Storage Lipid Synthesis Is Non-essential in Yeast

Abstract

Steryl esters and triacylglycerol (TAG) are the main storage lipids in eukaryotic cells. In the yeast Saccharomyces cerevisiae, these storage lipids accumulate during stationary growth phase within organelles known as lipid bodies. We have used single and multiple gene disruptions to study storage lipid synthesis in yeast. Four genes, ARE1, ARE2, DGA1, and LRO1, were found to contribute to TAG synthesis. The most significant contribution is made by DGA1, which encodes a novel acyl-CoA:diacylglycerol acyltransferase. Two of the genes, ARE1 and ARE2, are also involved in steryl ester synthesis. A yeast strain that lacks all four genes is viable and has no apparent growth defects under standard conditions. The strain is devoid of both TAG and steryl esters, and fluorescence microscopy revealed that it also lacks lipid bodies. We conclude that neither storage lipids nor lipid bodies are essential for growth in yeast.Lipids are important storage compounds in plants, animals, and fungi. The main storage lipids in eukaryotes are triacylglycerol (TAG) 1 and steryl esters (1). Storage lipids are usually found within special organelles known as lipid particles or lipid bodies (2). In yeast, these lipid bodies accumulate during stationary phase, and they can constitute up to 70% of the total lipid content of the cell (3-5). Several lipid-metabolizing enzymes are preferentially localized to the lipid bodies in yeast, and it has therefore been proposed that they do not solely serve as a depot for lipids but instead may have a more complex role in lipid biosynthesis, metabolism, degradation, and trafficking (6).Steryl esters, the esterified form of sterols linked to a long chain fatty acid, are synthesized by the enzyme acyl-CoA:sterol acyltransferase (ASAT) which is encoded by the duplicated ARE1 and ARE2 genes in the yeast Saccharomyces cerevisiae (7-9). Related genes encoding enzymes with ASAT activity exist also in higher eukaryotes (10, 11). It has been shown that the other major storage lipid, TAG, can be synthesized in at least two different ways. One is an acyl-CoA-dependent reaction that is catalyzed by acyl-CoA:diacylglycerol acyltransferase (DGAT). Two different DGATs are known to exist in eukaryotes. One DGAT, also known as DGAT1, was first described in mammals but is also found in plants and is related to ASAT (12,13). In addition, a second DGAT, DGAT2, was recently identified in the fungus Mortiella ramanniana (14). DGAT2 is not related to any previously known enzyme, but genes encoding homologues of DGAT2 exist in other eukaryotes (14,15). One such gene is the DGA1 gene in yeast (open reading frame YOR245c). In addition to the reaction catalyzed by the two types of DGATs, an acyl-CoA-independent pathway for TAG synthesis was also recently discovered in plants and yeast (16). This pathway involves the novel enzyme phospholipid: diacylglycerol acyltransferase (PDAT), which can synthesize TAG from phospholipids and diacylglycerol (16). PDAT is distantly related to the mammalian enzyme lecithin:cholesterol acyltr...

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