Conversation between Voltage Sensors and Gates of Ion Channels

Richard Horn

doi:10.1021/bi0020473

Structural dynamics determine voltage and pH gating in human voltage-gated proton channel

10.1101/2021.08.25.457625 ◽

2021 ◽

Author(s):

Shuo Han ◽

Sophia Peng ◽

Joshua Vance ◽

Kimberly Tran ◽

Nhu Do ◽

...

Keyword(s):

Ion Channels ◽

Real Time ◽

Conformational Dynamics ◽

Conformational Transitions ◽

Extracellular Ph ◽

Voltage Sensors ◽

Voltage Gated ◽

Voltage Gated Ion Channels ◽

S4 Segment ◽

Gated Ion Channels

AbstractVoltage-gated ion channels are key players of electrical signaling in cells. As a unique subfamily, voltage-gated proton (Hv) channels are standalone voltage sensors without separate ion conductive pores. They are gated by both voltage and transmembrane proton gradient (i.e ΔpH), serving as acid extruders in most cells. Amongst their many functions, Hv channels are known to regulate the intracellular pH of human spermatozoa and compensate for the charge and pH imbalances caused by NADPH oxidases in phagocytes. Like the canonical voltage sensors, the Hv channel is a bundle of 4 helices (named S1 through S4), with the S4 segment carrying 3 positively charged Arg residues. Extensive structural and electrophysiological studies on voltage-gated ion channels generally agree on an outwards movement of the S4 segment upon activating voltage, but the real time conformational transitions are still unattainable. With purified human voltage-gated proton (hHv1) channel reconstituted in liposomes, we have examined its conformational dynamics at different voltage and pHs using the single molecule fluorescence resonance energy transfer (smFRET). Here we provided the first glimpse of real time conformational trajectories of the hHv1 voltage sensor and showed that both voltage and pH gradient shift the conformational dynamics of the S4 segment to control channel gating. Our results suggested the biological gating is determined by the conformational distributions of the hHv1 voltage sensor, rather than the conformational transitions between the presumptive ‘resting’ and ‘activated’ conformations. We further identified H140 as the key residue sensing extracellular pH and showed that both the intracellular and extracellular pH sensors act on the voltage sensing S4 segment to enrich the resting conformations. Taken together, we proposed a model that explains the mechanisms underlying voltage and pH gating in Hv channels, which may also serve as a general framework to understand the voltage sensing and gating in other voltage-gated ion channels.

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Immobilizing the Moving Parts of Voltage-Gated Ion Channels

The Journal of General Physiology ◽

10.1085/jgp.116.3.461 ◽

2000 ◽

Vol 116 (3) ◽

pp. 461-476 ◽

Cited By ~ 97

Author(s):

Richard Horn ◽

Shinghua Ding ◽

Hermann J. Gruber

Keyword(s):

