Structures of archaebacterial membrane lipids

Structural data on archaebacterial lipids is presented with emphasis on the ether lipids of the methanogens. These ether lipids normally account for 80–95% of the membrane lipids with the remaining 5–20% of neutral squalenes and other isoprenoids. Genus-specific combinations of various lipid core structures found in methanogens include diether-tetraether, diether-hydroxydiether, or diether-macrocyclic diether-tetraether lipid moieties. Some species have only the standard diether core lipid, but none are known with predominantly tetraether lipids as found in certain sulfur-dependent archaebacteria. The relative proportions of these lipid cores are known to vary in relation to growth conditions inMethanococcus jannaschii andMethanobacterium thermoautotrophicum. Polar headgroups in glycosidic or phosphodiester linkage to thesn-1 orsn-1 carbons of glycerol consist of polyols, carbohydrates, and amino compounds. The available structural data indicate a close similarity among the polar lipids synthesized within the species of the same genus. Detection of lipid molecular ions by mass spectrometry of total polar lipid extracts is a promising technique to provide valuable comparative data. Since these lipid structures are stable within the extreme environments that many archaebacteria inhabit, there may be specific applications for their use in biotechnology.     

Sane in the membrane: designing systems to modulate membrane proteins

Proteins and lipids make sense in rational approaches to the design of systems for the study of membrane proteins. Lipids surround integral membrane proteins in their natural environment. Although lipids have always formed part of investigations into membrane proteins, it has generally been the proteins themselves that have taken the limelight. As knowledge of membrane proteins has increased, so has that of their interactions with lipids. This increased understanding of the interplay of proteins and lipids, together with existing knowledge of lipid properties, is enabling new approaches to be introduced for membrane protein study. The lipids can be used to control protein behaviour and as novel probes of protein motion.     

Lipids in membrane protein structures

This review describes the recent knowledge about tightly bound lipids in membrane protein structures and deduces general principles of the binding interactions. Bound lipids are grouped in annular, nonannular, and integral protein lipids. The importance of lipid binding for vertical positioning and tight integration of proteins in the membrane, for assembly and stabilization of oligomeric and multisubunit complexes, for supercomplexes, as well as their functional roles are pointed out. Lipid binding is stabilized by multiple noncovalent interactions from protein residues to lipid head groups and hydrophobic tails. Based on analysis of lipids with refined head groups in membrane protein structures, distinct motifs were identified for stabilizing interactions between the phosphodiester moieties and side chains of amino acid residues. Differences between binding at the electropositive and electronegative membrane side, as well as a preferential binding to the latter, are observed. A first attempt to identify lipid head group specific binding motifs is made. A newly identified cardiolipin binding site in the yeast cytochrome bc(1) complex is described. Assignment of unsaturated lipid chains and evolutionary aspects of lipid binding are discussed.     

Fatty acids of plant vacuolar membrane lipids

Fatty acid (FA) composition of vacuolar membrane lipids from storage tissues of umbelliferous plants, viz., parsnip (Pastinaca sativa L.), parsley (Petroselinium crispum L.), and carrot (Daucus carota L.) is studied by gas-liquid chromatography and the FA biosynthetic pathways are considered. Vacuolar membrane lipids are characterized by high (78% of the total FA pool) content of unsaturated FA among which linoleic acid is predominant. Its content in vacuolar lipids of parsnip, parsley and carrot is 53.5, 55.1, and 54.9%, respectively. Parsnip and parsley vacuolar lipids contain large amounts of hexadecadienoic C16:2ω6 acid (8.0 and 4.6%, respectively). The content of α-linolenic acid in vacuolar lipids of tested plants varies from 4.8 to 7.3%. Palmitic acid (18.0–20.7%) predominates among saturated FA. High content of linoleic and hexadecadienoic acid in parsnip and parsley vacuolar lipids is suggestive of a crucial role of the microsomal ω6 fatty-acid desaturase fad2 gene in resistance and acclimation of plants to low temperatures.     

