Structures of glucolipids from the membrane of Acholeplasma laidlawii strain A-EF22. II. Monoacylmonoglucosyldiacylglycerol

alpha-methylene ordering of acyl chains differs in glucolipids and phosphatidylglycerol from Acholeplasma laidlawii membranes: (2)H-NMR quadrupole splittings from individual lipids in mixed bilayers

A Acholeplasma laidlawii strain A-EF22 was grown in a medium supplemented with alpha-deuterated oleic acid. Phosphatidylglycerol (PG), the glucolipids monoglucosyldiacylglycerol (MGlcDAG), diglucosyldiacylglycerol (DGlcDAG) and monoacyldiglucosyldiacylglycerol, and the phosphoglucolipid glycerophosphoryldiglucosyldiacylglycerol (GPDGlcDAG) were purified, and the phase behaviour and molecular ordering for the individual lipids, as well as for mixtures of the lipids, were studied by (2)H-, (31)P-NMR and X-ray scattering methods. The chemical structure of all the A. laidlawii lipids, except PG, has been determined and verified previously; here also the chemical structure of PG was verified, utilising mass spectrometry and (1)H and (13)C high resolution NMR spectroscopy. For the first time, lipid dimers were found in the mass spectrometry measurements. The major findings in this work are: (1) addition of 50 mol% of PG to the non-lamellar-forming lipid MGlcDAG does not significantly alter the transition temperature between lamellar and non-lamellar phases; (2) the (2)H-NMR quadrupole splitting patterns obtained from the lamellar liquid crystalline phase are markedly different for PG on one hand, and DGlcDAG and GPDGlcDAG on the other hand; and (3) mixtures of PG and DGlcDAG or MGlcDAG give rise to (2)H-NMR spectra consisting of a superposition of splitting patterns of the individual lipids. These remarkable features show that the local ordering of the alpha-carbon of the acyl chains is different for PG than for MGlcDAG and DGlcDAG, and that this difference is preserved when PG is mixed with the glucolipids. The results obtained are interpreted in terms of differences in molecular shape and hydrophilicity of the different polar headgroups.

Total lipids with short and long acyl chains from Acholeplasma form nonlamellar phases

The cell-wall-less bacterium Acholeplasma laidlawii A-EF22 synthesizes eight glycerolipids. Some of them form lamellar phases, whereas others are able to form normal or reversed nonlamellar phases. In this study we examined the phase properties of total lipid extracts with limiting average acyl chain lengths of 15 and 19 carbon atoms. The temperature at which these extracts formed reversed hexagonal (HII) phases differed by 5-10 degreesC when the water contents were 20-30 wt%. Thus the cells adjust the ratio between lamellar-forming and nonlamellar-forming lipids to the acyl chain lengths. Because short acyl chains generally increase the potential of lipids to form bilayers, it was judged interesting to determine which of the A. laidlawii A lipids are able to form reversed nonlamellar phases with short acyl chains. The two candidates with this ability are monoacyldiglucosyldiacylglycerol (MADGlcDAG) and monoglucosyldiacylglycerol. The average acyl chain lengths were 14.7 and 15.1 carbon atoms, and the degrees of acyl chain unsaturation were 32 and 46 mol%, respectively. The only liquid crystalline phase formed by MADGlcDAG is an HII phase. Monoglucosyldiacylglycerol forms reversed cubic (Ia3d) and HII phases at high temperatures. Thus, even when the organism is grown with short fatty acids, it synthesizes two lipids that have the capacity to maintain the nonlamellar tendency of the lipid bilayer. MADGlcDAG in particular contributes very powerfully to this tendency.

Lipids in total extracts from Acholeplasma laidlawii A pack more closely than the individual lipids. Monolayers studied at the air-water interface

