Specificity of lipid-protein interactions as determined by spectroscopic techniques

Hydrophobic coupling of lipid bilayer energetics to channel function

The hydrophobic coupling between membrane-spanning proteins and the lipid bilayer core causes the bilayer thickness to vary locally as proteins and other "defects" are embedded in the bilayer. These bilayer deformations incur an energetic cost that, in principle, could couple membrane proteins to each other, causing them to associate in the plane of the membrane and thereby coupling them functionally. We demonstrate the existence of such bilayer-mediated coupling at the single-molecule level using single-barreled as well as double-barreled gramicidin channels in which two gramicidin subunits are covalently linked by a water-soluble, flexible linker. When a covalently attached pair of gramicidin subunits associates with a second attached pair to form a double-barreled channel, the lifetime of both channels in the assembly increases from hundreds of milliseconds to a hundred seconds--and the conductance of each channel in the side-by-side pair is almost 10% higher than the conductance of the corresponding single-barreled channels. The double-barreled channels are stabilized some 100,000-fold relative to their single-barreled counterparts. This stabilization arises from: first, the local increase in monomer concentration around a single-barreled channel formed by two covalently linked gramicidins, which increases the rate of double-barreled channel formation; and second, from the increased lifetime of the double-barreled channels. The latter result suggests that the two barrels of the construct associate laterally. The underlying cause for this lateral association most likely is the bilayer deformation energy associated with channel formation. More generally, the results suggest that the mechanical properties of the host bilayer may cause the kinetics of membrane protein conformational transitions to depend on the conformational states of the neighboring proteins.

Quantifying the lateral lipid domain properties in erythrocyte ghost membranes using EPR-spectra decomposition

Using EPR spectroscopy a typical lateral domain structure was detected in the membranes of spin-labeled bovine erythrocyte ghosts. The spectral parameters were determined by decomposing the EPR spectrum into three spectral components and tuned by a hybrid-evolutionary-optimization method. In our experiments the lateral domain structure and its properties were influenced by the variation in the temperature and by the addition of n-butanol. The specific responses of the particular domain types were detected. For the most-ordered domain type a break was seen in the temperature dependence of its order parameter, while the order parameters of the two less-ordered domain types exhibited a continuous decrease. Below the break-point temperature the alcohol-induced membrane fluidity variation is mainly a consequence of the change in the proportions of the least- and the most-ordered domain type and not the change of the domain-type ordering or dynamics (with n-butanol concentration). On the other hand, the fluidity variation above the break-point temperature arises from both types of changes. Interestingly, the proportion of the domain type that has its order parameter between that of the least- and the most-ordered domain type remains almost constant with concentration as well as with temperature, which implies its stability. Such characterization of the lateral membrane domain structure could be beneficial when considering the lipid-protein interactions, because it can be assumed that the activity of the membrane-bound enzyme depends on the properties of the particular domain type.

Membrane behavior as influenced by partitioning of amphiphiles during drying: a comparative study in anhydrobiotic plant systems

During cellular desiccation, reduction in volume can in principle cause amphiphilic compounds to partition from the cytoplasm into membranes, with structural perturbance as the result. Here, we studied the effect of partitioning of endogenous amphiphiles on membrane surface dynamics in desiccation-tolerant and -intolerant, higher and lower plant systems, using electron paramagnetic resonance (EPR) spin probe techniques. Labeling cells with the amphiphilic spin probe perdeuterated TEMPONE (PDT) enabled partitioning into the various phases to be followed. During drying, PDT molecules preferentially partitioned from the aqueous cytoplasm into the membrane surface and, at advanced stages of water loss, also into oil bodies. There was no specific partition behavior that could be correlated with lower/higher plants or with desiccation-tolerance. In vivo labeling with 5-doxylstearate (5-DS) enabled membrane surface fluidity to be characterized. In hydrated plants, the 5-DS spectra contained an immobile and a fluid component. The characteristics of the immobile component could not be specifically correlated with either lower or higher plants, or with desiccation tolerance. The relative contribution of the fluid component to the 5-DS spectra was higher in lower plants than in higher plants, but considerably decreased with drying in all desiccation-tolerant organisms. In contrast, the proportion of the fluid component in desiccation-sensitive wheat seedling root was higher than that in desiccation-tolerant wheat axis and considerably increased at the onset of water loss. We suggest that partitioning of amphipaths fluidize the membrane surface, but that in desiccation-tolerant systems the membranes are protected from excessive fluidization.

Association of acylated cationic decapeptides with dipalmitoylphosphatidylserine-dipalmitoylphosphatidylcholine lipid membranes

The interaction of three acylated and cationic decapeptides with lipid membranes composed of dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylserine (DPPS) has been studied by means of fluorescence spectroscopy and differential scanning calorimetry (DSC). The synthetic model decapeptides that are N-terminally linked with C(2), C(8), and C(14) acyl chains contain four basic histidine residues in their identical amino acid sequence. A binding model, based on changes in the intrinsic fluorescent properties of the peptides upon association with the DPPC-DPPS membranes, is used to estimate the peptide-membrane dissociation constants. The results clearly show that all three peptides have a higher affinity to liposomes containing DPPS lipids due to non-specific electrostatic interactions between the cationic peptides and the anionic DPPS lipids. Furthermore, it is found that the acyl chain length of the peptides plays a crucial role for the binding. A preference for fluid phase membranes as compared to gel phase membranes is generally observed for all three peptides. DSC is used to characterise the influence of the three peptides on the thermodynamic phase behaviour of the binary DPPC-DPPS lipid mixture. The extent of peptide association deduced from the heat capacity measurements suggests a strong binding and membrane insertion of the C(14) acylated peptide in accordance with the fluorescence measurements.

Ternary phase diagram of dipalmitoyl-PC/dilauroyl-PC/cholesterol: nanoscopic domain formation driven by cholesterol

A ternary phase diagram is proposed for the hydrated lamellar lipid mixture dipalmitoylphosphatidylcholine/dilauroylphosphatidylcholine/cholesterol (DPPC/DLPC/cholesterol) at room temperature. The entire composition space has been thoroughly mapped by complementary experimental techniques, revealing interesting phase behavior that has not been previously described. Confocal fluorescence microscopy shows a regime of coexisting DPPC-rich ordered and DLPC-rich fluid lamellar phases, having an upper boundary at apparently constant cholesterol mole fraction chi(chol) approximately 0.16. Fluorescence resonance energy transfer experiments confirm the identification and extent of this two-phase regime and, furthermore, reveal a 1-phase regime between chi(chol) approximately 0.16 and 0.25, consisting of ordered and fluid nanoscopic domains. Dipyrene-PC excimer/monomer measurements confirm the new regime between chi(chol) approximately 0.16 and 0.25 and also show that rigidly ordered phases seem to disappear around chi(chol) approximately 0.25. This study should be considered as a step toward a more complete understanding of lateral heterogeneity within biomembranes. Cholesterol may play a role in domain separation on the nanometer scale.

