Identification of a fatty acyl-CoA synthetase gene, lcf2+, which affects viability after entry into the stationary phase in Schizosaccharomyces pombe

The lcf1(+) gene, which encodes a long chain fatty acyl-CoA synthetase, is necessary for the maintenance of viability after entry into the stationary phase in Schizosaccharomyces pombe. In this study, we analyzed a paralogous gene, SPBP4H10.11c (named lcf2(+)), and we present evidence that the gene encodes a new fatty acyl-CoA synthetase. The enzyme preferentially recognized myristic acid as a substrate. A Deltalcf2 mutant showed increased viability after entry into the stationary phase in SD medium. A Deltalcf1Deltalcf2 double mutant showed a severe decrease in long-chain fatty acyl-CoA synthetase activity and a rapid loss of viability after entry into the stationary phase. These results suggest that fatty acid utilization and/or metabolism is important to determine viability in the stationary phase.     

Mutants of Saccharomyces cerevisiae deficient in acyl-CoA synthetases secrete fatty acids due to interrupted fatty acid recycling

In the present study, acyl-CoA synthetase mutants of Saccharomyces cerevisiae were employed to investigate the impact of this activity on certain pools of fatty acids. We identified a genotype responsible for the secretion of free fatty acids into the culture medium. The combined deletion of Faa1p and Faa4p encoding two out of five acyl-CoA synthetases was necessary and sufficient to establish mutant cells that secreted fatty acids in a growth-phase dependent manner. The mutants accomplished fatty acid export during exponential growth-phase followed by fatty acid re-import into the cells during the stationary phase. The data presented suggest that the secretion is driven by an active component. The fatty acid re-import resulted in a severely altered ultrastructure of the mutant cells. Additional strains deficient of any cellular acyl-CoA synthetase activity revealed an almost identical phenotype, thereby proving transfer of fatty acids across the plasma membrane independent of their activation with CoA. Further experiments identified membrane lipids as the origin of the observed free fatty acids. Therefore, we propose the recycling of endogenous fatty acids generated in the course of lipid remodelling as a major task of both acyl-CoA synthetases Faa1p and Faa4p.     

A novel gene, ecl1(+), extends the chronological lifespan in fission yeast.

We have identified a novel gene from Schizosaccharomyces pombe that we have named ecl1(+) (extender of the chronological lifespan). When ecl1(+) is provided on a high-copy number plasmid, it extends the viability of both the Deltasty1 MAP kinase mutant and the wild-type cells after entry into the stationary phase. ecl1(+) encodes an 80-amino acid polypeptide that had not been annotated in the current database. The ecl1(+)-mRNA increases transiently when the growth phase is changed from the log phase to the stationary phase. The Ecl1 protein is localized in the nucleus. Calorie restriction extends the chronological lifespan of wild-type and Deltaecl1 cells but not ecl1(+)-overproducing cells. The Deltapka1 mutant shows little, if any, additional extension of viability when Ecl1 is overproduced. The ste11(+) gene that is negatively controlled by Pka1 is up regulated when Ecl1 is overproduced. From these results we propose that the effect of Ecl1 overproduction may be mainly linked to and negatively affects the Pka1-dependent pathway.     

The novel gene trs1 encodes an essential protein for the transition from mitotic cell cycle to resting state in Schizosaccharomyces pombe

A novel gene trs1 in the fission yeast Schizosaccharomyces pombe has been genetically defined. The trs1 mutant showed several intriguing phenotypes. Cells were sensitive to starvation and rapidly lost viability in the stationary phase; cells in the stationary phase were sensitive to heat shock. Some heat-shock proteins were not induced and the heat-shock response in log-phase cells was defective. These mutant phenotypes strongly suggest a vital function of the trs1 gene product for transition from the G1 to G0 phase on starvation and for the normal heat-shock response.     

