Dataset Information

Open and Closed Structures of a Barium-Blocked Potassium Channel.

ABSTRACT: Barium (Ba²⁺) is a classic permeant blocker of potassium (K⁺) channels. The "external lock-in effect" in barium block experiments, whereby the binding of external K⁺ impedes the forward translocation of the blocker, provides a powerful avenue to investigate the selectivity of the binding sites along the pore of potassium channels. Barium block experiments show that the external lock-in site is highly selective for K⁺ over Na⁺. Wild-type KcsA was crystallized in low K⁺ conditions, and the crystals were soaked in solutions containing various concentrations of barium. Structural analysis reveals open and closed gate conformations of the KcsA channel. Anomalous diffraction experiments show that Ba²⁺ primarily binds to the innermost site S4 of the selectivity filter of the open-gate conformation and also the site S2, but no binding is detected with the closed-gate conformation. Alchemical free-energy perturbation calculations indicate that the presence of a Ba²⁺ ion in the selectivity filter boosts the specificity of K⁺ binding relative to Na⁺ in the external sites S0-S2.

SUBMITTER: Rohaim A

PROVIDER: S-EPMC7453360 | biostudies-literature | 2020 Aug

REPOSITORIES: biostudies-literature

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Publications

Open and Closed Structures of a Barium-Blocked Potassium Channel.

Rohaim Ahmed A Gong LiDong L Li Jing J Rui Huan H Blachowicz Lydia L Roux Benoît B

Journal of molecular biology 20200629 17

Barium (Ba2+) is a classic permeant blocker of potassium (K+) channels. The "external lock-in effect" in barium block experiments, whereby the binding of external K+ impedes the forward translocation of the blocker, provides a powerful avenue to investigate the selectivity of the binding sites along the pore of potassium channels. Barium block experiments show that the external lock-in site is highly selective for K+ over Na+. Wild-type KcsA ...[more]

PMID: 32615129

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Project description:Blockade of large-conductance Ca(2+)-activated K(+) (BK) channels by the bulky quaternary ammonium compound, N-(4-[benzoyl]benzyl)-N,N,N-tributylammonium (bbTBA), exhibits features consistent with blockade of both closed and open states. Here, we examine block of closed BK channels by bbTBA and how it may differ from block of open channels. Although our observations generally confirm earlier results, we describe three observations that are inconsistent with a model in which closed and open channels are equally accessible to blockade by bbTBA. First, block by bbTBA exhibits Ca(2+)-dependent features that are inconsistent with strictly state-independent block. Second, the steady-state voltage dependence of bbTBA block at negative potentials shows that any block of completely closed states either does not occur or is completely voltage independent. Third, determination of the fractional unblock by bbTBA at either low or high Ca(2+) reveals deviations from a model in which open- and closed-state block is identical. The results support the view that bbTBA blockade of fully closed channels does not occur. We imagine two general types of explanation. First, a stronger voltage dependence of closed-channel block may minimize the contribution of closed-channel block at negative potentials. Second, voltage-dependent conformational changes among closed-channel states may permit block by bbTBA. The analysis supports the latter view, suggesting that bbTBA blockade of fully closed channels does not occur, but the ability of bbTBA to block a closed channel requires movement of one or more voltage sensors. Models in which block is coupled to voltage sensor movement can qualitatively account for (1) the ability of open-channel block to better fit block of conductance-voltage curves at high Ca(2+); (2) the voltage dependence of fractional availability; and (3) the fractional unblock at different open probabilities. BK channels appear to undergo voltage-dependent conformational changes among closed states that are permissive for bbTBA block.

Dataset Information

Open and Closed Structures of a Barium-Blocked Potassium Channel.

Publications

Open and Closed Structures of a Barium-Blocked Potassium Channel.

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