INFLUENCE OF MONOGLUCOSYLDIACYLGLYCEROL AND MONOACYLMONOGLUCOSYLDIACYLGLYCEROL ON THE LIPID BILAYER OF THE MEMBRANE FROM ACHOLEPLASMA-LAIDLAWII STRAIN A-EF22

The ability for ,2-diacyl-3-O-(alpha-n-glucopyranosyl)-sn-glycerol (MG lcDAG) and -O-(6-O-acyl-(alpha-D-glucopyranosyl))-sn-glycerol (MAMGlcD AG) to induce non-lamellar phases in a lipid mixture with an in vivo c omposition, prepared from Acholeplasma laidlawii membranes, has been i nvestigated. The phase transition temperatures from lamellar to non-la mellar structures were studied with varying fractions of MGlcDAG and M AMGlcDAG. The transition temperature decreased from 73 +/- 2 degrees C for 20 mol% MGlcDAG to 43 +/- 1 degrees C for 63 mol% MGlcDAG, in lip id mixtures where the other lipids are the native bilayer-forming lipi ds. MAMGlcDAG behaved differently and the phase transition temperature s were found to be almost constant and between 51-53 degrees C as the fraction of MAMGlcDAG varied between 11-45 mol%. It was also found tha t MAMGlcDAG can only be solubilized in low concentrations in the lipid bilayer, which is in good agreement with the fractions of MAMGlcDAG f ound in the membrane of A. laidlawii. Higher concentrations of MAMGlcD AG resulted in phase separations of lamellar liquid crystalline and ge l/crystalline phases. It is concluded that MAMGlcDAG is far more capab le than MGlcDAG to induce non-lamellar structures at lower concentrati ons. The results are discussed in terms of the model of lipid regulati on previously proposed by this laboratory (Lindblom, G., Hauksson, J.B ., Rilfors, L., Bergenstahl, B., Wieslander, A. and Eriksson, P.-O. (1 993) J. Biol. Chem. 268, 16198-16207), and the importance for the bila yer stability in cell membranes. It is proposed that the phase behavio ur of the membrane lipids has far-reaching consequences for membrane f unction.