Design and characterization of gramicidin channels

Intrinsic Lipid Curvature and Bilayer Elasticity as Regulators of Channel Function: A Comparative Single-Molecule Study

Perturbations in bilayer material properties (thickness, lipid intrinsic curvature and elastic moduli) modulate the free energy difference between different membrane protein conformations, thereby leading to changes in the conformational preferences of bilayer-spanning proteins. To further explore the relative importance of curvature and elasticity in determining the changes in bilayer properties that underlie the modulation of channel function, we investigated how the micelle-forming amphiphiles Triton X-100, reduced Triton X-100 and the HII lipid phase promoter capsaicin modulate the function of alamethicin and gramicidin channels. Whether the amphiphile-induced changes in intrinsic curvature were negative or positive, amphiphile addition increased gramicidin channel appearance rates and lifetimes and stabilized the higher conductance states in alamethicin channels. When the intrinsic curvature was modulated by altering phospholipid head group interactions, however, maneuvers that promote a negative-going curvature stabilized the higher conductance states in alamethicin channels but destabilized gramicidin channels. Using gramicidin channels of different lengths to probe for changes in bilayer elasticity, we found that amphiphile adsorption increases bilayer elasticity, whereas altering head group interactions does not. We draw the following conclusions: first, confirming previous studies, both alamethicin and gramicidin channels are modulated by changes in lipid bilayer material properties, the changes occurring in parallel yet differing dependent on the property that is being changed; second, isolated, negative-going changes in curvature stabilize the higher current levels in alamethicin channels and destabilize gramicidin channels; third, increases in bilayer elasticity stabilize the higher current levels in alamethicin channels and stabilize gramicidin channels; and fourth, the energetic consequences of changes in elasticity tend to dominate over changes in curvature.

Lipid-Dependent Titration of Glutamic Acid at a Bilayer Membrane Interface

The ionization properties of protein side chains in lipid-bilayer membranes will differ from the canonical values of side chains exposed to an aqueous solution. While the propensities of positively charged side chains of His, Lys, and Arg to release a proton in lipid membranes have been rather well characterized, the propensity for a negatively charged Glu side chain to receive a proton and achieve the neutral state in a bilayer membrane has been less well characterized. Indeed, the ionization of the glutamic acid side chain has been predicted to depend on its depth of burial in a lipid membrane but has been difficult to verify experimentally. To address the issue, we incorporated an interfacial Glu residue at position 4 of a distinct 23-residue transmembrane helix and used ²H NMR to examine the helix properties as a function of pH. We observe that the helix tilt and azimuthal rotation vary little with pH, but the extent of helix unraveling near residues 3 and 4 changes as the Glu residue E4 titrates. Remarkably, the ²H quadrupolar splitting for the side chain of alanine A3 responds to pH with an apparent pK _a of 4.8 in 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC) and 6.3 in 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC), but is unchanged up to pH 8.0 in 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) in the presence of residue E4. With bilayers composed of alkali-stable ether-linked lipids, the side chain of A3 responds to pH with an apparent pK _a of 11.0 in the ether analogue of DOPC. These results suggest that the depth dependence of Glu ionization in lipid-bilayer membranes may be steeper than previously predicted or envisioned.

Examination of pH dependency and orientation differences of membrane spanning alpha helices carrying a single or pair of buried histidine residues

We have employed the peptide framework of GWALP23 (acetyl-GGALWLALALALALALALWLAGA-amide) to examine the orientation, dynamics and pH dependence of peptides having buried single or pairs of histidine residues. When residue L8 is substituted to yield GWALP23-H⁸, acetyl-GGALWLAH⁸ALALALALALWLAGA-amide, the deuterium NMR spectra of ²H-labeled core alanine residues reveal a helix that occupies a single transmembrane orientation in DLPC, or in DMPC at low pH, yet shows multiple states at higher pH or in bilayers of DOPC. Moreover, a single histidine at position 8 or 16 in the GWALP23 framework is sensitive to pH. Titration points are observed near pH 3.5 for the deprotonation of H8 in lipid bilayers of DLPC or DMPC, and for H16 in DOPC. When residues L8 and L16 both are substituted to yield GWALP23-H^8,16, the ²H NMR spectra show, interestingly, no titration dependence from pH 2-8, yet bilayer thickness-dependent orientation differences. The helix with H8 and H16 is found to adopt a transmembrane orientation in thin bilayers of DLPC, a combination of transmembrane and surface orientations in DMPC, and then a complete transition to a surface bound orientation in the thicker DPoPC and DOPC lipid bilayers. In the surface orientations, alanine A7 no longer fits within the core helix. These results along with previous studies with different locations of histidine residues suggest that lipid hydrophobic thickness is a first determinant and pH a second determinant for the helical orientation, along with possible side-chain snorkeling, when the His residues are incorporated into the hydrophobic region of a lipid membrane-associated helix.

Comparing Interfacial Trp, Interfacial His and pH Dependence for the Anchoring of Tilted Transmembrane Helical Peptides

Charged and aromatic amino acid residues, being enriched toward the terminals of membrane-spanning helices in membrane proteins, help to stabilize particular transmembrane orientations. Among them, histidine is aromatic and can be positively charge at low pH. To enable investigations of the underlying protein-lipid interactions, we have examined the effects of single or pairs of interfacial histidine residues using the constructive low-dynamic GWALP23 (acetyl-GG²ALW⁵LALALALALALALW¹⁹LAG²²A-amide) peptide framework by incorporating individual or paired histidines at locations 2, 5, 19 or 22. Analysis of helix orientation by means of solid-state ²H NMR spectra of labeled alanine residues reveals marked differences with H^2,22 compared to W^2,22. Nevertheless, the properties of membrane-spanning H^2,22WALP23 helices show little pH dependence and are similar to those having Gly, Arg or Lys at positions 2 and 22. The presence of H5 or H19 influences the helix rotational preference but not the tilt magnitude. H5 affects the helical integrity, as residue 7 unwinds from the core helix; yet once again the helix orientation and dynamic properties show little sensitivity to pH. The overall results reveal that the detailed properties of transmembrane helices depend upon the precise locations of interfacial histidine residues.

Influence of interfacial tryptophan residues on an arginine-flanked transmembrane helix

The transmembrane helices of membrane proteins often are flanked by interfacial charged or aromatic residues that potentially help to anchor the membrane-spanning protein. For isolated single-span helices, the interfacial residues may be especially important for stabilizing particular tilted transmembrane orientations. The peptide RWALP23 (acetyl-GR²ALW(LA)₆LWLAR²²A-amide) has been employed to investigate the interplay between interfacial arginines and tryptophans. Here we replace the tryptophans of RWALP23 with A5 and A19, to investigate arginines alone with respect to helix fraying and orientation in varying lipid bilayers. Deuterated alanines incorporated into the central sequence allow the orientation and stability of the core helix to be assessed by means of solid -state ²H NMR in bilayers of DOPC, DMPC and DLPC. The helix tilt from the bilayer normal is found to increase slightly when R2 and R22 are present, and increases still further when the tryptophans W5 and W19 are replaced by alanines. The extent of helix dynamic averaging remains low in all cases. The preferred helix azimuthal rotation is essentially constant for all of the helices in each of the lipid membranes considered here. The alanines located outside of the core region of the peptide are sensitive to helical integrity. The new alanines, A5 and A19, therefore, provide new information about the length of the core helix and the onset of unraveling of the terminals. Residue A19 remains essentially on the central helix in each lipid membrane, while residues A3, A5 and A21 deviate from the core helix to an extent that depends on the membrane thickness. Differential unraveling of the two ends to expose peptide backbone groups for hydrogen bonding therefore acts together with specific interfacial side chains to stabilize a transmembrane helix.

Antidepressants are modifiers of lipid bilayer properties

The two major classes of antidepressants, tricyclic antidepressants (TCAs) and selective serotonin reuptake inhibitors (SSRIs), inhibit neurotransmitter reuptake at synapses. They also have off-target effects on proteins other than neurotransmitter transporters, which may contribute to both desired changes in brain function and the development of side effects. Many proteins modulated by antidepressants are bilayer spanning and coupled to the bilayer through hydrophobic interactions such that the conformational changes underlying their function will perturb the surrounding lipid bilayer, with an energetic cost (ΔG _def) that varies with changes in bilayer properties. Here, we test whether changes in ΔG _def caused by amphiphilic antidepressants partitioning into the bilayer are sufficient to alter membrane protein function. Using gramicidin A (gA) channels to probe whether TCAs and SSRIs alter the bilayer contribution to the free energy difference for the gramicidin monomer⇔dimer equilibrium (representing a well-defined conformational transition), we find that antidepressants alter gA channel activity with varying potency and no stereospecificity but with different effects on bilayer elasticity and intrinsic curvature. Measuring the antidepressant partition coefficients using isothermal titration calorimetry (ITC) or cLogP shows that the bilayer-modifying potency is predicted quite well by the ITC-determined partition coefficients, and channel activity is doubled at an antidepressant/lipid mole ratio of 0.02-0.07. These results suggest a mechanism by which antidepressants could alter the function of diverse membrane proteins by partitioning into cell membranes and thereby altering the bilayer contribution to the energetics of membrane protein conformational changes.