Pressure-area curves were obtained at 25, 35 and 45 degrees C for total lipid extracts and four individual glucolipids isolated from Acholeplasma laidlawii strain A-EF22. The glucolipids are 1,2-diacyl-3-0-(alpha-D-glucopyranosyl)-sn-glycerol (MGlcDAG), 1,2 -diacyl-3-0-[alpha-D-glucopyranosyl-(1-->2)-0-alpha-D-glucopyranosyl] -sn-glycerol (DGlcDAG), 1,2-diacyl-3-0-[alpha-D-glucopyranosyl-(1-->2)-0-(6-0-acyl-alpha-D-gluco pyranosyl)]-sn-glycerol (MADGlcDAG), and 1,2-diacyl-3-0-[glycerophosphoryl-6-0-(alpha-D-glucopyranosyl-(1-- >)-0-alpha-D-glucopyranosyl)]-sn-glycerol (GPDGlcDAG). The total lipid extracts were obtained from A. laidlawii, grown at 37 degrees C with fatty acids of varying degrees of unsaturation and chain length. The mean surface area per molecule was obtained from these pressure-area curves at surface pressures equal to 10, 20, 30 and 40 mN/m. It was found that the interfacial area of the lipids increases with increasing degree of unsaturation, but is nearly independent of the acyl chain length at constant unsaturation. The surface charge density varied between 4.7 x 10(-3) e-/angstrom(2) and 9.4 x 10(-3) e-/angstrum(2) for the total lipid extracts studied, but did not exhibit any consistent dependence on variations in degree of unsaturation or acyl chain length. The mean area per molecule was found to be smaller for the total lipid extracts than for the individual lipids. It is concluded that the bacterium strives to regulate its lipid composition in such a way that the packing of the lipids in the membrane is appropriately tight, and/or to keep a slight negative spontaneous curvature of the lipid bilayer of the cell membrane ("optimal packing"). This is in accordance with the physico-chemical model for the regulation of the lipid composition in the membrane of A. laidlaiwii previously presented by us (see e.g. Andersson, A.-S., Riffors, L., Bergqvist, M., Persson, S. and Lindblom, G. (1996) Biochemistry 35, 11119-11130).

Occurrence of monoacyl-diglucosyl-diacyl-glycerol and monoacyl-bis-glycerophosphoryl-diglucosyl-diacyl-glycerol in membranes of Acholeplasma laidlawii strain B-PG9

It is shown by thin-layer and high-performance liquid chromatography that the two membrane lipids monoacyl-diglucosyl-diacyl-glycerol (MADGlcDAG) and monoacyl-bis-glycerophosphoryl-diglucosyl-diacyl-glycerol are synthesized by Acholeplasma laidlawii strain B-PG9 when the cells are grown in two different growth media. The two lipids are also synthesized by A. laidlawii strain A-EF22 and their chemical structures have been determined previously by NMR spectroscopy. Since a reversed hexagonal phase is the only liquid-crystalline phase formed by MADGlcDAG, it is concluded that A. laidlawii strain B-PG9, in resemblance to strain A-EF22, synthesizes three membrane lipids that are able to form reversed nonlamellar phases. A comparison of the membrane lipids from the two strains shows that there is essentially one lipid from each strain that differs. However, both these lipids have common physico-chemical properties, namely the ability to form reversed nonlamellar phases. Finally, it is also shown that novel lipids may be synthesized by A. laidlawii through long-time adaptation to altered growth conditions.

Cryo-TEM and NMR studies of a micelle-forming phosphoglucolipid from membranes of Acholeplasma laidlawii A and B

The chemical structure of a phosphoglucolipid from the membrane of the bacterium Acholeplasma laidlawii strain B-PG9 has been determined by high resolution NMR to be 1,2-diacyl-3-O-[glycerophosphoryl-6-O-(alpha-D-glucopyranosyl-(1 -->2)-O-alpha-D-glucopyranosyl)]-sn-glycerol (GPDGlcDAG). It was concluded that this lipid has exactly the same structure as one of the phosphoglucolipids from A. laidlawii strain A-EF22. By cryo transmission electron microscopy (cryo-TEM) and NMR diffusion techniques it was shown that, in highly diluted aqueous solutions, this membrane lipid forms long thread-like micelles in equilibrium with lipid vesicles. The cause of the occurrence of these different aggregates is discussed in terms of the varying molecular shapes of the lipid because of a heterogeneous composition of the acyl chains. A second membrane phosphoglucolipid from the bacterium, namely 1,2-diacyl-3-O-[glycerophosphoryl-6-O-(alpha-D- glucopyranosyl-(1 -->2)-monoacylglycerophosphoryl-6-O-alpha-D-glucopyranosyl)]-sn-gl ycerol (MABGPDGlcDAG), was found to form only a lamellar liquid crystalline phase coexisting with water.

