Ion Movements in Gramicidin Channels

Application of Peak Intensity Analysis to Measurements of Protein Binding to Lipid Vesicles and Erythrocytes Using Fluorescence Correlation Spectroscopy: Dependence on Particle Size

Fluorescence correlation spectroscopy (FCS) is a sensitive analytical tool for investigation of processes accompanied by changes in the mobility of molecules and complexes. In the present work, peak intensity analysis (PIA) in combination with the solution stirring using FCS setup was applied to explore the interaction between fluorescently labeled protein ligands and corresponding receptors located on membranes. In the system composed of biotinylated liposomes and fluorescently labeled streptavidin as a ligand, PIA allowed us to determine the optimum receptor concentration and demonstrate the essential dependence of the binding efficacy on the length of the linker between the biotin group and the polar head group of the lipid. The binding was dependent on the size of liposomes which was varied by lipid extrusion through filters of different pore diameters. The sensitivity of the method was higher with the liposomes of larger sizes. The PIA approach can be applied not only to liposomes but also to relatively large objects, e.g., erythrocytes or Sepharose beads derivatized with lactose as a receptor for the binding of viscumin and ricin.

Gramicidin A disassembles large conductive clusters of its lysine-substituted derivatives in lipid membranes

N-terminally substituted lysine derivatives of gramicidin A (gA), [Lys1]gA and [Lys3]gA, but not glutamate- or aspartate-substituted peptides have been previously shown to cause the leakage of carboxyfluorescein from liposomes. Here, the leakage induction was also observed for [Arg1]gA and [Arg3]gA, while [His1]gA and [His3]gA were inactive at neutral pH. The Lys3-containing analogue with all tryptophans replaced by isoleucines did not induce liposome leakage, similar to gA. This suggests that the presence of both tryptophans and N-terminal cationic residues is critical for pore formation. Remarkably, the addition of gA blocked the leakage induced by [Lys3]gA. By examining with fluorescence correlation spectroscopy the peptide-induced leakage of fluorescent markers from liposomes, we estimated the diameter of pores responsible for the leakage to be about 1.6 nm. Transmission electron cryo-microscopy imaging of liposomes with [Lys3]gA showed that the liposomal membranes contained high electron density particles with a size of about 40 Å, suggesting the formation of peptide clusters. No such clusterization was observed in liposomes incorporating gA or a mixture of gA with [Lys3]gA. Three-dimensional reconstruction of the clusters was compatible with their pentameric arrangement. Based on experimental data and computational modeling, we suggest that the large pore formed by [Lys3]gA represents a barrel-stave oligomeric cluster formed by antiparallel double-stranded helical dimers (DH). In a tentative model, the pentamer of dimers may be stabilized by aromatic Trp-Trp and cation-π Trp-Lys interactions between the neighboring DHs. The inhibiting effect of gA on the [Lys3]gA-induced leakage can be attributed to breaking of cation-π interactions, which prevents peptide clusterization and pore formation.

Unsaturated lipids protect the integral membrane peptide gramicidin A from singlet oxygen

In contrast to expectations that unsaturated fatty acids contribute to oxidative stress by providing a source of lipid peroxides, we demonstrated the protective effect of double bonds in lipids on oxidative damage to membrane proteins. Photodynamic inactivation of gramicidin channels was decreased in unsaturated lipid compared to saturated lipid bilayers. By estimating photosensitizer (boronated chlorine e6 amide) binding to the membrane with the current relaxation technique, the decrease in gramicidin photoinactivation was attributed to singlet oxygen scavenging by double bonds in lipids rather than to the reduction in photosensitizer binding. Gramicidin protection by unsaturated lipids was also observed upon induction of oxidative stress with tert-butyl hydroperoxide.

Paradoxical one-ion pore behavior of the long β-helical peptide of marine cytotoxic polytheonamide B

The cytotoxic 48-mer peptide, polytheonamide B (pTB), from a marine sponge forms a β^6.3-helix with an inner diameter of 4 Å and a length of 45 Å, features that allow the selective permeation of monovalent cations across targeted cell membranes. To characterize this long, narrow pore, electrophysiological examination using a planar lipid bilayer method was performed. The single-channel current amplitude exhibited saturation for concentrated Cs⁺ or K⁺ solution, and the reversal potential in mixed solutions did not exhibit any anomalous mole-fraction behavior. These results suggest the one-ion permeation mechanism. This is in contrast to the short (26 Å) β^6.3-helical gramicidin channel, which holds two ions simultaneously. The paradoxical one-ion permeation through the long pTB channel was modeled with a discrete-state Markov model. Ions permeate through the channel by stepping between two binding sites in the pore, but never occupy these sites simultaneously in either pure or mixed ion solution.