Transmembrane Helix Integrity versus Fraying To Expose Hydrogen Bonds at a Membrane-Water Interface

Transmembrane helices dominate the landscape for many membrane proteins. Often flanked by interfacial aromatic residues, these transmembrane helices also contain loops and interhelix segments, which could help in stabilizing a transmembrane orientation. Using ²H nuclear magnetic resonance spectroscopy to monitor bilayer-incorporated model GWALP23 family peptides, we address systematically the issue of helix fraying in relation to the dynamics and orientation of highly similar individual transmembrane helices. We inserted aromatic (Phe, Trp, Tyr, and His) or non-aromatic residues (Ala and Gly) into positions 4 and 5 adjacent to a core transmembrane helix to examine the side-chain dependency of the transmembrane orientation, dynamics, and helix integrity (extent and location of unraveling). Incorporation of [²H]alanine labels enables one to assess the helicity of the core sequence and the peptide termini. For most of the helices, we observed substantial unwinding involving at least three residues at both ends. For the unique case of histidine at positions 4 and 5, an extended N-terminal unwinding was observed up to residue 7. For further investigation of the onset of fraying, we employed A^4,5GWALP23 with ²H labels at residues 4 and 5 and found that the number of terminal residues involved in the unwinding depends on bilayer thicknesses and helps to govern the helix dynamics. The combined results enable us to compare and contrast the extent of fraying for each related helix, as reflected by the deviation of experimental ²H quadrupolar splitting magnitudes of juxta-terminal alanines A3 and A21 from those represented by an ideal helix geometry.

Exchange of Gramicidin between Lipid Bilayers: Implications for the Mechanism of Channel Formation

The canonical mechanism of gramicidin (gA) channel formation is transmembrane dimerization of nonconducting subunits that reside in opposite bilayer leaflets. The channels do not open and close; they appear and disappear due to subunit association and dissociation. Many different types of experiments support this monomer ↔ dimer mechanism. Recently, however, this mechanism was challenged, based on experiments with lipid vesicle-incorporated gA under conditions where vesicle fusion could be controlled. In these experiments, sustained channel activity was observed long after fusion had been terminated, which led to the proposal that gA single-channel current transitions result from closed-open transitions in long-lived bilayer-spanning dimers. This proposal is at odds with 40 years of experiments, but involves the key assumption that gA monomers do not exchange between bilayers. We tested the possibility of peptide exchange between bilayers using three different types of experiments. First, we demonstrated the exchange of gA between 1,2-dierucoyl-sn-glycero-3-phosphocholine (DC_22:1PC) or 1,2-dioleoyl-sn-glycero-3-phosphocholine (DC_18:1PC) lipid vesicles using a fluorescence assay for gA channel activity. Second, we added gA-free DC_22:1PC vesicles to both sides of planar DC_18:1PC bilayers preincubated with gA, which reduced channel activity up to 10-fold. Third, we added gA-containing DC_22:1PC vesicles to one or both sides of DC_18:1PC planar bilayers, which produced much higher channel activity when the gA-containing vesicles were added to both sides of the bilayer, as compared to one side only. All three types of experiments show that gA subunits can exchange between lipid bilayers. The exchange of subunits between bilayers thus is firmly established, which becomes a crucial consideration with respect to the mechanism of channel formation.

A general mechanism for drug promiscuity: Studies with amiodarone and other antiarrhythmics

Amiodarone is a widely prescribed antiarrhythmic drug used to treat the most prevalent type of arrhythmia, atrial fibrillation (AF). At therapeutic concentrations, amiodarone alters the function of many diverse membrane proteins, which results in complex therapeutic and toxicity profiles. Other antiarrhythmics, such as dronedarone, similarly alter the function of multiple membrane proteins, suggesting that a multipronged mechanism may be beneficial for treating AF, but raising questions about how these antiarrhythmics regulate a diverse range of membrane proteins at similar concentrations. One possible mechanism is that these molecules regulate membrane protein function by altering the common environment provided by the host lipid bilayer. We took advantage of the gramicidin (gA) channels' sensitivity to changes in bilayer properties to determine whether commonly used antiarrhythmics--amiodarone, dronedarone, propranolol, and pindolol, whose pharmacological modes of action range from multi-target to specific--perturb lipid bilayer properties at therapeutic concentrations. Using a gA-based fluorescence assay, we found that amiodarone and dronedarone are potent bilayer modifiers at therapeutic concentrations; propranolol alters bilayer properties only at supratherapeutic concentration, and pindolol has little effect. Using single-channel electrophysiology, we found that amiodarone and dronedarone, but not propranolol or pindolol, increase bilayer elasticity. The overlap between therapeutic and bilayer-altering concentrations, which is observed also using plasma membrane-like lipid mixtures, underscores the need to explore the role of the bilayer in therapeutic as well as toxic effects of antiarrhythmic agents.

Phytochemicals perturb membranes and promiscuously alter protein function

A wide variety of phytochemicals are consumed for their perceived health benefits. Many of these phytochemicals have been found to alter numerous cell functions, but the mechanisms underlying their biological activity tend to be poorly understood. Phenolic phytochemicals are particularly promiscuous modifiers of membrane protein function, suggesting that some of their actions may be due to a common, membrane bilayer-mediated mechanism. To test whether bilayer perturbation may underlie this diversity of actions, we examined five bioactive phenols reported to have medicinal value: capsaicin from chili peppers, curcumin from turmeric, EGCG from green tea, genistein from soybeans, and resveratrol from grapes. We find that each of these widely consumed phytochemicals alters lipid bilayer properties and the function of diverse membrane proteins. Molecular dynamics simulations show that these phytochemicals modify bilayer properties by localizing to the bilayer/solution interface. Bilayer-modifying propensity was verified using a gramicidin-based assay, and indiscriminate modulation of membrane protein function was demonstrated using four proteins: membrane-anchored metalloproteases, mechanosensitive ion channels, and voltage-dependent potassium and sodium channels. Each protein exhibited similar responses to multiple phytochemicals, consistent with a common, bilayer-mediated mechanism. Our results suggest that many effects of amphiphilic phytochemicals are due to cell membrane perturbations, rather than specific protein binding.

Phosphoinositides alter lipid bilayer properties

Phosphatidylinositol-4,5-bisphosphate (PIP₂), which constitutes ∼1% of the plasma membrane phospholipid, plays a key role in membrane-delimited signaling. PIP₂ regulates structurally and functionally diverse membrane proteins, including voltage- and ligand-gated ion channels, inwardly rectifying ion channels, transporters, and receptors. In some cases, the regulation is known to involve specific lipid–protein interactions, but the mechanisms by which PIP₂ regulates many of its various targets remain to be fully elucidated. Because many PIP₂ targets are membrane-spanning proteins, we explored whether the phosphoinositides might alter bilayer physical properties such as curvature and elasticity, which would alter the equilibrium between membrane protein conformational states—and thereby protein function. Taking advantage of the gramicidin A (gA) channels’ sensitivity to changes in lipid bilayer properties, we used gA-based fluorescence quenching and single-channel assays to examine the effects of long-chain PIP₂s (brain PIP₂, which is predominantly 1-stearyl-2-arachidonyl-PIP₂, and dioleoyl-PIP₂) on bilayer properties. When premixed with dioleoyl-phosphocholine at 2 mol %, both long-chain PIP₂s produced similar changes in gA channel function (bilayer properties); when applied through the aqueous solution, however, brain PIP₂ was a more potent modifier than dioleoyl-PIP₂. Given the widespread use of short-chain dioctanoyl-phosphoinositides, we also examined the effects of diC₈-phosphoinositol (PI), PI(4,5)P₂, PI(3,5)P₂, PI(3,4)P₂, and PI(3,4,5)P₃. The diC₈ phosphoinositides, except for PI(3,5)P₂, altered bilayer properties with potencies that decreased with increasing head group charge. Nonphosphoinositide diC₈ phospholipids generally were more potent bilayer modifiers than the polyphosphoinositides. These results show that physiological increases or decreases in plasma membrane PIP₂ levels, as a result of activation of PI kinases or phosphatases, are likely to alter lipid bilayer properties, in addition to any other effects they may have. The results further show that exogenous PIP₂, as well as structural analogues that differ in acyl chain length or phosphorylation state, alters lipid bilayer properties at the concentrations used in many cell physiological experiments.