Wild-type Escherichia coli cells regulate the membrane lipid composition in a "window" between gel and non-lamellar structures

Escherichia coli strain K12 was grown at 17, 27, and 37 degrees C. The acyl chain composition of the membrane lipids varied with the growth temperature; the fraction of cis-vaccenoyl chains decreased, and the fraction of palmitoyl chains increased, when the growth temperature was increased. However, the polar head group composition did not change significantly. The equilibria between lamellar and reversed non-lamellar phases of lipids extracted from the inner membrane (IM), and from both the membranes (IOM), were studied with NMR and x-ray diffraction. At temperatures above the growth temperature the lipid extracts formed a reversed hexagonal phase, or a bicontinuous cubic phase, depending on the degree of hydration of the lipids. It was observed that: 1) at equal elevations above the growth temperature, IM lipid extracts, as well as IOM lipid extracts, have a nearly equal ability to form non-lamellar phases; 2) IM extracts have a stronger tendency than IOM extracts to form non-lamellar phases; 3) non-lamellar phases are formed under conditions that are relatively close to the physiological ones; the membrane lipid monolayers are thus "frustrated"; and 4) as a consequence of the change of the acyl chain structures, the temperature for the lamellar gel to liquid crystalline phase transition is changed simultaneously, and in the same direction, as the temperature for the lamellar to non-lamellar phase transition. With a too large fraction of saturated acyl chains the membrane lipids enter a gel state, and with a too large fraction of unsaturated acyl chains the lipids transform to non-lamellar phases. It is thus concluded that the regulation of the acyl chain composition in wild-type cells of E. coli is necessary for the organism to be able to grow in a "window" between a lamellar gel phase and reversed non-lamellar phases.

Influence of monoglucosyldiacylglycerol and monoacylmonoglucosyldiacylglycerol on the lipid bilayer of the membrane from Acholeplasma laidlawii strain A-EF22

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.

Structures of glucolipids from the membrane of Acholeplasma laidlawii strain A-EF22. III. Monoglucosyldiacylglycerol, diglucosyldiacylglycerol, and monoacyldiglucosyldiacylglycerol

The structures of three glucolipids from the membrane of Acholeplasma laidlawii, strain A-EF22, were determined by high resolution 1H-NMR and 13C-NMR spectroscopy. The two most abundant glucolipids in this organism were shown to be 1,2-diacyl-3-O-(alpha-D-glucopyranosyl)-sn-glycerol (MGlcDAG) and 1,2-diacyl-3-O-[alpha-D-glucopyranosyl-(1 --> 2)-O-alpha-D- glucopyranosyl]-sn-glycerol (DGlcDAG). These structures agree with those determined previously by chemical analyses of the two most abundant glucolipids synthesized by the B strain of A. laidlawii. The structure of a newly discovered glucolipid in A. laidlawii strain A-EF22 was also determined. This lipid is an acylated derivative of DGlcDAG with the structure 1,2-diacyl-3-O-[alpha-D-glucopyranosyl-(1 --> 2)-O-(6-O-acyl-alpha-D- glucopyranosyl)]-sn-glycerol. The existence of this lipid was detected by 1H-NMR spectroscopy in preparations of MGlcDAG which had been judged by thin-layer chromatography to be pure. The biosynthesis of the glucolipids and their role in the metabolic lipid regulation are briefly discussed.

Lipid extracts from membranes of Acholeplasma laidlawii A grown with different fatty acids have a nearly constant spontaneous curvature

X-ray diffraction methods were used to explore the variation in the spontaneous curvature of lipid extracts from Acholeplasma laidlawii strain A-EF22 grown with different mixtures of palmitic acid and oleic acid. It was shown that the cells respond to the different growing conditions by altering the polar head group compositions in order to keep the phase transition between lamellar and nonlamellar structures within a narrow temperature range. This has been interpreted to mean that the membrane lipids are adjusted toward an optimal packing (Lindblom et al. (1986) Biochemistry 25, 7502). Here it is shown that for these extracts, the membrane curvature is kept within a narrow range (58-73 A), compared to the range in curvatures exhibited by pure lipids extracts from the membrane (17-123 A). These observations support the hypothesis (Gruner (1989) J. Phys. Chem. 93, 7562) that the spontaneous curvature is a functionally important membrane parameter which is regulated by the organism and is likely to be one of the constraints controlling the lipid composition of the bilayer.