Interaction of ions and water in gramicidin A channels: streaming potentials across lipid bilayer membranes

For very narrow channels in which ions and water cannot overtake one another (single-file transport), electrokinetic measurements provide information about the number of water molecules within a channel. Gramicidin A is believed to form such narrow channels in lipid bilayer membranes. In 0.01 and 0.1 M solutions of CsCl, KCL, and NaCl, streaming potentials of 3.0 mV per osmolal osmotic pressure difference (created by urea, glycerol, or glucose) appear across gramicidin A-treated membranes. This implies that there are six to seven water molecules within a gramicidin channel. Electroosmotic experiments, in which the water flux assoicated with current flow across gramicidin-treated membranes is measured, corroborate this result. In 1 M salt solutions, streaming potentials are 2.35 mV per osmolal osmotic pressure difference instead of 3.0 mV. The smaller value may indicate multiple ion occupancy of the gramicidin channel at high salt concentrations. Apparent deviations from ideal cationic selectivity observed while attempting to measure single-salt dilution potentials across gramicidin-treated membranes result from streaming potential effects.     

Ion movement through gramicidin A channels. Studies on the diffusion-controlled association step

The permeability characteristics of gramicidin A channels are generally considered to reflect accurately the intrinsic properties of the channels themselves; i.e., the aqueous convergence regions are assumed to be negligible barriers for ion movement through the channels. The validity of this assumption has been examined by an analysis of gramicidin A single-channel current-voltage characteristics up to very high potentials (500 mV). At low permeant ion concentrations the currents approach a voltage-independent limiting value, whose magnitude is proportional to the permeant ion concentration. The magnitude of this current is decreased by experimental maneuvers that decrease the aqueous diffusion coefficient of the ions. It is concluded that the magnitude of this limiting current is determined by the diffusive ion movement through the aqueous convergence regions up to the channel entrance. It is further shown that the small-signal (ohmic) permeability properties also reflect the existence of the aqueous diffusion limitation. These results have considerable consequences for the construction of kinetic models for ion movement through gramicidin A channels. It is shown that the simple two-site-three-barrier model commonly used to interpret gramicidin A permeability data may lead to erroneous conclusions, as biionic potentials will be concentration dependent even when the channel is occupied by at most one ion. The aqueous diffusion limitation must be considered explicitly in the analysis of gramicidin A permeability characteristics. Some implications for understanding the properties of ion-conducting channels in biological membranes will be considered.     

Dynamic ion-ion and water-ion interactions in ion channels.

The dynamic interactions among ions and water molecules in ion channels are treated based on an assumption that ions at binding sites can be knocked off by both transient entering ions and local water molecules. The theory, when applied to a single-site model K+ channel, provides solutions for super- and subsaturations, flux-ratio exponent (n') greater than 1, osmotic streaming current, activity-dependent reversal potentials, and anomalous mole-fraction behavior. The analysis predicts that: (a) the saturation may but, in general, does not follow the Michaelis-Menten relation; (b) streaming current results from imbalanced water-ion knock-off interactions; (c) n' greater than 1 even for single-site channels, but it is unlikely to exceed 1.4 unless the pore is occupied by one or more ion(s); (d) in the calculation involving two permeant ion species with similar radii, the heavier ions show higher affinity; the ion-ion knock-off dissociation from the site is more effective when two interacting ions are identical. Therefore, the "multi-ion behaviors" found in most ion channels are the consequences of dynamic ion-ion and water-ion interactions. The presence of these interactions does not require two or more binding sites in channels.     

Streaming potential measurements in Ca2+-activated K+ channels from skeletal and smooth muscle. Coupling of ion and water fluxes.

Streaming potentials arising across large-conductance Ca2+-activated K+ channels incorporated into planar lipid bilayers were measured. Ca2+-activated channels obtained either from skeletal muscle or from smooth muscle membranes were used. Streaming potentials were extracted from the current-voltage relationship for the open channel obtained in the presence of an osmotic gradient. The osmotic gradient was established by adding glucose to one side of the membrane. At 300 mM KCl, the average streaming potential was 0.72 mV/osmol per kg for t-tubule channels and 0.83 mV/osmol per kg for smooth muscle channels. Streaming potential values depend on KCl concentration, they decrease as KCl concentration increases, and the value obtained by extrapolation to zero KCl concentration is 0.85 mV/osmol per kg. Assuming that water and ions cannot pass each other, at least in a region of the channel, the streaming potential values obtained indicate that this region contains a minimum of two and a maximum of four water molecules. It is concluded that the channel has a narrow region with a length of 0.6-1.2 nm.     

The pore dimensions of gramicidin A.

