Syringomycin action gene SYR2 is essential for sphingolipid 4-hydroxylation in Saccharomyces cerevisiae

Syringomycin E inhibition of Saccharomyces cerevisiae: requirement for biosynthesis of sphingolipids with very-long-chain fatty acids and mannose- and phosphoinositol-containing head groups

Syringomycin E is an antifungal cyclic lipodepsinonapeptide that inhibits the growth of Saccharomyces cerevisiae by interaction with the plasma membrane. A screen conducted to find the yeast genes necessary for its fungicidal action identified two novel syringomycin E response genes, SYR3 and SYR4. A syr3 mutant allele was complemented by ELO2 and ELO3. These genes encode enzymes that catalyze the elongation of sphingolipid very long chain fatty acids. Tetrad analysis showed that SYR3 was ELO2. Strains with deletions of SYR3/ELO2 and ELO3 were resistant to syringomycin E, and lipid analyses of both mutants revealed shortened fatty acid chains and lower levels of sphingolipids. SYR4 was identified by Tn5 inactivation of genomic library plasmids that complemented a syr4 mutant allele. SYR4 was found to be identical to IPT1, which encodes the terminal sphingolipid biosynthetic enzyme, mannosyl-diinositolphosphoryl-ceramide synthase. Deletion Deltasyr4/ipt1 strains were viable, were resistant to syringomycin E, did not produce mannosyl-diinositolphosphoryl-ceramide, and accumulated mannosyl-inositolphosphoryl-ceramide. Accumulation of mannosyl-inositolphosphoryl-ceramide was not responsible for resistance since a temperature-sensitive secretory pathway mutant (sec14-3(ts)) accumulated this sphingolipid and was sensitive to syringomycin E. Finally, Deltacsg1/sur1 and Deltacsg2 strains defective in the transfer of mannose to inositolphosphoryl-ceramide were resistant to syringomycin E. These findings show that syringomycin E growth inhibition of yeast is promoted by the production of sphingolipids with fully elongated fatty acid chains and the mannosyl and terminal phosphorylinositol moieties of the polar head group.

Brave little yeast, please guide us to thebes: sphingolipid function in S. cerevisiae

Sphingolipids typically cover the exoplasmic leaflet of the plasma membrane of eukaryotic cells. They differ from the more abundant glycerophospholipids in that they contain ceramide instead of diacylglycerol as a hydrophobic anchor. Why did nature choose to invent this complex class of lipids, and why do eukaryotic cells follow elaborate remodelling pathways in order to generate dozens to hundreds of different molecular species of sphingolipid, depending on cell type? Yeast may, once again, serve as a model to dissect sphingolipid function at various levels. Almost the complete pathway for sphingolipid synthesis in yeast has been uncovered during the past two decades. More recently, key enzymes in sphingolipid degradation and signalling have been identified. Together with a wealth of genetic data obtained from the characterization of various suppressor mutants, this information now allows for an unprecedented analysis of sphingolipid function in this organism. This overview summarizes recent data on sphingolipid function in cell signalling, their role in the heat-stress response and Ca(2+) homeostasis, and addresses their function in transport of glycosylphosphatidylinositol-anchored proteins.

Antifungal activities of antineoplastic agents: Saccharomyces cerevisiae as a model system to study drug action

Recent evolutionary studies reveal that microorganisms including yeasts and fungi are more closely related to mammals than was previously appreciated. Possibly as a consequence, many natural-product toxins that have antimicrobial activity are also toxic to mammalian cells. While this makes it difficult to discover antifungal agents without toxic side effects, it also has enabled detailed studies of drug action in simple genetic model systems. We review here studies on the antifungal actions of antineoplasmic agents. Topics covered include the mechanisms of action of inhibitors of topoisomerases I and II; the immunosuppressants rapamycin, cyclosporin A, and FK506; the phosphatidylinositol 3-kinase inhibitor wortmannin; the angiogenesis inhibitors fumagillin and ovalicin; the HSP90 inhibitor geldanamycin; and agents that inhibit sphingolipid metabolism. In general, these natural products inhibit target proteins conserved from microorganisms to humans. These studies highlight the potential of microorganisms as screening tools to elucidate the mechanisms of action of novel pharmacological agents with unique effects against specific mammalian cell types, including neoplastic cells. In addition, this analysis suggests that antineoplastic agents and derivatives might find novel indications in the treatment of fungal infections, for which few agents are presently available, toxicity remains a serious concern, and drug resistance is emerging.

Metabolism and selected functions of sphingolipids in the yeast Saccharomyces cerevisiae

Our knowledge of sphingolipid metabolism and function in Saccharomyces cerevisiae is growing rapidly. Here we discuss the current status of sphingolipid metabolism including recent evidence suggesting that exogenous sphingoid long-chain bases must first be phosphorylated and then dephosphorylated before incorporation into ceramide. Phenotypes of strains defective in sphingolipid metabolism are discussed because they provide hints about the undiscovered functions of sphingolipids and are one of the major reasons for studying this model eukaryote. The long-chain base phosphates, dihydrosphingosine-1-phosphate and phytosphingosine-1-phosphate, have been hypothesized to play roles in heat stress resistance, perhaps acting as signaling molecules. We evaluate the data supporting this hypothesis and suggest future experiments needed to verify it. Finally, we discuss recent clues that may help to reveal how sphingolipid synthesis and total cellular sphingolipid content are regulated.

