Involvement of surface potential in regulation of polar membrane lipids in Acholeplasma laidlawii

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.

In vitro biosynthesis of ether-type glycolipids in the methanoarchaeon Methanothermobacter thermautotrophicus

The biosynthesis of archaeal ether-type glycolipids was investigated in vitro using Methanothermobacter thermautotrophicus cell-free homogenates. The sole sugar moiety of glycolipids and phosphoglycolipids of the organism is the beta-D-glucosyl-(1-->6)-D-glucosyl (gentiobiosyl) unit. The enzyme activities of archaeol:UDP-glucose beta-glucosyltransferase (monoglucosylarchaeol [MGA] synthase) and MGA:UDP-glucose beta-1,6-glucosyltransferase (diglucosylarchaeol [DGA] synthase) were found in the methanoarchaeon. The synthesis of DGA is probably a two-step glucosylation: (i) archaeol + UDP-glucose --> MGA + UDP, and (ii) MGA + UDP-glucose --> DGA + UDP. Both enzymes required the addition of K(+) ions and archaetidylinositol for their activities. DGA synthase was stimulated by 10 mM MgCl(2), in contrast to MGA synthase, which did not require Mg(2+). It was likely that the activities of MGA synthesis and DGA synthesis were carried out by different proteins because of the Mg(2+) requirement and their cellular localization. MGA synthase and DGA synthase can be distinguished in cell extracts greatly enriched for each activity by demonstrating the differing Mg(2+) requirements of each enzyme. MGA synthase preferred a lipid substrate with the sn-2,3 stereostructure of the glycerol backbone on which two saturated isoprenoid chains are bound at the sn-2 and sn-3 positions. A lipid substrate with unsaturated isoprenoid chains or sn-1,2-dialkylglycerol configuration exhibited low activity. Tetraether-type caldarchaetidylinositol was also actively glucosylated by the homogenates to form monoglucosyl caldarchaetidylinositol and a small amount of diglucosyl caldarchaetidylinositol. The addition of Mg(2+) increased the formation of diglucosyl caldarchaetidylinositol. This suggested that the same enzyme set synthesized the sole sugar moiety of diether-type glycolipids and tetraether-type phosphoglycolipids.

Calorimetric and spectroscopic studies of the phase behavior and organization of lipid bilayer model membranes composed of binary mixtures of dimyristoylphosphatidylcholine and dimyristoylphosphatidylglycerol

The thermotropic phase behavior of hydrated bilayers derived from binary mixtures of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylglycerol (DMPG) was investigated by differential scanning calorimetry, Fourier-transform infrared spectroscopy and 31P-nuclear magnetic resonance spectroscopy. Binary mixtures of DMPC and DMPG that have not been annealed at low temperatures exhibit broad, weakly energetic pretransitions (approximately 11-15 degrees C) and highly cooperative, strongly energetic gel/liquid-crystalline phase transitions (approximately 23-25 degrees C). After low temperature incubation, these mixtures also exhibit a thermotropic transition form a lamellar-crystalline to a lamellar gel phase at temperatures below the onset of the gel/liquid-crystalline phase transition. The midpoint temperatures of the pretransitions and gel/liquid-crystalline phase transitions of these lipid mixtures are both maximal in mixtures containing approximately 30 mol% DMPG but the widths and enthalpies of the same thermotropic events exhibit no discernable composition dependence. In contrast, thermotropic transitions involving the Lc phase exhibit a very strong composition dependence, and the midpoint temperatures and transition enthalpies are both maximal with mixtures containing equimolar amounts of the two lipids. Our spectroscopic studies indicate that the Lc phases formed are structurally similar as regards their modes of hydrocarbon chain packing, interfacial hydration and hydrogen-bonding interactions, as well as the range and amplitudes of the reorientational motions of their phosphate headgroups. Our results indicate that although DMPC and DMPG are highly miscible, their mixtures do not exhibit ideal mixing. We attribute the non-ideality in their mixing behavior to the formation of preferential PC/PG contacts in the Lc phase due to the combined effects of steric crowding of the DMPC headgroups and charge repulsion between the negatively charged DMPG molecules.

Irreversible binding and activity control of the 1,2-diacylglycerol 3-glucosyltransferase from Acholeplasma laidlawii at an anionic lipid bilayer surface

