Specificity of lipid-protein interactions as determined by spectroscopic techniques

Binding equations for the lipid composition dependence of peripheral membrane-binding proteins

The specific recognition of peripheral membrane-binding proteins for their target membranes is mediated by a complex constellation of various lipid contacts. Despite the inherent complexities of the heterogeneous protein-membrane interface, the binding dependence of such proteins is, surprisingly, often reliably described by simple models such as the Langmuir Adsorption Isotherm or the Hill equation. However, these models were not developed to describe associations with two-dimensional, highly concentrated heterogeneous ligands such as lipid membranes. In particular, these models fail to capture the dependence on the lipid composition, a significant determinant of binding that distinguishes target from non-target membranes. In this work, we present a model that describes the dependence of peripheral proteins on lipid composition through an analytic expression for their association. The resulting membrane-binding equation retains the features of these simple models but completely describes the binding dependence on multiple relevant variables in addition to the lipid composition, such as protein and vesicle concentration. Implicit in this lipid composition dependence is a new form of membrane-based cooperativity that significantly differs from traditional solution-based cooperativity. We introduce the Membrane-Hill number as a measure of this cooperativity and describe its unique properties. We illustrate the utility and interpretational power of our model by analyzing previously published data on two peripheral proteins that associate with phosphatidylserine-containing membranes: The transmembrane immunoglobulin and mucin domain-containing protein 3 (TIM3) that employs calcium in its association, and milk fat globulin epidermal growth factor VIII (MFG-E8) which is completely insensitive to calcium. We also provide binding equations for systems that exhibit more complexity in their membrane-binding.

PRODAN Photophysics as a Tool to Determine the Bilayer Properties of Different Unilamellar Vesicles Composed of Phospholipids

Vesicles formed by phospholipids are promising candidates for drug delivery. It is known that the lipid composition affects properties such as the rigidity-fluidity of the membrane and that it influences the bilayer permeability, but sometimes sophisticated techniques are selected to monitor them. In this work, we study the bilayer of different unilamellar vesicles composed of different lipids (1,2-dioleoyl-sn-glycero-3-phosphocholine, DOPC, and lecithin) and diverse techniques such as extruder and electrospun templates and using 6-propionyl-2-(N,N-dimethyl) aminonaphthalene (PRODAN) and its photophysics. Moreover, we were able to monitor the influence of cholesterol on the bilayers. We demonstrate that the bilayer properties can be evaluated using the emission feature of the molecular probe PRODAN. This fluorescent probe gives relevant information on the polarity and fluidity of the microenvironment for unilamellar vesicles formed by two different methods. The PRODAN emission at 434 nm suggests that the bilayer properties significantly change if DOPC or lecithin is used in the vesicle preparation especially in their fluidity. Moreover, cholesterol induces alterations in the bilayer's structural and microenvironmental properties to a greater or lesser degree in both vesicles. Thus, we propose an easy and elegant way to evaluate physicochemical properties, which is fundamental for manufacturing vesicles as a drug delivery system, simply by monitoring the molecular probe emission band centered at 434 nm, which corresponds to the PRODAN species deep inside the bilayer.

Cationic Proteins Rich in Lysine Residue Trigger Formation of Non-bilayer Lipid Phases in Model and Biological Membranes: Biophysical Methods of Study

Cationic membrane-active toxins are the most abundant group of proteins in the venom of snakes and insects. Cationic proteins such as cobra venom cytotoxin and bee venom melittin are known for their pharmacological reactions including anticancer and antimicrobial effects which arise from the toxin-induced alteration in the dynamics and structure of plasma membranes and membranes of organelles. It has been established that these cationic toxins trigger the formation of non-bilayer lipid phase transitions in artificial and native mitochondrial membranes. Remarkably, the toxin-induced formation of non-bilayer lipid phase increases at certain conditions mitochondrial ATP synthase activity. This observation opens an intriguing avenue for using cationic toxins in the development of novel drugs for the treatment of cellular energy deficiency caused by aging and diseases. This observation also warrants a thorough investigation of the molecular mechanism(s) of lipid phase polymorphisms triggered by cationic proteins. This article presents a review on the application of powerful biophysical methods such as resonance spectroscopy (31P-, 1H-, 2H-nuclear magnetic resonance, and electron paramagnetic resonance), luminescence, and differential scanning microcalorimetry in studies of non-bilayer lipid phase transitions triggered by cationic proteins in artificial and biological membranes. A phenomenon of the triggered by cationic proteins the non-bilayer lipid phase transitions occurring within 10-2-10-11 s is discussed in the context of potential pharmacological applications of cationic proteins. Next to the ATP dimer is an inverted micelle made of cardiolipin that serves as a vehicle for the transport of H+ ions from the intra-crista space to the matrix. It is proposed that such inverted micelles are triggered by the high density of H+ ions and the cationic proteins rich in lysine residue which compete with the conserved lysine residues of the ATP synthase rotor for binding to cardiolipin in the inner mitochondrial membrane and perturb the bilayer lipid packing of cristae. Phospholipids with a blue polar head represent cardiolipin and those with a red polar head represent other phospholipids found in the crista membrane.

Two dimensional crowding effects on protein folding at interfaces observed by chiral vibrational sum frequency generation spectroscopy

The crowding effect is prevalent in cellular environments due to high concentrations of biomacromolecules. It can alter the structures and dynamics of proteins and thus impact protein functions. The crowding effect is important not only in 3-dimensional cytoplasm but also for a 2-dimensional (2D) cell surface due to the presence of membrane proteins and glycosylation of membrane proteins and phospholipids. These proteins and phospholipids - with limited translational degrees of freedom along the surface normal - are confined in 2D space. Although the crowding effect at interfaces has been studied by adding crowding agents to bulk solution, the 2D crowding effect remains largely unexplored. This is mostly due to challenges in controlling 2D crowding and synergistic use of physical methods for in situ protein characterization. To address these challenges, we applied chiral vibrational sum frequency generation (SFG) spectroscopy to probe the sp1 zinc finger (ZnF), a 31-amino acid protein, folding into a β-hairpin/α-helix (ββα) motif upon binding to Zn2+. We anchored ZnF at the air/water interface via covalent linkage of ZnF to palmitic acid and controlled 2D crowding by introducing neutral lipid as a spacer. We obtained chiral amide I SFG spectra upon addition of Zn2+ and/or spacer lipid. The chiral SFG spectra show that interfacial crowding in the absence of spacer lipid hinders ZnF from folding into the ββα structure even in the presence of Zn2+. The results establish a paradigm for future quantitative, systematic studies of interfacial crowding effects.

