Lipid-mediated inactivation of colicin E1 channels by calcium ions

Based on the model of a toroidal protein-lipid pore, the effect of calcium ions on colicin E1 channel was predicted. In electrophysiological experiments Ca2+ suppressed the activity of colicin E1 channels in membranes formed of diphytanoylphosphatidylglycerol, whereas no desorption of the protein occurred from the membrane surface. The effect of Ca2+ was not observed on membranes formed of diphytanoylphosphatidylcholine. Single-channel measurements revealed that Ca2+-induced reduction of the colicin-induced current across the negatively charged membrane was due to a decrease in the number of open colicin channels and not changes in their properties. In line with the toroidal model, the effect of Ca2+ on the colicin E1 channel-forming activity is explained by alteration of the membrane lipid curvature caused by electrostatic interaction of Ca2+ with negatively charged lipid head groups.     

Major transmembrane movement associated with colicin Ia channel gating

Colicin Ia, a bacterial protein toxin of 626 amino acid residues, forms voltage-dependent channels in planar lipid bilayer membranes. We have exploited the high affinity binding of streptavidin to biotin to map the topology of the channel-forming domain (roughly 175 residues of the COOH-terminal end) with respect to the membrane. That is, we have determined, for the channel's open and closed states, which parts of this domain are exposed to the aqueous solutions on either side of the membrane and which are inserted into the bilayer. This was done by biotinylating cysteine residues introduced by site-directed mutagenesis, and monitoring by electrophysiological methods the effect of streptavidin addition on channel behavior. We have identified a region of at least 68 residues that flips back and forth across the membrane in association with channel opening and closing. This identification was based on our observations that for mutants biotinylated in this region, streptavidin added to the cis (colicin- containing) compartment interfered with channel opening, and trans streptavidin interfered with channel closing. (If biotin was linked to the colicin by a disulfide bond, the effects of streptavidin on channel closing could be reversed by detaching the streptavidin-biotin complex from the colicin, using a water-soluble reducing agent. This showed that the cysteine sulfur, not just the biotin, is exposed to the trans solution). The upstream and downstream segments flanking the translocated region move into and out of the bilayer during channel opening and closing, forming two transmembrane segments. Surprisingly, if any of several residues near the upstream end of the translocated region is held on the cis side by streptavidin, the colicin still forms voltage-dependent channels, indicating that a part of the protein that normally is fully translocated across the membrane can become the upstream transmembrane segment. Evidently, the identity of the upstream transmembrane segment is not crucial to channel formation, and several open channel structures can exist.     

Effects of anionic lipid and ion concentrations on the topology and segmental mobility of colicin Ia channel domain from solid-state NMR

Channel-forming colicins are bacterial toxins that spontaneously insert into the inner cell membrane of sensitive bacteria to form voltage-gated ion channels. It has been shown that the channel current and the conformational flexibility of colicin E1 channel domain depend on the membrane surface potential, which is regulated by the anionic lipid content and the ion concentration. To better understand the dependence of colicin structure and dynamics on the membrane surface potential, we have used solid-state NMR to investigate the topology and segmental motion of the closed state of colicin Ia channel-forming domain in membranes of different anionic lipid contents and ion concentrations. Colicin Ia channel domain was reconstituted into membranes with different POPG and KCl concentrations. 1H spin diffusion experiments indicate that the protein contains a small domain that inserts into the hydrophobic center of the 70% anionic membrane, similar to when it binds to the 25% anionic membrane. Measurements of C-H and N-H dipolar couplings indicate that, on the sub-microsecond time scale, the protein has the least segmental mobility under the high-salt and low-anionic lipid condition, which has the most physiological membrane surface potential. Measurement of millisecond time scale motions yielded similar results. These suggest that optimal channel activity requires the protein to have sufficient segmental rigidity so that entire helices can undergo cooperative conformational motions that are required for translocating the channel-forming helices across the lipid bilayer upon voltage activation.     