Characterization of the interaction between phospholipid and protein by capillary electrophoresis with laser-induced fluorescence detection

We report an electrophoretic mobility shift-based method to study the interactions between phospholipids and proteins by capillary electrophoresis with laser-induced fluorescence detection. A fluorogenic dye, 3-(2-furoyl)quinoline-2-carboxaldehyde (FQ), was used to label phosphatidylserine (PS). Then the FQ labeled PS (FQ-PS) was used as the fluorescent probe for monitoring the association between PS and bovine serum albumin (BSA). Two conjugates were observed to form between each PS species and BSA, indicating that two interactions exist between these PS species and BSA. We can also detect the competitive association with BSA between labeled PS and unlabeled PS. This method only needs a minute volume of sample. It is highly sensitive and can be used to detect the interaction between phospholipids and nanomolar concentrations of proteins, such as BSA.

Association of an acylated model peptide with DPPC-DPPS lipid membranes

The interaction between a small positively charged peptide with a N-terminally linked acyl chain and dipalmitoylphosphatidylcholine-dipalmitoylphosphatidylserine (DPPC-DPPS) lipid membranes has been studied by means of fluorescence resonance energy transfer. Two different lipid compositions were used: a neutral membrane (100 mol% DPPC), and a negatively charged membrane (30 mol% DPPS in DPPC). The fluorescence resonance energy transfer results reveal that the peptide associates with both types of membranes. Furthermore, it is found that the slope of the titration curve for the negatively charged membranes is much steeper than that for the neutral membranes. This indicates a higher binding affinity of the acylated peptide towards negatively charged lipid membranes as compared with neutral lipid membranes.

Suppression of red cell diffusional water permeability by lipophilic solutes

The inhibitory effect of a series of neutral lipophilic solutes (methanol, ethanol, n-propanol, iso-propanol, n-butanol, iso-amylalcohol, n-hexanol, diethylether, nitrobenzene, and pyridine) on the diffusional water permeability (Pd, tot) of bovine erythrocyte membrane at 25 degrees C was studied in comparison to that of p-chloromercuri benzoate (pCMB). Permeability data were obtained by measuring the transmembrane diffusional water exchange time tau(exch) using an 1H-T2 NMR technique. Maximal inhibition by approximately 50% of Pd, tot was produced by 2 mM pCMB which completely blocked the membrane water channels in 20 min, hence suggesting the channel-to-lipid diffusional water permeability ratio of about 1:1. Furthermore, the maximal inhibitory effect of pCMB in combination with the lipophilic solutes was lower than that of pCMB alone. As pCMB does not interfere with the lipid bilayer, and provided that it blocks the water channels in solute presence as well, this confirms that the solutes induce an increase in the lipid-mediated background water permeability contribution (Pd, lipid) by the formation of aqueous leaks in the membrane hydrophobic barrier. However, faster but less efficient in permeability inhibition than pCMB (either alone or combined with solutes) were the lipophilic solutes alone. Taken together, the results indicate that the lipophilic solutes suppress the membrane total permeability Pd, tot by two opposing effects: a reduction of its channel-mediated part (Pd, channel) to the extent exceeding that of a simultaneous Pd, lipid increase. The inhibitory potency of the solutes tested appears to be correlated with their solubility in the membrane medium.

Mechanisms underlying the cost of living in animals

The cost of living can be measured as an animal's metabolic rate. Basal metabolic rate (BMR) is factorially related to other metabolic rates. Analysis of BMR variation suggests that metabolism is a series of linked processes varying in unison. Membrane processes, such as maintenance of ion gradients, are important costs and components of BMR. Membrane bilayers in metabolically active systems are more polyunsaturated and less monounsaturated than metabolically less-active systems. Such polyunsaturated membranes have been proposed to result in an increased molecular activity of membrane proteins, and in this manner the amount of membrane and its composition can act as a pacemaker for metabolism. The potential importance of membrane acyl composition in metabolic depression, hormonal control of metabolism, the evolution of endothermy, as well as its implications for lifespan and human health, are briefly discussed.

Membrane cholesterol content modulates activation of volume-regulated anion current in bovine endothelial cells

Activation of volume-regulated anion current (VRAC) plays a key role in the maintenance of cellular volume homeostasis. The mechanisms, however, that regulate VRAC activity are not fully understood. We have examined whether VRAC activation is modulated by the cholesterol content of the membrane bilayer. The cholesterol content of bovine aortic endothelial cells was increased by two independent methods: (a) exposure to a methyl-beta-cyclodextrin saturated with cholesterol, or (b) exposure to cholesterol-enriched lipid dispersions. Enrichment of bovine aortic endothelial cells with cholesterol resulted in a suppression of VRAC activation in response to a mild osmotic gradient, but not to a strong osmotic gradient. Depletion of membrane cholesterol by exposing the cells to methyl-beta-cyclodextrin not complexed with cholesterol resulted in an enhancement of VRAC activation when the cells were challenged with a mild osmotic gradient. VRAC activity in cells challenged with a strong osmotic gradient were unaffected by depletion of membrane cholesterol. These observations show that changes in membrane cholesterol content shift VRAC sensitivity to osmotic gradients. Changes in VRAC activation were not accompanied by changes in anion permeability ratios, indicating that channel selectivity was not affected by the changes in membrane cholesterol. This suggests that membrane cholesterol content affects the equilibrium between the closed and open states of VRAC channel rather than the basic pore properties of the channel. We hypothesize that changes in membrane cholesterol modulate VRAC activity by affecting the membrane deformation energy associated with channel opening.

Protein-induced membrane disorder: a molecular dynamics study of melittin in a dipalmitoylphosphatidylcholine bilayer

A molecular dynamics simulation of melittin in a hydrated dipalmitoylphosphatidylcholine (DPPC) bilayer was performed. The 19, 000-atom system included a 72-DPPC phospholipid bilayer, a 26-amino acid peptide, and more than 3000 water molecules. The N-terminus of the peptide was protonated and embedded in the membrane in a transbilayer orientation perpendicular to the surface. The simulation results show that the peptide affects the lower (intracellular) layer of the bilayer more strongly than the upper (extracellular) layer. The simulation results can be interpreted as indicating an increased level of disorder and structural deformation for lower-layer phospholipids in the immediate vicinity of the peptide. This conclusion is supported by the calculated deuterium order parameters, the observed deformation at the intracellular interface, and an increase in fractional free volume. The upper layer was less affected by the embedded peptide, except for an acquired tilt relative to the bilayer normal. The effect of melittin on the surrounding membrane is localized to its immediate vicinity, and its asymmetry with respect to the two layers may result from the fact that it is not fully transmembranal. Melittin's hydrophilic C-terminus anchors it at the extracellular interface, leaving the N-terminus "loose" in the lower layer of the membrane. In general, the simulation supports a role for local deformation and water penetration in melittin-induced lysis. As for the peptide, like other membrane-embedded polypeptides, melittin adopts a significant 25 degree tilt relative to the membrane normal. This tilt is correlated with a comparable tilt of the lipids in the upper membrane layer. The peptide itself retains an overall helical structure throughout the simulation (with the exception of the three N-terminal residues), adopting a 30 degree intrahelical bend angle.