Effects of GM1 on brain spectrin-aminophospholipid interactions

Spectrin, a major component of the membrane skeletal meshwork of metazoan cells, is implicated to associate with membrane domains and is known to act as a scaffold for stabilization and activation of different signalling modules. We have studied the effect of GM1 (monosialotetrahexosyl ganglioside), a well-known model ganglioside and a signalling moiety, on the interaction of non-erythroid brain spectrin with both saturated and unsaturated aminophospholipids by spectroscopic methods. We observe that GM1 modulates brain spectrin-aminophospholipid interaction to the greatest degree whereas its effect on erythroid spectrin is not as pronounced. Fluorescence quenching studies show that brain spectrin interacts with DMPC/DMPE-based vesicles with a 10-fold increased affinity in presence of very low amounts of 2% and 5% GM1, and the extent of quenching decreases progressively in presence of increasing amounts of GM1. Interaction of brain spectrin with unsaturated membrane systems of DOPC/DOPE weakens in presence GM1. Increase in the mean lifetime of the Trp residues of brain spectrin in presence of GM1 indicates change in the microenvironment of spectrin, without affecting the secondary structure of the protein significantly. Studies on pressure - area isotherm of Langmuir-Blodgett monolayer and Brewster's angle microscopy show that GM1 has an expanding effect on the aminophospholipid monolayers, and ordered regions in DMPC/DMPE mixed monolayers are formed and are stabilized at higher pressure. GM1-induced fluidization of the phospholipid membranes and probable physical contact between bulky sugar head group of GM1 and spectrin, may explain the modulatory role of GM1 on aminophospholipid interactions with nonerythroid brain spectrin.

Quantifying the Interaction between EGFR Dimers and Grb2 in Live Cells

Adaptor proteins are a class of cytoplasmic proteins that bind to phosphorylated residues in receptor tyrosine kinases and trigger signaling cascades that control critically important cellular processes, such as cell survival, growth, differentiation, and motility. Here, we seek to characterize the interaction between epidermal growth factor receptor (EGFR) and the cytoplasmic adaptor protein growth factor receptor-bound protein 2 (Grb2) in a cellular context. To do so, we explore the utility of a highly biologically relevant model system, mammalian cells under reversible osmotic stress, and a recently introduced Förster resonance energy transfer microscopy method, fully quantified spectral imaging. We present a method that allows us to quantify the stoichiometry and the association constant of the EGFR-Grb2 binding interaction in the plasma membrane, in the presence and absence of activating ligand. The method that we introduce can have broad utility in membrane protein research, as it can be applied to different membrane protein-cytoplasmic protein pairs.

High resolution solid-state NMR spectroscopy of the Yersinia pestis outer membrane protein Ail in lipid membranes

The outer membrane protein Ail (Adhesion invasion locus) is one of the most abundant proteins on the cell surface of Yersinia pestis during human infection. Its functions are expressed through interactions with a variety of human host proteins, and are essential for microbial virulence. Structures of Ail have been determined by X-ray diffraction and solution NMR spectroscopy, but those samples contained detergents that interfere with functionality, thus, precluding analysis of the structural basis for Ail's biological activity. Here, we demonstrate that high-resolution solid-state NMR spectra can be obtained from samples of Ail in detergent-free phospholipid liposomes, prepared with a lipid to protein molar ratio of 100. The spectra, obtained with 13C or 1H detection, have very narrow line widths (0.40-0.60 ppm for 13C, 0.11-0.15 ppm for 1H, and 0.46-0.64 ppm for 15N) that are consistent with a high level of sample homogeneity. The spectra enable resonance assignments to be obtained for N, CO, CA and CB atomic sites from 75 out of 156 residues in the sequence of Ail, including 80% of the transmembrane region. The 1H-detected solid-state NMR 1H/15N correlation spectra obtained for Ail in liposomes compare very favorably with the solution NMR 1H/15N TROSY spectra obtained for Ail in nanodiscs prepared with a similar lipid to protein molar ratio. These results set the stage for studies of the molecular basis of the functional interactions of Ail with its protein partners from human host cells, as well as the development of drugs targeting Ail.

Electrochemical and photophysical behavior of 1-naphthol in benzyl-n-hexadecyldimethylammonium 1,4-bis(2-ethylhexyl)sulfosuccinate large unilamellar vesicles

The behavior of 1-naphthol and its partition process in LUVs formed from a new catanionic surfactant were studied by electrochemical and spectroscopic techniques.

Naja naja oxiana Cobra Venom Cytotoxins CTI and CTII Disrupt Mitochondrial Membrane Integrity: Implications for Basic Three-Fingered Cytotoxins

Cobra venom cytotoxins are basic three-fingered, amphipathic, non-enzymatic proteins that constitute a major fraction of cobra venom. While cytotoxins cause mitochondrial dysfunction in different cell types, the mechanisms by which cytotoxins bind to mitochondria remain unknown. We analyzed the abilities of CTI and CTII, S-type and P-type cytotoxins from Naja naja oxiana respectively, to associate with isolated mitochondrial fractions or with model membranes that simulate the mitochondrial lipid environment by using a myriad of biophysical techniques. Phosphorus-31 nuclear magnetic resonance (³¹P-NMR) spectroscopy data suggest that both cytotoxins bind to isolated mitochondrial fractions and promote the formation of aberrant non-bilayer structures. We then hypothesized that CTI and CTII bind to cardiolipin (CL) to disrupt mitochondrial membranes. Collectively, ³¹P-NMR, electron paramagnetic resonance (EPR), proton NMR (¹H-NMR), deuterium NMR (²H-NMR) spectroscopy, differential scanning calorimetry, and erythrosine phosphorescence assays suggest that CTI and CTII bind to CL to generate non-bilayer structures and promote the permeabilization, dehydration and fusion of large unilamellar phosphatidylcholine (PC) liposomes enriched with CL. On the other hand, CTII but not CTI caused biophysical alterations of large unilamellar PC liposomes enriched with phosphatidylserine (PS). Mechanistically, single molecule docking simulations identified putative CL, PS and PC binding sites in CTI and CTII. While the predicted binding sites for PS and PC share a high number of interactive amino acid residues in CTI and CTII, the CL biding sites in CTII and CTI are more divergent as it contains additional interactive amino acid residues. Overall, our data suggest that cytotoxins physically associate with mitochondrial membranes by binding to CL to disrupt mitochondrial structural integrity.

