YEH2/YLR020c Encodes a Novel Steryl Ester Hydrolase of the Yeast Saccharomyces cerevisiae

Abstract

Previous work from our laboratory (Zinser, E., Paltauf, F., and Daum, G. (1993) J. Bacteriol. 175, 2853-2858) demonstrated steryl ester hydrolase activity in the plasma membrane of the yeast Saccharomyces cerevisiae. Here, we show that the gene product of YEH2/ YLR020c, which is homologous to several known mammalian steryl ester hydrolases, is the enzyme catalyzing this reaction. Deletion of yeast YEH2 led to complete loss of plasma membrane steryl ester hydrolase activity whereas overexpression of the gene resulted in a significant elevation of the activity. Purification of enzymatically active Yeh2p close to homogeneity unambiguously identified this protein as a steryl ester hydrolase and thus as the first enzyme of this kind characterized in S. cerevisiae. In addition to evidence obtained in vitro experiments in vivo contributed to the characterization of this novel enzyme. Sterol analysis of yeh2⌬ unveiled a slightly elevated level of zymosterol suggesting that the esterified form of this sterol precursor is a preferred substrate of Yeh2p. However, in strains bearing hybrid proteins with strongly enhanced Yeh2p activity decreased levels of all steryl esters were observed. Thus, it appears that Yeh2p activity is not restricted to distinct steryl esters but rather has broad substrate specificity. The fact that in a yeh2⌬ deletion strain bulk steryl ester mobilization occurred at a similar rate as in wild type suggested that Yeh2p is not the only steryl ester hydrolase but that other enzymes with overlapping function exist in the yeast.Most eukaryotic cells contain and synthesize sterols, which are essential lipid components of membranes. Besides their role in maintaining membrane permeability and fluidity, effects of sterols on aerobic metabolism (1, 2), completion of the cell cycle (3) as well as on sterol uptake (4) and sterol transport (5) have been described. Considering the different functions of sterols it is obvious that sterol homeostasis, including sterol biosynthesis, uptake, transport, storage, utilization, and efflux, has to be a strictly regulated process. Specific sterols present in different eukaryotic cells vary, ergosterol being the major sterol in the yeast Saccharomyces cerevisiae.Besides other regulatory mechanisms esterification of sterols and hydrolysis of steryl esters (STE) 1 play an important role in cellular sterol homeostasis. These processes do not only allow cells to store chemical energy, which can be used in times of deprivation, but also provide an additional means to balance the concentrations of free sterols and fatty acids, which are essential and critical for cell structure and function. In mammalian cells esterification of sterols is catalyzed either by lecithin:cholesteryl acyltransferase (reviewed in Refs. 6 -8) or by the two acyl-CoA:cholesteryl acyltransferases ACAT1 and ACAT2 (reviewed in Ref. 9). Cholesteryl ester hydrolysis occurs by a number of mammalian enzymes many of which have already been identified, among them carboxyl ester lipase (CEL), lysosomal acid lipase...

A role for seipin in lipid droplet dynamics and inheritance in yeast

“…One population of lipid droplets is dispersed towards the periphery of cells upon stimulation of growth, which is also accompanied by active lipolysis (Kurat et al, 2006). Lipid droplets that contain both TAG and steryl esters have been implicated in lipid trafficking , in particular to supply the plasma membrane with ergosterol that might be liberated from the lipid droplets by the plasma-membrane-resident steryl ester hydrolase Yeh2 (Koffel et al, 2005;Mullner et al, 2005). Thus it could be speculated that the two populations of lipid droplets are indeed characterized by different, specific lipid compositions, harbouring either Tag or SE.…”

Multiple Functions as Lipase, Steryl Ester Hydrolase, Phospholipase, and Acyltransferase of Tgl4p from the Yeast Saccharomyces cerevisiae

“…Enzyme Analyses-The affinity chromatography-purified Tgl4p was assayed for calcium-independent PLA 2 , lipase, and (55,56). All reactions were terminated by lipid extraction with 200 l of butanol.…”

Structural and Biochemical Properties of Lipid Particles from the Yeast Saccharomyces cerevisiae

“…The highest SE hydrolase activity present in the plasma membrane (7) was attributed to Yeh2p (12,13). The physiological role of this SE hydrolase in the plasma membrane is still a matter of dispute.…”

“…Strikingly, most of these proteins are enzymes involved in lipid metabolism. As prominent examples, LP from S. cerevisiae harbor enzymes of phosphatidic acid biosynthesis (3), fatty acid activation (4 -6), sterol biosynthesis (7), but also TAG biosynthesis and degradation (8 -11) and SE hydrolysis (12)(13)(14).…”