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.

Cycle flux algebra for ion and water flux through the KcsA channel single-file pore links microscopic trajectories and macroscopic observables

In narrow pore ion channels, ions and water molecules diffuse in a single-file manner and cannot pass each other. Under such constraints, ion and water fluxes are coupled, leading to experimentally observable phenomena such as the streaming potential. Analysis of this coupled flux would provide unprecedented insights into the mechanism of permeation. In this study, ion and water permeation through the KcsA potassium channel was the focus, for which an eight-state discrete-state Markov model has been proposed based on the crystal structure, exhibiting four ion-binding sites. Random transitions on the model lead to the generation of the net flux. Here we introduced the concept of cycle flux to derive exact solutions of experimental observables from the permeation model. There are multiple cyclic paths on the model, and random transitions complete the cycles. The rate of cycle completion is called the cycle flux. The net flux is generated by a combination of cyclic paths with their own cycle flux. T.L. Hill developed a graphical method of exact solutions for the cycle flux. This method was extended to calculate one-way cycle fluxes of the KcsA channel. By assigning the stoichiometric numbers for ion and water transfer to each cycle, we established a method to calculate the water-ion coupling ratio (CR(w-i)) through cycle flux algebra. These calculations predicted that CR(w-i) would increase at low potassium concentrations. One envisions an intuitive picture of permeation as random transitions among cyclic paths, and the relative contributions of the cycle fluxes afford experimental observables.

Noncontact dipole effects on channel permeation. III. Anomalous proton conductance effects in gramicidin

Proton transport on water wires, of interest for many problems in membrane biology, is analyzed in side-chain analogs of gramicidin A channels. In symmetrical 0.1N HCl solutions, fluorination of channel Trp(11), Trp-(13), or Trp(15) side chains is found to inhibit proton transport, and replacement of one or more Trps with Phe enhances proton transport, the opposite of the effects on K(+) transport in lecithin bilayers. The current-voltage relations are superlinear, indicating that some membrane field-dependent process is rate limiting. The interfacial dipole effects are usually assumed to affect the rate of cation translocation across the channel. For proton conductance, however, water reorientation after proton translocation is anticipated to be rate limiting. We propose that the findings reported here are most readily interpreted as the result of dipole-dipole interactions between channel waters and polar side chains or lipid headgroups. In particular, if reorientation of the water column begins with the water nearest the channel exit, this hypothesis explains the negative impact of fluorination and the positive impact of headgroup dipole on proton conductance.

A hierarchical approach for predicting the transport properties of the gramicidin A channel

A hierarchical computational approach combining results of molecular dynamics (MD) simulations with continuous Poisson-Nernst-Planck (PNP) theory was used to investigate ion transport in a gramicidin A (gA) channel embedded within a 1,2-dimyristoylphosphatidylcholine (DMPC) bilayer. Molecular dynamics (MD) employing the CHARMM force field was used to investigate the diffusion of Na and K at different locations along the gA channel in both singly- and doubly-occupied states. Self-diffusion coefficients for single Na and K cations were determined to be 4.7 × 10 cm s and 6.2 × 10 cm s, respectively. Using these values, maximum ionic conductivities calculated from the Nernst-Einstein equation were 37 pS and 49 pS for Na and K, respectively, in the singly-occupied gA channel. These values agree with experimental data within an order of magnitude. Conductance of the gA channel was calculated from simulation results using the three-dimensional Poisson-Nernst-Planck (3D-PNP) model. Partial charge distributions for gA and for DMPC were assigned using the Poisson-Boltzmann module available in CHARMM. Diffusion coefficients were those obtained from the MD simulation. Results confirm that DMPC electrostatics have significant influence on channel conductivity. At low electrolyte concentrations, the channel cannot be occupied by more than one monovalent cation. Using ion diffusion coefficients obtained at different locations along the channel, current-voltage values obtained using 3D-PNP predictions for a channel immersed in an aqueous NaCl solution show good agreement with experimental values.

Free energy simulations of single and double ion occupancy in gramicidin A

Simultaneous occupancy of the two binding sites in gramicidin A by monovalent cations is a well known property of this channel, but the energetic feasibility of this process in molecular dynamics simulations has not been established so far. Here the authors study the energetics of single and double ion occupancy in gramicidin A by constructing the potential of mean force for single and pair of cations. As representatives of small and large ions, they consider both Na+ and K+ ions in the calculations. Binding constants of ions are estimated from the free energy profiles. Comparisons with the experimental results indicate 3-4 kT discrepancy in the binding energies. They also study the coordination of the ions in their respective binding sites and the dynamic behavior of the channel water during the double ion binding process.