A Comparative Study of Phase States of the Peribacteroid Membrane from Yellow Lupin and Broad Bean Nodules

A comparative study of the lipid bilayer phase status and structure of the outer membrane of free-living Bradyrhizobium strain 359a (Nod⁺Fix⁺) and 400 (Nod⁺FixL) or Rhizobium leguminosarum 97 (Nod⁺Fix⁺, effective) and 87 (Nod⁺FixL, ineffective) has been carried out. Also, the effect of the symbiotic pair combination on the lipid bilayer structure of the bacteroid outer membrane and peribacteroid membrane, isolated from the nodules of Lupinus luteus L. or Vicia faba L., has been studied. As a result, it is shown that the lipid bilayer status of the bacteroid outer membrane is mainly determined by microsymbiont, but not the host plant. In the contrast, the lipid bilayer status of the peribacteroid membrane and, as a consequence, its properties depend on interaction of both symbiotic partners.

Propagation of MinCDE waves on free-standing membranes

As a spatial modulator of cytokinesis in Escherichia coli, the Min system cooperates with the nucleoid occlusion mechanism to target the divisome assembly towards mid-cell. Based on a reaction-diffusion mechanism powered by ATP (adenosine triphosphate) hydrolysis, the Min proteins propagate in waves on the cell membrane, resulting in oscillations between the cell poles, thus preventing the formation of the division ring everywhere but in the cell centre. The dynamic behaviour of Min proteins has been successfully reconstructed in vitro on supported lipid bilayers (SLBs), reproducing many of the features observed in the cell. However, there has been a marked discrepancy between the speed of propagation of Min protein waves in vitro, compared with the cellular system. A very plausible explanation is the different mobility of proteins on model membranes, compared with the inner membrane of bacteria. To quantitatively demonstrate how membrane diffusion influences Min wave propagation, we compared Min waves on SLBs with free-standing giant unilamellar vesicles (GUV) membranes which display higher fluidity. Intriguingly, the propagation velocity and wavelength on GUVs are three times higher than those reported on supported bilayers, but the wave period is conserved. This suggests that the shorter spatial period of the patterns in vivo might indeed be primarily explained by lower diffusion coefficients of proteins on the bacterial inner membrane.

Macromolecular interactions of the bacterial division FtsZ protein: from quantitative biochemistry and crowding to reconstructing minimal divisomes in the test tube

The division of Escherichia coli is an essential process strictly regulated in time and space. It requires the association of FtsZ with other proteins to assemble a dynamic ring during septation, forming part of the functionally active division machinery, the divisome. FtsZ reversibly interacts with FtsA and ZipA at the cytoplasmic membrane to form a proto-ring, the first molecular assembly of the divisome, which is ultimately joined by the rest of the division-specific proteins. In this review we summarize the quantitative approaches used to study the activity, interactions, and assembly properties of FtsZ under well-defined solution conditions, with the aim of furthering our understanding of how the behavior of FtsZ is controlled by nucleotides and physiological ligands. The modulation of the association and assembly properties of FtsZ by excluded-volume effects, reproducing in part the natural crowded environment in which this protein has evolved to function, will be described. The subsequent studies on the reactivity of FtsZ in membrane-like systems using biochemical, biophysical, and imaging technologies are reported. Finally, we discuss the experimental challenges to be met to achieve construction of the minimum protein set needed to initiate bacterial division, without cells, in a cell-like compartment. This integrated approach, combining quantitative and synthetic strategies, will help to support (or dismiss) conclusions already derived from cellular and molecular analysis and to complete our understanding on how bacterial division works.

Isolation, characterization and lipid-binding properties of the recalcitrant FtsA division protein from Escherichia coli

We have obtained milligram amounts of highly pure Escherichia coli division protein FtsA from inclusion bodies with an optimized purification method that, by overcoming the reluctance of FtsA to be purified, surmounts a bottleneck for the analysis of the molecular basis of FtsA function. Purified FtsA is folded, mostly monomeric and interacts with lipids. The apparent affinity of FtsA binding to the inner membrane is ten-fold higher than to phospholipids, suggesting that inner membrane proteins could modulate FtsA-membrane interactions. Binding of FtsA to lipids and membranes is insensitive to ionic strength, indicating that a net contribution of hydrophobic interactions is involved in the association of FtsA to lipid/membrane structures.

Desmosterol replaces cholesterol for ligand binding function of the serotonin(1A) receptor in solubilized hippocampal membranes: support for nonannular binding sites for cholesterol?

The serotonin(1A) receptor is an important member of the G-protein coupled receptor family, and is involved in the generation and modulation of a variety of cognitive and behavioral functions. Solubilization of the hippocampal serotonin(1A) receptor by CHAPS is accompanied by loss of cholesterol that results in a reduction in specific agonist binding activity. Replenishment of cholesterol to solubilized membranes restores membrane cholesterol content and significantly recovers specific agonist binding. In order to test the stringency of cholesterol requirement, we solubilized native hippocampal membranes followed by replenishment with desmosterol. Desmosterol is the immediate biosynthetic precursor of cholesterol in the Bloch pathway differing only in a double bond at the 24th position. Our results show that replenishment with desmosterol restores ligand binding of serotonin(1A) receptors. This is consistent with earlier results showing that desmosterol can replace cholesterol in a large number of cases. However, these results appear to be contradictory to our earlier findings, performed by sterol manipulation utilizing methyl-β-cyclodextrin, in which we observed that replacing cholesterol with desmosterol is unable to restore specific ligand binding of the hippocampal serotonin(1A) receptor. We discuss the possible molecular mechanism, in terms of nonannular lipid binding sites around the receptor, giving rise to these differences.

