Showing papers on "Membrane lipids published in 1998"

Functions of lipid rafts in biological membranes.

Abstract: ▪ Abstract Recent studies showing that detergent-resistant membrane fragments can be isolated from cells suggest that biological membranes are not always in a liquid-crystalline phase. Instead, sph...

Lipid Domain Structure of the Plasma Membrane Revealed by Patching of Membrane Components

Abstract: Lateral assemblies of glycolipids and cholesterol, “rafts,” have been implicated to play a role in cellular processes like membrane sorting, signal transduction, and cell adhesion. We studied the structure of raft domains in the plasma membrane of non-polarized cells. Overexpressed plasma membrane markers were evenly distributed in the plasma membrane. We compared the patching behavior of pairs of raft markers (defined by insolubility in Triton X-100) with pairs of raft/non-raft markers. For this purpose we cross-linked glycosyl-phosphatidylinositol (GPI)-anchored proteins placental alkaline phosphatase (PLAP), Thy-1, influenza virus hemagglutinin (HA), and the raft lipid ganglioside GM1 using antibodies and/or cholera toxin. The patches of these raft markers overlapped extensively in BHK cells as well as in Jurkat T–lymphoma cells. Importantly, patches of GPI-anchored PLAP accumulated src-like protein tyrosine kinase fyn, which is thought to be anchored in the cytoplasmic leaflet of raft domains. In contrast patched raft components and patches of transferrin receptor as a non-raft marker were sharply separated. Taken together, our data strongly suggest that coalescence of cross-linked raft elements is mediated by their common lipid environments, whereas separation of raft and non-raft patches is caused by the immiscibility of different lipid phases. This view is supported by the finding that cholesterol depletion abrogated segregation. Our results are consistent with the view that raft domains in the plasma membrane of non-polarized cells are normally small and highly dispersed but that raft size can be modulated by oligomerization of raft components.

Structure and origin of ordered lipid domains in biological membranes

Abstract: The clearest function of membrane lipids is to form amphipathic bilayers that surround cells and organelles and block leakage of hydrophilic compounds while housing membrane proteins. However, the wide variety of lipids observed in biological membranes would not be required for a simple barrier function. Phospholipids alone display a variety of headgroup and acyl chain structures, and eukaryotic cell membranes often contain sphingolipids and sterols as well. Functional consequences of this lipid heterogeneity are starting to emerge. One such consequence is the possibility of nonrandom mixing in the bilayer and the formation of lipid microdomains. It is clear that microdomains can form in artificial bilayers [55]. However, despite much interest in the subject, convincing evidence that lipids can cluster in cell membranes has been slow to emerge. This review summarizes evidence that one type of microdomain may exist in cell membranes. Most of this evidence has come from studies of membrane fragments that are insoluble in cold non-ionic detergents such as Triton X-100. These detergent-resistant membranes (DRMs) are rich in cholesterol and sphingolipids, and may exist in membranes in the liquid-ordered (l o) phase or a phase with similar properties. These studies may provide some of the first evidence for phase separation in biological membranes. Readers are also referred to our recent reviews of DRMs and the ordered-domain model [5, 6], and to two other insightful reviews of these domains [8, 82]. Increasing evidence suggests that another type of domain, formed by electrostatic interactions between membrane-associated components, may exist in membranes [44]. One intriguing example is the ability of a membrane-associated positively charged peptide derived from the MARCKS protein to organize domains rich in phosphatidylinositol bisphosphate [19]. Though potentially very important, formation of these domains will not be covered here.

Sphingolipid organization in biomembranes: what physical studies of model membranes reveal.

Abstract: Recent cell biological studies suggest that sphingolipids and cholesterol may cluster in biomembranes to form raft-like microdomains. Such lipid domains are postulated to function as platforms involved in the lateral sorting of certain proteins during their trafficking within cells as well as during signal transduction events. Here, the physical interactions that occur between cholesterol and sphingolipids in model membrane systems are discussed within the context of microdomain formation. A model is presented in which the role of cholesterol is refined compared to earlier models.

