Evidence for titratable gating charges controlling the voltage dependence of the outer mitochondrial membrane channel,VDAC

Summary A voltage-dependent anion-selective channel, VDAC, is found in outer mitochondrial membranes. VDAC's conductance is known to decrease as the transmembrane voltage is increased in either the positive or negative direction. Charged groups on the channel may be responsible for this voltage dependence by allowing the channel to respond to an applied electric field. If so, then neutralization of these charges would eliminate the voltage dependence. Channels in planar lipid bilayers which behaved normally at pH 6 lost much of their voltage dependence at high pH. Raising the pH reduced the steepness of the voltage dependence and raised the voltage needed to close half the channels. In contrast, the energy difference between the open and closed state in the absence of a field was changed very little by the elevated pH. The groups being titrated had an apparent pK of 10.6. From the pK and chemical modification, lysine epsilon amino groups are the most likely candidates responsible for VDAC's ability to respond to an applied electric field.     

Patch clamp study of the voltage-dependent anion channel in the thylakoid membrane

Measurements of single channel currents were performed on isolated membrane patches from osmotically swollen thylakoids of the Charophyte alga Nitellopsis obtusa. A channel with a high selectivity for anions over cations and a conductance of 100 to 110 pS (114 mM Cl–) was revealed. The channel has a bells-haped voltage-dependence of the open probability, with a maximum at about 0 mV. This dependence was explained by two gating processes, one causing channel closure at positive and one at negative potentials. The steepness of the voltage-dependence corresponded to approximately 2 elementary charges to be transferred across the entire membrane in each of the two gating processes. The analysis of the anion channel kinetics in the millisecond time domain revealed an e-fold increase of mean open and decrease of mean closed times when the membrane voltage was made more positive by 20 and 36 mV, respectively. Concert transitions of two identical anion channels between open and long inactivated states were observed, while the millisecond closed-open transitions of the two channels within a burst of activity were kinetically independent.This work was financially supported by the Deutsche Forschungsgemeinschaft (SFB 176 TP B11) and by a research fellowship from the Alexander von Humboldt Foundation to I.I. Pottosin.     

The mitochondrial voltage-dependent channel,VDAC, is modified asymmetrically by succinic anhydride

Summary In the accompanying paper, succinic anhydride was shown to react with the outer mitochondrial membrane channel-forming protein, VDAC, resulting in the loss of its voltage dependence. In this paper, the anhydride was added to VDAC held in a particular conformational state by means of an applied electric field. VDAC was inserted into the membranes from thecis side and the anhydride was added either to thecis ortrans side. Channels modified in the open state behaved similarly whether anhydride was added to thecis ortrans side. Modifications of VDAC in either of the two closed states did not. Modifications resulting in the loss of voltage-dependence occurred primarily when anhydride was added to the negative side of the membrane irrespective of which closed state the VDAC was in indicating that the accessibility of the gating charges alternated between thecis andtrans sides as the channel's conformation was changed from one closed state to the other. Despite the pronounced asymmetry, in general the resulting channels behaved in the same way in response to either positive or negative fields. A model consistent with the results is presented which proposes that the same gating charges are responsible for channel closure at both positive and negative fields.     

Voltage gating in VDAC is markedly inhibited by micromolar quantities of aluminum

Summary The mitochondrial outer membrane contains voltagegated channels called VDAC that are responsible for the flux of metabolic substrates and metal ions across this membrane. The addition of micromolar quantities of aluminum chloride to phospholipid membranes containing VDAC channels greatly inhibits the voltage dependence of the channels' permeability. The channels remain in their high conducting (open) state even at high membrane potentials. An analysis of the change in the voltage-dependence parameters revealed that the steepness of the voltage dependence decreased while the voltage needed to close half the channels increased. The energy difference between the open and closed states in the absence of an applied potential did not change. Therefore, the results are consistent with aluminum neutralizing the voltage sensor of the channel. pH shift experiments showed that positively charged aluminum species in solution were not involved. The active form was identified as being either (or both) the aluminum hydroxide or the tetrahydroxoaluminate form. Both of these could reasonably be expected to neutralize a positively charged voltage sensor. Aluminum had no detectable effect of either single-channel conductance or selectivity, indicating that the sensor is probably not located in the channel proper and is distinct from the selectivity filter.     