Using micropatterned lipid bilayer arrays to measure the effect of membrane composition on merocyanine 540 binding

The lipophilic dye merocyanine 540 (MC540) was used to model small molecule-membrane interactions using micropatterned lipid bilayer arrays (MLBAs) prepared using a 3D Continuous Flow Microspotter (CFM). Fluorescence microscopy was used to monitor MC540 binding to fifteen different bilayer compositions simultaneously. MC540 fluorescence was two times greater for bilayers composed of liquid-crystalline (l.c.) phase lipids (1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC),1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (SOPC), and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)) compared to bilayers in the gel phase (1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC)). The effect cholesterol (CHO) had on MC540 binding to the membrane was found to be dependent on the lipid component; cholesterol decreased MC540 binding in DMPC, DPPC and DSPC bilayers while having little to no effect on the remaining l.c. phase lipids. MC540 fluorescence was also lowered when 1,2-dioleoyl-sn-glycero-3-phospho-L-serine (sodium salt) (DOPS) was incorporated into DOPC bilayers. The increase in the surface charge density appears to decrease the occurrence of highly fluorescent monomers and increase the formation of weakly fluorescent dimers via electrostatic repulsion. This paper demonstrates that MLBAs are a useful tool for preparing high density reproducible bilayer arrays to study small molecule-membrane interactions in a high-throughput manner.

Surface plasmon resonance analysis on interactions of food components with a taste epithelial cell model

A new device for evaluating the continuity of taste was developed with the use of surface plasmon resonance (SPR). The model of lingual cells was constructed with liposomes immobilized onto an L1 sensor chip for SPR. Using this device, we classified food components into three categories according to the sensorgram pattern and residual ratio on lipid bilayer. Samples in group A strongly interacted with lipid bilayer, those in group B poorly interacted, and those in group C belong to neither group A nor group B. Sweet proteins and gymnemic acids that prolonged sweet perception were categorized in group A. Almost all the carbohydrates investigated and aspartame, of which the taste perception does not continue, belonged to group B. This device made it possible to detect the interaction with lipid bilayer and dissected the mechanism of taste continuity.

Identification of specific lipid-binding sites in integral membrane proteins

Protein-lipid interactions are increasingly recognized as central to the structure and function of membrane proteins. However, with the exception of simplified models, specific protein-lipid interactions are particularly difficult to highlight experimentally. Here, we used molecular dynamics simulations to identify a specific protein-lipid interaction in lactose permease, a prototypical model for transmembrane proteins. The interactions can be correlated with the functional dependence of the protein to specific lipid species. The technique is simple and widely applicable to other membrane proteins, and a variety of lipid matrices can be used.

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.

Fluorescence and UV resonance Raman study of peptide-vesicle interactions of human cathelicidin LL-37 and its F6W and F17W mutants

LL-37 is a broad-spectrum human antimicrobial peptide in the cathelicidin family. Potency assays in the form of minimal inhibitory concentration and vesicle leakage indicate that the single-tryptophan mutants, F6W and F17W, are as effective at killing bacteria and disrupting membranes as the native, tryptophan-free LL-37 peptide. Steady-state fluorescence and UV resonance Raman spectroscopy of F6W and F17W reveal molecular details of these tryptophan residues. The local environment polarity, hydrogen bond strength of the indole N-H moiety, and rotational freedom decrease for both F6W and F17W in the presence of carbonate ions relative to in pure distilled water; these results are consistent with burial of the hydrophobic region of alpha-helical LL-37 in oligomeric cores induced in the presence of carbonate ions. Differences in the spectroscopic properties of the carbonate-induced alpha-helical forms of F6W and F17W reflect the presence of a local lysine residue near F6W that makes the microenvironment of F6W more polar than that of F17W. In the presence of lipid vesicles, the mutants undergo additional loss of environment polarity, hydrogen bond strength, and rotational freedom. Quenching experiments utilizing brominated lipids reveal that the tryptophan residues in both mutants are essentially equidistant from the bilayer center and that bromines closer to the bilayer center, in the 9,10 positions, quench fluorescence more efficiently than those closer to the headgroups (6,7 positions). These results support carpeting or toroidal pore mechanisms of membrane disruption by LL-37 and demonstrate that the combination of tryptophan mutants and sensitive spectroscopic tools may provide important molecular clues about antimicrobial action.

Interaction of glucagon with artificial lipid bilayer membranes

The enhancement of fluorescence emission from the tryptophan residue of glucagon, the quenching of that emission with acrylamide and with 5-doxyl and 16-doxyl stearic acid, circular dichroism spectra, the release of 6-carboxyfluorescein, and polarized infrared attenuated total reflection (IR-ATR) spectra were used to study the interaction of glucagon with intact lipid vesicles and flat bilayers. Dimyristoylphosphatidylcholine bound the peptide only below the main transition temperature, thus confirming earlier results of Epand et al. (1977). However, the peptide is also bound by vesicles of unsaturated lipids above their transition temperature, suggesting an influence of lipid area on the binding process. Circular dichroism showed that binding to such vesicles also increases the helix content of glucagon. The IR-ATR study and a comparison with dynorphin-A-(1-13)-tridecapeptide revealed profound differences in orientation of the two peptides. The dichroic ratios and the derived order parameters indicated an isotropic orientation of the helical segments of glucagon, but did not exclude a principal orientation of the molecules lying flat on the membrane surface. In contrast, the axis of the dynorphin helix is clearly oriented normal to the interface. The two peptides also differ in their rates of 6-carboxyfluorescein release, suggesting a deeper penetration of the primary amphiphilic helix of dynorphin A-(1-13) than of the secondary amphiphilic helix of glucagon.