Distribution of a glycosylphosphatidylinositol-anchored protein at the apical surface of MDCK cells examined at a resolution of <100 A using imaging fluorescence resonance energy transfer.

Abstract: Membrane microdomains (“lipid rafts”) enriched in glycosylphosphatidylinositol (GPI)-anchored proteins, glycosphingolipids, and cholesterol have been implicated in events ranging from membrane trafficking to signal transduction. Although there is biochemical evidence for such membrane microdomains, they have not been visualized by light or electron microscopy. To probe for microdomains enriched in GPI- anchored proteins in intact cell membranes, we used a novel form of digital microscopy, imaging fluorescence resonance energy transfer (FRET), which extends the resolution of fluorescence microscopy to the molecular level (<100 A). We detected significant energy transfer between donor- and acceptor-labeled antibodies against the GPI-anchored protein 5′ nucleotidase (5′ NT) at the apical membrane of MDCK cells. The efficiency of energy transfer correlated strongly with the surface density of the acceptor-labeled antibody. The FRET data conformed to theoretical predictions for two-dimensional FRET between randomly distributed molecules and were inconsistent with a model in which 5′ NT is constitutively clustered. Though we cannot completely exclude the possibility that some 5′ NT is in clusters, the data imply that most 5′ NT molecules are randomly distributed across the apical surface of MDCK cells. These findings constrain current models for lipid rafts and the membrane organization of GPI-anchored proteins.

Lipids in photosynthesis: structure, function and genetics.

Abstract: Preface. 1. Lipids in Photosynthesis: An Overview N. Murata, P.-A. Siegenthaler. 2. Structure, Distribution and Biosynthesis of Glycerolipids from Higher Plant Chloroplasts J. Joyard, et al. 3. Membrane Lipids in Algae J.L. Harwood. 4. Membrane Lipids in Cyanobacteria H. Wada, N. Murata. 5. Membrane Lipids in Anoxygenic Photosynthetic Bacteria C. Benning. 6. The Physical Properties of Thylakoid Membrane Lipids and Their Relation to Photosynthesis W.P. Williams. 7. Molecular Organization of Acyl Lipids in Photosynthetic Membranes of Higher Plants P.-A. Siegenthaler. 8. Role of Acyl Lipids in the Function of Photosynthetic Membranes in Higher Plants P.-A. Siegenthaler, A. Tremolieres. 9. Reconstitution of Photosynthetic Structures and Activities with Lipids A. Tremolieres, P.-A. Siegenthaler. 10. Lipid-Protein Interactions in Chloroplast Protein Import B. de Kruijff, et al. 11. Development of Thylakoid Membranes with Respect to Lipids E. Selstam. 12. Triglycerides as Products of Photosynthesis. Genetic Engineering, Fatty Acid Composition and Structure of Triglycerides D. Facciotti, V. Knauf. 13. Genetic Engineering of the Unsaturation of Membrane Glycerolipid: Effects on the Ability of the Photosynthetic Machinery to Tolerate Temperature Stress Z. Gombos, N. Murata. 14. A Genetic Approach to Investigating Membrane Lipid Structure and Photosynthetic Function P. Vijayan, et al. 15. Involvement of Chloroplast Lipids in the Reaction of Plants Submitted to Stress J.L. Harwood. Index.

The essence of being extremophilic: the role of the unique archaeal membrane lipids.

Abstract: In extreme environments, mainly Archaea are encountered. The archaeal cytoplasmic membrane contains unique ether lipids that cannot easily be degraded, are temperature- and mechanically resistant, and highly salt tolerant. Moreover, thermophilic and extreme acidophilic Archaea possess membrane-spanning tetraether lipids that form a rigid monolayer membrane which is nearly impermeable to ions and protons. These properties make the archaeal lipid membranes more suitable for life and survival in extreme environments than the ester-type bilayer lipids of Bacteria or Eukarya.