The ion channel forming peptide gramicidin A adopts a number of distinct conformations in different environments. We have developed a new method to analyze and display the pore dimensions of ion channels. The procedure is applied to two x-ray crystal structures of gramicidin that adopt distinct antiparallel double helical dimer conformations and a nuclear magnetic resonance (NMR) structure for the beta6.3 NH2-terminal to NH2-terminal dimer. The results are discussed with reference to ion conductance properties and dependence of pore dimensions on the environment.     

The effect of gramicidin A on the temperature dependence of water permeation through liposomal membranes prepared from phosphatidylcholines with different chains lengths.

The permeation of water through liposomal membranes composed of various saturated phosphatidylcholine plus gramicidin A was studied as a function of temperature. 1. The presence of gramicidin in the liposomal bilayers caused an increase in water permeability. Below the phase transition temperature this effect could be measured quite clearly in all the systems we tested, but the extent of the increase was largely dependent on the length of the hydrocarbon chains. 2. Increasing amounts of gramicidin caused a gradual disappearance of the abrupt change in the rate of water permeation near the gel-liquid crystalline phase transition temperature of dipalmitoyl phosphatidylcholine liposomes. Differential scanning calorimetry analysis of the system containing these relatively small amounts of gramicidin still showed a clear transition from the liquid crystalline to the gel state with only a slight reduction in the enthalpy change. 3. In liposomes composed of dimyristoyl, dipalmitoyl and saturated egg phosphatidylcholine there was a concomitant decrease in the activation energy of water permeation in the presence of gramicidin below and above the phase transition temperature. The activation energy for water permeation through longer chained distearoyl phosphatidylcholine liposomal bilayers was the same with or without gramicidin in the bilayer. 4. It is concluded that the ability of gramicidin to form conducting channels in a gel state bilayer depends on the thickness of the paraffin core.     

Gramicidin A channel as a test ground for molecular dynamics force fields

We use the well-known structural and functional properties of the gramicidin A channel to test the appropriateness of force fields commonly used in molecular dynamics (MD) simulations of ion channels. For this purpose, the high-resolution structure of the gramicidin A dimer is embedded in a dimyristoylphosphatidylcholine bilayer, and the potential of mean force of a K(+) ion is calculated along the channel axis using the umbrella sampling method. Calculations are performed using two of the most common force fields in MD simulations: CHARMM and GROMACS. Both force fields lead to large central barriers for K(+) ion permeation, that are substantially higher than those deduced from the physiological data by inverse methods. In long MD simulations lasting over 60 ns, several ions are observed to enter the binding site but none of them crossed the channel despite the presence of a large driving field. The present results, taken together with many earlier studies, highlights the shortcomings of the standard force fields used in MD simulations of ion channels and calls for construction of more appropriate force fields for this purpose.     

Water permeability of gramicidin A-treated lipid bilayer membranes

In membranes containing aqueous pores (channels), the osmotic water permeability coefficient, P f, is greater than the diffusive water permeability coefficient, P d. In fact, the magnitude of P f/P d is commonly used to determine pore radius. Although, for membranes studied to date, P f/P d monotonically declines with decreasing pore radius, there is controversy over the value it theoretically assumes when that radius is so small that water molecules cannot overtake one another within the channel (single-file transport). In one view it should equal 1, and in another view it should equal N, the number of water molecules in the pore. Gramicidin A forms, in lipid bilayer membranes, narrow aqueous channels through which single-file transport may occur. For these channels we find that P f/P d approximately 5. In contrast, for the wider nystatin and amphotericin B pores, P f/P d approximately 3. These findings offer experimental support for the view that P f/P d = N for single-file transport, and we therefore conclude that there are approximately five water molecules in a gramicidin A channel. A similar conclusion was reached independently from streaming potential data. Using single-channel conductance data, we calculate the water permeability of an individual gramicidin A channel. In the Appendix we report that there is a wide range of channel sizes and lifetimes in cholesterol-containing membranes.     

Electrogenic behavior of synaptic vesicles from Torpedo californica.

Electrical potential changes in pure synaptic vesicles from Torpedo californica were monitored with the fluorescent dye 3,3'-dipropylthiadicarbocyanine iodide. Vesicles resuspended in variable external sodium ion in the presence of gramicidin established sodium ion membrane diffusion potentials. Vesicles resuspended in choline or acetylcholine chloride became hyperpolarized upon addition of gramicidin. Hyperpolarization was subsequently partially reversed spontaneously by choline or acetylcholine influx, which was confirmed by gel filtration, to yield a new, less negative, stable membrane potential. Thus, acetylcholine and choline are taken up electrogenically by synaptic vesicles.     

Coupling of water and ion fluxes in a K+-selective channel of sarcoplasmic reticulum.