Conductive properties and gating of channels formed by syringopeptin 25A, a bioactive lipodepsipeptide from Pseudomonas syringae pv. syringae, in planar lipid membranes

Syringopeptin 25A, a pseudomonad lipodepsipeptide, can form ion channels in planar lipid membranes. Pore conductance is around 40 pS in 0.1 M NaCl. Channel opening is strongly voltage dependent and requires a negative potential on the same side of the membrane where the toxin was added. These pores open and close with a lifetime of several seconds. At negative voltages, an additional pore state of around 10 pS and a lifetime of around 30 ms is also present. The voltage dependence of the rates of opening and closing of the stable pores is exponential. This allows estimation of the equivalent charge that is moved across the membrane during the process of opening at about 2.6 elementary charges. When NaCl is present, the pore is roughly 3 times more permeant for anions than for cations. The current voltage characteristic of the pore is nonlinear, i.e., pore conductance is larger at negative than at positive voltages. The maximal conductance of the pore depends on the concentration of the salt present, in a way that varies almost linearly with the conductivity of the solution. From this, an estimate of a minimal pore radius of 0.4 nm was derived.     

Properties of Voltage-gated Ion Channels Formed by Syringomycin E in Planar Lipid Bilayers

Using the planar lipid bilayer technique we demonstrate that the lipodepsipeptide antibiotic, syringomycin E, forms voltage-sensitive ion channels of weak anion selectivity. The formation of channels in bilayers made from dioleoylglycerophosphatidylserine doped with syringomycin E at one side (1–40 μg/ml) was greatly affected by cis-positive voltage. A change of voltage from a positive to a negative value resulted in (i) an abrupt increase in the single channel conductance (the rate of increase was voltage dependent) simultaneous with (ii) a closing of these channels and an exponential decrease in macroscopic conductance over time. The strong voltage dependence of multichannel steady state conductance, the single channel conductance, the rate of opening of channels at positive voltages and closing them at negative voltages, as well as the observed abrupt increase of single channel conductance after voltage sign reversal suggest that the change of the transmembrane field induces a significant rearrangement of syringomycin E channels, including a change in the spacing of charged groups that function as voltage sensors. The conductance induced by syringomycin E increased with the sixth power of syringomycin E concentration suggesting that at least six monomers are required for channel formation. Received: 3 April 1995/Revised: 24 August 1995     

Channels formed by colicin E1 in planar lipid bilayers are large and exhibit pH-dependent ion selectivity

Summary The E1 subgroup (E1, A, Ib, etc.) of antibacterial toxins called colicins are known to form voltage-dependent channels in planar lipid bilayers. The genes for colicins E1, A and Ib have been cloned and sequenced, making these channels interesting models for the widespread phenomenon of voltage dependence in cellular channels. In this paper we investigate ion selectivity and channel size—properties relevant to model building. Our major finding is that the colicin E1 channel is large, having a diameter ofat least 8 Å at its narrowest point. We established this from measurements of reversal potentials for gradients formed by salts of large cations or large anions. In so doing, we exploited the fact that the colicin channel is permeable to both cations and anions, and its relative selectivity to them is a functions and anions, and its relative selectivity to them is a function of pH. The channel is anion selective (Cl^– over K⁺) in neutral membranes, and the degree of selectivity is highly dependent on pH. In negatively charged membranes, it becomes cation selective at pH's higher than about 5. Experiments with pH gradients cross the membrane suggest that titratable groups both within the channel lumen and near the channel ends affect the selectivity. Individual E1 channels have more than one open conductance state, all displaying comparable ion selectivity. Colicins A and Ib also exhibit pH-dependent ion selectivity, and appear to have even larger lumens than E1.     

