Modulation of dolichyl-phosphomannose synthase activity by changes in the lipid environment of the enzyme

Curvature forces in membrane lipid-protein interactions

Membrane biochemists are becoming increasingly aware of the role of lipid-protein interactions in diverse cellular functions. This review describes how conformational changes in membrane proteins, involving folding, stability, and membrane shape transitions, potentially involve elastic remodeling of the lipid bilayer. Evidence suggests that membrane lipids affect proteins through interactions of a relatively long-range nature, extending beyond a single annulus of next-neighbor boundary lipids. It is assumed the distance scale of the forces is large compared to the molecular range of action. Application of the theory of elasticity to flexible soft surfaces derives from classical physics and explains the polymorphism of both detergents and membrane phospholipids. A flexible surface model (FSM) describes the balance of curvature and hydrophobic forces in lipid-protein interactions. Chemically nonspecific properties of the lipid bilayer modulate the conformational energetics of membrane proteins. The new biomembrane model challenges the standard model (the fluid mosaic model) found in biochemistry texts. The idea of a curvature force field based on data first introduced for rhodopsin gives a bridge between theory and experiment. Influences of bilayer thickness, nonlamellar-forming lipids, detergents, and osmotic stress are all explained by the FSM. An increased awareness of curvature forces suggests that research will accelerate as structural biology becomes more closely entwined with the physical chemistry of lipids in explaining membrane structure and function.

N-glycans in cell survival and death: cross-talk between glycosyltransferases

Asparagine-linked (N-linked) protein glycosylation is one of the most important protein modifications. N-glycans with "high mannose", "hybrid", or "complex" type sugar chains participate in a multitude of cellular processes. These include cell-cell/cell-matrix/receptor-ligand interaction, cell signaling/growth and differentiation, to name a few. Many diseases such as disorders of blood clotting, congenital disorder of glycosylation, diseases of blood vessels, cancer, neo-vascularization, i.e., angiogenesis essential for breast and other solid tumor progression and metastasis are associated with N-glycan expression. Biosynthesis of N-glycans requires multiple steps and multiple cellular compartments. Following transcription and translation the proteins migrate to the endoplasmic reticulum (ER) lumen to acquire glycan chain(s) with a defined glycoform, i.e., a tetradecasaccharide. These are further modified, i.e., edited in ER lumen and in Golgi prior to moving to their respective destinations. The tetradecasaccharide is pre-assembled on a poly-isoprenoid lipid called dolichol, and becomes an essential component of the supply chain. Therefore, dolichol cycle synthesizing the lipid-linked oligosaccharide (LLO) is a hallmark for all N-linked glycoproteins. It is expected that there is a great deal of cross-talk between the participating glycosyltransferases and any missed step would express defective N-glycans that could have fatal consequences. The positive impact of the structurally altered N-glycans could lead to discovery of an N-glycan signature for a disease and/or help developing glycotherapeutic treating cancer or other human diseases. The purpose of this review is to identify the gaps of N-glycan biology and help developing appropriate technology for biomedical applications. This article is part of a Special Issue entitled Glycoproteomics.

Multiple alkane hydroxylase systems in a marine alkane degrader, Alcanivorax dieselolei B-5

Alcanivorax dieselolei strain B-5 is a marine bacterium that can utilize a broad range of n-alkanes (C(5) -C(36) ) as sole carbon source. However, the mechanisms responsible for this trait remain to be established. Here we report on the characterization of four alkane hydroxylases from A. dieselolei, including two homologues of AlkB (AlkB1 and AlkB2), a CYP153 homologue (P450), as well as an AlmA-like (AlmA) alkane hydroxylase. Heterologous expression of alkB1, alkB2, p450 and almA in Pseudomonas putida GPo12 (pGEc47ΔB) or P. fluorescens KOB2Δ1 verified their functions in alkane oxidation. Quantitative real-time RT-PCR analysis showed that these genes could be induced by alkanes ranging from C(8) to C(36) . Notably, the expression of the p450 and almA genes was only upregulated in the presence of medium-chain (C(8) -C(16) ) or long-chain (C(22) -C(36) ) n-alkanes, respectively; while alkB1 and alkB2 responded to both medium- and long-chain n-alkanes (C(12) -C(26) ). Moreover, branched alkanes (pristane and phytane) significantly elevated alkB1 and almA expression levels. Our findings demonstrate that the multiple alkane hydroxylase systems ensure the utilization of substrates of a broad chain length range.

Tryptophan residues promote membrane association for a plant lipid glycosyltransferase involved in phosphate stress

Chloroplast membranes contain a substantial excess of the nonbilayer-prone monogalactosyldiacylglycerol (GalDAG) over the biosynthetically consecutive, bilayer-forming digalactosyldiacylglycerol (GalGalDAG), yielding a high membrane curvature stress. During phosphate shortage, plants replace phospholipids with GalGalDAG to rescue phosphate while maintaining membrane homeostasis. Here we investigate how the activity of the corresponding glycosyltransferase (GT) in Arabidopsis thaliana (atDGD2) depends on local bilayer properties by analyzing structural and activity features of recombinant protein. Fold recognition and sequence analyses revealed a two-domain GT-B monotopic structure, present in other plant and bacterial glycolipid GTs, such as the major chloroplast GalGalDAG GT atDGD1. Modeling led to the identification of catalytically important residues in the active site of atDGD2 by site-directed mutagenesis. The DGD synthases share unique bilayer interface segments containing conserved tryptophan residues that are crucial for activity and for membrane association. More detailed localization studies and liposome binding analyses indicate differentiated anchor and substrate-binding functions for these separated enzyme interface regions. Anionic phospholipids, but not curvature-increasing nonbilayer lipids, strongly stimulate enzyme activity. From our studies, we propose a model for bilayer "control" of enzyme activity, where two tryptophan segments act as interface anchor points to keep the substrate region close to the membrane surface. Binding of the acceptor substrate is achieved by interaction of positive charges in a surface cluster of lysines, arginines, and histidines with the surrounding anionic phospholipids. The diminishing phospholipid fraction during phosphate shortage stress will then set the new GalGalDAG/phospholipid balance by decreasing stimulation of atDGD2.

