Action Of The Antimicrobial Peptide Novicidin: Divorcing Folding From Function

Abstract

Many small cationic peptides have antimicrobial properties. This is assumed to be linked to their ability to permeabilize bacterial membrane. Membrane binding is usually accompanied by the transition from an unstructured conformation to an α-helical state. To investigate further the link between folding and membrane permeabilization we have studied the effect of acylating the N-terminus of the antimicrobial peptide Novicidin with C8, C12 and C16 chains. Acylation increases the ability to form α-helical structure in the presence of zwitterionic vesicles but reduces the ability to permeabilize these vesicles, even at concentrations sufficiently low to prevent formation of peptide micelles mediated by the acyl chains. Laser confocal scanning microscopy studies that show Novicidin's preference for DOPC vesicles among populations of different vesicles. The divorce between folding and function is further emphasized by stopped-flow studies using fluorophor-labelled peptide which indicate that a more superficial mode of binding is more efficient in releasing vesicle contents. Rapid kinetic measurements showed a significant increase in the vesicle disruption lag time as a function of acyl chain length indicating that acylation actually decreases the kinetics of interaction. We suggest that induction of α-helical structure is not a prerequisite for membrane disruption but may in fact inhibit disruption by sequestering the peptide in less membrane-active conformations inserted deeper into the membrane than the non-acylated form. This is corroborated by surface-measurements using Quartz Crystal Microbalances with Dissipation and Dual Polarization Interferometry. Our microscopy studies also reveal multiple modes of interaction between AMPs and simple model membranes, namely fusion, pore-formation and lysis, and indicate that peptide-membrane interactions may be even more varied in the complex environment of live bacterial membranes.