The functional properties of Ompβ, the regularly arrayed porin of the hyperthermophilic bacterium Thermotoga maritima

Abstract The regularly arrayed outer membrane protein, Ompβ, of Thermotoga maritima was purified to homogeneity and was characterized functionally by incorporation into artificial lipid bilayers. The polypeptide has an apparent molecular mass ( M _r) of approx. 40 000 and forms stable trimers in the presence of 1% octyl-polyoxyethylene or 2% SDS which dissociate when boiling the sample. The protein has a secondary structure (predominantly β-sheet) and an amino acid composition characteristic for porins. Pore-forming activity was demonstrated by porin incorporation into artificial bilayers proving that Ompβ is a true porin: selectivity measurements showed a 4.4-fold selectivity for cations over anions. Conductivity of the porin is influenced by surface charges and also depends on the applied voltage.     

The first extracellular loop domain is a major determinant of charge selectivity in connexin46 channels

Intercellular channels formed of members of the gene family of connexins (Cxs) vary from being substantially cation selective to being anion selective. We took advantage of the ability of Cx46 to function as an unopposed hemichannel to examine the basis of Cx charge selectivity. Previously we showed Cx46 hemichannels to be large pores that predominantly conduct cations and inwardly rectify in symmetric salts, properties suggesting selectivity is influenced by fixed negative charges located toward the extracellular end of the pore. Here we demonstrate that high ionic strength solutions applied to the extracellular, but not the intracellular, side of Cx46 hemichannels substantially reduce the ratio of cation to anion permeability. Substitution of the first extracellular loop (E1) domain of Cx32, an anion-preferring Cx, reduces conductance, converts Cx46 from cation to anion preferring, and changes the I-V relation form inwardly to outwardly rectifying. These data suggest that fixed negative charges influencing selectivity in Cx46 are located in E1 and are substantially reduced and/or are replaced with positive charges from the Cx32 E1 sequence. Extending studies to Cx46 cell-cell channels, we show that they maintain a strong preference for cations, have a conductance nearly that expected by the series addition of hemichannels, but lack rectification in symmetric salts. These properties are consistent with preservation of the fixed charge region in E1 of hemichannels, which upon docking, become symmetrically placed near the center of the cell-cell channel pore. Furthermore, heterotypic cell-cell channels formed by pairing Cx46 with Cx32 or Cx43 rectify in symmetric salts in accordance with the differences in the charges we ascribed to E1. These data are consistent with charged residues in E1 facing the channel lumen and playing an important role in determining Cx channel conductance and selectivity.     

Neutralization of gating charges in domain II of the sodium channel alpha subunit enhances voltage-sensor trapping by a beta-scorpion toxin

beta-Scorpion toxins shift the voltage dependence of activation of sodium channels to more negative membrane potentials, but only after a strong depolarizing prepulse to fully activate the channels. Their receptor site includes the S3-S4 loop at the extracellular end of the S4 voltage sensor in domain II of the alpha subunit. Here, we probe the role of gating charges in the IIS4 segment in beta-scorpion toxin action by mutagenesis and functional analysis of the resulting mutant sodium channels. Neutralization of the positively charged amino acid residues in the IIS4 segment by mutation to glutamine shifts the voltage dependence of channel activation to more positive membrane potentials and reduces the steepness of voltage-dependent gating, which is consistent with the presumed role of these residues as gating charges. Surprisingly, neutralization of the gating charges at the outer end of the IIS4 segment by the mutations R850Q, R850C, R853Q, and R853C markedly enhances beta-scorpion toxin action, whereas mutations R856Q, K859Q, and K862Q have no effect. In contrast to wild-type, the beta-scorpion toxin Css IV causes a negative shift of the voltage dependence of activation of mutants R853Q and R853C without a depolarizing prepulse at holding potentials from -80 to -140 mV. Reaction of mutant R853C with 2-aminoethyl methanethiosulfonate causes a positive shift of the voltage dependence of activation and restores the requirement for a depolarizing prepulse for Css IV action. Enhancement of sodium channel activation by Css IV causes large tail currents upon repolarization, indicating slowed deactivation of the IIS4 voltage sensor by the bound toxin. Our results are consistent with a voltage-sensor-trapping model in which the beta-scorpion toxin traps the IIS4 voltage sensor in its activated position as it moves outward in response to depolarization and holds it there, slowing its inward movement on deactivation and enhancing subsequent channel activation. Evidently, neutralization of R850 and R853 removes kinetic barriers to binding of the IIS4 segment by Css IV, and thereby enhances toxin-induced channel activation.     

