Interaction of diphtheria toxin with model membranes

Area of Science:

• Biochemistry
• Molecular Biology
• Membrane Biophysics

Background:

• Diphtheria toxin's membrane penetration is crucial for its toxicity.
• Low pH is hypothesized to initiate toxin entry into host cells.

Purpose of the Study:

• To investigate the effect of pH on diphtheria toxin binding to model membranes.
• To determine how lipid composition influences toxin-membrane interactions and binding.
• To elucidate the role of electrostatic interactions in toxin binding.

Main Methods:

• Utilized a fluorescence quenching assay to measure toxin binding.
• Employed small unilamellar vesicles (SUVs) with varying zwitterionic and anionic lipid compositions.
• Varied pH and ionic strength to assess their impact on binding kinetics and affinity.

Main Results:

• Diphtheria toxin binding to SUVs increased significantly at low pH.
• The transition pH for binding was higher in membranes with a greater fraction of anionic lipids.
• Binding was rapid and tight at low pH, with enhanced kinetics and affinity observed with anionic lipids.

Conclusions:

• Membrane lipid composition, particularly the presence of anionic lipids, significantly affects the pH-dependent binding of diphtheria toxin.
• Electrostatic interactions and the ionic double layer around anionic vesicles contribute to the observed shift in transition pH.
• These findings provide insights into the mechanism of diphtheria toxin-induced membrane penetration.