Interaction of polymyxin B nonapeptide with anionic phospholipids

Area of Science:

• Biochemistry
• Biophysics
• Microbiology

Background:

• Polymyxin B (PMB) is a potent antibiotic targeting Gram-negative bacteria.
• Polymyxin B nonapeptide (PMBN) shares structural similarities with PMB but lacks bactericidal activity.
• Understanding their differential interactions with bacterial membranes is crucial for developing new antimicrobial strategies.

Purpose of the Study:

• To investigate the molecular interactions between PMB, PMBN, and various anionic phospholipids.
• To elucidate the mechanisms by which these peptides affect lipid bilayer structure and function.
• To correlate observed biophysical interactions with their known physiological effects on bacterial cell envelopes.

Main Methods:

• Calorimetry and electron spin resonance spectroscopy to study lipid phase transitions.
• Fluorescence spectrometry and electron microscopy for structural analysis.
• Stopped-flow assays and leakage experiments to assess vesicle fusion and membrane permeabilization.

Main Results:

• PMB and PMBN binding caused similar phase transition temperatures in DPPA and DPPG lipids.
• Both peptides induced bilayer interdigitation and vesicle fusion, with PMB forming stable lipid-peptide domains.
• PMBN-induced domain formation was reversible and rapid, unlike the stable domains formed by PMB.

Conclusions:

• The penetration of peptide hydrophobic side chains into lipid bilayers is sufficient to induce structural changes like interdigitation.
• Differences in the stability of lipid-peptide domains correlate with the distinct biological activities of PMB and PMBN.
• These findings provide insights into the mechanism of action of polymyxins on bacterial membranes.