Diacylglycerols, multivalent membrane modulators

Diacylglycerols are second messengers confined to biomembranes and, although relatively simple molecules from the structural point of view, they are able of triggering a surprisingly wide range of biological responses. Diacylglycerols are recognized by a well conserved protein motif, such as the C1 domain. This domain was observed for the first time in protein kinases C but is now known to be present in many other proteins. The effect of diacylglycerols is not limited to binding to C1 domains and they are able to alter the biophysical properties of biomembranes and hence modulate the activity of membrane associated proteins and also facilitate some processes like membrane fusion.

Proteomic insights into metabolic adaptations in Alcanivorax borkumensis induced by alkane utilization

Alcanivorax borkumensis is a ubiquitous marine petroleum oil-degrading bacterium with an unusual physiology specialized for alkane metabolism. This "hydrocarbonoclastic" bacterium degrades an exceptionally broad range of alkane hydrocarbons but few other substrates. The proteomic analysis presented here reveals metabolic features of the hydrocarbonoclastic lifestyle. Specifically, hexadecane-grown and pyruvate-grown cells differed in the expression of 97 cytoplasmic and membrane-associated proteins whose genes appeared to be components of 46 putative operon structures. Membrane proteins up-regulated in alkane-grown cells included three enzyme systems able to convert alkanes via terminal oxidation to fatty acids, namely, enzymes encoded by the well-known alkB1 gene cluster and two new alkane hydroxylating systems, a P450 cytochrome monooxygenase and a putative flavin-binding monooxygenase, and enzymes mediating beta-oxidation of fatty acids. Cytoplasmic proteins up-regulated in hexadecane-grown cells reflect a central metabolism based on a fatty acid diet, namely, enzymes of the glyoxylate bypass and of the gluconeogenesis pathway, able to provide key metabolic intermediates, like phosphoenolpyruvate, from fatty acids. They also include enzymes for synthesis of riboflavin and of unsaturated fatty acids and cardiolipin, which presumably reflect membrane restructuring required for membranes to adapt to perturbations induced by the massive influx of alkane oxidation enzymes. Ancillary functions up-regulated included the lipoprotein releasing system (Lol), presumably associated with biosurfactant release, and polyhydroxyalkanoate synthesis enzymes associated with carbon storage under conditions of carbon surfeit. The existence of three different alkane-oxidizing systems is consistent with the broad range of oil hydrocarbons degraded by A. borkumensis and its ecological success in oil-contaminated marine habitats.

In vitro phosphorylation by cAMP-dependent protein kinase up-regulates recombinant Saccharomyces cerevisiae mannosylphosphodolichol synthase

DPM1 is the structural gene for mannosylphosphodolichol synthase (i.e. Dol-P-Man synthase, DPMS) in Saccharomyces cerevisiae. Earlier studies with cDNA cloning and sequence analysis have established that 31-kDa DPMS of S. cerevisiae contains a consensus sequence (YRRVIS141) that can be phosphorylated by cAMP-dependent protein kinase (PKA). We have been studying the up-regulation of DPMS activity by protein kinase A-mediated phosphorylation in higher eukaryotes, and used the recombinant DPMS from S. cerevisiae in this study to advance our knowledge further. DPMS catalytic activity was indeed enhanced severalfold when the recombinant protein was phosphorylated in vitro. The rate as well as the magnitude of catalysis was higher with the phosphorylated enzyme. A similar increase in the catalytic activity was also observed when the in vitro phosphorylated recombinant DPMS was assayed as a function of increasing concentrations of exogenous dolichylmonophosphate (Dol-P). Kinetic studies indicated that there was no change in the Km for GDP-mannose between the in vitro phosphorylated and control recombinant DPMS, but the Vmax was increased by 6-fold with the phosphorylated enzyme. In vitro phosphorylated recombinant DPMS also exhibited higher enzyme turnover (kcat) and enzyme efficiency (kcat/Km). SDS-PAGE followed by autoradiography of the 32P-labeled DPMS detected a 31-kDa phosphoprotein, and immunoblotting with anti-phosphoserine antibody established the presence of a phosphoserine residue in in vitro phosphorylated recombinant DPMS. To confirm the phosphorylation activation of recombinant DPMS, serine 141 in the consensus sequence was replaced with alanine by PCR site-directed mutagenesis. The S141A DPMS mutant exhibited more than half-a-fold reduction in catalytic activity compared with the wild type when both were analyzed after in vitro phosphorylation. Thus, confirming that S. cerevisiae DPMS activity is indeed regulated by the cAMP-dependent protein phosphorylation signal, and the phosphorylation target is serine 141.