Acylation of Escherichia coli Hemolysin: A Unique Protein Lipidation Mechanism Underlying Toxin Function

Abstract: The pore-forming hemolysin (HlyA) of Escherichia coli represents a unique class of bacterial toxins that require a posttranslational modification for activity. The inactive protoxin pro-HlyA is activated intracellularly by amide linkage of fatty acids to two internal lysine residues 126 amino acids apart, directed by the cosynthesized HlyC protein with acyl carrier protein as the fatty acid donor. This action distinguishes HlyC from all bacterial acyltransferases such as the lipid A, lux-specific, and nodulation acyltransferases, and from eukaryotic transferases such as N-myristoyl transferases, prenyltransferases, and thioester palmitoyltransferases. Most lipids directly attached to proteins may be classed as N-terminal amide-linked and internal ester-linked acyl groups and C-terminal ether-linked isoprenoid groups. The acylation of HlyA and related toxins does not equate to these but does appear related to a small number of eukaryotic proteins that include inflammatory cytokines and mitogenic and cholinergic receptors. While the location and structure of lipid moieties on proteins vary, there are common effects on membrane affinity and/or protein-protein interactions. Despite being acylated at two residues, HlyA does not possess a “double-anchor” motif and does not have an electrostatic switch, although its dependence on calcium binding for activity suggests that the calcium-myristoyl switch may have relevance. The acyl chains on HlyA may provide anchorage points onto the surface of the host cell lipid bilayer. These could then enhance protein-protein interactions either between HlyA and components of a host signal transduction pathway to influence cytokine production or between HlyA monomers to bring about oligomerization during pore formation.

Assembly mechanism of the oligomeric streptolysin O pore: the early membrane lesion is lined by a free edge of the lipid membrane and is extended gradually during oligomerization

Abstract: Streptolysin O (SLO) is a bacterial exotoxin that binds to cell membranes containing cholesterol and then oligomerizes to form large pores. Along with rings, arc-shaped oligomers form on membranes. It has been suggested that each arc represents an incompletely assembled oligomer and constitutes a functional pore, faced on the opposite side by a free edge of the lipid membrane. We sought functional evidence in support of this idea by using an oligomerization-deficient, non-lytic mutant of SLO. This protein, which was created by chemical modification of a single mutant cysteine (T250C) with N-(iodoacetaminoethyl)-1-naphthylamine-5-sulfonic acid, formed hybrid oligomers with active SLO on membranes. However, incorporation of the modified T250C mutant inhibited subsequent oligomerization, so that the hybrid oligomers were reduced in size. These appeared as typical arc lesions in the electron microscope. They formed pores that permitted passage of NaCl and calcein but restricted permeation of large dextran molecules. The data indicate that the SLO pore is formed gradually during oligomerization, implying that pores lined by protein on one side and an edge of free lipid on the other may be created in the plasma membrane. Intentional manipulation of the pore size may extend the utility of SLO as a tool in cell biological experiments.

A new look at lipid-membrane structure in relation to drug research

Abstract: Lipid-bilayer membranes are key objects in drug research in relation to (i) interaction of drugs with membrane-bound receptors, (ii) drug targeting, penetration, and permeation of cell membranes, and (iii) use of liposomes in micro-encapsulation technologies for drug delivery. Rational design of new drugs and drug-delivery systems therefore requires insight into the physical properties of lipid-bilayer membranes. This mini-review provides a perspective on the current view of lipid-bilayer structure and dynamics based on information obtained from a variety of recent experimental and theoretical studies. Special attention is paid to trans-bilayer structure, lateral molecular organization of the lipid bilayer, lipid-mediated protein assembly, and lipid-bilayer permeability. It is argued that lipids play a major role in lipid membrane-organization and functionality.

Membrane Lipids in Algae

Abstract: Eukaryotic algae are a diverse group of organisms. Their lipid compositions have been less studied than those of higher plants but, nevertheless, we now have sufficient data to be able to make some broad generalizations. Algae contain many of the major lipids of plants, such as the glycosylglycerides and the usual phosphoglycerides. In addition, more unusual compounds such as the betaine lipids, chlorosulfolipids or various other sulfolipids may be major components of some species or orders.

