Tethered-bilayer lipid membranes as a support for membrane-active peptides.

An immunosensing device, comprising a lipid membrane incorporating ion channels tethered to the surface of a gold electrode, has been reported [Cornell, Braach-Maksvytis, King, Osman, Raguse, Wieczorek and Pace (1997) Nature (London) 387, 580-583]. The present article describes key steps in the assembly of the device and provides further evidence for its proposed sensing mechanism.

The gramicidin-based biosensor: a functioning nano-machine.

Biosensors combine a biological recognition mechanism with a physical transduction technique. In nature, the transduction mechanism for high sensitivity molecular detection is the modulation of the cell membrane ionic conductivity through specific ligand-receptor binding-induced switching of ion channels. This effects an inherent signal amplification of six to eight orders of magnitude, corresponding to the total ion flow arising from the single channel gating event. Here we describe the first reduction of this principle to a practical sensing device, which is a planar impedance element composed of a macroscopically supported synthetic bilayer membrane incorporating gramicidin ion channels. The membrane and an ionic reservoir are covalently attached to an evaporated gold surface. The channels have specific receptor groups attached (usually antibodies) that permit switching of gramicidin channels by analyte binding to the receptors. The device may then be made specific for the detection of a wide range of analytes, including proteins, drugs, hormones, antibodies, DNA, etc., currently in the 10(-7)-10(-13) M range. It also lends itself readily to microelectronic fabrication and signal transduction. By adjusting the surface density of the receptors/channel components during fabrication, the optimum sensitivity range of the device may be tuned over several orders of magnitude.

Kinetics of the competitive response of receptors immobilised to ion-channels which have been incorporated into a tethered bilayer.

A competitive ion channel switch (ICS) biosensor has been modelled yielding ligand mediated monomer-dimer reaction kinetics of gramicidin (gA) ion-channels within a tethered bilayer lipid membrane. Through employing gramicidin A, functionalized with the water-soluble hapten digoxigenin, it is possible to cross-link gramicidin to antibody fragments tethered at the membrane/aqueous interface. The change in ionic conductivity of the channel dimers may then be used to measure the binding kinetics of hapten-protein interactions at the membrane surface. The approach involves measuring the time dependence of the increase in impedance following the addition of a biotinylated antibody fragment (b-Fab'), which cross-links the functionalized gramicidin monomers in the outer layer of the lipid bilayer to tethered membrane spanning lipid. The subsequent addition of the small molecule digoxin, (M(r) 781 Da), competes with and reverses this interaction. The model provides a quantitative description of the response to both the cross-linking following the addition of the b-Fab' and the competitive displacement of the hapten by a water-soluble small analyte. Good agreement is obtained with independent measures of the cross-linking reaction rates of the gramicidin monomer-dimer and the b-Fab: hapten complex. The rate and amplitude of the competitive response is dependent on concentration and provides a fast and sensitive detection technique. Estimates are made of the concentration of gramicidin monomers in both the inner and outer monolayer leaflets of the membrane. This is used in the calculation of the gramicidin monomer/dimer equilibrium constant, K2D3. Other considerations include the membrane impedance limit set by the membrane leakage which is also a function of the concentration of the gA monomer concentration, and the two-dimensional kinetic association constant k2D2, of the hapten: b-Fab' complex. The gA dimer concentration is dependent on both the concentration of gA-dig and of the tethered streptavidin: b-Fab' complexes. The model shows that the 2D dissociation constant k2D3(-1), must be at least 10 times faster than the 3D dissociation constant k3D2(-1) for digoxin to completely reverse the cross-linked hapten-receptor interaction at the membrane interface.

A biosensor that uses ion-channel switches.