Docosahexaenoic acid: membrane properties of a unique fatty acid

Docosahexaenoic acid (DHA) with 22-carbons and 6 double bonds is the extreme example of an omega-3 polyunsaturated fatty acid (PUFA). DHA has strong medical implications since its dietary presence has been positively linked to the prevention of numerous human afflictions including cancer and heart disease. The PUFA, moreover, is essential to neurological function. It is remarkable that one simple molecule has been reported to affect so many seemingly unrelated biological processes. Although details of a molecular mode of action remain elusive, DHA must be acting at a fundamental level common to many tissues that is related to the high degree of conformational flexibility that the multiple double bonds have been identified to confer. One likely target for DHA action is at the cell membrane where the fatty acid is known to readily incorporate into membrane phospholipids. Once esterified into phospholipids DHA has been demonstrated to significantly alter many basic properties of membranes including acyl chain order and "fluidity", phase behavior, elastic compressibility, permeability, fusion, flip-flop and protein activity. It is concluded that DHA's interaction with other membrane lipids, particularly cholesterol, may play a prominent role in modulating the local structure and function of cell membranes.

Phosphatidylcholine synthesis is elevated in neuronal models of Gaucher disease due to direct activation of CTP:phosphocholine cytidylyltransferase by glucosylceramide

Glucosylceramide (GlcCer) accumulates in the inherited metabolic disorder, Gaucher disease, because of the defective activity of lysosomal glucocerebrosidase. We previously demonstrated that upon GlcCer accumulation, cultured hippocampal neurons exhibit modified growth patterns, altered endoplasmic reticulum density, and altered calcium release from intracellular stores. We here examined the relationship between GlcCer accumulation and phospholipid synthesis. After treatment of neurons with an active site-directed inhibitor of glucocerebrosidase, or in neurons obtained from a mouse model of Gaucher disease, [14C]methyl choline incorporation into [14C]phosphatidylcholine ([14C]PC) and [14C]sphingomyelin was elevated, as were [14C]CDP-choline levels, suggesting that CTP:phosphocholine cytidylyltransferase (CCT) is activated. Indeed, CCT activity was elevated in neurons that had accumulated GlcCer. GlcCer, but not galactosylceramide (GalCer), stimulated CCT activity in rat brain homogenates, and significantly higher levels of CCT were membrane associated in cortical homogenates from a mouse model of Gaucher disease compared with wild-type mice. Because CCT mRNA and protein levels were unaltered in either neurons or brain tissue that had accumulated GlcCer, it appeared likely that GlcCer activates CCT by a post-translational mechanism. This was verified by examination of the effect of GlcCer on CCT purified about 1200-fold from rat brain. GlcCer stimulated CCT activity, with stimulation observed at levels as low as 2.5 mol% and with maximal activation reached at 10 mol%. In contrast, GalCer had no effect. Together, these data demonstrate that GlcCer directly activates CCT, which results in elevated PC synthesis, which may account for some of the changes in growth rates observed upon neuronal GlcCer accumulation.

Electrostatic properties of membrane lipids coupled to metarhodopsin II formation in visual transduction

Changes in lipid composition have recently been shown to exert appreciable influences on the activities of membrane-bound proteins and peptides. We tested the hypothesis that the conformational states of rhodopsin linked to visual signal transduction are related to biophysical properties of the membrane lipid bilayer. For bovine rhodopsin, the meta I-meta II conformational transition was studied in egg phosphatidylcholine (PC) recombinants versus the native rod outer segment (ROS) membranes by means of flash photolysis. Formation of metarhodopsin II was observed by the change in absorbance at 478 nm after a single actinic flash was delivered to the sample. The meta I/meta II ratio was investigated as a function of both temperature and pH. The data clearly demonstrated thermodynamic reversibility of the transition for both the egg PC recombinants and the native ROS membranes. A significant shift of the apparent pK(a) for the acid-base equilibrium to lower values was evident in the egg PC recombinant, with little meta II produced under physiological conditions. Calculations of the membrane surface pH using a Poisson-Boltzmann model suggested the free energies of the meta I and meta II states were significantly affected by electrostatic properties of the bilayer lipids. In the ROS membranes, phosphatidylserine (PS) is needed for full formation of meta II, in combination with phosphatidylethanolamine (PE) and polyunsaturated docosahexaenoic acid (DHA; 22:6omega3) chains. We propose that the PS surface potential leads to an accumulation of hydronium ions, H(3)O(+), in the electrical double layer, which drive the reaction together with the large negative spontaneous curvature (H(0)) conferred by PE plus DHA chains. The elastic stress/strain of the bilayer arises from an interplay of the approximately zero H(0) from PS and the negative H(0) due to the PE headgroups and polyunsaturated chains. The lipid influences are further explained in terms of matching of the bilayer spontaneous curvature to the curvature at the lipid/rhodopsin interface, as formulated by the Helfrich bending energy. These new findings guide current ideas as to how bilayer properties govern the conformational energetics of integral membrane proteins. Moreover, they yield knowledge of how membrane lipid-protein interactions involving acidic phospholipids such as PS and neutral polyunsaturated DHA chains are implicated in key biological functions such as vision.