1,2-Diacylglycerol 3-glucosyltransferase is associated with the membrane surface catalyzing the synthesis of the major nonbilayer-prone lipid alpha-monoglucosyl diacylglycerol (MGlcDAG) from 1,2-DAG in the cell wall-less Acholeplasma laidlawii. Phosphatidylglycerol (PG), but not neutral or zwitterionic lipids, seems to be essential for an active conformation and function of the enzyme. Surface plasmon resonance analysis was employed to study association of the enzyme with lipid bilayers. Binding kinetics could be well fitted only to a two-state model, implying also a (second) conformational step. The enzyme bound less efficiently to liposomes containing only zwitterionic lipids, whereas increasing molar fractions of the anionic PG or cardiolipin (CL) strongly promoted binding by improved association (k(a1)), and especially a decreased rate of return (k(d2)) from the second state. This yielded a very low overall dissociation constant (K(D)), corresponding to an essentially irreversible membrane association. Both liposome binding and consecutive activity of the enzyme correlated with the PG concentration. The importance of the electrostatic interactions with anionic lipids was shown by quenching of both binding and activity with increasing NaCl concentrations, and corroborated in vivo for an active enzyme-green fluorescent protein hybrid in Escherichia coli. Nonbilayer-prone lipids substantially enhanced enzyme-liposome binding by promoting a changed conformation (decreasing k(d2)), similar to the anionic lipids, indicating the importance of hydrophobic interactions and a curvature packing stress. For CL and the nonbilayer lipids, effects on enzyme binding and consecutive activity were not correlated, suggesting a separate lipid control of activity. Similar features were recorded with polylysine (cationic) and polyglutamate (anionic) peptides present, but here probably dependent on the selective charge interactions with the enzyme N- and C-domains, respectively. A lipid-dependent conformational change and PG association of the enzyme were verified by circular dichroism, intrinsic tryptophan, and pyrene-probe fluorescence analyses, respectively. It is concluded that an electrostatic association of the enzyme with the membrane surface is accompanied by hydrophobic interactions and a conformational change. However, specific lipids, the curvature packing stress, and proteins or small molecules bound to the enzyme can modulate the activity of the bound A. laidlawii MGlcDAG synthase.

Effect of independent variations in fatty acid structure and chain length on lipid polar headgroup composition in Acholeplasma laidlawii B membranes: regulation of lamellar/nonlamellar phase propensity

We have studied the biosynthetic regulation of the membrane lipid polar headgroup distribution in Acholeplasma laidlawii B cells made fatty acid auxotrophic by growth in the presence of the biotin-binding agent avidin to test whether this organism has the ability to coherently regulate the lamellar/nonlamellar phase propensity of its membrane lipids. The addition of various single normal growth-supporting exogenous fatty acids to such cell cultures produces fatty acid-homogeneous cells in which the hydrocarbon chain length and structure of the fatty acyl chains of the membrane lipids can be independently varied. Moreover, in analyzing our results, we consider the fact that the individual membrane lipid classes of this organism can form either normal micellar, lamellar, or reversed cubic or hexagonal phases in isolation (Lewis, R. N. A. H., and McElhaney, R. N. (1995) Biochemistry 34, 13818-13824). When A. laidlawii cells are highly enriched in one of a homologous series of methyl isobranched, methyl anteisobranched, or omega-cyclohexyl fatty acids, neither the ratio of normal micellar/lamellar nor of inverted cubic or hexagonal/lamellar phase-forming lipids are coherently regulated, and in fact in the former case, the changes in lipid polar headgroup composition observed are generally in a direction opposite to that required to maintain the overall lamellar/nonlamellar phase preference of the total membrane lipids constant when hydrocarbon chain length is varied. Similarly, when lipid hydrocarbon structure is varied at a constant effective chain length, a similar lack of coherent regulation of membrane lipid polar headgroup distribution is also observed, although in this case a weak overall trend in the expected direction occurs. We also confirm our previous finding (Foht, P. J., Tran, Q. M., Lewis, R. N. A. H., and McElhaney, R. N. (1995) Biochemistry 34, 13811-13817) that the ratio of inverted phase-forming monoglucosyl diacylglycerol to the lamellar phase-forming glycolipid diglucosyl diacylglycerol, previously used to estimate membrane lipid phase preference in A. laidlawii A and B, is not by itself a reliable indicator of the overall lamellar/nonlamellar phase propensity of the total membrane lipids of these organisms. Our results indicate that A. laidlawii B lacks a coherent mechanism to biosynthetically regulate the polar headgroup distribution of its membrane lipids to maintain the micellar/lamellar/inverted phase propensity constant in the face of induced variations in either the chain length or the structure of its lipid hydrocarbon chains. Finally, we suggest that the lack of a coherent regulatory mechanism to regulate the overall phase-forming propensity of the total membrane lipids of this organism under these circumstances may result in part from its inability to optimize all of the biologically relevant physical properties of its membrane lipid bilayer simultaneously.

Regulation of lipid composition in Acholeplasma laidlawii and Escherichia coli membranes: NMR studies of lipid lateral diffusion at different growth temperatures

Lipid lateral diffusion coefficients have been directly determined by pulsed field gradient NMR spectroscopy on macroscopically aligned, fully hydrated lamellar phases containing dimyristoylphosphatidylcholine and total lipid extracts from Acholeplasma laidlawii and Escherichia coli. The temperature dependence of the diffusion coefficient was of the Arrhenius type in the temperature interval studied. The sharp increase in the diffusion coefficient at the growth temperature of E. coli obtained by FRAP measurements, using a fluorescent probe molecule (Jin, A. J., Edidin, M., Nossal, R., and Gershfeld, N. L. (1999) Biochemistry 38, 13275-13278), was not observed. Thus, we conclude that the lipid structural properties (i.e., those affecting the lipid phase behavior), rather than the lipid dynamics, are involved in the adjustment of the membrane lipid composition. Further support for this conclusion is given by the finding that lipid extracts from A. laidlawii grown at different temperatures have about the same diffusion coefficients. Finally, the lipid lateral diffusion in bilayers of phospholipids was found to be much faster than that in bilayers of mainly glucolipids, which can be understood in terms of a free volume theory for the diffusion process.

