Abstract
The membrane lipid composition in several strains of Acholeplasma laidlawii is regulated upon a change in the growth conditions. Monoglucosyldiacylglycerol (MGlcDAG) and diglucosyldiacylglycerol (DGlcDAG) are the most abundant lipids in the A. laidlawii membrane. A third glucolipid, 3-O-acyl-monoglucosyldiacylglycerol (MAMGlcDAG) is synthesized by strain A-EF22 when the membrane lipids contain large amounts of saturated acyl chains. The lipid regulation can be understood from a simple theoretical model, in which the cells strive to maintain a balance between the lipids constituting a bilayer and those forming reversed non-lamellar liquid crystalline phases. Thus, the physical chemistry of membrane lipids, in particular their ability to form different aggregate structures, constitutes the basis for the lipid regulation, and therefore an understanding of the phase equilibria of membrane lipids is crucial. MGlcDAG and MAMGlcDAG isolated from A. laidlawii strain A-EF22 membranes were studied mainly by 2H NMR, 1H NMR, and 1H NMR diffusion measurements. MAMGlcDAG, containing 96 mol % saturated acyl chains formed a gel/crystalline phase up to about 80 degrees C, where a transition occurred to a reversed micellar (L2) phase. This is an unexpected finding for a membrane lipid. However, this lipid homogeneously mixes with the other membrane lipids at physiological temperatures. Previous and new data on MGlcDAG show that the lamellar phase is stabilized when the length and the degree of unsaturation of the acyl chains are decreased. The physicochemical properties of MAMGlcDAG and MGlcDAG were compared and found to be of great significance for the physiological regulation of the lipids in the membrane. MAMGlcDAG is synthesized under conditions when the phase equilibria of MGlcDAG are shifted from a non-lamellar toward a lamellar phase. Apart from MAMGlcDAG, MGlcDAG is the major lipid in A. laidlawii strain A-EF22 which is able to form reversed aggregate structures. MAMGlcDAG probably assists MGlcDAG in maintaining an optimal molecular packing, or negative curvature, of the lipids in the membrane.
Highlights
Per-Olof Erikssonz From the Departments of $Physical Chemistry and 1)Biochemistry, University of Umeii, S-90187 Urn&,Sweden and the lllnstitute of Surface Chembtry, P. 0.Box 5607, 8-114 86 Stockholm, Sweden
The membrane lipid composition in several strains In recent years overwhelming evidence has been presented of Acholeplasma laidlawii is regulated upon a change that the membrane lipids participate in cell membrane funcin the growthconditions
MGlcDAG and MAMGlcDAG isolated from A. laidlawisitrain A-EF22 membranes were studiedmainly by 2HNMR, ‘H NMR, and ‘HNMR diffusion measurements
Summary
Pulse width of 14 ps, a spectral width of 166 kHz, and an interpulse spacing of 50 ps. Cell Growth-For preparation of MGlcDAG, A. laidlawii strain A- described previously (Eriksson etal., 1991). EF22 (Wieslander and Rilfors, 1977) was grown in a medium con- ' H NMR Diffmion Measurements-Lipid translational diffusion taining lipid-depleted tryptose andfatty acid-poor bovine serum in isotropic phases, like the cubic and micellar ones, can be convenalbumin (Eriksson etal., 1991).The medium was supplemented with iently studied using thestandard pulsed magnetic field gradient. Preparation of MAMGlcDAG,the cells were grown in 15 liters of the above mentioned medium supplemented with 120 PM palmitic acid and 30 p M oleic acid.A significant synthesis of MAMGlcDAG occurs only when A. laidlawiistrain A-EF22 incorporates a large fraction of saturated, straight-chain fatty acids into the membrane lipids.
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Published Version
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