Role of membrane organization and membrane domains in endocytic lipid trafficking

Lipid compositions vary greatly among organelles, and specific sorting mechanisms are required to establish and maintain these distinct compositions. In this review, we discuss how the biophysical properties of the membrane bilayer and the chemistry of individual lipid molecules play a role in the intracellular trafficking of the lipids themselves, as well as influencing the trafficking of transmembrane proteins. The large diversity of lipid head groups and acyl chains lead to a variety of weak interactions, such as ionic and hydrogen bonding at the lipid/water interfacial region, hydrophobic interactions, and van-der-Waals interactions based on packing density. In simple model bilayers, these weak interactions can lead to large-scale phase separations, but in more complex mixtures, which mimic cell membranes, such phase separations are not observed. Nevertheless, there is growing evidence that domains (i.e., localized regions with non-random lipid compositions) exist in biological membranes, and it is likely that the formation of these domains are based on interactions similar to those that lead to phase separations in model systems. Sorting of lipids appears to be based in part on the inclusion or exclusion of certain types of lipids in vesicles or tubules as they bud from membrane organelles.     

Fluid lipid fraction in rod outer segment membrane

Rod outer segment membrane is analyzed using the spin label technique by means of two probes. The solubility of the first label, 2,2,6,6-tetramethylpiperidin-1-oxyl, is correlated with the membrane fluidity which is measured using a stearic acid spin probe. The two values are compared to the solubility-fluidity relationship which characterizes a model system in which all lipids are in a fluid state. The analysis leads to the conclusion that only two thirds of the membrane lipids are fluid. This conclusion is reinforced by the observation that partial lipid removal leaves rigid lipids associated with the rhodopsin molecules.     

Lipids in endocytic membrane transport and sorting

Protein complexes associated to specific membrane lipids and protein-lipid domains contribute to regulate protein sorting and membrane dynamics in the endocytic pathway. It is also becoming apparent that different lipid territories are distributed along the pathway, and that some lipids segregate into specialised microdomains.     

How proteins produce cellular membrane curvature

Biological membranes exhibit various function-related shapes, and the mechanism by which these shapes are created is largely unclear. Here, we classify possible curvature-generating mechanisms that are provided by lipids that constitute the membrane bilayer and by proteins that interact with, or are embedded in, the membrane. We describe membrane elastic properties in order to formulate the structural and energetic requirements of proteins and lipids that would enable them to work together to generate the membrane shapes seen during intracellular trafficking.     

Molecular dynamics simulation of a phosphatidylglycerol membrane

Although molecular dynamics simulations are an important tool for studying membrane systems, relatively few simulations have used anionic lipids. This paper reports the first simulation of a pure phosphatidylglycerol (PG) bilayer. The properties of this equilibrated palmitoyloleoylphosphatidylglycerol membrane agree with experimental observations of PG membranes and with previous simulations of monolayers and mixed bilayers containing PG lipids. These simulations also provide interesting insights into hydrogen bonding interactions in PG membranes. This equilibrated membrane will be a useful starting point for simulations of membrane proteins interacting with PG lipids.     

Electron spin resonance in membrane research: protein-lipid interactions

Different electron spin resonance (ESR) methods are described that allow determination of the stoichiometry and selectivity of interaction of spin-labelled lipids with integral transmembrane peptides or proteins, and also with peripheral surface-binding membrane proteins or peptides. In addition, ESR methods for determining the exchange rates of spin-labelled lipids at the protein-lipid interface are described, as well as methods to detect penetration of surface-binding peptides into the hydrophobic membrane core. Instrumental requirements are considered, and also sample handling, spin-labelling techniques and the synthesis of spin-labelled lipids.     

