Anders Christiansson

Lipid Bilayer Stability in Biological Membranes

One of the most important problems in biophysics today is the self-assembly of membrane components, in particular lipids and proteins. It has long been known that many membrane lipids spontaneously form a bilayer when mixed with water (Luzzati, 1968). Recently, it was also indicated that even a membrane protein may form a bilayer structure (Carlsson, 1981). The Singer and Nicolson (1972) model of a biomembrane is based on the assumption that the lipids form a bilayer matrix in which the proteins are incorporated and are able to diffuse more or less freely in two dimensions. If the function of the lipids is only to form this fluid matrix, why does a biological membrane often contain more than 100 different lipid species? Furthermore, many lipids do not spontaneously form bilayers with water. Sometimes not even the major lipid in a membrane forms a bilayer, e.g., monogalactosyldiglyceride of chloroplasts (Shipley et al., 1973; Brentel et al., 1984c). However, the membrane lipids together with the proteins form a stable, functioning membrane, with the properties necessary for a living cell, i.e., to be both a barrier and a communicator to the surroundings. It can thus be expected that the membrane lipids are not working just as a fluid matrix, as is suggested by the Singer and Nicolson model. Most probably they play an important structural role in biological membranes as will be discussed in this chapter.

Lipid phase structure governs the regulation of lipid composition in membranes of acholeplasma laidlawii

great variety of lipid molecules are present in biologic membr~es - a rn~rn~an cell membrane may contain 100 different lipids including intra- molecular variants. Many of these form lamellar liquid crystalline phases together with water [ I]_ However, most biological membr~es contain at least one major lipid species forming a non-lamellar phase. Monog~actosyl~~y~~de and mono~ucosyldi- glyceride (MGDG) form a reversed hexagonal (HE) phase [ 2,3], while

Control of membrane polar lipid composition in Acholeplasma laidlawii a by the extent of saturated fatty acid synthesis

The low level of endogenous fatty acid synthesis in Acholeplasma laidlawii A strain EF22 was found to be caused by a deficiency of pantetheine in the lipid-depleted growth medium. By supplementing the oleic acid-containing medium with increasing concentrations of pantethein, saturated fatty acid synthesis was stimulated (having an apparent Km of 5 microM for pantetheine) and the incorporation of endogenously synthesized fatty acids in membrane lipids increased markedly. Furthermore, carotenoid biosynthesis was stimulated. Exogenous palmitic acid was found to inhibit partially the endogenous fatty acid synthesis. A gradual stimulation of fatty acid synthesis was accompanied by a linear increase in the molar proportion between the two dominating membrane glucolipids, monoglucosyldiacylglycerol and diglucosyldiacylglycerol. The total amount of charged membrane lipids decreased upon increasing the degree of fatty acid saturation. These regulations are discussed in terms of membrane stability, and influence of membrane molecular ordering and surface charge density on lipid polar head group synthesis.

Fractionation of membranes from Acholeplasma laidlawii A on the basis of their surface properties by partition in two-polymer aqueous phase systems

Acholeplasma laidlawii A consists of pleomorphic cell clusters surrounded by a single membrane. When lysed, a cell gives rise to several membrane fragments which cannot be separated from each other by isopycnic sucrose gradient centrifugation. A heterogeneous lateral organization of the cell membranes was detected by countercurrent distribution of membrane fragments in a two-polymer aqueous phase system. It revealed that the membranes consist of at least two subpopulations with respect to surface properties. Changes in the fatty acid and cholesterol content of the membranes revealed that the resolution of different subpopulations was predominantly due to a critical ratio of monoglucosyldiglyceride to diglucosyldiglyceride. The heterogeneity of the membrane probably depends on lipid-lipid and lipid-protein steric interactions. Charged lipids, an apolar monoglucolipid and the ratio between lipids and proteins also affect membrane partition. The differences in the subpopulations were further reflected by different specific activities of NADH dehydrogenase, NADH oxidase and ATPase. These activities varied independently. Minor quantitative differences in the protein patterns of different subpopulations were apparent. The origin and the preservation of the membrane subpopulations are discussed in terms of lipid-lipid and lipid-protein interactions, their age and energy metabolism.

Effects of anesthetics on water permeability and lipid metabolism in acholeplasma laidlawii membranes

The addition of tetracaine and diethyl ether to Acholeplasma laidlawii at concentrations commonly used for local anesthesia did not affect water permeability over the cell membrane, as measured by a pulsed magnetic field gradient NMR method. However, A. laidlawii changed its membrane lipid composition upon treatment with these anesthetics. Both tetracaine and diethyl ether addition resulted in a decrease in the molar ratio between the major membrane glucolipids, monoglucosyldiacylglycerol and diglucosyldiacylglycerol. The ratio between saturated and unsaturated acyl chains did not change. The results are in accordance with our proposal that A. laidlawii regulates its lipid composition in order to maintain optimal packing stability in the membrane (Wieslander, A., Christiansson, A., Rilfors. L. and Lindblom, G. (1980) Biochemistry 19, 3650--3655). Introduction of anesthetics into the hydrophobic region of a bilayer is likely to affect the lipid packing. A membrane which contains lipids like monoglucosyldiacylglycerol, which forms a reversed hexagonal phase, will be destabilized unless the amounts of such lipids are reduced. The membrane concentration of anesthetics was estimated to one molecule per 12--15 lipid molecules. The fact that A. laidlawii regulates its lipid composition as a response to these concentrations, despite their negligible effect on water permeability, indicates a high sensitivity of this regulatory system.

Membrane potential, lipid regulation and adenylate energy charge in acyl chain modified Acholeplasma laidlawii.

In Acholeplasma laidlawii variations induced in the transmembrane electrical potential have been shown to affect the membrane lipid composition. Particularly the molar ratio between the predominant glucolipids, monoglucosyldiacylglycerol and diglucosyldiacylglycerol, decreases upon hyperpolarization and increases upon depolarization (Clementz et al. (1986) Biochemistry 25, 823-830). Upon variation of the degree of membrane fatty acyl chain unsaturation, known to affect the passive permeability for a number of small molecules, there was no significant correlation between acyl chain composition and the magnitude of the electrical potential. Hyperpolarization by valinomycin decreased the glucolipid ratio for all kinds of membranes, but the size of the decrease was not correlated to the acyl chain composition. However, a clear relationship, independent of acyl chain composition, was found between the extent of hyperpolarization and the size of the decrease in the glucolipid ratio. The adenylate energy charge value (Ec) of the cells was affected by the acyl chain composition, although not exclusively by the proportion of unsaturation. Furthermore, a larger hyperpolarization upon valinomycin addition was accompanied by a stronger reduction in Ec.