Altered molecular dynamics and antioxidant status in the spermatozoa in testosterone-induced oligospermia in mouse

Though supraphysiological doses testosterone (T) and its derivatives are known to suppress spermatogenesis in mammals by interfering with the hypothalamus-pituitary axis leading to oligozoospermia, no study has been performed to evaluate the integrity of the sperm cells produced by such individuals. In T-induced oligozoospermia in the mouse, the spermatozoa showed suppressed zona-binding ability though the motility and viability remained unchanged. In order to assess whether this decreased zona-binding ability is due to perturbations in the mechanical properties of the sperm membranes, we attempted to examine the molecular dynamics employing a lipophilic spin label (16-doxyl stearate) and a protein-binding label (Mal-Net) in two sets of independent experiments. The results showed that the rotational freedom of lipophilic molecules reduced significantly within the first week of T-treatment. During weeks 1 through 4, the protein rotation was found to be retarded significantly. We observed a sharp increase in the ascorbyl radical associated with the cauda epididymal spermatozoa and epididymal fluid of testosterone-treated mice. Moreover, the glutathione (GSH) content in the spermatozoa and the epididymal fluid increased significantly after testosterone-treatment. Further, there was a elevation in the superoxide dismutase (SOD) activity and suppression in the superoxide anion radical generated by the cauda epididymal spermatozoa of testosterone-treated animals. A change in the mechanical properties of a bilayer could modify both the mechanical properties and the function of incorporated proteins. In many instances, a liquid-crystalline bilayer is necessary for protein function. It is likely that the change in the physical properties of sperm membranes might cause the inhibition of enzymes associated with spermatozoa after T-treatment. The alterations in the sperm membrane structure and the antioxidant potentials of both the spermatozoa and the cauda epididymal fluid could also account for the decrease in the zona-binding index of the spermatozoa in T-treated animals. Thus, this study demonstrates for the first time that supraphysiological doses of testosterone could modify the mechano-dynamic properties of sperm membranes and could perturb the redox status of both spermatozoa and the epididymal fluid.

Catalytic properties and conformation of hydrophobized alpha-chymotrypsin incorporated into a bilayer lipid membrane

A set of artificially hydrophobized alpha-chymotrypsin derivatives, carrying 2-11 stearoyl residues per enzyme molecule, were synthesized and their catalytic parameters and conformation in water solution and in the liposome-bound state were investigated. Hydrophobization of alpha-chymotrypsin and its further incorporation into phosphatidylcholine (PC) liposomes have no effect on the rate constant of the N-acetyl-L-tyrosine ethyl ester (ATEE) ester bond hydrolysis (k(cat)). At the same time, an increase in the number of stearoyl residues attached to the enzyme results in a drastic decrease of ATEE binding to the active center (K(M) increase). Incorporation of the hydrophobized enzyme into the PC liposome membrane results in K(M) recovery to nearly that of native alpha-chymotrypsin. The above changes are accompanied by partial unfolding of the enzyme molecules observed by fluorescence measurements. The obtained results are of interest to mimic the contribution of surface hydrophobic sites in the functioning of membrane proteins.

Simulation of the distribution and diffusion of a rigid amphipathic particle embedded in a model membrane

We simulate, by Brownian dynamics, the distribution, orientation and diffusion of a rigid molecule, represented as a dumbbell, with amphipathic nature, embedded in a model membrane. The significant features of a biological membrane are reproduced by means of a Maier-Saupe orienting potential, an enclosing potential and a lipophobic potential. We also evaluate the equilibrium quantities, such as order parameter, and dynamic features, such as rotational or translational diffusivity, of the embedded molecule in terms of the system parameters and compare the obtained results with those obtained from model independent theory.

Control of lipid membrane stability by cholesterol content

Cholesterol has a concentration-dependent effect on membrane organization. It is able to control the membrane permeability by inducing conformational ordering of the lipid chains. A systematic investigation of lipid bilayer permeability is described in the present work. It takes advantage of the transmembrane potential difference modulation induced in vesicles when an external electric field is applied. The magnitude of this modulation is under the control of the membrane electrical permeability. When brought to a critical value by the external field, the membrane potential difference induces a new membrane organization. The membrane is then permeable and prone to solubilized membrane protein back-insertion. This is obtained for an external field strength, which depends on membrane native permeability. This approach was used to study the cholesterol effect on phosphatidylcholine bilayers. Studies have been performed with lipids in gel and in fluid states. When cholesterol is present, it does not affect electropermeabilization and electroinsertion in lipids in the fluid state. When lipids are in the gel state, cholesterol has a dose-dependent effect. When present at 6% (mol/mol), cholesterol prevents electropermeabilization and electroinsertion. When cholesterol is present at more than 12%, electropermeabilization and electroinsertion are obtained under milder field conditions. This is tentatively explained by a cholesterol-induced alteration of the hydrophobic barrier of the bilayer core. Our results indicate that lipid membrane permeability is affected by the cholesterol content.

Spring constants for channel-induced lipid bilayer deformations. Estimates using gramicidin channels

Hydrophobic interactions between a bilayer and its embedded membrane proteins couple protein conformational changes to changes in the packing of the surrounding lipids. The energetic cost of a protein conformational change therefore includes a contribution from the associated bilayer deformation energy (DeltaGdef0), which provides a mechanism for how membrane protein function depends on the bilayer material properties. Theoretical studies based on an elastic liquid-crystal model of the bilayer deformation show that DeltaGdef0 should be quantifiable by a phenomenological linear spring model, in which the bilayer mechanical characteristics are lumped into a single spring constant. The spring constant scales with the protein radius, meaning that one can use suitable reporter proteins for in situ measurements of the spring constant and thereby evaluate quantitatively the DeltaGdef0 associated with protein conformational changes. Gramicidin channels can be used as such reporter proteins because the channels form by the transmembrane assembly of two nonconducting monomers. The monomerleft arrow over right arrow dimer reaction thus constitutes a well characterized conformational transition, and it should be possible to determine the phenomenological spring constant describing the channel-induced bilayer deformation by examining how DeltaGdef0 varies as a function of a mismatch between the hydrophobic channel length and the unperturbed bilayer thickness. We show this is possible by analyzing experimental studies on the relation between bilayer thickness and gramicidin channel duration. The spring constant in nominally hydrocarbon-free bilayers agrees well with estimates based on a continuum analysis of inclusion-induced bilayer deformations using independently measured material constants.

Electron-spin resonance study of aggregation of gramicidin in dipalmitoylphosphatidylcholine bilayers and hydrophobic mismatch

The effect of aggregation of gramicidin A' (GA) on the phase structure of dipalmitoylphosphatidylcholine (DPPC) multilamellar vesicles was studied by cw-ESR using a chain-labeled lipid (16PC) at temperatures between 30 degrees and 45 degreesC that span the main phase transition of DPPC. Boundary lipids were observed only in dispersions with GA/DPPC molar ratios >1:15, where GA aggregates. Detailed fits by nonlinear least squares (NLLS) methods are consistent with the boundary lipid being characterized by a large negative order parameter ( approximately -0.4), indicative of a dynamic bending of the end of the acyl chain, and a substantially reduced motion, about an order of magnitude slower than that of the bulk lipid. The NLLS analysis compares favorably with a recent two-dimensional Fourier transform ESR study on DPPC/GA vesicles, which accurately discerned the bulk lipid. The detailed ESR observables are discussed in terms of the ordering effect of GA at low concentration of GA, the dissociation of the GA channel and the dynamic bending of the end chain segment of boundary lipid at high concentration of GA, and of HII phase formation induced by GA. It is suggested that these phenomena can be interpreted in terms of the combined effects of partial dehydration of the lipid headgroup by the GA and of the hydrophobic mismatch between GA and DPPC molecules. Substantial hysteresis is observed for heating versus cooling cycles, but only for a GA/DPPC molar ratio >1:15. This is consistent with the aggregation of GA molecules at high concentrations.

