Protein translocation through the anthrax toxin transmembrane pore is driven by a proton gradient

Area of Science:

• Biochemistry
• Cell Biology
• Toxicology

Background:

• Anthrax toxin employs protective antigen (PA) to deliver lethal factor (LF) and edema factor (EF) into host cells.
• PA assembles into a heptameric pore in endosomal membranes, facilitating protein translocation upon acidification.

Purpose of the Study:

• To investigate the role of proton gradients (DeltapH) in driving LF and EF translocation through the PA63 pore.
• To elucidate the mechanism by which the PA pore facilitates protein passage into the cytosol.

Main Methods:

• Planar lipid bilayer experiments were conducted to measure translocation.
• Proton gradients (DeltapH) and membrane potential were controlled to assess their influence on translocation.

Main Results:

• A physiological proton gradient (DeltapH) is a significant driving force for LF, EF, and LFN translocation through the PA63 pore.
• Translocation rates increase under acidic endosomal conditions and are synergistically enhanced by the PA's Phe427 residues (phi clamp).
• DeltapH-driven translocation occurs independently of membrane potential, suggesting a proton/protein symporter mechanism.

Conclusions:

• The study reveals that proton gradients are crucial for anthrax toxin translocation into host cells.
• A combined mechanism involving a Brownian ratchet and a protonation-state-driven ratchet facilitates protein passage.
• The PA pore functions as a proton/protein symporter, mediating toxin entry.