Lipid-protein interactions in membranes

Phospholipid Membranes as Chemically and Functionally Tunable Materials

The sheet-like lipid bilayer is the fundamental structural component of all cell membranes. Its building blocks are phospholipids and cholesterol. Their amphiphilic structure spontaneously leads to the formation of a bilayer in aqueous environment. Lipids are not just structural elements. Individual lipid species, the lipid membrane structure, and lipid dynamics influence and regulate membrane protein function. An exciting field is emerging where the membrane-associated material properties of different bilayer systems are used in designing innovative solutions for widespread applications across various fields, such as the food industry, cosmetics, nano- and biomedicine, drug storage and delivery, biotechnology, nano- and biosensors, and computing. Here, the authors summarize what is known about how lipids determine the properties and functions of biological membranes and how this has been or can be translated into innovative applications. Based on recent progress in the understanding of membrane structure, dynamics, and physical properties, a perspective is provided on how membrane-controlled regulation of protein functions can extend current applications and even offer new applications.

Identifying Membrane Protein-Lipid Interactions with Lipidomic Lipid Exchange-Mass Spectrometry

Lipids can play important roles in modulating membrane protein structure and function. However, it is challenging to identify natural lipids bound to membrane proteins in complex bilayers. Here, we developed lipidomic lipid exchange-mass spectrometry (LX-MS) to study the lipid affinity for membrane proteins on a lipidomic scale. We first mix membrane protein nanodiscs with empty nanodiscs that have no embedded membrane proteins. After allowing lipids to passively exchange between the two populations, we separate the two types of nanodiscs and perform lipidomic analysis on each with liquid chromatography and MS. Enrichment of lipids in the membrane protein nanodiscs reveals the affinity of individual lipids for binding the target membrane protein. We apply this approach to study three membrane proteins. With the Escherichia coli ammonium transporter AmtB and aquaporin AqpZ in nanodiscs with E. coli polar lipid extracts, we detected binding of cardiolipin and phosphatidyl-glycerol lipids to the proteins. With the acetylcholine receptor in nanodiscs with brain polar lipid extracts, we discovered a complex set of lipid interactions that depended on the head group and tail composition. Overall, lipidomic LX-MS provides a detailed understanding of the lipid-binding affinity and thermodynamics for membrane proteins in complex bilayers and provides a unique perspective on the chemical environment surrounding membrane proteins.

The Role of the Membrane in Transporter Folding and Activity

The synthesis, folding, and function of membrane transport proteins are critical factors for defining cellular physiology. Since the stability of these proteins evolved amidst the lipid bilayer, it is no surprise that we are finding that many of these membrane proteins demonstrate coupling of their structure or activity in some way to the membrane. More and more transporter structures are being determined with some information about the surrounding membrane, and computational modeling is providing further molecular details about these solvation structures. Thus, the field is moving towards identifying which molecular mechanisms - lipid interactions, membrane perturbations, differential solvation, and bulk membrane effects - are involved in linking membrane energetics to transporter stability and function. In this review, we present an overview of these mechanisms and the growing evidence that the lipid bilayer is a major determinant of the fold, form, and function of membrane transport proteins in membranes.

Activation of G-protein-coupled receptors is thermodynamically linked to lipid solvation

Preferential lipid solvation of the G-protein-coupled A_2A adenosine receptor (A_2AR) is evaluated from 35 μs of all-atom molecular dynamics simulation. A coarse-grained transition matrix algorithm is developed to overcome slow equilibration of the first solvation shell, obtaining estimates of the free energy of solvation by different lipids for the receptor in different activation states. Results indicate preference for solvation by unsaturated chains, which favors the active receptor. A model for lipid-dependent G-protein-coupled receptor activity is proposed in which the chemical potential of lipids in the bulk membrane modulates receptor activity. The entropies associated with moving saturated and unsaturated lipids from bulk to A_2AR's first solvation shell are evaluated. Overall, the acyl chains are more disordered (i.e., obtain a favorable entropic contribution) when partitioning to the receptor surface, and this effect is augmented for the saturated chains, which are relatively more ordered in bulk.

Hepatitis C Virus p7 Induces Membrane Permeabilization by Interacting with Phosphatidylserine

Hepatitis C virus (HCV) p7 is known to be a nonselective cation channel for HCV maturation. Because the interaction of HCV proteins with host lipids in the endoplasmic reticulum membrane is crucial for the budding process, the identification of p7–lipid interactions could be important for understanding the HCV life cycle. Here, we report that p7 interacts with phosphatidylserine (PS) to induce membrane permeabilization. The interaction of p7 with PS was not inhibited by Gd³⁺ ions, which have been known to interact with negatively charged lipids, but channel activity and p7-induced mitochondrial depolarization were inhibited by Gd³⁺ ions. From the present results, we suggest that the p7–PS interaction plays an essential role in regulating its ion channel function and could be a potential molecular target for anti-HCV therapy.

Studying Lipid-Protein Interactions with Electron Paramagnetic Resonance Spectroscopy of Spin-Labeled Lipids