Membrane Lipids in Cyanobacteria

Abstract: Cyanobacteria, a large group of microorganisms in the prokaryotic kingdom, perform oxygenic photosynthesis using two photosystems that resemble those in the chloroplasts of eukaryotic plants. Cyanobacteria contain three glycolipids, monogalactosyldiacylglycerol, digalactosyldiacylglycerol and sulfoquinovosyldiacylglycerol, and a phospholipid, phosphatidylglycerol, as major glycerolipids. The lipid composition of most cyanobacteria is similar to that of the inner envelope membranes and thylakoid membranes of the chloroplasts of higher plants, and it is different from that of the membranes of most bacteria, which contain phospholipids as major glycerolipids. Cyanobacteria can be classified into four groups with respect to the composition of the fatty acids of their glycerolipids. Since some strains, such as Synechocystis sp. PCC 6803 and Synechococcus sp. PCC 7942, take up exogenous DNA autonomously, they are naturally transformable. Thus, molecular biological techniques, for example, transformation and gene targeting, can easily be applied to these strains and they provide useful systems for studying the molecular aspects of the biosynthesis of lipids and fatty acids, as well as of the functions of membrane lipids in oxygenic photosynthesis. Furthermore, since cyanobacteria respond to changes in a variety of environmental conditions by altering their membrane lipids, they are also useful systems for studying the acclimation of the photosynthetic machinery to environmental factors such as temperature.

The fats of Escherichia coli during infancy and old age: regulation by global regulators, alarmones and lipid intermediates

Abstract: The fluidity and phase state of bacterial lipid bilayers commonly change in response to ambient environmental conditions to maintain the critical functions of the envelope as a semipermeable and selective boundary. A special, and intricate, set of alterations in membrane lipid metabolism is elicited by conditions causing growth arrest. Under such conditions, specific alterations in the membrane lipid-fatty acid composition are required for survival of the cell and, concurrently, the membrane lipids are suggested to serve as endogenous reserves providing carbon/energy for maintenance requirements. It appears that the global regulator FadR is required for both of these activities to be performed properly and that the FadR regulon is interconnected to the universal stress response of Escherichia coli. FadR, in conjunction with long-chain fatty acyl-CoA, long-chain acyl-ACP, ppGpp and cAMP, are key players in regulating the activities of enzymes and expression of genes involved in fatty acid and phospholipid metabolism in dividing and ageing E. coli cells.

Stress tolerance in the marsh plant Spartina patens: Impact of NaCl on growth and root plasma membrane lipid composition

Abstract: The C-4 salt marsh grass, Spartina patens, thrives in the upper portion of the marsh where soil salinities may be equal to coastal seawater. Spartina patens was grown in hydroponic culture in a greenhouse at 0, 340, and 510 mM NaCl, and measured for growth, tissue cation content, and root plasma membrane (PM) lipid composition. From 0 to 340 and 510 mM, the shoot growth decreased, but root growth was not affected. The Na + content increased in both shoots and roots when plants were grown in salt, while the shoots had a decreased K + content and the roots had a decreased Ca 2+ content. Spartina patens root plasma membrane was isolated with an aqueous polymer two-phase system. The purity of the plasma membrane was verified with cytochemical tests on membrane enzyme markers. Plasma membrane lipids were stable relative to the membrane protein content. Molar percentages of sterols (including free sterols) and phospholipid decreased with increasing salinity. However, glycolipid showed a statistically significant increase in the total lipid as salinity in the medium was increased from 0 to 510 mM. Even at a salinity of 510 mM, the plasma membrane sterol/phospholipid ratio was unaffected by NaCI. When the plants were grown in NaCI media, the plasma membrane had a decreased phosphatidylcholine (PC) and phosphatidylethanolamine (PE) content, but the PC/PE ratios were not affected. The plasma membrane molar percentage of sitosterol in total free sterol increased when plants were grown in salt media. The predominant membrane fatty acids were C11 and C14, and the major unsaturated one was C14:1. An increase in growth medium salinity resulted in a decreased root plasma membrane fluidity.