Gating of a voltage-dependent channel (colicin E1) in planar lipid bilayers: translocation of regions outside the channel-forming domain

Summary C-terminal fragments of colicin E1, ranging in mol wt from 14.5 to 20kD, form channels with voltage dependence and ion selectivity qualitatively similar to those of whole E1, placing an upper limit on the channel-forming domain. Under certain conditions, however, the gating kinetics and ion selectivity of channels formed by these different E1 peptides can be distinguished. The differences in channel behavior appear to be correlated with peptide length. Enzymatic digestion with trypsin of membrane-bound E1 peptides converts channel behavior of longer peptides to that characteristic of channels formed by shorter fragments. Apparently trypsin removes segments of protein N-terminal to the channel-forming region, since gating behavior of the shortest fragment is little affected by the enzyme. The success of this conversion depends on the side of the membrane to which trypsin is added and on the state, open or closed, of the channel. Trypsin modifies only closed channels from thecis side (the side to which protein has been added) and only open channels from thetrans side. These results suggest that regions outside the channel-forming domain affect ion selectivity and gating, and they also provide evidence that large protein segments outside the channel-forming domain are translocated across the membrane with channel gating.     

Colicin N forms voltage- and pH-dependent channels in planar lipid bilayer membranes

The protein antibiotic colicin N forms ion-permeable channels through planar lipid bilayers. Channels are induced when positive voltages higher than +60 mV are applied. Incorporated channels activate and inactivate in a voltage-dependent fashion. It is shown that colicin N undergoes a transition between an “acidic” and a “basic” channel form which are distinguishable by different voltage dependences. The single-channel conductance is non-ohmic and strongly dependent on pH, indicating that titratable groups control the passage of ions through the channel. The ion selectivity of colicin N channels is influenced by the pH and the lipid composition of the bilayer membrane. In neutral membranes the channel undergoes a transition from slightly cation-selective to slightly anion-selective when the pH is changed from 7 to 5. In lipid membranes bearing a negative surface charge the channel shows a more pronounced cation selectivity which decreases but does not reverse upon lowering the pH from 7 to 5. The high degree of similarity between the channel characteristics of colicin A and N suggests that the channels share common features in their molecular structure. Offprint requests to: F. Pattus     

On the role of lipid in colicin pore formation

Insights into the protein-membrane interactions by which the C-terminal pore-forming domain of colicins inserts into membranes and forms voltage-gated channels, and the nature of the colicin channel, are provided by data on: (i) the flexible helix-elongated state of the colicin pore-forming domain in the fluid anionic membrane interfacial layer, the optimum anionic surface charge for channel formation, and voltage-gated translocation of charged regions of the colicin domain across the membrane; (ii) structure-function data on the voltage-gated K(+) channel showing translocation of an arginine-rich helical segment through the membrane; (iii) toroidal channels formed by small peptides that involve local participation of anionic lipids in an inverted phase. It is proposed that translocation of the colicin across the membrane occurs through minimization of the Born charging energy for translocation of positively charged basic residues across the lipid bilayer by neutralization with anionic lipid head groups. The resulting pore structure may consist of somewhat short, ca. 16 residues, trans-membrane helices, in a locally thinned membrane, together with surface elements of inverted phase lipid micelles.     

Protein Translocation across Planar Bilayers by the Colicin Ia Channel-Forming Domain: Where Will It End?

Colicin Ia, a 626-residue bactericidal protein, consists of three domains, with the carboxy-terminal domain (C domain) responsible for channel formation. Whole colicin Ia or C domain added to a planar lipid bilayer membrane forms voltage-gated channels. We have shown previously that the channel formed by whole colicin Ia has four membrane-spanning segments and an approximately 68-residue segment translocated across the membrane. Various experimental interventions could cause a longer or shorter segment within the C domain to be translocated, making us wonder why translocation normally stops where it does, near the amino-terminal end of the C domain (approximately residue 450). We hypothesized that regions upstream from the C domain prevent its amino-terminal end from moving into and across the membrane. To test this idea, we prepared C domain with a ligand attached near its amino terminus, added it to one side of a planar bilayer to form channels, and then probed from the opposite side with a water-soluble protein that can specifically bind the ligand. The binding of the probe had a dramatic effect on channel gating, demonstrating that the ligand (and hence the amino-terminal end of the C domain) had moved across the membrane. Experiments with larger colicin Ia fragments showed that a region of more than 165 residues, upstream from the C domain, can also move across the membrane. All of the colicin Ia carboxy-terminal fragments that we examined form channels that pass from a state of relatively normal conductance to a low-conductance state; we interpret this passage as a transition from a channel with four membrane-spanning segments to one with only three.     

