A charged view of voltage-gated ion channels

Electrical Field Regulation of Ion Transport in Polyethylene Terephthalate Nanochannels

Rectified ion transport in nanochannels is the basis of ion channels in biological cells and has inspired emerging nanochannel applications in ion separation, Coulter counters, and biomolecule detection and nanochannel energy harvesters. In this work we fabricated a polyethylene terephthalate (PET) conical nanochannel using latent ion track etching technique and then systematically studied the ion transport and influence of cation species on the nanochannel surface with cyclic I-V measurement. We discovered the electrical regulation of the reversible and irreversible modification of the nanochannel transportation by bivalent and trivalent cations, revealing the existence of the switching threshold voltage which can control the current rectification in bivalent solution. The proposed mechanism of the transport state transition in the PET nanochannel mimics behaviors of voltage-gated biological ion channels. These findings provide new insight into the understanding of the ion channel signaling and translocation control of charged particles in nanochannel applications.

bSUM: A bead-supported unilamellar membrane system facilitating unidirectional insertion of membrane proteins into giant vesicles

KvAP conjugated to beads via a C-terminal His-tag seeds formation of a supported bilayer with unidirectional channel orientation for functional studies.

Fused or giant vesicles, planar lipid bilayers, a droplet membrane system, and planar-supported membranes have been developed to incorporate membrane proteins for the electrical and biophysical analysis of such proteins or the bilayer properties. However, it remains difficult to incorporate membrane proteins, including ion channels, into reconstituted membrane systems that allow easy control of operational dimensions, incorporation orientation of the membrane proteins, and lipid composition of membranes. Here, using a newly developed chemical engineering procedure, we report on a bead-supported unilamellar membrane (bSUM) system that allows good control over membrane dimension, protein orientation, and lipid composition. Our new system uses specific ligands to facilitate the unidirectional incorporation of membrane proteins into lipid bilayers. Cryo–electron microscopic imaging demonstrates the unilamellar nature of the bSUMs. Electrical recordings from voltage-gated ion channels in bSUMs of varying diameters demonstrate the versatility of the new system. Using KvAP as a model system, we show that compared with other in vitro membrane systems, the bSUMs have the following advantages: (a) a major fraction of channels are orientated in a controlled way; (b) the channels mediate the formation of the lipid bilayer; (c) there is one and only one bilayer membrane on each bead; (d) the lipid composition can be controlled and the bSUM size is also under experimental control over a range of 0.2–20 µm; (e) the channel activity can be recorded by patch clamp using a planar electrode; and (f) the voltage-clamp speed (0.2–0.5 ms) of the bSUM on a planar electrode is fast, making it suitable to study ion channels with fast gating kinetics. Our observations suggest that the chemically engineered bSUMs afford a novel platform for studying lipid–protein interactions in membranes of varying lipid composition and may be useful for other applications, such as targeted delivery and single-molecule imaging.

A model for the hysteresis observed in gating of lysenin channels

The pore-forming toxin lysenin self-inserts to form conductance channels in natural and artificial lipid membranes containing sphingomyelin. The inserted channels exhibit voltage regulation and hysteresis of the macroscopic current during the application of positive periodic voltage stimuli. We explored the bi-stable behavior of lysenin channels and present a theoretical approach for the mechanism of the hysteresis to explain its static and dynamic components. This investigation develops a model to incorporate the role of charge accumulation on the bilayer lipid membrane in influencing the channel conduction state. Our model is supported by experimental results and also provides insight into the temperature dependence of lysenin channel hysteresis. Through this work we gain perspective into the mechanism of how the response of a channel protein is determined by previous stimuli.

Controlled gating of lysenin pores

Lysenin forms unitary large conductance pores in artificial bilayer membranes containing sphingomyelin. A population of lysenin pores inserted into such a bilayer membrane exhibited a dynamic negative conductance region, as predicted by a simple two-state model for voltage-gated channels. The recorded I-V curves demonstrated that lysenin pores inserted into the bilayer are uniformly oriented. Additionally, the transition between the two-states was affected by changes in the monovalent ion concentration and pH, pointing towards an electrostatic interaction governing the gating mechanism.