Streaming potentials arising across a K+-selective channel from fragmented sarcoplasmic reticulum were measured by incorporating the channel into planar bilayer membranes and imposing osmotic gradients across the membranes by addition of sorbitol or urea to only one side. Single-channel zero-current potentials were determined, and dilution artifacts were corrected for by addition of valinomycin to the bilayer. The streaming potentials were found to be unusually small, 1.1 mV per osmolal. The potentials were linearly related to the osmotic gradient across the bilayer, and were identical for sorbitol and urea. The results imply that the channel cannot be envisioned as a long tube, like gramicidin, but rather as a short constriction of less than 10 A in length opening out into wider mouths on either side of the membrane.     

Radiation inactivation of ion channels formed by gramicidin A. Protection by lipid double bonds and by alpha-tocopherol.

The conductance induced by the channel-forming peptide gramicidin A in lipid membranes is reduced by many orders of magnitude on exposure of the membrane and its aqueous environment to ionizing radiation. This results from an interaction of free radicals of water radiolysis with the tryptophan residues of gramicidin A. The sensitivity of the ion channels towards irradiation is strongly reduced in the presence of either vitamin E or of highly unsaturated lipids. An increase of the D37 dose up to a factor of 50 was found. The phenomena are interpreted via a reduction of the effective concentration of free radicals (such as OH.) in the membrane by reaction with unsaturated fatty acid residues or with vitamin E.     

Coupled K+-water flux through the HERG potassium channel measured by an osmotic pulse method

The streaming potential (V(stream)) is a signature feature of ion channels in which permeating ions and water molecules move in a single file. V(stream) provides a quantitative measure of the ion and water flux (the water-ion coupling ratio), the knowledge of which is a prerequisite for elucidating the mechanisms of ion permeation. We have developed a method to measure V(stream) with the whole-cell patch-clamp configuration. A HEK293 cell stably expressing the HERG potassium channel was voltage clamped and exposed to hyperosmotic solutions for short periods of time (<1 s) by an ultrafast solution switching system (the osmotic pulse [quick jump-and-away] method). The reversal potentials were monitored by a series of voltage ramps before, during, and after the osmotic pulse. The shifts of the reversal potentials immediately after the osmotic jump gave V(stream). In symmetrical K+ solutions (10 mM), the V(stream)s measured at different osmolalities showed a linear relationship with a slope of -0.7 mV/DeltaOsm, from which the water-ion coupling ratio (n, the ratio of the flux of water to the flux of cations; Levitt, D.G., S.R. Elias, and J.M. Hautman. 1978. Biochim. Biophys. Acta. 512:436-451) was calculated to be 1.4. In symmetrical 100 mM K+ solutions, the coupling ratio was decreased significantly (n = 0.9), indicating that the permeation process through states with increased ion occupancy became significant. We presented a diagrammatic representation linking the water-ion coupling ratio to the mode of ion permeation and suggested that the coupling ratio of one may represent the least hydrated ion flux in the single-file pore.     

Simulation of voltage-driven hydrated cation transport through narrow transmembrane channels.

Molecular dynamics studies for the voltage-driven transport of the alkali metal ions lithium, sodium, and potassium through gramicidin A-type channels filled with water molecules are presented. The number of water molecules in the channel is obtained from a previous study (Skerra, A., and J. Brickmann, 1987, Biophys. J., 51:969-976). It is shown that the selectivity of the intrachannel ion diffusion through our model pore conforms to the experimentally observed selectivity of the gramicidin A channel. It is demonstrated that the number of water molecules in the channel plays a key role for the selectivity.     

Uniformly oriented gramicidin channels embedded in thick monodomain lecithin multilayers.

Phosphatidylcholine multilayers, containing 20% water by total sample weight and gramicidin/lipid molar ratios up to 1:40 were aligned by low temperature annealing (less than 60 degrees C) and mechanical stressing. We were able to obtain large (greater than 80 micron thick X 40 mm2 area) monodomain defect-free multilayers containing approximately 10(17) uniformly oriented gramicidin channels. The alignment of lipid multilayers was monitored by conoscopy and polarized microscopy. The smectic defects, which appeared during the alignment process, were identified and dissolved. The incorporation of gramicidin into the multilayers in the form of transmembrane channels was indicated by its circular dichroic (CD) spectrum. A well-defined CD spectrum of uniformly oriented gramicidin channels was obtained. The oriented samples will allow spectroscopic studies of the ion channel in its conducting state and diffraction studies of the channel-channel organization in the membrane.     