Spin label electron paramagnetic resonance (EPR) of lipid-protein interactions reveals crucial features of the structure and assembly of integral membrane proteins. Spin-label EPR spectroscopy is the technique of choice to characterize the protein solvating lipid shell in its highly dynamic nature, because the EPR spectra of lipids that are spin-labeled close to the terminal methyl end of their acyl chains display two spectral components, those corresponding to lipids directly contacting the protein and those corresponding to lipids in the bulk fluid bilayer regions of the membrane. In this chapter, typical spin label EPR procedures are presented that allow determination of the stoichiometry of interaction of spin-labeled lipids with the intramembranous region of membrane proteins or polypeptides, as well as the association constant of the spin-labeled lipid with respect to the host lipid. The lipids giving rise to a so-called immobile spectral component in the EPR spectrum of such samples are identified as the motionally restricted first-shell lipids solvating membrane proteins in biomembranes. Stoichiometry and selectivity are directly related to the structure of the intramembranous sections of membrane-associated proteins or polypeptides and can be used to study the state of assembly of such proteins in the membrane. Since these characteristics of lipid-protein interactions are discussed in detail in the literature (see ref. Marsh, Eur Biophys J 39:513-525, 2010 for a recent review), here we focus more on how to spin label model membranes and biomembranes and how to measure and analyze the two-component EPR spectra of spin-labeled lipids in phospholipid bilayers that contain proteins or polypeptides. After a description of how to prepare spin-labeled model and native biological membranes, we present the reader with computational procedures for determining the molar fraction of motionally restricted lipids when both, one or none of the pure isolated-mobile or immobile-spectral components are available. With these topics, this chapter complements a previous methodological paper (Marsh, Methods 46:83-96, 2008). The interpretation of the data is discussed briefly, as well as other relevant and recent spin label EPR techniques for studying lipid-protein interactions, not only from the point of view of lipid chain dynamics.

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.

Studying lipid-protein interactions with electron paramagnetic resonance spectroscopy of spin-labeled lipids

Spin label electron paramagnetic resonance (EPR) of lipid-protein interactions reveals crucial features of the structure and assembly of integral membrane proteins. Spin label EPR spectroscopy is the technique of choice to characterize the protein-solvating lipid shell in its highly dynamic nature, because the EPR spectra of lipids that are spin labeled close to the terminal methyl end of their acyl chains display two spectral components, those corresponding to lipids directly contacting the protein and those corresponding to lipids in the bulk fluid bilayer regions of the membrane. In this chapter, typical spin label EPR procedures are presented that allow determination of the stoichiometry of interaction of spin-labeled lipids with the intra-membranous region of membrane proteins or polypeptides, as well as the association constant of the spin-labeled lipid with respect to the host lipid. The lipids giving rise to the so-called immobile spectral component in the EPR spectrum of such samples are identified as the motionally restricted first-shell lipids solvating membrane proteins in biomembranes. Stoichiometry and selectivity are directly related to the structure of the intra-membranous sections of membrane-associated proteins or polypeptides and can be used to study the state of assembly of such proteins in the membrane. Since these characteristics of lipid-protein interactions are discussed in detail in the literature [see Marsh (Eur Biophys J 39:513-525, 2010) for a most recent review], here we focus more on how to spin label model and biomembranes and how to measure and analyze the two-component EPR spectra of spin-labeled lipids in phospholipid bilayers that contain proteins or polypeptides. After a description of how to prepare spin-labeled model and native biological membranes, we present the reader with computational procedures for determining the molar fraction of motionally restricted lipids when both, one, or none of the pure isolated-mobile or immobile-spectral components are available. With these topics, this chapter complements a recent methodological paper [Marsh (Methods 46:83-96, 2008)]. The interpretation of the data is discussed briefly, as well as other relevant and recent spin label EPR techniques for studying lipid-protein interactions, not only from the point of view of lipid chain dynamics.

Binding of cationic pentapeptides with modified side chain lengths to negatively charged lipid membranes: Complex interplay of electrostatic and hydrophobic interactions

Basic amino acids play a key role in the binding of membrane associated proteins to negatively charged membranes. However, side chains of basic amino acids like lysine do not only provide a positive charge, but also a flexible hydrocarbon spacer that enables hydrophobic interactions. We studied the influence of hydrophobic contributions to the binding by varying the side chain length of pentapeptides with ammonium groups starting with lysine to lysine analogs with shorter side chains, namely omithine (Orn), alpha, gamma-diaminobutyric acid (Dab) and alpha, beta-diaminopropionic acid (Dap). The binding to negatively charged phosphatidylglycerol (PG) membranes was investigated by calorimetry, FT-infrared spectroscopy (FT-IR) and monolayer techniques. The binding was influenced by counteracting and sometimes compensating contributions. The influence of the bound peptides on the lipid phase behavior depends on the length of the peptide side chains. Isothermal titration calorimetry (ITC) experiments showed exothermic and endothermic effects compensating to a different extent as a function of side chain length. The increase in lipid phase transition temperature was more significant for peptides with shorter side chains. FTIR-spectroscopy revealed changes in hydration of the lipid bilayer interface after peptide binding. Using monolayer techniques, the contributions of electrostatic and hydrophobic effects could clearly be observed. Peptides with short side chains induced a pronounced decrease in surface pressure of PG monolayers whereas peptides with additional hydrophobic interactions decreased the surface pressure much less or even lead to an increase, indicating insertion of the hydrophobic part of the side chain into the lipid monolayer.

Electrochemistry of cytochrome c immobilized on cardiolipin-modified electrodes: a probe for protein-lipid interactions

Electrochemistry of cytochrome c (cyt c) immobilized on a cardiolipin (CL)/phosphatidylcholine (PC) film supported on a glassy carbon electrode was investigated using variable-frequency AC voltammetry. At low ionic strength, we observed two redox-active subpopulations characterized by distinct values of potential (E1/2) and electron transfer rate constant (k(ET)). At high ionic strength, only one subpopulation was detected, consistent with the existence of very stable cyt c-CL adducts, most probably formed by hydrophobic interactions between the protein and the fatty acid (FA) chains carried by CL. This subpopulation exhibits a comparatively high k(ET) value (> 300 s(-1)) apparently changing with the structure of the FA chains of CL, i.e. 18:2(n - 6) or 14:0. Our study suggests that electrochemistry can be a useful technique for probing protein-lipid interactions, and more particularly the role played by the specific structure of the FA chains of CL on cyt c binding.