Redox-regulated ion channel activity of a cysteine-containing gramicidin A analogue

According to recent data, gramicidin A analogues having positively charged amino acid sequences at the C-termini exhibit two types of channel activity in lipid membranes: classical cation-selective channels and large unselective pores. The induction of unselective pores was shown here to strongly depend on the redox state of the membrane-bathing solution, if the gramicidin analogue contained a cysteine residue in the sequence GSGPKKKRKVC attached to the C-terminus. In particular, the addition of H2O2 led to an increase in the transmembrane current and the loss of cationic selectivity on planar bilayer lipid membranes and an increase in the carboxyfluorescein leakage of liposomes. The effect was observed at high concentration of the peptide while was absent at the single-channel level. It was concluded that oxidation led to possible formation of dimers of the peptide, which promoted the formation of large unselective pores.

Large unselective pore in lipid bilayer membrane formed by positively charged peptides containing a sequence of gramicidin A

Ion-channel activity of a series of gramicidin A analogues carrying charged amino-acid sequences on the C-terminus of the peptide was studied on planar bilayer lipid membranes and liposomes. It was found that the analogue with the positively charged sequence GSGRRRRSQS forms classical cationic pores at low concentrations and large unselective pores at high concentrations. The peptide was predominantly in the right-handed beta(6.3)-helical conformation in liposomes as shown by circular dichroism spectroscopy. The single-channel conductance of the large pore was estimated to be 320pS in 100mM choline chloride as judged from the fluctuation analysis of the multi-channel current. The analogue with the negatively charged sequence GSGEEEESQS exhibited solely classical cationic channel activity. The ability of a peptide to form different type of channels can be used in the search for broad-spectrum antibiotics.

Noncontact dipole effects on channel permeation. VI. 5F- and 6F-Trp gramicidin channel currents

Fluorination of peptide side chains has been shown to perturb gramicidin channel conductance without significantly changing the average side chain structure, which, it is hoped, will allow detailed analysis of electrostatic modulation of current flow. Here we report a 1312-point potassium current-voltage-concentration data set for homodimeric channels formed from gramicidin A (gA) or any of eight fluorinated Trp analogs in both lecithin and monoglyceride bilayers. We fit the data with a three-barrier, two-site, two-ion (3B2S) kinetic model. The fluorination-induced changes in the rate constants were constrained by the same factor in both lipids. The rate constant changes were converted to transition-state free-energy differences for comparison with previous electrostatic potential energy differences based on an ab initio force field. The model allowed a reasonably good fit (chi = 8.29 with 1271 degrees of freedom). The measured changes were subtle. Nevertheless, the fitted energy perturbations agree well with electrostatic predictions for five of the eight peptides. For the other three analogs, the fitted changes suggested a reduced translocation barrier rather than the reduced exit barrier as predicted by electrostatics.

Electrophysiologic properties of channels induced by Abeta25-35 in planar lipid bilayers

Abeta25-35, a fragment of the neurotoxic amyloid beta protein Abeta1-42 found in the brain of Alzheimer patients, possesses amyloidogenic, neurotoxins and channel forming abilities similar to that of Abeta1-42. We have previously reported that Abeta25-35 formed voltage-dependent, relatively nonselective, ion-permeable channels in planar lipid bilayers. Here, we show that Abeta25-35 formed channels in both solvent-containing and solvent-free bilayers. We also report that for Abeta25-35, channel forming activity was dependent on ionic strength, membrane lipid composition, and peptide concentration, but not on pH. Lower ionic strength and negatively charged lipids increased channel formation activity, while cholesterol decreased activity. The nonlinear function relating [Abeta25-35] and membrane activity suggests that aggregation of at least three monomers is required for channel formation.