Role of the Schizosaccharomyces pombe F-Box DNA helicase in processing recombination intermediates

In an effort to identify novel genes involved in recombination repair, we isolated fission yeast Schizosaccharomyces pombe mutants sensitive to methyl methanesulfonate (MMS) and a synthetic lethal with rad2. A gene that complements such mutations was isolated from the S. pombe genomic library, and subsequent analysis identified it as the fbh1 gene encoding the F-box DNA helicase, which is conserved in mammals but not conserved in Saccharomyces cerevisiae. An fbh1 deletion mutant is moderately sensitive to UV, MMS, and gamma rays. The rhp51 (RAD51 ortholog) mutation is epistatic to fbh1. fbh1 is essential for viability in stationary-phase cells and in the absence of either Srs2 or Rqh1 DNA helicase. In each case, lethality is suppressed by deletion of the recombination gene rhp57. These results suggested that fbh1 acts downstream of rhp51 and rhp57. Following UV irradiation or entry into the stationary phase, nuclear chromosomal domains of the fbh1Delta mutant shrank, and accumulation of some recombination intermediates was suggested by pulsed-field gel electrophoresis. Focus formation of Fbh1 protein was induced by treatment that damages DNA. Thus, the F-box DNA helicase appears to process toxic recombination intermediates, the formation of which is dependent on the function of Rhp51.     

Decrease in cell viability in an RMF,sigma(38), and OmpC triple mutant of Escherichia coli

In a speG-disrupted Escherichia coli mutant, which cannot metabolize spermidine to acetylspermidine, addition of spermidine to the medium caused a decrease in cell viability at the late stationary phase of growth. There were parallel decreases in the levels of ribosome modulation factor (RMF), the sigma(38) subunit of RNA polymerase, and the outer membrane protein C (OmpC). To clarify that these three proteins are strongly involved in cell viability, the rmf, rpoS (encoding sigma(38)), and ompC genes were disrupted. Viability of the triple mutant decreased to less than 1% of normal cells. The triple mutant had a reduced cell viability compared to any combination of double mutants, which also had a reduced cell viability. The single rmf and rpoS, but not ompC, mutant only slightly reduced cell viability. The results indicate that cooperative functions of these three proteins are necessary for cell viability at the late stationary phase. The triple mutant had a reduced level of ribosomes and of intracellular cations.     

SMC6 is required for MMS-induced interchromosomal and sister chromatid recombinations in Saccharomyces cerevisiae

SMC6 (RHC18) in Saccharomyces cerevisiae, which is a homologue of the Schizosaccharomyces pombe rad18+ gene and essential for cell viability, encodes a structural maintenance of chromosomes (SMC) family protein. In contrast to the rest of the SMC family of proteins, Smc1-Smc4, which are the components of cohesin or condensin, little is known about Smc6. In this study, we generated temperature sensitive (ts) smc6 mutants of budding yeast and characterized their properties. One ts-mutant, smc6-56, ceased growth soon after up-shift to a non-permissive temperature, arrested in the late S and G2/M phase, and gradually lost viability. smc6-56 cells at a permissive temperature showed a higher sensitivity than wild-type cells to various DNA damaging agents including methyl methanesulfonate (MMS). The rad52 smc6-56 double mutant showed a sensitivity to MMS similar to that of the rad52 single mutant, indicating that Smc6 is involved in a pathway that requires Rad52 to function. Moreover, no induction of interchromosomal recombination and sister chromatid recombination was observed in smc6-56 cells, which occurred in wild-type cells upon exposure to MMS.     