Biosensors are molecular sensors that combine a biological recognition mechanism with a physical transduction technique. They provide a new class of inexpensive, portable instrument that permit sophisticated analytical measurements to be undertaken rapidly at decentralized locations. However, the adoption of biosensors for practical applications other than the measurement of blood glucose is currently limited by the expense, insensitivity and inflexibility of the available transduction methods. Here we describe the development of a biosensing technique in which the conductance of a population of molecular ion channels is switched by the recognition event. The approach mimics biological sensory functions and can be used with most types of receptor, including antibodies and nucleotides. The technique is very flexible and even in its simplest form it is sensitive to picomolar concentrations of proteins. The sensor is essentially an impedance element whose dimensions can readily be reduced to become an integral component of a microelectronic circuit. It may be used in a wide range of applications and in complex media, including blood. These uses might include cell typing, the detection of large proteins, viruses, antibodies, DNA, electrolytes, drugs, pesticides and other low-molecular-weight compounds.

Sodium ion binding in the gramicidin A channel. Solid-state NMR studies of the tryptophan residues.

Gramicidin A analogs, labeled with 13C in the backbone carbonyl groups and the C-2 indole carbons of the tryptophan-11 and tryptophan-13 residues, were synthesized using t-Boc-protected amino acids. The purified analogs were incorporated into phosphatidylcholine bilayers at a 1:15 molar ratio and macroscopically aligned between glass coverslips. The orientations of the labeled groups within the channel were investigated using solid-state NMR and the effect of a monovalent ion (Na+) on the orientation of these groups determined. The presence of sodium ions did not perturb the 13C spectra of the tryptophan carbonyl groups. These results contrast with earlier results in which the Leu-10, Leu-12, and Leu-14 carbonyl groups were found to be significantly affected by the presence of sodium ions and imply that the tryptophan carbonyl groups are not directly involved in ion binding. The channel form of gramicidin A has been demonstrated to be the right-handed form of the beta 6.3 helix: consequently, the tryptophan carbonyls would be directed away from the entrance to the channel and take part in internal hydrogen bonding, so that the presence of cations in the channel would have less effect than on the outer leucine residues. Sodium ions also had no effect on the C-2 indole resonance of the tryptophan side chains. However, a small change was observed in Trp-11 when the ether lipid, ditetradecylphosphatidylcholine, was substituted for the ester lipid, dimyristoylphosphatidylcholine, indicating some sensitivity of the gramicidin side chains to the surrounding lipid.

Structure and orientation of the pore-forming peptide, melittin, in lipid bilayers.

Ten analogues of the 26-residue, bee venom peptide, melittin (H3N(+)-GIGAVLKVLTTGLPALISWIKRKRQQ-CONH2), were synthesized, each with 13C enrichment of a single peptide carbonyl carbon. These peptides were incorporated into bilayers of the diether lipid, ditetradecylphosphatidylcholine, aligned between stacked glass plates. Solid-state 13C nuclear magnetic resonance spectra were obtained as a function of the angle between the bilayer planes and the magnetic field of the spectrometers, and at temperatures above and below the lipid gel-to-liquid crystalline transition temperature, Tc. For bilayers aligned with the normal along the applied magnetic field there was no shift in the carbonyl resonances of residues Ile2, Ala4, Leu9, Leu13, or Ala15, with minor changes for residues Val8 and Ile20, and small changes at Val5, Leu6 and Ile17 on immobilization of the peptide below Tc. In contrast, the spectra for bilayers aligned at right angles to the field showed greatly increased anisotropy below Tc for all analogues. From these experiments it was evident that the peptide was well-aligned in the bilayers and reoriented about the bilayer normal. The observed reduced chemical shift anisotropies and the chemical shifts were consistent with melittin adopting a helical conformation with a transbilayer orientation in the lipid membranes. With the exception of Ile17, there was no apparent difference between the behaviour of residues in the two segments that form separate helices in the water-soluble form of the peptide, suggesting that in membranes the angle between the helices is greater than the 120 degrees observed in the crystal form.

Melittin-induced changes in lipid multilayers. A solid-state NMR study.

