Effect of endosomal acidification on small ion transport through the anthrax toxin PA₆₃ channel

Abstract

Tight regulation of pH is critical for the structure and function of cells and organelles. The pH environment changes dramatically along the endocytic pathway, an internalization transport process that is 'hijacked' by many intracellularly active bacterial exotoxins, including the anthrax toxin. Here we investigate the role of pH on singlechannel properties of the anthrax toxin channel, PA 63 . Using conductance and current noise analysis, blocker binding, ion selectivity, and PEG partitioning measurements, we show that the channel exists in two different open states ('maximum' and 'main') at pH  5.5, while only a maximum conductance state is detected at pH  5.5. We describe two substantially distinct patterns of PA 63 conductance dependence on KCl concentration uncovered at pH 6.5 and 4.5.

Hydrophobic Gating and 1/f Noise of the Anthrax Toxin Channel

“…The 1/ f noise in the PA 63 current has been observed at a variety of experimental conditions, such as different bathing electrolyte concentrations (Figure ), compositions (Figure ), and pH (Figure S2). It has been detected in PA 63 type I and type II insertions (Figure S3), in both octameric , and heptameric PA 63 channels (Figure S4), when PA 63 was reconstituted in bilayer membranes made using different neutral and negatively charged, lamellar and nonlamellar lipids (Figure ). All these factors do not appear to have any substantial effect on the frequency, duration, and amplitude of the 1/ f fluctuations.…”

Effect of late endosomal DOBMP lipid and traditional model lipids of electrophysiology on the anthrax toxin channel activity

“…Note that the grand canonical Monte Carlo/Brownian dynamics simulations, performed previously by Lee and colleagues [58], resulted in PA 63 simulated ion conductance values overestimating the experimental data by a factor of 1.5–7 and showed a significant current asymmetry depending on transmembrane voltage sign, which was not observed experimentally [94]. The calculations also predicted a significant drop in cation/anion permeability ratios when all His residues in the channel lumen are protonated; yet PA 63 shows nearly no change in cation preference in the 4.5–7.5 pH range [95]. Therefore the small ion transport through PA 63 deserves a separate computational investigation, which we did not intend to perform in this study.…”