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

Biophysical regulation of lipid biosynthesis in the plasma membrane

We present a cellular model of lipid biosynthesis in the plasma membrane that couples biochemical and biophysical features of the enzymatic network of the cell-wall-less Mycoplasma Acholeplasma laidlawii. In particular, we formulate how the stored elastic energy of the lipid bilayer can modify the activity of curvature-sensitive enzymes through the binding of amphipathic alpha-helices. As the binding depends on lipid composition, this results in a biophysical feedback mechanism for the regulation of the stored elastic energy. The model shows that the presence of feedback increases the robustness of the steady state of the system, in the sense that biologically inviable nonbilayer states are less likely. We also show that the biophysical and biochemical features of the network have implications as to which enzymes are most efficient at implementing the regulation. The network imposes restrictions on the steady-state balance between bilayer and nonbilayer lipids and on the concentrations of particular lipids. Finally, we consider the influence of the length of the amphipathic alpha-helix on the efficacy of the feedback and propose experimental measurements and extensions of the modeling framework.

Structural preferences of dioleoyl glycolipids with mono- and disaccharide head groups

The structural preferences of 1,2-dioleoyl-sn-glycerol glycolipids with glucose, galactose, maltose, and cellobiose as sugar head group were investigated under near physiological conditions with Fourier-transform infrared spectroscopy (FT-IR) and synchrotron radiation small-angle X-ray scattering (SAXS). Whereas all glycolipids have a very high fluidity at temperatures above 0 degrees C, the mono- and disaccharide compounds differ considerably in their aggregate structures. The monosaccharide compounds adopt only inverted hexagonal (H(II)) structures in the temperature range 5-70 degrees C, while the disaccharide compounds adopt only multilamellar structures. Since these and similar glycolipids are frequently found in nature, these data should be of relevance for the function of their host cell membranes.

Stable isotope probing reveals the dominant role of Burkholderia species in aerobic degradation of PCBs

The active bacteria of a biofilm community grown directly on polychlorinated biphenyl (PCB) droplets were analyzed by 16S rRNA fingerprinting, identified by their 16S rRNA gene sequences and fatty acid profiling, and compared with isolates from the biofilm. Although, the multi-species biofilm degraded di- and trichlorinated PCB-congeners these substrates were not attacked by its individual isolated members, which suggests that a metabolic network is responsible for PCB degradation in the biofilm. The community metabolized [U-13C]-2,2'-dichlorobiphenyl incorporating the label into certain phospholipid fatty acids matching those found in Burkholderia species. In contrast, abundant biofilm community members, like Methylobacterium species, did not incorporate the label. These results provide prima faciae evidence for Burkholderia species as the main degraders of PCBs in this type of aerobic soils.

Lateral organization in Acholeplasma laidlawii lipid bilayer models containing endogenous pyrene probes

In membranes of the small prokaryote Acholeplasma laidlawii bilayer- and nonbilayer-prone glycolipids are major species, similar to chloroplast membranes. Enzymes of the glucolipid pathway keep certain important packing properties of the bilayer in vivo, visualized especially as a monolayer curvature stress ('spontaneous curvature'). Two key enzymes depend in a cooperative fashion on substantial amounts of the endogenous anionic lipid phosphatidylglycerol (PG) for activity. The lateral organization of five unsaturated A. laidlawii lipids was analyzed in liposome model bilayers with the use of endogenously produced pyrene-lipid probes, and extensive experimental designs. Of all lipids analyzed, PG especially promoted interactions with the precursor diacylglycerol (DAG), as revealed from pyrene excimer ratio (Ie/Im) responses. Significant interactions were also recorded within the major nonbilayer-prone monoglucosylDAG (MGlcDAG) lipids. The anionic precursor phosphatidic acid (PA) was without effects. Hence, a heterogeneous lateral lipid organization was present in these liquid-crystalline bilayers. The MGlcDAG synthase when binding at the PG bilayer interface, decreased acyl chain ordering (increase of membrane free volume) according to a bis-pyrene-lipid probe, but the enzyme did not influence the bulk lateral lipid organization as recorded from DAG or PG probes. It is concluded that the concentration of the substrate DAG by PG is beneficial for the MGlcDAG synthase, but that binding in a proper orientation/conformation seems most important for activity.

Structural features of glycosyltransferases synthesizing major bilayer and nonbilayer-prone membrane lipids in Acholeplasma laidlawii and Streptococcus pneumoniae