Characterization of the leukotriene B4 receptor in porcine leukocytes. Separation and reconstitution with heterotrimeric GTP-binding proteins

Leukotriene B4 (LTB4) is a potent chemoattractant derived from arachidonic acid. When cDNAs for LTB4 receptor (BLT) were cloned it was found that they belong to a guanine nucleotide-binding regulatory protein (G-protein)-coupled receptor superfamily. However, purification of BLT from inflammatory cells and reconstitution with various types of G-proteins have not been successful. In the present study, BLT from porcine leukocytes was solubilized, separated from associated G-proteins by Ricinus communis agglutinin (RCA) 120 chromatography, and reconstituted with several endogenous and exogenous G-proteins, in combination with the fraction which contained endogenous phospholipids and Gbeta gamma. Kinetic studies of LTB4 were performed to determine the association with G-proteins. A partially purified BLT fraction (retained on an RCA120 column) free of G-proteins showed a lower affinity for LTB4 (Kd = 500 nm), but reconstitution of the BLT fraction with a G-protein-rich fraction (flow-through of an RCA column) increased the affinity for LTB4 10-fold (Kd = 50 nm). The partially purified BLT fraction was also reconstituted with exogenous G-proteins such as a heterotrimeric Gi2 purified from bovine brain or recombinant alpha subunits of Gi1, Gi2, Gi3, and Go expressed in Spodoptera frugiperda-9 cells. These increases in LTB4 bindings demonstrate that the BLT of porcine leukocytes can interact with pertussis toxin-sensitive G-proteins in vitro. The method is useful for the purification and reconstitution of other, as yet unisolated, G-protein-coupled receptors.

The lipid/protein interface as a target site for general anesthetics: a multiple-site kinetic analysis of synaptosomal Ca2+-ATPase

There is a long-standing controversy on whether membrane lipids or proteins are the target for general anesthetics. The plasma membrane-associated Ca2+-ATPase of synaptosomes has recently been established as a model system for general anesthesia, the protein interior being the proposed target site (M.M. Lopez, D. Kosk-Kosicka, J. Biol. Chem. 270 (1995) 28239-28245). Multiple-site kinetics is now applied as a mechanistic tool to analyze inhibition by organic solvents and general anesthetics. A close fit to the experimental data points was achieved using the complex equations for a competitive displacement of lipid activators from multiple sites on the protein surface. Inhibitor dissociation constants were about 1. 6x105-fold higher than the microscopic lipid dissociation binding constants that are derived here for the first time. Binding of lipid therefore is by -7.1 kcal/mole favored over that of the tested inhibitors. The latter are nevertheless effective because in the model used displacement of only few of the lipid solvation molecules cause complete inhibition. The lipid/protein interface rather than protein or lipid alone appeared to be the anesthetic target site.

Binding sites for exogenous and endogenous non-competitive inhibitors of the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor (AChR) is the paradigm of the neurotransmitter-gated ion channel superfamily. The pharmacological behavior of the AChR can be described as three basic processes that progress sequentially. First, the neurotransmitter acetylcholine (ACh) binds the receptor. Next, the intrinsically coupled ion channel opens upon ACh binding with subsequent ion flux activity. Finally, the AChR becomes desensitized, a process where the ion channel becomes closed in the prolonged presence of ACh. The existing equilibrium among these physiologically relevant processes can be perturbed by the pharmacological action of different drugs. In particular, non-competitive inhibitors (NCIs) inhibit the ion flux and enhance the desensitization rate of the AChR. The action of NCIs was studied using several drugs of exogenous origin. These include compounds such as chlorpromazine (CPZ), triphenylmethylphosphonium (TPMP+), the local anesthetics QX-222 and meproadifen, trifluoromethyl-iodophenyldiazirine (TID), phencyclidine (PCP), histrionicotoxin (HTX), quinacrine, and ethidium. In order to understand the mechanism by which NCIs exert their pharmacological properties several laboratories have studied the structural characteristics of their binding sites, including their respective locations on the receptor. One of the main objectives of this review is to discuss all available experimental evidence regarding the specific localization of the binding sites for exogenous NCIs. For example, it is known that the so-called luminal NCIs bind to a series of ring-forming amino acids in the ion channel. Particularly CPZ, TPMP+, QX-222, cembranoids, and PCP bind to the serine, the threonine, and the leucine ring, whereas TID and meproadifen bind to the valine and extracellular rings, respectively. On the other hand, quinacrine and ethidium, termed non-luminal NCIs, bind to sites outside the channel lumen. Specifically, quinacrine binds to a non-annular lipid domain located approximately 7 A from the lipid-water interface and ethidium binds to the vestibule of the AChR in a site located approximately 46 A away from the membrane surface and equidistant from both ACh binding sites. The non-annular lipid domain has been suggested to be located at the intermolecular interfaces of the five AChR subunits and/or at the interstices of the four (M1-M4) transmembrane domains. One of the most important concepts in neurochemistry is that receptor proteins can be modulated by endogenous substances other than their specific agonists. Among membrane-embedded receptors, the AChR is one of the best examples of this behavior. In this regard, the AChR is non-competitively modulated by diverse molecules such as lipids (fatty acids and steroids), the neuropeptide substance P, and the neurotransmitter 5-hydroxytryptamine (5-HT). It is important to take into account that the above mentioned modulation is produced through a direct binding of these endogenous molecules to the AChR. Since this is a physiologically relevant issue, it is useful to elucidate the structural components of the binding site for each endogenous NCI. In this regard, another important aim of this work is to review all available information related to the specific localization of the binding sites for endogenous NCIs. For example, it is known that both neurotransmitters substance P and 5-HT bind to the lumen of the ion channel. Particularly, the locus for substance P is found in the deltaM2 domain, whereas the binding site for 5-HT and related compounds is putatively located on both the serine and the threonine ring. Instead, fatty acid and steroid molecules bind to non-luminal sites. More specifically, fatty acids may bind to the belt surrounding the intramembranous perimeter of the AChR, namely the annular lipid domain, and/or to the high-affinity quinacrine site which is located at a non-annular lipid domain. Additionally, steroids may bind to a site located on the extracellular hydrophi

1H- and 2H-NMR studies of a fragment of PMP1, a regulatory subunit associated with the yeast plasma membrane H(+)-ATPase. Conformational properties and lipid-peptide interactions

PMP1 is a 38-residue polypeptide associated with the yeast plasma membrane H(+)-ATPase, found to regulate the enzyme activity. To investigate the molecular basis of the PMP1 biological function, the conformational properties of a synthetic PMP1 fragment, A18-F38, comprising the predicted C-terminal cytoplasmic domain and a part of the transmembrane anchor have been studied by 1H- and 2H-NMR spectroscopies. High resolution 1H-NMR experiments showed that, in deuterated DPC micelles, the A18-G34 segment adopts a well defined helix conformation. Our data suggest that the whole PMP1 molecule forms a unique helix whose axis might be slightly tilted with respect to the bilayer normal. Protonated DPC, DMPC and DMPS were incorporated in deuterated micelles containing the PMP1 fragment for studying lipid-peptide interactions. Unusually strong and selective intermolecular NOEs between lipid chain and peptide side chain protons, especially those of the unique Trp residue, were observed. Solid state 2H-NMR experiments performed on pure deuterated POPC and mixed deuterated POPC:POPS (5:1) bilayers revealed that the PMP1 fragment specifically interacts with negatively charged PS lipids.

