Abstract
The electrochemical gradients established across cell membranes are paramount for the execution of biological functions. Besides ion channels, other transporters, such as exogenous pore-forming toxins, may present ionic selectivity upon reconstitution in natural and artificial lipid membranes and contribute to the electrochemical gradients. In this context, we utilized electrophysiology approaches to assess the ionic selectivity of the pore-forming toxin lysenin reconstituted in planar bilayer lipid membranes. The membrane voltages were determined from the reversal potentials recorded upon channel exposure to asymmetrical ionic conditions, and the permeability ratios were calculated from the fit with the Goldman–Hodgkin–Katz equation. Our work shows that lysenin channels are ion-selective and the determined permeability coefficients are cation and anion-species dependent. We also exploited the unique property of lysenin channels to transition to a stable sub-conducting state upon exposure to calcium ions and assessed their subsequent change in ionic selectivity. The observed loss of selectivity was implemented in an electrical model describing the dependency of reversal potentials on calcium concentration. In conclusion, our work demonstrates that this pore-forming toxin presents ionic selectivity but this is adjusted by the particular conduction state of the channels.
Highlights
The biological activity of ion channels is a direct consequence of three common characteristics: high transport rate, regulation, and selectivity [1]
Ionic selectivity is a salient feature of ion channels leading to creating and maintaining electrochemical gradients needed for the correct functionality of cells [2,3,4,5,6], and the modulation of these gradients by physical and chemical stimuli is essential for excitability [1,7,8]
We focused our work on investigating the ionic selectivity of lysenin channels
Summary
The biological activity of ion channels is a direct consequence of three common characteristics: high transport rate, regulation, and selectivity [1]. The ionic selectivity of PFTs is generally far under that of ion channels, prior investigations provided valuable information on the nature of the selectivity filters and even allowed intentional selectivity modulation by chemical modifications [16] Such achievements are anticipated to improve our understanding of the selectivity mechanisms and physiological relevance and for developing new drugs, therapeutic strategies, and applications in synthetic biology. For a better understanding of how intermediate conductance states adjust the transport properties of protein pores, we exploited a unique feature of lysenin channels, which is the attainment of stable sub-conducting states in the presence of divalent ions (i.e., Ca2+ ) [12,14] In this line, our experimental work demonstrates that the selectivity of lysenin channels to monovalent ions is significantly diminished when they are subconducting, which leads to vanishing membrane voltages. The diminished selectivity was assessed by employing a simple electrical model, which in conjunction with the Langmuir isothermal adsorption model describing the divalent ion–channel interaction [12] provided a good match of the experimental data
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