Thallous ion movements through gramicidin channels incorporated in lipid monolayers supported by mercury

The potential independent limiting flux of hydrated Tl(+) ions through gramicidin (GR) channels incorporated in phospholipid monolayers self assembled on a hanging mercury-drop electrode is shown to be controlled both by diffusion and by a dehydration step. Conversely, the potential independent limiting flux of dehydrated Tl(+) ions stemming from Tl amalgam electro-oxidation is exclusively controlled by diffusion of thallium atoms within the amalgam. Modulating the charge on the polar heads of dioleoylphosphatidylserine (DOPS) by changing pH affects the limiting flux of hydrated Tl(+) ions to a notable extent, primarily by electrostatic interactions. The dipole potential of DOPS and dioleoylphosphatidylcholine (DOPC), positive toward the hydrocarbon tails, does not hinder the translocation step of Tl(+) ions to such an extent as to make it rate limiting. Consequently, incorporation in the lipid monolayer of phloretin, which decreases such a positive dipole potential, does not affect the kinetics of Tl(+) flux through GR channels. In contrast, the increase in the positive dipole potential produced by the incorporation of ketocholestanol causes the translocation step to contribute to the rate of the overall process. A model providing a quantitative interpretation of the kinetics of diffusion, dehydration-hydration, translocation, and charge transfer of the Tl(+)/Tl(0)(Hg) couple through GC channels incorporated in mercury-supported phospholipid monolayers is provided. A cut-off disk model yielding the profile of the local electrostatic potential created by an array of oriented dipoles located in the lipid monolayer along the axis of a cylindrical ion channel is developed.

Noncontact dipole effects on channel permeation. IV. Kinetic model of 5F-Trp(13) gramicidin A currents

Nonlinear least squares fitting was used to assign rate constants for the three-barrier, two-site, double-occupancy, single-filing kinetic model for previously reported current-voltage relations of (5F-Indole)Trp(13) gramicidin A and gramicidin A channels (, 75:2830-2844). By judicious coupling of parameters, it was possible to reduce the parameter space from 64 parameters to 24, and a reasonable fit consistent with other experimental data was obtained. The main features of the fit were that fluorination increased the rate constant for translocation by a factor of 2.33, consistent with a free energy change in the translocation barrier of -0.50 kcal/mol, and increased first-ion binding affinity by a factor of 1.13, primarily by decreasing the first-ion exit rate constant. The translocation rate constant was 5.62 times slower in diphytanoyl phosphatidylcholine (DPhPC) bilayers than in monoolein (GMO) bilayers (coupled for the four combinations of peptide and salt), suggesting a 44.2-mV difference in the projection of the interfacial dipole into the channel. Thus fluorination caused increased currents in DPhPC bilayers, where a high interfacial dipole potential makes translocation more rate limiting because the translocation barrier was reduced, and decreased currents in GMO bilayers, where ion exit or entry is rate limiting because these barriers were increased.

Noncontact dipole effects on channel permeation. V. Computed potentials for fluorinated gramicidin

Experimental and theoretical calculations indicate that the dipole moment of the four Trp side chains in gramicidin A (gA) channels modify channel conductance through long-range electrostatic interactions. Electrostatic ion/side-chain interaction energies along the channel were computed with CHARMM using ab initio atom charges for native and 4-, 5-, or 6-fluorinated Trp side chains. The bulk water reaction to the polar side chains was included using the method of images as implemented by, and channel waters in idealized structures were included. Ion/Trp interaction energies were approximately -0.6 kcal/mol throughout the channel for all four of the native Trp pairs. Channel waters produced a modest reduction in the magnitude of interactions, essentially offsetting images representing the bulk water outside the channel. The effects of side-chain fluorination depended on ring position and, to a lesser extent, residue number. Compared with native Trp, 5-fluorination reduces the translocation barrier with minor effects on the exit barrier. In contrast, 6-fluorination primarily reduces exit barrier. 4-Fluorination produces a more complex double-well energy profile. Effects of measured side-chain movements resulting from fluorination or change in lipid bilayer were negligible whereas thermal side chain librations cause large effects, especially in the region of the ion-binding sites.

Conducting gramicidin channel activity in phospholipid monolayers

Potential step amperometry (chronoamperometry) of the Tl(I)/Tl(Hg) electrochemical reduction process has been used to investigate the underlying mechanisms of gramicidin activity in phospholipid monolayers. The experiments were carried out at gramicidin-modified dioleoyl phosphatidylcholine (DOPC)-coated electrodes. Application of a potential step to the coated electrode system results in a current transient that can be divided into two regions. An initial exponential decay of current corresponds to the inactivation of monomer channel conductance and a longer time scale quasi-steady-state represents the diffusion of ions to a bimolecular surface reaction. Concentrations of monomer conducting channels are relatively low, and the results indicate that two or more forms of gramicidin are in equilibrium with each other in the layer. Aromatic/conjugated compounds incorporated into the monolayer increase the reduction current by decreasing the rate of channel inactivation and increasing the stability of the conducting channel. This effect is positively correlated with the degree of the compound's aromaticity. The anomalous influence of alkali metal ions on the reduction current is consistent with the model of gramicidin being speciated in the monolayer in more than one form. The results have implications on the lability of the peptide conformation in biological membranes and its dependence on lipid environment, solution composition, and applied potential.