Lipid dynamics and protein-lipid interactions in 2D crystals formed with the β-barrel integral membrane protein VDAC1

We employ a combination of (13)C/(15)N magic angle spinning (MAS) NMR and (2)H NMR to study the structural and functional consequences of different membrane environments on VDAC1 and, conversely, the effect of VDAC1 on the structure of the lipid bilayer. MAS spectra reveal a well-structured VDAC1 in 2D crystals of dimyristoylphosphatidylcholine (DMPC) and diphytanoylphosphatidylcholine (DPhPC), and their temperature dependence suggests that the VDAC structure does not change conformation above and below the lipid phase transition temperature. The same data show that the N-terminus remains structured at both low and high temperatures. Importantly, functional studies based on electrophysiological measurements on these same samples show fully functional channels, even without the presence of Triton X-100 that has been found necessary for in vitro-refolded channels. (2)H solid-state NMR and differential scanning calorimetry were used to investigate the dynamics and phase behavior of the lipids within the VDAC1 2D crystals. (2)H NMR spectra indicate that the presence of protein in DMPC results in a broad lipid phase transition that is shifted from 19 to ~27 °C and show the existence of different lipid populations, consistent with the presence of both annular and bulk lipids in the functionally and structurally homogeneous samples.

Influence of hydrophobic mismatch and amino acid composition on the lateral diffusion of transmembrane peptides

We investigated the effect of amino acid composition and hydrophobic length of alpha-helical transmembrane peptides and the role of electrostatic interactions on the lateral diffusion of the peptides in lipid membranes. Model peptides of varying length and composition, and either tryptophans or lysines as flanking residues, were synthesized. The peptides were labeled with the fluorescent label Alexa Fluor 488 and incorporated into phospholipid bilayers of different hydrophobic thickness and composition. Giant unilamellar vesicles were formed by electroformation, and the lateral diffusion of the transmembrane peptides (and lipids) was determined by fluorescence correlation spectroscopy. In addition, we performed coarse-grained molecular-dynamics simulations of single peptides of different hydrophobic lengths embedded in planar membranes of different thicknesses. Both the experimental and simulation results indicate that lateral diffusion is sensitive to membrane thickness between the peptides and surrounding lipids. We did not observe a difference in the lateral diffusion of the peptides with respect to the presence of tryptophans or lysines as flanking residues. The specific lipid headgroup composition of the membrane has a much less pronounced impact on the diffusion of the peptides than does the hydrophobic thickness.

Electron spin resonance in membrane research: protein-lipid interactions from challenging beginnings to state of the art

Conventional electron paramagnetic resonance (EPR) spectra of lipids that are spin-labelled close to the terminal methyl end of the acyl chains are able to resolve the lipids directly contacting the protein from those in the fluid bilayer regions of the membrane. This allows determination of both the stoichiometry of lipid-protein interaction (i.e., number of lipid sites at the protein perimeter) and the selectivity of the protein for different lipid species (i.e., association constants relative to the background lipid). Spin-label EPR data are summarised for 20 or more different transmembrane peptides and proteins, and 7 distinct species of lipids. Lineshape simulations of the two-component conventional spin-label EPR spectra allow estimation of the rate at which protein-associated lipids exchange with those in the bulk fluid regions of the membrane. For lipids that do not display a selectivity for the protein, the intrinsic off-rates for exchange are in the region of 10 MHz: less than 10x slower than the rates of diffusive exchange in fluid lipid membranes. Lipids with an affinity for the protein, relative to the background lipid, have off-rates for leaving the protein that are correspondingly slower. Non-linear EPR, which depends on saturation of the spectrum at high radiation intensities, is optimally sensitive to dynamics on the timescale of spin-lattice relaxation, i.e., the microsecond regime. Both progressive saturation and saturation transfer EPR experiments provide definitive evidence that lipids at the protein interface are exchanging on this timescale. The sensitivity of non-linear EPR to low frequencies of spin exchange also allows the location of spin-labelled membrane protein residues relative to those of spin-labelled lipids, in double-labelling experiments.

Membrane cholesterol in the function and organization of G-protein coupled receptors

Cholesterol is an essential component of higher eukaryotic membranes and plays a crucial role in membrane organization, dynamics and function. The G-protein coupled receptors (GPCRs) are the largest class of molecules involved in signal transduction across membranes, and represent major targets in the development of novel drug candidates in all clinical areas. Membrane cholesterol has been reported to have a modulatory role in the function of a number of GPCRs. Two possible mechanisms have been previously suggested by which membrane cholesterol could influence the structure and function of GPCRs (i) through a direct/specific interaction with GPCRs, or (ii) through an indirect way by altering membrane physical properties in which the receptor is embedded, or due to a combination of both. Recently reported crystal structures of GPCRs have shown structural evidence of cholesterol binding sites. Against this backdrop, we recently proposed a novel mechanism by which membrane cholesterol could affect structure and function of GPCRs. According to our hypothesis, cholesterol binding sites in GPCRs could represent 'nonannular' binding sites. Interestingly, previous work from our laboratory has demonstrated that membrane cholesterol is required for the function of the serotonin(1A) receptor (a representative GPCR), which could be due to specific interaction of the receptor with cholesterol. Based on these results, we envisage that there could be specific/nonannular cholesterol binding site(s) in the serotonin(1A) receptor. We have analyzed putative cholesterol binding sites from protein databases in the serotonin(1A) receptor. Our analysis shows that cholesterol binding sites are inherent characteristic features of serotonin(1A) receptors and are conserved through natural evolution. Progress in deciphering molecular details of the GPCR-cholesterol interaction in the membrane would lead to better insight into our overall understanding of GPCR function in health and disease, thereby enhancing our ability to design better therapeutic strategies to combat diseases related to malfunctioning of GPCRs.