The subcellular distribution of protein kinase Calpha, -epsilon, and -zeta isoforms during cardiac cell differentiation

There is little information on the molecular events that control the subcellular distribution of protein kinase C during cardiac cell differentiation. We examined protein kinase C activity and the subcellular distribution of representatives of the "classical," "novel," and "atypical" protein kinase C's in P19 murine teratoma cells induced to undergo differentiation into cardiac myocytes by the addition of dimethylsulfoxide to the medium (Grepin et al., Development 124, 2387-2395, 1997). Differentiation was assessed by the presence of striated myosin, a morphological marker for cardiac cells. Addition of dimethyl sulfoxide to the medium resulted in the appearance of striated myosin by 10 days postincubation. Immunolocalization and Western blot studies revealed that a significant proportion of protein kinase Calpha, -epsilon, and -zeta were associated with the particulate fraction in P19 cells prior to differentiation. Differentiation into cardiac cells resulted in a translocation of protein kinase C activity from the particulate fraction to cytosol and localization of most of protein kinase Calpha, -epsilon, and -zeta to the cytoplasmic compartment. The total cellular protein kinase C activity was unaltered during differentiation. The translocation of protein kinase C activity during differentiation of P19 cells into cardiac myocytes was associated with a decrease in the levels of cellular 1, 2-diacyl-sn-glycerol. The cellular levels of phosphatidylserine and phosphatidylinositol did not change during differentiation. Addition of 1,2-dioctanoyl-sn-glycerol, a cell-permeant 1, 2-diacyl-sn-glycerol analog, reversed the differentiation-induced switch in the relative distribution of protein kinase C activity and dramatically increased the association of protein kinase Calpha with the particulate fraction. Addition of 1,2-dioctanoyl-sn-glycerol did not reverse the pattern of distribution for protein kinase Cepsilon or -zeta. The results indicate that protein kinase C activity and protein kinase Calpha, -epsilon and -zeta isoforms are redistributed from the particulate to the cytosolic fraction during differentiation of P19 cells into cardiomyocytes. The mechanism for the redistribution of protein kinase Calpha may be related to the reduction in the cellular 1,2-diacyl-sn-glycerol levels that accompany differentiation.

Lipid polymorphism and protein-lipid interactions

Non-lamellar-forming lipids play an important role in determining the physical properties of membranes. They affect the activity of membrane proteins and peptides. In addition, peptides which lyse membranes as well as those which promote membrane fusion facilitate the formation of non-lamellar phases, either micelles, cubic or hexagonal phases. The relationship of these diverse effects on membrane curvature is discussed in relation to the function of certain peptides and proteins. Specific examples of ionophoric peptides, cytotoxic peptides and viral fusion peptides are given. In addition, we compare the modulation of the rate of photoisomerisation of an integral membrane protein, rhodopsin, by non-lamellar-forming lipids with the effects of these lipids on an amphitropic protein, protein kinase C. Among these diverse systems it is frequently observed that the modulation of biological activity can be described in terms of the effect of the peptide or protein on the relative stability of lamellar and non-lamellar structures.

Influence of membrane-spanning alpha-helical peptides on the phase behavior of the dioleoylphosphatidylcholine/water system

The effect of solubilized hydrophobic peptides on the phase behavior of dioleoylphosphatidylcholine (DOPC)/water system was studied by 2H- and 31P-NMR spectroscopy and by x-ray diffraction, and partial phase diagrams were constructed. The utilized peptides were HCO-AWW(LA)5WWA-NHCH2CH2OH (WALP16), which is an artificial peptide designed to resemble a transmembrane part of a membrane protein; and VEYAGIALFFVAAVLTLWSMLQYLSAAR (Pgs peptide E), a peptide that is identical to one of the putative transmembrane segments of the membrane-associated protein phosphatidylglycerophosphate synthase (Pgs) in Escherichia coli. Circular dichroism spectroscopy suggests that both peptides are mostly alpha-helical in DOPC vesicles. The most striking features in the phase diagram of the WALP16/DOPC/water system are 1) a single lamellar liquid crystalline (L alpha) phase forms only at very low peptide concentrations. 2) At low water content and above a peptide/lipid molar ratio of approximately 1:75 a reversed hexagonal liquid crystalline (H[II]) phase coexists with an L alpha phase, while in excess water this phase forms at a peptide/lipid molar ratio of approximately 1:25. 3) At peptide/lipid ratios > or =1:6 a single H(II) phase is stable. Also, the Pgs peptide E strongly affects the phase behavior, and a single L alpha phase is only found at low peptide concentrations (peptide/lipid molar ratios <1:50), and water concentrations <45% (w/w). Higher peptide content results in coexistence of L alpha and isotropic phases. Generally, the fraction of the isotropic phase increases with increasing temperature and water concentration, and at 80% (w/w) water content only a single isotropic phase is stable at 55 degrees C. Thus, both peptides were found to be able to induce nonlamellar phases, although different in structure, in the DOPC/water system. The phase transitions, the extensions of the one-phase regions, and the phase structures observed for the two systems are discussed in terms of the molecular structure of the two peptides and the matching between the hydrophobic lengths of the peptides and the bilayer thickness of DOPC.

Human and Saccharomyces cerevisiae dolichol phosphate mannose synthases represent two classes of the enzyme, but both function in Schizosaccharomyces pombe

Dolichol phosphate mannose (Dol-P-Man), formed upon transfer of Man from GDPMan to Dol-P, is a mannosyl donor in pathways leading to N-glycosylation, glycosyl phosphatidylinositol membrane anchoring, and O-mannosylation of protein. Dol-P-Man synthase is an essential protein in Saccharomyces cerevisiae. We have cloned cDNAs encoding human and Schizosaccharomyces pombe proteins that resemble S. cerevisiae Dol-P-Man synthase. Disruption of the gene for the S. pombe Dol-P-Man synthase homolog, dpm1(+), is lethal. The known Dol-P-Man synthase sequences can be divided into two classes. One contains the S. cerevisiae, Ustilago maydis, and Trypanosoma brucei enzymes, which have a COOH-terminal hydrophobic domain, and the other contains the human, S. pombe, and Caenorhabditis synthases, which lack a hydrophobic COOH-terminal domain. The two classes of synthase are functionally equivalent, because S. cerevisiae DPM1 and its human counterpart both complement the lethal null mutation in S. pombe dpm1(+). The findings that Dol-P-Man synthase is essential in yeast and that the Ustilago and Trypanosoma synthases are in a different class from the human enzyme raise the possibility that Dol-P-Man synthase could be exploited as a target for inhibitors of pathogenic eukaryotic microbes.