Post-Episode Depression of GABAergic Transmission in Spinal Neurons of the Chick Embryo

Whole cell recordings were obtained from ventral horn neurons in spontaneously active spinal cords isolated from the chick embryo [ embryonic days 10 to 11 ( E10–E11)] to examine the post-episode depression of GABAergic transmission. Spontaneous activity occurred as recurrent, rhythmic episodes approximately 60 s in duration with 10- to 15-min quiescent inter-episode intervals. Current-clamp recording revealed that episodes were followed by a transient hyperpolarization (7 ± 1.2 mV, mean ± SE), which dissipated as a slow (0.5–1 mV/min) depolarization until the next episode. Local application of bicuculline 8 min after an episode hyperpolarized spinal neurons by 6 ± 0.8 mV and increased their input resistance by 13%, suggesting the involvement of GABAergic transmission. Gramicidin perforated-patch recordings showed that the GABAa reversal potential was above rest potential ( E GABAa = −29 ± 3 mV) and allowed estimation of the physiological intracellular [Cl−] = 50 mM. In whole cell configuration (with physiological electrode [Cl−]), two distinct types of endogenous GABAergic currents ( I GABAa) were found during the inter-episode interval. The first comprised TTX-resistant, asynchronous miniature postsynaptic currents (mPSCs), an indicator of quantal GABA release (up to 42% of total mPSCs). The second (tonic I GABAa) was complimentary to the slow membrane depolarization and may arise from persistent activation of extrasynaptic GABAa receptors. We estimate that approximately 10 postsynaptic channels are activated by a single quantum of GABA release during an mPSC and that about 30 extrasynaptic GABAa channels are required for generation of the tonic I GABAa in ventral horn neurons. We investigated the post-episode depression of I GABAa by local application of GABA or isoguvacine (100 μM, for 10–30 s) applied before and after an episode at holding potentials ( V hold) −60 mV. The amplitude of the evoked I GABA was compared after clamping the cell during the episode at one of three different V hold: −60 mV, below E GABAa resulting in Cl− efflux; −30 mV, close to E GABAa with minimal Cl− flux; and 0 mV, above E GABAa resulting in Cl− influx during the episode. The amplitude of the evoked I GABA changed according to the direction of Cl− flux during the episode: at −60 mV a 41% decrease, at −30 mV a 4% reduction, and at 0 mV a 19% increase. These post-episode changes were accompanied by shifts of E GABAa of −10, −1.2, and +7 mV, respectively. We conclude that redistribution of intracellular [Cl−] during spontaneous episodes is likely to be an important postsynaptic mechanism involved in the post-episode depression of GABAergic transmission in chick embryo spinal neurons.

Design and characterization of gramicidin channels with side chain or backbone mutations

Mutations and chemical substitutions of amino acid side chains and backbone atoms have proved vital for understanding the folding, structure and function of gramicidin channels in phospholipid membranes. The channel's pore is lined by peptide backbone groups; their importance for channel structure and function is shown by a single amide-to-ester replacement within the backbone, which greatly reduces the resulting channel conductance and lifetime. The four tryptophans and the intervening leucines together govern the formation and dissociation of conducting channels from single-stranded subunits. Conducting double-stranded gramicidin conformations (channels) occur rarely in membranes--except when the sequence has been altered to permit special arrangements of tryptophans or (infrequently) in unusually thick membranes. The tryptophans anchor the single-stranded channels to the membrane/solution interface, and the indole dipoles promote cation transport through the channels. Removal of any indole dipole reduces ion conductance; whereas 5-fluorination of an indole, which increases its dipole moment, enhances ion conductance. Some sequence changes at the formyl-NH-terminus (in the membrane interior, away from the tryptophans), including fluorination of the formyl-NH-terminal valine, introduce voltage-dependent channel gating. Gramicidin channels are not just static conductors, but also dynamic entities whose structure and function can be manipulated by backbone and side chain modifications.