Comparison of human red cell lysis by hypochlorous and hypobromous acids: insights into the mechanism of lysis.

Abstract: Human red blood cells are lysed by the neutrophil-derived oxidant hypochlorous acid (HOCl), although the mechanism of lysis is unknown. Hypobromous acid (HOBr), a similarly reactive oxidant, lysed red cells approx. 10-fold faster than HOCl. Therefore we compared the effects of these oxidants on thiols, membrane lipids and proteins to determine which reactions are associated with lysis. There was no difference in the loss of reduced glutathione or membrane thiols with either oxidant, but HOBr reacted more readily with membrane lipids and proteins. Bromohydrin derivatives of phospholipids and cholesterol were seen at approx. one-tenth the level of oxidant than chlorohydrins were. However, these products were detected only with high concentrations of HOCl or HOBr, which caused instant haemolysis. Membrane protein modification occurred at much lower doses of oxidant and was more closely correlated with lysis. SDS/PAGE analysis showed that band 3, the anion transport protein, was lost at the lowest dose of HOBr and at the higher concentrations of HOCl. Labelling the red cells with eosin 5-maleimide, a fluorescent label for band 3, suggested possible clustering of this protein in oxidant-exposed cells. There was also irreversible cross-linking of all the major membrane proteins; this reaction occurred more readily with HOBr. The results indicate that membrane protein modification is the reaction responsible for HOCl-mediated lysis. These effects, and particularly cross-link formation, might result in clustering of band 3 and other membrane and cytoskeletal proteins to form haemolytic pores.

Variations of the Envelope Composition of Bacillus subtilis During Growth in Hyperosmotic Medium

Abstract: The envelope properties of B. subtilis cultures grown in LB and LBN hyperosmotic media (LB + 1.5 M NaCl) were compared. Since hypertonic cultures showed a Spo-phenotype, a Spo-mutant grown in LB was also analyzed. LBN cultures showed extensive filamentation and presented different sensitivities toward phage infection (φ29 and φ105), or antibiotics whose targets are at wall (lysozyme, penicillin G) or membrane level (polymyxin B, phosphonomycin). Results of the biochemical composition revealed that during hyperosmotic growth, the cell wall increased in thickness, and among the membrane lipids, glycolipid and cardiolipin increased in parallel with a decrease in phosphatidylglycerol. The fatty acid composition was also modified, and an increase in saturated straight chain with a decrease of saturated iso-branched fatty acids was observed. The increase of monounsaturated 18-1 (ω-9) fatty acid was probably related to the absence of sporulation observed in hypertonic media, since its increase has been shown to inhibit the KinA sensor of sporulation. The significance of the other wall and membrane composition variations (and hydrophobic surface properties) in relation to the osmotic adaptation are discussed.

Lipid-binding characteristics of the polybasic carboxy-terminal sequence of K-ras4B.

Abstract: We have examined the association with lipid vesicles of fluorescent lipidated peptides based on the farnesylated, polybasic carboxy-terminal region of mature K-ras4B, which functions physiologically as an autonomous plasma membrane-targeting motif. While the peptides bind to neutral lipid (phosphatidylcholine/phosphatidylethanolamine) vesicles with relatively low affinity, the vesicle-binding affinity increases exponentially as increasing amounts of anionic lipids are incorporated into the vesicle bilayers. Competitive vesicle-binding experiments reveal that the K-ras4B carboxy-terminal sequence accordingly discriminates strongly between lipid surfaces of differing surface charge, such that two lipid bilayers differing in anionic lipid content by 10 mol % will show a 45-fold preferential accumulation of the lipidated peptide in the more negatively charged surface. At the same time, the carboxyl-terminal region of K-ras4B exhibits no preferential binding to particular anionic lipids, including the polyanionic species phosphatidylinositol-4'-phosphate and phosphatidylinositol-4',5'-bisphosphate, beyond that predicted on the basis of surface-charge effects. The K-ras4B carboxyl-terminal sequence dissociates rapidly (with half-times of seconds or less) from lipid bilayers containing up to 40 mol % anionic lipid. These results suggest that the targeting of the mature K-ras4B carboxy-terminus to the plasma membrane, if it is based on interactions with plasma membrane lipids, is not mediated by a kinetic-trapping mechanism or by specific binding to particular anionic lipids but may rest on the sensitive surface potential-sensing function of this region of the protein.

