The effect of altered membrane sterol composition on the temperature dependence of yeast mitochondrial ATPase

The sterol content of cells of $\text{Saccharomyces cerevisiae}$ was manipulated by growing the organism anaerobically in a medium containing excess supplements of unsaturated fatty acids and a range of supplements of ergosterol. Anaerobic mitochondrial precursor structures were isolated whose membrane lipids contain the same fatty acid composition but whose sterol content varies from 7 to 105 mg/g mitochondrial protein. Arrhenius plots of the mitochondrial ATPase activity of the different preparations show a discontinuity with Arrhenius activation energies of about +40 and +80 KJ/mole, respectively, above and below the transition temperature. However, the temperature of the transition is markedly dependent on sterol composition, and increases by up to 17° as the sterol content of the mitochondria is progressively decreased. These results support the concepts that membrane lipid composition influences the activity of membrane-bound enzymes, and that sterols promote the gel to liquid phase transition in biological membranes.