Energetics of inclusion-induced bilayer deformations

The material properties of lipid bilayers can affect membrane protein function whenever conformational changes in the membrane-spanning proteins perturb the structure of the surrounding bilayer. This coupling between the protein and the bilayer arises from hydrophobic interactions between the protein and the bilayer. We analyze the free energy cost associated with a hydrophobic mismatch, i.e., a difference between the length of the protein's hydrophobic exterior surface and the average thickness of the bilayer's hydrophobic core, using a (liquid-crystal) elastic model of bilayer deformations. The free energy of the deformation is described as the sum of three contributions: compression-expansion, splay-distortion, and surface tension. When evaluating the interdependence among the energy components, one modulus renormalizes the other: e.g., a change in the compression-expansion modulus affects not only the compression-expansion energy but also the splay-distortion energy. The surface tension contribution always is negligible in thin solvent-free bilayers. When evaluating the energy per unit distance (away from the inclusion), the splay-distortion component dominates close to the bilayer/inclusion boundary, whereas the compression-expansion component is more prominent further away from the boundary. Despite this complexity, the bilayer deformation energy in many cases can be described by a linear spring formalism. The results show that, for a protein embedded in a membrane with an initial hydrophobic mismatch of only 1 A, an increase in hydrophobic mismatch to 1.3 A can increase the Boltzmann factor (the equilibrium distribution for protein conformation) 10-fold due to the elastic properties of the bilayer.

Gramicidin channel kinetics under tension

We have measured the effect of tension on dimerization kinetics of the channel-forming peptide gramicidin A. By aspirating large unilamellar vesicles into a micropipette electrode, we are able to simultaneously monitor membrane tension and electrical activity. We find that the dimer formation rate increases by a factor of 5 as tension ranges from 0 to 4 dyn/cm. The dimer lifetime also increases with tension. This behavior is well described by a phenomenological model of membrane elasticity in which tension modulates the mismatch in thickness between the gramicidin dimer and membrane.

Surface charge control of electropermeabilization and glycophorin electroinsertion with 1,2-diacyl-sn-glycero-3-phosphocholine (lecithin) liposomes

Back insertion of a solubilized membrane protein, glycophorin A, has been obtained in lipid multilamellar vesicles by applying calibrated electric field pulses on a lipid/protein mixture. Experimental evidence for insertion is given by means of immunofluorescence. Insertion was obtained only under field conditions that induced the leakage of a soluble hydrophilic molecule, calcein, which was trapped between the lipid layers. Studies were performed on mixed liposomes where charged species were present. The critical permeabilizing field is the same whatever the composition, but with overcritical fields the associated calcein transmembraneous flow is higher with positively charged lipids. Field conditions that where prone to trigger glycophorin insertion were similar to those that induced electropermeabilization. No electroinsertion has been obtained with stearylamine (SteNH2)/1,2-dipalmitoyl-sn-glycero-3-phosphocholine (Pam2GroPCho) liposomes under the same conditions. Calcein efflux as well as glycophorin insertion are controlled by the electric surface charge of the host liposome. These observations confirm our previous conclusions that spontaneous membrane protein insertion is obtained when the host membrane is brought to its electropermeabilized state, but show that a strong control due to the surface charges is nevertheless present.

In situ aggregational state of M13 bacteriophage major coat protein in sodium cholate and lipid bilayers

The in situ aggregational behavior of the bacteriophage M13 major coat protein was determined for the protein isolated in sodium cholate and reconstituted into DOPC lipid bilayers. For this purpose, the cysteine mutants A49C and T36C of the major coat protein were labeled with either a maleimido spin-label or a fluorescence label (IAEDANS). The steric restrictions sensed by the spin-label were used to evaluate the local protein conformation and the extent of protein-protein interactions at the position of the labeled residue. In addition, fluorescent labels covalently attached to the protein were used to determine the polarity of the local environment. The labeled coat protein mutants were examined under different conditions of protein association (amphiphile environment, ionic strength, temperature, and pH). The aggregational state of the major coat protein solubilized from the phage particle in sodium cholate was not dependent on the ionic strength, but was strongly dependent on cholate concentration and pH during sample preparation. At pH 7.0 and high sodium cholate concentration, the protein was in a dimeric form. The unusually strong association properties of the protein dimer in sodium cholate at pH 7.0 were attributed to the inability of sodium cholate to disrupt the strong hydrophobic forces between neighboring protein subunits in the phage particle. Such a "structural protein dimer" was, however, completely and irreversibly disrupted at pH 10.0. Qualitatively the same aggregational tendency was found upon changing the pH for the coat protein reconstituted in DOPC lipid bilayers. This reveals that the dimer disruption process is primarily a protein property, because there are no titratable groups on DOPC in the experimental pH range. The results are interpreted in terms of a model relating the protein aggregational state in the assembled phage to the protein aggregational behavior in sodium cholate and lipid bilayers.

Molecular sorting of lipids by bacteriorhodopsin in dilauroylphosphatidylcholine/distearoylphosphatidylcholine lipid bilayers

A combined experimental and theoretical study is performed on binary dilauroylphosphatidylcholine/distearoylphosphatidylcholine (DLPC/DSPC) lipid bilayer membranes incorporating bacteriorhodopsin (BR). The system is designed to investigate the possibility that BR, via a hydrophobic matching principle related to the difference in lipid bilayer hydrophobic thickness and protein hydrophobic length, can perform molecular sorting of the lipids at the lipid-protein interface, leading to lipid specificity/selectivity that is controlled solely by physical factors. The study takes advantage of the strongly nonideal mixing behavior of the DLPC/DSPC mixture and the fact that the average lipid acyl-chain length is strongly dependent on temperature, particularly in the main phase transition region. The experiments are based on fluorescence energy transfer techniques using specifically designed lipid analogs that can probe the lipid-protein interface. The theoretical calculations exploit a microscopic molecular interaction model that embodies the hydrophobic matching as a key parameter. At low temperatures, in the gel-gel coexistence region, experimental and theoretical data consistently indicate that BR is associated with the short-chain lipid DLPC. At moderate temperatures, in the fluid-gel coexistence region, BR remains in the fluid phase, which is mainly composed of short-chain lipid DLPC, but is enriched at the interface between the fluid and gel domains. At high temperatures, in the fluid phase, BR stays in the mixed lipid phase, and the theoretical data suggest a preference of the protein for the long-chain DSPC molecules at the expense of the short-chain DLPC molecules. The combined results of the experiments and the calculations provide evidence that a molecular sorting principle is active because of hydrophobic matching and that BR exhibits physical lipid selectivity. The results are discussed in the general context of membrane organization and compartmentalization and in terms of nanometer-scale lipid-domain formation.