Carbon dioxide transport through membranes

Several membrane channels, like aquaporin-1 (AQP1) and the RhAG protein of the rhesus complex, were hypothesized to be of physiological relevance for CO(2) transport. However, the underlying assumption that the lipid matrix imposes a significant barrier to CO(2) diffusion was never confirmed experimentally. Here we have monitored transmembrane CO(2) flux (J(CO2)) by imposing a CO(2) concentration gradient across planar lipid bilayers and detecting the resulting small pH shift in the immediate membrane vicinity. An analytical model, which accounts for the presence of both carbonic anhydrase and buffer molecules, was fitted to the experimental pH profiles using inverse problems techniques. At pH 7.4, the model revealed that J(CO2) was entirely rate-limited by near-membrane unstirred layers (USL), which act as diffusional barriers in series with the membrane. Membrane tightening by sphingomyelin and cholesterol did not alter J(CO2) confirming that membrane resistance was comparatively small. In contrast, a pH-induced shift of the CO(2) hydration-dehydration equilibrium resulted in a relative membrane contribution of about 15% to the total resistance (pH 9.6). Under these conditions, a membrane CO(2) permeability (3.2 +/- 1.6 cm/s) was estimated. It indicates that cellular CO(2) uptake (pH 7.4) is always USL-limited, because the USL size always exceeds 1 mum. Consequently, facilitation of CO(2) transport by AQP1, RhAG, or any other protein is highly unlikely. The conclusion was confirmed by the observation that CO(2) permeability of epithelial cell monolayers was always the same whether AQP1 was overexpressed in both the apical and basolateral membranes or not.

Electrostatically-controlled protein adsorption onto lipid bilayer: modeling adsorbate aggregation behavior

Using adsorption models based on scaled particle (SPT) and double layer theories the electrostatically-controlled protein adsorption onto membrane surface has been simulated for non-associating and self-associating ligands. The binding isotherms of monomeric and oligomeric protein species have been calculated over a range of variable parameters including lipid and protein concentrations, protein and membrane charges, pH and ionic strength. Adsorption behavior of monomers appeared to be the most sensitive to the changes in the protein aggregation state. The hallmarks of the protein oligomerization are identified. The practical guides for optimal design of binding experiments focused on obtaining proofs of protein self-association are suggested.

New insights on the behavior of PRODAN in homogeneous media and in large unilamellar vesicles

The behavior of 6-propionyl-2-dimethylaminonaphthalene (PRODAN) was studied in homogeneous media and in large unilamellar vesicles (LUVs) of the phospholipid 1,2-di-oleoyl-sn-glycero-3-phosphatidylcholine (DOPC), using absorption, emission, depolarization, and time-resolved spectroscopies. In homogeneous media, the Kamlet and Taft solvatochromic comparison method quantified solute-solvent interactions from the absorption and emission PRODAN bands. These studies demonstrate that the absorption band is sensitive to the polarity-polarizability (pi) and the hydrogen bond donor ability (alpha) parameters of the media. PRODAN in the excited state is even more sensitive to these parameters and to the hydrogen bond acceptor ability (beta) of the media. The transition energy (expressed in kcal/mol) for both absorption and emission bands gives a linear correlation with the well-known polarity parameter E(T30). The results from the absorption and emission bands also reveal that PRODAN aggregates in water. The monomer has two fluorescence lifetimes, 2.27 and 0.65 ns, while the aggregate has a lifetime of 14.6 ns. Using steady-state anisotropy measurements, the calculated volumes of the aggregate and the monomer are 5590 and 222 mL mol(-1), respectively. In DOPC LUVs, PRODAN undergoes a partition process between the water bulk and the DOPC bilayer. We show that the partition constant (K(p)) value is large enough that only at [DOPC] below 0.15 mg/mL PRODAN in water can be detected. PRODAN dissolved in LUVs at [DOPC] > 1 mg/mL exists completely incorporated in its monomer form and senses two different microenvironments within the bilayer: a polar region in the interface near the water and a less polar and also less viscous environment, between the phospholipid tails. These environments were characterized by their fluorescence lifetimes (tau), showing that PRODAN in the polar microenvironment has a tau value of approximately 4 ns while in the less polar region gives a value of 1.2 ns. Moreover, this probe also senses the micropolarity of these two different regions of the bilayer and yields values similar to that of methanol and tetrahydrofuran.

Single-molecule methods for monitoring changes in bilayer elastic properties

Membrane-spanning proteins perturb the organization and dynamics of the adjacent bilayer lipids. For example, when the hydrophobic length (l) of a bilayer-spanning protein differs from the average thickness (d0) of the host bilayer, the bilayer thickness will vary locally in the vicinity of the protein in order to "match" the length of the protein's hydrophobic exterior to the thickness of the bilayer hydrophobic core. Such bilayer deformations incur an energetic cost, the bilayer deformation energy (DeltaG0def), which will vary as a function of the protein shape, the protein-bilayer hydrophobic mismatch (d0 - l), the lipid bilayer elastic properties, and the lipid intrinsic curvature (c0). Thus, if the membrane protein conformational changes underlying protein function involve the protein/bilayer interface, the ensuing changes in DeltaG0def (DeltaDeltaG0def) will contribute to the overall free-energy change of the conformational changes (DeltaG0tot)-meaning that the host lipid bilayer will modulate protein function. For a given protein, (DeltaDeltaG0def) varies as a function of the bilayer geometric properties (thickness and intrinsic curvature) and the elastic (bending and compression) moduli, which vary as a function of changes in lipid composition or with the adsorption of amphiphiles at the bilayer/solution interface. To understand how changes in bilayer properties modulate the function of bilayer-spanning proteins, single-molecule methods have been developed to probe changes in bilayer elastic properties using gramicidins as molecular force transducers. Different approaches to measuring the deformation energy are described: (1) measurements of changes in channel lifetimes and appearance rates as the lipid bilayer thickness or channel length are varied, (2) measurements of the equilibrium distribution among channels of different lengths, formed by homo- and heterodimers between gramicidin subunits of different lengths, and (3) measurements of the ratio of the appearance rates of heterodimer channels relative to parent homodimer channels formed by gramicidin subunits of different lengths.

