Ionic Coulomb blockade and anomalous mole fraction effect in the NaChBac bacterial ion channel and its charge-varied mutants

Abstract

Abstract. Background. The selectivity of biological cation channels is defined by a short, narrow selectivity filter, having a negative net fixed charge Q f . Voltage gated bacterial channels (NaChBac and some others) are frequently used in biophysics as simplified models of mammalian calcium and sodium channels. We report an experimental, analytic and numerical study of the effects of Q f and bulk ionic concentrations of Ca 2+ and Na + on conduction and selectivity of NaChBac channels, wild type and Q f -varied mutants. Methods. Site-directed mutagenesis and voltage clamp recordings were used to investigate the Na + /Ca 2+ selectivity, divalent blockade and anomalous mole fraction effect (AMFE) for different NaChBac wild type/ mutants channels and the properties dependence on Q f . Experimental results were compared with Brownian dynamics simulations and with analytic predictions of the ionic Coulomb blockade (ICB) model, which was extended to encompass bulk concentration effects. Results. It was shown that changing of Q f from -4e (for LESWAS wild type) to -8e (for LEDWAS mutant) leads to strong divalent blockade of the Na + current by micromolar amounts of Ca 2+ ions, similar to the effects seen in mammalian calcium channels. The BD simulations revealed a concentration-related logarithmic shift of the conduction bands. These results were shown to be consistent with ICB model predictions. Conclusions. The extended ICB model explains the experimental (divalent blockade and AMFE) and simulated (multi-ion bands and their concentration-related shifts) selectivity phenomena of NaChBac channel and its charge-varied mutants. These results extend the understanding of ion channel selectivity and may also be applicable to biomimetic nanopores with charged walls.

Ionic Coulomb blockade as a fractional Wien effect

“…However, it is only recently that experimental progress in nanoscience has allowed fabrication of artificial pores with controlled material properties and (1D or 2D) geometries. These new systems have reached unprecedented nanometre and even angström scale confinements [8][9][10][11], yet they are still far from exhibiting the same functions as biological ionic machines.…”

Effect of local binding on stochastic transport in ion channels

“…The effect of local binding on conduction and occupancy (analytic result (9)) is found to agree with Brownian dynamics simulations, thus putatively accounting for how the position of the resonant point M 0 is influenced by the radius of the Glutamate/Aspartate ring in NaChBac channels and their mutants [18], [23], [24].…”

Changes in Ion Selectivity Following the Asymmetrical Addition of Charge to the Selectivity Filter of Bacterial Sodium Channels

“…Despite this, there is still no widely accepted predictive model for cation selectivity, representing gaps in our knowledge of the molecular mechanism of ion permeation and selectivity. Mutation studies suggest the fixed charge (Q f ) of the SF to be one of the major determinants of selectivity and permeation [ 15 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 ]. The Q f charge is at the core of many theoretical models attempting to explain the physical origins of cation selectivity in ion channels.…”

“…The Q f charge is at the core of many theoretical models attempting to explain the physical origins of cation selectivity in ion channels. For example, cation conduction has recently been modelled within the framework of Ionic Coulomb Blockade (ICB) [ 24 , 25 ], an electrostatic model with the aim of predicting Na + and Ca 2+ permeability based on knowing the actual Q f value of the SF.…”

“…Monte Carlo simulations predicted this D residue (termed D2p51 in [ 37 ]) to occupy a position in close proximity to the EEEE locus. This observation led to the hypothesis that the locus imparting Ca 2+ permeability is actually EEEED, with a Q f value of −5 e [ 25 , 37 ]. Moreover, when this conserved D residue in domain 2 of Cav1.2 (referred to as D707 in [ 29 ]) was replaced with neutral residues, a striking reduction in Ca 2+ binding to the SF was measured [ 29 ].…”