Reconstitution of ion channels

In vitro reconstitution of eukaryotic ion channels using droplet interface bilayers

The ability to routinely study eukaryotic ion channels in a synthetic lipid environment would have a major impact on our understanding of how different lipids influence ion channel function. Here, we describe a straightforward, detergent-free method for the in vitro reconstitution of eukaryotic ion channels and ionotropic receptors into droplet interface bilayers and measure their electrical activity at both the macroscopic and single-channel level. We explore the general applicability of this method by reconstitution of channels from a wide range of sources including recombinant cell lines and native tissues, as well as preparations that are difficult to study by conventional methods including erythrocytes and mitochondria.

Gangliosides affect membrane-channel activities dependent on ambient temperature

1. The functional properties of biological membranes depend on their molecular composition. In regard to this, charged glycosphingolipids play an outstanding role in the functional adaptation of membranes to different temperatures. 2. In order to shed some light on the respective functional properties of complex membraneous glycosphingolipids, the effects of altered temperatures (5-40 degrees C) on planar lipid bilayers made from diphytanoylphosphatidylcholine (DPPC) and alamethicin as an ion channel was analyzed in the presence of either a sialoglycosphingolipid (less polar disialoganglioside GD1a or highly polar tetrasialoganglioside GQ1b) or phosphatidylserine (PS; as control). 3. Different to the control bilayers made from DPPC or DPPC + PS, the bilayers containing gangliosides had specific maxima in alamethicin conductance and stabile life times. Changes in pore-state conductances indicate structural effects based on an interaction of the large (negatively charged) ganglioside headgroups with the alamethicin pores. 4. The results concerning open time and closed time of channels seem to be based on the gangliosides changing the viscosity of the bilayer and possibly introducing phase transitions. 5. Thus, the findings suggest that gangliosides (1) directly affect channel molecules via their headgroups and (2) may additionally affect the fluidity of membranes in order to maintain membrane homeoviscosity in areas surrounding ion channels independent from the environmental temperature. 6. The effects of gangliosides may be of special interest in describing the ability of neuronal adaptation of vertebrates to temperature and more general regarding the functional adaptation of neurons.

Pharmacology and biophysical properties of alpha 7 and alpha 7-alpha 8 alpha-bungarotoxin receptor subtypes immunopurified from the chick optic lobe

Two chick optic lobe alpha-bungarotoxin receptor subtypes (alpha 7 and alpha 7-alpha 8) were immunopurified using polyclonal antibodies raised against synthetic peptides of chick alpha 7 and alpha 8 alpha-bungarotoxin receptor subunits. The alpha 7 subtype contained the M(r) 57,000 alpha 7 subunit, and represented 60-70% of the alpha-bungarotoxin receptors; the alpha 7-alpha 8 subtype contained the M(r) 57,000 alpha 7 and alpha 8 subunits, and represented only 20-25% of the receptors. Both subtypes also had an additional M(r) 52,000 subunit. The affinity of these subtypes for alpha-bungarotoxin as well as antagonists was similar. However, the alpha 7-alpha 8 subtype displayed consistently higher affinities for agonists. When reconstituted in planar lipid bilayers, the alpha 7-alpha 8 subtype displayed several conductance states of 10-50 pS; the alpha 7 subtype had only one conductance state of 45 pS. The alpha 7-alpha 8 subtype was activated by lower agonist concentrations than the alpha 7 subtype. When expressed in Xenopus oocytes, the alpha 8 subunit formed functional homomeric receptors that desensitized rapidly. These channels were blocked by alpha-bungarotoxin and displayed a higher affinity for agonists than the alpha 7 homomeric receptor. Taken together, these data indicate that at least two alpha-bungarotoxin subtypes are present in the chick optic lobe. They operate as ligand-gated channels and display different agonist sensitivities and kinetics/conductance properties.

Isolation of a potassium-selective ion channel from the plasma membrane of the broad bean Vicia faba L

Potassium ions are essential for plant nutrition and play an important role in osmoregulation and plant movement. Recent patch clamp investigations and molecular analysis have identified voltage-dependent potassium channels responsible for translocation of this ion across plant membranes. In order to characterize these transporters on the biochemical level, a potassium channel protein was isolated from Vicia faba under preservation of its functional integrity. Potassium channel activity was monitored by tracer flux and black lipid bilayer measurements. The capability for K+ transport copurified with a 67 kDa protein from V. faba plasma membranes. A monoclonal antibody directed against the 67 kDa protein blocked channel activity. Single channels with a unit conductance of 40 pS were highly selective for K+ over Na+.