Comparison of biophysical and biologic properties of alpha-helical enantiomeric antimicrobial peptides

In our previous study (Chen et al. J Biol Chem 2005, 280:12316-12329), we utilized an alpha-helical antimicrobial peptide V(681) as the framework to study the effects of peptide hydrophobicity, amphipathicity, and helicity on biologic activities where we obtained several V(681) analogs with dramatic improvement in peptide therapeutic indices against gram-negative and gram-positive bacteria. In the present study, the D-enantiomers of three peptides--V(681), V13A(D) and V13K(L) were synthesized to compare biophysical and biologic properties with their enantiomeric isomers. Each D-enantiomer was shown by circular dichroism spectroscopy to be a mirror image of the corresponding L-isomer in benign conditions and in the presence of 50% trifluoroethanol. L- and D-enantiomers exhibited equivalent antimicrobial activities against a diverse group of Pseudomonas aeruginosa clinical isolates, various gram-negative and gram-positive bacteria and a fungus. In addition, L- and D-enantiomeric peptides were equally active in their ability to lyse human red blood cells. The similar activity of L- and D-enantiomeric peptides on prokaryotic or eukaryotic cell membranes suggests that there are no chiral receptors and the cell membrane is the sole target for these peptides. Peptide D-V13K(D) showed significant improvements in the therapeutic indices compared with the parent peptide V(681) by 53-fold against P. aeruginosa strains, 80-fold against gram-negative bacteria, 69-fold against gram-positive bacteria, and 33-fold against Candida albicans. The excellent stability of D-enantiomers to trypsin digestion (no proteolysis by trypsin) compared with the rapid breakdown of the L-enantiomers highlights the advantage of the D-enantiomers and their potential as clinical therapeutics.

Coexisting domains in the plasma membranes of live cells characterized by spin-label ESR spectroscopy

The importance of membrane-based compartmentalization in eukaryotic cell function has become broadly appreciated, and a number of studies indicate that these eukaryotic cell membranes contain coexisting liquid-ordered (L(o)) and liquid-disordered (L(d)) lipid domains. However, the current evidence for such phase separation is indirect, and so far there has been no direct demonstration of differences in the ordering and dynamics for the lipids in these two types of regions or their relative amounts in the plasma membranes of live cells. In this study, we provide direct evidence for the presence of two different types of lipid populations in the plasma membranes of live cells from four different cell lines by electron spin resonance. Analysis of the electron spin resonance spectra recorded over a range of temperatures, from 5 to 37 degrees C, shows that the spin-labeled phospholipids incorporated experience two types of environments, L(o) and L(d), with distinct order parameters and rotational diffusion coefficients but with some differences among the four cell lines. These results suggest that coexistence of lipid domains that differ significantly in their dynamic order in the plasma membrane is a general phenomenon. The L(o) region is found to be a major component in contrast to a model in which small liquid-ordered lipid rafts exist in a 'sea' of disordered lipids. The results on ordering and dynamics for the live cells are also compared with those from model membranes exhibiting coexisting L(o) and L(d) phases.

Protein thermal stability and phospholipid–protein interaction investigated by capillary isoelectric focusing with whole column imaging detection

CIEF with whole column imaging detection (WCID) is an attractive technique for studying protein reaction and protein-ligand interaction due to its fast separation, simple operation, and high efficiency. In this study, two interesting applications by the CIEF-WCID were developed, involving the study of protein thermal stability and phospholipid-protein interaction. Four proteins (beta-lactoglobulin B, trypsin inhibitor, phosphorylase b, and trypsinogen) with different pI, and two types of phospholipids, including phosphatidylcholine (PC) and phosphatidylserine (PS), were used for this purpose. First, the altered CIEF profiles of four proteins were exhibited due to conformational changes resulting from protein denaturation induced by a high incubation temperature at 60 degrees C. It was demonstrated that the addition of a zwitterionic phospholipid (PC) played a crucial role in the thermal stability of targeted proteins, especially for trypsin inhibitor whose thermal stability was promoted with the addition of the PC vesicles at 60 degrees C. Second, the zwitterionic (PC) and acidic (PS) phospholipids displayed completely different interactions with the proteins. The addition of PS vesicles modified the zwitterionic phospholipids to carry negative charges, which correspondingly changed the interaction between the phospholipid and the protein. Our study demonstrates that the CIEF-WCID is a powerful approach to study protein reaction and protein-ligand interaction with high efficiency, high selectivity, and fast separation.

Phospholipid requirement and pH optimum for the in vitro enzymatic activity of the E. coli P-type ATPase ZntA

Detergent solubilization and purification of the E. coli heavy metal P-type ATPase ZntA yields an enzyme with reduced hydrolytic activity in vitro. Here, it is shown that the in vitro hydrolytic activity of detergent solubilized ZntA is increased in the presence of negatively charged phospholipids and at slightly acidic pH. The protein-lipid interaction of ZntA was characterized by enzyme-coupled ATPase assays and fluorescence spectroscopy. Among the most abundant naturally occurring phospholipids, only phosphatidyl-glycerol lipids (PG) enhance the in vitro enzymatic ATPase activity of ZntA. Re-lipidation of detergent purified ZntA with 1,2-dioleoylphosphatidyl-glycerol (DOPG) increases the ATPase activity four-fold compared to the purified state. All other E. coli phospholipids fail to activate the ATPase. Among the phosphatidyl-glycerol family, highest activity was observed for 1,2-dioleoyl-PG followed by 1,2-dimyristoyl-PG, 1,2-dipalmitoyl-PG and 1,2-distearoyl-PG. Increasing intrinsic Trp fluorescence quantum yield upon relipidation of ZntA was used to determine a pH maximum for lipid binding at pH 6.7. The pH dependence of the lipid binding was confirmed by pH-dependent ATPase assays showing maximum activity at pH 6.7. The biophysical characterization of detergent solubilized membrane proteins crucially relies on the conformational stability and functional integrity of the protein under investigation. The present study describes how the E. coli ZntA P-type ATPase can be stabilized and functionally activated in a detergent solubilized system.