Solid-state 1H, 13C, 14N, and 31P NMR spectroscopy was used to study the effects of the bee venom peptide, melittin, on aligned multilayers of dimyristoyl-, dilauryl- and ditetradecyl-phosphatidylcholines above the gel to liquid-crystalline transition temperature, Tc. Both 31P spectra from the lipid headgroups and 1H resonances from the lipid acyl chain methylene groups indicate that the peptide does not affect the mosaic spread of the lipid molecules at lipid:peptide molar ratios of 10:1, or higher. None of the samples prepared above Tc showed any evidence of the formation of hexagonal or isotropic phases. Melittin-induced changes in the chemical shift anisotropy of the headgroup phosphate and the lipid carbonyl groups, and in the choline 14N quadrupole splittings, show that the peptide has effects on the headgroup order and on the molecular organization in the sections of the acyl chains nearest to the bilayer surface. The spin-lattice relaxation time for the lipid acyl chain methylene protons was found to increase and the rotating-frame longitudinal relaxation time to markedly decrease with the addition of melittin, suggesting that motions on the nanosecond time scale are restricted, whereas the slower, collective motions are enhanced in the presence of the peptide.

A 35Cl and 37Cl NMR study of chloride binding to the erythrocyte anion transport protein.

Band 3, the erythrocyte anion transport protein, mediates the one-for-one exchange of bicarbonate and chloride ions across the membrane and consequently plays an important role in respiration. Binding to the protein forms the first step in the translocation of the chloride across the membrane. 35Cl and 37Cl NMR relaxation measurements at various field strengths were used to study chloride binding to the protein in the presence and absence of the transport inhibitor 4,4'-dinitrostilbene-2,2'-disulfonate. Significant differences occurred in the NMR relaxation rates depending on whether the inhibitor was present or not. The results indicate that the rate of chloride association and dissociation at each external binding site occurs on a time scale of less than or equal to 5 microseconds. This implies that the transmembrane flux is not limited by the rate of chloride binding to the external chloride binding site of band 3. The rotational correlation-time of chloride bound to band 3 was found to be greater than 20 ns with a quadrupole coupling constant of approximately 2 MHz.

Actin dynamics studied by solid-state NMR spectroscopy.

Solid-state nuclear magnetic resonance spectroscopy was used to study the motion of 2H and 19F probes attached to the skeletal muscle actin residues Cys-10, Lys-61 and Cys-374. The probe resonances were observed in dried and hydrated G-actin, F-actin and F-actin-myosin subfragment-1 complexes. Restricted motion was exhibited by 19F probes attached to Cys-10 and Cys-374 on actin. The dynamics of probes attached to dry cysteine powder or F-actin were very similar and the binding of myosin had little effect indicating that the local probe environment imposes the major influence on motion in the solid state. Correlation times determined for the solid state probes indicated that they were undergoing some rapid internal motion in both G-actin and F-actin such as domain twisting. The probe size influenced the motion in G-actin and appeared to sense monomer rotation but not in F-actin where segmental mobility and intramonomer co-ordination appeared to dominate.

Solid-state 13C-NMR studies of the effects of sodium ions on the gramicidin A ion channel.

End-to-end helical dimers of gramicidin A form transmembrane pores in lipid bilayers, through which monovalent ions may pass. The groups within the peptide that interact with these ions have been studied by application of solid-state spectroscopic methods to a series of gramicidin A analogues synthesized with 13C in selected peptide carbonyl groups. The resonances of D-Leu10, D-Leu12 and D-Leu14 analogues were perturbed in the presence of 0.16 M sodium ions, whereas the resonances of the carbonyls of Gly2, Ala3, D-Leu4 and Val7, which are closer to the formylated N-terminal end of the peptide, were unaffected. The observed changes in chemical shift anisotropy are indicative of a change in orientation of the abovementioned leucine carbonyls.

Effect of acyl chain length on the structure and motion of gramicidin A in lipid bilayers.