Lipid-gramicidin interactions using two-dimensional Fourier-transform electron spin resonance

The application of two-dimensional Fourier-transform electron-spin-resonance (2D-FT-ESR) to the study of lipid/gramicidin A (GA) interactions is reported. It is shown that 2D-FT-ESR spectra provide substantially enhanced spectral resolution to changes in the dynamics and ordering of the bulk lipids (as compared with cw-ESR spectra), that result from addition of GA to membrane vesicles of dipalmitoylphosphatidylcholine (DPPC) in excess water containing 16-PC as the lipid spin label. The agreement between the theory of Lee, Budil, and Freed and experimental results is very good in the liquid crystalline phase. Both the rotational and translational diffusion rates of the bulk lipid are substantially decreased by addition of GA, whereas the ordering is only slightly increased, for a 1:5 ratio of GA to lipid. The slowing effect on the diffusive rates of adding GA in the gel phase is less pronounced. It is suggested that the spectral fits in this phase would be improved with a more detailed dynamic model. No significant evidence is found in the 2D-FT-ESR spectra for a second immobilized component upon addition of GA, which is in contrast to cw-ESR. It is shown from simulations of the observed 2D-FT-ESR spectra that the additional component seen in cw-ESR spectra, and usually attributed to "immobilized" lipid, is inconsistent with its being characterized by increased ordering, according to a model proposed by Ge and Freed, but it would be consistent with the more conventional model of a significantly reduced diffusional rate. This is because the 2D-FT-ESR spectra exhibit a selectivity, favoring components with longer homogeneous relaxation times, T2. The homogeneous linewidths of the 2D-FT-ESR autopeaks appear to broaden as a function of mixing time. This apparent broadening is very likely due to the process of cooperative order director fluctuations (ODF) of the lipids in the vesicle. This real-time observation of ODF is distinct from, but appears in reasonable agreement with, NMR results. It is found that addition of GA to give the 1:5 ratio has only a small effect on the ODF, but there is a significant temperature dependence.

Bending elasticities of model membranes: influences of temperature and sterol content

Giant liposomes obtained by electroformation and observed by phase-contrast video microscopy show spontaneous deformations originating from Brownian motion that are characterized, in the case of quasispherical vesicles, by two parameters only, the membrane tension sigma and the bending elasticity k(c). For liposomes containing dimyristoyl phosphatidylcholine (DMPC) or a 10 mol% cholesterol/DMPC mixture, the mechanical property of the membrane, k(c), is shown to be temperature dependent on approaching the main (thermotropic) phase transition temperature T(m). In the case of DMPC/cholesterol bilayers, we also obtained evidence for a relation between the bending elasticity and the corresponding temperature/cholesterol molecular ratio phase diagram. Comparison of DMPC/cholesterol with DMPC/cholesterol sulfate bilayers at 30 degrees C containing 30% sterol ratio shows that k(c) is independent of the surface charge density of the bilayer. Finally, bending elasticities of red blood cell (RBC) total lipid extracts lead to a very low k(c) at 37 degrees C if we refer to DMPC/cholesterol bilayers. At 25 degrees C, the very low bending elasticity of a cholesterol-free RBC lipid extract seems to be related to a phase coexistence, as it can be observed by solid-state (31)P-NMR. At the same temperature, the cholesterol-containing RBC lipid extract membrane shows an increase in the bending constant comparable to the one observed for a high cholesterol ratio in DMPC membranes.

Interactions of the nicotinic acetylcholine receptor transmembrane segments with the lipid bilayer in native receptor-rich membranes

Proper ion channel function of the nicotinic acetylcholine receptor (nAChR) requires the interaction of the protein with distinct lipid species present in the receptor's membrane microenvironment. Two classes of lipid binding sites present at the protein-membrane interface have been postulated: annular binding sites primarily occupied by phospholipids and non-annular binding sites mainly occupied by cholesterol [Jones & McNamee (1988) Biochemistry 27, 2364-2374]. We investigated the binding of these lipids to the nAChR and potential dynamics of these interactions during events associated with signal transduction by electron spin resonance spectroscopy (ESR) using spin-labeled analogues of phospholipids, androstane, and stearic acid. Protein-lipid interactions were characterized in receptor-rich membranes prepared from Torpedo californica electric tissue preserving the native lipid environment of the nAChR. We found a strong preference of the receptor for the phosphatidylserine (PS) analogue as compared to the other probes. Up to 57% of PS were perturbed by the membrane protein, while the fraction of motionally restricted lipid for the other analogues was on the order of 30%. After removal of the extramembrane portions of the membrane-bound receptor, we observed a loss of binding sites for the spin-labeled analogue of androstane and for stearic acid, but not for phospholipids and sphingomyelin analogues. Our results demonstrate the existence of topologically distinct lipid binding sites for different lipid species. In the case of cholesterol, extramembrane portions of the receptor are involved, whereas the transmembrane segments meet the requirements for the binding of phospholipids. Tyrosine phosphorylation of the nAChR did not affect protein-lipid interactions in samples of intact nAChR. Similarly, no significant changes were observed in the presence of carbamoylcholine at concentrations that caused rapid and quantitative desensitization of the nAChR.

Spectroscopic studies of the C-terminal secretion signal of the Serratia marcescens haem acquisition protein (HasA) in various membrane-mimetic environments

The structure of a peptide comprising the last 56 C-terminal residues of the Serratia marcescens haem acquisition protein (HasA) secreted by an ATP-binding cassette exporter was examined by 1H-NMR, circular dichroic and fluorescence spectroscopies. The peptide, which contains the secretion signal of HasA, is efficiently secreted by the HasA transporter. It is largely unstructured and flexible in aqueous buffer solution, but its helical content increases upon addition of trifluoroethanol, detergents and lipids. By circular dichroism, a stable helical conformation is observed between 20% and 70% (by vol.) trifluoroethanol. The 1H-NMR spectrum was analysed at these two trifluoroethanol concentrations; residues 7-15, 21-30 and 40-50 were shown to form relatively stable helices. In the presence of neutral detergent, alpha-helix is induced to a similar extent upon micelle formation; in this case, fluorescence data indicate that at least the N-terminus of the peptide interacts with the micelle. In the presence of negatively charged detergent, alpha-helix is induced before micelle formation and the N-terminus of the peptide seems not to be involved in this interaction. In the presence of negatively charged liposomes, the peptide interacts with the vesicle, again inducing a helical conformation. However, the helical content remains lower than upon addition of trifluoroethanol or neutral micelles. These results are compared to those previously obtained with the secretion signal of one of the Erwinia chrysanthemi metalloproteases which are transported efficiently by the HasA transporter. Both signals exhibit similar conformational features, despite their low sequence similarity.

