Lipids of mitochondria.

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.

Biochemistry, Cell Biology and Molecular Biology of Lipids of Saccharomyces cerevisiae

https://www.cell.com/trends/biochemical-sciences/pdf/S0968-0004(98)01349-8.pdf

Mechanisms of lipid-body formation.

Storage Lipid Synthesis Is Non-essential in Yeast

Cargo-selected transport from the mitochondria to peroxisomes is mediated by vesicular carriers.

Lipid particles of the yeast, Saccharomyces cerevisiae, were isolated to high purity and their components were analysed. The hydrophobic core of this organelle consists of triacylglycerols and steryl esters, which are almost exclusively located to that compartment. Lipid particles are stabilized by a surface membrane consisting of phospholipids and proteins. Electron microscopy confirmed the purity of the preparations and the proposed structure deduced from biochemical experiments. Major proteins of lipid particles have molecular weights of 72, 52, 43 and 34 kDa, respectively. The 43 kDa protein reacts with an antiserum against human apolipoprotein AII. In lipid particles of the yeast mutant strain S. cerevisiae erg6, which is deficient in sterol delta 24-methyltransferase, this protein is missing thereby identifying the protein and confirming our previous finding (Zinser et al., 1993) that sterol delta 24-methylation is associated with lipid particles. A possible involvement of surface proteins of lipid particles in the interaction with other organelles is discussed with respect to sterol translocation in yeast.

Characterization of lipid particles of the yeast, Saccharomyces cerevisiae.

Squalene epoxidase, encoded by the ERG1 gene in yeast, is a key enzyme of sterol biosynthesis. Analysis of subcellular fractions revealed that squalene epoxidase was present in the microsomal fraction (30,000 x g) and also cofractionated with lipid particles. A dual localization of Erg1p was confirmed by immunofluorescence microscopy. On the basis of the distribution of marker proteins, 62% of cellular Erg1p could be assigned to the endoplasmic reticulum and 38% to lipid particles in late logarithmic-phase cells. In contrast, sterol Delta24-methyltransferase (Erg6p), an enzyme catalyzing a late step in sterol biosynthesis, was found mainly in lipid particles cofractionating with triacylglycerols and steryl esters. The relative distribution of Erg1p between the endoplasmic reticulum and lipid particles changes during growth. Squalene epoxidase (Erg1p) was absent in an erg1 disruptant strain and was induced fivefold in lipid particles and in the endoplasmic reticulum when the ERG1 gene was overexpressed from a multicopy plasmid. The amount of squalene epoxidase in both compartments was also induced approximately fivefold by treatment of yeast cells with terbinafine, an inhibitor of the fungal squalene epoxidase. In contrast to the distribution of the protein, enzymatic activity of squalene epoxidase was only detectable in the endoplasmic reticulum but was absent from isolated lipid particles. When lipid particles of the wild-type strain and microsomes of an erg1 disruptant were mixed, squalene epoxidase activity was partially restored. These findings suggest that factor(s) present in the endoplasmic reticulum are required for squalene epoxidase activity. Close contact between lipid particles and endoplasmic reticulum may be necessary for a concerted action of these two compartments in sterol biosynthesis.

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Regina Maria Leber

Papers

Characterization of lipid particles of the yeast, Saccharomyces cerevisiae.

Dual Localization of Squalene Epoxidase, Erg1p, in Yeast Reflects a Relationship between the Endoplasmic Reticulum and Lipid Particles

Export of steryl esters from lipid particles and release of free sterols in the yeast, Saccharomyces cerevisiae