Bilayers containing gangliosides develop channels when exposed to cholera toxin

The role of bacterial and non-bacterial toxins in the induction of changes in membrane transport: implications for diarrhea

Bacterial toxins induce changes in membrane transport which underlie the loss of electrolyte homeostasis associated with diarrhea. Bacterial- and their secreted toxin-types which have been linked with diarrhea include: (a) Vibrio cholerae (cholera toxin, E1 Tor hemolysin and accessory cholera enterotoxin); (b) Escherichia coli (heat stable enterotoxin, heat-labile enterotoxin and colicins); (c) Shigella dysenteriae (shiga-toxin); (d) Clostridium perfringens (C. perfringens enterotoxin, alpha-toxin, beta-toxin and theta-toxin); (e) Clostridium difficile (toxins A and B); (f) Staphylococcus aureus (alpha-haemolysin); (g) Bacillus cereus (cytotoxin K and haemolysin BL); and (h) Aeromonas hydrophila (aerolysin, heat labile cytotoxins and heat stable cytotoxins). The mechanisms of toxin-induced diarrhea include: (a) direct effects on ion transport in intestinal epithelial cells, i.e. direct toxin interaction with intrinsic ion channels in the membrane and (b) indirect interaction with ion transport in intestinal epithelial cells mediated by toxin binding to a membrane receptor. These effects consequently cause the release of second messengers, e.g. the release of adenosine 3',5'-cyclic monophosphate/guanosine 3',5'-monophosphate, IP(3), Ca2+ and/or changes in second messengers that are the result of toxin-formed Ca2+ and K+ permeable channels, which increase Ca2+ flux and augment changes in Ca2+ homeostasis and cause depolarisation of the membrane potential. Consequently, many voltage-dependent ion transport systems, e.g. voltage-dependent Ca2+ influx, are affected. The toxin-formed ion channels may act as a pathway for loss of fluid and electrolytes. Although most of the diarrhea-causing toxins have been reported to act via cation and anion channel formation, the properties of these channels have not been well studied, and the available biophysical properties that are needed for the characterization of these channels are inadequate.

Partial cloning of putative G-proteins modulating mechanotransduction in the ciliate stentor

Signal transduction systems known to utilize G-proteins in higher eukaryotes undoubtedly evolved prior to the development of metazoa. Pharmacological evidence indicates that the ciliates Paramecium, Stentor, and Tetrahymena all utilize signaling systems similar to those found in mammals. However, there has been relatively little direct evidence for the existence of G-proteins in ciliates. Since highly conserved heterotrimeric G-proteins form the basis of receptor-coupled signal transduction systems in a wide variety of metazoa, it is of interest to know if these important signaling molecules were early to evolve and are present and functionally important in a wide variety of unicellular organisms. We have previously shown that mechanotransduction in Stentor is modulated by opiates in a manner that may involve pertussis toxin-sensitive G-proteins. Here we utilize drugs known to interact with G-proteins to further test for the involvement of these important signaling molecules in Stentor mechanotransduction. We present behavioral and electrophysiological data demonstrating that putative G-proteins in Stentor decrease mechanical sensitivity by modulating the mechanotransduction process. In addition, we report the partial cloning of 4 G-protein alpha-subunits from Stentor. We confirm that these clones are of Stentor origin and are transcribed. Furthermore, we employ antisense oligodeoxynucleotide-mediated knockout to demonstrate that these ciliate G-proteins exert a modulatory influence on Stentor behavior, and that a G1/G0-like clone mediates the inhibitory action of beta-endorphin on mechanotransduction.

Interaction with a lipid membrane: a key step in bacterial toxins virulence

Bacterial toxins are secreted as soluble proteins. However, they have to interact with a cell lipid membrane either to permeabilize the cells (pore forming toxins) or to enter into the cytosol to express their enzymatic activity (translocation toxins). The aim of this review is to suggest that the strategies developed by toxins to insert in a lipid membrane is mediated by their structure. Two categories, which contains both pore forming and translocation toxins, are emerging: alpha helical proteins containing hydrophobic domains and beta sheets proteins in which no hydrophobicity can be clearly detected. The first category would rather interact with the membrane through multi-spanning helical domains whereas the second category would form a beta barrel in the membrane.

