Ion Binding to KcsA: Implications in Ion Selectivity and Channel Gating

ROMK channels are regulated by internal pH (pHi) and extracellular K+ (K+o). The mechanisms underlying this regulation were studied in these channels after expression in Xenopus oocytes. Replacement of the COOH-terminal portion of ROMK2 (Kir1.1b) with the corresponding region of the pH-insensitive channel IRK1 (Kir 2.1) produced a chimeric channel (termed C13) with enhanced sensitivity to inhibition by intracellular H+, increasing the apparent pKa for inhibition by ∼0.9 pH units. Three amino acid substitutions at the COOH-terminal end of the second transmembrane helix (I159V, L160M, and I163M) accounted for these effects. These substitutions also made the channels more sensitive to reduction in K+o, consistent with coupling between the responses to pHi and K+o. The ion selectivity sequence of the activation of the channel by cations was K+ ≅ Rb+ > NH4+ >> Na+, similar to that for ion permeability, suggesting an interaction with the selectivity filter. We tested a model of coupling in which a pH-sensitive gate can close the pore from the inside, preventing access of K+ from the cytoplasm and increasing sensitivity of the selectivity filter to removal of K+o. We mimicked closure of this gate using positive membrane potentials to elicit block by intracellular cations. With K+o between 10 and 110 mM, this resulted in a slow, reversible decrease in conductance. However, additional channel constructs, in which inward rectification was maintained but the pH sensor was abolished, failed to respond to voltage under the same conditions. This indicates that blocking access of intracellular K+ to the selectivity filter cannot account for coupling. The C13 chimera was 10 times more sensitive to extracellular Ba2+ block than was ROMK2, indicating that changes in the COOH terminus affect ion binding to the outer part of the pore. This effect correlated with the sensitivity to inactivation by H+. We conclude that decreasing pHI increases the sensitivity of ROMK2 channels to K+o by altering the properties of the selectivity filter.

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Temperature dependence of ion permeation at the endplate channel.

The Journal of General Physiology ◽

10.1085/jgp.81.5.687 ◽

1983 ◽

Vol 81 (5) ◽

pp. 687-703 ◽

Cited By ~ 18

Author(s):

H M Hoffmann ◽

V E Dionne

Keyword(s):

Free Energy ◽

Single Channel ◽

Ion Selectivity ◽

Boltzmann Distribution ◽

Type Model ◽

Fluctuation Analysis ◽

Ion Binding ◽

Ion Permeation ◽

Channel Conductance ◽

Extracellular Side

The dependence of acetylcholine receptor mean single-channel conductance on temperature was studied at garter snake twitch-muscle endplates using fluctuation analysis. In normal saline under conditions where most of the endplate current was carried by Na+, the channel conductance increased continuously from near 0 degrees C to approximately 23 degrees C with a Q10 of 1.97 +/- 0.14 (mean +/- SD). When 50% of the bath Na+ was replaced by either Li+, Rb+, or Cs+, the Q10 did not change significantly; however, at any temperature the channel conductance was greatest in Cs-saline and decreased with the ion sequence Cs greater than Rb greater than Na greater than Li. The results were fit by an Eyring-type model consisting of one free-energy well on the extracellular side of a single energy barrier. Ion selectivity appeared to result from ion-specific differences in the well and not in the barrier of this model. With a constant barrier enthalpy for different ions, well free-energy depth was greatest for Cs+ and graded identical to the permeability sequence. The correlation between increased well depth (i.e., ion binding) and increased channel conductance can be accounted for by the Boltzmann distribution of thermal energy.

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K2P channel C-type gating involves asymmetric selectivity filter order-disorder transitions

10.1101/2020.03.20.000893 ◽

2020 ◽

Cited By ~ 3

Author(s):

Marco Lolicato ◽

Andrew M. Natale ◽

Fayal Abderemane-Ali ◽

David Crottès ◽

Sara Capponi ◽

...

Keyword(s):