Interactions of drugs and amphiphiles with membranes: modulation of lipid bilayer elastic properties by changes in acyl chain unsaturation and protonation

Poly-unsaturated fatty acids (PUFAs) alter the function of many membrane proteins, whereas monounsatured fatty acids generally are inert. We previously showed that docosahexaenoic acid (DHA) at pH 7 decreases the bilayer stiffness, consistent with an amphiphile-induced increase in elasticity, but not with a negative change in curvature; oleic acid (OA) was inert (Bruno, Koeppe and Andersen, Proc. Natl. Acad. Sci., 2007, 104, 9638-9643). To further explore how PUFAs and other amphiphiles may alter lipid bilayer properties, and thus membrane protein function, we examined how changes in acyl chain unsaturation and head group charge and size alter bilayer properties, as sensed by bilayer-spanning gramicidin A (gA) channels of different lengths. Compared to DHA, the neutral DHA-methyl ester has reduced effects on bilayer properties and 1-palmitoyl-2-docosahexaenoyl-phosphatidylcholine (PDPC) forms bilayers that are softer than dioleoylphosphatidylcholine (DOPC). The changes in channel function are larger for the short gA channels, indicating that changes in elasticity dominate over changes in curvature. We altered the fatty acid protonation by titration: docosahexaenoic acid (DHA) is more potent at pH 9 (relative to pH 7) and is inert at pH 4; OA, which was inert at pH 7, becomes a potent modifier of bilayer properties at pH 9. At both pH 7 and 9, DHA and OA produced larger changes in the lifetimes of the short gA channels, demonstrating that they increase lipid bilayer elasticity when deprotonated--though OA promotes the formation of inverted hexagonal phases at pH 7. The positively charged oleylamine (OAm), which has a small head-group and therefore should be a negative curvature promoter, inhibited gA channel function with similar reductions in the lifetimes of the short and long gA channels, indicating a curvature-dominated effect. Monitoring the single-channel conductance, we find that the negatively charged fatty acids increase the conductance by increasing the local negative charge around the channel, whereas the positively charged OAm has no effect. These results suggest that deprotonated fatty acids increase bilayer elasticity by reversibly adsorbing at the bilayer/solution interface.

N-terminally glutamate-substituted analogue of gramicidin A as protonophore and selective mitochondrial uncoupler

Limited uncoupling of oxidative phosphorylation could be beneficial for cells by preventing excessive generation of reactive oxygen species. Typical uncouplers are weak organic acids capable of permeating across membranes with a narrow gap between efficacy and toxicity. Aimed at designing a nontoxic uncoupler, the protonatable amino acid residue Glu was substituted for Val at the N-terminus of the pentadecapeptide gramicidin A (gA). The modified peptide [Glu1]gA exhibited high uncoupling activity in isolated mitochondria, in particular, abolishing membrane potential at the inner mitochondrial membrane with the same or even larger efficacy as gA. With mitochondria in cell culture, the depolarizing activity of [Glu1]gA was observed at concentrations by an order of magnitude lower than those of gA. On the contrary, [Glu1]gA was much less potent in forming proton channels in planar lipid bilayers than gA. Remarkably, at uncoupling concentrations, [Glu1]gA did not alter cell morphology and was nontoxic in MTT test, in contrast to gA showing high toxicity. The difference in the behavior of [Glu1]gA and gA in natural and artificial membranes could be ascribed to increased capability of [Glu1]gA to permeate through membranes and/or redistribute between different membranes. Based on the protective role of mild uncoupling, [Glu1]gA and some other proton-conducting gA analogues may be considered as prototypes of prospective therapeutic agents.

Proline kink angle distributions for GWALP23 in lipid bilayers of different thicknesses

By using selected (2)H and (15)N labels, we have examined the influence of a central proline residue on the properties of a defined peptide that spans lipid bilayer membranes by solid-state nuclear magnetic resonance (NMR) spectroscopy. For this purpose, GWALP23 (acetyl-GGALW(5)LALALALALALALW(19)LAGA-ethanolamide) is a suitable model peptide that employs, for the purpose of interfacial anchoring, only one tryptophan residue on either end of a central α-helical core sequence. Because of its systematic behavior in lipid bilayer membranes of differing thicknesses [Vostrikov, V. V., et al. (2010) J. Biol. Chem. 285, 31723-31730], we utilize GWALP23 as a well-characterized framework for introducing guest residues within a transmembrane sequence; for example, a central proline yields acetyl-GGALW(5)LALALAP(12)ALALALW(19)LAGA-ethanolamide. We synthesized GWALP23-P12 with specifically placed (2)H and (15)N labels for solid-state NMR spectroscopy and examined the peptide orientation and segmental tilt in oriented DMPC lipid bilayer membranes using combined (2)H GALA and (15)N-(1)H high-resolution separated local field methods. In DMPC bilayer membranes, the peptide segments N-terminal and C-terminal to the proline are both tilted substantially with respect to the bilayer normal, by ~34 ± 5° and 29 ± 5°, respectively. While the tilt increases for both segments when proline is present, the range and extent of the individual segment motions are comparable to or smaller than those of the entire GWALP23 peptide in bilayer membranes. In DMPC, the proline induces a kink of ~30 ± 5°, with an apparent helix unwinding or "swivel" angle of ~70°. In DLPC and DOPC, on the basis of (2)H NMR data only, the kink angle and swivel angle probability distributions overlap those of DMPC, yet the most probable kink angle appears to be somewhat smaller than in DMPC. As has been described for GWALP23 itself, the C-terminal helix ends before Ala(21) in the phospholipids DMPC and DLPC yet remains intact through Ala(21) in DOPC. The dynamics of bilayer-incorporated, membrane-spanning GWALP23 and GWALP23-P12 are less extensive than those observed for WALP family peptides that have more than two interfacial Trp residues.

Thiazolidinedione insulin sensitizers alter lipid bilayer properties and voltage-dependent sodium channel function: implications for drug discovery

The thiazolidinediones (TZDs) are used in the treatment of diabetes mellitus type 2. Their canonical effects are mediated by activation of the peroxisome proliferator–activated receptor γ (PPARγ) transcription factor. In addition to effects mediated by gene activation, the TZDs cause acute, transcription-independent changes in various membrane transport processes, including glucose transport, and they alter the function of a diverse group of membrane proteins, including ion channels. The basis for these off-target effects is unknown, but the TZDs are hydrophobic/amphiphilic and adsorb to the bilayer–water interface, which will alter bilayer properties, meaning that the TZDs may alter membrane protein function by bilayer-mediated mechanisms. We therefore explored whether the TZDs alter lipid bilayer properties sufficiently to be sensed by bilayer-spanning proteins, using gramicidin A (gA) channels as probes. The TZDs altered bilayer elastic properties with potencies that did not correlate with their affinity for PPARγ. At concentrations where they altered gA channel function, they also altered the function of voltage-dependent sodium channels, producing a prepulse-dependent current inhibition and hyperpolarizing shift in the steady-state inactivation curve. The shifts in the inactivation curve produced by the TZDs and other amphiphiles can be superimposed by plotting them as a function of the changes in gA channel lifetimes. The TZDs’ partition coefficients into lipid bilayers were measured using isothermal titration calorimetry. The most potent bilayer modifier, troglitazone, alters bilayer properties at clinically relevant free concentrations; the least potent bilayer modifiers, pioglitazone and rosiglitazone, do not. Unlike other TZDs tested, ciglitazone behaves like a hydrophobic anion and alters the gA monomer–dimer equilibrium by more than one mechanism. Our results provide a possible mechanism for some off-target effects of an important group of drugs, and underscore the importance of exploring bilayer effects of candidate drugs early in drug development.