A novel method for structure-based prediction of ion channel conductance properties

A rapid and easy-to-use method of predicting the conductance of an ion channel from its three-dimensional structure is presented. The method combines the pore dimensions of the channel as measured in the HOLE program with an Ohmic model of conductance. An empirically based correction factor is then applied. The method yielded good results for six experimental channel structures (none of which were included in the training set) with predictions accurate to within an average factor of 1.62 to the true values. The predictive r2 was equal to 0.90, which is indicative of a good predictive ability. The procedure is used to validate model structures of alamethicin and phospholamban. Two genuine predictions for the conductance of channels with known structure but without reported conductances are given. A modification of the procedure that calculates the expected results for the effect of the addition of nonelectrolyte polymers on conductance is set out. Results for a cholera toxin B-subunit crystal structure agree well with the measured values. The difficulty in interpreting such studies is discussed, with the conclusion that measurements on channels of known structure are required.

HOLE: a program for the analysis of the pore dimensions of ion channel structural models

A method (HOLE) that allows the analysis of the dimensions of the pore running through a structural model of an ion channel is presented. The algorithm uses a Monte Carlo simulated annealing procedure to find the best route for a sphere with variable radius to squeeze through the channel. Results can be displayed in a graphical fashion or visualized with most common molecular graphical packages. Advances include a method to analyze the anisotropy within a pore. The method can also be used to predict the conductance of channels using a simple empirically corrected ohmic model. As an example the program is applied to the cholera toxin B-subunit pentamer. The compatibility of the crystal structure and conductance data is established.

Low conductance states of a single ion channel are not 'closed'

We have used a polymer-exclusion method to estimate the sizes of the high- and low-conductance states of Staphylococcus aureus alpha-toxin channels across planar lipid bilayers. Despite a > 10-fold difference in conductance between high- and low-conductance states, the size differs by < 2-fold. We conclude that factors other than the dimensions have a strong influence on the conductance of alpha-toxin channels. We also show that the high conductance state is destabilized by the presence of high molecular weight polymers outside the channel, compatible with the removal of channel water as the high conductance state "shrinks" to the low conductance state.

Fluorescence analysis of the interaction between ganglioside GM1-containing phospholipid vesicles and the B subunit of cholera toxin

Binding of cholera toxin B protomer (CT-B) to a pyrene-labeled analogue of its ganglioside GM1 receptor (pyrene-GM1) in the absence and presence of phosphatidylcholine vesicles was monitored using steady-state fluorescence spectroscopy. CT-B association with pyrene-GM1 micelles induces changes in the fluorescence properties of this ganglioside analogue that are consistent with its conversion from an excimer to a monomer form. Incubation of pyrene-GM1 with preformed vesicles of phosphatidylcholine (PC) results in complete conversion of pyrene-GM1 to its monomer form, however, unlike with CT-B binding, incorporation of pyrene-GM1 into PC vesicles occurs with a concomitant loss of fluorescence quenching by the small polar quenching agent acrylamide. Subsequent binding of CT-B to the PC-GM1 composite vesicles causes no further change in the pyrene fluorescence emission spectrum but does appear to increase acrylamide accessibility. These data lead to the conclusion that cholera toxin binding to a cell membrane alters membrane packing at the site of attachment. Furthermore, this phenomenon appears to be influenced by environmental conditions such as pH. A pH of about 4.0 or less causes acrylamide quenching to decrease to approximately the levels observed in the absence of CT-B. These results may be useful in describing the dynamics of the interaction between cholera toxin and target cell membranes. Moreover, these data could provide clues to the mechanism by which the toxic portion of CT is able to enter the cytoplasm of target cells.

Interaction of a cholera toxin derivative containing a reduced number of receptor binding sites with intact cells in culture