Ionic partition and transport in multi-ionic channels: a molecular dynamics simulation study of the OmpF bacterial porin

We performed all-atom molecular dynamics simulations studying the partition of ions and the ionic current through the bacterial porin OmpF and two selected mutants. The study is motivated by new, interesting experimental findings concerning their selectivity and conductance behavior at neutral pH. The mutations considered here are designed to study the effect of removal of negative charges present in the constriction zone of the wild-type OmpF channel (which contains, on one side, a cluster with three positive residues, and on the other side, two negatively charged residues). Our results show that these mutations induce an exclusion of cations from the constriction zone of the channel, substantially reducing the flow of cations. In fact, the partition of ions inside the mutant channels is strongly inhomogeneous, with regions containing an excess of cations and regions containing an excess of anions. Interestingly, the overall number of cations inside the channel is larger than the number of anions, this excess being different for each protein channel. We found that the differences in ionic charge inside these channels are justified by the differences in electric charge between the wild-type OmpF and the mutants, following an electroneutral balance.     

Polyvalent cations constitute the voltage gating particle in human connexin37 hemichannels

Connexins oligomerize to form intercellular channels that gate in response to voltage and chemical agents such as divalent cations. Historically, these are believed to be two independent processes. Here, data for human connexin37 (hCx37) hemichannels indicate that voltage gating can be explained as block/unblock without the necessity for an independent voltage gate. hCx37 hemichannels closed at negative potentials and opened in a time-dependent fashion at positive potentials. In the absence of polyvalent cations, however, the channels were open at relatively negative potentials, passing current linearly with respect to voltage. Current at negative potentials could be inhibited in a concentration-dependent manner by the addition of polyvalent cations to the bathing solution. Inhibition could be explained as voltage-dependent block of hCx37, with the field acting directly on polyvalent cations, driving them through the pore to an intracellular site. At positive potentials, in the presence of polyvalent cations, the field favored polyvalent efflux from the intracellular blocking site, allowing current flow. The rate of appearance of current depended on the species and valence of the polyvalent cation in the bathing solution. The rate of current decay upon repolarization depended on the concentration of polyvalent cations in the bathing solution, consistent with deactivation by polyvalent block, and was rapid (time constants of tens of milliseconds), implying a high local concentration of polyvalents in or near the channel pore. Sustained depolarization slowed deactivation in a flux-dependent, voltage- and time-independent fashion. The model for hCx37 voltage gating as polyvalent block/unblock can be expanded to account for observations in the literature regarding hCx37 gap junction channel behavior.     

X-ray crystallographic and mass spectrometric structure determination and functional characterization of succinylated porin from Rhodobacter capsulatus: implications for ion selectivity and single-channel conductance.

The role of charges near the pore mouth has been discussed in theoretical work about ion channels. To introduce new negative charges in a channel protein, amino groups of porin from Rhodobacter capsulatus 37b4 were succinylated with succinic anhydride, and the precise extent and sites of succinylations and structures of the succinylporins determined by mass spectrometry and X-ray crystallography. Molecular weight and peptide mapping analyses using matrix-assisted laser desorption-ionization mass spectrometry identified selective succinylation of three lysine-epsilon-amino groups (Lys-46, Lys-298, Lys-300) and the N-terminal alpha-amino group. The structure of a tetra-succinylated porin (TS-porin) was determined to 2.4 A and was generally found unchanged in comparison to native porin to form a trimeric complex. All succinylated amino groups found in a mono/di-succinylated porin (MS-porin) and a TS-porin are localized at the inner channel surface and are solvent-accessible: Lys-46 is located at the channel constriction site, whereas Lys-298, Lys-300, and the N-terminus are all near the periplasmic entrance of the channel. The Lys-46 residue at the central constriction loop was modeled as succinyl-lysine from the electron density data and shown to bend toward the periplasmic pore mouth. The electrical properties of the MS-and TS-porins were determined by reconstitution into black lipid membranes, and showed a negative charge effect on ion transport and an increased cation selectivity through the porin channel. The properties of a typical general diffusion porin changed to those of a channel that contains point charges near the pore mouth. The single-channel conductance was no longer a linear function of the bulk aqueous salt concentration. The substantially higher cation selectivity of the succinylated porins compared with the native protein is consistent with the increase of negatively charged groups introduced. These results show tertiary structure-selective modification of charged residues as an efficient approach in the structure-function evaluation of ion channels, and X-ray crystallography and mass spectrometry as complementary analytical tools for defining precisely the chemically modified structures.     

