Ionic Coulomb blockade and the determinants of selectivity in the NaChBac bacterial sodium channel

O.A Fedorenko,I.Kh Kaufman,W.A.T Gibby,M.L Barabash,D.G Luchinsky,S.K Roberts,P.V.E Mcclintock

doi:10.1016/j.bbamem.2020.183301

O.A Fedorenko, I.Kh Kaufman + Show 5 more

Open Access

https://doi.org/10.1016/j.bbamem.2020.183301

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Abstract

Mutation-induced transformations of conductivity and selectivity in NaChBac bacterial channels are studied experimentally and interpreted within the framework of ionic Coulomb blockade (ICB), while also taking account of resonant quantised dehydration (QD) and site protonation. Site-directed mutagenesis and whole-cell patch-clamp experiments are used to investigate how the fixed charge Qf at the selectivity filter (SF) affects both valence selectivity and same-charge selectivity. The new ICB/QD model predicts that increasing ∣Qf∣ should lead to a shift in selectivity sequences toward larger ion sizes, in agreement with the present experiments and with earlier work. Comparison of the model with experimental data leads to the introduction of an effective charge Qf∗ at the SF, which was found to differ between Aspartate and Glutamate charged rings, and also to depend on position within the SF. It is suggested that protonation of the residues within the restricted space of the SF is important in significantly reducing the effective charge of the EEEE ring. Values of Qf∗ derived from experiments on divalent blockade agree well with expectations based on the ICB/QD model and have led to the first demonstration of ICB oscillations in Ca2+ conduction as a function of the fixed charge. Preliminary studies of the dependence of Ca2+ conduction on pH are qualitatively consistent with the predictions of the model.

Highlights

Biological ion channels provide for the highly-selective passive transport of physiologically important ions (e.g. Na+, K+ and Ca2+) through the bilipid membranes of living cells
We perform an experimental study of mutation-induced transformations of conductivity and selectivity in NaChBac voltage-gated bacterial channels, including both Qf varying mutations and Qf -conserved (D⇀ ↽E) substitutions within the selectivity filter (SF), in order to see whether the results could be understood within the framework of the ionic Coulomb blockade (ICB) model
We have made divalent blockade measurements, providing us with the experimental information needed for application of the extended ICB model incorporating the effect of quantised dehydration (QD)

Summary

Introduction

Biological ion channels provide for the highly-selective passive transport of physiologically important ions (e.g. Na+, K+ and Ca2+) through the bilipid membranes of living cells. Following Eisenman [2], ionic selectivity arises through a balance between repulsion by the dehydration/self-energy barrier and electrostatic attraction/affinity to the binding site. It results in resonant barrier-less conduction for the Preprint submitted to BBA – Biomembranes selected ion [3,4,5,6,7,8], leading to selectivity phenomena such as divalent blockade of the sodium current [9, 10] and the anomalous mole fraction effect (AMFE) [10, 11] where the channel conductance is lower in a mixture of salts than in either of the pure salts at the same concentration. ICB is closely similar to its electronic counterpart in quantum dots and nanostructures [20,21,22,23]

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