Conformational Changes ◽

Ion Selectivity ◽

Transmembrane Helix ◽

Selectivity Filter ◽

Ion Binding ◽

Anomalous Scattering ◽

X Ray Crystallography ◽

Functional Studies ◽

Ion Binding Sites ◽

Physical And Chemical

K2P channels regulate nervous, cardiovascular, and immune system functions1,2 through the action of their selectivity filter (C-type) gate3-6. Although structural studies show K2P conformations that impact activity7-13, no selectivity filter conformational changes have been observed. Here, combining K2P2.1 (TREK-1) X-ray crystallography in different potassium concentrations, potassium anomalous scattering, molecular dynamics, and functional studies, we uncover the unprecedented, asymmetric, potassium-dependent conformational changes underlying K2P C-type gating. Low potassium concentrations evoke conformational changes in selectivity filter strand 1 (SF1), selectivity filter strand 2 (SF2), and the SF2-transmembrane helix 4 loop (SF2-M4 loop) that destroy the S1 and S2 ion binding sites and are suppressed by C-type gate activator ML335. Shortening the uniquely long SF2-M4 loop to match the canonical length found in other potassium channels or disrupting the conserved Glu234 hydrogen bond network supporting this loop blunts C-type gate response to various physical and chemical stimuli. Glu234 network destabilization also compromises ion selectivity, but can be reversed by channel activation, indicating that the ion binding site loss reduces selectivity similar to other channels14. Together, our data establish that C-type gating occurs through potassium-dependent order-disorder transitions in the selectivity filter and adjacent loops that respond to gating cues relayed through the SF2-M4 loop. These findings underscore the potential for targeting the SF2-M4 loop for the development of new, selective K2P channel modulators.

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Free energy calculations suggest a mechanism for Na+/K+-ATPase ion selectivity

10.1101/106724 ◽

2017 ◽

Author(s):

Asghar M. Razavi ◽

Lucie Delemotte ◽

Joshua R. Berlin ◽

Vincenzo Carnevale ◽

Vincent A. Voelz

Keyword(s):

Mechanistic Model ◽

Ion Selectivity ◽

Chloride Ion ◽

Conformational Transitions ◽

Free Energy Calculations ◽

Ion Binding ◽

Transmembrane Helices ◽

Protonation State ◽

Extracellular Milieu ◽

Binding Stoichiometry

AbstractNa+/K+-ATPase transports Na+and K+ions across the cell membrane via an ion binding site made alternatively accessible to the intra- and extracellular milieu by conformational transitions that confer marked changes in ion binding stoichiometry and selectivity. To probe the mechanism of these changes, we used molecular simulation approaches to identify the protonation state of Na+and K+coordinating residues in E1P and E2P conformations. Further analysis of these simulations revealed a novel molecular mechanism responsible for the change in protonation state: the conformation-dependent binding of an anion (a chloride ion in our simulations) to a previously unrecognized cytoplasmic site in the loop between transmembrane helices 8 and 9, which influences the electrostatic potential of the crucial Na+-coordinating residue D926. This mechanistic model is consistent with experimental observations and provides a molecular-level picture of how E1P to E2P enzyme conformational transitions are coupled to changes in ion binding stoichiometry and selectivity.

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Cooperation of Hydrophobic Gating, Knock-on Effect, and Ion Binding Determines Ion Selectivity in the p7 Channel

The Journal of Physical Chemistry B ◽

10.1021/acs.jpcb.6b00684 ◽

2016 ◽

Vol 120 (19) ◽

pp. 4351-4356 ◽

Cited By ~ 4

Author(s):

Siladitya Padhi ◽

U. Deva Priyakumar

Keyword(s):

Ion Selectivity ◽

Ion Binding ◽

P7 Channel

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Gating mechanisms of a natural anion channelrhodopsin

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1513602112 ◽

2015 ◽

Vol 112 (46) ◽

pp. 14236-14241 ◽

Cited By ~ 36

Author(s):

Oleg A. Sineshchekov ◽

Elena G. Govorunova ◽

Hai Li ◽

John L. Spudich

Keyword(s):

Amino Acid ◽

Positive Charge ◽

Ion Selectivity ◽

Channel Gating ◽

Structural Basis ◽

Amino Acid Residues ◽

Gating Mechanisms ◽

Anion Conductance ◽

Guillardia Theta ◽

Sensitive Group

Anion channelrhodopsins (ACRs) are a class of light-gated channels recently identified in cryptophyte algae that provide unprecedented fast and powerful hyperpolarizing tools for optogenetics. Analysis of photocurrents generated byGuillardia thetaACR 1 (GtACR1) and its mutants in response to laser flashes showed thatGtACR1 gating comprises two separate mechanisms with opposite dependencies on the membrane voltage and pH and involving different amino acid residues. The first mechanism, characterized by slow opening and fast closing of the channel, is regulated by Glu-68. Neutralization of this residue (the E68Q mutation) specifically suppressed this first mechanism, but did not eliminate it completely at high pH. Our data indicate the involvement of another, yet-unidentified pH-sensitive group X. Introducing a positive charge at the Glu-68 site (the E68R mutation) inverted the channel gating so that it was open in the dark and closed in the light, without altering its ion selectivity. The second mechanism, characterized by fast opening and slow closing of the channel, was not substantially affected by the E68Q mutation, but was controlled by Cys-102. The C102A mutation reduced the rate of channel closing by the second mechanism by ∼100-fold, whereas it had only a twofold effect on the rate of the first. The results show that anion conductance by ACRs has a fundamentally different structural basis than the relatively well studied conductance by cation channelrhodopsins (CCRs), not attributable to simply a modification of the CCR selectivity filter.

