In the yeast Saccharomyces cerevisiae, two acyl-CoA:sterol acyltransferases (ASATs) that catalyze the synthesis of steryl esters have been identified, namely Are2p (Sat1p) and Are1p (Sat2p). Deletion of either ARE1 or ARE2 has no effect on cell viability, and are1are2 double mutants grow in a similar manner to wild-type despite the complete lack of cellular ASAT activity and steryl ester formation Here we show that both Are2p and Are1p reside in the endoplasmic reticulum as demonstrated by measuring ASAT activity in subcellular fractions of are1 and are2 deletion strains. This localization was confirmed by fluorescence microscopy using hybrid proteins of Are2p and Are1p fused to green fluorescent protein (GFP). Lipid analysis of are1 and are2 deletion strains revealed that Are2p and Are1p utilize sterol substrates in vivo with different efficiency; Are2p has a significant preference for ergosterol as a substrate, whereas Are1p esterifies sterol precursors, mainly lanosterol, as well as ergosterol. The specificity towards fatty acids is similar for both isoenzymes. The lack of steryl esters in are1are2 mutant cells is largely compensated by an increased level of free sterols. Nevertheless, terbinafine, an inhibitor of ergosterol biosynthesis, inhibits growth of are1are2 cells more efficiently than growth of wild-type. In a growth competition experiment are1are2 cells grow more slowly than wild-type after several rounds of cultivation, suggesting that Are1p and Are2p or steryl esters, the product formed by these two enzymes, are more important in the natural environment than under laboratory conditions.

Squalene is a natural triterpene and an important intermediate of sterol and hopanoid biosynthesis in various types of cell from bacteria to human. Synthesis and further conversion of squalene are key steps in the metabolism of sterols and related components. Here we summarize the recent knowledge of squalene biochemistry, its molecular properties, and its physiological effects. We compare squalene biosynthetic pathways in different cell types and describe biotechnological strategies to isolate this lipid. Finally, applications of squalene in nutrition, pharmacy, and medicine are discussed.

This study was aimed at a better understanding of organelle organization in the yeast Saccharomyces cerevisiae with special emphasis on the interaction and physical association of organelles. For this purpose, a computer aided method was employed to generate three-dimensional ultrastructural reconstructions of chemically and cryofixed yeast cells. This approach showed at a high level of resolution that yeast cells were densely packed with organelles that had a strong tendency to associate at a distance of <30 nm. The methods employed here also allowed us to measure the total surface area and volume of organelles, the number of associations between organelles, and the ratio of associations between organelles per surface area. In general, the degree of organelle associations was found to be much higher in chemically fixed cells than in cryofixed cells, with endoplasmic reticulum/plasma membrane, endoplasmic reticulum/mitochondria and lipid particles/nuclei being the most prominent pairs of associated fractions. In cryofixed cells, similar preferences for organelle association were seen, although at lower frequency. The occurrence of specific organelle associations is believed to be important for intracellular translocation and communication. Membrane contact as a possible means of interorganelle transport of cellular components, especially of lipids, is discussed.

Waste not, want not! This saying also applies to the yeast. In times of excess this microorganism synthesizes non-polar lipids, triacylglycerols (TAG) and steryl esters (SE), which serve as energy source and/or as a source of building blocks for the formation of membrane phospholipids when nutriments become limiting. Due to the lack of charged groups substantial amounts of non-polar lipids cannot be incorporated into cellular membranes, whereas the hydrophobic nature of these molecules also deprives cytosolic localization. Consequently, TAG and SE are stored in specialized compartments known as lipid particles or lipid droplets. TAG synthases and SE synthases which catalyze the final steps in storage lipid formation occur redundantly. Hydrolytic enzymes, TAG lipases and SE hydrolases, initiate the breakdown of storage lipids and make sterols, diacylglycerols, and free fatty acids available for anabolic processes and energy production. In this paper we describe routes of non-polar lipid metabolism, focus on molecular properties of enzymes involved and discuss regulatory aspects of the respective pathways. The value of the yeast as a eukaryotic model system is advocated.

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