Hybrid CTB (hCTB), having only one or two functional binding sites, has been constructed from two chemically inactivated derivatives of CTB. One inactive derivative consisted of CTB formylated in the lone Trp-88 of each beta-chain (fCTB), whereas the other inactive derivative consisted of CTB specifically succinylated in three amino groups located in or near the receptor binding site (sssCTB). hCTB, fCTB and sssCTB were able to reassociate with CTA and form the corresponding holotoxins hCT, fCT and sssCT as measured by gel filtration chromatography. In contrast to fCT and sssCT, hCT could increase the cAMP content of intact Vero cells in a time- and dose-dependent way: concentrations as low as a few nanograms of hCT per milliliter caused a significant increase in the intracellular cAMP level. The maximal cAMP level induced by hCT (1 microgram/ml) was, however, more than 2-fold lower than that elicited by its native counterpart. At saturating ligand concentrations and at 37 degrees C, the lag periods and rates of CT and hCT induced cAMP accumulation were essentially the same. Treatment of Vero and HeLa cells with GM1 did not affect their difference in response to CT and hCT. When Vero cells treated with hCT were incubated for longer periods of time, a further slow accumulation of cAMP occurred until after about 20 h cAMP levels of cells exposed to CT or hCT were essentially the same. In contrast to Vero and HeLa cells, human skin fibroblasts exhibited an almost identical response to CT as well as to hCT. Acidotropic agents such as chloroquine and monensin affected the CT and hCT induced increase in cAMP content of Vero cells, fibroblasts and GM1 treated Hela cells in a similar way. The results are consistent with the view that CT receptor recognition domains are shared between adjacent beta-chains, that pentavalent binding appears not to be essential for cytotoxicity and that in the cell types studied intracellular processing of CT, hCT is involved.

Structure and function of cholera toxin and the related Escherichia coli heat-labile enterotoxin

Cholera and the related Escherichia coli-associated diarrheal disease are important problems confronting Third World nations and any area where water supplies can become contaminated. The disease is extremely debilitating and may be fatal in the absence of treatment. Symptoms are caused by the action of cholera toxin, secreted by the bacterium Vibrio cholerae, or by a closely related heat-labile enterotoxin, produced by Escherichia coli, that causes a milder, more common traveler's diarrhea. Both toxins bind receptors in intestinal epithelial cells and insert an enzymatic subunit that modifies a G protein associated with the adenylate cyclase complex. The consequent stimulated production of cyclic AMP, or other factors such as increased synthesis of prostaglandins by intoxicated cells, initiates a metabolic cascade that results in the excessive secretion of fluid and electrolytes characteristic of the disease. The toxins have a very high degree of structural and functional homology and may be evolutionarily related. Several effective new vaccine formulations have been developed and tested, and a growing family of endogenous cofactors is being discovered in eukaryotic cells. The recent elucidation of the three-dimensional structure of the heat-labile enterotoxin has provided an opportunity to examine and compare the correlations between structure and function of the two toxins. This information may improve our understanding of the disease process itself, as well as illuminate the role of the toxin in studies of signal transduction and G-protein function.

The ionic channels formed by cholera toxin in planar bilayer lipid membranes are entirely attributable to its B-subunit

The interaction of cholera toxin with planar bilayer lipid membranes (BLM) at low pH results in the formation of ionic channels, the conductance of which can be directly measured in voltage-clamp experiments. It is found that the B-subunit of cholera toxin (CT-B) also is able to induce ionic channels in BLM whereas the A-subunit is not able to do it. The increase of pH inhibited the channel-forming activity of CT-B. The investigation of pH-dependences of both the conductance and the cation-anion selectivity of the CT-B channel allowed us to suggest that the water pore of this channel is confined to the B-subunit of cholera toxin. The effective diameter of the CT-B channels water pores was directly measured in BLM and is equal to 2.1 +/- 0.2 nm. The channels formed by whole toxin and its B-subunit exhibit voltage-dependent activity. We believe these channels are relevant to the mode of action of cholera toxin and especially to the endosomal pathway of the A-subunit into cells.

Effect of gangliosides on membrane permeability studied by enzymic and fluorescence-spectroscopy techniques

The effect of gangliosides on membrane permeability was investigated by studying the kinetic properties of cytochrome c oxidase, the activity of which, when the enzyme is reconstituted in phospholipid vesicles, is dependent on membrane permeability to H+ and K+. The experiments indicate that three different gangliosides (GM1, DD1a, GT1b) incorporated into cytochrome c oxidase-containing phospholipid vesicles stimulate enzymic activity, in the absence of ionophores, most probably by disorganizing the bilayer lipid assembly and increasing its permeability to ions. This interpretation was confirmed by fluorescence-spectroscopy experiments in which the rate of passive leakage of carboxyfluorescein entrapped in the vesicles was measured. Cholera toxin, or its isolated B-subunit, added to GM1-containing proteoliposomes inhibited cytochrome c oxidase activity, indicating the lack of formation, under these experimental conditions, of channels freely permeable to H+ or K+.