Involvement of Moc1 in sexual development and survival of Schizosaccharomyces pombe

The moc1 gene in Schizosaccharomyces pombe was found as to overcome sterility caused by high expression of adenylyl cyclase. The moc1 gene was found to be identical with sds23 and psp1. Although psp1 has been reported to be essential for growth, sds23 has not been. To clarify this apparent discrepancy, we first assessed independently the phenotypes of the moc1 disruptant. We confirmed that the deletion mutant of moc1 is sterile, sensitive to high salt, and grows slowly at higher and lower temperatures, and that mutant cells are elongated. Besides these phenotypes, we found that viability of the moc1 disruptant was rapidly lost at the stationary phase. We confirmed that the Moc1 protein is phosphorylated in the stationary phase and also under nitrogen-starved conditions. We examined the significance of this phosphorylation of Moc1 by creating the S333A or S333D mutant Moc1. Interestingly, while S333D mutant Moc1 is lower in inducing sexual development, S333A mutant Moc1 is higher in this than the wild type, suggesting that phosphorylation of Moc1 affects sexual development. The other phenotypes, such as sensitivity to high salt and higher temperature and elongation of cells, were not affected by mutation of S333A nor S333D. We found that Moc1-GFP localized to both the cytosol and the nucleus during mitotic growth, but accumulated in the nucleus in mating cells and then enriched in spores, and that this localization shift was negatively regulated by the cAMP pathway. This and the observations above suggest that nuclear localized Moc1 is an inducer of sexual development. Thus, in addition to the roles of moc1/sds23/psp1 in mitosis and stress response, it is also important for the survival and sexual development of fission yeast, but phosphorylation of Moc1 only affects the sexual development.     

Mechanistic studies of the long chain acyl-CoA synthetase Faa1p from Saccharomyces cerevisiae

Long chain acyl-CoA synthetase (ACSL; fatty acid CoA ligase: AMP forming; EC 6.2.1.3) catalyzes the formation of acyl-CoA through a process, which requires fatty acid, ATP and coenzymeA as substrates. In the yeast Saccharomyces cerevisiae the principal ACSL is Faa1p (encoded by the FAA1 gene). The preferred substrates for this enzyme are cis-monounsaturated long chain fatty acids. Our previous work has shown Faa1p is a principal component of a fatty acid transport/activation complex that also includes the fatty acid transport protein Fat1p. In the present work hexameric histidine tagged Faa1p was purified to homogeneity through a two-step process in the presence of 0.1% eta-dodecyl-beta-maltoside following expression at 15 degrees C in Escherichia coli. In order to further define the role of this enzyme in fatty acid transport-coupled activation (vectorial acylation), initial velocity kinetic studies were completed to define the kinetic parameters of Faa1p in response to the different substrates and to define mechanism. These studies showed Faa1p had a Vmax of 158.2 nmol/min/mg protein and a Km of 71.1 microM oleate. When the concentration of oleate was held constant at 50 microM, the Km for CoA and ATP were 18.3 microM and 51.6 microM respectively. These initial velocity studies demonstrated the enzyme mechanism for Faa1p was Bi Uni Uni Bi Ping Pong.     

Vectorial acylation in Saccharomyces cerevisiae. Fat1p and fatty acyl-CoA synthetase are interacting components of a fatty acid import complex

In Saccharomyces cerevisiae Fat1p and fatty acyl-CoA synthetase (FACS) are hypothesized to couple import and activation of exogenous fatty acids by a process called vectorial acylation. Molecular genetic and biochemical studies were used to define further the functional and physical interactions between these proteins. Multicopy extragenic suppressors were selected in strains carrying deletions in FAA1 and FAA4 or FAA1 and FAT1. Each strain is unable to grow under synthetic lethal conditions when exogenous long-chain fatty acids are required, and neither strain accumulates the fluorescent long-chain fatty acid C(1)-BODIPY-C(12) indicating a fatty acid transport defect. By using these phenotypes as selective screens, plasmids were identified encoding FAA1, FAT1, and FAA4 in the faa1Delta faa4Delta strain and encoding FAA1 and FAT1 in the faa1Delta fat1Delta strain. Multicopy FAA4 could not suppress the growth defect in the faa1Delta fat1Delta strain indicating some essential functions of Fat1p cannot be performed by Faa4p. Chromosomally encoded FAA1 and FAT1 are not able to suppress the growth deficiencies of the fat1Delta faa1Delta and faa1Delta faa4Delta strains, respectively, indicating Faa1p and Fat1p play distinct roles in the fatty acid import process. When expressed from a 2-mu plasmid, Fat1p contributes significant oleoyl-CoA synthetase activity, which indicates vectorial esterification and metabolic trapping are the driving forces behind import. Evidence of a physical interaction between Fat1p and FACS was provided using three independent biochemical approaches. First, a C-terminal peptide of Fat1p deficient in fatty acid transport exerted a dominant negative effect against long-chain acyl-CoA synthetase activity. Second, protein fusions employing Faa1p as bait and portions of Fat1p as trap were active when tested using the yeast two-hybrid system. Third, co-expressed, differentially tagged Fat1p and Faa1p or Faa4p were co-immunoprecipitated. Collectively, these data support the hypothesis that fatty acid import by vectorial acylation in yeast requires a multiprotein complex, which consists of Fat1p and Faa1p or Faa4p.     