Alcohol's effects on lipid bilayer properties

Alcohols are known modulators of lipid bilayer properties. Their biological effects have long been attributed to their bilayer-modifying effects, but alcohols can also alter protein function through direct protein interactions. This raises the question: Do alcohol's biological actions result predominantly from direct protein-alcohol interactions or from general changes in the membrane properties? The efficacy of alcohols of various chain lengths tends to exhibit a so-called cutoff effect (i.e., increasing potency with increased chain length, which that eventually levels off). The cutoff varies depending on the assay, and numerous mechanisms have been proposed such as: limited size of the alcohol-protein interaction site, limited alcohol solubility, and a chain-length-dependent lipid bilayer-alcohol interaction. To address these issues, we determined the bilayer-modifying potency of 27 aliphatic alcohols using a gramicidin-based fluorescence assay. All of the alcohols tested (with chain lengths of 1-16 carbons) alter the bilayer properties, as sensed by a bilayer-spanning channel. The bilayer-modifying potency of the short-chain alcohols scales linearly with their bilayer partitioning; the potency tapers off at higher chain lengths, and eventually changes sign for the longest-chain alcohols, demonstrating an alcohol cutoff effect in a system that has no alcohol-binding pocket.

Effects of green tea catechins on gramicidin channel function and inferred changes in bilayer properties

Green tea's health benefits have been attributed to its major polyphenols, the catechins: (-)-epigallocatechin gallate (EGCG), (-)-epicatechin gallate (ECG), (-)-epigallocatechin (EGC), and epicatechin (EC). Catechins (especially EGCG) modulate a wide range of biologically important molecules, including many membrane proteins. Yet, little is known about their mechanism(s) of action. We tested the catechins' bilayer-modifying potency using gramicidin A (gA) channels as molecular force probes. All the catechins alter gA channel function and modify bilayer properties, with a 500-fold range in potency (EGCG>ECG≫EGC>EC). Additionally, the gallate group causes current block, as evident by brief downward current transitions (flickers).

Response of GWALP transmembrane peptides to changes in the tryptophan anchor positions

While the interfacial partitioning of charged or aromatic anchor residues may determine the preferred orientations of transmembrane peptide helices, the dependence of helix orientation on anchor residue position is not well understood. When anchor residue locations are changed systematically, some adaptations of the peptide-lipid interactions may be required to compensate for the altered interfacial interactions. Recently, we have developed a novel transmembrane peptide, termed GW(5,19)ALP23 (acetyl-GGALW(5)LALALALALALALW(19)LAGA-ethanolamide), which proves to be a well-behaved sequence for an orderly investigation of protein-lipid interactions. Its roughly symmetric nature allows for shifting the anchoring Trp residues by one Leu-Ala pair inward (GW(7,17)ALP23) or outward (GW(3,21)ALP23), thus providing fine adjustments of the formal distance between the tryptophan residues. With no other obvious anchoring features present, we postulate that the inter-Trp distance may be crucial for aspects of the peptide-lipid interaction. Importantly, the amino acid composition is identical for each of the resulting related GWALP23 sequences, and the radial separation between the pairs of Trp residues on each side of the transmembrane α-helix remains similar. Here we address the adaptation of the aforementioned peptides to the varying Trp locations by means of solid-state (2)H nuclear magnetic resonance experiments in varying lipid bilayer membrane environments. All of the GW(x,y)ALP23 sequence isomers adopt transmembrane orientations in DOPC, DMPC, and DLPC environments, even when the Trp residues are quite closely spaced, in GW(7,17)ALP23. Furthermore, the dynamics for each peptide isomer are less extensive than for peptides possessing additional interfacial Trp residues. The helical secondary structure is maintained more strongly within the Trp-flanked core region than outside of the Trp boundaries. Deuterium-labeled tryptophan indole rings in the GW(x,y)ALP23 peptides provide additional insights into the behavior of the Trp side chains. A Trp side chain near the C-terminus adopts a different orientation and undergoes somewhat faster dynamics than a corresponding Trp side chain located an equivalent distance from the N-terminus. In contrast, as the inter-Trp distance changes, the variations among the average orientations of the Trp indole rings at either terminus are systematic yet fairly small. We conclude that subtle adjustments to the peptide tilt, and to the N- and C-terminal Trp side chain torsion angles, permit the GW(x,y)ALP23 peptides to maintain preferred transmembrane orientations while adapting to lipid bilayers with differing hydrophobic thicknesses.

Linear rate-equilibrium relations arising from ion channel-bilayer energetic coupling

Linear rate-equilibrium (RE) relations, also known as linear free energy relations, are widely observed in chemical reactions, including protein folding, enzymatic catalysis, and channel gating. Despite the widespread occurrence of linear RE relations, the principles underlying the linear relation between changes in activation and equilibrium energy in macromolecular reactions remain enigmatic. When examining amphiphile regulation of gramicidin channel gating in lipid bilayers, we noted that the gating process could be described by a linear RE relation with a simple geometric interpretation. This description is possible because the gating process provides a well-understood reaction, in which structural changes in a bilayer-embedded model protein can be studied at the single-molecule level. It is thus possible to obtain quantitative information about the energetics of the reaction transition state and its position on a spatial coordinate. It turns out that the linear RE relation for the gramicidin monomer-dimer reaction can be understood, and the quantitative relation between changes in activation energy and equilibrium energy can be interpreted, by considering the effects of amphiphiles on the changes in bilayer elastic energy associated with channel gating. We are not aware that a similar simple mechanistic explanation of a linear RE relation has been provided for a chemical reaction in a macromolecule. RE relations generally should be useful for examining how amphiphile-induced changes in bilayer properties modulate membrane protein folding and function, and for distinguishing between direct (e.g., due to binding) and indirect (bilayer-mediated) effects.

The membrane interface dictates different anchor roles for "inner pair" and "outer pair" tryptophan indole rings in gramicidin A channels

We investigated the effects of substituting two of the four tryptophans (the "inner pair" Trp(9) and Trp(11) or the "outer pair" Trp(13) and Trp(15)) in gramicidin A (gA) channels. The conformational preferences of the doubly substituted gA analogues were assessed using circular dichroism spectroscopy and size-exclusion chromatography, which show that the inner tryptophans 9 and 11 are critical for the gA's conformational preference in lipid bilayer membranes. [Phe(13,15)]gA largely retains the single-stranded helical channel structure, whereas [Phe(9,11)]gA exists primarily as double-stranded conformers. Within this context, the (2)H NMR spectra from labeled tryptophans were used to examine the changes in average indole ring orientations, induced by the Phe substitutions and by the shift in conformational preference. Using a method for deuterium labeling of already synthesized gAs, we introduced deuterium selectively onto positions C2 and C5 of the remaining tryptophan indole rings in the substituted gA analogues for solid-state (2)H NMR spectroscopy. The (least possible) changes in orientation and overall motion of each indole ring were estimated from the experimental spectra. Regardless of the mixture of backbone folds, the indole ring orientations observed in the analogues are similar to those found previously for gA channels. Both Phe-substituted analogues form single-stranded channels, as judged from the formation of heterodimeric channels with the native gA. [Phe(13,15)]gA channels have Na(+) currents that are ~50% and lifetimes that are ~80% of those of native gA channels. The double-stranded conformer(s) of [Phe(9,11)]gA do not form detectable channels. The minor single-stranded population of [Phe(9,11)]gA forms channels with Na(+) currents that are ~25% and single-channel lifetimes that are ~300% of those of native gA channels. Our results suggest that Trp(9) and Trp(11), when "reaching" for the interface, tend to drive both monomer folding (to "open" a channel) and dimer dissociation (to "close" a channel). Furthermore, the dipoles of Trp(9) and Trp(11) are relatively more important for the single-channel conductance than are the dipoles of Trp(13) and Trp(15).

Charged or aromatic anchor residue dependence of transmembrane peptide tilt

The membrane-spanning segments of integral membrane proteins often are flanked by aromatic or charged amino acid residues, which may "anchor" the transmembrane orientation. Single spanning transmembrane peptides such as those of the WALP family, acetyl-GWW(LA)(n)LWWA-amide, furthermore adopt a moderate average tilt within lipid bilayer membranes. To understand the anchor residue dependence of the tilt, we introduce Leu-Ala "spacers" between paired anchors and in some cases replace the outer tryptophans. The resulting peptides, acetyl-GX(2)ALW(LA)(6)LWLAX(22)A-amide, have Trp, Lys, Arg, or Gly in the two X positions. The apparent average orientations of the core helical sequences were determined in oriented phosphatidylcholine bilayer membranes of varying thickness using solid-state (2)H NMR spectroscopy. When X is Lys, Arg, or Gly, the direction of the tilt is essentially constant in different lipids and presumably is dictated by the tryptophans (Trp(5) and Trp(19)) that flank the inner helical core. The Leu-Ala spacers are no longer helical. The magnitude of the apparent helix tilt furthermore scales nicely with the bilayer thickness except when X is Trp. When X is Trp, the direction of tilt is less well defined in each phosphatidylcholine bilayer and varies up to 70° among 1,2-dioleoyl-sn-glycero-3-phosphocholine, 1,2-dimyristoyl-sn-glycero-3-phosphocholine, and 1,2-dilauroyl-sn-glycero-3-phosphocholine bilayer membranes. Indeed, the X = Trp case parallels earlier observations in which WALP family peptides having multiple Trp anchors show little dependence of the apparent tilt magnitude on bilayer thickness. The results shed new light on the interactions of arginine, lysine, tryptophan, and even glycine at lipid bilayer membrane interfaces.

