Ion Channels In Biological Membranes Research Articles

Саморегуляція в іонних каналах біологічних мембран

Саморегуляція в іонних каналах біологічних мембран

Bcl-x(L) forms an ion channel in synthetic lipid membranes.

Bcl-2-related proteins are critical regulators of cell survival that are localized to the outer mitochondrial, outer nuclear and endoplasmic reticulum membranes. Despite their physiological importance, the biochemical function of Bcl-2-related proteins has remained elusive. The three-dimensional structure of Bcl-xL, an inhibitor of apoptosis, was recently shown to be similar to the structures of the pore-forming domains of bacterial toxins. A key feature of these pore-forming domains is the ability to form ion channels in biological membranes. Here we demonstrate that Bcl-xL shares this functional feature. Like the bacterial toxins, Bcl-xL can insert into either synthetic lipid vesicles or planar lipid bilayers and form an ion-conducting channel. This channel is pH-sensitive and becomes cation-selective at physiological pH. The ion-conducting channel(s) formed by Bcl-xL display multiple conductance states that have identical ion selectivity. Together, these data suggest that Bcl-xL may maintain cell survival by regulating the permeability of the intracellular membranes to which it is distributed.

Pathway for Ca 2+ influx into cells by trichosporin-B-VIa, an α-aminoisobutyric acid-containing peptide, from the fungus Trichoderma polysporum

Trichosporin (TS) -B-VIa, a fungal α-aminoisobutyric acid (Aib) -containing peptide consisting of 19 amino acid residues and a phenylalaninol, produced both 45Ca 2+ influx into bovine adrenal chromaffin cells and catecholamine secretion from the cells. The secretion induced by TS-B-VIa at lower concentrations (2–5 μM) was completely dependent on the external Ca 2+, while that induced by TS-B-VIa at higher concentrations (10–30 μM) was partly independent of the Ca 2+. The concentration-response curves (2–5 μM) for the TS-B-VIa-induced Ca 2+ influx and secretion correlated well. The TS-B-VIa (at 5 μM) -induced secretion was not antagonized by diltiazem, a blocker of L-type voltage-sensitive Ca 2+ channels. The treatment of fura-2-loaded C 6 glioma cells with TS-B-VIa (2–5 μM) led to an increase in the intracellular free Ca 2+ concentration ([Ca 2+] i) in a concentration-dependent manner but the stimulatory effects of TS-B-VIa on [Ca 2+]; were only slightly observed in Ca 2+-free medium, indicating that TS-B-VIa causes Ca 21 influx from the external medium into the C 6 cells. The TS-B-VIa-induced increase in [Ca 2+] i in the C 6 cells was not antagonized by diltiazem and by SK&F 96365, a novel blocker of receptor-mediated Ca 2+ entry. High K + increased neither [Ca 2+] i in the C6 cells nor Mn 2+ influx into the cells, while TS-B-VIa increased Mn 2+ influx. Also in other non-excitable cells, bovine platelets, similar results were obtained. These results strongly suggest that the mechanism of Ca 2+ influx by TS-B-VIa at the lower concentrations is distinct from the event of Ca 2+ influx through receptor-operated or L-type voltage-sensitive Ca 2+ channels in both excitable cells (the chromaffin cells) and non-excitable cells (the C 6 cells and the platelets) and that TS-B-VIa per se may form Ca 2+-permeable ion channels in biological membranes. On the other hand, the peptide at the higher concentrations seems to damage cell membranes.

An electrical model for force activated ion channels in biological membranes

An electrical model for force activated ion channels in biological membranes

Radiation-induced and free radical-mediated inactivation of ion channels formed by the polyene antibiotic amphotericin B in lipid membranes: effect of radical scavengers and single-channel analysis.

This paper is part of a study on the effect of ionizing radiation on ion channels in biological membranes. Ion channels formed by polyene antibiotics amphotericin B or nystatin represent clusters of conjugated double bonds. As a consequence of this structural peculiarity, the conductance of lipid membranes--in the presence of polyene channels--has been found to decrease by several orders of magnitude at comparatively small doses of ionizing radiation. The phenomenon shows an inverse dose-rate behaviour similar to that of radiation-induced lipid peroxidation. We report on experiments performed in the presence of various radical scavengers, at varying cholesterol concentrations, and with different lipids. They support the view that channel inactivation is due to free radical-induced peroxidation of the polyenes leading to a destabilization of the barrel-like structure of the ion channels. Radiation-induced channel closing is shown for the first time at the level of single-ion channels.

Monte Carlo simulations of ionic channel selectivity

Monte Carlo simulations have been developed to study the selectivity of ionic channels in biological membranes. The channel is considered to be in either of two possible states: (i) densely packed with ions, the ions moving in single file in one direction, or alternatively, (ii) sparsely packed, where holes could appear at any particular time thereby allowing bidirectional movement of ions. The two models enable us to envisage a quantitative flux of permeable ions in the presence of smaller sized ions, taking into consideration their concentrations in the bulk solutions, the ion-channel interactions and probability with which they fill up the channel. The programs are written in FORTRAN-77 (MS-FORTRAN) for an IBM-compatible personal computer. From the simulation results we observe an enzymatic function of the channel and also note that the smaller sized ions tend to block the movement of permeable ions. The simulations represent a technique for visualization of the factors that decide ionic permeability and help in manipulating their effects with ease and speed which would otherwise involve intricate experimental setups.