The transmembrane ion transport properties of gramicidin A have previously been shown to dependent on the nature of its lipid environment. Solid-state NMR spectroscopic studies of 13C-labelled analogues of gramicidin in oriented multilayers of phosphatidylcholine have shown that variation of the lipid hydrocarbon chain length has no effect on the structure or orientation of the peptide backbone.

Determination of the structure of a membrane-incorporated ion channel. Solid-state nuclear magnetic resonance studies of gramicidin A.

Solid-state nuclear magnetic resonance (NMR) measurements on 13C-labeled analogues of the ion channel-forming peptide, gramicidin A, have been used to directly determine the structure of this peptide in lipid membranes. Seven gramicidin analogues, each labeled in a single carbonyl group of gly2, L-ala3, D-leu4, L-val7, D-leu10, D-leu12, or D-leu14 were synthesized by the solid-phase method. These gramicidin analogues were incorporated into aligned multilayers of dimyristoylphosphatidylcholine, or diether lipid bearing 14- or 16-carbon chains, at a 1:15 peptide:lipid mole ratio. Proton-enhanced, 13C, solid-state spectra were obtained at several temperatures and over a range of sample orientations with respect to the spectrometer magnetic field to permit accurate measurement of the chemical shift anisotropies. The observed anisotropies indicate that all of the labeled carbonyl bonds are oriented almost parallel to the molecular long axis and perpendicular to the lipid bilayer plane. These orientations are consistent with gramicidin forming a beta 6.3 single-strand helix that is oriented parallel to the methylene chains of the lipid molecules. Comparison of the linewidths from labeled residues that are in the innermost turn of the helix (gly2, ala3, and D-leu4), in the center of the molecule (val7), and in the turn nearest the lipid bilayer surface (D-leu10, D-leu12, and D-leu14) suggests that although the peptide behaves largely as a rigid barrel, segments of the peptide close to the membrane surface possess greater motional freedom. At temperatures above the gel-to-liquid crystalline transition temperature (Tc) the gramicidin molecules rotate, with a less than millisecond correlation time, about the bilayer normal: several degrees below Tc they become immobile on the NMR timescale, without change in the channel conformation. In the L beta' phase the linewidths of the D-leu10, D-leu'2, and D-leu" resonances become equal to those of the other labeled sites, indicating reduced but equivalent motion for all of the peptide carbonyl groups.

Microviscosity of human erythrocytes studied with hypophosphite and 31P-NMR.

A 31P-NMR method, which complements earlier 13C-NMR procedures for probing the intra-erythrocyte microenvironment, is described. Hypophosphite is an almost unique probe of the erythrocyte microenvironment, since it is rapidly transported into the cell via the band 3 protein, and intra- and extracellular populations give rise to distinct resonances in the 31P-NMR spectrum. Relaxation mechanisms of the 31P nucleus in the hypophosphite ion were shown to be spin-rotation and dipole-dipole. Analysis of longitudinal relaxation rates in human erythrocytes, haemolysates and concentrated glycerol solutions allowed the determination of microviscosity using the Debye equation. Bulk viscosities of lysates and glycerol solutions were measured using Ostwald capillary viscometry. Translational diffusion coefficients were then calculated from the viscosity estimates using the Stokes-Einstein equation. The results with a range of solvent systems showed that 'viscosity' is a relative phenomenon and that bulk (i.e., macro-) viscosity is therefore not necessarily related to the NMR-determined viscosity. The intracellular NMR-determined viscosities from red cells, ranging in volume from 65.5 to 100.1 fl, varied from 2.10 to 2.67 mPa s. This is consistent with the translational diffusion coefficients of the hypophosphite ion altering by only 20%, whereas the values determined from bulk viscosity measurements conducted on lysates of these cells are consistent with a 230% change.

Chemical shift anisotropies obtained from aligned egg yolk phosphatidylcholine by solid-state 13C nuclear magnetic resonance.