Biophysical characterization of a new phospholipid analogue with a spin-labeled unsaturated fatty acyl chain

Spin-labeled analogs of phospholipids have been used widely to characterize the biophysical properties of membranes. We describe synthesis and application of a new spin-labeled phospholipid analog, SL-POPC. The advantage of this molecule is that the EPR active doxyl group is linked to an unsaturated fatty acyl chain different to saturated phospholipid analogs used so far. The need for those analogs arises from the fact that biological membranes contain unsaturated phospholipids to a large extent. The biophysical properties of SL-POPC in membranes were characterized using EPR and NMR spectroscopy and compared with those of the saturated spin-labeled phospholipid, SL-PSPC. To this end, POPC membranes were labeled with either analog to assess whether the spin-labeled counterpart SL-POPC mimics the membrane properties better than the often used SL-PSPC. The results show that SL-POPC and SL-PSPC explore different molecular environments of the bilayer, and that the type and degree of perturbation of bilayer caused by the label moiety also differs between both analogs. We found that SL-POPC is more appropriate to assess the versatile dynamics of POPC membranes than SL-PSPC.

Folding kinetics of the outer membrane proteins OmpA and FomA into phospholipid bilayers

The folding mechanism of outer membrane proteins (OMPs) of Gram-negative bacteria into lipid bilayers has been studied using OmpA of E. coli and FomA of F. nucleatum as examples. Both, OmpA and FomA are soluble in unfolded form in urea and insert and fold into phospholipid bilayers upon strong dilution of the denaturant urea. OmpA is a structural protein and forms a small ion channel, composed of an 8-stranded transmembrane beta-barrel domain. FomA is a voltage-dependent porin, predicted to form a 14 stranded beta-barrel. Both OMPs fold into a range of model membranes of very different phospholipid compositions. Three membrane-bound folding intermediates of OmpA were discovered in folding studies with dioleoylphosphatidylcholine bilayers that demonstrated a highly synchronized mechanism of secondary and tertiary structure formation of beta-barrel membrane proteins. A study on FomA folding into lipid bilayers indicated the presence of parallel folding pathways for OMPs with larger transmembrane beta-barrels.

The major outer membrane protein of Fusobacterium nucleatum (FomA) folds and inserts into lipid bilayers via parallel folding pathways

Membrane protein insertion and folding was studied for the major outer membrane protein of Fusobacterium nucleatum (FomA), which is a voltage-dependent general diffusion porin. The transmembrane domain of FomA forms a beta-barrel that is predicted to consist of 14 beta-strands. Here, unfolded FomA is shown to insert and fold spontaneously and quantitatively into phospholipid bilayers upon dilution of the denaturant urea, which was shown previously only for outer membrane protein A (OmpA) of Escherichia coli. Folding of FomA is demonstrated by circular dichroism and fluorescence spectroscopy, by SDS-polyacrylamide gel electrophoresis, and by single-channel recordings. Refolded FomA had a single-channel conductance of 1.1 nS at 1 M KCl, in agreement with the conductance of FomA isolated from membranes in native form. In contrast to OmpA, which forms a smaller eight-stranded beta-barrel domain, folding kinetics of the larger FomA were slower and provided evidence for parallel folding pathways of FomA into lipid bilayers. Two pathways were observed independent of membrane thickness with two different lipid bilayers, which were either composed of dicapryl phosphatidylcholine or dioleoyl phosphatidylcholine. This is the first observation of parallel membrane insertion and folding pathways of a beta-barrel membrane protein from an unfolded state in urea into lipid bilayers. The kinetics of both folding pathways depended on the chain length of the lipid and on temperature with estimated activation energies of 19 kJ/mol (dicapryl phosphatidylcholine) and 70 kJ/mol (dioleoyl phosphatidylcholine) for the faster pathways.

Surface thermodynamics study of monolayers formed with heteroacid phospholipids of biological interest

The interaction of 1-palmitoy-2-oleoyl-sn-glycero-3-phosphocoline (POPC) and 1-palmitoy-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), two of the major components in biological membranes, were investigated using the monolayer technique at the air-water interface. The pressure-area isotherms indicate that both phospholipids are miscible through all range of compositions. POPE-POPC form stable mixtures, with a minimum for the Gibbs energy of mixing at X(POPC) = 0.4. A virial equation of state was fitted to the experimental values. Positive values found for the second virial coefficient indicate repulsion between POPC and POPE. The interaction parameter was evaluated which indicated that a corresponding decrease in the repulsion occurs when POPC molar fraction is low. This effect suggests the existence of hydrogen bonds between POPE and the water beneath the interface.

Cytochrome c?dimyristoylphosphatidylglycerol interactions studied by asymmetrical flow field-flow fractionation

Lipid membranes are well recognized ligands that bind peripheral and integral proteins in a specific manner and regulate their function. Cytochrome c (cyt c) is one of the partner peripheral protein that binds to the lipid membranes via electrostatic and hydrophobic interactions. In this study, asymmetrical flow field-flow fractionation (AsFlFFF) was used to compare the interactions of cyt c with the acidic phospholipid 1,2-dimyristoyl-sn-glycero-3-phospho-rac-glycerol (DMPG), oleic acid (OA), and sodium dodecyl sulfate (SDS). The influence of pH and the cyt c-lipid molar mass ratios were evaluated by monitoring the diffusion coefficients and particle diameter distributions obtained for the free and lipid-bound protein. The hydrodynamic particle diameter of cyt c (pI 10) was 4.1 nm at pH 11.4 and around 4.2 nm at pH 7.0 and 8.0. Standard molar mass marker proteins were used for calibration to obtain the molar masses of free cyt c and its complexes with lipids. AsFlFFF revealed the binding of cyt c to DMPG and to OA to be mainly electrostatic. In the absence of electrostatic interactions, minor complex formation occurred, possibly due to the extended lipid anchorage involving the hydrophobic cavity of cyt c and the hydrocarbon chains of DMPG or SDS. The possibility of the formation of the molten globule state of cyt c, induced by the interaction between cyt c and lipids, is discussed.

