The effect of cyclization of magainin 2 and melittin analogues on structure, function, and model membrane interactions: implication to their mode of action.

The amphipathic alpha-helical structure is a common motif found in membrane binding polypeptides including cell lytic peptides, antimicrobial peptides, hormones, and signal sequences. Numerous studies have been undertaken to understand the driving forces for partitioning of amphipathic alpha-helical peptides into membranes, many of them based on the antimicrobial peptide magainin 2 and the non-cell-selective cytolytic peptide melittin, as paradigms. These studies emphasized the role of linearity in their mode of action. Here we synthesized and compared the structure, biological function, and interaction with model membranes of linear and cyclic analogues of these peptides. Cyclization altered the binding of melittin and magainin analogues to phospholipid membranes. However, at similar bound peptide:lipid molar ratios, both linear and cyclic analogues preserved their high potency to permeate membranes. Furthermore, the cyclic analogues preserved approximately 75% of the helical structure of the linear peptides when bound to membranes. Biological activity studies revealed that the cyclic melittin analogue had increased antibacterial activity but decreased hemolytic activity, whereas the cyclic magainin 2 analogue had a marked decrease in both antibacterial and hemolytic activities. The results indicate that the linearity of the peptides is not essential for the disruption of the target phospholipid membrane, but rather provides the means to reach it. In addition, interfering with the coil-helix transition by cyclization, while maintaining the same sequence of hydrophobic and positively charged amino acids, allows a separated evaluation of the hydrophobic and electrostatic contributions to binding of peptides to membranes.