Cardiotoxin II segregates phosphatidylglycerol from mixtures with phosphatidylcholine: (31)P and (2)H NMR spectroscopic evidence

The interaction of the cationic protein cardiotoxin II (CTX II) with mixtures of zwitterionic 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and anionic 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG) was investigated using phosphorus ((31)P) and deuterium ((2)H) nuclear magnetic resonance (NMR) spectroscopy. Adding CTX II to 1:1 POPC/POPG mixtures produced a two-component (31)P NMR spectrum, in which the second component had a decreased chemical shift anisotropy. Simultaneously, the (2)H NMR quadrupolar splitting measured from both POPC-alpha-d(2) and POPC-beta-d(2) decreased. Thus, CTX II produces an altered macroscopic phase state of the lipid bilayers, and this obscures any effects on bilayer surface electrostatics observed by (2)H NMR. Using magic angle spinning (MAS) (31)P NMR spectroscopy, two isotropic resonances were resolved in the absence of CTX II and were assigned to POPG (0.47 ppm) and POPC (-.58 ppm). Adding CTX II produced two new isotropic resonances shifted approximately 0.5 ppm downfield. Quantifying the intensities of the various resonance lines revealed that the binding isotherms for different POPC/POPG mixtures shifted onto a universal curve when expressed as a function of the CTX II/POPG ratio. The results indicate that CTX II binds preferentially to POPG and is able to laterally segregate POPG from POPC. Fitting of the binding isotherms was achieved using a two-site model derived from statistical-thermodynamic considerations. One class of binding site is specific for POPG and the other is nonspecific, capable of binding both POPC and POPG.

Effect of hydration upon the fluidity of intercellular membranes of stratum corneum: an EPR study

The principal mechanisms controlling the molecular permeability through the skin are associated to the intercellular membranes of stratum corneum (SC), the outermost layer of mammalian skin. It is generally accepted that an increase in fluidity of these membranes leads to a reduction of the physical barrier exerted by SC with a consequent enhancement in permeation of different compounds. It is known that water diffusion in SC increases with the increase in the water content in SC. Using the spin labeling method we evaluate the effect of hydration on the fluidity of intercellular membranes at three depths of the alkyl chain. Increase in the water content in SC leads to a drastic increase in membrane fluidity especially in the region near the membrane/water interface; the effect decreases on going deeper inside the hydrophobic core. Analysis of electron paramagnetic resonance (EPR) parameters as a function of temperature showed that the rotational motion at depth of the 16th carbon atom of the chain experienced a phase transition at 45 and 60 degrees C. These phase transition temperatures were not altered by changes in the water content of SC. A phase transition between 28 and 48 degrees C was observed from the segmental motion in the region near the polar headgroup (up to 12th carbon in the chain) and was strongly dependent upon the hydration of SC. Our results give a better characterization of the fluidity of SC, the main parameter involved in the mechanisms that control the permeability of different compounds through skin.

Modification of fluid lipid and mobile protein fractions of reticulocyte plasma membranes affects agonist-stimulated adenylate cyclase. Application of the percolation theory

The technique of fluorescence recovery after photobleaching was used to measure the lateral mobility of membrane integral proteins in reticulocyte plasma membranes which were treated to modify the 'fluid' lipid or immobilized protein fractions, hence increasing the relative prevalence of obstacles to protein lateral motion. This was achieved by either: (1) treating the plasma membranes with phospholipase A2 followed by extraction of the hydrolysis products using fatty-acid-free bovine serum albumin, resulting in a decrease in the membrane 'fluid' lipid portion; or (2) preincubating the plasma membranes with polylysines, resulting in plasma membrane protein aggregation and immobilization. As the prevalence of obstacles to lateral motion increased in plasma membranes through the treatments described above, the mobility of the membrane integral proteins diminished. Experimental results for the dependence of protein mobility on the prevalence of obstacles to lateral motion were compared to theoretical data in order to verify the applicability of the percolation theory to reticulocyte plasma membranes. The influence of a decrease in the 'fluid' lipid and an increase in the immobilized membrane protein fractions upon the hormone-stimulated adenylate cyclase activity has been studied as well. As the 'solid' lipid and immobilized membrane protein fractions decreased, both the hormone-stimulated adenylate cyclase activity and the fraction of beta-adrenergic receptors with high affinity to hormone diminished. It was shown that this correlation can be caused by a decrease in membrane fraction accessible to the movement of the interacting proteins of the adenylate cyclase complex. Hormonal stimulation of adenylate cyclase is discussed in terms of the percolation theory.

Interaction of cytochrome c with cardiolipin: an infrared spectroscopic study

The interactions of cytochrome c (cyt c) with cardiolipin, a major anionic phospholipid of mitochondrial membranes, and dioleoylphosphatidylglycerol (DOPG), have been compared by infrared (IR) spectroscopy. The Fourier self-deconvoluted IR spectra of the lipid carbonyl groups indicate that both cyt c3+ and cyt c2+ perturb and/or dehydrate the interfacial region of cardiolipin bilayers. Only a slight perturbation, if any, is observed in the interfacial region of DOPG bilayers. However, the phosphate head region of DOPG is perturbed by cyt c3+, which was not detected in cardiolipin. The results suggest that cytochrome c in both redox states can partially penetrate into cardiolipin but not into DOPG bilayers. The interaction of cyt c with cardiolipin and DOPG is mainly hydrophobic and electrostatic, respectively. The Fourier self-deconvoluted IR spectra in the amide I region reveal that ca. 10% of the cyt c3+ alpha-helix unfolds to random coil upon binding to cardiolipin bilayers. However, only very slight secondary structural changes, if any, were detected when cyt c3+ binds to DOPG bilayers.

Assembly of antibodies in lipid membranes for biosensor development

An investigation of the incorporation of antibody in lipid films of a composition that has been used for biosensor preparation is reported. IgG that is incorporated into lipid monolayers prepared from 7:3 mixtures of dipalmitoyl phosphatidylcholine and dipalmitoyl phosphatidic acid is edge-active, and enters and penetrates the fluid region of the mixed-phase system when monolayers are held at low pressure (< 20 mN/m). It was found that there is an "exclusion pressure" observed in pressure-area (pi-A) curves that are collected for monolayers that contain antibody. This term refers to a specific threshold of lateral pressure (which is reached by monolayer compression) that can cause explusion of antibody from the interior of a membrane. Microscopic images of monolayers containing the fluorescent phospholipid nitrobenzoxadiazole dipalmitoyl phosphatidylethanolamine (NBD-PE), or antibody labeled with tetramethylrhodamine isothiocyanate (TRITC), were used to determine the structure of membranes, and the location of effects on structure caused by IgG. Ellipsometric measurements of lipid monolayers that were cast onto silicon wafers by the Langmuir-Blodgett method were used to study the thickness of monolayers and to investigate the structural changes that occurred at the "exclusion pressure." Both the use of fluorescent antigen and ellipsometry indicated that antibody binding activity was present and was dependent on compression pressure. The effects of pH and ionic strength of subphase, antibody concentration, incubation time, and lateral pressure have been examined. The results may indicate the conditions that can be used to improve the incorporation of active IgG for preparation of biosensors that are based on lipid membranes.