Three-dimensional structure of cholera toxin penetrating a lipid membrane

Two-dimensional crystals of cholera toxin bound to receptors in a lipid membrane give diffraction extending to 15 A resolution. Three-dimensional structure determination reveals a ring of five B subunits on the membrane surface, with one-third of the A subunit occupying the center of the ring. The remaining mass of the A subunit appears to penetrate the hydrophobic interior of the membrane. Cleavage of a disulfide bond in the A subunit, which activates the toxin, causes a major conformational change, with the A subunit mostly exiting from the B ring.

Calcium transport affinity, ion competition and cholera toxin effects on cytosolic Ca concentration

The physiological relevance of an apparent ionophore activity of cholera toxin towards Ca2+ has been examined in several different systems designed to measure affinity, specificity, rates of ion transfer, and effects on intracellular ion concentrations. Half-maximal transfer rates across porcine jejunal brush-border vesicles were obtained at a concentration of 0.20 microM Ca2+. When examined in the presence of competing ions the transfer process was blocked by very low concentrations of La3+ or Cd2+, Sr2+, Ba2+ and Mg2+ were relatively inefficient competitors for Ca2+ transport mediated by cholera toxin. The relative affinities observed would be compatible with a selectivity for Ca2+ transfer at physiological ion concentrations, as well as an inhibition of this ionophore activity by recognized antagonists of cholera toxin such as lanthanum ions. Entry rates of Ca2+ into brush-border vesicles exposed to cholera toxin were large enough to accelerate the collapse of a Ca2+ gradient generated by endogenous Ca, Mg-ATPase activity. The treatment of isolated jejunal enterocytes with cholera toxin caused a significant elevation in cytosolic Ca2+ concentrations as measured by Quin-2 fluorescence. This effect was specifically prevented by prior exposure of the cholera toxin to excess ganglioside GM1. We conclude that cholera toxin has many of the properties required for promoting transmembranes Ca2+ movement in membrane vesicles and appears to be an effective Ca2+ ionophore in isolated mammalian cells.

Complexity of the influence of gangliosides on histamine release from human basophils and rat mast cells

The influence of exogenous addition of gangliosides on histamine release from human basophils and rat mast cells was examined in vitro. Gangliosides dose-dependently inhibited histamine release, and this inhibition was dependent on the ganglioside sialic acid content, since GT1b, having 3 sialic acid moieties, was more potent than gangliosides with 2 moieties (GD1a and GD1b), which again were more potent inhibitors than GM1 with one moiety. Asialo-GM1 was without effect. In high concentrations the gangliosides potentiated basophil histamine release. The modulation of histamine release was reflected in the sensitivity of the cells to extracellular calcium, since inhibition of the release could be counteracted by increasing the extracellular concentration of calcium.

New chemical trends in ganglioside research

A report is given of recent progress in the methodology for isolation of gangliosides from natural sources, for the preparation of molecular species of gangliosides homogeneous in both the oligosaccharide and ceramide portions of the molecule, for chemical manipulation and derivatization of gangliosides, and for the preparation of gangliosides radiolabelled in different parts of the molecule. Particular emphasis has been given to: high performance liquid chromatographic procedures capable to separate gangliosides on the basis of their oligosaccharide or ceramide moieties and yielding completely homogeneous compounds, that is gangliosides with a single oligosaccharide, a single long chain base and a single fatty acid; two-dimensional thin-layer chromatographic procedures, provided with a fully computerized quantification system, particularly suitable to identifying gangliosides containing alkali-labile linkages, including ganglioside lactones; chemical procedures of high yield for reducing gangliosides at the double bond of long chain base, for selective removal of the fatty acyl moiety and replacement with a novel fatty acid, and for the synthesis of ganglioside lactones; chemical procedures for inserting fluorescent, paramagnetic or photoreactive probes at the fatty acyl part of the ganglioside molecule; procedures for chemical isotopic radiolabelling of gangliosides at the level of sialic acid acetyl group and at the fatty acid moiety. Examples are provided evidencing the significance and potential use of a variety of ganglioside derivatives in the study of ganglioside metabolism and functional implications.