Biophysical and functional consequences of receptor-mediated nerve fiber transformation

Stimulation of the nervous system by substance P, a G protein-coupled receptor, and subsequent receptor internalization causes dendrites to change their shape from homogeneous cylinders to a heterogeneous string of swollen varicosities (beads) connected by thin segments. In this paper we have analyzed this phenomenon and propose quantitative mechanisms to explain this type of physical shape transformation. We developed a mathematical solution to describe the relationship between the initial radius of a cylindrical nerve fiber and the average radii of the subsequently created varicosities and connecting segments, as well as the periodicity of the varicosities along the nerve fiber. Theoretical predictions are in good agreement with our own and published experimental data from dorsal root ganglion neurons, spinal cord, and brain. Modeling the electrical properties of these beaded fibers has led to an understanding of the functional biophysical consequences of nerve fiber transformation. Several hypotheses for how this shape transformation can be used to process information within the nervous system have been put forth.

The role of the lipid matrix in the biosynthesis of dolichyl-linked oligosaccharides

The enzymes in the dolichol pathway are membrane-proteins that utilize a combination of hydrophilic and extremely hydrophobic substrates. The enzymes in this pathway that have been purified and characterized to any extent have either been shown to be stabilized by mixed phospholipid/detergent micelles, or else require a lipid matrix for catalytic activity. Further understanding of the mechanisms of these essential enzymes may require developing methods for the reconstitution of the glycosyltransferases and their hydrophobic substrates in appropriate lipid matrices.

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.

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.

Effect of propensity of hexagonal II phase formation on the activity of mitochondrial ubiquinol-cytochrome c reductase and H(+)-ATPase

The propensity of hexagonal II phase formation plays an important role in the activity of mitochondrial ubiquinol-cytochrome c reductase or H(+)-ATPase. The respiratory control ratio of reconstituted ubiquinol-cytochrome c reductase or the ATP-induced membrane potential of reconstituted H(+)-ATPase became higher as the non-bilayer phospholipid phosphatidylethanolamine content of proteoliposomes increased. The highest respiratory control ratio or ATP-induced membrane potential was obtained in the case of 60-80% phosphatidylethanolamine-containing proteoliposomes. Dioleoylphosphatidylethanolamine could significantly enhance the respiratory control ratio of ubiquinol-cytochrome c reductase and ATP-induced membrane potential of H(+)-ATPase, while no obvious change could be observed when dielaidoylphosphatidylethanolamine was used. The bilayer to hexagonal II phase transition temperature of ubiquinol-cytochrome c reductase-containing proteoliposomes reconstituted with phosphatidylcholine+phosphatidylethanolamine increases with decreasing content of phosphatidylethanolamine. Several additives such as the bilayer stabilizers, cholesterol 3-sulfate and carbobenzoxy-D-Phe-L-PheGly, or hexagonal II phase-forming promoters, such as diolein or eicosane, can decrease or increase the activity of these two enzyme complexes.

Membrane thickness and molecular ordering in Acholeplasma laidlawii strain A studied by 2H NMR spectroscopy

Since Acholeplasma laidlawii can be restricted to incorporating fatty acids from the growth medium into its membrane lipids, it is possible to study the effects of the length of the acyl chains on the properties of the membrane of the organism. A. laidlawii strain A-EF22 was grown with mixtures of one perdeuterated saturated fatty acid and one monounsaturated fatty acid. The average length (<Cn>) of the acyl chains in the membrane lipids varied from 14.6 to 19.9, and the degree of unsaturation ranged from 21 to 79 mol %. 2H nuclear magnetic resonance (NMR) spectra were recorded on whole cells, on intact membranes, and on lipids extracted from these membranes. It was found that the NMR spectra for all three cases were very similar, yielding deuterium quadrupolar splittings typical for the lamellar liquid-crystalline phase (L alpha) found in model membrane systems. The use of a perdeuterated acyl chain as a reporter molecule allowed for the calculation of order parameters averaged over the entire system. These measurements yielded a wide range of average order parameters varying from 0.136 to 0.186 for the membranes and from 0.137 to 0.181 for the extracted lipids. From the order parameters the average acyl chain length can be calculated, which is related to the average membrane thickness. This value ranged from 23.2 to 30.6 A. When either the order or the membrane thickness of the intact membranes was compared to that of the extracted lipids, only slight or even undetectable differences were found. This implies that the proteins associated with the membranes do not have any large effect on the overall packing of the membrane lipids, even though the membrane thickness varied by approximately 8 A over the series studied. A decrease in the ordering of the acyl chains was observed when the length of the acyl chains incorporated from the growth medium was increased in either the membranes or the extracted lipids. This decrease correlated with the decrease in the fraction of monoglucosyldiacylglycerol (MGlcDAG) found in the membrane. Since both the average order and the membrane thickness varied, it is proposed that by changing the mole fraction of MGlcDAG the organism regulates either the membrane curvature energy or the permeability, both of which are related to lipid packing in the bilayer.

