Ion Conduction in the Stem of the PA63 Channel of Anthrax Toxin during Lethal Factor (LF) Translocation

Abstract

Protective antigen (PA), a critical 83kDa protein component of the anthrax toxin, forms channels in cell membranes which allow for the transport of two toxin components, lethal factor (LF) and edema factor (EF), into cells. The heptameric channel formed by PA63 (the 63 kDa, C-terminal part of PA) is thought to be a mushroom-shaped structure, with seven phenylalanine residues forming a ring (known as the phenylalanine clamp) at the junction between the cap and the stem. It is known that when LF is driven through the channel by an applied ΔV across the membrane, the seal created by the phenylalanine clamp causes an essentially complete block of conduction, but it is not known whether the stem of the channel allows for any conduction when occupied by LF. We are studying the PA63 channel at both the microscopic and the macroscopic level in an effort to determine how much conductance, if any, is allowed when the channel is occupied by LF and the phenylalanine clamp has been mutated to less bulky alanine residues. Preliminary data suggest that when LF is added to the cis compartment containing the mutant channel, the block in conductance (at positive voltages) may not be as complete as when the phenylalanine clamp is present, but it is nonetheless significant. If this is the case, then the electric field created by the ΔV across the membrane is focused at the phenylalanine clamp.