Stabilization of O-pyromellitylgramicidin channels in bilayer lipid membranes through electrostatic interaction with polylysines of different chain lengths

Functioning of membrane proteins, in particular ionic channels, can be modulated by alteration of their arrangement in membranes. We addressed this issue by studying the effect of different chain length polylysines on the kinetics of ionic channels formed in a bilayer lipid membrane (BLM) by O-pyromellitylgramicidin carrying three negative charges at the C-terminus. The method of sensitized photoinactivation was applied to the analysis of the channel association-dissociation kinetics (characterized by the exponential factor of the curve describing the time course of the flash-induced decrease in the transmembrane current, tau). Addition of polylysine to the bathing solutions of BLM led to the deceleration of the photoinactivation kinetics, i.e. to the increase in tau. It was shown here that for a series of polylysines differing in their chain lengths, the value of tau grew as their concentration increased above a threshold level until at a certain concentration of each polylysine tau reached maximum. At higher polylysine concentrations tau began to decrease and finally became close to the control level observed in the absence of polylysine. With lengthening of the polylysine chain the maximum value of tau increased, the concentration dependence became steeper, and the threshold concentration decreased. The increase in the ionic strength of the medium shifted the concentration dependence of tau to higher polylysine concentrations and decreased the maximum value of tau. It was concluded that the increase in tau was caused by the formation of domains of O-pyromellitylgramicidin molecules induced by binding of polylysines. This can be related to functional aspects of polycation-induced sequestering of negatively charged transmembrane peptides in neutral membranes.

Desformylgramicidin: a model channel with an extremely high water permeability

The water conductivity of desformylgramicidin exceeds the permeability of gramicidin A by two orders of magnitude. With respect to its single channel hydraulic permeability coefficient of 1.1.10(-12) cm(3) s(-1), desformylgramicidin may serve as a model for extremely permeable aquaporin water channel proteins (AQP4 and AQPZ). This osmotic permeability exceeds the conductivity that is predicted by the theory of single-file transport. It was derived from the concentration distributions of both pore-impermeable and -permeable cations that were simultaneously measured by double barreled microelectrodes in the immediate vicinity of a planar bilayer. From solvent drag experiments, approximately five water molecules were found to be transported by a single-file process along with one ion through the channel. The single channel proton, potassium, and sodium conductivities were determined to be equal to 17 pS (pH 2.5), 7 and 3 pS, respectively. Under any conditions, the desformyl-channel remains at least 10 times longer in its open state than gramicidin A.

Photosensitizer binding to lipid bilayers as a precondition for the photoinactivation of membrane channels

The photodynamic activity of sulfonated aluminum phthalocyanines (AlPcS(n), 1 </= n </= 4) was found to correlate with their affinity for membrane lipids. Adsorbing to the surface of large unilamellar vesicles (LUVs), aluminum phthalocyanine disulfonate induced the highest changes in their electrophoretic mobility. AlPcS(2) was also most efficient in mediating photoinactivation of gramicidin channels, as revealed by measurements of the electric current across planar lipid bilayers. The increase in the degree of sulfonation of phthalocyanine progressively reduced its affinity for the lipid bilayer as well as its potency of sensitizing gramicidin channel photoinactivation. The portion of photoinactivated gramicidin channels, alpha, increased with rising photosensitizer concentration up to some optimum. The concentration at which alpha was at half-maximum amounted to 80 nM, 30 nM, 200 nM, and 2 microM for AlPcS(1), AlPcS(2), AlPcS(3), and AlPcS(4), respectively. At high concentrations alpha was found to decrease, which was attributed to quenching of reactive oxygen species and self-quenching of the photosensitizer triplet state by its ground state. Fluoride anions were observed to inhibit both AlPcS(n) (2 </= n </= 4) binding to LUVs and sensitized photoinactivation of gramicidin channels. It is concluded that photosensitizer binding to membrane lipids is a prerequisite for the photodynamic inactivation of gramicidin channels.

Solvent drag across gramicidin channels demonstrated by microelectrodes

The competition of ion and water fluxes across gramicidin channels was assessed from the concentration distributions of both pore-impermeable and -permeable cations that were simultaneously measured by double-barreled microelectrodes in the immediate vicinity of a planar bilayer. Because water movement across the membrane led to accumulation of solutes on one side of the membrane and depletion on the other, the permeable cation was not only pushed by water across the channel (true solvent drag); it also flowed along its concentration gradient (pseudo-solvent drag). For the demonstration of true solvent drag, a difference between the bulk concentrations on the hypertonic and the hypotonic sides of the membrane was established. It was adjusted to get equal cation concentrations at both membrane/water interfaces. From the sodium and potassium fluxes measured along with membrane conductivity under these conditions, approximately five water molecules were found to be transported simultaneously with one ion through the channel. In diphytanoyl phosphatidylcholine membranes, a single-channel hydraulic permeability coefficient of 1.6 x 10(-14) cm(3) s(-1) was obtained.