The temperature-composition phase diagram and mesophase structure characterization of monopentadecenoin in water

The temperature-composition phase diagram of monopentadecenoin, a monoacylglycerol with a cis monounsaturated fatty acid 15 carbon atoms long (C15:1c10) in water was constructed using x-ray diffraction. Low- and wide-angle diffraction patterns were collected from samples of fixed hydration as a function of temperature in the heating direction on x-ray-sensitive film. The temperature and hydration ranges investigated were 0-104 degrees C and 0-60% (w/w) water, respectively. The phases identified in the system include the lamellar crystalline phase, the lamellar liquid crystalline phase, the fluid isotropic phase, and two inverted cubic phases belonging to space groups la3d (Q230) and Pn3m (Q244). Particular attention has been devoted to the issues of phase equilibrium, phase boundary verification, and structure characterization. The phase diagrams of monopentadecenoin, monomyristolein (C14:1c9), and monoolein (C18:1c9) are compared, and the impact of molecular structure on mesophase stability and structure is discussed.

Evidence that binding of CTP:phosphocholine cytidylyltransferase to membranes in rat hepatocytes is modulated by the ratio of bilayer- to non-bilayer-forming lipids

The mechanism by which phospholipase C (PLC) digestion of cultured cells mediates binding of CTP:phosphocholine cytidylyltransferase to cellular membranes was investigated. Incubation of choline-depleted rat hepatocytes with PLC caused a translocation of enzyme from cytosol to membranes concomitant with a decrease in the concentration of phosphatidylcholine with no effect on the concentration of other phospholipids. Removal of PLC and supplementation with choline restored the amount of phosphatidylcholine in the cells and translocated cytidylyltransferase to the cytosol. However, when phosphatidylcholine levels were decreased by incubation with phospholipase A2 (PLA2), there was no significant redistribution of cytidylyltransferase activity. With PLA2 the concentration of phosphatidylethanolamine, as well as of phosphatidylcholine, was significantly decreased. Since PLC, but not phospholipase A2, raised the cellular concentration of diacylglycerol, possibly diacylglycerol mediated the binding of cytidylyltransferase to membranes. This possibility was examined, but is unlikely, since addition of lysophosphatidylcholine to PLC-treated cells restored the concentration of phosphatidylcholine and released cytidylyltransferase into the cytosol, but did not lower diacylglycerol levels to normal values. Studies in vitro, incubations of cells with choline analogues and a survey of the literature suggested that the over-riding common factor in regulation of cytidylyltransferase binding to membranes may be the ratio of bilayer to non-bilayer lipids in that membrane.

Lipid headgroup and acyl chain composition modulate the MI-MII equilibrium of rhodopsin in recombinant membranes

A current paradigm for visual function centers on the metarhodopsin I (MI) to metarhodopsin II (MII) conformational transition as the trigger for an intracellular enzyme cascade leading to excitation of the retinal rod. We investigated the influences of the membrane lipid composition on this key triggering event in visual signal transduction using flash photolysis techniques. Bovine rhodopsin was combined with various phospholipids to form membrane recombinants in which the lipid acyl chain composition was held constant at that of egg phosphatidylcholine (PC), while the identity of the lipid headgroups was varied. The ratio of MII/MI produced in these recombinants by an actinic flash at 28 degrees C was studied as a function of pH. The results were compared to the photochemical function observed for rhodopsin in native retinal rod outer segment (ROS) membranes, in total native ROS lipid recombinants, and in dimyristoylphosphatidylcholine (DMPC) recombinants. In membrane recombinants incorporating lipids derived from egg PC, as well as in the total ROS lipids control and the native ROS disk membranes, MI and MII were found to coexist in a pH-dependent, acid-base equilibrium on the millisecond time scale. The recombinants of rhodopsin with egg PC, either alone or in combination with egg PC-derived phosphatidylethanolamine (PE) or phosphatidylserine (PS), exhibited substantially reduced photochemical activity at pH 7.0. However, all recombinants comprising phospholipids with unsaturated acyl chains were capable of full native-like MII production at pH 5.0, confirming previous results [Gibson, N.J.. & Brown, M.F. (1990) Biochem. Biophys. Res. Commun. 169, 1028-1034]. It follows that energetic constraints on the MI and MII states imposed by egg PC-derived acyl chains can be offset by increased activity of H+ ions. The data reveal that the major effect of the membrane lipid composition is to alter the apparent pK for the MI-MII conformational equilibrium of rhodopsin [Gibson, N.J., & Brown, M.F. (1991) Biochem. Biophys. Res. Commun. 176, 915-921]. Recombinants containing only phosphocholine headgroups exhibited the lowest apparent pK values, whereas the presence of either 50 mol % PE or 15 mol % PS increased the apparent pK. The inability to obtain full native-like function in recombinants having egg PC-derived chains and a native-like headgroup composition indicates a significant role of the polyunsaturated docosahexaenoic acid (DHA) chains (22:6 omega 3) of the native retinal rod membrane lipids. Temperature studies of the MI-MII transition enabled an investigation of lipid influences on the thermodynamic parameters of a membrane protein conformational change linked directly to function.(ABSTRACT TRUNCATED AT 400 WORDS)

Membrane lipid influences on the energetics of the metarhodopsin I and metarhodopsin II conformational states of rhodopsin probed by flash photolysis