Stochastic dynamics and function of proteins. Modelling the membrane ionic channels

The role of stochastic dynamics in protein molecules is considered in order to explain the specific functions of ionic channels in biological membranes: conductivity, selectivity and gating. The events that govern the recognition and transfer of specific ions are very fast and should involve the small-scale dynamics. The essential features of these events may be described on the basis of the Debye model of liquid dielectrics. In contrast, the processes which control the gating (switching the channel from closed to open state) are probably associated with the large-scale fluctuations. Their mechanism is suggested to involve the non-linear interactions between the ion flux and protein conformational modes.

A model of the gating of ion channels

The gating of ion channels in biological membranes has usually been described in termsof Markov transitions between a few discrete open or closed states. Such models predict that the distributions of open and closed durations decay as a sum of exponential terms. Recent experimental data have indicated that certain channels are not easily described by these models. We show that distributions of open and closed times similar to those seen experimentally are predicted by a model that involves only one open and closed state but that assumes the activation energy of the gating process to be stochastic. This model involves only a few parameters and these have direct physical interpretations. Measurements of the correlation between the durations of successive open or closed events is shown to provide an experimental method for distinguishing between this and other models.

Analyzing stochastic events in multi-channel patch clamp data

In an effort to understand the gating properties of ionic channels in biological membranes, an efficient method was developed to estimate the kinetic constants from opening-closing events of the channels. Our method is suitable to single channel patch-clamp recordings that contain several ionic channels functioning simultaneously. It is different from the maximum likelihood method previous developed by Horn and Lange, in that our method is a continuum approach and makes uses of analytic expressions of the probability density functions of the event times. Combinatorial analysis was necessary to correctly include more typical multi-channel recordings. This yields computationally quicker results than the method of Horn and Lange, which uses a discretized time series. Model-dependent portions of the code are minimal and easily modified. To illustrate the goodness of our method, we have generated the open-close processes of the patch-clamp records on a digital computer using the exponential random number generators. For multi-channel patches, we have introduced a few plausible approximations to make our algorithm more efficient. The soundness of the our approximations were tested with such measures as the fraction of the open state at time t, Popen (t), and frequencies of the number of openings per run. A copy of the computer code implementing this algorithm can be obtained from the authors.

Reconstitution of Ion Channel

The first model of the structure of cell membranes was developed very early from indirect evidence.' It postulated the existence of a bimolecular lipid membrane which serves as the main and electrical isolating part of the cell membrane. The proteins were either associated with or incorporated into the membrane.Z Following this idea, it was postulated that the proteins being incorporated into the bimolecular lipid matrix (integral membrane proteins) were the source of ion transport across the electrical isolating lipid membrane.3 Ion channels formed by these proteins were one of the mechanisms being considered to be involved in the ion transport across cell membranes. These ion channel-forming proteins were proposed to be involved in the electrical excitation of biological membranes very early.4 For a long time, however, it was impossible to investigate directly the properties of these channels, i.e., in single channel experiments. Limited by the low current and time resolution of electrophysiological measurements, only multichannel experiments were possible. Due to the fact that multichannel data can be explained by a larger number of single channel models there was a need for detailed single channel data to obtain a more in-depth view of channel function. In addition, all of the single channel parameters calculated from the electrophysiological experiments were scattered over a wide range of values. Nevertheless, from electrophysiological experiments a large number of different ion channels were postulated to be active in biological membranes, and highly specific properties were given to these hypothetical ion channels. However, until 1962 not even the existence of single ion channels could be shown experimentally. Following that line of thought, the first technique for the formation of planar lipid bilayers was established in 1962 by Mueller et al.,' using large amounts of organic solvents to stabilize the artificial membrane. The first single channel fluctuations shown were those of EIM and gramicidin in planar lipid bilayers which were formed according to that t echn iq~e .~ -~ For a period of time, single channel fluctuations being induced by different polypeptide antibiotics and related substameslo in planar bilayers were the only really measured single channel fluctuations. At that time it was not possible to measure current fluctuations of single ion channels in cell membranes directly. It was obvious that the incorporation of channel-forming proteins into planar lipid bilayers would be a useful technique for finding the proof of the existence of single ion channels in biological membranes and for investigating the properties of these hypothetical channels in singleand multichannel experiments. Most of the early reconstitution experiments were not very successful. Problems occurred because these bilayers contained large amounts of organic solvents, techniques had to be developed to incorporate C ri tic al R ev ie w s in B io ch em is tr y an d M ol ec ul ar B io lo gy D ow nl oa de d fr om in fo rm ah ea lth ca re .c om b y 41 .2 20 .2 8. 51 o n 05 /2 0/ 14

International seminar on structure and function of receptor and ion channels in biological membranes

International seminar on structure and function of receptor and ion channels in biological membranes