In membranes of Acholeplasma laidlawii two consecutively acting glucosyltransferases, the (i) alpha-monoglucosyldiacylglycerol (MGlcDAG) synthase (alMGS) (EC ) and the (ii) alpha-diglucosyl-DAG (DGlcDAG) synthase (alDGS) (EC ), are involved in maintaining (i) a certain anionic lipid surface charge density and (ii) constant nonbilayer/bilayer conditions (curvature packing stress), respectively. Cloning of the alDGS gene revealed related uncharacterized sequence analogs especially in several Gram-positive pathogens, thermophiles and archaea, where the encoded enzyme function of a potential Streptococcus pneumoniae DGS gene (cpoA) was verified. A strong stimulation of alDGS by phosphatidylglycerol (PG), cardiolipin, or nonbilayer-prone 1,3-DAG was observed, while only PG stimulated CpoA. Several secondary structure prediction and fold recognition methods were used together with SWISS-MODEL to build three-dimensional model structures for three MGS and two DGS lipid glycosyltransferases. Two Escherichia coli proteins with known structures were identified as the best templates, the membrane surface-associated two-domain glycosyltransferase MurG and the soluble GlcNAc epimerase. Differences in electrostatic surface potential between the different models and their individual domains suggest that electrostatic interactions play a role for the association to membranes. Further support for this was obtained when hybrids of the N- and C-domain, and full size alMGS with green fluorescent protein were localized to different regions of the E. coli inner membrane and cytoplasm in vivo. In conclusion, it is proposed that the varying abilities to bind, and sense lipid charge and curvature stress, are governed by typical differences in charge (pI values), amphiphilicity, and hydrophobicity for the N- and (catalytic) C-domains of these structurally similar membrane-associated enzymes.

Sequence properties of the 1,2-diacylglycerol 3-glucosyltransferase from Acholeplasma laidlawii membranes. Recognition of a large group of lipid glycosyltransferases in eubacteria and archaea

Synthesis of the nonbilayer-prone alpha-monoglucosyldiacylglycerol (MGlcDAG) is crucial for bilayer packing properties and the lipid surface charge density in the membrane of Acholeplasma laidlawii. The gene for the responsible, membrane-bound glucosyltransferase (alMGS) (EC ) was sequenced and functionally cloned in Escherichia coli, yielding MGlcDAG in the recombinants. Similar amino acid sequences were encoded in the genomes of several Gram-positive bacteria (especially pathogens), thermophiles, archaea, and a few eukaryotes. All of these contained the typical EX(7)E catalytic motif of the CAZy family 4 of alpha-glycosyltransferases. The synthesis of MGlcDAG by a close sequence analog from Streptococcus pneumoniae (spMGS) was verified by polymerase chain reaction cloning, corroborating a connection between sequence and functional similarity for these proteins. However, alMGS and spMGS varied in dependence on anionic phospholipid activators phosphatidylglycerol and cardiolipin, suggesting certain regulatory differences. Fold predictions strongly indicated a similarity for alMGS (and spMGS) with the two-domain structure of the E. coli MurG cell envelope glycosyltransferase and several amphipathic membrane-binding segments in various proteins. On the basis of this structure, the alMGS sequence charge distribution, and anionic phospholipid dependence, a model for the bilayer surface binding and activity is proposed for this regulatory enzyme.

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.

Cytotoxic polyisoprenes and glycosides of long-chain fatty alcohols from Dimocarpus fumatus

The ethanolic extract from the stem bark of Dimocarpus fumatus, showed in vitro cytotoxic activity against KB cells. Fractionation of the extract gave compounds belonging to different classes. The two major components have been identified as a benzoquinone, sargaquinone, and a chromene, sargaol. One sphingolipid, soyacerebroside I, two glycosides of sitosterol, and fatty acids were also identified. Besides these known compounds, two new glycosides of long-chain fatty alcohols have been identified as 1-O-[alpha-L-rhamnopyranosyl-(1-->2)-beta-D- glucopyranosyl-(1-->3)-alpha-L-rhamnopyranosyl-(1-->6)-beta-D- glucopyranosyl]hexadecanol and 1-O-[[alpha-L-arabinopyranosyl-(1-->3)]-alpha-L-rhamnopyranosyl-(1 -->2)- beta-D-glucopyranosyl-(1-->3)-alpha-L-rhamnopyranosyl-(1-->6)-beta-D- glucopyranosyl] hexadecanol, and a mixture of three new diacylglycerylglucosides has been isolated. These structures were elucidated by analysis of 2D-NMR and mass spectra.

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.

Structure of a novel phosphocholine-containing aminoglycoglycerolipid of Mycoplasma fermentans

Mycoplasma fermentans is thought to be a pathogen of rheumatoid arthritis or a cofactor of AIDS (acquired immunodeficiency syndrome). To elucidate the possible involvement of membrane constituents in the pathogenesis of these diseases, we studied its lipid components. Several alkali labile glycophospholipids were detected and named glycoglycerophospholipids (GGPLs). Previously, we purified and determined the structure of one of them as 6'-O-phosphocholine-alpha-glucopyranosyl-(1'-3)-1,2-diacyl-sn-glycerol (GGPL-I). The present paper describes the purification and structural characterization of GGPL-III, the major GGPL of M. fermentans using 1H-, 13C- and 31P-nuclear magnetic resonance spectroscopy, and mass-spectroscopy as 1"-phosphocholine,2"-amino dihydroxypropane-3"-phospho-6'-alpha-glucopyranosyl-(1'-3)-1,2-dia cyl-glycerol.