The influence of membrane lipid structure on plasma membrane Ca2+ -ATPase activity

Lipid composition and Ca(2+)-ATPase activity both change with age and disease in many tissues. We explored relationships between lipid composition/structure and plasma membrane Ca(2+)-ATPase (PMCA) activity. PMCA was purified from human erythrocytes and was reconstituted into liposomes prepared from human ocular lens membrane lipids and synthetic lipids. Lens lipids were used in this study as a model for naturally ordered lipids, but the influence of lens lipids on PMCA function is especially relevant to the lens since calcium homeostasis is vital to lens clarity. Compared to fiber cell lipids, epithelial lipids exhibited an ordered to disordered phase transition temperature that was 12 degrees C lower. Reconstitution of PMCA into lipids was essential for maximal activity. PMCA activity was two to three times higher when the surrounding phosphatidylcholine molecules contained acyl chains that were ordered (stiff) compared to disordered (fluid) acyl chains. In a completely ordered lipid hydrocarbon chain environment, PMCA associates more strongly with the acidic lipid phosphatidylserine in comparison to phosphatidylcholine. PMCA associates much more strongly with phosphatidylcholine containing disordered hydrocarbon chains than ordered hydrocarbon chains. PMCA activity is influenced by membrane lipid composition and structure. The naturally high degree of lipid order in plasma membranes such as those found in the human lens may serve to support PMCA activity. The absence of PMCA activity in the cortical region of human lenses is apparently not due to a different lipid environment. Changes in lipid composition such as those observed with age or disease could potentially influence PMCA function.     

Non-covalent binding of membrane lipids to membrane proteins

Polar lipids and membrane proteins are major components of biological membranes, both cell membranes and membranes of enveloped viruses. How these two classes of membrane components interact with each other to influence the function of biological membranes is a fundamental question that has attracted intense interest since the origins of the field of membrane studies. One of the most powerful ideas that driven the field is the likelihood that lipids bind to membrane proteins at specific sites, modulating protein structure and function. However only relatively recently has high resolution structure determination of membrane proteins progressed to the point of providing atomic level structure of lipid binding sites on membrane proteins. Analysis of X-ray diffraction, electron crystallography and NMR data over 100 specific lipid binding sites on membrane proteins. These data demonstrate tight lipid binding of both phospholipids and cholesterol to membrane proteins. Membrane lipids bind to membrane proteins by their headgroups, or by their acyl chains, or binding is mediated by the entire lipid molecule. When headgroups bind, binding is stabilized by polar interactions between lipid headgroups and the protein. When acyl chains bind, van der Waals effects dominate as the acyl chains adopt conformations that complement particular sites on the rough protein surface. No generally applicable motifs for binding have yet emerged. Previously published biochemical and biophysical data link this binding with function. This Article is Part of a Special Issue Entitled: Membrane Structure and Function: Relevance in the Cell's Physiology, Pathology and Therapy.     

Turnover of brain mitochondrial membrane lipids

1. The turnover of lipids, of myelin and other brain subcellular particles has been studied in double-labelling experiments on intact rats. 2. Overall metabolism of brain mitochondrial lipids was three times slower than that of the liver. 3. Individual lipids of brain mitochondria and myelin were also separated and their metabolism was studied. 4. All myelin lipids examined undergo very slow turnover. Two pools of brain mitochondrial lipid were identified. The slowly metabolized lipids were cholesterol, cardiolipin plus phosphatidic acid and possibly sphingomyelin; the remaining phosphatides underwent more rapid turnover. 5. The possible significance of these results is discussed.     

Fluid lipid fraction in rod outer segment membrane.

Rod outer segment membrane is analyzed using the spin resonance label technique by means of two probes. The solubility of the first label,2,2,6,6-tetramethylpiperidin-1-oxyl, is correlated with the membrane fluidity which is measured using a stearic acid spin probe. The two values are compared to the solubility-fluidity relationship which characterizes a model system in which all lipids are in a fluid state. The analysis leads to the conclusion that only two thirds of the membrane lipids are fluid. This conclusion is reinforced by the observation that partial lipid removal leaves rigid lipids associated with the rhodopsin molecules.     

Glycosphingolipids in endocytic membrane transport

Endocytosis leads to the internalisation of both lipids and proteins and their delivery to specific subcellular locations. This involves sorting processes that are not completely understood, but may involve interactions between lipids and proteins as well as pH and calcium gradients. This article discusses the importance of endocytosis in glycosphingolipid (GSL) synthesis as well as the potential roles of GSLs in endocytic membrane transport. Although the accumulation of GSLs in storage diseases clearly disrupts endocytic transport, increasing evidence also supports a role for GSLs in endocytosis in normal cells.     