The biosynthetic incorporation of short-chain linear saturated fatty acids by Acholeplasma laidlawii B may suppress cell growth by perturbing membrane lipid polar headgroup distribution

Acholeplasma laidlawii B cells made fatty acid auxotrophic by growth in the presence of the biotin-binding agent avidin grow increasingly poorly at 37 degrees C when supplemented with single exogenous linear saturated fatty acids of decreasing hydrocarbon chain length. Interestingly, this progressive decrease in growth yields with decreasing hydrocarbon chain length is not observed when cells are cultured in the presence of other classes of exogenous fatty acids. Moreover, normal growth is observed is other types of fatty acids with equivalent or shorter hydrocarbon chain lengths, indicating that poor growth in the presence of short-chain linear saturated fatty acids cannot be due to a decrease in membrane lipid bilayer thickness per se. To understand the molecular basis of such growth inhibition, we determined the growth yields, membrane lipid fatty acid and polar headgroups compositions, and phase state and fluidity of the membrane lipids in cells progressively biosynthetically enriched in tridecanoic acid (13:0) or dodecanoic acid (12:0). The growth of fatty acid auxotrophic A. laidlawii B cells grown in the presence of binary combinations of an exogenous fatty acid which supports normal growth on its own and 13:0 or 12:0 revealed that growth inhibition is not observed until 13:0 and 12:0 biosynthetic incorporation levels reach about 90 and 60 mol %, respectively, after which growth is markedly inhibited. Differential scanning calorimetric analyses of membranes from cells maximally enriched in 13:0 indicate that the lipid gel/liquid-crystalline phase transition temperature is unexpectedly high but that at the growth temperature of 37 degrees C, the membrane lipid bilayer is almost exclusively in the liquid-crystalline state but is certainly not excessively fluid. However, high levels of 13:0 incorporation produce a greatly elevated level of the high melting, reversed nonlamellar phase-preferring lipid component monoglucosyl diacylglycerol, and greatly reduced levels of all other membrane lipid components. This marked elevation of monoglucosyl diacylglycerol levels can be rationalized as a regulatory response which maintains the lamellar/nonlamellar phase-forming propensity of the total membrane lipid mixture relatively constant in the face of the biosynthetic incorporation of increasing quantities of short-chain saturated fatty acids, which favor the lamellar phase. However, this lipid biosynthetic response produces a marked decline in the levels of anionic phospholipid and phosphoglycolipid which are probably required to maintain the minimal negative surface charge density of the lipid bilayer, which we suggest is responsible for the observed growth inhibition. This work shows that the lipid biosynthetic regulatory mechanisms present in this organism may sometimes operate at cross purposes such that it is not possible to simultaneously optimize all of the biologically relevant physical properties of the membrane lipid bilayer.

Lateral organisation of membrane lipids. The superlattice view

Most biological membranes are extremely complex structures consisting of hundreds or even thousands of different lipid and protein molecules. The prevailing view regarding the organisation of these membranes is based on the fluid-mosaic model proposed by Singer and Nicholson in 1972. According to this model, phospholipids together with some other lipids form a fluid bilayer in which these lipids are diffusing very rapidly laterally. The idea of rapid lateral diffusion implies that, in general, the different lipid species would be randomly distributed in the plain of the membrane. However, there are recent data indicating that the components tend to adopt regular (superlattice-like) distributions in fluid, mixed bilayers. Based on this, a superlattice model of membranes has been proposed. This superlattice model is intriguing because it allows only a limited certain number of 'critical' compositions. These critical compositions could play a key role in the regulation of the lipid compositions of biological membranes. Furthermore, such putative critical compositions could explain how compositionally distinct organelles can exist despite of rapid inter-organelle membrane traffic. In this review, these intriguing predictions are discussed along with the basic principles of the model and the evidence supporting it.

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.

Purification of a phosphatase which hydrolyzes phosphatidic acid, a key intermediate in glucolipid synthesis in Acholeplasma laidlawii A membranes

A phosphatidic acid phosphatase (PAP; EC 3.1.3.4.), dephosphorylating phosphatidic acid (PA) to diacylglycerol (DAG), was identified and purified from the plasma membrane of Acholeplasma laidlawii A. After four purification steps, including membrane preparation, Tween 20 solubilization, preparative gel electrophoresis and electro-elution, PAP was purified about 400 times to near homogeneity. The molecular weight of PAP was according to SDS-polyacrylamide gel electrophoresis approximately 25 kDa and the enzyme was a stable and integral membrane protein. It is proposed to catalyze the first enzymatic step in the important glucolipid pathway of A. laidlawii. No essential cofactors or activator lipids were found. However, some divalent cations and phosphate analogues were potent inhibitors. Beside the in vivo substrate (PA), PAP was found to dephosphorylate p-nitrophenylphosphate. This less stringent specificity makes alternative in vivo functions for PAP plausible, the importance which is discussed.