Can we use rate constants and state models to describe ion transport through gramicidin channels?

Can we use rate constants and state models to describe ion transport through gramicidin channels? Maybe, but only if rate constants are just proportionality constants between rates and probabilities of observing states of the channel. This approach is natural if the system of channel plus ions (plus water) is almost always in one or another of a small number of identifiable states. Many features of ion transport through gramicidin, including the conductance-concentration relationship, concentration-dependent permeability ratios, anomalous mole fraction effect and to some extent flux ratio exponents, are consistent with a description in which there are four occupation 'states' of the pore: only water; an ion at one end; an ion at the other; and ions at both ends. Current-voltage relationships can (and must) also be fitted, but until there is a theory to predict the potential dependence of the rate constants this success will remain hollow. Other features have resisted interpretation. These include the failures to determine 'binding constants' consistent with all the data; the variation of flux ratio exponents with ion type; and, probably, the variation of the currents with asymmetrical ion concentrations. Nevertheless, state models still have one attractive feature, they allow consideration of the effects that one ion within the pore has on the movements of another.

Effect of gramicidin A on the dipole potential of phospholipid membranes

The effect of channel-forming peptide gramicidin A on the dipole potential of phospholipid monolayers and bilayers has been studied. Surface pressure and surface potential isotherms of monolayers have been measured with a Langmuir trough equipped with a Wilhelmy balance and a surface potential meter (Kelvin probe). Gramicidin has been shown to shift pressure-area isotherms of phospholipids and to reduce their monolayer surface potentials. Both effects increase with the increase in gramicidin concentration and depend on the kind of phosphatidylcholine used. Application of the dual-wavelength ratiometric fluorescence method using the potential-sensitive dye RH421 has revealed that the addition of gramicidin A to dipalmitoylphosphatidylcholine liposomes leads to a decrease in the fluorescence ratio of RH421. This is similar to the effect of phloretin, which is known to decrease the dipole potential. The comparison of the concentration dependences of the fluorescence ratio for gramicidin and phloretin shows that gramicidin is as potent as phloretin in modifying the membrane dipole potential.

Noncontact dipole effects on channel permeation. I. Experiments with (5F-indole)Trp13 gramicidin A channels

Gramicidin A (gA), with four Trp residues per monomer, has an increased conductance compared to its Phe replacement analogs. When the dipole moment of the Trp13 side chain is increased by fluorination at indole position 5 (FgA), the conductance is expected to increase further. gA and FgA conductances to Na+, K+, and H+ were measured in planar diphytanoylphosphatidylcholine (DPhPC) or glycerylmonoolein (GMO) bilayers. In DPhPC bilayers, Na+ and K+ conductances increased upon fluorination, whereas in GMO they decreased. The low ratio in the monoglyceride bilayer was not reversed in GMO-ether bilayers, solvent-inflated or -deflated bilayers, or variable fatty acid chain monoglyceride bilayers. In both GMO and DPhPC bilayers, fluorination decreased conductance to H+ but increased conductance in the mixed solution, 1 M KCl at pH 2.0, where K+ dominates conduction. Eadie-Hofstee plot slopes suggest similar destabilization of K+ binding in both lipids. Channel lifetimes were not affected by fluorination in either lipid. These observations indicate that fluorination does not change the rotameric conformation of the side chain. The expected difference in the rate-limiting step for transport through channels in the two bilayers qualitatively explains all of the above trends.

Streaming potentials in gramicidin channels measured with ion-selective microelectrodes

Streaming potentials have been measured for gramicidin channels with a new method employing ion-selective microelectrodes. It is shown that ideally ion-selective electrodes placed at the membrane surface record the true streaming potential. Using this method for ion concentrations below 100 mM, approximately seven water molecules are transported whenever a sodium, potassium, or cesium ion, passes through the channel. This new method confirms earlier measurements (Rosenberg, P.A., and A. Finkelstein. 1978. Interaction of ions and water in gramicidin A channels. J. Gen. Physiol. 72:327-340) in which the streaming potentials were calculated as the difference between electrical potentials measured in the presence of gramicidin and in the presence of the ion carriers valinomycin and nonactin.