Nonbilayer lipids affect peripheral and integral membrane proteins via changes in the lateral pressure profile

Nonbilayer lipids can be defined as cone-shaped lipids with a preference for nonbilayer structures with a negative curvature, such as the hexagonal phase. All membranes contain these lipids in large amounts. Yet, the lipids in biological membranes are organized in a bilayer. This leads to the question: what is the physiological role of nonbilayer lipids? Different models are discussed in this review, with a focus on the lateral pressure profile within the membrane. Based on this lateral pressure model, predictions can be made for the effect of nonbilayer lipids on peripheral and integral membrane proteins. Recent data on the catalytic domain of Leader Peptidase and the potassium channel KcsA are discussed in relation to these predictions and in relation to the different models on the function of nonbilayer lipids. The data suggest a general mechanism for the interaction between nonbilayer lipids and membrane proteins via the membrane lateral pressure.     

Synthetic peptides as models for intrinsic membrane proteins

There are many ways in which lipids can modulate the activity of membrane proteins. Simply a change in hydrophobic thickness of the lipid bilayer, for example, already can have various consequences for membrane protein organization and hence for activity. By using synthetic transmembrane peptides, it could be established that these consequences include peptide oligomerization, tilt of transmembrane segments, and reorientation of side chains, depending on the specific properties of the peptides and lipids used. The results illustrate the potential of the use of synthetic model peptides to establish general principles that govern interactions between membrane proteins and surrounding lipids.     

Interaction of membrane skeletal proteins with membrane lipid domain

The object of this paper is to review briefly the studies on the interaction of red blood cell membrane skeletal proteins and their non-erythroid analogues with lipids in model systems as well as in natural membranes. An important question to be addressed is the physiological significance and possible regulatory molecular mechanisms in which these interactions are engaged.     

Lactoperoxidase-catalyzed iodination of surface membrane lipids

Lactoperoxidase-catalyzed iodination of intact cells, is known to label predominantly, if not exclusively, the exposed tyrosine residues of cell surface proteins. The present study demonstrates that during this iodination process surface membrane lipids are also iodinated through an enzyme-dependent step. Phosphatidylcholine, cholesterol-phosphatidylcholine liposomes and confluent secondary cultures of chick embryo cells were iodinated by the lactoperoxidase-glucose oxidase-glucose [¹²⁵I] procedure. Liposomes were efficiently labeled. In the cells, 20–30% of the radioactivity was found in proteins and 20–30% in the lipids. Both neutral and polar lipids were found to bind [¹²⁵I] covalently. Controls in which lactoperoxidase was omitted showed < 6% of the radioactivity found in liposomes or cells labeled with the enzyme.     

Perylenoyl- and anthrylvinyl-labeled lipids as membrane probes

The properties of a new family of lipid-specific fluorescent probes, a fatty acid, a phosphatidylcholine and a sphingomyelin, bearing a 3-perylenoyl-labeled hydrophobic chain, are described. Perylenoyl-labeled lipids readily enter the lipid bilayer, the fluorophore being localized in the apolar region of the membrane. The perylenoyl fluorophore is characterized by a high quantum yield, its fluorescence parameters (λ_ex 446 nm, λ_em 479–545 nm) permit to apply it as an acceptor of excitation energy from the 9-anthrylvinyl fluorophore used earlier for phospholipid labeling (Molotkovsky, Jul. G.; Manevich, Y.M., Gerasimova, E.N., Molotkovskaya, I.M., Polessky, V.A. and Bergelson, L.D. (1982) Eur. J. Biochem. 122, 573–579). The anthrylvinyl-labeled lipids were shown to be capable to report phase segregation between the corresponding prototype lipids in model systems. The combined use of anthrylvinyl- and perylenoyl-labeled lipids opens additional possibilities for investigation of lipid-lipid and lipid-protein interactions in artificial and biological membranes. Perylenoyl-labeled lipids appeared also to be useful as fluorescent dyes in cytological studies.