Properties of the leak permeability induced by a cytotoxic protein from Pseudomonas aeruginosa (PACT) in rat erythrocytes and black lipid membranes

A cytotoxic protein, isolated from Pseudomonas aeruginosa (PACT), was tested on red blood cells of rats and on black lipid membranes for changes of membrane permeability. In rat erythrocytes PACT induces lysis indicative of the formation of a leak permeable to monovalent ions. The dose response curve for the PACT-induced hemolysis demonstrates that the rate of lysis as well as the fraction of lytic cells increases with increasing toxin concentration. Furthermore, the leak pathway discriminates hydrophilic non-electrolytes according to their molecular weight. The findings indicate formation by PACT of a pore with an apparent radius of about 1.2 nm. In pure lipid membranes PACT forms hydrophilic pathways with moderate selectivity for small cations over small anions. The presence of cholesterol is a prerequisite for the occurrence of these PACT-induced permeability changes.

Tetanus toxin interaction with human erythrocytes. II. Kinetic properties of toxin association and evidence for a ganglioside-toxin macromolecular complex formation

The properties of tetanus toxin interaction with human erythrocytes supplemented with disialo- and trisialo-gangliosides have been investigated. Binding of toxin is linear with time for 1 h and is 3-4-fold higher at 37 degrees C than at 4 degrees C during incubation of long duration. It exhibits saturation at toxin concentrations between 0.1 and 1 microgram/ml; however, it is nonsaturable between 1 and up to 50 micrograms/ml. It is effectively prevented by free gangliosides and antibodies or by pretreatment with sialidase but is unaffected by a number of closely related ligands including toxoid and toxin fragments. NaCl (1 M) removes a great portion (86%) of cell-associated toxin while Triton X-100 extracts an additional fraction (30%) of the salt-resistant cell-bound toxin. The residual sequestred toxin after detergent extraction is sensitive to proteolytic degradation. The trypsin-stable fraction (1.5%) is biotoxic and may be indicative of internalization of toxin. A macromolecular complex of about 700 kDa containing toxin and gangliosides has been isolated and characterized by Sephacryl S-300 gel permeation chromatography, SDS-gel electrophoresis, immunoprecipitability and biotoxicity. This complex is obtained only in ganglioside-supplemented cells and not when free 3H-labeled GD1b is reacted with 125I-labeled toxin in solution in the absence of cells. The hydrophobicity properties acquired as a result of ganglioside-toxin interaction, presumably at the cell surface, suggest a conformational change of the toxin which may enable its penetration into the bilayer.

Micellar gangliosides mediate the lipid insertion of cholera toxin protomer A

The topology of the interaction of cholera toxin with ganglioside and detergent micelles was studied with the technique of hydrophobic photolabelling. Cholera toxin alpha and gamma polypeptide chains appear to penetrate into the hydrophobic core of ganglioside micelles. Micelles of SDS cause the labelling also of the beta polypeptide chains, while Triton X-100 micelles have little ability to mediate the labelling of the toxin. The specific reduction of the alpha-gamma disulfide bond allows the penetration of the alpha polypeptide chain into Triton X-100 micelles, but does not affect the interaction of cholera toxin with either ganglioside or SDS micelles. Thus, ganglioside micelles appear to cause a conformational change of the native toxin, such as to induce the penetration of the alpha chain into the micelle hydrophobic core.

Role of membrane gangliosides in the binding and action of bacterial toxins

Gangliosides are complex glycosphingolipids that contain from one to several residues of sialic acid. They are present in the plasma membrane of vertebrate cells with their oligosaccharide chains exposed to the external environment. They have been implicated as cell surface receptors and several bacterial toxins have been shown to interact with them. Cholera toxin, which mediates its effects on cells by activating adenylate cyclase, bind with high affinity and specificity to ganglioside GM1. Toxin-resistant cells which lack GM1 can be sensitized to cholera toxin by treating them with GM1. Cholera toxin specifically protects GM1 from cell surface labeling procedures and only GM1 is recovered when toxin-receptor complexes are isolated by immunoadsorption. These results clearly demonstrate that GM1 is the specific and only receptor for cholera toxin. Although cholera toxin binds to GM1 on the external side of the plasma membrane, it activates adenylate cyclase on the cytoplasmic side of the membrane by ADP-ribosylation of the regulatory component of the cyclase. GM1 in addition to functioning as a binding site for the toxin appears to facilitate its transmembrane movement. The heat-labile enterotoxin of E. coli is very similar to cholera toxin in both form and function and can also use GM1 as a cell surface receptor. The potent neurotoxin, tetanus toxin, has a high affinity for gangliosides GD1b and GT1b and binds to neurons which contain these gangliosides. It is not yet clear whether these gangliosides are the physiological receptors for tetanus toxin. By applying the techniques that established GM1 as the receptor for cholera toxin, the role of gangliosides as receptors for tetanus toxin as well as physiological effectors may be elucidated.