Voltage dependence and ion selectivity of the mitochondrial channel,VDAC, are modified by succinic anhydride

Summary The mitochondrial protein VDAC forms voltage-dependent anion-selective channels in planar phospholipid membranes. When succinic anhydride was added to these membranes, it virtually eliminated VDAC's voltage-dependence and changed its selectivity from anion to cation. These results were obtained without large changes in open-channel conductance or in energy difference between the open and closed states in the absence of a field. Since succinic anhydride converts amino groups into carboxyl groups, we propose that amino groups are responsible for VDAC's voltage-dependence and anion selectivity. Perhaps the same charges are responsible for both functions.     

Voltage-dependent gating properties of the channel formed byE. coli hemolysin in planar lipid membranes

Escherichia coli hemolysin forms cation selective, ion-permeable channels of large conductance in planar phospholipid bilayer membranes. The pore formation mechanism is voltage dependent resembling that of some colicins and of diphtheria toxin: pores open when negative voltages are applied and close with positive potentials. The pH dependence of this gating process suggests that it is mediated by a negative fixed charge present in the lumen of the pore. A simple physical model of how the channel opens and closes in response to the applied voltage is given.     

Properties of chemically modified porin from Escherichia coli in lipid bilayer membranes

Purified porin OmpF from Escherichia coli outer membrane was chemically modified by acetylation and succinylation of amino groups and by amidation of the carboxyl groups. Native and chemically modified porins were incorporated into lipid bilayer membranes and the permeability properties of the pores were studied. Acetylation and succinylation of the porin trimers had almost no influence on the single channel conductance in the presence of small cations and anions and the cation selectivity remained essentially unchanged as compared with the native porin. Amidation had also only little influence on the single channel conductance and changed the pore conductance at maximum by less than 50%, whereas the cation selectivity of the porin is completely lost after amidation. The results suggest that the structure of the porin pore remains essentially unchanged after chemical modification of the pores and that their cation selectivity is caused by an excess of negatively charged groups inside the pore and/or on the surface of the protein. Furthermore, it seems very unlikely that the pore contains any positively charged group at neutral pH.     

Voltage sensitivity and gating charge in Shaker and Shab family potassium channels.

The members of the voltage-dependent potassium channel family subserve a variety of functions and are expected to have voltage sensors with different sensitivities. The Shaker channel of Drosophila, which underlies a transient potassium current, has a high voltage sensitivity that is conferred by a large gating charge movement, approximately 13 elementary charges. A Shaker subunit's primary voltage-sensing (S4) region has seven positively charged residues. The Shab channel and its homologue Kv2.1 both carry a delayed-rectifier current, and their subunits have only five positively charged residues in S4; they would be expected to have smaller gating-charge movements and voltage sensitivities. We have characterized the gating currents and single-channel behavior of Shab channels and have estimated the charge movement in Shaker, Shab, and their rat homologues Kv1.1 and Kv2.1 by measuring the voltage dependence of open probability at very negative voltages and comparing this with the charge-voltage relationships. We find that Shab has a relatively small gating charge, approximately 7.5 e(o). Surprisingly, the corresponding mammalian delayed rectifier Kv2.1, which has the same complement of charged residues in the S2, S3, and S4 segments, has a gating charge of 12.5 e(o), essentially equal to that of Shaker and Kv1.1. Evidence for very strong coupling between charge movement and channel opening is seen in two channel types, with the probability of voltage-independent channel openings measured to be below 10(-9) in Shaker and below 4 x 10(-8) in Kv2.1.     

Asymmetric electrostatic effects on the gating of rat brain sodium channels in planar lipid membranes.