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

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

Lysophospholipids modulate channel function by altering the mechanical properties of lipid bilayers

Lipid metabolites, free fatty acids and lysophospholipids, modify the function of membrane proteins including ion channels. Such alterations can occur through signal transduction pathways, but may also result from "direct" effects of the metabolite on the protein. To investigate possible mechanisms for such direct effects, we examined the alterations of gramicidin channel function by lysophospholipids (LPLs): lysophosphatidylcholine (LPC), lysophosphatidylethanolamine (LPE), lysophosphatidylserine (LPS), and lysophosphatidylinositol (LPI). The experiments were done on planar bilayers formed by diphytanoylphosphatidylcholine in n-decane a system where receptor-mediated effects can be excluded. At aqueous concentrations below the critical micelle concentration (CMC), LPLs can increase the dimerization constant for membrane-bound gramicidin up to 500-fold (at 2 microM). The relative potency increases as a function of the size of the polar head group, but does not seem to vary as a function of head group charge. The increased dimerization constant results primarily from an increase in the rate constant for channel formation, which can increase more than 100-fold (in the presence of LPC and LPI), whereas the channel dissociation rate constant decreases only about fivefold. The LPL effect cannot be ascribed to an increased membrane fluidity, which would give rise to an increased channel dissociation rate constant. The ability of LPC to decrease the channel dissociation rate constant varies as a function of channel length (which is always less than the membrane's equilibrium thickness): as the channel length is decreased, the potency of LPC is increased. LPC has no effect on membrane thickness or the surface tension of monolayers at the air/electrolyte interface. The bilayer-forming glycerolmonooleate does not decrease the channel dissociation rate constant. These results show that LPLs alter gramicidin channel function by altering the membrane deformation energy, and that the changes in deformation energy can be related to the molecular "shape" of the membrane-modifying compounds. Similar alterations in the mechanical properties of biological membranes may form a general mechanism by which one can alter membrane protein function.

Divalent metals of myelin and their differential binding by myelin basic protein of bovine central nervous system

Divalent metal ions are being implicated in the compaction of myelin. Levels of Cd, Co, Cu, Hg, Mn, Pb, Zn, Ca and Mg of isolated myelin of bovine central nervous system (CNS) were measured by flame atomic absorption spectrophotometry. Binding of these metal ions by isolated myelin basic protein (MBP, M(r) 18,500) of bovine CNS was concurrently assessed by centrifugal equilibrium dialysis. Metals were bound in the order of Hg > Cu > Zn > Mg > Cd > Co, exempting Mn, Pb and Ca. The results are indicative of differential metal affinity of MBP which may account for the immobilization or anchoring of MBP in myelin by zinc and other divalent metal cations.

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.

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.

The kinetic aspects of intracellular fluorescence labeling with TMA-DPH support the maturation model for endocytosis in L929 cells

TMA-DPH (1-(4-trimethylammonium)-6-phenyl-1,3,5-hexatriene), a hydrophobic fluorescent membrane probe, interacts with living cells by instantaneous incorporation into the plasma membrane, where it becomes fluorescent. It then follows the intracellular constitutive membrane traffic and acts as a bulk membrane marker of the endocytic pathway (Illinger, D., P. Poindron, P. Fonteneau, M. Modolell, and J. G. Kuhry. 1990. Biochim. Biophys. Acta. 1030:73-81; Illinger, D., P. Poindron, and J. G. Kuhry. 1991. Biol. Cell. 73:131-138). As such, TMA-DPH displays particular properties mainly due to partition between membranes and aqueous media. From these properties, original arguments can be inferred in favor of the maturation model for the endocytic pathway, against that of pre-existing compartments, in L929 cultured mouse fibroblasts. (a) TMA-DPH labeling is seen to progress from the cell periphery to perinuclear regions during endocytosis without any noticeable loss in fluorescence intensity; with a vesicle shuttle model this evolution would be accompanied by probe dilution with a decrease in the overall intracellular fluorescence intensity, and the labeling of the inner (late) compartments could in no way become more intense than that of the peripheral (early) ones. (b) From TMA-DPH fluorescence anisotropy assays, it is concluded that membrane fluidity is the same in the successive endocytic compartments as in the plasma membrane, which probably denotes a similar phospholipidic membrane composition, as might be expected in the maturation model. (c) TMA-DPH internalization and release kinetics are more easily described with the maturation model.

Studies on lipid membranes by two-dimensional Fourier transform ESR: Enhancement of resolution to ordering and dynamics

The first two-dimensional Fourier-transform electron spin resonance (2D-FT-ESR) studies of nitroxide-labeled lipids in membrane vesicles are reported. The considerable enhancement this experiment provides for extracting rotational and translational diffusion rates, as well as orientational ordering parameters by means of ESR spectroscopy, is demonstrated. The 2D spectral analysis is achieved using theoretical simulations that are fit to experiments by an efficient and automated nonlinear least squares approach. These methods are applied to dispersions of 1-palmitoyl-2oleoyl-sn-glycerophosphatidylcholine (POPC) model membranes utilizing spin labels 1-palmitoyl-2-(16-doxyl stearoyl) phosphatidylcholine and the 3-doxyl derivative of cholestan-3-one (CSL). Generally favorable agreement is obtained between the results obtained by 2D-FT-ESR on vesicles with the previous results on similar systems studied by continuous wave (cw) ESR on aligned samples. The precision in determining the dynamic and ordering parameters is significantly better for 2D-FT-ESR, even though the cw ESR spectra from membrane vesicles are resolved more poorly than those from well aligned samples. Some small differences in results by the two methods are discussed in terms of limitations of the methods and/or theoretical models, as well as possible differences between dynamic molecular structure in vesicles versus aligned membranes. An interesting observation with CSL/POPC, that the apparent homogeneous linewidths seem to increase in "real time," is tentatively attributed to the effects of slow director fluctuations in the membrane vesicles.

Steady-state fluorescence studies on lipase-vesicle interactions

The interaction of lipase from Candida cylindracea (CCL) with positively charged polymerizable surfactant vesicles was studied by the use of steady-state fluorescence techniques. The phase transition of vesicles composed of nonpolymerized and polymerized N-allylbis[2-(hexadecanoyloxy)ethyl]methylammonium bromide (ABHEMA Br) was determined in the absence of lipase, by measuring the change in fluorescence anisotropy of the membrane probe 1,6-diphenyl-1,3,5-hexatriene (DPH). The phase transition temperature for nonpolymerized vesicles is 49 degrees C and for the polymerized analogues 45 degrees C. Fluorescence anisotropy and resonance energy transfer measurements were used to illustrate the incorporation of the lipase in the vesicle membrane. These studies demonstrated that CCL is incorporated into the hydrophobic bilayer of the vesicle. By using an interfacial membrane probe 1-[4-(trimethylammonium)phenyl]-6-phenyl-1,3,5-hexatriene p-toluene sulphonate, TMA-DPH) and an internal membrane probe (DPH), it could be determined that the enzyme is incorporated more efficiently into nonpolymerized vesicles, and that the penetration of the enzyme into the bilayer is less deep in the case of the polymerized vesicles.