Activating amphiphiles cause a conformational change of the 1,2-diacylglycerol 3-glucosyltransferase from Acholeplasma laidlawii membranes according to proteolytic digestion

1,2-Diacylglycerol 3-glucosyltransferase synthesizes the major nonbilayer-prone lipid monoglucosyldiacylglycerol (MGlcDAG) in the membrane of Acholeplasma laidlawii, which is important for the spontaneous curvature, and is a regulatory site for the lipid surface charge density. A potential connection between activity and a conformational change of this enzyme, governed by essential lipid activators, was studied with purified MGlcDAG synthase in different lipid aggregates. Critical fractions of anionic phospholipids 1, 2-dioleoyl-phosphatidylglycerol (DOPG) and 1,2-dioleoyl-phosphatidylserine (DOPS) were essential for the restoration of enzyme activity, while the zwitterionic 1,2-dioleoyl-phosphatidylcholine (DOPC) and the uncharged diglucosyldiacylglycerol (DGlcDAG) were not. Proteolytic resistance had a very good correlation with the enzyme activity in various lipid-CHAPS mixed micelles. Anionic lipids DOPG and DOPS could protect the exposed MGlcDAG synthase from digestion, whereas DOPC and DGlcDAG could not. Similar features were observed in liposome bilayers. Likewise, the detergent dodecylphosphoglycerol (PGD), with a phosphatidylglycerol-like headgroup, could also stimulate the MGlcDAG synthase activity efficiently with a concomitant protection toward proteolytic digestion. Neither proteolytic resistance nor restored enzyme activity was observed using soluble glycerol 3-phosphate. It is concluded that in addition to critical amounts, both the negatively charged headgroup and hydrophobic chains of the activator amphiphiles, but not a certain aggregate curvature, seem necessary for a proper conformation and the resulting active state of the MGlcDAG synthase.

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.

New aspects on membrane lipid regulation in Acholeplasma laidlawii A and phase equilibria of monoacyldiglucosyldiacylglycerol

A new membrane lipid, monoacyldiglucosyldiacylglycerol (MADGlcDAG), was recently discovered in Acholeplasma laidlawii strain A-EF22, demanding a new study of the biosynthetic regulation, and the phase behavior, of the glucolipids in this organism. The only liquid-crystalline phase formed by MADGlcDAG is a reversed hexagonal phase. A. laidlawii A-EF22 synthesizes four lipids that have the ability to induce the formation of reversed nonlamellar phases: MADGlcDAG, monoglucosyldiacylglycerol (MGlcDAG), monoacylmonoglucosyldiacylglycerol (MAMGlcDAG), and diacylglycerol (DAG). A Cn value of approximately 16 seems to be a critical value for the fractions of these lipids in the membrane: the fractions of MADGlcDAG and MGlcDAG are largest when the Cn values are lower than 16, while the fractions of MAMGlcDAG and DAG are largest when the Cn values are higher than 16. The fraction of nonlamellar-forming lipids was 55 mol% when the Cn value was 14.8 and the degree of unsaturation was 33 mol%. This fraction was reduced to 7 mol% when the Cn value and the degree of unsaturation were increased to 17.8 and 92 mol%, respectively, i.e., at conditions that markedly favor the formation of reversed nonlamellar phases. These observations convincingly show that a balance between lamellar- and nonlamellar-forming lipids is maintained in the membrane and strongly support the validity of the lipid regulation model proposed by us. From earlier biochemical data, obtained with short acyl chains, that were difficult to reconcile with our regulation model, it could be predicted that a lipid ought to be synthesized that assists MGlcDAG to maintain the nonlamellar-forming properties with the short chains. It is shown in the present work that this lipid is MADGlcDAG and that the regulation of the balance between lamellar- and nonlamellar-forming lipids is even more complex and sophisticated in A. laidlawii A-EF22 than previously proposed.

Correlation between bilayer lipid dynamics and activity of the diglucosyldiacylglycerol synthase from Acholeplasma laidlawii membranes

