Kinetic Defects Induced by Melittin in Model Lipid Membranes: A Solution Atomic Force Microscopy Study

Area of Science:

• Biophysics
• Membrane Biology
• Materials Science

Background:

• Understanding membrane defects is crucial for studying membrane-active peptides.
• Melittin is a well-known peptide that interacts with lipid bilayers.

Purpose of the Study:

• To quantitatively characterize kinetic membrane defects induced by melittin.
• To investigate the influence of lipid composition and cholesterol on defect dynamics.
• To propose a kinetic defect growth model based on experimental observations.

Main Methods:

• Fluorescence spectroscopy to measure calcein leakage (membrane permeabilization).
• Solution atomic force microscopy (AFM) to visualize and measure membrane defect radii.
• Studies on various lipid compositions including DLPC, DOPC, and DOPC/cholesterol bilayers.

Main Results:

• Melittin induces time-dependent calcein leakage, indicating kinetic defects.
• Defect radii increase with incubation time, with initial radii of ~3.8 nm (DLPC) and ~4.7 nm (DOPC).
• Cholesterol initially suppresses defect kinetics in DOPC bilayers, but this effect is overcome with longer incubation.
• The kinetic rate of defect development follows DLPC > DOPC > DOPC/cholesterol.
• Melittin affects lipid bilayer phase behavior, initially suppressing Lo domains, which later emerge.
• Defects in phase-coexisting bilayers are localized in the Ld phase with a radius of ~5 nm.
• Defects can occupy up to 40% of the bilayer surface.

Conclusions:

• Melittin-induced membrane defects are not static but exhibit spontaneous growth.
• Defect growth is intrinsically linked to melittin-mediated membrane permeabilization.
• A kinetic defect growth model is proposed to explain the observed phenomena.