Ion Channels ◽

Potassium Channels ◽

Sodium Channel ◽

Ultraviolet Irradiation ◽

Voltage Sensors ◽

Voltage Gated ◽

Voltage Gated Ion Channels ◽

S4 Segment ◽

Moving Parts ◽

Gated Ion Channels

Voltage-gated ion channels have at least two classes of moving parts, voltage sensors that respond to changes in the transmembrane potential and gates that create or deny permeant ions access to the conduction pathway. To explore the coupling between voltage sensors and gates, we have systematically immobilized each using a bifunctional photoactivatable cross-linker, benzophenone-4-carboxamidocysteine methanethiosulfonate, that can be tethered to cysteines introduced into the channel protein by mutagenesis. To validate the method, we first tested it on the inactivation gate of the sodium channel. The benzophenone-labeled inactivation gate of the sodium channel can be trapped selectively either in an open or closed state by ultraviolet irradiation at either a hyperpolarized or depolarized voltage, respectively. To verify that ultraviolet light can immobilize S4 segments, we examined its relative effects on ionic and gating currents in Shaker potassium channels, labeled at residue 359 at the extracellular end of the S4 segment. As predicted by the tetrameric stoichiometry of these potassium channels, ultraviolet irradiation reduces ionic current by approximately the fourth power of the gating current reduction, suggesting little cooperativity between the movements of individual S4 segments. Photocross-linking occurs preferably at hyperpolarized voltages after labeling residue 359, suggesting that depolarization moves the benzophenone adduct out of a restricted environment. Immobilization of the S4 segment of the second domain of sodium channels prevents channels from opening. By contrast, photocross-linking the S4 segment of the fourth domain of the sodium channel has effects on both activation and inactivation. Our results indicate that specific voltage sensors of the sodium channel play unique roles in gating, and suggest that movement of one voltage sensor, the S4 segment of domain 4, is at least a two-step process, each step coupled to a different gate.

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Slow Photo-Cross-Linking Kinetics of Benzophenone-Labeled Voltage Sensors of Ion Channels†

Biochemistry ◽

10.1021/bi010709y ◽

2001 ◽

Vol 40 (35) ◽

pp. 10707-10716 ◽

Cited By ~ 7

Author(s):

Shinghua Ding ◽

Richard Horn

Keyword(s):

Ion Channels ◽

Cross Linking ◽

Voltage Sensors ◽

Kinetics Of

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Cryo-EM structure of the KvAP channel reveals a non-domain-swapped voltage sensor topology

eLife ◽

10.7554/elife.52164 ◽

2019 ◽

Vol 8 ◽

Cited By ~ 5

Author(s):

Xiao Tao ◽

Roderick MacKinnon

Keyword(s):

Ion Channels ◽

Conformational Changes ◽

Voltage Sensor ◽

Structural Domains ◽

Structural Class ◽

Voltage Sensors ◽

Voltage Gated ◽

Voltage Dependent ◽

Voltage Gated Ion Channels ◽

Gated Ion Channels

Conductance in voltage-gated ion channels is regulated by membrane voltage through structural domains known as voltage sensors. A single structural class of voltage sensor domain exists, but two different modes of voltage sensor attachment to the pore occur in nature: domain-swapped and non-domain-swapped. Since the more thoroughly studied Kv1-7, Nav and Cav channels have domain-swapped voltage sensors, much less is known about non-domain-swapped voltage-gated ion channels. In this paper, using cryo-EM, we show that KvAP from Aeropyrum pernix has non-domain-swapped voltage sensors as well as other unusual features. The new structure, together with previous functional data, suggests that KvAP and the Shaker channel, to which KvAP is most often compared, probably undergo rather different voltage-dependent conformational changes when they open.

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Ion channel depolarization increases repulsions between positive S4 charges to drive activation

10.1101/691881 ◽

2019 ◽

Author(s):

H. R. Leuchtag

Keyword(s):

