis in the pgi1 strain by a combination of competition between glucose and fructose at the uptake and/or phosphorylation level and inhibition of fructose uptake and/or phosphorylation by glucose 6-phosphate. Fructose pulses administered to the pgi1 strain transiently decreased the glycolytic flux downstream of fructose-1,6-bisphosphate. Transcriptional induction of the PDC1 gene (encoding pyruvate decarboxylase) was observed after glucose or galactose pulses were applied to the pgi1 strain, demonstrating that metabolism of these sugars beyond glucose 6-phosphate is dispensable for PDC1 induction. Fructose also induced PDC1 transcription, indicating that intracellular sugars could act as trigger for PDC1 induction or, alternatively, that two inductors are present. In contrast to the wild-type transcriptional inhibition of the glucose-repressible genes, HXK1 and GAL10 (encoding hexokinase isoenzyme 1 and uridine diphosphoglucose-4-epimerase, respectively) did not occur upon addition of glucose or fructose to the pgi1 mutant. Transcriptional repression was observed after application of the fructose pulse when the yeast had resumed metabolism of fructose. These results demonstrate that the initial signal for catabolite repression is not generated by high sugar concentrations or high concentrations of intermediates; moreover a simple role for the hexokinases can also be excluded. The absence of an increased glycolytic flux in the pgi1 mutant after administration of the sugar pulses while the concentrations of sugar and glycolytic intermediates were high, suggests that the initial signal for glucose repression could be linked to an increased glycolytic flux. The occurrence of PDC1 induction in the pgi1 strain while GAL10/HXKI repression is absent, demonstrates that the initial signals for catabolite induction and catabolite repression are different.

Suppression of temperature sensitive mutations in oncogene-related CDC genes in Saccharomyces cerevisiae by catabolite repression resistance and cytoplasmic petite mutations

The "start" cell division control genes CDC36 and CDC28 have been reported to contain a certain sequence homology to tissue oncogenes (ets and some protein kinase encoding oncogenes respectively). Here we report that temperature sensitive mutations in these genes are suppressed in cytoplasmic "petite" mutants and catabolite repression resistant mutants.

Pleiotropic glucose repression-resistant mutation in Saccharomyces carlesbergensis

We describe the characterization of a mutation of the locus GLR1. This mutation allowed for (i) the glucose repression-insensitive synthesis ot the enzymes maltase, galactokinase, alpha-galactosidase, reduced nicotinamide adenine dinucleotide-cytochrome c reductase, and cytochrome c oxidase and (ii) growth on maltose in the presence of the gratuitous glucose repressor D-glucosamine. The glucosamine resistance cosegregated with the glucose-insensitive synthesis of the enzymes listed above. In addition, crosses between the glucosamine-resistant mutant and isogenic sensitive strains gave only tetrads containing two resistant and two sensitive spores. Thus, a single pleiotropic mutation is responsible for both phenotypes. We call the locus GLR1, for glucose regulation, and the glucose repression-insensitive mutation glr1-1.