The effects of ionic strength (10-1,000 mM) on the gating of batrachotoxin-activated rat brain sodium channels were studied in neutral and in negatively charged lipid bilayers. In neutral bilayers, increasing the ionic strength of the extracellular solution, shifted the voltage dependence of the open probability (gating curve) of the sodium channel to more positive membrane potentials. On the other hand, increasing the intracellular ionic strength shifted the gating curve to more negative membrane potentials. Ionic strength shifted the voltage dependence of both opening and closing rate constants of the channel in analogous ways to its effects on gating curves. The voltage sensitivities of the rate constants were not affected by ionic strength. The effects of ionic strength on the gating of sodium channels reconstituted in negatively charged bilayers were qualitatively the same as in neutral bilayers. However, important quantitative differences were noticed: in low ionic strength conditions (10-150 mM), the presence of negative charges on the membrane surface induced an extra voltage shift on the gating curve of sodium channels in relation to neutral bilayers. It is concluded that: (a) asymmetric negative surface charge densities in the extracellular (1e-/533A2) and intracellular (1e-/1,231A2) sides of the sodium channel could explain the voltage shifts caused by ionic strength on the gating curve of the channel in neutral bilayers. These surface charges create negative electric fields in both the extracellular and intracellular sides of the channel. Said electric fields interfere with gating charge movements that occur during the opening and closing of sodium channels; (b) the voltage shifts caused by ionic strength on the gating curve of sodium channels can be accounted by voltage shifts in both the opening and closing rate constants; (c) net negative surface charges on the channel's molecule do not affect the intrinsic gating properties of sodium channels but are essential in determining the relative position of the channel's gating curve; (d) provided the ionic strength is below 150 mM, the gating machinery of the sodium channel molecule is able to sense the electric field created by surface changes on the lipid membrane. I propose that during the opening and closing of sodium channels, the gating charges involved in this process are asymmetrically displaced in relation to the plane of the bilayer. Simple electrostatic calculations suggest that gating charge movements are influenced by membrane electrostatic potentials at distances of 48 and 28 A away from the plane of the membrane in the extracellular sides of the channel, respectively.     

The gate of mitochondrial porin channel is controlled by a number of negative and positive charges

Negatively charged carboxyl groups of mitochondrial porin have been converted into positively charged ones by means of reaction with water-soluble carbodiimide in the presence of ethylenediamine. Properties of channels formed in a planar lipid bilayer by native and modified porins are compared. Amidation has only little influence on the porin channel-forming activity as well as on the open-state conductance of the channel. However, the modification results in a significant enhancement of the voltage dependence of the channel gating and in an increase of the anionic selectivity. It is suggested that the voltage sensor of the porin channel gate is composed of a number of negative (greater than 14) and positive (greater than 22) charges.     

Conductance and permeability of the residual state of connexin43 gap junction channels

We used cell lines expressing wild-type connexin43 and connexin43 fused with the enhanced green fluorescent protein (Cx43-EGFP) to examine conductance and perm-selectivity of the residual state of Cx43 homotypic and Cx43/Cx43-EGFP heterotypic gap junction channels. Each hemichannel in Cx43 cell-cell channel possesses two gates: a fast gate that closes channels to the residual state and a slow gate that fully closes channels; the transjunctional voltage (V(j)) closes the fast gate in the hemichannel that is on the relatively negative side. Here, we demonstrate macroscopically and at the single-channel level that the I-V relationship of the residual state rectifies, exhibiting higher conductance at higher V(j)s that are negative on the side of gated hemichannel. The degree of rectification increases when Cl(-) is replaced by Asp(-) and decreases when K(+) is replaced by TEA(+). These data are consistent with an increased anionic selectivity of the residual state. The V(j)-gated channel is not permeable to monovalent positively and negatively charged dyes, which are readily permeable through the fully open channel. These data indicate that a narrowing of the channel pore accompanies gating to the residual state. We suggest that the fast gate operates through a conformational change that introduces positive charge at the cytoplasmic vestibule of the gated hemichannel, thereby producing current rectification, increased anionic selectivity, and a narrowing of channel pore that is largely responsible for reducing channel conductance and restricting dye transfer. Consequently, the fast V(j)-sensitive gating mechanism can serve as a selectivity filter, which allows electrical coupling but limits metabolic communication.