We have investigated the relationship between rhodopsin photochemical function and the retinal rod outer segment (ROS) disk membrane lipid composition using flash photolysis techniques. Bovine rhodopsin was combined with various phospholipids to form recombinant membrane vesicles, in which the lipid acyl chain composition was maintained at that of egg phosphatidylcholine (PC), while the nature of the headgroups was varied. The ratio of metarhodopsin II (MII)/metarhodopsin I (MI) in these recombinants produced by an actinic flash was investigated as a function of pH, and compared with the photochemical activity observed for rhodopsin in native ROS membranes and dimyristoylphosphatidylcholine recombinants. In recombinants made with lipids derived from egg PC, as well as in native ROS membranes, MI and MII were found to be present in a pH-dependent, acid-base equilibrium on the millisecond timescale. The recombinants made with phospholipids containing unsaturated acyl chains were capable of full native-like MII production, but each demonstrated a titration curve with a different pK. In addition, some of the recombinants exhibited apparent deviations from the Henderson-Hasselbalch curve shape. The presence of either phosphatidylethanolamine (PE) or phosphatidylserine (PS) headgroups appeared to increase the amount of MII produced. This may result from alteration of the curvature free energy, in the case of PE, and from the influence of the membrane surface potential in the case of PS. An investigation of the effects of temperature on the MI-MII transition in native ROS membranes and the recombinants was also carried out.(ABSTRACT TRUNCATED AT 250 WORDS)

Specific lipids enhance the activity of UDP-GlcNAc: dolichol phosphate GlcNAc-1-phosphate transferase in rat liver endoplasmic reticulum membrane vesicles

The rate of the reaction catalyzed by UDP-N-acetylglucosamine (GlcNAc):dolichol phosphate GlcNAc-1-phosphate transferase in rat liver endoplasmic reticulum vesicles was shown to be influenced by particular lipids. Utilizing in vitro assay conditions where the membrane vesicles retained latency of glucose-6-phosphatase activity, the addition of phosphatidylethanolamine, cardiolipin, or monogalactosyldiglyceride resulted in severalfold increases in the rate of dolichol pyrophosphate N-acetylglucosamine synthesis. Other phospholipids were not stimulatory. These rates were dependent on the concentrations of the exogenous lipids and of the substrate dolichol phosphate. In the presence of cardiolipin, the membrane-bound enzyme became more susceptible to inactivation by protease K and to inhibition by tunicamycin. Titration of cardiolipin-containing endoplasmic reticulum vesicles with adriamycin indicated that the majority of the cardiolipin was exposed on the outer surface. These results suggest that the particular lipids altered membrane structure in a way that allowed further access of the enzyme to substrate, inhibitor, and other molecules. Lipids observed in these studies to be stimulatory are known to exist in the macromolecular hexagonal phase and may therefore be affecting the GlcNAc-1-phosphate transferase by locally disrupting the bilayer structure of the membrane. As other dolichol-utilizing enzymes have been previously observed by other investigators to be similarly influenced by such lipids, the effects may be common to enzymes of the dolichol cycle.

Incorporation and activation of a membrane-bound enzyme in bilayers of liposomes

The effects of lipid composition and fluidity of lipid bilayers on incorporation and activation of membrane-bound D-fructose dehydrogenase are described in this study. The incorporation of the enzyme into bilayers of small unilamellar vesicles (SUV) made of several phospholipids resulted in enzyme activation with magnitudes higher than that observed in the presence of Triton X-100, indicating that this higher activation is due to lipid-protein interaction. The activity was highest in the presence of SUV formed by the addition of 10% DL-alpha-dipalmitoylphosphatidylethanolamine to L-alpha-dimyristoylphosphatidylcholine, which resulted in eightfold higher activation compared with that of the enzyme in its free state. This activation did not appear to be due to the degree of incorporation of the enzyme, indicating that incorporation is distinct from the activation event. Thus, it is probably the lipid environment that leads to higher activation of the enzyme. A break in the Arrhenius plot of the activity of the membrane-bound enzyme at temperatures close to the phase transition of the phospholipid implies that changes in the physical state of the lipid bilayer influence the enzyme activity. Furthermore, immobilization of D-fructose dehydrogenase, previously adsorbed to SUV, on urethane prepolymer also resulted in about eightfold higher activation than that of the free enzyme.

Is the mechanism of general anesthesia related to lipid membrane spontaneous curvature?

Lipid bilayers in biomembranes may exist in a state of elastic curvature stress which may couple to the conformation of integral membrane proteins in a logical, and energetically significant, fashion. Many biomembranes contain sufficiently large fractions of nonlamellar-prone lipids to have monolayers under substantial curvature stress. Although very few experiments have been performed that can be used to correlate protein activity with curvature stress, the literature does contain a small number of studies that indicate that some protein function is nonspecifically modulated by the amounts of nonlamellar-prone lipid in the imbedding bilayers. The spontaneous curvature, is altered by the presence of anesthetics in physiologically relevant concentrations. This leads us to suggest that anesthetic action may be coupled to protein function via alteration of the tensions leading to the spontaneous curvature of biomembrane layers. The spontaneous curvature is also sufficiently sensitive to pressure that a mechanism for the pressure reversal of anesthesia follows if the effects of pressure are to counter changes in membrane lateral tension induced by anesthetics. It is emphasized that many more experimental data must be acquired to determine whether the ideas presented in this paper have validity. In particular, there is a need for data on the effects of different anesthetics and pressure on the spontaneous curvature and, more generally, lipid monolayer lateral tensions. Most importantly, experiments must be performed to investigate whether protein function correlates with these quantities.