Rational design of alpha-helical antimicrobial peptides with enhanced activities and specificity/therapeutic index

In the present study, the 26-residue peptide sequence Ac-KWKSFLKTFKSAVKTVLHTALKAISS-amide (V681) was utilized as the framework to study the effects of peptide hydrophobicity/hydrophilicity, amphipathicity, and helicity (induced by single amino acid substitutions in the center of the polar and nonpolar faces of the amphipathic helix) on biological activities. The peptide analogs were also studied by temperature profiling in reversed-phase high performance liquid chromatography, from 5 to 80 degrees C, to evaluate the self-associating ability of the molecules in solution, another important parameter in understanding peptide antimicrobial and hemolytic activities. A higher ability to self-associate in solution was correlated with weaker antimicrobial activity and stronger hemolytic activity of the peptides. Biological studies showed that strong hemolytic activity of the peptides generally correlated with high hydrophobicity, high amphipathicity, and high helicity. In most cases, the D-amino acid substituted peptides possessed an enhanced average antimicrobial activity compared with L-diastereomers. The therapeutic index of V681 was improved 90- and 23-fold against Gram-negative and Gram-positive bacteria, respectively. By simply replacing the central hydrophobic or hydrophilic amino acid residue on the nonpolar or the polar face of these amphipathic derivatives of V681 with a series of selected D-/L-amino acids, we demonstrated that this method has excellent potential for the rational design of antimicrobial peptides with enhanced activities.

Transmembrane asymmetry and lateral domains in biological membranes

It is generally assumed that rafts exist in both the external and internal leaflets of the membrane, and that they overlap so that they are coupled functionally and structurally. However, the two monolayers of the plasma membrane of eukaryotic cells have different chemical compositions. This out-of-equilibrium situation is maintained by the activity of lipid translocases, which compensate for the slow spontaneous transverse diffusion of lipids. Thus rafts in the outer leaflet, corresponding to domains enriched in sphingomyelin and cholesterol, cannot be mirrored in the inner cytoplasmic leaflet. The extent to which lipids contribute to raft properties can be conveniently studied in giant unilamellar vesicles. In these, cholesterol can be seen to condense with saturated sphingolipids or phosphatidylcholine to form microm scale domains. However, such rafts fail to model biological rafts because they are symmetric, and because their membranes lack the mechanism that establishes this asymmetry, namely proteins. Biological rafts are in general of nm scale, and almost certainly differ in size and stability in inner and outer monolayers. Any coupling between rafts in the two leaflets, should it occur, is probably transient and dependent not upon the properties of lipids, but on transmembrane proteins within the rafts.

Membrane interaction of erythroid spectrin: surface-density-dependent high-affinity binding to phosphatidylethanolamine

Density-dependent spectrin binding to dimyristoylphosphatidylcholine/dimyristoylphosphatidylethanolamine (DMPC/DMPE) small uni-lamellar vesicles (SUVs) has been directly evaluated in this work from the increase in the extent of quenching of the tryptophan fluorescence of spectrin at two different temperatures, above and below the main phase transition temperatures (Tm). Results from the binding studies of spectrin to phospholipid SUVs indicated that the binding dissociation constant Kd, increased from 45 +/- 7 nM in pure DMPC SUVs to 219 +/- 20 nM in DMPC/DMPE (50:50) SUVs, both in the gel and liquid crystalline phase. However, in pure DMPE SUVs the Kd decreased drastically to 0.7 +/- 0.2 nM in the gel phase at 18 degrees C and to 2.6 +/- 0.7 nM in the fluid phase at 55 degrees C indicating a high affinity binding of spectrin for the bilayer-forming DMPE. The maximum extent of phospholipid-induced quenching and the number of spectrin molecules associated with one SUV particle, evaluated in the present work, led to a model in DMPC/DMPE bilayer membranes indicating the PE-binding site of spectrin to localize at one of the terminal domains of the dimeric spectrin. A direct evidence of the localization of the PE-binding site at one of the terminal ends of the spectrin dimer also came from electron microscopic observation in fluid membranes made of bovine brain PE.

Effect of variations in the structure of a polyleucine-based alpha-helical transmembrane peptide on its interaction with phosphatidylethanolamine Bilayers

High-sensitivity differential scanning calorimetry and Fourier transform infrared spectroscopy were used to study the interaction of a cationic alpha-helical transmembrane peptide, acetyl-Lys2-Leu24-Lys2-amide (L24), and members of the homologous series of zwitterionic n-saturated diacyl phosphatidylethanolamines (PEs). Analogs of L24, in which the lysine residues were replaced by 2,3-diaminopropionic acid (acetyl-DAP2-Leu24-DAP2-amide (L24DAP)) or in which a leucine residue at each end of the polyleucine sequence was replaced by a tryptophan (Ac-K2-W-L22-W-K2-amide (WL22W)), were also studied to investigate the roles of lysine side-chain snorkeling and aromatic side-chain interactions with the interfacial region of phospholipid bilayers. The gel/liquid-crystalline phase transition temperature of the PE bilayers is altered by these peptides in a hydrophobic mismatch-independent manner, in contrast to the hydrophobic mismatch-dependent manner observed previously with zwitterionic phosphatidylcholine (PC) and anionic phosphatidylglycerol (PG) bilayers. Moreover, all three peptides reduce the phase transition temperature to a greater extent in PE bilayers than in PC and PG bilayers, indicating a greater disruption of PE gel-phase bilayer organization. Moreover, the lysine-anchored L24 reduces the phase transition temperature, enthalpy, and the cooperativity of PE bilayers to a much greater extent than DAP-anchored L24DAP, whereas replacement of the terminal leucines by tryptophan residues (Ac-K2-W-L22-W-K2-amide) only slightly attenuates the effects of this peptide on the chain-melting phase transition of the host PE bilayers. All three peptides form very stable alpha-helices in PE bilayers, but small conformational changes occur in response to mismatch between peptide hydrophobic length and gel-state lipid bilayer hydrophobic thickness. These results suggest that the lysine snorkeling plays a significant role in the peptide-PE interactions and that cation-pi-interactions between lysine and tryptophan residues may modulate these interactions. Altogether, these results suggest that the lipid-peptide interactions are affected not only by the hydrophobic mismatch between these peptides and the host lipid bilayer but also by the electrostatic and hydrogen-bonding interactions between the positively charged lysine residues at the termini of these peptides and the polar headgroups of PE bilayers.