Inhibition of chloroplast ATPase by the K+ channel blocker alpha-dendrotoxin

Possible structural and functional similarities between the channel part, CF0, of chloroplast ATPase (CF0CF1) and ion channels permeable to monovalent cations were investigated using high-affinity toxins mainly targeted against voltage-sensitive K+ channels. In particular, the effect of the K(+)-channel blocker alpha-dendrotoxin and the crude scorpion venom of Leiurus quinquestriatus hebraeus (LQ venom) on ATP synthesis in thylakoid membranes and in CF0CF1-containing liposomes was characterised. Alpha-dendrotoxin (K(i) approximately 5.05 microM) and the LQ venom (K(i) approximately 1.55 micrograms/ml) specifically inhibited ATP synthesis in thylakoid membranes and in CF0CF1-containing liposomes. Our results show that alpha-dendrotoxin and peptides of the LQ venom with an apparent molecular mass of about 4.0 kDa, probably isoforms of charybdotoxin, specifically bind to CF0CF1. This binding was reversible and induced a high leak conductance for H+ through CF0. The Ca(2+)-dependent ATPase activity of the isolated soluble part of CF0CF1 (CF1) was completely inhibited by 1 microM alpha-dendrotoxin, while the crude LQ venom, at concentrations up to 10 micrograms/ml, had no affect on ATPase activity. The concentration dependence of the inhibition by alpha-dendrotoxin indicates that approximately 2 mol alpha-dendrotoxin bind/mol CF0CF1 and 1 mol alpha-dendrotoxin/mol CF1. Known inhibitors of H(+)-flow-through CF0 acted in the presence of alpha-dendrotoxin synergistically. Dicyclohexylcarbodiimide and venturicidin, in contrast to their known effect of blocking H(+)-flow-through CF0, increased the leak conductance through CF0 in the presence of alpha-dendrotoxin drastically. This uncoupling effect indicates that their normal mode of blocking is a secondary effect. Binding of the inhibitors to their respective sites apparently does not affect the proton pathway in CF0, but induces a conformation which closes the channel part for H+. Protein sequence comparison between the known binding site of charybdotoxin in the shaker K+ channel from Drosophila [MacKinnon, R. & Heginbotham, L. (1990) Neuron 5, 767-771] and the choroplast ATPase showed that subunit III reveals a significant similarity (64%) in parts of its sequence (Gln28-Leu53) to the helix 5 and helix 6 (S5-S6) linker region (Ala413-Cys462; the charybdotoxin-binding site) of the shaker K+ channel. According to secondary-structure predictions, the homologous sequences in subunit III and the shaker K+ channel represent putative hydrophilic loops connecting two transmembrane alpha-helices. Apparently the shaker K+ channel and subunit III share significant topological features in these hydrophilic loops which may be part of the respective channel entrance.

A functional α-bungarotoxin receptor is present in chick cerebellum: Purification and characterization

It has recently been demonstrated that alpha-bungarotoxin receptors, which behave as functional nicotinic receptors, are present in chick CNS. In this paper, we report the purification and characterization of a functional alpha-bungarotoxin receptor from chick cerebellum, a nervous tissue in which a clear inhibition of induced nicotine effects has been reported in vivo. This receptor contains at least three subunits of apparent mol. wt 52,000, 57,000 and 67,000. The use of monoclonal antibodies specific for the alpha 7 subunit demonstrated that 75% of the molecules present in our purified preparation belong to the alpha 7 subtype and that this antibody labels the 57,000 band in western blot, thus indicating that this is the toxin binding subunit. Reconstruction experiments in planar lipid bilayers show that this alpha-bungarotoxin receptor forms a cation selective channel whose opening is blocked by d-tubocurarine. Binding experiments on immobilized receptors over an alpha-bungarotoxin-Sepharose affinity column show that the ligand binding subunit is present in vivo in two copies per receptor. Immunological, pharmacological and functional experiments show that this purified receptor is very similar, but not identical, to the previously characterized chick optic lobe receptor, thus indicating the heterogeneity of these alpha-bungarotoxin receptors in the CNS.