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Permeation and interaction of monovalent cations with the cGMP-gated channel of cone photoreceptors.

The Journal of General Physiology ◽

10.1085/jgp.100.4.647 ◽

1992 ◽

Vol 100 (4) ◽

pp. 647-673 ◽

Cited By ~ 83

Author(s):

A Picones ◽

J I Korenbrot

Keyword(s):

Binding Site ◽

Ion Selectivity ◽

Membrane Surface ◽

Outward Current ◽

Rate Theory ◽

Ion Binding ◽

Membrane Thickness ◽

Monovalent Cations ◽

Cone Photoreceptors ◽

Ionic Solutions

We measured the ion selectivity of cGMP-dependent currents in detached membrane patches from the outer segment of cone photoreceptors isolated from the retina of striped bass. In inside-out patches excised from either single or twin cones the amplitude of these currents, under symmetric ionic solutions, changed with the concentration of cGMP with a dependence described by a Hill equation with average values, at +80 mV, of Km = 42.6 microM and n = 2.49. In the absence of divalent cations, and under symmetric ionic solutions, the I-V curves of the currents were linear over the range of -80 to +80 mV. The addition of Ca altered the form of the I-V curve to a new function well described by an empirical equation that also describes the I-V curve of the photocurrent measured in intact photoreceptors. The monovalent cation permeability sequence of the cGMP-gated channels in the absence of divalent ions was PK > PNa = PLi = PRb > PCs (1.11 > 1.0 = 0.99 = 0.96 > 0.82). The conductance selectivity sequence at +80 mV was GNa = GK > GRb > GCs > GLi (1.0 = 0.99 > 0.88 > 0.74 > 0.60). The organic cations tetramethylammonium (TMA) and arginine partially blocked the current, but the larger ion, arginine, was permeant, whereas the smaller ion, TMA, was not. The amplitude of the outward current through the channels increased with the concentration of monovalent cations on the cytoplasmic membrane surface, up to a saturating value. The increase was well described by the adsorption isotherm of a single ion binding site within the channel with average binding constants, at +80 mV, of 104 mM for Na and 37.6 mM for Li. By assuming that the ion channel contains a single ion binding site in an energy trough separated from each membrane surface by an energy barrier, and using Eyring rate theory, we simulated I-V curves that fit the experimental data measured under ionic concentration gradients. From this fit we conclude that the binding site interacts with one ion at a time and that the energy barriers are asymmetrically located within the membrane thickness. Comparison of the quantitative features of ion permeation and interaction between the cGMP-gated channels of rod and cone photoreceptors reveals that the ion binding sites are profoundly different in the two types of channels. This molecular difference may be particularly important in explaining the differences in the transduction signal of each receptor type.

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Contribution of Ion Binding Affinity to Ion Selectivity and Permeation in KcsA, a Model Potassium Channel

Biochemistry ◽

10.1021/bi201497n ◽

2012 ◽

Vol 51 (18) ◽

pp. 3891-3900 ◽

Cited By ~ 9

Author(s):

M. L. Renart ◽

E. Montoya ◽

A. M. Fernández ◽

M. L. Molina ◽

J. A. Poveda ◽

...

Keyword(s):

Potassium Channel ◽

Binding Affinity ◽

Ion Selectivity ◽

Ion Binding

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Structure, function, and ion-binding properties of a K+ channel stabilized in the 2,4-ion–bound configuration

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1901888116 ◽

2019 ◽

Vol 116 (34) ◽

pp. 16829-16834 ◽

Cited By ~ 6

Author(s):

Cholpon Tilegenova ◽

D. Marien Cortes ◽

Nermina Jahovic ◽

Emily Hardy ◽

Parameswaran Hariharan ◽

...

Keyword(s):

Single Channel ◽

Ion Selectivity ◽

Streaming Potential ◽

Point Of View ◽

Selectivity Filter ◽

K Channel ◽

Crystallographic Structure ◽

Ion Binding ◽

Coupled Transport ◽

Binding Properties

Here, we present the atomic resolution crystallographic structure, the function, and the ion-binding properties of the KcsA mutants, G77A and G77C, that stabilize the 2,4-ion–bound configuration (i.e., water, K+, water, K+-ion–bound configuration) of the K+ channel’s selectivity filter. A full functional and thermodynamic characterization of the G77A mutant revealed wild-type–like ion selectivity and apparent K+-binding affinity, in addition to showing a lack of C-type inactivation gating and a marked reduction in its single-channel conductance. These structures validate, from a structural point of view, the notion that 2 isoenergetic ion-bound configurations coexist within a K+ channel’s selectivity filter, which fully agrees with the water–K+-ion–coupled transport detected by streaming potential measurements.

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