Natural abundance solid-state 13C NMR spectra were obtained from orientated egg yolk phosphatidylcholine multilayers in which peaks from the different types of carbon in the lipid were resolved. The residual chemical shift anisotropy of the choline, glycerol, and olefinic carbons, as well as the carbonyl and acyl chain methylene carbons, were estimated. This information provided the basis for a qualitative description of the order and conformation of egg yolk phosphatidylcholine in the L alpha phase. The results suggested the gauche conformation for the C alpha-C beta bond in the choline moiety, a constrained glycerol region, a magic angle orientation for the sn-2 carbonyl, and a preferred orientation close to the bilayer normal for the plane of the sn-1 carbonyl bond and acyl chain C = C bond. The orientations of the carbon nuclei are in accord with the molecular conformation derived from previous 2H, 31P, and 13C NMR studies.

A model for gramicidin A'-phospholipid interactions in bilayers.

A model is proposed for the effect of gramicidin A' on the order and structure of phospholipid dispersions. According to this model, the addition of gramicidin A' influences the surrounding lipids via two independent mechanisms. The first arises from a drop in surface pressure for those lipids substantially bounded by gramicidin A'. The second mechanism arises from the increase in the phospholipid headgroup spacing due to the small polar region of the polypeptide. The model provides an explanation for the currently available NMR, X-ray diffraction and Langmuir monolayer results. The model also suggests mechanisms for the ability of gramicidin A' to trigger a transition of the lipid from the lamellar to hexagonal II phase, the dependence of this transition on the lipid chain length and the formation of a lamellar phase with lysophosphatidylcholine.

The effect of gramicidin A on phospholipid bilayers.

The helical polypeptide, gramicidin A has been widely studied as a model for the interactions of hydrophobic proteins with lipid bilayer membranes. Many reports are now available of the physical effects of mixing gramicidin A with phospholipid membranes, however, the interpretation of these data remains unclear. The purpose of this communication is to examine the controversial claim that high concentrations of gramicidin A' cause disorder within the L alpha phase of phosphatidylcholine-water dispersions. Solid-state nuclear magnetic resonance (NMR), density gradient and X-ray diffraction techniques are used to confirm the existence of such an effect and mechanisms are discussed which account for the known effects of gramicidin A on lipid bilayers.

Characterization of dodecylphosphocholine/myelin basic protein complexes.

The stoichiometry of myelin basic protein (MBP)/dodecylphosphocholine (DPC) complexes and the location of protein segments in the micelle have been investigated by electron paramagnetic resonance (EPR), ultracentrifugation, photon correlation light scattering, 31P, 13C, and 1H nuclear magnetic resonance (NMR), and electron microscopy. Ultracentrifugation measurements indicate that MBP forms stoichiometrically well-defined complexes consisting of 1 protein molecule and approximately 140 detergent molecules. The spin-labels 5-, 12-, and 16-doxylstearate have been incorporated into DPC/MBP aggregates. EPR spectral parameters and 13C and 1H NMR relaxation times indicate that the addition of MBP does not affect the environment and location of the labels or the organization of the micelles except for a slight increase in size. Previous results indicating that the protein lies primarily near the surface of the micelle have been confirmed by comparing 13C NMR spectra of the detergent with and without protein with spectra of protein/detergent aggregates containing spin-labels. Electron micrographs of the complexes taken by using the freeze-fracture technique confirm the estimated size obtained by light-scattering measurements. Overall, these results indicate that mixtures of MBP and DPC can form highly porous particles with well-defined protein and lipid stoichiometry. The structural integrity of these particles appears to be based on protein-lipid interactions. In addition, electron micrographs of aqueous DPC/MBP suspensions show the formation of a small amount of material consisting of large arrays of detergent micelles, suggesting that MBP is capable of inducing large changes in the overall organization of the detergent.