Reevaluation of the role of gangliosides in the binding and action of thyrotropin

A cloned line of normal rat thyroid cells, FRTL, contained a small number of high-affinity binding sites for thyrotropin (TSH) when measured under physiological conditions. The cells also bound small amounts of cholera toxin, and both hormone and toxin stimulated cyclic AMP production by the cells. The major ganglioside of FRTL cells was N-acetylneuraminylgalactosylglucosylceramide (GM3), with minor amounts of gangliosides corresponding to galactosyl-N-acetylgalactosaminyl-[N-acetyl-neuraminyl]-galactosylglucosylceramide (GM1) and N-acetylneuraminylgalactosyl-N-acetylgalactosaminyl-[N-acetylneuraminyl]-galactosylglucosylceramide (GD1a). Treatment of these cells with neuraminidase (acylneuraminyl hydrolase, EC 3.2.1.18) converted most of the GD1a to GM1. After neuraminidase treatment, the binding of cholera toxin, which binds to GM1, was increased, but there was no change in the binding of TSH. Preincubation of neuraminidase-treated FRTL cells with the B (binding) component of cholera toxin completely prevented cholera toxin binding but had no effect on the binding of TSH. Neuraminidase treatment also somewhat enhanced, rather than decreased, the cyclic AMP response to TSH. Pretreatment of FRTL cells with mixed brain gangliosides resulted in a 10-fold increase in cholera toxin binding. Again there was no enhancement of TSH binding or adenylate cyclase stimulation. Finally, prolonged exposure of FRTL cells to TSH induced down-regulation of TSH receptors but had no effect on gangliosides or cholera toxin receptors. The results indicate that more complex gangliosides do not serve as a component of the TSH receptor nor are they involved in the transmission of the hormone signal across the cell membrane of these cultured rat thyroid cells.

Actions of cholera toxin and the prevention and treatment of cholera

The drastic intestinal secretion of fluid and electrolytes that is characteristic of cholera is the result of reasonably well understood cellular and biochemical actions of the toxin secreted by Vibrio cholerae. Based on this understanding it is possible to devise new techniques for the treatment and prophylaxis of cholera to complement those based on fluid replacement therapy and sanitation.

Photolabelling of cholera toxin subunits during membrane penetration

There has been much speculation about the mechanism by which cholera toxin exerts its effect on the cytoplasmic side of the membranes with which it interacts. After the pentamer of B subunits (5B) binds to membrane receptors, particularly the monosialylganglioside GM1, the disulphide-linked dimer A1SSA2 (which together with 5B constitutes the complete toxin) is thought to penetrate the membrane, perhaps through a channel formed by 5B and become reduced so that A1SH units reach the cytoplasm and stimulate adenylate cyclase. Evidence for this mechanism is circumstantial. If it is correct, a compound which will specifically label intramembranous sections of the toxin should label the channel-forming B subunits but not the channel-contained A1 subunit. We have tested this prediction with a photoreactive glycolipid compound and have obtained the opposite result. Therefore, we propose that only the A1 subunit enters the membrane and we provide here data on the kinetics of that process.

Diphtheria toxin forms transmembrane channels in planar lipid bilayers

When exposed to a lipid bilayer, diphtheria toxin binds to it and forms transmembrane, voltage-dependent, anion-selective channels. The mean (+/- SD) conductance of these channels in asolectin membranes is 6.2 +/- 0.7 pmho (pS) in 0.2 M NaCl and 20 +/- 2 pmho in 1.0 M NaCl. The rate of channel formation depends on the pH in the toxin-containing compartment; it is very low at pH greater than 5.0 and increases abruptly as the pH decreases from 4.9 to 4.0. Binding of toxin to the membrane is also pH dependent, being unmeasurable at pH 7 and increasing monotonically with decreasing pH. The rate of channel formation depends upon membrane potential as well; channels form only at negative potentials. These channels are permanent in the time scale of the experiments (about 1 hr). The membrane conductance caused by the channels is also voltage dependent, being constant at positive potentials and decreasing at negative potentials. Hence, the current-voltage curve is linear at positive potentials and sublinear at negative potentials. The conditions necessary for insertion of toxin into the bilayer and formation of channels are similar to those that prevail inside the lysosome. Thus, these results lend credence to the idea that toxin enters the cytoplasm from the lysosomal compartment.