Polar groups in membrane channels: consequences of replacing alanines with serines in membrane-spanning gramicidin channels

To explore the consequences of burying polar, hydrogen-bonding hydroxyl groups within the hydrocarbon core of lipid bilayer membranes, we examined the structural and functional effects of alanine-to-serine substitutions in bilayer-spanning gramicidin channels. A native Ala was replaced by Ser at position 3 or 5 in the gramicidin A (gA) sequence: formyl-VG(2)A(3)LA(5)VVVWLWLWLW-ethanolamide (d-residues underlined). In the head-to-head dimers that form the conducting, membrane-spanning gA channels, these sequence positions are located near the lipid bilayer center (and subunit interface). The sequence substitutions at positions 3 and 5 were tested within the context of having either Gly or d-Ala at position 2, because d-Ala(2) causes the channel lifetimes to increase 3-fold relative to Gly(2) [Mattice et al. (1995) Biochemistry 34, 6827]. Size-exclusion chromatograms and circular dichroism spectra show that the Ala --> Ser replacements are well tolerated and have little effect on channel structure. In planar bilayers, the Ser-substituted gramicidins form well-defined channels, with cation conductances that are approximately 60% of those of the reference channels. The Ser-substituted channels are structurally equivalent to native gramicidin channels, as demonstrated by the formation of heterodimeric channels between a Ser-containing subunit and a native gramicidin subunit. These hybrid channels exhibit rectification, attributable to asymmetric placement of the single Ser hydroxyl group with respect to the bilayer center. Compared to the corresponding Ala-containing reference channels, the polar Ser residues decrease the analogues' channel-forming potency by 3 orders of magnitude, indicating a substantial energetic penalty ( approximately 15 kJ/mol) for burying the polar Ser side chain in the bilayer hydrophobic core.

Amphiphile regulation of ion channel function by changes in the bilayer spring constant

Many drugs are amphiphiles that, in addition to binding to a particular target protein, adsorb to cell membrane lipid bilayers and alter intrinsic bilayer physical properties (e.g., bilayer thickness, monolayer curvature, and elastic moduli). Such changes can modulate membrane protein function by altering the energetic cost (DeltaG(bilayer)) of bilayer deformations associated with protein conformational changes that involve the protein-bilayer interface. But amphiphiles have complex effects on the physical properties of lipid bilayers, meaning that the net change in DeltaG(bilayer) cannot be predicted from measurements of isolated changes in such properties. Thus, the bilayer contribution to the promiscuous regulation of membrane proteins by drugs and other amphiphiles remains unknown. To overcome this problem, we use gramicidin A (gA) channels as molecular force probes to measure the net effect of amphiphiles, at concentrations often used in biological research, on the bilayer elastic response to a change in the hydrophobic length of an embedded protein. The effects of structurally diverse amphiphiles can be described by changes in a phenomenological bilayer spring constant (H(B)) that summarizes the bilayer elastic properties, as sensed by a bilayer-spanning protein. Amphiphile-induced changes in H(B), measured using gA channels of a particular length, quantitatively predict changes in lifetime for channels of a different length--as well as changes in the inactivation of voltage-dependent sodium channels in living cells. The use of gA channels as molecular force probes provides a tool for quantitative, predictive studies of bilayer-mediated regulation of membrane protein function by amphiphiles.

Influence of proline upon the folding and geometry of the WALP19 transmembrane peptide

The orientations, geometries, and lipid interactions of designed transmembrane (TM) peptides have attracted significant experimental and theoretical interest. Because the amino acid proline will introduce a known discontinuity into an alpha helix, we have sought to measure the extent of helix kinking caused by a single proline within the isolated TM helical domain of WALP19. For this purpose, we synthesized acetyl-GWWLALALAP(10)ALALALWWA-ethanolamide and included pairs of deuterated alanines by using 60-100% Fmoc-l-Ala-d(4) at selected sequence positions. Solid-state deuterium ((2)H) magnetic resonance spectra from oriented, hydrated samples (1/40, peptide/lipid; using several lipids) reveal signals from many of the alanine backbone C(alpha) deuterons as well as the alanine side-chain C(beta) methyl groups, whereas signals from C(alpha) deuterons generally have not been observed for similar peptides without proline. It is conceivable that altered peptide dynamics may be responsible for the apparent "unmasking" of the backbone resonances in the presence of the proline. Data analysis using the geometric analysis of labeled alanines (GALA) method reveals that the peptide helix is distorted due to the presence of the proline. To provide additional data points for evaluating the segmental tilt angles of the two halves of the peptide, we substituted selected leucines with l-Ala-d(4). Using this approach, we were able to deduce that the apparent average tilt of the C-terminal increases from approximately 4 degrees to approximately 12 degrees when Pro(10) is introduced. The segment N-terminal to proline is more complex and possibly is more dynamically flexible; Leu to Ala mutations within the N-terminal segment alter the average orientations of alanines in both segments. Nevertheless, in DOPC, we could estimate an apparent kink angle of approximately 19 degrees . Together, the results suggest that the central proline influences not only the geometry but also the dynamics of the membrane-spanning peptide. The results make up an important basis for understanding the functional role of proline in several families of membrane proteins.

The preference of tryptophan for membrane interfaces: insights from N-methylation of tryptophans in gramicidin channels

To better understand the structural and functional roles of tryptophan at the membrane/water interface in membrane proteins, we examined the structural and functional consequences of Trp --> 1-methyl-tryptophan substitutions in membrane-spanning gramicidin A channels. Gramicidin A channels are miniproteins that are anchored to the interface by four Trps near the C terminus of each subunit in a membrane-spanning dimer. We masked the hydrogen bonding ability of individual or multiple Trps by 1-methylation of the indole ring and examined the structural and functional changes using circular dichroism spectroscopy, size exclusion chromatography, solid state (2)H NMR spectroscopy, and single channel analysis. N-Methylation causes distinct changes in the subunit conformational preference, channel-forming propensity, single channel conductance and lifetime, and average indole ring orientations within the membrane-spanning channels. The extent of the local ring dynamic wobble does not increase, and may decrease slightly, when the indole NH is replaced by the non-hydrogen-bonding and more bulky and hydrophobic N-CH(3) group. The changes in conformational preference, which are associated with a shift in the distribution of the aromatic residues across the bilayer, are similar to those observed previously with Trp --> Phe substitutions. We conclude that indole N-H hydrogen bonding is of major importance for the folding of gramicidin channels. The changes in ion permeability, however, are quite different for Trp --> Phe and Trp --> 1-methyl-tryptophan substitutions, indicating that the indole dipole moment and perhaps also ring size and are important for ion permeation through these channels.

Lipid interactions of acylated tryptophan-methylated lactoferricin peptides by solid-state NMR

Lactoferricin (LfB) is a 25-residue innate immunity peptide released by pepsin from the N-terminal region of bovine lactoferrin. A smaller amidated peptide, LfB6 (RRWQWR-NH2) retains antimicrobial activity and is thought to constitute the "antimicrobial active-site" (Tomita, Acta Paediatr Jpn. 1994; 36: 585-91). Here we report on N-acylation of 1-Me-Trp5-LfB6, Cn-RRWQ[1-Me-W]R-NH2, where Cn is an acyl chain having n = 0, 2, 4, 6 or 12 carbons. Tryptophan 5 (Trp5) was methylated to enhance membrane binding and to allow for selective deuteration at that position. Peptide/lipid interactions of Cn-RRWQ[1-Me-W]R-NH2 (deuterated 1-Me-Trp5 underlined), were monitored by solid state 31P NMR and 2H NMR. The samples consisted of macroscopically oriented bilayers of mixed neutral (dimyristoylphosphatidylcholine, DMPC) and anionic (dimyristoylphosphatidylglycerol, DMPG) lipids in a 3:1 ratio with Cn-RRWQ[&1-Me-W]R-NH2 peptides added at a 1:25 peptide to lipid ratio. 2H-NMR spectra reveal that the acylated peptides are well aligned in DMPC:DMPG bilayers. The 2H NMR quadrupolar splittings suggest that the 1-Me-Trp is located in a motionally restricted environment, indicating partial alignment at the membrane interface. 31P-NMR spectra reveal that the lipids are predominantly in a bilayer configuration, with little perturbation by the peptides. Methylation alone, in C0-RRWQ[1-Me-W]R-NH2, resulted in a 3-4 fold increase in antimicrobial activity against E. coli. N-acylation with a C12 fatty acid enhanced activity almost 90 fold.