Signal Sequence Recognition in Cotranslational Translocation by Protein Components of the Endoplasmic Reticulum Membrane

Abstract: We have investigated the role of membrane proteins and lipids during early phases of the cotranslational insertion of secretory proteins into the translocation channel of the endoplasmic reticulum (ER) membrane. We demonstrate that all steps, including the one during which signal sequence recognition occurs, can be reproduced with purified translocation components in detergent solution, in the absence of bulk lipids or a bilayer. Photocross-linking experiments with native membranes show that upon complete insertion into the channel signal sequences are both precisely positioned with respect to the protein components of the channel and contact lipids. Together, these results indicate that signal sequences are bound to a specific binding site at the interface between the channel and the surrounding lipids, and are recognized ultimately by protein–protein interactions. Our data also suggest that at least some signal sequences reach the binding site by transfer through the interior of the channel.

Enzymatic degradation of polar lipids in Vigna unguiculata leaves and influence of drought stress

Abstract: 14C-labelled polar lipids (monogalactosyl-diacylglycerol [MGDG], digalactosyl-diacylglycerol [DGDG], phosphatidylcholine [PC] and phosphatidylglycerol [PG]), purified from Vigna unguiculata leaves, were used as substrates to study the lipolytic activities of Vigna unguiculata leaf extracts. Analysis of the radioactive degradation products revealed the presence of at least three enzyme activities contributing to the hydrolysis of the four main leaf membrane lipids: Lipolytic acyl hydrolase (LAH) activities responsible for the deacylation of galactolipids and phospholipids. phospholipase D (PLD, EC 3.1.4.4) activity which gives rise to phosphatidic acid, and as suggested by the presence of diacylglycerols in minor quantities after phospholipid hydrolysis, phosphatidate phosphohydrolase (PAP, EC 3.1.3.4) and/or phospholipase C (PLC, EC 3.1.4.3.) activity. Under the conditions described in the present paper, the presence of phospholipase A (PLA 1 , EC 3.1.1.3 and PLA 2 , EC 3.1.1.4) activities remains hypothetical, due to the absence of lysophospholipids. LAH and PLD were partially soluble and partially associated with the membranes. When Vigna unguiculata plants were submitted to drought, the enzymatic degradation of galactolipids and phospholipids increased. The stimulation of lipolytic activities was greater in the drought-sensitive cultivar of Vigna unguiculata (cv. 1183) than in the drought-tolerant (cv. EPACE-I) one. In cv. 1183, MGDG- and DGDG-LAH activities in the membrane fractions were dramatically stimulated at a rather moderate water deficit ( -0.75 MPa). A sharp increase in membrane phospholipolytic activities was also observed at mild drought stress (-1,2 MPa). In contrast, in cv. EPACE-I, the stimulation of lipolytic activities was less drastic and occurred at lower leaf water potentials (below - 1.2 MPa for galactolipases, and below - 1,4 MPa for phospholipases). Our results confirm the presence in leaves of higher plants of a very active LAH acting on galactolipids, whereas PLD is the main enzyme responsible for the degradation of phospholipids, particularly when plants are submitted to drought stress. The differences in stimulation of lipolytic activities between the two Vigna cultivars was in accordance with the different levels of membrane lipid degradation shown previously and could explain their different capacity to sustain drought.