Secondary Structure Formation by Bilayer-Active Peptides Studied by Freeze-Fracture Electron Microscopy: From Disc Micelles to Cochleate Cylinder

Abstract

Melittin, a 26 amino acid, amphipatic peptide, is known for its hemolytic activity but some short-chain, cationic peptides show antimicrobial properties. Employing freeze-fracture transmission electron microscopy (ff-TEM) we investigated structural modifications within bilayers caused by these two membrane-active peptide groups. Ff-TEM as a cryofixation replica technique is a powerful tool to explore bilayer alterations in a probe-free mode down to a resolution limit of 2nm. Moreover, the fact that the fracture plane follows the area of weakest forces allows insides into the hydrophobic center of lipid bilayer [1]. Here we report the formation of disc micelles as a result of the reversible bilayer to micelle transformation caused by melittin in synthetic lipid bilayer [2]. Furthermore, we explored lipid domain formation in lipid films mimicking the lipid pattern of cytoplasmic membranes of Gram negative bacteria by arginine-rich antimicrobial peptides [3]. Additionally to lipid domains we observed secondary structure formation such as doughnut-type structures caused by arginine-rich peptides and cochleate cylinder (CCC) triggered by lysine-rich peptides. The new type of CCC has been proven to encapsulate and transport traditional, ineffective antibiotics such as erythromycin to resistant bacteria cells. The double-function of these antimicrobial peptides in forming CCC and promoting encapsulation and effective delivery of traditional antibiotics in/by these assemblies provides an interesting approach combating bacterial multidrug resistance [4].