Neuronal acetylcholine receptor channels from insects: a comparative electrophysiological study

The channel properties of nicotinic acetylcholine receptor subtypes in the nervous system of insects (Locusta migratoria) have been characterized. Single channel measurements were performed using patch-clamp techniques as well as planar lipid bilayer reconstitution approaches. In reconstitution experiments using receptor-preparations isolated from neuronal membranes by alpha-toxin affinity chromatography, a ligand-gated channel type was found, which showed a high conductance and a short mean lifetime. Patch-clamp experiments on synapse-free somata of isolated nerve cells revealed an acetylcholine-gated channel type with a lower conductance but a longer lifetime. The two different agonist-activated channel types are supposed to represent synaptic and extrasynaptic acetylcholine receptors.

Pore-forming protein from Entamoeba histolytica forms voltage- and pH-controlled multi-state channels with properties similar to those of the barrel-stave aggregates

Pore-forming protein from Entamoeba histolytica forms cation-selective channels in planar bilayers. With increasing potentials, the open-state probability of these channels decreases, and channel aggregates collapse (Young, J.D.-E. and Cohn, Z.A. (1985) J. Cell. Biochem. 29, 299-308). In this communication we report the following observations: (i) incorporation of the pore in black-lipid membranes was stimulated by membrane potential, (ii) pores were rectifying, (iii) breakdown of pores resulted in a continuous spectrum of subconductance states, (iv) the open-state probability increased strongly with pH. This pattern of behaviour is similar to that of the barrel-stave aggregates (alamethicin and related toxins). We therefore conclude that the amebal pores, like those of the barrel-stave class, may consist of complexes involving variable numbers of membrane-spanning subunits.

Reconstitution of acetylcholine receptors into planar lipid bilayers

Obviously, bilayer reconstitution experiments have largely contributed to the understanding of the AChR-channel function. Nevertheless, at present there are many unanswered questions concerning the minimum structural requirements for AChR-channel function, agonist cooperativity, and different types of AChR. Another complex of parameters important for receptor function which must be explored in much more detail, is the dependence of AChR-channel function on membrane composition and its physical state. This important but rather neglected field is predestined to be explored by reconstitution techniques. All the results on AChRs reconstituted in planar lipid bilayers cannot adequately be discussed without the data obtained by other techniques, thus coming back to the statements already mentioned in the introductory section about strategies for investigating ion channels in general. Only such a strategy can lead to a molecular understanding of channel function.

cGMP-dependent channel protein from photoreceptor membranes: single-channel activity of the purified and reconstituted protein

The cGMP-dependent channel protein has been purified from bovine rod photoreceptor membranes and incorporated into planar lipid membranes. At low divalent cation concentrations, cGMP stimulated single-channel current fluctuations. The probability Po of the channel being open strongly depended on the cGMP concentration (EC50 = 31 microM; Hill coefficient, n = 2.3); whereas the single-channel conductance (lambda = 26 pS) was independent of the agonist concentration. The agonist-stimulated increase in the probability of an open channel was largely due to shorter closed times and, to a lesser extent, due to the channel staying open for a longer time. The current-voltage relationship of the single open channel deviated from ohmic behavior, and the open probability decreased at more negative membrane potentials. The rectification of the macroscopic cGMP-induced current in artificial bilayers that contained many channel copies can be accounted for by the voltage dependence of channel gating together with the nonlinearity of the current-voltage curve of an open channel. Current fluctuations exhibited a variety of sublevels, indicating that the channel may exist in more than one conductive state.

Identification, purification, and functional reconstitution of the cyclic GMP-dependent channel from rod photoreceptors

The cyclic GMP-dependent cation channel from bovine rod outer segments has been purified to greater than 90% homogeneity by a rapid two-step chromatographic procedure. The purified channel has an apparent molecular mass of 63 kDa as determined by NaDodSO4/gel electrophoresis. When incorporated into the membrane of liposomes, the purified protein mediates the cyclic GMP-dependent efflux of entrapped Ca2+. The reconstituted channel protein exhibits properties similar to the cyclic GMP-dependent channel observed in excised patches of the plasma membrane and in disk membranes. Cyclic GMP activated the channel cooperatively (Hill coefficient n = 3.1) with an apparent Michaelis constant of approximately 11 microM. After reconstitution of the purified protein into a planar lipid bilayer, we recorded cyclic GMP-stimulated single-channel activity. The single-channel conductance at physiological salt concentrations and in the absence of divalent cations was 26 pS. The drug l-cis-diltiazem, shown to block the cyclic GMP-dependent channel in excised patches of the plasma membrane and in isolated disks of rod outer segments, was ineffective against the purified channel.