Import-defective colicin B derivatives mutated in the TonB box

The pore-forming colicin B is taken up into Escherichia coli by a receptor and TonB-dependent process. The receptor and colicin B both contain a similar amino acid sequence, close to the N-terminal end, termed the TonB box. Point mutations were introduced into the TonB-box region of the colicin B structural gene cba resulting in colicin B derivatives which were partially or totally inactive against E. coli cells. All derivatives still bound to the receptor. An inactive derivative killed cells when translocated across the outer membrane by osmotic shock treatment, and formed pores in planar lipid bilayer membranes identical to the wild-type colicin. Some of the mutations were partially suppressed by mutations in the tonB structural gene. It was concluded that the TonB-box mutations define a region that is involved in the uptake of colicin B across the outer membrane.     

Gating of Connexin Channels by transjunctional-voltage: Conformations and models of open and closed states

Voltage is an important physiologic regulator of channels formed by the connexin gene family. Connexins are unique among ion channels in that both plasma membrane inserted hemichannels (undocked hemichannels) and intercellular channels (aggregates of which form gap junctions) have important physiological roles. The hemichannel is the fundamental unit of gap junction voltage-gating. Each hemichannel displays two distinct voltage-gating mechanisms that are primarily sensitive to a voltage gradient formed along the length of the channel pore (the transjunctional voltage) rather than sensitivity to the absolute membrane potential (V_m or V_i-o). These transjunctional voltage dependent processes have been termed V_j- or fast-gating and loop- or slow-gating. Understanding the mechanism of voltage-gating, defined as the sequence of voltage-driven transitions that connect open and closed states, first and foremost requires atomic resolution models of the end states. Although ion channels formed by connexins were among the first to be characterized structurally by electron microscopy and x-ray diffraction in the early 1980′s, subsequent progress has been slow. Much of the current understanding of the structure-function relations of connexin channels is based on two crystal structures of Cx26 gap junction channels. Refinement of crystal structure by all-atom molecular dynamics and incorporation of charge changing protein modifications has resulted in an atomic model of the open state that arguably corresponds to the physiologic open state. Obtaining validated atomic models of voltage-dependent closed states is more challenging, as there are currently no methods to solve protein structure while a stable voltage gradient is applied across the length of an oriented channel. It is widely believed that the best approach to solve the atomic structure of a voltage-gated closed ion channel is to apply different but complementary experimental and computational methods and to use the resulting information to derive a consensus atomic structure that is then subjected to rigorous validation. In this paper, we summarize our efforts to obtain and validate atomic models of the open and voltage-driven closed states of undocked connexin hemichannels.This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.     

Structure and dynamics of the colicin E1 channel

The toxin-like and bactericidal colicin E1 molecule is of interest for problems of toxin action, polypeptide translocation across membranes, voltage-gated channels, and receptor function. Colicin E1 binds to a receptor in the outer membrane and is translocated across the cell envelope to the inner membrane. Import of the colicin channel-forming domain into the inner membrane involves a translocation-competent intermediate state and a membrane potential-dependent movement of one third to one half of the channel peptide into the membrane bilayer. The voltage-gated channel has a conductance sufficiently large to depolarize the Escherichia coli cytoplasmic membrane. Amino acid residues that affect the channel ion selectivity have been identified by site-directed mutagenesis. The colicin E1 channel is one of a few membrane proteins whose secondary structures in the membrane, predominantly alpha-helix, have been determined by physico-chemical techniques. Hypothesis for the identity of the trans-membrane helices, and the mechanism of binding to the membrane, are influenced by the solved crystal structure of the soluble colicin A channel peptide. The protective action of immunity protein is a unique aspect of the colicin problem, and information has been obtained, by genetic techniques, about the probable membrane topography of the imm gene product.     