Cell growth and differentiation in vitro in mouse macrophages transformed by a tsA mutant of simian virus 40. I. Cellular response in proliferative and phagocytic activities to the shift of temperature differs depending on the culture state in mouse bone marrow cells transformed by the tsA640 mutant of simian virus 40

It was shown previously that mouse bone marrow cells transformed by simian virus 40 (SV40) show a reversible cell density-dependent phenotypic transition between the nonmacrophage (rapidly growing) and the macrophage (stationary) states; cells in low-density cultures are in the growing phase, express SV40 T antigen strongly as revealed by immunofluorescence, and lose typical macrophage properties such as immune phagocytosis; whereas cells in high-density cultures are in the stationary (nongrowing) phase, express SV40 T antigen weakly, and recover their macrophage properties (Takayama, 1980). In the hope of clarifying the relationship between T antigen, cell growth, and macrophage-specific cellular function, we examined the behavior at 33 and 39 degrees C of mouse bone marrow cells transformed by an SV40 gene A mutant (tsA640) whose mutation renders the molecular weight of 90K (large) T antigen temperature sensitive. The results presented in this paper suggest that functional large T antigen is required for cells in the stationary phase to initiate multiplication when transferred at lower density and is not necessary for a majority of them to maintain the nongrowing state (viability) at both high and lower cell densities, whereas it is required for cells in the growing phase to keep multiplying without losing their viability. The results also suggest that the functional large T antigen does not play a direct role in maintaining the cells as either phagocytic or nonphagocytic. It is also suggested that the physiological or tsA mutation-mediated arrest of growth may or may not be accompanied by induction and/or maintenance of cellular phagocytic activity depending on the culture state.     

The Sjögren-Larsson syndrome gene encodes a hexadecenal dehydrogenase of the sphingosine 1-phosphate degradation pathway

Sphingosine 1-phosphate (S1P) functions not only as a bioactive lipid molecule, but also as an important intermediate of the sole sphingolipid-to-glycerolipid metabolic pathway. However, the precise reactions and the enzymes involved in this pathway remain unresolved. We report here that yeast HFD1 and the?Sj?gren-Larsson syndrome (SLS)-causative mammalian gene ALDH3A2 are responsible for conversion of?the S1P degradation product hexadecenal to hexadecenoic acid. The absence of ALDH3A2 in CHO-K1 mutant cells caused abnormal metabolism of S1P/hexadecenal to ether-linked glycerolipids. Moreover, we demonstrate that yeast Faa1 and Faa4 and mammalian ACSL family members are acyl-CoA synthetases involved in the sphingolipid-to-glycerolipid metabolic pathway and that hexadecenoic acid accumulates in Δfaa1 Δfaa4 mutant cells.?These results unveil the entire S1P metabolic pathway: S1P is metabolized to glycerolipids via hexadecenal, hexadecenoic acid, hexadecenoyl-CoA, and palmitoyl-CoA. From our results we propose a possibility that accumulation of the S1P metabolite hexadecenal contributes to the pathogenesis of SLS.     

