Small Ion Transport Properties of the Anthrax Toxin Channels

Abstract

Anthrax toxin channel, protective antigen (PA63), is known to be strongly cation selective, the property that has determined the development of positively charged inhibitors as channel blockers. Successful interpretation of small ion transport through PA63 can also contribute to the description of the translocase activity of the channel that transports the enzymatic factors of anthrax toxin, lethal factor (LF) and edema factor (EF), into the cytosol. It was recently suggested by Kalu et al (FEBS Letters, 2012) that PA63 conductance is determined by the two contributing factors: ionic strength-dependent electrical potential and access resistance, with PA63 dependence on KCl concentration showing two distinct patterns at pH 6.5 and 4.5. The model also offered an explanation for the fact that the protonation of the cavity residues does not change PA63 small ion selectivity. The authors suggested that the channel conductance is determined by the narrowest φ clamp part of the channel and the ionizable residues in proximity to it. In contrast, selectivity is governed by the concerted action of many residues in the channel lumen. To validate this, so far putative model, here we investigate PA63 small ion transport properties using different electrolyte solutions, including LiCl, NaCl, KCl, RbCl, CsCl, NH4Cl, and CaCl2. We demonstrate that PA63 conductance shows an unexpectedly strong dependence on the nature of the electrolyte solution cation, compared to the bulk conductivity change. At the same time, the channel's selectivity remains essentially unchanged for monovalent cation solutions. In CaCl2, the selectivity is reversed. We discuss the influence of PA63 lumen residues, cation hydration shells, and access resistance on small ion transport properties of the channel.