Comparison of the macroscopic and single channel conductance properties of colicin E1 and its COOH-terminal tryptic peptide

A COOH-terminal tryptic fragment (Mr approximately equal to 20,000) of colicin E1 has been proposed to contain the membrane channel-forming domain of the colicin molecule. A comparison is made of the conductance properties of colicin E1 and its COOH-terminal fragment in planar bilayer membranes. The macroscopic and single channel properties of colicin E1 and its COOH-terminal tryptic fragment are very similar, if not indistinguishable, implying that the NH2-terminal, two-thirds of the colicin E1 molecule, does not significantly influence its channel properties. The channel-forming activity of both polypeptides is dependent upon the presence of a membrane potential, negative on the trans side of the membrane. The average single channel conductance of colicin E1 and the COOH-terminal fragment is 20.9 +/- 3.9 and 19.1 +/- 2.9 picosiemens, respectively. The rate at which both proteins form conducting channels increases as the pH is lowered from 7 to 5. Both molecules require negatively charged lipids for activity to be expressed, exhibit the same ion selectivity, and rectify the current to the same extent. Both polypeptides associate irreversibly with the membrane in the absence of voltage, but subsequent formation of conducting channels requires a negative membrane potential.     

The haemolysin-secreting ShlB protein of the outer membrane of Serratia marcescens : determination of surface-exposed residues and formation of ion-permeable pores by ShlB mutants in artificial lipid bilayer membranes

The ShlB protein in the outer membrane of Serratia marcescens is the only protein known to be involved in secretion of the ShlA protein across the outer membrane. At the same time, ShlB converts ShlA into a haemolytic and a cytolytic toxin. Surface-exposed residues of ShlB were determined by reaction of an M2 monoclonal antibody with the M2 epitope DYKDDDDK inserted at 25 sites along the entire ShlB polypeptide. The antibody bound to the M2 epitope at 17 sites in intact cells, which indicated surface exposure of the epitope, and to 23 sites in isolated outer membranes. Two insertion mutants contained no ShlB(M2) protein in the outer membrane. The ShlB derivatives activated and/or secreted ShlA. To gain insights into the secretion mechanism, we studied whether highly purified ShlB and ShlB deletion derivatives formed pores in artificial lipid bilayer membranes. Wild-type ShlB formed channels with very low single channel conductance that rarely assumed an open channel configuration. In contrast, open channels with a considerably higher single channel conductance were observed with the deletion mutants ShlB(Delta65-186), ShlB(Delta87-153), and ShlB(Delta126-200). ShlB(Delta126-200) frequently formed permanently open channels, whereas the conductance caused by ShlB(Delta65-186) and ShlB(Delta87-153) did not assume a stationary value, but fluctuated rapidly between open and closed configurations. The results demonstrate the orientation of large portions of ShlB in the outer membrane and suggest that ShlB may function as a specialized pore through which ShlA is secreted.     

The C-terminal half of the colicin A pore-forming domain is active in vivo and in vitro

The pore-forming domain of colicin A (pfColA) fused to a prokaryotic signal peptide (sp-pfColA) is transported across and inserts into the inner membrane of Escherichia coli from the periplasmic side and forms a functional channel. The soluble structure of pfColA consists of a ten-helix bundle containing a hydrophobic helical hairpin. Here, we generated a series of mutants in which an increasing number of sp-pfColA alpha-helices was deleted. These peptides were tested for their ability to form ion channels in vivo and in vitro. We found that the shortest sp-pfColA mutant protein that killed Escherichia coli was composed of the five last alpha-helices of sp-pfColA, whereas the shortest peptide that formed a channel in planar lipid bilayer membranes similar to that of intact pfColA was the protein composed of the last six alpha-helices. The peptide composed of the last five alpha-helices of pfColA generated a voltage-independent conductance in planar lipid bilayer with properties very different from that of intact pfColA. Thus, helices 1 to 4 are unnecessary for channel formation, while helix 5, or some part of it, is important but not absolutely necessary. Voltage-dependence of colicin is evidently controlled by the first four alpha-helices of pfColA.     