In the single membrane of Acholeplasma laidlawii a specific glucosyltransferase synthesize the major, lamellar-forming lipid diglucosyldiacylglycerol (DGlcDAG) from the major, nonlamellar-prone monoglucosyldiacylglycerol (MGlcDAG). This is crucial for the maintenance of phase equilibria close to a bilayer-nonbilayer transition and a nearly constant spontaneous curvature in the membrane lipid bilayer. Acyl chain order is also affected, but not kept constant. Phosphatidylglycerol (PG) is an essential activator, needed in substantial amounts by the DGlcDAG synthase, and likely to affect bilayer properties. A potential connection was investigated between the (i) lateral diffusion, (ii) domain formation of the PG activator and (iii) bilayer chain ordering (i.e., the hydrocarbon free volume), revealed in unilamellar liposomes by lipid probes containing one or two (fluorescent) pyrene acyl chains, and (iv) activity of the DGlcDAG synthase. Different activator, nonbilayer perturbant, and bilayer matrix conditions were employed. Diffusion of PG was substantially slower in a DGlcDAG compared to a phosphatidylcholine (PC) matrix with 18:1c chains but increased with the PG content in both. No obvious correlation between diffusion and enzyme activity, and no local concentration of PG as a function of chain ordering or curvature, was detected. However, an enrichment of PG activator into domains could be induced by a chain length mismatch between 18:1c-PG and 14:1c-PC (but not 22:1c-PC), even at small PG fractions. Patching was sufficient to stimulate enzyme activity 4-fold in relation to the activities normally valid at low PG concentrations. Chain order was substantially lower (i.e., free volumes larger) in bilayers of DGlcDAG than in bilayers of PC and increased in an additive fashion in both by the content of especially the nonbilayer-prone 1,3-18:1c-DAG but also by PG. At physiological concentrations of PG in DGlcDAG bilayers (approximately 20%) a good correlation was evident between increased DAG content and chain ordering and strongly enhanced enzyme activities, with maxima close to a bilayer-nonbilayer transition. It is concluded that regulation of packing conditions in A. laidlawii membranes by the DGlcDAG synthase seems to be governed not by the absolute extent of chain order but more by the spontaneous curvature within a certain range of conditions. Domain formation of the essential PG activator due to bilayer conditions is a second mechanism, potentially overriding the curvature effects.

Membrane lipid composition and cell size of Acholeplasma laidlawii strain A are strongly influenced by lipid acyl chain length

The small, cell-wall-less prokaryote Acholeplasma laidlawii strain A-EF22 could grow with membrane lipids having an average acyl chain length Cn varying over 14.5- almost 20 carbons by exogenous supplementation with selected fatty acids. For 16 < Cn < 18, the cells grew with lipids containing 100% (mol/100 mol) monounsaturated acyl chains, whereas for Cn < 16 and Cn > 18, cell growth only occurred with gradually lower fractions of unsaturated chains. Cn was actively increased and decreased by chain elongation or de novo fatty acid synthesis upon incorporation of short-chain and long-chain fatty acids, respectively. The membrane lipid composition was strongly affected by the acyl chain length and unsaturation, and the metabolic responses are readily explained as a regulation mechanism based on the established phase equilibria of the individual lipids in the A. laidlawii membrane. Monoglucosyldiacylglycerol (Glc-acyl2-Gro) was the dominating lipid with short chains but the fraction of this lipid decreased with increasing Cn, correlating with the decreasing lamellar to nonlamellar phase transition temperatures for this lipid. The fractions of diglucosyldiacylglycerol (Glc2-acyl2Gro) and phosphatidylglycerol (PtdGro), forming lamellar phases only, increased with increasing Cn over the entire chain-length interval. A weaker correlation was usually observed between the relative amount of a lipid and the extent of chain unsaturation; however, the fractions of Glc2-acyl2Gro and PtdGro increased clearly with an increasing degree of unsaturation. Moreover, the synthesis of the nonbilayer-forming lipids acyl2Gro and monoacyl-Glc-acyl2Gro was strongly stimulated by a high degree of chain saturation. Concomitantly, the phase equilibria of Glc-acyl2Gro are shifted towards lamellar phases at the growth temperature. The fraction of the three potentially nonbilayer-forming lipids varied over 10-80% (mol/100 mol) total lipids as a function of the acyl chain composition. The combined molar fractions of the three phospholipids increased strongly with chain unsaturation. However, the fraction of phosphate moieties in the different lipids was constant over the entire chain-length interval. It is concluded that the regulation of the membrane lipid composition aims at maintaining similar phase equilibria and surface charge densities of the lipid bilayer. The size of A. laidlawii cells was changed in a systematic manner and correlated qualitatively with the packing properties of the lipids. Cell diameters were increased by an increase in acyl chain length and saturation, and was affected by additives such an n-dodecane and acyl2Gro.

Similar regulatory mechanisms despite differences in membrane lipid composition in Acholeplasma laidlawii strains A-EF22 and B-PG9. A multivariate data analysis

Mycoplasmas are small, cell wall-deficient bacteria. The metabolic regulation of the lipid composition in the membrane of the species Acholeplasma laidlawii, strains A-EF22 and B-JU, is governed mainly by the balance between the potential formation of lamellar and nonlamellar phase structures. However, the regulatory features have not been consistently observed in the B-PG9 strain. A comparison has been performed between the membrane lipid composition for strains A-EF22 and B-PG9, simultaneously changing eight experimental conditions known to affect the regulation and packing properties of the A-EF22 lipids. Multiple regression and partial least-square discriminant analyses of many variables showed: (i) quantitative differences in membrane lipid and protein composition, and in membrane protein molecular masses of the two strains; (ii) different molar fractions of the major polar lipids monoglucosyldiacylglycerol (nonlamellar) and diglucosyldiacylglycerol (lamellar), which were caused by differences in lipid acyl chain length and unsaturation inherent in the strains and by the type of growth medium used; and (iii) similar regulatory mechanisms for changes in the lipid composition under most conditions, responding to the experimentally varied bilayer and nonbilayer properties of the lipid matrix. These regulatory principles are probably valid in other bacteria as well.