Role of tryptophan residues in gramicidin channel organization and function

The linear peptide gramicidin forms prototypical ion channels specific for monovalent cations and has been used extensively to study the organization, dynamics, and function of membrane-spanning channels. The tryptophan residues in gramicidin channels are crucial for maintaining the structure and function of the channel. We explored the structural basis for the reduction in channel conductance in the case of single-tryptophan analogs of gramicidin with three Trp --> hydrophobic substitutions using a combination of fluorescence approaches, which include red edge excitation shift and membrane penetration depth analysis, size-exclusion chromatography, and circular dichroism spectroscopy. We show here that the gramicidin analogs containing single-tryptophan residues adopt a mixture of nonchannel and channel conformations, as evident from analysis of membrane penetration depth, size-exclusion chromatography, and backbone circular dichroism data. These results are potentially useful in analyzing the effect of tryptophan substitution on the functioning of other ion channels and membrane proteins.

Membrane-thinning effect of curcumin

Interaction of curcumin with lipid bilayers is not well understood. A recent experiment showed that curcumin significantly affected the single-channel lifetime of gramicidin in a 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) bilayer without affecting its single-channel conductance. We performed two experiments to understand this result. By isothermal titration calorimetry, we measured the partition coefficient of curcumin binding to DOPC bilayers. By x-ray lamellar diffraction, we measured the thickness change of DOPC bilayers as a function of the curcumin/lipid ratio. A nonlinear membrane-thinning effect by curcumin was discovered. The gramicidin data were qualitatively interpreted by the combination of isothermal titration calorimetry and x-ray results. We show that not only does curcumin thin the lipid bilayer, it might also weaken its elasticity moduli. The result implies that curcumin may affect the function of membrane proteins by modifying the properties of the host membrane.

Is there a preferential interaction between cholesterol and tryptophan residues in membrane proteins?

Recently, several indications have been found that suggest a preferential interaction between cholesterol and tryptophan residues located near the membrane-water interface. The aim of this study was to investigate by direct methods how tryptophan and cholesterol interact with each other and what the possible consequences are for membrane organization. For this purpose, we used cholesterol-containing model membranes of dimyristoylphosphatidylcholine (DMPC) in which a transmembrane model peptide with flanking tryptophans [acetyl-GWW(LA)8LWWA-amide], called WALP23, was incorporated to mimic interfacial tryptophans of membrane proteins. These model systems were studied with two complementary methods. (1) Steady-state and time-resolved Förster resonance energy transfer (FRET) experiments employing the fluorescent cholesterol analogue dehydroergosterol (DHE) in combination with a competition experiment with cholesterol were used to obtain information about the distribution of cholesterol in the bilayer in the presence of WALP23. The results were consistent with a random distribution of cholesterol which indicates that cholesterol and interfacial tryptophans are not preferentially located next to each other in these bilayer systems. (2) Solid-state 2H NMR experiments employing either deuterated cholesterol or indole ring-deuterated WALP23 peptides were performed to study the orientation and dynamics of both molecules. The results showed that the quadrupolar splittings of labeled cholesterol were not affected by an interaction with tryptophan-flanked peptides and, vice versa, that the quadrupolar splittings of labeled indole rings in WALP23 are not significantly influenced by addition of cholesterol to the bilayer. Therefore, both NMR and fluorescence spectroscopy results independently show that, at least in the model systems studied here, there is no evidence for a preferential interaction between cholesterol and tryptophans located at the bilayer interface.

Helical distortion in tryptophan- and lysine-anchored membrane-spanning alpha-helices as a function of hydrophobic mismatch: a solid-state deuterium NMR investigation using the geometric analysis of labeled alanines method

We used solid-state deuterium NMR spectroscopy and geometric analysis of labeled alanines to investigate the structure and orientation of a designed synthetic hydrophobic, membrane-spanning alpha-helical peptide that is anchored within phosphatidylcholine (PC) bilayers using both Trp and Lys side chains near the membrane/water interface. The 23-amino-acid peptide consists of an alternating Leu/Ala core sequence that is expected to be alpha-helical, flanked by aromatic and then cationic anchors at both ends of the peptide: acetyl-GKALW(LA)(6)LWLAKA-amide (KWALP23). The geometric analysis of labeled alanines method was elaborated to permit the incorporation and assignment of multiple alanine labels within a single synthetic peptide. Peptides were incorporated into oriented bilayers of dilauroyl- (di-C12:0-), dimyristoyl- (di-C14:0-), or dioleoyl- (di-C18:1c-) PC. In the C12:0 and C14:0 lipids, the (2)H-NMR quadrupolar splittings for the set of six core alanines could not be fit to a canonical undistorted alpha-helix. Rather, we found that a model containing a helical distortion, such as a localized discontinuity or "kink" near the peptide and bilayer center, could fit the data for KWALP23 in these shorter lipids. The suggestion of helix distortion was confirmed by (2)H-NMR spectra for KWALP23 in which Leu(8) was changed to deuterated Ala(8). Further analysis involving reexamination of earlier data led to a similar conclusion that acetyl-GWW(LA)(8)LWWA-amide (WALP23) is distorted in dilauroyl-PC, allowing significant improvement in the fitting of the (2)H-NMR results. In contrast, WALP23 and KWALP23 are well represented as undistorted alpha-helices in dioleoyl-PC, suggesting that the distortion could be a response to hydrophobic mismatch between peptide and lipids.

The gramicidin ion channel: A model membrane protein

The linear peptide gramicidin forms prototypical ion channels specific for monovalent cations and has been extensively used to study the organization, dynamics and function of membrane-spanning channels. In recent times, the availability of crystal structures of complex ion channels has challenged the role of gramicidin as a model membrane protein and ion channel. This review focuses on the suitability of gramicidin as a model membrane protein in general, and the information gained from gramicidin to understand lipid-protein interactions in particular. Special emphasis is given to the role and orientation of tryptophan residues in channel structure and function and recent spectroscopic approaches that have highlighted the organization and dynamics of the channel in membrane and membrane-mimetic media.

The inhibitory effect of (-)-epigallocatechin gallate on activation of the epidermal growth factor receptor is associated with altered lipid order in HT29 colon cancer cells

(-)-Epigallocatechin gallate (EGCG), a major biologically active constituent of green tea, inhibits activation of the epidermal growth factor (EGF) receptor (EGFR) and downstream signaling pathways in several types of human cancer cells, but the precise mechanism is not known. Because several plasma membrane-associated receptor tyrosine kinases (RTK) including EGFR are localized in detergent-insoluble ordered membrane domains, so-called "lipid rafts," we examined whether the inhibitory effect of EGCG on activation of the EGFR is associated with changes in membrane lipid order in HT29 colon cancer cells. First, we did cold Triton X-100 solubility assays. Phosphorylated (activated) EGFR was found only in the Triton X-100-insoluble (lipid raft) fraction, whereas total cellular EGFR was present in the Triton X-100-soluble fraction. Pretreatment with EGCG inhibited the binding of Alexa Fluor 488-labeled EGF to the cells and also inhibited EGF-induced dimerization of the EGFR. To examine possible effects of EGCG on membrane lipid organization, we labeled the cells with the fluorescent lipid analogue 1, 1'-dihexadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate, which preferentially incorporates into ordered membrane domains in cells and found that subsequent treatment with EGCG caused a marked reduction in the Triton X-100-resistant membrane fraction. Polyphenon E, a mixture of green tea catechins, had a similar effect but (-)-epicatechin (EC), the biologically inactive compound, did not significantly alter the Triton X-100 solubility properties of the membrane. Furthermore, we found that EGCG but not EC caused dramatic changes in the function of bilayer-incorporated gramicidin channels. Taken together, these findings suggest that EGCG inhibits the binding of EGF to the EGFR and the subsequent dimerization and activation of the EGFR by altering membrane organization. These effects may also explain the ability of EGCG to inhibit activation of other membrane-associated RTKs, and they may play a critical role in the anticancer effects of this and related compounds.