Polyunsaturated fatty acid deficiency reverses effects of alcohol on mitochondrial energy metabolism

BACKGROUND/AIMS

Polyunsaturated fatty acids (PUFA) deficiency is common in patients with alcoholic liver disease. The suitability of reversing such deficiency remains controversial. The aim was to investigate the role played by PUFA deficiency in the occurrence of alcohol-related mitochondrial dysfunction.

METHODS

Wistar rats were fed either a control diet with or without alcohol (control and ethanol groups) or a PUFA deficient diet with or without alcohol (PUFA deficient and PUFA deficient+ethanol groups). After 6 weeks, liver mitochondria were isolated for energetic studies and fatty acid analysis.

RESULTS

Mitochondria from ethanol fed rats showed a dramatic decrease in oxygen consumption rates and in cytochrome oxidase activity. PUFA deficiency showed an opposite picture. PUFA deficient+ethanol group roughly reach control values, regarding cytochrome oxidase activity and respiratory rates. The relationship between ATP synthesis and respiratory rate was shifted to the left in ethanol group and to the right in PUFA-deficient group. The plots of control and PUFA deficient+ethanol groups were overlapping. Phospholipid arachidonic over linoleic ratio closely correlated to cytochrome oxidase and oxygen uptake.

CONCLUSIONS

PUFA deficiency reverses alcohol-related mitochondrial dysfunction via an increase in phospholipid arachidonic over linoleic ratio, which raises cytochrome oxidase activity. Such deficiency may be an adaptive mechanism.

A wave of IP3 production accompanies the fertilization Ca2+ wave in the egg of the frog, Xenopus laevis: theoretical and experimental support

The fertilization Ca2+ wave in Xenopus laevis is a single, large wave of elevated free Ca2+ that is initiated at the point of sperm-egg fusion and traverses the entire width of the egg. This Ca2+ wave involves an increase in inositol-1,4,5-trisphosphate (IP3) resulting from the interaction of the sperm and egg, which then results in the activation of the endoplasmic reticulum Ca2+ release machinery. The extraordinarily large size of this cell (1.2 mm diameter) together with the small surface region of sperm-receptor activation makes special demands on the IP3-dependent Ca2+ mobilizing machinery. We propose a detailed model of the fertilization Ca2+ wave in Xenopus eggs that requires an accompanying wave of IP3 production. While the Ca2+ wave is initiated by a localized increase of IP3 near the site of sperm-egg fusion, the Ca2+ wave propagates via IP3 production correlated with the Ca2+ wave-possibly via Ca(2+)-mediated PLC activation. Such a Ca(2+)-mediated IP(3) production wave has not been required previously to explain the fertilization Ca2+ wave in eggs; we argue this is necessary to explain the observed IP3 dynamics in Xenopus eggs. To test our hypothesis, we have measured the IP3 levels from 20 nl "sips" of the egg cortex during wave propagation. We were unable to detect the low IP3 levels in unfertilized eggs, but after fertilization, [IP3] ranged from 175 to 430 nM at the sperm entry point and from 120 to 700 nM 90 degrees away once the Ca2+ wave passed that region about 2 min after fertilization. Prior to the Ca2+ wave reaching that region the IP3 levels were undetectable. Since significant IP3 could not diffuse to this region from the sperm entry point within 2 min, this observation is consistent with a regenerative wave of IP3 production.

In vitro folding of alpha-helical membrane proteins

For large-scale production, as required in structural biology, membrane proteins can be expressed in an insoluble form as inclusion bodies and be refolded in vitro. This requires refolding conditions where the native form is thermodynamically stable and where nonproductive pathways leading to aggregation are avoided. Examples of successful refolding are reviewed and general guidelines to establish refolding protocols of membrane proteins are presented.

Binding of sea anemone pore-forming toxins sticholysins I and II to interfaces--modulation of conformation and activity, and lipid-protein interaction

Sticholysins I and II (St I and St II) are water-soluble toxins produced by the sea anemone Stichodactyla helianthus. St I and St II bind to biological and model membranes containing sphingomyelin (SM), forming oligomeric pores that lead to leakage of internal contents. Here we describe functional and structural studies of the toxins aiming at the understanding at a molecular level of their mechanism of binding, as well as their effects on membrane permeabilization. St I and St II caused potassium leakage from red blood cells and temperature-dependent hemolysis, the activation energy of the process being lower for the latter toxin. Protein intrinsic fluorescence measurements provided evidence for toxin binding to model membranes composed of 1:1 (mol:mol) egg phosphatidyl choline (ePC):SM. The fluorescence intensity increased and the maximum emission wavelength decreased as a result of binding. The changes were quantitatively different for both toxins. Circular dichroism spectra showed that both St I and St II exhibit a high content of beta-sheet structure and that binding to model membranes did not alter the toxin's conformation to a large extent. Changing the lipid composition by adding 5 mol% of negatively charged phosphatidic acid (PA) or phosphatidyl glycerol (PG) had small, but detectable, effects on protein conformation. The influence of lipid composition on toxin-induced membrane permeabilization was assessed by means of fluorescence measurements of calcein leakage. The effect was larger for ePC:SM bilayers containing 5 mol% of negative curvature-inducing lipids. Electron paramagnetic resonance (EPR) spectra of intercalated fatty acid spin probes carrying the nitroxide moiety at different carbons (5, 7, 12, and 16) evidenced the occurrence of lipid-protein interaction. Upon addition of the toxins, two-component spectra were observed for the probe labeled at C-12. The broader component, corresponding to a population of strongly immobilized spin probes, was ascribed to boundary lipid. The contribution of this component to the total spectrum was larger for St II than for St I. Moreover, it was clearly detectable for the C-12-labeled probe, but it was absent when the label was at C-16, indicating a lack of lipid-protein interaction close to the lipid terminal methyl group. This effect could be either due to the fact that the toxins do not span the whole bilayer thickness or to the formation of a toroidal pore leading to the preferential interaction with acyl chain carbons closer to the phospholipids head groups.