The enzymatic synthesis of membrane glucolipids in Acholeplasma laidlawii

In membranes of the prokaryote Acholeplasma laidlawii, the physiological regulation of the two major membrane lipids, monoglucosyldiacylglycerol (MGlcDAG) and diglucosyldiacylglycerol (DGlcDAG), is governed by factors affecting the equilibria between lamellar and non-lamellar phases of the membrane lipids. The synthesis of the glucolipids is considered to be a two-step glucosylation: (i) DAG+UDP-Glc----MGlcDAG+UDP; and (ii) MGlcDAG+UDP-Glc----DGlcDAG+UPD. This was corroborated by in vivo pulse labelling experiments showing turnover of MGlcDAG but not DGlcDAG. The enzymatic synthesis of MGlcDAG was localized to fresh or freeze-dried membranes in vitro. Synthesis of DGlcDAG was minor in such membranes but of substantial magnitude in intact cells. Synthesis of MGlcDAG was stimulated by small amounts of SDS but completely inhibited upon solubilization of the membranes by a variety of detergents. The inhibitory effect of several UDP-Glc analogs on glucolipid synthesis demonstrated the importance of UDP-Glc as the sugar donor. Synthesis of both glucolipids was lost in freeze-dried plus lipid-extracted cells but restored when lipids were transferred back to the extracted cell membrane. By selectively adding specific lipids, a strong dependence on the acceptor lipid DAG, as well as the need for general matrix lipids for enzyme activity, was established. In addition, the anionic phosphatidylglycerol (PG), but not the other phospholipids, had a strong stimulatory effect. The presence of different phosphorylating agents stimulated the synthesis of DGlcDAG and partially inhibited that of MGlcDAG. This, together with the lipid dependency, may constitute mechanisms for the regulation of the enzyme activities in vivo.

Membrane protein acylation. Preference for exogenous myristic acid or endogenous saturated chains in Acholeplasma laidlawii

Mycoplasmas are small bacteria without a cell wall, often found as surface parasites on eukaryotic cells. Of the more than 200 membrane proteins from Acholeplasma laidlawii resolved by two-dimensional PAGE, 23 were covalently modified with acyl chains. These acyl proteins had lower pI values than average and were all labelled by different exogenously supplied radioactive fatty acids attached by O-ester bonds. The fatty acids were selectively incorporated in the order myristic acid (14:0) greater than palmitic acid (16:0) greater than stearic acid (18:0) greater than oleic acid (18:1). However, endogenously synthesised saturated fatty acids, most of which were 16:0, were preferred over the supplied ones. A fraction of the exogenous 14:0 was elongated to 16:0. Absence of saturated fatty acids increased the incorporation of 18:1. The maximum extent of modification was one acyl chain for protein T2, on the exterior surface and two acyl chains for protein D12, spanning them membrane. Exogenously supplied fatty acids were incorporated into membrane lipids in proportion to their occurrence. However, the acylated proteins always contained 8-10 times more saturated chains than did the lipids. When exogenously supplied, all A. laidlawii polar membrane lipids could donate acyl chains to the acylated proteins but the neutral fraction (fatty acids and diacylglycerol) was most efficient. An incorporation into the acylated proteins of labelled cysteine, but not glucose or glycerol, was observed. Acylated proteins with different chains interacted similarly with a Triton X-114 detergent phase, and no full-size proteins (or acylated fragments) were released from cells by proteolytic enzymes. The results indicate an anchoring with peptide segments in addition to the acyl chains. Both 14:0 and 16:0 were attached at one end of both T2 and D12, but the N-terminal methionine of T2 was not acylated. The extent of modification and preference for saturated chains in the A. laidlawii membrane acylated proteins is more similar to eukaryotic than to eubacterial proteins.

Effect of head-group structure and counterion condensation on phase equilibria in anionic phospholipid-water systems studied by 2H, 23Na, and 31P NMR and X-ray diffraction