Docosahexaenoic acid alters bilayer elastic properties

At low micromolar concentrations, polyunsaturated fatty acids (PUFAs) alter the function of many membrane proteins. PUFAs exert their effects on unrelated proteins at similar concentrations, suggesting a common mode of action. Because lipid bilayers serve as the common "solvent" for membrane proteins, the common mechanism could be that PUFAs adsorb to the bilayer/solution interface to promote a negative-going change in lipid intrinsic curvature and, like other reversibly adsorbing amphiphiles, increase bilayer elasticity. PUFA adsorption thus would alter the bilayer deformation energy associated with protein conformational changes involving the protein/bilayer boundary, which would alter protein function. To explore the feasibility of such a mechanism, we used gramicidin (gA) analogues of different lengths together with bilayers of different thicknesses to assess whether docosahexaenoic acid (DHA) could exert its effects through a bilayer-mediated mechanism. Indeed, DHA increases gA channel appearance rates and lifetimes and decreases the free energy of channel formation. The appearance rate and lifetime changes increase with increasing channel-bilayer hydrophobic mismatch and are not related to differing DHA bilayer absorption coefficients. DHA thus alters bilayer elastic properties, not just lipid intrinsic curvature; the elasticity changes are important for DHA's bilayer-modifying actions. Oleic acid (OA), which has little effect on membrane protein function, exerts no such effects despite OA's adsorption coefficient being an order of magnitude greater than DHA's. These results suggest that DHA (and other PUFAs) may modulate membrane protein function by bilayer-mediated mechanisms that do not involve specific protein binding but rather changes in bilayer material properties.

Orientation and motion of tryptophan interfacial anchors in membrane-spanning peptides

The tryptophans of integral membrane proteins have been suggested to play specific roles as "interfacial anchors", based on their preference for a location near the lipid head groups. Still, the underlying mechanism behind this behavior remains unclear. NMR experiments can provide an important tool to study this interaction in an actual bilayer environment. Here solid-state deuterium nuclear magnetic resonance was used to study the tryptophans in membrane-spanning model peptides from the WALP family (acetyl-GWW(LA)nWWA-ethanolamide with n = 5 and 6.5) in samples of mechanically aligned dimyristoylphosphatidylcholine (DMPC) bilayers. The data indicate that the tryptophans near the C-terminal end of the peptide display a significantly different behavior from those near the N-terminus. This is reflected prominently in a large difference in the motion experienced by the indoles at either end of the peptide, highlighting the directionality of the helix. Nevertheless, our observations indicate high levels of motional freedom for all tryptophans in these membrane spanning domains that exceed the dynamics for the helix itself. These observations signify that steric and dynamic features of the polypeptide context modulate the tryptophan anchoring in the membrane interface. Measurements of WALP19 in the ether-linked DMPC analogue ditetradecylphosphatidylcholine (missing the lipid carbonyls) show very similar Trp dynamics and suggest similar orientations for some or all of the tryptophans. This suggests that the lipid acyl chain carbonyls play at most a minor role in the anchoring interaction between these Trp residues and the DMPC interfacial region.

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.

2,3-butanedione monoxime affects cystic fibrosis transmembrane conductance regulator channel function through phosphorylation-dependent and phosphorylation-independent mechanisms: the role of bilayer material properties

2,3-Butanedione monoxime (BDM) is widely believed to act as a chemical phosphatase. We therefore examined the effects of BDM on the cystic fibrosis transmembrane regulator (CFTR) Cl(-) channel, which is regulated by phosphorylation in a complex manner. In guinea pig ventricular myocytes, forskolin-activated whole-cell CFTR currents responded biphasically to external 20 mM BDM: a rapid approximately 2-fold current activation was followed by a slower (tau approximately 20 s) inhibition (to approximately 20% of control). The inhibitory response was abolished by intracellular dialysis with the phosphatase inhibitor microcystin, suggesting involvement of endogenous phosphatases. The BDM-induced activation was studied further in Xenopus laevis oocytes expressing human epithelial CFTR. The concentration for half-maximal BDM activation (K(0.5)) was state-dependent, approximately 2 mM for highly and approximately 20 mM for partially phosphorylated channels, suggesting a modulated receptor mechanism. Because BDM modulates many different membrane proteins with similar K(0.5) values, we tested whether BDM could alter protein function by altering lipid bilayer properties rather than by direct BDM-protein interactions. Using gramicidin channels of different lengths (different channel-bilayer hydrophobic mismatch) as reporters of bilayer stiffness, we found that BDM increases channel appearance rates and lifetimes (reduces bilayer stiffness). At 20 mM BDM, the appearance rates increase approximately 4-fold (for the longer, 15 residues/monomer, channels) to approximately 10-fold (for the shorter, 13 residues/monomer channels); the lifetimes increase approximately 50% independently of channel length. BDM thus reduces the energetic cost of bilayer deformation, an effect that may underlie the effects of BDM on CFTR and other membrane proteins; the state-dependent changes in K(0.5) are consistent with such a bilayer-mediated mechanism.

SFG studies on interactions between antimicrobial peptides and supported lipid bilayers

The mode of action of antimicrobial peptides (AMPs) in disrupting cell membrane bilayers is of fundamental importance in understanding the efficiency of different AMPs, which is crucial to design antibiotics with improved properties. Recent developments in the field of sum frequency generation (SFG) vibrational spectroscopy have made it a powerful and unique biophysical technique in investigating the interactions between AMPs and a single substrate supported planar lipid bilayer. We will review some of the recent progress in applying SFG to study membrane lipid bilayers and discuss how SFG can provide novel information such as real-time bilayer structure change and AMP orientation during AMP-lipid bilayer interactions in a very biologically relevant manner. Several examples of applying SFG to monitor such interactions between AMPs and a dipalmitoyl phosphatidylglycerol (DPPG) bilayer are presented. Different modes of actions are observed for melittin, tachyplesin I, d-magainin 2, MSI-843, and a synthetic antibacterial oligomer, demonstrating that SFG is very effective in the study of AMPs and AMP-lipid bilayer interactions.

Regulation of membrane protein function by lipid bilayer elasticity-a single molecule technology to measure the bilayer properties experienced by an embedded protein

Membrane protein function is generally regulated by the molecular composition of the host lipid bilayer. The underlying mechanisms have long remained enigmatic. Some cases involve specific molecular interactions, but very often lipids and other amphiphiles, which are adsorbed to lipid bilayers, regulate a number of structurally unrelated proteins in an apparently non-specific manner. It is well known that changes in the physical properties of a lipid bilayer (e.g., thickness or monolayer spontaneous curvature) can affect the function of an embedded protein. However, the role of such changes, in the general regulation of membrane protein function, is unclear. This is to a large extent due to lack of a generally accepted framework in which to understand the many observations. The present review summarizes studies which have demonstrated that the hydrophobic interactions between a membrane protein and the host lipid bilayer provide an energetic coupling, whereby protein function can be regulated by the bilayer elasticity. The feasibility of this 'hydrophobic coupling mechanism' has been demonstrated using the gramicidin channel, a model membrane protein, in planar lipid bilayers. Using voltage-dependent sodium channels, N-type calcium channels and GABA(A) receptors, it has been shown that membrane protein function in living cells can be regulated by amphiphile induced changes in bilayer elasticity. Using the gramicidin channel as a molecular force transducer, a nanotechnology to measure the elastic properties experienced by an embedded protein has been developed. A theoretical and technological framework, to study the regulation of membrane protein function by lipid bilayer elasticity, has been established.

Importance of tensor asymmetry for the analysis of 2H NMR spectra from deuterated aromatic rings

We have used ab initio calculations to compute all of the tensor elements of the electric field gradient for each carbon-deuterium bond in the ring of deuterated 3-methyl-indole. Previous analyses have ignored the smaller tensor elements perpendicular to principal component Vzz which is aligned with the C-2H bond (local bond z-axis). At each ring position, the smallest element Vxx is in the molecular plane and Vyy is normal to the plane of the ring. The asymmetry parameter = (Vyy - Vxx)/Vzz ranges from 0.07 at C4 to 0.11 at C2. We used the perpendicular (off-bond) tensor elements, in concert with an improved understanding of the indole ring geometry, to analyze prototype 2H NMR spectra from well-oriented, hydrated peptide/lipid samples. For each of the four tryptophans of membrane-spanning gramicidin A (gA) channels, the inclusion of the perpendicular elements changes the deduced ring tilt by nearly 10 and increases the ring principal order parameter Szz for overall "wobble" with respect to the membrane normal (molecular z-axis). With the improved analysis, the magnitude of Szz for the outermost indole rings of Trp13 and Trp15 is indistinguishable from that observed previously for backbone atoms (0.93 +/- 0.03). For the Trp9 and Trp11 rings, which are slightly more buried within the membrane, Szz is slightly lower (0.86 +/- 0.03). The results show that the perpendicular elements are important for the detailed analysis of 2H NMR spectra from aromatic ring systems.