Amino Acids ◽

Ion Channels ◽

Ion Channel ◽

Nerve Impulse ◽

Electrostatic Repulsion ◽

Channel Activation ◽

Voltage Sensors ◽

Repulsion Energy ◽

Repulsive Forces ◽

The Way

AbstractThe positively charged residues, arginine and lysine, of the S4 segments of voltage-sensitive ion channels repel each other with Coulomb forces inversely proportional to the mean channel dielectric permittivity ε. Dipole moments induced at rest potential in the branched sidechains of leucine, isoleucine and valine lend high values of ε to the channel. High ε keeps electrostatic forces small at rest, leaving the channel in a compact conformation closed to ion conduction. On membrane depolarization beyond threshold, the repulsive forces between positive S4 charges increase greatly on a sharp decrease in ε due to the collapse of induced dipoles, causing an expansion of the S4 segments, which drives the channel into activation. Model calculations based on α helical S4 geometry, neglecting the small number of negative charges, provide estimates of electrostatic energy for different values of open-channel ε and numbers of positive S4 charges. When theShakerK+channel is depolarized, the repulsion energy in each S4 segment increases from about 0.2 kcal/mol to about 120 kJ/mol (30 kcal/mol). The S4 expansions lengthen and widen the pore domain, expanding the hydrogen bonds of its α helices, thus providing sites for permeant ions. Ion percolation via these sites produces the stochastic ion currents observed in activated channels. The model proposed, Channel Activation by Electrostatic Repulsion (CAbER), explains observed features of voltage-sensitive channel behavior and offers predictions that can be tested by experiment.SIGNIFICANCE STATEMENTScience walks on two legs, experiment and theory. Experiment provides the facts that theory seeks to explain; the predictions of a theoretical model are then tested in the laboratory.Rigid adherence to an inadequate model can lead to stagnation of a field.The way in which a protein molecule straddling a lipid membrane in a nerve or muscle fiber responds to a voltage change by allowing certain ions to cross it is currently modeled by simple devices such as gated pores, screws and paddles. Since molecules and everyday objects are worlds apart, these devices don’t provide productive models of the way a voltage-sensitive ion channel is activated when the voltage across the resting membrane is eliminated in a nerve impulse. A change of paradigm is needed.Like all matter, ion channels obey the laws of physics. One such law says that positive charges repel other positive charges. Since each of these ion channels has four “voltage sensors” studded with positive charges, they store repulsion energy in a membrane poised to conduct an impulse. To see how that stored energy is released in activation, we must turn to condensed-state physics. Recent advances in materials called ferroelectric liquid crystals, with structures resembling those of voltage-sensitive ion channels, provide a bridge between physics and biology. This bridge leads to a new model, Channel Activation by Electrostatic Repulsion,Three amino acids scattered throughout the molecules have side chains split at their ends, which makes them highly sensitive to changing electric fields. The calculations that form the core of this report examine the effect of these branched-chain amino acids on the repulsions between the positive charges in the voltage sensors. The numbers tell us that the voltage sensors expand on activation, popping the ion channel into a porous structure through which specific ions are able to cross the membrane and so carry the nerve impulse along.This model may someday enable us to learn more about diseases caused by mutations in voltage-sensitive ion channels. But for now, the ball is in the court of the experimentalists to test whether the predictions of this model are confirmed in the laboratory.

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Tarantula Toxins Interact with Voltage Sensors within Lipid Membranes

The Journal of General Physiology ◽

10.1085/jgp.200709869 ◽

2007 ◽

Vol 130 (5) ◽

pp. 497-511 ◽

Cited By ~ 90

Author(s):

Mirela Milescu ◽

Jan Vobecky ◽

Soung H. Roh ◽

Sung H. Kim ◽

Hoi J. Jung ◽

...

Keyword(s):

Ion Channels ◽

Lipid Membranes ◽

Voltage Sensor ◽

Structural Motif ◽

Membrane Interaction ◽

Voltage Sensors ◽

Kv Channels ◽

Channel Interactions ◽

The Stability ◽

Electrical Signaling

Voltage-activated ion channels are essential for electrical signaling, yet the mechanism of voltage sensing remains under intense investigation. The voltage-sensor paddle is a crucial structural motif in voltage-activated potassium (Kv) channels that has been proposed to move at the protein–lipid interface in response to changes in membrane voltage. Here we explore whether tarantula toxins like hanatoxin and SGTx1 inhibit Kv channels by interacting with paddle motifs within the membrane. We find that these toxins can partition into membranes under physiologically relevant conditions, but that the toxin–membrane interaction is not sufficient to inhibit Kv channels. From mutagenesis studies we identify regions of the toxin involved in binding to the paddle motif, and those important for interacting with membranes. Modification of membranes with sphingomyelinase D dramatically alters the stability of the toxin–channel complex, suggesting that tarantula toxins interact with paddle motifs within the membrane and that they are sensitive detectors of lipid–channel interactions.

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