The phase equilibria, hydration, and sodium counterion association for the systems DOPA-2H2O, DOPS-2H2O, DOPG-2H2O, and DPG-2H2O were investigated with 2H, 23Na, and 31P NMR and X-ray diffraction. The following one-phase regions were found in the DOPA-water system: a reversed hexagonal liquid-crystalline (HII) phase up to about 35 wt % water and a lamellar liquid-crystalline (L alpha) phase between about 55 and 98 wt % water. The area per DOPA molecule was 36-65 A2 in the HII phase (10-40 wt % water) and 69 A2 in the L alpha phase (60 wt % water). DOPS and DOPG with 10-98 wt % water, and DPG with 20-95 wt % water formed an L alpha phase at temperatures between 25 and 55 degrees C. At temperatures above 55 degrees C, DPG with 20 and 30 wt % water formed a mixture of L alpha, HII, and cubic liquid-crystalline phases, the mole percent of lipid forming nonlamellar phases being smaller at 30 wt % water than at 20 wt % water. DPG with 10 wt % water probably formed a mixture of an L alpha phase and at least one nonlamellar liquid-crystalline phase at 25 and 35 degrees C, and a pure HII phase at 45 degrees C and higher temperatures. At water concentrations above about 50 wt % the 23Na quadrupole splitting was constant for all four lipid-water systems studied, implying that the counterion association to the charged lipid aggregates did not change upon dilution. These experimental observations can be described with an ion condensation model but not with a simple equilibrium model. The fraction of counterions located close to the lipid-water interface was calculated to be greater than 95%. The 2H and 23Na NMR quadrupole splittings of 2H2O and sodium counterions, respectively, indicate that the molecular order in the polar head-group region decreases for the L alpha phase in the order DOPA approximately DPG greater than DOPS greater than DOPG.

Cation-induced aggregation of acidic phospholipid vesicles: the role of fatty acid unsaturation and cholesterol

Cation-induced aggregation of acidic phospholipid vesicles consisting of dimyristoylphosphatidylglycerol (DMPG), dipalmitoylphosphatidylserine (DPPS), phosphatidylserine from bovine brain (brPS), and phosphatidylglycerol from egg yolk (eggPG) was studied. Significant differences were evident in the NaCl-induced aggregation of fully saturated and unsaturated acidic phospholipid vesicles. The threshold NaCl concentration of vesicle aggregation ([NaCl]Thr) for DPPS vesicles was 320 mM compared to 610 mM observed for brPS vesicles. For DMPG vesicles the [NaCl]Thr was 430 mM and no aggregation of eggPG vesicles could be observed upon addition of NaCl. The threshold CaCl2 concentrations of aggregation of DMPG and eggPG vesicles were 2.3 and 4.9 mM, respectively. The corresponding threshold CaCl2 concentrations for DPPS and brPS vesicles were 0.85 mM and 1.3 mM, respectively. The inclusion of cholesterol into vesicles attenuated NaCl- and CaCl2-induced aggregation of DMPG and DPPS vesicles. However, enhancement of aggregation by inclusion of cholesterol was observed in the case of NaCl-induced aggregation of brPS vesicles. It is concluded that cation mediated membrane-membrane interactions depend, in addition to polar headgroup structure, on the fatty acid composition of the phospholipids also.

The effect of variations in growth temperature, fatty acid composition and cholesterol content on the lipid polar head-group composition of Acholeplasma laidlawii B membranes

We have systematically investigated the effect of variations in growth temperature, fatty acid composition and cholesterol content on the membrane lipid polar headgroup composition of Acholeplasma laidlawii B. Two important lipid compositional parameters have been determined from such an analysis. The first parameter studied was the ratio of the two major neutral glycolipids of this organism, monoglucosyldiacylglycerol (MGDG) and diglucosyldiacylglycerol (DGDG). As the former lipid prefers to exist in a reversed hexagonal phase at higher temperatures, with unsaturated fatty acyl chains or in the presence of cholesterol, the ratio of these two lipids reflects the phase state preference of the total A. laidlawii membrane lipids. Although we find that the MGDG/DGDG ratio is reduced in response to an increase in fatty acid unsaturation, increases in growth temperature or cholesterol content reduce this ratio only in cells enriched in a saturated but not an unsaturated fatty acid. The second parameter studied was the ratio of these neutral glycolipids to the only phosphatide in the A. laidlawii membrane, phosphatidylglycerol (PG); this parameter reflects the relative balance of uncharged and charged lipids in the membrane of this organism. We find that the MGDG + DGDG/PG ratio is lowest in cells enriched in the saturated fatty acid even though these cells already have the highest lipid bilayer surface charge density. Moreover, this ratio is not consistently related to growth temperature or changes in cholesterol levels, as expected. We therefore conclude that A. laidlawii strain B, apparently unlike strain A, does not possess coherent regulatory mechanisms for maintaining either the phase preference or the surface charge density of its membrane lipid constant in response to variations in growth temperature, fatty acid composition or cholesterol content.

Neutron diffraction studies of digalactosyldiacylglycerol

The structure of the digalactosyldiacylglycerol bilayer is calculated using neutron diffraction data. The polar head group of this lipid is oriented parallel to the plane of the bilayer such that the galactose moieties are tightly packed at the bilayer surface into a 0.8 nm thick polar layer. The thickness of this layer is independent of water activity over a wide range (15-100% relative humidity). The constant thickness of both the galactose layer and the hydrocarbon layer constrain the structure factor amplitudes to lie on a single continuous transform for repeat periods between 4 and 5 nm.