Novel paradigms on scorpion toxins that affects the activating mechanism of sodium channels

Scorpion toxins classified as beta-class are reviewed using a new paradigm. Four distinct sub types are recognized: "classical", "Tsgamma-like", "excitatory" and "depressant"beta-scorpion toxins. Recent experimental data have made possible to identify the interacting interfaces of the Na(+) channel-receptor site 4 with some of these toxins. The voltage-sensor trapping mechanism proposed for the action of these toxic peptides is analyzed in the context of what causes a modification of the activating mechanism of Na(+) channels. A cartoon model is presented with the purpose of summarizing the most current knowledge on the field. Finally, the recent advances on the knowledge of the specific interactions of beta-toxins and different sub types of Na(+) channels are also reviewed.

NMR-derived dynamic aspects of N-type inactivation of a Kv channel suggest a transient interaction with the T1 domain

Some eukaryotic voltage-gated K+ (Kv) channels contain an N-terminal inactivation peptide (IP), which mediates a fast inactivation process that limits channel function during membrane depolarization and thus shapes the action potential. We obtained sequence-specific nuclear magnetic resonance (NMR) assignments for the polypeptide backbone of a tetrameric N-terminal fragment (amino acids 1-181) of the Aplysia Kv1.1 channel. Additional NMR measurements show that the tetramerization domain 1 (T1) has the same globular structure in solution as previously determined by crystallography and that the IP (residues 1-20) and the linker (residues 21-65) are in a flexibly disordered, predominantly extended conformation. A potential contact site between the T1 domain and the flexible tail (residues 1-65) has been identified on the basis of chemical-shift changes of individual T1 domain amino acids, which map to the T1 surface near the interface between adjacent subunits. Paramagnetic perturbation experiments further indicate that, in the ensemble of solution conformers, there is at least a small population of species with the IP localized in close proximity to the proposed interacting residues of the T1 tetramer. Electrophysiological measurements show that all three mutations in this pocket that we tested slow the rate of inactivation and speed up recovery, as predicted from the preinactivation site model. These results suggest that specific, short-lived transient interactions between the T1 domain and the IP or the linker segment may play a role in defining the regulatory kinetics of fast channel inactivation.

Has the code for protein translocation been broken?

Polypeptides chains are segregated by the translocon channel into secreted or membrane-inserted proteins. Recent reports claim that an in vivo system has been used to break the "amino acid code" used by translocons to make the determination of protein type (i.e. secreted or membrane-inserted). However, the experimental setup used in these studies could have confused the derivation of this code, in particular for polar amino acids. These residues are likely to undergo stabilizing interactions with other protein components in the experiment, shielding them from direct contact with the inhospitable membrane. Hence, it is our view that the "code" for protein translocation has not yet been deciphered and that further experiments are required for teasing apart the various energetic factors contributing to protein translocation.

Models of the structure and voltage-gating mechanism of the shaker K+ channel

In the preceding, accompanying article, we present models of the structure and voltage-dependent gating mechanism of the KvAP bacterial K+ channel that are based on three types of evidence: crystal structures of portions of the KvAP protein, theoretical modeling criteria for membrane proteins, and biophysical studies of the properties of native and mutated voltage-gated channels. Most of the latter experiments were performed on the Shaker K+ channel. Some of these data are difficult to relate directly to models of the KvAP channel's structure due to differences in the Shaker and KvAP sequences. We have dealt with this problem by developing new models of the structure and gating mechanism of the transmembrane and extracellular portions of the Shaker channel. These models are consistent with almost all of the biophysical data. In contrast, much of the experimental data are incompatible with the "paddle" model of gating that was proposed when the KvAP crystal structures were first published. The general folding pattern and gating mechanisms of our current models are similar to some of our earlier models of the Shaker channel.

