The Influence of Membrane Lipid Composition and Physical Properties of Membrane Structure and Function inAcholeplasma Laidlawii

The unique properties of the A. luidlawii membrane were utilized by Steim et to show for the first time that biological membranes can undergo a gel-to-liquid-crystalline lipid phase transition similar to that previously reported for lamellar phospholipid-water systems. These workers demonstrated that when whole cells or isolated membranes are analyzed by differential scanning calorimetry (DSC), two relatively broad endothermic transitions are observed on the initial heating scan. The lowertemperature transition is fully reversible, varies markedly in position with changes in the length and degree of unsaturation of the membrane lipid fatty acyl chains, is broadened and eventually abolished by cholesterol incorporation, and exhibits a transition enthalpy characteristic of the mixed-acid synthetic phospholipids. Moreover, an endothermic transition having essentially identical properties is observed for the protein-free total membrane lipid extract dispersed in excess water or aqueous buffer, indicating that the presence of membrane proteins has little effect on the thennotropic phase behavior of most of the membrane lipids. The higher-temperature transition, in contrast, is irreversible, is independent on membrane lipid fatty acid composition or cholesterol content, and is absent in total membrane lipid extracts, indicating that it is due to an irreversible thermal denaturation of the membrane proteins. A comparison of the enthalpies of transition of the lipids in the membrane and in water dispersions indicates that at least 75% of the total membrane lipids participate in this transition. Evidence was also presented that the lipids must be predominantly in the fluid state to support normal growth. These results were later confirmed and extended by Reinert and Steim3 and by Melchior et al. ,4 who showed that the gel-to-liquid-crystalline