Maintenance of the peroxisomal compartment in glucose-repressed and anaerobically grown Saccharomyces cerevisiae cells

The stress-regulatory transcription factors Msn2 and Msn4 regulate fatty acid oxidation in budding yeast

The transcription factors Msn2 and Msn4 (multicopy suppressor of SNF1 mutation proteins 2 and 4) bind the stress-response element in gene promoters in the yeast Saccharomyces cerevisiae However, the roles of Msn2/4 in primary metabolic pathways such as fatty acid β-oxidation are unclear. Here, in silico analysis revealed that the promoters of most genes involved in the biogenesis, function, and regulation of the peroxisome contain Msn2/4-binding sites. We also found that transcript levels of MSN2/MSN4 are increased in glucose-depletion conditions and that during growth in nonpreferred carbon sources, Msn2 is constantly localized to the nucleus in wild-type cells. Of note, the double mutant msn2Δmsn4Δ exhibited a severe growth defect when grown with oleic acid as the sole carbon source and had reduced transcript levels of major β-oxidation genes. ChIP indicated that Msn2 has increased occupancy on the promoters of β-oxidation genes in glucose-depleted conditions, and in vivo reporter gene analysis indicated reduced expression of these genes in msn2Δmsn4Δ cells. Moreover, mobility shift assays revealed that Msn4 binds β-oxidation gene promoters. Immunofluorescence microscopy with anti-peroxisome membrane protein antibodies disclosed that the msn2Δmsn4Δ strain had fewer peroxisomes than the wild type, and lipid analysis indicated that the msn2Δmsn4Δ strain had increased triacylglycerol and steryl ester levels. Collectively, our data suggest that Msn2/Msn4 transcription factors activate expression of the genes involved in fatty acid oxidation. Because glucose sensing, signaling, and fatty acid β-oxidation pathways are evolutionarily conserved throughout eukaryotes, the msn2Δmsn4Δ strain could therefore be a good model system for further study of these critical processes.

Good fat, essential cellular requirements for triacylglycerol synthesis to maintain membrane homeostasis in yeast

Storage triacylglycerols (TAG) and membrane phospholipids share common precursors, i.e. phosphatidic acid and diacylglycerol, in the endoplasmic reticulum. In addition to providing a biophysically rather inert storage pool for fatty acids, TAG synthesis plays an important role to buffer excess fatty acids (FA). The inability to incorporate exogenous oleic acid into TAG in a yeast mutant lacking the acyltransferases Lro1p, Dga1p, Are1p, and Are2p contributing to TAG synthesis results in dysregulation of lipid synthesis, massive proliferation of intracellular membranes, and ultimately cell death. Carboxypeptidase Y trafficking from the endoplasmic reticulum to the vacuole is severely impaired, but the unfolded protein response is only moderately up-regulated, and dispensable for membrane proliferation, upon exposure to oleic acid. FA-induced toxicity is specific to oleic acid and much less pronounced with palmitoleic acid and is not detectable with the saturated fatty acids, palmitic and stearic acid. Palmitic acid supplementation partially suppresses oleic acid-induced lipotoxicity and restores carboxypeptidase Y trafficking to the vacuole. These data show the following: (i) FA uptake is not regulated by the cellular lipid requirements; (ii) TAG synthesis functions as a crucial intracellular buffer for detoxifying excess unsaturated fatty acids; (iii) membrane lipid synthesis and proliferation are responsive to and controlled by a balanced fatty acid composition.