Regulation of sodium channel function by bilayer elasticity: the importance of hydrophobic coupling. Effects of Micelle-forming amphiphiles and cholesterol

Membrane proteins are regulated by the lipid bilayer composition. Specific lipid–protein interactions rarely are involved, which suggests that the regulation is due to changes in some general bilayer property (or properties). The hydrophobic coupling between a membrane-spanning protein and the surrounding bilayer means that protein conformational changes may be associated with a reversible, local bilayer deformation. Lipid bilayers are elastic bodies, and the energetic cost of the bilayer deformation contributes to the total energetic cost of the protein conformational change. The energetics and kinetics of the protein conformational changes therefore will be regulated by the bilayer elasticity, which is determined by the lipid composition. This hydrophobic coupling mechanism has been studied extensively in gramicidin channels, where the channel–bilayer hydrophobic interactions link a “conformational” change (the monomer↔dimer transition) to an elastic bilayer deformation. Gramicidin channels thus are regulated by the lipid bilayer elastic properties (thickness, monolayer equilibrium curvature, and compression and bending moduli). To investigate whether this hydrophobic coupling mechanism could be a general mechanism regulating membrane protein function, we examined whether voltage-dependent skeletal-muscle sodium channels, expressed in HEK293 cells, are regulated by bilayer elasticity, as monitored using gramicidin A (gA) channels. Nonphysiological amphiphiles (β-octyl-glucoside, Genapol X-100, Triton X-100, and reduced Triton X-100) that make lipid bilayers less “stiff”, as measured using gA channels, shift the voltage dependence of sodium channel inactivation toward more hyperpolarized potentials. At low amphiphile concentration, the magnitude of the shift is linearly correlated to the change in gA channel lifetime. Cholesterol-depletion, which also reduces bilayer stiffness, causes a similar shift in sodium channel inactivation. These results provide strong support for the notion that bilayer–protein hydrophobic coupling allows the bilayer elastic properties to regulate membrane protein function.

An Evolutionarily Conserved Network of Amino Acids Mediates Gating in Voltage-dependent Potassium Channels

A novel sequence-analysis technique for detecting correlated amino acid positions in intermediate-size protein families (50-100 sequences) was developed, and applied to study voltage-dependent gating of potassium channels. Most contemporary methods for detecting amino acid correlations within proteins use very large sets of data, typically comprising hundreds or thousands of evolutionarily related sequences, to overcome the relatively low signal-to-noise ratio in the analysis of co-variations between pairs of amino acid positions. Such methods are impractical for voltage-gated potassium (Kv) channels and for many other protein families that have not yet been sequenced to that extent. Here, we used a phylogenetic reconstruction of paralogous Kv channels to follow the evolutionary history of every pair of amino acid positions within this family, thus increasing detection accuracy of correlated amino acids relative to contemporary methods. In addition, we used a bootstrapping procedure to eliminate correlations that were statistically insignificant. These and other measures allowed us to increase the method's sensitivity, and opened the way to reliable identification of correlated positions even in intermediate-size protein families. Principal-component analysis applied to the set of correlated amino acid positions in Kv channels detected a network of inter-correlated residues, a large fraction of which were identified as gating-sensitive upon mutation. Mapping the network of correlated residues onto the 3D structure of the Kv channel from Aeropyrum pernix disclosed correlations between residues in the voltage-sensor paddle and the pore region, including regions that are involved in the gating transition. We discuss these findings with respect to the evolutionary constraints acting on the channel's various domains. The software is available on our website

Computer simulation of the KvAP voltage-gated potassium channel: steered molecular dynamics of the voltage sensor

The recent crystal structures of the voltage-gated potassium channel KvAP and its isolated voltage-sensing 'paddle' (composed of segments S1-S4) challenge existing models of voltage gating and raise a number of questions about the structure of the physiologically relevant state. We investigate a possible gating mechanism based on the crystal structures in a 10 ns steered molecular dynamics simulation of KvAP in a membrane-mimetic octane layer. The structure of the full KvAP protein has been modified by restraining the S2-S4 domain to the conformation of the isolated high-resolution paddle structure. After an initial relaxation, the paddle tips are pulled through the membrane from the intracellular to the extracellular side, corresponding to a putative change from closed to open. We describe the effect of this large-scale motion on the central pore domain, which remains largely unchanged, on the protein hydrogen-bonding network and on solvent. We analyze the motion of the S3b-S4 portion of the protein and propose a possible coupling mechanism between the paddle motion and the opening of the channel. Interactions between the arginine residues in S4, solvent and chloride ions are likely to play a role in the gating charge.