Identification of a translocated gating charge in a voltage-dependent channel. Colicin E1 channels in planar phospholipid bilayer membranes.

The availability of primary sequences for ion-conducting channels permits the development of testable models for mechanisms of voltage gating. Previous work on planar phospholipid bilayers and lipid vesicles indicates that voltage gating of colicin E1 channels involves translocation of peptide segments of the molecule into and across the membrane. Here we identify histidine residue 440 as a gating charge associated with this translocation. Using site-directed mutagenesis to convert the positively charged His440 to a neutral cysteine, we find that the voltage dependence for turn-off of channels formed by this mutant at position 440 is less steep than that for wild-type channels; the magnitude of the change in voltage dependence is consistent with residue 440 moving from the trans to the cis side of the membrane in association with channel closure. The effect of trans pH changes on the ion selectivity of channels formed by the carboxymethylated derivative of the cysteine 440 mutant independently establishes that in the open channel state, residue 440 lies on the trans side of the membrane. On the basis of these results, we propose that the voltage-gated opening of colicin E1 channels is accompanied by the insertion into the bilayer of a helical hairpin loop extending from residue 420 to residue 459, and that voltage-gated closing is associated with the extrusion of this loop from the interior of the bilayer back to the cis side.     

Helical alpha-synuclein forms highly conductive ion channels

Alpha-synuclein (alphaS) is a cytosolic protein involved in the etiology of Parkinson's disease (PD). Disordered in an aqueous environment, alphaS develops a highly helical conformation when bound to membranes having a negatively charged surface and a large curvature. It exhibits a membrane-permeabilizing activity that has been attributed to oligomeric protofibrillar forms. In this study, monomeric wild-type alphaS and two mutants associated with familial PD, E46K and A53T, formed ion channels with well-defined conductance states in membranes containing 25-50% anionic lipid and 50% phosphatidylethanolamine (PE) in the presence of a trans-negative potential. Another familial mutant, A30P, known to have a lower membrane affinity, did not form ion channels. Ca2+ prevented channel formation when added to membranes before alphaS and decreased channel conductance when added to preformed channels. In contrast to the monomer, membrane permeabilization by oligomeric alphaS was not characterized by formation of discrete channels, a requirement for PE lipid, or a membrane potential. Channel activity, alpha-helical content, thermal stability of membrane-bound alphaS determined by far-UV CD, and lateral mobility of alphaS bound to planar membranes measured by fluorescence correlation spectroscopy were correlated. It was inferred that discrete ion channels with well-defined conductance states were formed in the presence of a membrane potential by one or several molecules of monomeric alphaS in an alpha-helical conformation and that such channels may have a role in the normal function and/or pathophysiology of the protein.     

Adventures in membrane protein topology. A study of the membrane-bound state of colicin E1.

The molecular aggregate size of the closed state of the colicin E1 channel was determined by fluorescence resonance energy transfer experiments involving a fluorescence donor (three tryptophans, wild-type protein) and a fluorescence acceptor (5-(((acetyl)amino)ethyl)aminonaphthalene-1-sulfonic acid (AEDANS), Trp-deficient protein). There was no evidence of energy transfer between the donor and acceptor species when bound to membrane large unilamellar vesicles. These experiments led to the conclusion that the colicin E1 channel is monomeric in the membrane-bound closed channel state. Experiments were also conducted to study the membrane topology of the closed colicin channel in membrane large unilamellar vesicles using acrylamide as the membrane-impermeant, nonionic quencher of tryptophan fluorescence in a battery of single tryptophan mutant proteins. Furthermore, additional fluorescence parameters, including fluorescence emission maximum, fluorescence quantum yield, and fluorescence decay times, were used to assist in mapping the topology of the closed channel. Results suggest that the closed channel comprises most of the polypeptide of the channel domain and that the hydrophobic anchor domain does not transverse the membrane bilayer but nonetheless is deeply embedded within the hydrocarbon core of the membrane. Finally, a model is proposed which features at least two states that are in rapid equilibrium with each other and in which one state is more heavily populated than the other.     