Proteomic analysis of Aspergillus nidulans cultured under hypoxic conditions

The fungus Aspergillus nidulans reduces nitrate to ammonium and simultaneously oxidizes ethanol to acetate to generate ATP under hypoxic conditions in a mechanism called ammonia fermentation (Takasaki, K. et al.. J. Biol. Chem. 2004, 279, 12414-12420). To elucidate the mechanism, the fungus was cultured under normoxic and hypoxic (ammonia fermenting) conditions, intracellular proteins were resolved by 2-DE, and 332 protein spots were identified using MALDI MS after tryptic digestion. Alcohol and aldehyde dehydrogenases that play key roles in oxidizing ethanol were produced at the basal level under hypoxic conditions but were obviously provoked by ethanol under normoxic conditions. Enzymes involved in gluconeogenesis, as well as the tricarboxylic and glyoxylate cycles, were downregulated. These results indicate that the mechanism of fungal energy conservation is altered under hypoxic conditions. The results also showed that proteins in the pentose phosphate pathway as well as the metabolism of both nucleotide and thiamine were upregulated under hypoxic conditions. Levels of xanthine and hypoxanthine, deamination products of guanine and adenine were increased in DNA from hypoxic cells, indicating an association between hypoxia and intracellular DNA base damage. This study is the first proteomic comparison of the hypoxic responses of A. nidulans.

Distinct intracellular localization of Gpd1p and Gpd2p, the two yeast isoforms of NAD+-dependent glycerol-3-phosphate dehydrogenase, explains their different contributions to redox-driven glycerol production

During anaerobiosis Saccharomyces cerevisiae strongly increases glycerol production to provide for non-respiratory oxidation of NADH to NAD(+). We here report that respiratory-deficient cells become strictly dependent on the Gpd2p isoform of the NAD(+)-linked glycerol-3-phosphate dehydrogenase (Gpd). The growth inhibition of respiratory incompetent cox18Delta cells lacking GPD2 is reversed by the addition of acetoin, an alternative sink for NADH oxidation. Growth is also restored by addition of lysine or glutamic acid/glutamine, the synthesis of which involves production of mitochondrial NADH. Lysine produced a stronger growth stimulating effect than glutamic acid consistent with an upregulated expression of the IDP3 gene for peroxisomal synthesis of the glutamate precursor alpha-ketoglutarate. Gpd2p is known to be a cytosolic protein but possesses a classical mitochondrial presequence, which we show is sufficient for mitochondrial targeting. A partial mitochondrial localization of Gpd2p will provide for establishment of intramitochondrial redox balance under non-respiratory conditions. Gpd1p, the other Gpd isoform, is partly cytosolic and partly peroxisomal and becomes more strictly peroxisomal in respiratory-deficient mutants. The different cellular distribution of Gpd1p and Gpd2p thus appears to be the main reason Gpd1p cannot substitute for Gpd2p in cox18Deltagpd2Delta cells, despite similar kinetic characteristics of the two iso-enzymes.

The Gef1 protein of Saccharomyces cerevisiae is associated with chloride channel activity

The Gef1 protein of the yeast Saccharomyces cerevisiae (Gef1p) has amino acid homology to the voltage-gated CLC chloride channel family. It has been postulated that it provides the compensatory transport of Cl- anions to the lumen of the Golgi thereby regulating the pH of this compartment. Using GEF1 fusion with heterologous promoter we obtained a yeast strain highly overproducing Gef1p. The electrophysiological properties of the microsomal fraction obtained from this strain were measured using lipid bilayer system. Our data indicate that Gef1p is associated with the chloride channel activity. This anion-selective channel has a unitary conductance of 42 pS when measured in symmetrical 600/600 mM TEA-Cl solutions, is voltage-dependent, and closes at high negative voltages.

Saccharomyces cerevisiae IRR1 protein is indirectly involved in colony formation

The ability of a microorganism to adhere to a solid support and to initiate a colony is often the first stage of microbial infections. To date, studies on S. cerevisiae cell-cell and cell-solid support interactions concerned only cell agglutination during mating and flocculation. Colony formation has not been studied before probably because this species is not pathogenic. However, S. cerevisiae can be a convenient model to study this process, thanks to well-developed genetics and the full knowledge of its nucleotide sequence. A preliminary characterization of the recently cloned essential IRR1 gene indicated that it may participate in cell-cell/substrate interactions. Here we show that lowering the level of expression of IRR1 (after fusion with a regulatory catalase A gene promoter) affects colony formation and disturbs zygote formation and spore germination. All these processes involve cell-cell or cell-solid support contacts. The IRR1 protein is localized in the cytosol as verified by immunofluorescence microscopy, and confirmed by cell fractionation and Western blotting. This indicates that Irr1p is not directly involved in the cell-solid support adhesion, but may be an element of a communication pathway between the cell and its surroundings.