Unusual lipid composition of the gram-negative, freshwater, stalked bacterium Caulobacter bacteroides NP-105

The extractable lipids of the gram-negative, stalked, freshwater bacterium Caulobacter bacteroides NP-105 account for about 9.5% by weight of dry cells, polar lipids comprising up to 95% of the total. The polar lipids consist of five glycolipids, namely, 1,2-diacyl-3-alpha-D-glucopyranosyl-sn-glycerol (I) (34% of the total), 1,2-diacyl-3-alpha-D-[6'-(1",2"-diacyl-sn-glycero-3-phospho)]glucopyranosyl-sn-glycerol (II) (7%), 1,2-diacyl-3-a-D-glucuronopyranosyl-sn-glycerol (III) (17%), 1,2-diacyl-3-alpha-D-(6'-sulfo)quinovopyranosyl-sn-glycerol (V) (9%), and 1,2-diacyl-3-alpha-D-[4'-(alpha-D-glucopyranosyl)] glucuronopyranosyl-sn-glycerol (VI) (28%), and one glycerophospholipid, 1,2-diacyl-sn-glycero-3-phosphoglycerol (IV) (5%). The main fatty acyls of the lipids are n-16:0, cis-18:1 omega7, and cis-11,12-methyleneoctadecanoic (lactobacillic) acid residue. Of the 6 lipids listed, only 3 (I, IV, and V) can be treated as widely encountered. However, sulfonic glycolipid V is a characteristic lipid component of photosynthetic organisms rather than non-photosynthetic ones. Phosphatidyl derivatives of glycosyldiacylglycerols of type II normally occur in gram-positive bacteria, among gram-negative bacteria they have been revealed only in two species of the Pseudomonas genus. Glucuronosyl and alpha-glucosyl-a-glucuronosyl diacylglycerols such as III and VI, respectively, are very rare lipids, the latter being found so far only in a Streptomyces strain.

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.

Lipid dependence and basic kinetics of the purified 1,2-diacylglycerol 3-glucosyltransferase from membranes of Acholeplasma laidlawii

UDP-glucose: 1,2-diacylglycerol 3-glucosyltransferase (EC 2.4.1.157), catalyzes the transfer of glucose from UDP-glucose to diacylglycerol (DAG) to yield monoglucosyldiacylglycerol (MGlcDAG) and UDP. MGlcDAG is the first glucolipid along the glucolipid pathway, and a major (nonbilayer-prone) lipid in the single membrane of Acholeplasma laidlawii. MGlcDAG is further glucosylated to give the major diglucosyldiacylglycerol (DGlc-DAG). The bilayer fractions of these lipids are crucial for the metabolic maintenance of phase equilibria close to a potential bilayer-nonbilayer transition and a nearly constant spontaneous curvature. The glucolipid syntheses are also balanced against the phosphatidylglycerol pathway, competing for the common minor precursor phosphatidic acid, to retain a constant lipid surface charge density. The 1,2-diacylglycerol 3-glucosyltransferase was purified to homogeneity from detergent-solubilized A. laidlawii cells by three column chromatography methods (enrichment approximately 9000 x), and identified as a minor 40-kDa protein by using SDS-polyacrylamide gel electrophoresis. In CHAPS detergent, mixed micelles, a cooperative dependence on anionic lipids for activity was confirmed. Dependence of the enzyme on UDP-glucose followed Michaelis-Menten kinetics while the other hydrophobic substrate dioleoylglycerol stimulated the enzyme by an activating, potentially cooperative mechanism. Physiological concentrations of the activator lipid dioleoyl-phosphatidylglycerol influenced the turnover number of the enzyme but not the interaction with UDP-glucose, as inferred from variable and constant values of the apparent Vmax and Km, respectively. Dipalmitoylglycerol was a better substrate than the oleoyl species, supporting earlier in vivo and crude enzyme data. The responses of the purified 1,2-diacylglycerol 3-glucosyltransferase indicated that (i) the regulatory features of the MGlcDAG synthesis is held by the catalytic enzyme itself, and (ii) this strongly corroborates the "homeostasis" model for lipid bilayer properties in A. laidlawii proposed earlier.

Chromatographic resolution of partially perdeuterated diglucosyldiacylglycerols from Acholeplasma laidlawii

Diglucosyldiacylglycerol from Acholeplasma laidlawii was isolated by adsorption chromatography and subsequently subjected to molecular species analysis by reversed-phase high-performance liquid chromatography. Twenty-three molecular species were resolved. The bacterium was grown in a medium supplemented with a mixture of oleic acid and perdeuterated palmitic acid, and incorporation of perdeuterated acyl moieties occurred in several molecular species. Short and odd-numbered acyl chains were found in combinations with palmitic acid, perdeuterated palmitic acid and oleic acid. Also, molecular species differing only by perdeuterated and protonated acyl chain content were resolved chromatographically. Separation between diglucosyldiacylglycerol and digalactosyldiacylglycerol isolated from a plant source was achieved with adsorption chromatography. The polarity of diglucosyldiacylglycerol as compared to digalactosyldiacylglycerol is discussed.

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.