Conformational changes of colicin Ia channel-forming domain upon membrane binding: a solid-state NMR study

Channel-forming colicins are bactericidal proteins that spontaneously insert into hydrophobic lipid bilayers. We have used magic-angle spinning solid-state nuclear magnetic resonance spectroscopy to examine the conformational differences between the water-soluble and the membrane-bound states of colicin Ia channel domain, and to study the effect of bound colicin on lipid bilayer structure and dynamics. We detected ¹³C and ¹⁵N isotropic chemical shift differences between the two forms of the protein, which indicate structural changes of the protein due to membrane binding. The Val Cα signal, unambiguously assigned by double-quantum experiments, gave a 0.6 ppm downfield shift in the isotropic position and a 4 ppm reduction in the anisotropic chemical shift span after membrane binding. These suggest that the α-helices in the membrane-bound colicin adopt more ideal helical torsion angles as they spread onto the membrane. Colicin binding significantly reduced the lipid chain order, as manifested by ²H quadrupolar couplings. These results are consistent with the model that colicin Ia channel domain forms an extended helical array at the membrane-water interface upon membrane binding.     

Conformational changes of colicin Ia channel-forming domain upon membrane binding: a solid-state NMR study

Channel-forming colicins are bactericidal proteins that spontaneously insert into hydrophobic lipid bilayers. We have used magic-angle spinning solid-state nuclear magnetic resonance spectroscopy to examine the conformational differences between the water-soluble and the membrane-bound states of colicin Ia channel domain, and to study the effect of bound colicin on lipid bilayer structure and dynamics. We detected (13)C and (15)N isotropic chemical shift differences between the two forms of the protein, which indicate structural changes of the protein due to membrane binding. The Val C(alpha) signal, unambiguously assigned by double-quantum experiments, gave a 0.6 ppm downfield shift in the isotropic position and a 4 ppm reduction in the anisotropic chemical shift span after membrane binding. These suggest that the alpha-helices in the membrane-bound colicin adopt more ideal helical torsion angles as they spread onto the membrane. Colicin binding significantly reduced the lipid chain order, as manifested by (2)H quadrupolar couplings. These results are consistent with the model that colicin Ia channel domain forms an extended helical array at the membrane-water interface upon membrane binding.     

Lipid dependence of the channel properties of a colicin E1-lipid toroidal pore

Colicin E1 belongs to a group of bacteriocins whose cytotoxicity toward Escherichia coli is exerted through formation of ion channels that depolarize the cytoplasmic membrane. The lipid dependence of colicin single-channel conductance demonstrated intimate involvement of lipid in the structure of this channel. The colicin formed "small" conductance 60-picosiemens (pS) channels, with properties similar to those previously characterized, in 1,2-dieicosenoyl-sn-glycero-3-phosphocholine (C20) or thinner membranes, whereas it formed a novel "large" conductance 600-pS state in thicker 1,2-dierucoyl-sn-glycero-3-phosphocholine (C22) bilayers. Both channel states were anion-selective and voltage-gated and displayed a requirement for acidic pH. Lipids having negative spontaneous curvature inhibited the formation of both channels but increased the ratio of open 600 pS to 60 pS conductance states. Different diameters of small and large channels, 12 and 16 A, were determined from the dependence of single-channel conductance on the size of nonelectrolyte solute probes. Colicin-induced lipid "flip-flop" and the decrease in anion selectivity of the channel in the presence of negatively charged lipids implied a significant contribution of lipid to the structure of the channel, most readily described as toroidal organization of lipid and protein to form the channel pore.     

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.