Role of phosphatidylserine in the MI-MII equilibrium of rhodopsin

Bovine rhodopsin was recombined with various phospholipids in which the lipid acyl chain composition was held constant at that of egg phosphatidylcholine (PC), while the identity of the headgroups was varied. The ratio of MII/MI produced in the recombinant membrane vesicles by an actinic flash was studied as a function of pH, and compared to the photochemical activity observed for rhodopsin in native ROS membranes. MI and MII were found to coexist in a pH-dependent, acid-base equilibrium on the millisecond timescale. Recombinants made with phospholipids containing unsaturated acyl chains were capable of full native-like MII production, but demonstrated titration curves with different pK values. The presence of phosphoethanolamine or phosphoserine headgroups increased the amount of MII produced. In the case of phosphatidylserine this may result from alteration of the membrane surface potential, leading to an increase in the local H+ activity. The results indicate that the Gibbs free energies of the MI and MII conformational states are influenced by the membrane bilayer environment, suggesting a possible role of lipids in visual excitation.

Nonbilayer phases of membrane lipids

Numerous liquid crystalline biomembrane lipids are known to exhibit non-lamellar phases characterized by curvature of their component lipid monolayers. An understanding of the phase stability of these systems begins with analysis of the energy of bending the monolayers, the interactions which lead to the bending energy, and the geometrical constraints which lead to competing energy terms which arise when the monolayers are bent and packed onto lattices with different structures. Diffraction and other techniques suitable for probing lipid phase structure are described. A phenomenological model is reviewed which successfully explains many of the qualitative features of lipid mesomorphic phase behavior. A key result of this model is that lipid bilayer compositions which are close to the non-lamellar phase boundaries of their phase diagrams are characterized by a frustrated elastic stress which may modulate the activity of imbedded membrane proteins and which may provide a rationale for the prevalence of non-lamellar-tending lipid species in biomembrane bilayers. Areas in need of future research are discussed.

Triggering of mannosyltransferase activity in inner mitochondrial membranes by dolichyl-monophosphate incorporation mediated through phospholipids or fatty acids

The activity of GDPmannose:dolichyl monophosphate mannosyltransferase in inner mitochondrial membranes can be triggered by dolichyl-monophosphate incorporation mediated through phospholipids or fatty acids. The efficiency of this incorporation and the efficiency of the enzyme activity are not equivalent. Among a variety of amphiphiles which were tested, the highest mannosyltransferase activity was obtained with the mixture of lipids extracted from the outer mitochondrial membranes. The results presented here appear consistent only with a mechanism involving collisional contacts of the phospholipid vesicles and fusion with the membranes. ESR spectroscopy confirms that (a) the incorporation process is followed by solubilization of dolichyl monophosphate molecules in the lipid phase and (b) the general organization of the inner mitochondrial membranes is not perturbed by the addition of dolichyl monophosphate.

Bilayer membrane destabilization induced by dolichylphosphate

Small vesicles containing the fluorescent probe calcein were used to investigate the effect of dolichyl phosphate (Dol-P) on phospholipid bilayer stability. In the absence of Dol-P, phospholipid vesicles retained the fluorescent probe upon the addition of divalent cations. Small vesicles containing Dol-P, however, exhibited calcein leakage when incubated in the presence of divalent cations. This effect was observed in liposomes composed of a mixture of phosphatidylethanolamine (PE), phosphatidylcholine (PC) and Dol-P, but not in PC/Dol-P liposomes. The rate of calcein leakage was proportional to divalent cation concentration and to temperature, but was independent of vesicle concentration. These results demonstrate that Dol-P has significant effects on the stability of PE containing phospholipid bilayers. Vesicle leakage was also promoted by the addition of rat liver Dol-P-mannose synthase (EC 2.4.1.83) to intact PE/PC/Dol-P vesicles. Enzyme induced leakage from phospholipid vesicles required the presence of both unsaturated PE and Dol-P. The phospholipid composition of leaky vesicles could be correlated with the lipid matrix required for maximal transferase activity of the rat liver synthase. The destabilizing effects of Dol-P on phospholipid bilayers may therefore be involved in the translocation of activated sugars across biological membranes.

Role of the stereochemistry of the hydroxyl group of cholesterol and the formation of nonbilayer structures in phosphatidylethanolamines

The phase behavior of mixtures of cholesterol or epicholesterol with phosphatidylethanolamine was studied by differential scanning calorimetry and by X-ray diffraction. Discrete domains of cholesterol are detected by X-ray diffraction in the L alpha phase of phosphatidylethanolamine from egg yolk and synthetic dielaidoylphosphatidylethanolamine beginning at mole fractions of 0.35-0.4 cholesterol. Separate domains of crystalline epicholesterol can also be detected in the L alpha phase of dielaidoylphosphatidylethanolamine by X-ray diffraction at as little as 0.16 mole fraction of epicholesterol. This is a result of poor miscibility of the epicholesterol with dielaidoylphosphatidylethanolamine. Epicholesterol does not alter the L beta----L alpha transition or bilayer spacing. Epicholesterol also has little effect on the diameter of the cylinders in the hexagonal phase. Formation of the inverted hexagonal phase is facilitated by addition of small amounts of cholesterol (mole fraction less than 0.2) in both egg phosphatidylethanolamine and dielaidoylphosphatidylethanolamine. However, at higher mole fractions of cholesterol, the stability of the liquid-crystalline phase is found to increase markedly for dielaidoylphosphatidylethanolamine but not for egg phosphatidylethanolamine, indicating the importance of the structure of the acyl chains in controlling the relative stability of the lamellar and nonlamellar phases in these systems. In contrast to cholesterol, epicholesterol markedly lowers the L alpha----HII phase transition temperature at low mole fraction of sterol. This result demonstrates the importance of the orientation and motional properties of an additive in determining the L alpha----HII transition temperature.