Pseudomonas syringae phytotoxins: mode of action, regulation, and biosynthesis by peptide and polyketide synthetases

Coronatine, syringomycin, syringopeptin, tabtoxin, and phaseolotoxin are the most intensively studied phytotoxins of Pseudomonas syringae, and each contributes significantly to bacterial virulence in plants. Coronatine functions partly as a mimic of methyl jasmonate, a hormone synthesized by plants undergoing biological stress. Syringomycin and syringopeptin form pores in plasma membranes, a process that leads to electrolyte leakage. Tabtoxin and phaseolotoxin are strongly antimicrobial and function by inhibiting glutamine synthetase and ornithine carbamoyltransferase, respectively. Genetic analysis has revealed the mechanisms responsible for toxin biosynthesis. Coronatine biosynthesis requires the cooperation of polyketide and peptide synthetases for the assembly of the coronafacic and coronamic acid moieties, respectively. Tabtoxin is derived from the lysine biosynthetic pathway, whereas syringomycin, syringopeptin, and phaseolotoxin biosynthesis requires peptide synthetases. Activation of phytotoxin synthesis is controlled by diverse environmental factors including plant signal molecules and temperature. Genes involved in the regulation of phytotoxin synthesis have been located within the coronatine and syringomycin gene clusters; however, additional regulatory genes are required for the synthesis of these and other phytotoxins. Global regulatory genes such as gacS modulate phytotoxin production in certain pathovars, indicating the complexity of the regulatory circuits controlling phytotoxin synthesis. The coronatine and syringomycin gene clusters have been intensively characterized and show potential for constructing modified polyketides and peptides. Genetic reprogramming of peptide and polyketide synthetases has been successful, and portions of the coronatine and syringomycin gene clusters could be valuable resources in developing new antimicrobial agents.

Yeast sphingolipids

Many advances in our understanding of fungal sphingolipids have been made in recent years. This review focuses on the types of sphingolipids that have been found in fungi and upon the genes in Saccharomyces cerevisiae, the common baker's yeast, that are necessary for sphingolipid metabolism. While only a small number of fungi have been examined, most contain sphingolipids composed of ceramide derivatized at carbon-1 with inositol phosphate. Further additions include mannose and then other carbohydrates. The second major class of fungal sphingolipids is the glycosylceramides, having either glucose or galactose attached to ceramide rather than inositol phosphate. The glycosylceramides sometimes contain additional carbohydrates. Knowledge of the genome sequence has expedited identification of S. cerevisiae genes necessary for sphingolipid metabolism. At least one gene is known for most steps in S. cerevisiae sphingolipid metabolism, but more are likely to be identified so that the 13 known genes are likely to grow in number. The AUR1 gene is necessary for addition of inositol phosphate to ceramide and has been identified as a target of several potent antifungal compounds. This essential step in yeast sphingolipid synthesis, which is not found in humans, appears to be an excellent target for the development of more effective antifungal compounds, both for human and for agricultural use.

Biochemistry, cell biology and molecular biology of lipids of Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae is a powerful experimental system to study biochemical, cell biological and molecular biological aspects of lipid synthesis. Most but not all genes encoding enzymes involved in fatty acid, phospholipid, sterol or sphingolipid biosynthesis of this unicellular eukaryote have been cloned, and many gene products have been functionally characterized. Less information is available about genes and gene products governing the transport of lipids between organelles and within membranes, turnover and degradation of complex lipids, regulation of lipid biosynthesis, and linkage of lipid metabolism to other cellular processes. Here we summarize current knowledge about lipid biosynthetic pathways in S. cerevisiae and describe the characteristic features of the gene products involved. We focus on recent discoveries in these fields and address questions on the regulation of lipid synthesis, subcellular localization of lipid biosynthetic steps, cross-talk between organelles during lipid synthesis and subcellular distribution of lipids. Finally, we discuss distinct functions of certain key lipids and their possible roles in cellular processes.

Role of ergosterol in growth inhibition of Saccharomyces cerevisiae by syringomycin E

The antifungal activity of the lipodepsipeptide syringomycin E from Pseudomonas syringae pv. syringae is modulated by sterols. To study the requirement of the predominant fungal sterol, ergosterol, in syringomycin E action, the sterol composition of Saccharomyces cerevisiae sterol auxotroph strain FY-14 was modified and sensitivity to syringomycin E examined. Cells containing solely ergosterol, cholesterol, beta-sitosterol or stigmasterol were sensitive to syringomycin E with the latter two being the most sensitive. Cells containing growth-promoting cholesterol were the most sensitive and those with growth-promoting ergosterol the least sensitive. It is concluded that sensitivity to syringomycin E is modulated by growth-promoting sterols and does not necessarily require ergosterol.

Membrane sterol composition modulates the pore forming activity of syringomycin E in human red blood cells

The effect of lipopeptide antifungal agent, syringomycin E (SRE) on the membrane permeability of human red blood cells (RBCs) was studied. SRE added to RBCs above a concentration of 2x106 molecules/cell (50 microgram/ml RBCs) caused a rapid and concentration dependent lysis of a small subpopulation of RBCs; the extent of this lysis remained unchanged as long as 100 min. During this time period the membranes of the unlysed cells had enhanced permeability for ions which was monitored by direct measurement of 86Rb flux. Both the extent of cell lysis and ion transport rate showed linear relationships with SRE concentration demonstrating a random distribution of SRE molecules in red blood cells. The kinetics of the 86Rb efflux suggested pore formation by syringomycin E. The pores had discrete life times and were eventually inactivated. The pores were also a pathway for efflux of monomeric haemoglobin. Alteration of the membrane sterol composition, i.e. depletion of cholesterol by 50% or partial ergosterol substitution of the cholesterol increased the SRE induced membrane permeability for 86Rb by two orders compared to membranes with unaltered sterol composition. This modification of the sterol composition promotes the pore forming activity of this lipopeptide in the membrane.