Insulin-induced lipid binding to hemoglobin

Under hypoglycemic conditions, concomitant hyperinsulinism causes an apparent modification of hemoglobin (Hb) which is manifested by its aggregation (Niketic et al., Clin. Chim. Acta 197 (1991) 47). In the present work the causes and mechanisms underlying this Hb modification were studied. Hemoglobin isolated from normal erythrocytes incubated with insulin was analyzed by applying 31P-spectrometry and lipid extraction and analysis. To study the dynamics of the plasma membrane during hyperinsulinism, a fluorescent lipid-analog was applied. In the presence of insulin phosphatidylserine (PS), phosphatidylethanolamine (PE) and cholesterol were found to bind to Hb. Lipid binding resulted in Hb aggregation, a condition that can be reproduced when phospholipids are incubated with Hb in vitro. Using a fluorescent lipid-analog, it was also shown that exposing erythrocytes to supraphysiological concentrations of insulin in vitro resulted in the internalization of lipids. The results presented in this work may have relevance to cases of diabetes mellitus and hypoglycemia.

Secondary and tertiary structure formation of the beta-barrel membrane protein OmpA is synchronized and depends on membrane thickness

The mechanism of membrane insertion and folding of a beta-barrel membrane protein has been studied using the outer membrane protein A (OmpA) as an example. OmpA forms an eight-stranded beta-barrel that functions as a structural protein and perhaps as an ion channel in the outer membrane of Escherichia coli. OmpA folds spontaneously from a urea-denatured state into lipid bilayers of small unilamellar vesicles. We have used fluorescence spectroscopy, circular dichroism spectroscopy, and gel electrophoresis to investigate basic mechanistic principles of structure formation in OmpA. Folding kinetics followed a second-order rate law and is strongly depended on the hydrophobic thickness of the lipid bilayer. When OmpA was refolded into model membranes of dilaurylphosphatidylcholine, fluorescence kinetics were characterized by a rate constant that was about fivefold higher than the rate constants of formation of secondary and tertiary structure, which were determined by circular dichroism spectroscopy and gel electrophoresis, respectively. The formation of beta-sheet secondary structure and closure of the beta-barrel of OmpA were correlated with the same rate constant and coupled to the insertion of the protein into the lipid bilayer. OmpA, and presumably other beta-barrel membrane proteins therefore do not follow a mechanism according to the two-stage model that has been proposed for the folding of alpha-helical bundle membrane proteins. These different folding mechanisms are likely a consequence of the very different intramolecular hydrogen bonding and hydrophobicity patterns in these two classes of membrane proteins.

Annexin V binding perturbs the cardiolipin fluidity gradient in isolated mitochondria. Can it affect mitochondrial function?

The phosholipid bilayer fluidity of isolated mitochondria and phospholipid vesicles after calcium-dependent binding of annexin V was studied using EPR spectroscopy. The membranes were probed at different depths by alternatively using cardiolipin, phosphatidylcholine, or phosphatidylethanolamine spin labeled at position C-5 or C-12 or C-16 of the beta acyl chain. Computer-aided spectral titration facilitated observing and quantitating the EPR spectrum from phospholipid spin labels affected by annexin binding, and spectral mobility was calibrated by comparison with standard spectra scanned at various temperatures. In most cases it was found that binding of the protein to the membranes makes the inner bilayer more rigid up to acyl position C-12 than afterward, in agreement with the previously observed effect in SUVs [Megli, F. M., Selvaggi, M., Liemann, S., Quagliariello, E., and Huber, R. (1998) Biochemistry 37, 10540-10546]. Moreover, in isolated mitochondrial membranes, cardiolipin apparently is more readily affected than the other main phospholipids, while in vesicles made from mitochondrial phospholipids, the different species are affected in essentially the same way. This behavior is consistent with the existence of distinct cardiolipin pools in mitochondria, and with the already advanced hypothesis that these domains are the binding site for annexin V to the isolated organelles [Megli, F. M., Selvaggi, M., De Lisi, A., and Quagliariello, E. (1995) Biochim. Biophys. Acta 1236, 273-278]. Keeping in mind the funcional importance of cardiolipin in the mitochondrial membrane, the question is raised as to whether the observed influence of annexin V binding to this phospholipid and its consequent local fluidity alteration might affect the mitochondrial functionality, at least in vitro.

Reversible penetration of alpha-glutathione S-transferase into biological membranes revealed by photosensitized labelling in situ

Fluorescent lipid analogue 3,3'-dioctadecyloxacarbocyanine incorporated into biological membranes was used to induce photoactivation of a hydrophobic probe 5-[125I]iodonaphthyl-1-azide (125INA) by energy transfer and to thereby confine subsequent radiolabelling of proteins to the lipid bilayer. This approach was applied in bovine chromaffin cells to discover cytosolic proteins that reversibly penetrate into membrane domains. alpha-Glutathione S-transferase (alpha-GST) was identified as the only labelled protein in bovine chromaffin-cell cytosol, indicating that it inserts reversibly into the membrane lipid bilayer. The selectivity of the labelling towards the lipid bilayer is demonstrated by showing that influenza virus haemagglutinin becomes labelled by 125INA only after the insertion of this protein into the target membrane. The molar 125INA:protein ratio was used as a quantitative criterion for evaluation of the penetration of proteins into the membrane lipid bilayer. This ratio was calculated for four integral membrane proteins and four soluble proteins that interact with biological membranes. The values for four integral membrane proteins (erythrocyte anion transporter, multidrug transporter gp-170, dopamine transporter and fusion-competent influenza virus haemagglutinin) were 1, 8, 2 and 2, respectively, whereas for soluble proteins (annexin VII, protein kinase C, BSA and influenza virus haemagglutinin) the values were 0.002, 0, 0.002 and 0.02, respectively. The molar ratio for alpha-GST was found to be 1, compatible with the values obtained for integral membrane proteins.

