Sterol control of the phosphatidylethanolamine-phosphatidylcholine conversion in the yeast mutant GL7.

Abstract

The relatively slow growth rate of the yeast mutant GL7, a sterol auxotroph, on medium containing cholesterol is markedly accelerated by supplementation with small amounts of ergosterol. Under these conditions (sterol synergism) cellular phospholipid synthesis is enhanced. We now find that one of the ergosterol-stimulated processes is the methylation of phosphatidylethanolamine to phosphatidylcholine. This is shown by comparing methyltransferase activities of membrane preparations derived from cells grown on either ergosterol, cholesterol, or the synergistic sterol pair. Incorporation of 32p from [y-32P]ATP into the yeast membranes is rapid and greater when ergosterol-grown cells rather than cholesterol-grown cells are the source of membranes.Under defined culture conditions, selected pairs of sterols promote substantially faster growth of the sterol auxotrophs Mycoplasma capricolum (1) and the yeast mutant GL7 (2) than do the corresponding sterols individually. We refer to this growth response, which is greater than additive, as sterol synergism. In M. capricolum a marked synergistic effect is seen with a 20:1 mixture of lanosterol and cholesterol, whereas for the yeast mutant a 3:1 mixture of cholesterol and ergosterol is the most effective pair. The optimal single sterol for supporting the growth of mycoplasma is cholesterol; for yeast it is ergosterol. In our view the synergistic growth responses we have observed with pairs of sterols are manifestations of dual or multiple functions of sterols in certain membrane systems (1-3). The conventional function, modulation of the membrane physical state, is assigned to the major component and distinguished from the role of the minor sterol component, which involves control of some metabolic membrane-associated processes. Evidence for such sterol-specific metabolic interventions in phospholipid biosynthesis has been reported. In M. capricolum cholesterol lowers the Km for oleic acid uptake and incorporation into phosphatidylglycerol severalfold (4). In the yeast mutant GL7, cellular ergosterol accelerates the rates of synthesis of all membrane phospholipids, as judged by incorporation of oleic acid (2).In exploratory experiments designed to identify ergosterolsensitive partial reactions of yeast phospholipid synthesis the first trial was negative. Fatty acyl-CoA synthetase activities in extracts of cells grown either on ergosterol or cholesterol were identical. Examining the transmethylation pathway for the synthesis of phosphatidylcholine (PtdCho) we observed, however, an ergosterol-specific promotion of enzyme activity. First, we found that yeast cells grown on ergosterol or ergosterol/cholesterol mixtures incorporate substantially more radioactivity from [methyl-3H]methionine into PtdCho and its partially methylated precursors than cells grown on cholesterol alone. This observation pointed to a role for the prototypic yeast sterol in the S-adenosylmethionine-mediated methylation of phosphatidylethanolamine (PtdEtn) to PtdCho. In accord with this expectatio...

Importance and Role of Sterols in Fungal Membranes

“…This will promote more effective interaction between the phospholipid acyl chains and the sterol a-face (Buttke and Bloch 1980). Examining the transmethylation pathway for the synthesis of phosphatidy1chol-ine, Kawasaki et al (1985) observed an ergosterol-specific promotion of enzyme activity. They found that GL 7 cells grown on ergosterol or ergosterol/cholesterol (1: 3) mixtures, incorporate several times more 3H from [methyl-3Hj methionine into phosphatidylcholine than cells that have been raised on cholesterol alone.…”

A protein kinase antigenically related to pp60v-src possibly involved in yeast cell cycle control: positive in vivo regulation by sterol.

“…Phosphotyrosine was not detected. The membrane fraction of yeast has been shown to possess protein kinase activity (22,23).…”

Lipid acyl chain-dependent effects of sterols in Acholeplasma laidlawii membranes

“…The ability of 12 different sterols to affect (i) cell growth, (ii) lipid head group composition, (iii) the order parameter of the acyl chains, and (iv) the phase equilibria of in vivo lipid mixtures was studied. The following two effects were observed with respect to cell growth: (i) with a given acyl chain composition of the membrane lipids, growth was stimulated, unaffected, reduced, or completely inhibited (lysis), depending on the sterol structure; and (ii) the effect of a certain sterol depended on the acyl chain composition (most striking for epicoprostanol, cholest-4-en-3-one, and cholest-5-en-.3-one, which stimulated growth with saturated acyl chains but caused lysis with unsaturated chains (8,18,26), Of the membrane physical properties affected by sterols, the most commonly investigated are the gel-to-liquid crystalline phase transition temperature (Tm) of the lipids (11,17,20), the order parameter of the lipid acyl chains (17,30,32), and the permeability of the membranes or the derived lipids (1, 10, 12). Recently, studies have also been performed on the ability of some sterols to induce the transition between a lamellar phase and a nonlamellar phase, most often a reversed hexagonal (HI,) phase, in lipid-water model systems (7,15,25 …”

“…However, more specific functions of some sterols have recently been observed for two organisms auxotrophic for sterols, Mycoplasma capricolum and Saccharomyces cerevisiae. Addition of minor concentrations of a certain sterol to a growth medium already containing another sterol in a manifold higher concentration enhances the rate of important metabolic processes (8,18,26), Of the membrane physical properties affected by sterols, the most commonly investigated are the gel-to-liquid crystalline phase transition temperature (Tm) of the lipids (11,17,20), the order parameter of the lipid acyl chains (17,30,32), and the permeability of the membranes or the derived lipids (1,10,12). Recently, studies have also been performed on the ability of some sterols to induce the transition between a lamellar phase and a nonlamellar phase, most often a reversed hexagonal (HI,) phase, in lipid-water model systems (7,15,25).…”