Effect of streptavidins with varying biotin binding affinities on the properties of biotinylated gramicidin channels

The pentadecapeptide gramicidin A, which is known to form highly conductive ion channels in a bilayer lipid membrane by assembling as transmembrane head-to-head dimers, can be modified by attaching a biotin group to its C-terminus through an aminocaproyl spacer. Such biotinylated gramicidin A analogues also form ion channels in a hydrophobic lipid bilayer, exposing the biotin group to the aqueous bathing solution. Interaction of the biotinylated gramicidin channels with (strept)avidin has previously been shown to result in the appearance of a long-lasting open state with a doubled transition amplitude in single-channel traces and a deceleration of the macroscopic current kinetics as studied by the sensitized photoinactivation method. Here this interaction was studied further by using streptavidin mutants with weakened biotin binding affinities. The Stv-F120 mutant, having a substantially reduced biotin binding affinity, exhibited an efficacy similar to that of natural streptavidin in inducing both double-conductance channel formation and deceleration of the photoinactivation kinetics of the biotinylated gramicidin having a long linker arm. The Stv-A23D27 mutant with a severely weakened biotin binding affinity was ineffective in eliciting the double-conductance channels, but decelerated noticeably the photoinactivation kinetics of the long linker biotinylated gramicidin. However, the marked difference in the effects of the mutant and natural streptavidins was smaller than expected on the basis of the substantially reduced biotin binding affinity of the Stv-A23D27 mutant. This may suggest direct interaction of this mutant streptavidin with a lipid membrane in the process of its binding to biotinylated gramicidin channels. The role of linker arm length in the interaction of biotinylated gramicidins with streptavidin was revealed in experiments with a short linker gramicidin. This gramicidin analogue appeared to be unable to form double-conductance channels, though several lines of evidence were indicative of its binding by streptavidin. The data obtained show the conditions under which the interaction of streptavidin with biotinylated gramicidin leads to the formation of the double-conductance tandem channels composed of two cross-linked transmembrane dimers.     

Conformation of gramicidin a in water: inference from analysis of hydrogen/deuterium exchange behavior by matrix assisted laser desorption ionization mass spectrometry

Gramicidin A (the major component of gramicidin D) is a highly hydrophobic peptide with very little solubility in water. Hence, the conformation of this peptide has been extensively investigated in organic solvents and model membranes, but not in water. The peptide adopts a beta6.3-helical conformation in the monomeric and dimeric forms. We have investigated the conformation of gramicidin A in water by monitoring hydrogen-deuterium exchange by matrix-assisted laser desorption ionization-time-of-flight mass spectrometry. Our results indicate that gramicidin A is monomeric and exists in a highly folded conformation. The metal ion bound forms are clearly discernible in the monomers. The presence of the dimeric form is not observed. It is unlikely this is due to the operating conditions or the method used, as both hetero- and homodimers in gramicidin D are detected when methanol is used as a solvent. The present study also establishes that the linear gramicidins retain a history of solvent environment when ions are generated by matrix-assisted laser desorption ionization and analyzed by time-of-flight.     

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 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.     

Theoretical analysis of hydrophobic matching and membrane-mediated interactions in lipid bilayers containing gramicidin.

We present a quantitative analysis of the effects of hydrophobic matching and membrane-mediated protein-protein interactions exhibited by gramicidin embedded in dimyristoylphosphatidylcholine (DMPC) and dilauroylphosphatidylcholine (DLPC) bilayers (Harroun et al., 1999. Biophys. J. 76:937-945). Incorporating gramicidin, at 1:10 peptide/lipid molar ratio, decreases the phosphate-to-phosphate (PtP) peak separation in the DMPC bilayer from 35.3 A without gramicidin to 32.7 A. In contrast, the same molar ratio of gramicidin in DLPC increases the PtP from 30.8 A to 32.1 A. Concurrently, x-ray in-plane scattering showed that the most probable nearest-neighbor separation between gramicidin channels was 26.8 A in DLPC, but reduced to 23.3 A in DMPC. In this paper we review the idea of hydrophobic matching in which the lipid bilayer deforms to match the hydrophobic surface of the embedded proteins. We use a simple elasticity theory, including thickness compression, tension, and splay terms to describe the membrane deformation. The energy of membrane deformation is compared with the energy cost of hydrophobic mismatch. We discuss the boundary conditions between a gramicidin channel and the lipid bilayer. We used a numerical method to solve the problem of membrane deformation profile in the presence of a high density of gramicidin channels and ran computer simulations of 81 gramicidin channels to find the equilibrium distributions of the channels in the plane of the bilayer. The simulations contain four parameters: bilayer thickness compressibility 1/B, bilayer bending rigidity Kc, the channel-bilayer mismatch Do, and the slope of the interface at the lipid-protein boundary s. B, Kc, and Do were experimentally measured; the only free parameter is s. The value of s is determined by the requirement that the theory produces the experimental values of bilayer thinning by gramicidin and the shift in the peak position of the in-plane scattering due to membrane-mediated channel-channel interactions. We show that both hydrophobic matching and membrane-mediated interactions can be understood by the simple elasticity theory.