PubMed · 4592575
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D Chapman. 1972. Protein-lipid interactions.. https://doi.org/10.1002/9780470719909.ch15
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The ability for 1,2-diacyl-3-O-(alpha-D-glucopyranosyl)-sn-glycerol (MGlcDAG) and 1,2-diacyl-3-O-(6-O-acyl-(alpha-D-glucopyranosyl))-sn-glycerol (MAMGlcDAG) to induce non-lamellar phases in a lipid mixture with an in vivo composition, prepared from Acholeplasma laidlawii membranes, has been investigated. The phase transition temperatures from lamellar to non-lamellar structures were studied with varying fractions of MGlcDAG and MAMGlcDAG. The transition temperature decreased from 73 +/- 2 degrees C for 20 mol% MGlcDAG to 43 +/- 1 degree C for 63 mol% MGlcDAG, in lipid mixtures where the other lipids are the native bilayer-forming lipids. MAMGlcDAG behaved differently and the phase transition temperatures 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 that MAMGlcDAG can only be solubilized in low concentrations in the lipid bilayer, which is in good agreement with the fractions of MAMGlcDAG found in the membrane of A. laidlawii. Higher concentrations of MAMGlcDAG resulted in phase separations of lamellar liquid crystalline and gel/crystalline phases. It is concluded that MAMGlcDAG is far more capable than MGlcDAG to induce non-lamellar structures at lower concentrations. The results are discussed in terms of the model of lipid regulation previously proposed by this laboratory (Lindblom, G., Hauksson, J.B., Rilfors, L., Bergenståhl, B., Wieslander, A. and Eriksson, P.O. (1993) J. Biol. Chem. 268, 16198-16207), and the importance for the bilayer stability in cell membranes. It is proposed that the phase behaviour of the membrane lipids has far-reaching consequences for membrane function.
We have quantitated the phase preferences of all of the quantitatively significant lipids of fatty acid-homogeneous Acholeplasma laidlawii membranes by determining the effect of small amounts of each lipid on the lamellar/reversed hexagonal phase transition temperature of a phosphatidylethanolamine matrix of identical fatty acid composition using differential scanning calorimetry. We find that the incorporation of small amounts of these lipids produce effects ranging from a moderate depression to a marked elevation of the lamellar/reversed hexagonal phase transition temperature of the corresponding phosphatidylethanolamine. Thus, although the total membrane lipids from this organism form only lamellar phases under physiological conditions, the individual membrane lipids appear to exhibit a wide range of phase preferences. Phosphatidylglycerol and diglucosyldiacylglycerol seem to have relatively strong and weak preferences for the lamellar liquid-crystalline phase, respectively, while monoglucosyldiacylglycerol, and especially acyl polyprenyl glucoside, strongly prefers the reversed hexagonal phase. Most notable in this regard is the phase preference of glycerylphosphoryldiglucosyldiacylglycerol, which strongly destabilizes the reversed hexagonal phase and which we show in the accompanying paper [Lewis, R. N. A. H., & McElhaney, R. N. (1995) Biochemistry 34, 13818-13824] actually prefers the normal micellar phase in isolation. The presence of normal, lamellar, and reversed phase-preferring lipids in a single membrane has important implications for understanding the physical basis of lipid organization and biosynthetic regulation in this and possibly in other organisms. We also show that the characteristic effect of the individual A. laidlawii membrane lipids on the lamellar/reversed hexagonal phase transition temperature of the phosphatidylethanolamine matrix is not well correlated with their polar headgroup intrinsic volumes.(ABSTRACT TRUNCATED AT 250 WORDS)
It has been proposed that each of the lipids from the Acholeplasma laidlawii membrane prefers to form either a lamellar or a reversed cubic or hexagonal phase when dispersed in excess water at physiologically relevant temperatures and ionic strengths. In this study, we have reinvestigated the thermotropic phase behavior of all the major membrane lipids from A. laidlawii B membranes derived from cells grown in equimolar palmitic and elaidic acids. We confirm that phosphatidylglycerol (PG) and diglucosyldiacylglycerol (DGDG) from such membranes do indeed form only lamellar phases over the temperature range 5-80 degrees C. We also confirm that the monoglucosyldiacylglycerol (MGDG) and acyl polyprenyl glucoside (APG) exist in lamellar phases at lower temperatures but do form reversed phases at higher temperatures. However, we present here optical, differential scanning calorimetric, quasielastic light scattering, and 2H- and 31P-nuclear magnetic resonance spectroscopic evidence indicating that one lipid component of the A. laidlawii B membrane, namely, glycerylphosphoryldiglucosyldiacylglycerol (GPDGDG), actually forms normal micellar rather than lamellar or reversed phases when dispersed in excess water at physiological temperatures. To the best of our knowledge, this is the first demonstration of the existence of a micellar phase-preferring lipid in a prokaryotic cell membrane, and only the second demonstration of the existence of a micellar phase-forming lipid in any biological membrane. We also show that GPDGDG levels change greatly depending on the fatty acid composition of the membrane lipids of this organism.(ABSTRACT TRUNCATED AT 250 WORDS)