Soft picture of lateral heterogeneity in biomembranes

Standard methods of characterization of electron paramagnetic resonance (EPR) spectra of spin-labeled biomembranes limit the resolution of lateral heterogeneity to only two or three domain types. This disables examination of the structure-function relationship in complex membranes, which might be composed of a larger number of different domain types. To enable exploration of this kind, a new approach based on analysis of EPR spectra with multi-run, hybrid evolutionary optimization is proposed here. From the multiple runs a quasi-continuous distribution of membrane spectral parameters (order parameter, proportion of spectral component, polarity correction factor, rotational correlation time and broadening constant) can be constructed and presented by a new presentation technique CODE (colored distribution of E PR spectral parameters). Through this the concept of a "soft" picture of membrane heterogeneity is introduced, in contrast to the standard "discrete" domain picture. The "soft" characterization method, established on synthetic spectra, was used to examine the lateral heterogeneity of liposome membranes as well as of membranes of neutrophils from healthy and asthmatic horses. In liposome membranes the determined number of domain types was the same as already established by standard procedures of EPR spectra line-shape interpretation. In membranes of neutrophils a quasi-continuous distribution of membrane domain properties was detected by the new method.

Membrane inclusions as coupled harmonic oscillators: Effects due to anisotropic membrane slope relaxation

Membrane-mediated interaction between membrane-spanning peptides or protein segments plays an important role in their function and stability. Our rigorous "coupled harmonic oscillators" representation is extended to account for the complex boundary conditions permitting anisotropic relaxation of the membrane slope along the contours of the inclusions. Using this representation and applying a highly efficient finite-difference algorithm, we have analyzed the membrane-mediated interaction triggered by deformation of the hydrophobic tails of lipid molecules to match the lipophilic exterior of the inserted peptide. We establish that anisotropic relaxation crucially affects the interaction energy, leading to a short-range attraction between two inclusions, while conventional isotropic boundary conditions result in their strong repulsion. In a multi-inclusion cluster, this attraction is further enhanced and modified due to nonpairwise interactions. The results for dimyristoyl phosphatidylcholine and glyceryl monooleate membranes are compared, and the effects of the inclusion radius are considered. The possible role of slope relaxation in the reported stabilization of linked gramicidin channels and in proteins' functional cooperativity is outlined.

Fluorescence study on rat epithelial cells and liposomes exposed to aromatic nitroxides

This study was performed to evaluate the effects, if any, of aromatic nitroxides, namely, indolinic nitroxides, on membrane fluidity of rat epithelial cells using steady-state fluorescence. These nitroxides are being increasingly considered as new and versatile compounds to reduce oxidative stress in biological systems. Hence, the results obtained in this study will give more insights on the interaction of these compounds with biological structures which at present is lacking, especially in view of their possible application as antioxidant therapeutic agents. The probes DPH and Laurdan which give information on the hydrophobic and hydrophilic-hydrophobic regions of the membrane bilayer, respectively, showed that nitroxide 1 (1,2-dihydro-2-methyl-3H-indole-3-one-1-oxyl) significantly increases membrane fluidity, whereas the corresponding phenylimino nitroxide derivative 2 (1,2-dihydro-2-methyl-3H-indole-3-phenylimino-1-oxyl) leads to membrane rigidification. The aliphatic nitroxide TEMPO included in this study for comparison produced no modifications. Consequently, it appears that the structure of the heterocyclic rings (aromatic or aliphatic) and the substituents may affect membrane fluidity differently.

Identification of the dimer interface of the lactose transport protein from Streptococcus thermophilus

The lactose transporter from Streptococcus thermophilus catalyses the symport of galactosides and protons. The carrier domain of the protein harbours the contact sites for dimerization, and the individual subunits in the dimer interact functionally during the transport reaction. As a first step towards the elucidation of the mechanism behind the cooperation between the subunits, regions involved in the dimer interface were determined by oxidative and chemical cross-linking of 12 cysteine substitution mutants. Four positions in the protein were found to be susceptible to intermolecular cross-linking. To ensure that the observed cross-links were not the result of randomly colliding particles, the cross-linking was studied in samples in which either the concentration of LacS in the membrane was varied or the oligomeric state was manipulated. These experiments showed that the cross-links were formed specifically within the dimer. The four regions of the protein located at the dimer interface are close to the extracellular ends of transmembrane segments V and VIII and the intracellular ends of transmembrane segments VI and VII.

Three-dimensional crystallization of the Escherichia coli glycerol-3-phosphate transporter: a member of the major facilitator superfamily

Here we report the successful three-dimensional crystallization of GlpT, the glycerol-3-phosphate transporter from Escherichia coli inner membrane. GlpT possesses 12 transmembrane alpha-helices and is a member of the major facilitator superfamily. It mediates the exchange of glycerol-3-phosphate for inorganic phosphate across the membrane. Approximately 20 phospholipid molecules per protein, identified as negatively charged phosphatidylethanolamine, phosphatidylglycerol, and cardiolipin, were required for the monodispersity of purified GlpT. Analytical size-exclusion chromatography proved to be efficient in identifying detergents for GlpT monodispersity. Nine such detergents were later used for GlpT crystallization. Screening for crystal nucleation was carried out with a variety of polyethylene glycols as the precipitant over a wide pH range. Subsequent identification of a rigid protein core by limited proteolysis and mass spectroscopy resulted in better-ordered crystals. These crystals exhibited order to 3.7 A resolution in two dimensions. However, the stacking in the third dimension was partially disordered. This stacking problem was overcome by using a detergent mixture and manipulating the ionic interactions in the crystallization solution. The resulting GlpT crystals diffracted isotropically to 3.3 A resolution and were suitable for structure determination by X-ray crystallography.

Quantitative analysis of membrane protein-amphiphile interactions using resonance energy transfer

This work describes a simple method for determining the association constant of amphiphiles to membrane proteins. The method uses a fluorescent phospholipid probe, which senses the competition among unlabeled amphiphiles for positions on the transmembrane surface of the protein. The contact between the probe and the protein surface is detected through resonance energy transfer. We have analyzed theoretically this process deriving a general equation for the dependence of the energy transfer efficiency on the composition of the micelles/bilayers in which the protein is inserted. This equation includes an exchange constant for each amphiphile, which gives a measure of its affinity for the protein with respect to that of an amphiphile set as the reference. We applied this method to determine the exchange constant of different phospholipids for the plasma membrane calcium pump.