Conformation and orientation of gramicidin a in oriented phospholipid bilayers measured by solid state carbon-13 NMR.

Three analogues of the helical ionophore gramicidin A have been synthesized with (13)C-labeled carbonyls ((13)C=O) incorporated at either Gly(2), Ala(3), or Val(7). A fourth compound incorporated (13)C at both the carbonyl and alpha-carbon of Gly(2) within the same molecule. These labels were studied using solid-state, proton-enhanced, (13)C nuclear magnetic resonance (NMR) in hydrated dispersions of dimyristoylphosphatidylcholine (DMPC)-gramicidin A. The dispersions were aligned on glass coverslips whose orientation to the magnetic field could be varied through 180 degrees . The orientation dependence of the NMR spectrum was used to obtain an accurate measurement of the (13)C chemical shift anisotropy (CSA), and in the case of the fourth compound, the (13)C-(13)C dipolar coupling constant. From the measured CSA and estimates of the orientation of the (13)C shielding tensor, we are able to determine the direction of the (13)C=O bonds and to compare these with the predictions of the various reported models for the configuration of gramicidin A in phospholipid bilayers. Our results are consistent with the left-handed pipi(6.3) (LD) single-stranded helix (Urry, D. W., J. T. Walker, and T. L. Trapane. 1982. J. Membr. Biol. 69:225-231). The right-handed pipi(6.3) (LD) single-stranded helix observed for gramicidin A in sodium dodecyl sulfate micelles (Arseniev, A. S., I. L. Barsukov, V. F. Bystrov, A. L. Loize, and Yu A. Ovchinnikov. 1985. FEBS (Fed. Eur. Biochem. Soc.) Lett. 186:168-174) yields a poorer fit to the data. However, the width of the carbonyl resonances suggests a distribution of molecular geometries possibly resulting from a spread in the helix pitch and handedness. Double-stranded helices and beta sheet structures are excluded. In dispersions in which the lipid is in the L(alpha) phase, the gramicidin A undergoes rapid reorientation about an axis which is centered on the normal to the plane of the coverslips. When the supporting lipid is in the L(beta') phase the helices are rigid on the timescale of (13)C-NMR. The configuration of gramicidin A is unaltered by L(alpha)-L(beta') phase transition of the bilayer lipid.

Location and activity of ubiquinone 10 and ubiquinone analogues in model and biological membranes.

Deuteriated analogues of ubiquinone 10 (Q10) have been dispersed with plasma membranes of Escherichia coli and with the inner membranes of beetroot mitochondria. Orientational order at various deuteriated sites was measured by solid-state deuterium nuclear magnetic resonance (2H NMR). Similar measurements were made, using the compounds dispersed in dimyristoylphosphatidylcholine (DMPC) and egg yolk lecithin and dispersions prepared from the lipid extracts of beetroot mitochondria. In all cases only a single unresolved 2H NMR spectrum (typically 1000-Hz full width at half-height) was observed at concentrations down to 0.02 mol % Q10 per membrane lipid. This result shows that most Q10 is in a mobile environment which is physically separate from the orientational constraints of the bilayer lipid chains. In contrast, a short-chain analogue of Q10, in which the 10 isoprene groups have been replaced by a perdeuteriated tridecyl chain, showed 2H NMR spectra with quadrupolar splittings typical of an ordered lipid that is intercalated into the bilayer. The NADH oxidase activity and O2 uptake in Escherichia coli and in mitochondria were independent of which analogue was incorporated into the membrane. Thus, despite the major difference in their physical association with membranes, or their lipid extracts, the electron transport function of the long- and short-chain ubiquinones is similar, suggesting that the bulk of the long-chain ubiquinone does not have a direct function in electron transporting activity. The physiologically active Q10 may only be a small fraction of the total ubiquinone, a fraction that is below the level of detection of the present NMR equipment.(ABSTRACT TRUNCATED AT 250 WORDS)