Capsaicin regulates voltage-dependent sodium channels by altering lipid bilayer elasticity

At submicromolar concentrations, capsaicin specifically activates the TRPV1 receptor involved in nociception. At micro- to millimolar concentrations, commonly used in clinical and in vitro studies, capsaicin also modulates the function of a large number of seemingly unrelated membrane proteins, many of which are similarly modulated by the capsaicin antagonist capsazepine. The mechanism(s) underlying this widespread regulation of protein function are not understood. We investigated whether capsaicin could regulate membrane protein function by changing the elasticity of the host lipid bilayer. This was done by studying capsaicin's effects on lipid bilayer stiffness, measured using gramicidin A (gA) channels as molecular force-transducers, and on voltage-dependent sodium channels (VDSC) known to be regulated by bilayer elasticity. Capsaicin and capsazepine (10-100 microM) increase gA channel appearance rate and lifetime without measurably altering bilayer thickness or channel conductance, meaning that the changes in bilayer elasticity are sufficient to alter the conformation of an embedded protein. Capsaicin and capsazepine promote VDSC inactivation, similar to other amphiphiles that decrease bilayer stiffness, producing use-dependent current inhibition. For capsaicin, the quantitative relation between the decrease in bilayer stiffness and the hyperpolarizing shift in inactivation conforms to that previously found for other amphiphiles. Capsaicin's effects on gA channels and VDSC are similar to those of Triton X-100, although these amphiphiles promote opposite lipid monolayer curvature. We conclude that capsaicin can regulate VDSC function by altering bilayer elasticity. This mechanism may underlie the promiscuous regulation of membrane protein function by capsaicin and capsazepine-and by amphiphilic drugs generally.

Electrospray ionization-mass spectrometry and tandem mass spectrometry reveal self-association and metal-ion binding of hydrophobic peptides: a study of the gramicidin dimer

Gramicidin is a membrane pentadecapeptide that acts as a channel, allowing the passage of monovalent metal ions and assisting in bacterial cell death. The active form is a noncovalently bound dimer. One means to study the self-assembly of this peptide has been to compare the state of the peptide in various solvents ranging from hydrophilic (e.g., trifluoroethanol) to hydrophobic (e.g., n-propanol). In this article, we report the use of electrospray mass spectrometry to study the self-association of gramicidin in various organic and mixed solvents that are introduced directly into the mass spectrometer. The dimer (both homo and hetero) can survive the introduction into the gas phase, and the amount in the gas phase increases with the decreasing dielectric constant of the solvent, reflecting solution-phase behavior. Tandem mass spectrometry data reveal that the stability of dimer in the gas phase decreases with increasing metal ion size, strongly suggesting that the metal ion binds inside the dimer between the monomers.

Bilayer-dependent inhibition of mechanosensitive channels by neuroactive peptide enantiomers

The peptide GsMTx4, isolated from the venom of the tarantula Grammostola spatulata, is a selective inhibitor of stretch-activated cation channels (SACs). The mechanism of inhibition remains unknown; but both GsMTx4 and its enantiomer, enGsMTx4, modify the gating of SACs, thus violating a trademark of the traditional lock-and-key model of ligand-protein interactions. Suspecting a bilayer-dependent mechanism, we examined the effect of GsMTx4 and enGsMTx4 on gramicidin A (gA) channel gating. Both peptides are active, and the effect increases with the degree of hydrophobic mismatch between bilayer thickness and channel length, meaning that GsMTx4 decreases the energy required to deform the boundary lipids adjacent to the channel. GsMTx4 decreases inward SAC single-channel currents but has no effect on outward currents, suggesting it is located within a Debye length of the outer vestibule of the SACs, but significantly farther from the inner vestibule. Likewise, GsMTx4 decreases gA single-channel currents. Our results suggest that modulation of membrane proteins by amphipathic peptides--mechanopharmacology--involves not only the protein itself but also the surrounding lipids. The surprising efficacy of the d form of GsMTx4 peptide has important therapeutic implications, because d peptides are not hydrolysed by endogenous proteases and may be administered orally.

Synthesis and functional studies of THF-gramicidin hybrid ion channels

THF-gramicidin hybrids 2-4 with the L-THF amino acid 1 in positions 11 and 12 and compounds 5-8 with the D-THF amino acid ent-1 in positions 10 and 11 were synthesized and their ion channel properties were studied by single-channel-current analysis. The replacement of positions 11 and 12 by the L-THF amino acid 1 gave a strongly reduced channel performance. In contrast, replacement of positions 10 and 11 by the D-THF amino acid ent-1 gave rise to new and interesting channel properties. For the permeability ratios, the ion selectivity shifts from Eisenman I towards Eisenman III selectivity and the channels display ms-dynamics. Most remarkable is the asymmetric compound 8, which inserts selectively into a DPhPC membrane and displays voltage-directed gating dynamics.

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.

Simultaneous optical and electrical recording of single gramicidin channels

We report here an approach for simultaneous fluorescence imaging and electrical recording of single ion channels in planar bilayer membranes. As a test case, fluorescently labeled (Cy3 and Cy5) gramicidin derivatives were imaged at the single-molecule level using far-field illumination and cooled CCD camera detection. Gramicidin monomers were observed to diffuse in the plane of the membrane with a diffusion coefficient of 3.3 x 10(-8) cm(2)s(-1). Simultaneous electrical recording detected gramicidin homodimer (Cy3/Cy3, Cy5/Cy5) and heterodimer (Cy3/Cy5) channels. Heterodimer formation was observed optically by the appearance of a fluorescence resonance energy transfer (FRET) signal (irradiation of Cy3, detection of Cy5). The number of FRET signals was significantly smaller than the number of Cy3 signals (Cy3 monomers plus Cy3 homodimers) as expected. The number of FRET signals increased with increasing channel activity. In numerous cases the appearance of a FRET signal was observed to correlate with a channel opening event detected electrically. The heterodimers also diffused in the plane of the membrane with a diffusion coefficient of 3.0 x 10(-8) cm(2)s(-1). These experiments demonstrate the feasibility of simultaneous optical and electrical detection of structural changes in single ion channels as well as suggesting strategies for improving the reliability of such measurements.

Geometry and intrinsic tilt of a tryptophan-anchored transmembrane alpha-helix determined by (2)H NMR

We used solid-state deuterium NMR spectroscopy and an approach involving geometric analysis of labeled alanines (GALA method) to examine the structure and orientation of a designed synthetic hydrophobic, membrane-spanning alpha-helical peptide in phosphatidylcholine (PC) bilayers. The 19-amino-acid peptide consists of an alternating leucine and alanine core, flanked by tryptophans that serve as interfacial anchors: acetyl-GWW(LA)(6)LWWA-ethanolamine (WALP19). A single deuterium-labeled alanine was introduced at different positions within the peptide. Peptides were incorporated in oriented bilayers of dilauroyl- (di-C12:0-), dimyristoyl- (di-C14:0-), or dioleoyl- (di-C18:1(c)-) phosphatidylcholine. The NMR data fit well to a WALP19 orientation characterized by a distinctly nonzero tilt, approximately 4 degrees from the membrane normal, and rapid reorientation about the membrane normal in all three lipids. Although the orientation of WALP19 varies slightly in the different lipids, hydrophobic mismatch does not seem to be the dominant factor causing the tilt. We suggest rather that the peptide itself has an inherently preferred tilted orientation, possibly related to peptide surface characteristics or the disposition of tryptophan indole anchors relative to the lipids, the peptide backbone, and the membrane/water interface. Additionally, the data allow us to define more precisely the local alanine geometry in this membrane-spanning alpha-helix.

Gramicidin derivatives as membrane-based pH sensors

Ion channels provide a means for sensitive pH measurement at membrane interfaces. Detailed knowledge of the structure and function of gramicidin channels permits the engineering of pH-sensitive derivatives. Two derivatives, gramicidin-ethylenediamine and gramicidin-histamine, are shown to exhibit pH-dependent single-channel behaviour over the pH ranges 9-11 and 6.5-8.5, respectively. Thermal isomerization of a carbamate group at the entrance of the channels leads to a pattern of steps in single-channel recordings. The size of the steps depends on the time-averaged degree of protonation of the appended group (ethylenediamine or histamine). Measurement of the size of the steps thus permits single-molecule pH sensing under symmetrical pH conditions or in the presence of a pH gradient.