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

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.

The calcium-dependent binding of annexin V to phospholipid vesicles influences the bilayer inner fluidity gradient

The fluidity of the hydrophobic interior of phospholipid vesicles after calcium-dependent binding of human annexin V (AVH) was studied using EPR spectroscopy. Vesicles (SUVs) composed of PC or PE and an acidic phospholipid (alternatively PS, PA, or CL) were probed at different bilayer depths by either phosphatidylcholine, or the accompanying acidic phospholipid, bearing a spin label probe at position C-5, C-12, or C-16 of the sn-2 acyl chain. Alternatively, the vesicle surface was probed with a polar head spin labeled PE (PESL). The EPR spectra of annexin-bound bilayer domain(s) were obtained by computer spectral subtraction. The order parameter values (S) from the resulting difference spectra revealed that the bilayer hydrophobic interior has a greatly altered fluidity gradient, with an increased rigidity up to the C-12 position. Thereafter, the rigidification progressively vanished. The effect is not linked to the phospholipid class, since all the acidic phospholipid spectra, as well as phosphatidylcholine, shared the same sensitivity to the bound protein. The observed membrane rigidification appears to parallel the "crystallizing" tendency of vesicle-bound annexin V, but may not be involved in the calcium channeling activity of this protein.

The cholesterol dependence of activation and fast desensitization of the nicotinic acetylcholine receptor

When nicotinic acetylcholine receptors are reconstituted into lipid bilayers lacking cholesterol, agonists no longer stimulate cation flux. The kinetics of this process are difficult to study because variations in vesicle morphology cause errors in flux measurements. We developed a new stopped-flow fluorescence assay to study activation independently of vesicle morphology. When receptors were rapidly mixed with agonist plus ethidium, the earliest fluorescence increase reported the fraction of channels that opened and their apparent rate of fast desensitization. These processes were absent when the receptor was reconstituted into dioleoylphosphatidylcholine or into a mixture of that lipid with dioleoylphosphatidic acid (12 mol%), even though a fluorescent agonist reported that resting-state receptors were still present. The agonist-induced channel opening probability increased with bilayer cholesterol, with a midpoint value of 9 +/- 1.7 mol% and a Hill coefficient of 1.9 +/- 0.69, reaching a plateau above 20-30 mol% cholesterol that was equal to the native value. On the other hand, the observed fast desensitization rate was comparable to that for native membranes from the lowest cholesterol concentration examined (5 mol%). Thus the ability to reach the open state after activation varies with the cholesterol concentration in the bilayer, whereas the rate of the open state to fast desensitized state transition is unaffected. The structural basis for this is unknown, but an interesting corollary is that the channels of newly synthesized receptors are not fully primed by cholesterol until they are inserted into the plasma membrane--a novel form of posttranslational processing.

Spin-label electron spin resonance studies on the interactions of lysine peptides with phospholipid membranes

The interactions of lysine oligopeptides with dimyristoyl phosphatidylglycerol (DMPG) bilayer membranes were studied using spin-labeled lipids and electron spin resonance spectroscopy. Tetralysine and pentalysine were chosen as models for the basic amino acid clusters found in a variety of cytoplasmic membrane-associating proteins, and polylysine was chosen as representative of highly basic peripherally bound proteins. A greater motional restriction of the lipid chains was found with increasing length of the peptide, while the saturation ratio of lipids per peptide was lower for the shorter peptides. In DMPG and dimyristoylphosphatidylserine host membranes, the perturbation of the lipid chain mobility by polylysine was greater for negatively charged spin-labeled lipids than for zwitterionic lipids, but for the shorter lysine peptides these differences were smaller. In mixed bilayers composed of DMPG and dimyristoylphosphatidylcholine, little difference was found in selectivity between spin-labeled phospholipid species on binding pentalysine. Surface binding of the basic lysine peptides strongly reduced the interfacial pK of spin-labeled fatty acid incorporated into the DMPG bilayers, to a greater extent for polylysine than for tetralysine or pentalysine at saturation. The results are consistent with a predominantly electrostatic interaction with the shorter lysine peptides, but with a closer surface association with the longer polylysine peptide.

Opiates inhibit ion conductances elicited by cell swelling and cAMP in cultured cells

The effect of several opiate compounds on I- efflux was investigated in cultured cell lines. I- efflux was evoked by two distinct stimuli, namely cell swelling and elevation of cellular cAMP levels by prostaglandin E2. Cells expressing the multidrug resistance P-glycoprotein were found to have increased I- efflux in response to hypo-osmotic challenge. This increased I- efflux in P-glycoprotein containing cells was reduced to levels found in parental cells by the opiates morphine, pentazocine and naloxone. Addition of prostaglandin E2 to T84 cells resulted in elevated cellular cAMP levels and a significant I- efflux. This cAMP stimulated efflux was also inhibited by several opiates. None of the opiates was able to alter cAMP levels or protein kinase A mediated phosphorylation of immunoprecipitated cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel in T84 cells. The ability of opiates to alter ion conductances is discussed in relation to the anti-diarrheal effects of these compounds.