Lipid intermolecular hydrogen bonding: influence on structural organization and membrane function

The great variety of different lipids in membranes, with modifications to the hydrocarbon chains, polar groups and backbone structure suggests that many of these lipids may have unique roles in membrane structure and function. Acidic groups on lipids are clearly important, since they allow interaction with basic groups on proteins and with divalent cations. Another important property of certain lipids is their ability to interact intermolecularly with other lipids via hydrogen bonds. This interaction occurs through acidic and basic moieties in the polar head groups of phospholipids, and the amide moiety and hydroxyl groups on the acyl chain, sphingosine base and sugar groups of sphingo- and glycolipids. The putative ability of different classes of lipids to interact by intermolecular hydrogen bonding, the molecular groups which may participate and the effect of these interactions on some of their physical properties are summarized in Table IX. It is frequently questioned whether intermolecular hydrogen bonding could occur between lipids in the presence of water. Correlations of their properties with their molecular structures, however, suggest that it can. Participation in intermolecular hydrogen bonding increases the lipid phase transition temperature by approx. 8-16 Cdeg relative to the electrostatically shielded state and by 20-30 Cdeg relative to the repulsively charged state, while having variable effects on the enthalpy. It increases the packing density in monolayers, possibly also in the liquid-crystalline phase in bilayers, and decreases the lipid hydration. These effects can probably be accounted for by transient, fluctuating hydrogen bonds involving only a small percentage of the lipid at any one time. Thus, rotational and lateral diffusion of the lipids may take place but at a slower rate, and the lateral expansion is limited. Intermolecular hydrogen bonding between lipids in bilayers may be significantly stabilized, despite the presence of water, by the fact that the lipids are already intermolecularly associated as a result of the hydrophobic effect and the Van der Waals' interactions between their chains. The tendency of certain lipids to self-associate, their asymmetric distribution in SUVs, their preferential association with cholesterol in non-cocrystallizing mixtures, their temperature-induced transitions to the hexagonal phase and their inhibitory effect on penetration of hydrophobic residues of proteins partway into the bilayer can all be explained by their participation in intermolecular hydrogen bonding interactions.(ABSTRACT TRUNCATED AT 400 WORDS)

Membrane potential, lipid regulation and adenylate energy charge in acyl chain modified Acholeplasma laidlawii

In Acholeplasma laidlawii variations induced in the transmembrane electrical potential have been shown to affect the membrane lipid composition. Particularly the molar ratio between the predominant glucolipids, monoglucosyldiacylglycerol and diglucosyldiacylglycerol, decreases upon hyperpolarization and increases upon depolarization (Clementz et al. (1986) Biochemistry 25, 823-830). Upon variation of the degree of membrane fatty acyl chain unsaturation, known to affect the passive permeability for a number of small molecules, there was no significant correlation between acyl chain composition and the magnitude of the electrical potential. Hyperpolarization by valinomycin decreased the glucolipid ratio for all kinds of membranes, but the size of the decrease was not correlated to the acyl chain composition. However, a clear relationship, independent of acyl chain composition, was found between the extent of hyperpolarization and the size of the decrease in the glucolipid ratio. The adenylate energy charge value (Ec) of the cells was affected by the acyl chain composition, although not exclusively by the proportion of unsaturation. Furthermore, a larger hyperpolarization upon valinomycin addition was accompanied by a stronger reduction in Ec.

Lipid acyl chain-dependent effects of sterols in Acholeplasma laidlawii membranes

Acholeplasma laidlawii was grown with different fatty acids for membrane lipid synthesis (saturated straight- and branched-chain acids and mono- and di-unsaturated acids). The ability of 12 different sterols to affect cell growth, lipid head group composition, the order parameter of the acyl chains, and the phase equilibria of in vivo lipid mixtures was studied. The following two effects were observed with respect to cell growth: with a given acyl chain composition of the membrane lipids, growth was stimulated, unaffected, reduced, or completely inhibited (lysis), depending on the sterol structure; and the effect of a certain sterol depended on the acyl chain composition (most striking for epicoprostanol, cholest-4-en-3-one, and cholest-5-en-3-one, which stimulated growth with saturated acyl chains but caused lysis with unsaturated chains). The three lytic sterols were the only sterols that caused a marked decrease in the ratio between the major lipids monoglucosyldiglyceride and diglucosyldiglyceride and hence a decrease in bilayer stability when the membranes were enriched in saturated (palmitoyl) chains. With these chains correlations were found for several sterols between the glucolipid ratio and the order parameter of the acyl chains, as well as the lamellar-reversed hexagonal phase transition, in model systems. A shaft experiment revealed a marked decrease in the ratio of monoglucosyldiglyceride to diglucosyldiglyceride with the lytic sterols in unsaturated (oleoyl) membranes. The two cholestenes induced nonlamellar phases in in vivo mixtures of oleoyl A. laidlawii lipids. The order parameters of the oleoyl chains were almost unaffected by the sterols. Generally, the observed effects cannot be explained by an influence of the sterols on the gel-to-liquid crystalline phase transition.