Schizosaccharomyces pombe cells deficient in triacylglycerols synthesis undergo apoptosis upon entry into the stationary phase

Triacylglycerols (TAG) are important energy storage molecules for nearly all eukaryotic organisms. In this study, we found that two gene products (Plh1p and Dga1p) are responsible for the terminal step of TAG synthesis in the fission yeast Schizosaccharomyces pombe through two different mechanisms: Plh1p is a phospholipid diacylglycerol acyltransferase, whereas Dga1p is an acyl-CoA:diacylglycerol acyltransferase. Cells with both dga1+ and plh1+ deleted (DKO cells) lost viability upon entry into the stationary phase and demonstrated prominent apoptotic markers. Exponentially growing DKO cells also underwent dramatic apoptosis when briefly treated with diacylglycerols (DAGs) or free fatty acids. We provide strong evidence suggesting that DAG, not sphingolipids, mediates fatty acids-induced lipoapoptosis in yeast. Lastly, we show that generation of reactive oxygen species is essential to lipoapoptosis.     

SakA MAP kinase is involved in stress signal transduction,sexual development and spore viability in Aspergillus nidulans

In eukaryotic cells, environmental stress signals are transmitted by evolutionarily conserved MAPKs, such as Hog1 in the budding yeast Saccharomyces cerevisiae, Spc1 in the fission yeast Schizosaccharomyces pombe and p38/JNK in mammalian cells. Here, we report the identification of the Aspergillus nidulans sakA gene, which encodes a member of the stress MAPK family. The sakA gene is able to complement the S. pombe spc1- defects in both osmo-regulation and cell cycle progression. Moreover, SakA MAPK is activated in response to osmotic and oxidative stress in both S. pombe and A. nidulans. However, in contrast to hog1 and spc1 mutants, the sakA null mutant is not sensitive to high osmolarity stress, indicating a different regulation of the osmostress response in this fungus. On the other hand, the DeltasakA mutant shows development and cell-specific phenotypes. First, it displays premature steA-dependent sexual development. Second, DeltasakA mutant produces asexual spores that are highly sensitive to oxidative and heat shock stress and lose viability upon storage. Indeed, SakA is transiently activated early after induction of conidiation. Our results indicate that SakA MAPK is involved in stress signal transduction and repression of sexual development, and is required for spore stress resistance and survival.     

zds1, a novel gene encoding an ortholog of Zds1 and Zds2, controls sexual differentiation, cell wall integrity and cell morphology in fission yeast

While screening for genes that reverse the sporulation-deficient phenotype of the ras1delta diploid Schizosaccharomyces pombe strain, we identified zds1. This gene shares sequence homology with the ZDS1 and ZDS2 genes from Saccharomyces cerevisiae, which appear to be involved in multiple cellular events. Expression of Zds1 in ras1delta diploid cells elevated their sporulation rate from 0.3 to 11.2%. Expression of the Zds1 C-terminal region increased the sporulation rate further (to 21.9%) while introduction of the Zds1 N-terminal region had no effect. zds1 expression did not induce sporulation in strains with mutations in genes participating in the downstream MAP kinase cascade. The zds1-disrupted strain is sensitive to CaCl2, and this effect is suppressed by the C-terminal region of Zds1. The growth of the zds1delta strain is markedly inhibited by cold temperatures, while its viability decreased in the stationary phase. Moreover, the zds1delta strain is round in shape and very sensitive to zymolyase, and its cell wall becomes thicker than that of wild type. Thus, zds1 must be required to maintain cell wall integrity. The Zds1-GFP fusion protein localized to the cytosol, the septum, and the cell cortex. Its localization in the septum was dependent on its C-terminal region. Overexpression of the C-terminal region of Zds1 induced multi-septa and abnormal zygotes. We propose that the C-terminal region is the functional domain of Zds1 while the N-terminal region is a negative regulatory region. Thus, Zds1 is involved in multiple cellular events in fission yeast, including sexual differentiation, Ca2+ tolerance, cell wall integrity, viability in the stationary phase, and cell morphology.