Magnesium ions promote assembly of channel-like structures from beticolin 0, a non-peptide fungal toxin purified from Cercospora beticola

Beticolins are toxins produced by the fungus Cercospora beticola. Using beticolin 0 (B0), we have produced a strong and Mg(2+)-dependent increase in the membrane conductance of Arabidopsis protoplasts and Xenopus oocytes. In protein-free artificial bilayers, discrete deflexions of current were observed (12 pS unitary conductance in symmetrical 100 mM KCl) in the presence of B0 (approximately 10 microM) and in the presence of nominal Mg2+. Addition of 50 microM Mg2+ induced a macroscopic current which could be reversed to single channel current by chelating Mg2+ with EDTA. Both unitary and macroscopic currents were ohmic. The increase in conductance of biological membranes triggered by B0 is therefore likely to originate from the ability of this toxin to organize itself into transmembrane pores in the presence of Mg2+. The pore is poorly selective, displaying permeability ratios PCl/PK, PNa/PK and PCa/PK close to 0.3, 0.65 and 0.4, respectively. Such channel-like activity could be involved in the deleterious biological activity of the toxin, by causing the collapse of ionic and electrical gradients through biological membranes together with Ca2+ influx and scrambling of cellular signals.

Gramicidin-perforated patch revealed depolarizing effect of GABA in cultured frog melanotrophs

1. In frog pituitary melanotrophs, GABA induces a transient stimulation followed by prolonged inhibition of hormone secretion. This biphasic effect is inconsistent with the elevation of cytosolic calcium and the inhibition of electrical activity also provoked by GABA in single melanotrophs. In the present study, standard patch-clamp configurations and gramicidin-perforated patches were used to investigate the physiological GABAA receptor-mediated response and intracellular chloride concentration ([Cl-]i) in cultured frog melanotrophs. 2. In the gramicidin-perforated patch configuration, 1 microM GABA caused a depolarization associated with an action potential discharge and a slight fall of membrane resistance. In contrast, at a higher concentration (10 microM) GABA elicited a depolarization accompanied by a transient volley of action potentials, followed by a sustained inhibitory plateau and a marked fall of membrane resistance. Isoguvacine mimicked the GABA-evoked responses, indicating a mediation by GABAA receptors. 3. In gramicidin-perforated cells, the depolarizing excitatory effect of 1 microM GABA was converted into a depolarizing inhibitory action when 0.4 microM allopregnanolone was added to the bath solution. 4. After gaining the whole-cell configuration, the amplitude and/or direction of the GABA-evoked current (IGABA) rapidly changed before stabilizing. After stabilization, the reversal potential of IGABA followed the values predicted by the Nernst equation for chloride ions when [Cl-]i was varied. 5. In gramicidin-perforated cells, the steady-state I-V relationships of 10 microM GABA- or isoguvacine-evoked currents yielded reversal potentials of -37.5 +/- 1.6 (n = 17) and -38.6 +/- 2.0 mV (n = 8), respectively. These values were close to those obtained by using a voltage-ramp protocol in the presence of Na+, K+ and Ca2+ channel blockers. The current evoked by 1 microM GABA also reversed at these potentials. 6. We conclude that, in frog pituitary melanotrophs, chloride is the exclusive charge carrier of IGABA. In intact cells, the reversal potential of IGABA is positive to the resting potential because of a relatively high [Cl-]i (26.5 mM). Under these conditions, GABA induces a chloride efflux responsible for a depolarization triggering action potentials. However, GABA at a high concentration or in the presence of the potentiating steroid allopregnanolone exerts a concomitant shunting effect leading to a rapid inhibition of the spontaneous firing.

Perforated-patch recording with gramicidin avoids artifactual changes in intracellular chloride concentration

The antibiotic gramicidin, when incorporated into lipid membranes, forms pores that are exclusively permeable to monovalent cations and small unchanged molecules. We report the use of gramicidin for perforated patch-clamp recordings in the whole-cell mode. Recordings were performed in cultured rat spinal cord dorsal horn neurons. Cells had stable resting potentials and series resistances for times routinely exceeding 60 min. To test if intracellular chloride concentration ([Cl]i) remains stable with this technique, we measured responses to agonists of glycine and GABAA receptors, both of which gate chloride conductances. The driving force for these responses remained stable at values that differed significantly from values that would be expected if [Cl-]i were biased towards pipette [Cl-]. We conclude that gramicidin perforated-patch recording, in addition to other properties of the perforated-patch recording technique, has the advantage of not altering [Cl-]i. It is, therefore, an electrophysiological method particularly suitable for studies of anionic channels when [Cl-]i is a variable of interest, as well as for studies of homeostatic [Cl-]i regulation.

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.