Disruption of cellular signaling pathways by daunomycin through destabilization of nonlamellar membrane structures

Albeit anthracyclines are widely used in the treatment of solid tumors and leukemias, their mechanism of action has not been elucidated. The present study gives relevant information about the role of nonlamellar membrane structures in signaling pathways, which could explain how anthracyclines can exert their cytocidal action without entering the cell [Tritton, T. R. & Yee, G. (1982) Science 217, 248-250]. The anthracycline daunomycin reduced the formation of the nonlamellar hexagonal (HII) phase (i.e., the hexagonal phase propensity), stabilizing the bilayer structure of the plasma membrane by a direct interaction with membrane phospholipids. As a consequence, various cellular events involved in signal transduction, such as membrane fusion and membrane association of peripheral proteins [e.g., guanine nucleotide-binding regulatory proteins (G proteins and protein kinase C-alpha beta)], where nonlamellar structures (negative intrinsic monolayer curvature strain) are required, were altered by the presence of daunomycin. Functionally, daunomycin also impaired the expression of the high-affinity state of a G protein-coupled receptor (ternary complex for the alpha 2-adrenergic receptor) due to G-protein dissociation from the plasma membrane. In vivo, daunomycin also decreased the levels of membrane-associated G proteins and protein kinase C-alpha beta in the heart. The occurrence of such nonlamellar structures favors the association of these peripheral proteins with the plasma membrane and prevents daunomycin-induced dissociation. These results reveal an important role of the lipid component of the cell membrane in signal transduction and its alteration by anthracyclines.

Peptides in membranes: helicity and hydrophobicity

Synthetic model membrane-interactive peptides--both of natural and designed sequence--have become convenient and systematic tools for determination of how the membrane-spanning segments within integral membrane proteins confer protein structure and biology. Conformational studies on these peptides demonstrate that the alpha-helix is the natural choice of conformation for a peptide segment in a membrane, and that a helical conformation will arise "automatically" in a peptide above a threshold hydrophobicity that allows it to associate stably with the membrane. Environmental and sequential contexts thus impart conformational versatility to many of the amino acids, thereby providing a mechanism for producing the diverse structural and functional properties of proteins.

Lipid domains and lipid/protein interactions in biological membranes

In the fluid mosaic model of membranes, lipids are organized in the form of a bilayer supporting peripheral and integral proteins. This model considers the lipid bilayer as a two-dimensional fluid in which lipids and proteins are free to diffuse. As a direct consequence, both types of molecules would be expected to be randomly distributed within the membrane. In fact, evidences are accumulating to indicate the occurrence of both a transverse and lateral regionalization of membranes which can be described in terms of micro- and macrodomains, including the two leaflets of the lipid bilayer. The nature of the interactions responsible for the formation of domains, the way they develop and the time- and space-scale over which they exist represent today as many challenging problems in membranology. In this report, we will first consider some of the basic observations which point to the role of proteins in the transverse and lateral regionalization of membranes. Then, we will discuss some of the possible mechanisms which, in particular in terms of lipid/protein interactions, can explain lateral heterogenities in membranes and which have the merit of providing a thermodynamic support to the existence of lipid domains in membranes.

Signals determining protein tyrosine kinase and glycosyl-phosphatidylinositol-anchored protein targeting to a glycolipid-enriched membrane fraction

Glycosyl-phosphatidylinositol (GPI)-anchored membrane proteins and certain protein tyrosine kinases associate with a Triton X-100-insoluble, glycolipid-enriched membrane fraction in MDCK cells. Also, certain protein tyrosine kinases have been shown to associate with GPI-anchored proteins in other cell types. To characterize the interaction between GPI-anchored proteins and protein tyrosine kinases, GPI-anchored proteins were coexpressed with p56lck in HeLa cells. Both proteins were shown to target independently to the glycolipid-enriched membranes. Coimmunoprecipitation of GPI-anchored proteins and p56lck occurred only when both proteins were located in the glycolipid-enriched membranes, and gentle disruption of these membranes abolished the interaction. The GPI anchor was found to be the targeting signal for this membrane fraction in GPI-anchored proteins. Analysis of mutants indicated that p56lck was nearly quantitatively palmitoylated at Cys-5 but not palmitoylated at Cys-3. The nonpalmitoylated cysteine at position 3 was very important for association of p56lck with the membrane fraction, while palmitoylation at Cys-5 promoted only a low level of interaction. Because other src family protein tyrosine kinases that are associated with GPI-anchored proteins always contain a Cys-3, we propose that this residue, in addition to the N-terminal myristate, is part of a common signal targeting these proteins to a membrane domain that has been linked to transmembrane signaling.

Effects of partial delipidation of rat ovarian membranes on thermal stability of LH/hCG receptors

The role of lipids and of possible structure-functional properties of the LH/hCG receptor were analyzed by thermal perturbation techniques in its native membrane environment. A method for the reversible removal of lipids from membranes with a mild detergent Tween 20 was developed. The receptor was reactivated with phosphatidylcholine (PC) by its reconstitution into proteoliposomes. The heat inactivation profile of LH/hCG binding sites in delipidated membranes was shifted to a temperature lower by approx. 8 C degrees (T50 values). Thermal inactivation of the delipidated LH/hCG receptor was found to be a quick process. Occupation of receptor binding sites by the agonist before thermal perturbation induced stabilization of the receptor. Thermal inactivation of the receptor by delipidation was fully reversed by treatment with soybean PC, dioleoylphosphatidylcholine (DOPC) and dipalmitoylphosphatidylcholine (DPPC), partly with phosphatidylethanolamine (PE) and sphingomyelin (SpM), but not with phosphatidylserine (PS), phosphatidylglycerol (PGl) or cholesterol. Delipidation modified the differential scanning calorimetric profile characteristic of control membranes. Delipidation of ovarian membranes also increased membrane lipid rigidity. The addition of PC, DOPC and PS to delipidated membranes decreased, that of DPPC and SpM increased, while PGl did not change the degree of fluorescence polarization of DPH, suggesting that membrane lipid fluidity was not involved in the stabilizing action of specific phospholipids against thermal inactivation of the ovarian LH/hCG receptor.