Elementary processes and mechanisms of nanopore formation induced by antimicrobial peptides and other membrane-active peptides

The activity of membrane-active peptides/proteins (MAPs) involves interactions with lipid bilayer regions of cell membranes. For example, antimicrobial peptides, lytic peptides, pore-forming toxins, lipidated peptides, and cell-penetrating peptides are all MAPs. Most MAPs induce damage in cell membranes/lipid bilayers, such as nanopore formation. Various methods have been employed to examine the interactions between MAPs and lipid bilayers, as well as MAP-induced membrane damage. Methods using giant unilamellar vesicles (GUVs) are particularly versatile techniques because they provide useful information regarding both MAP-lipid bilayer interactions and MAP activities such as membrane damage. GUV studies have revealed many aspects of elementary processes of MAP-induced membrane damage and their correlations, thus clarifying the mechanisms of MAPs-induced membrane damage. Here, we focus on GUV-based studies of MAP-induced nanopore formation in lipid bilayers. First, we review the binding of MAPs to the lipid bilayers. Second, we review the rate of MAP-induced nanopore formation and the rate of membrane permeation of fluorescent probes through the nanopores. Third, we review the relationships between several elementary processes involved in MAP-induced nanopore formation (i.e., binding of MAPs, nanopore formation, translocation of MAPs across lipid bilayers) and factors that induce membrane instability to facilitate nanopore formation. The pre-pore model of translocation of MAPs across lipid bilayers is also reviewed. Fourth, we review the effects of membrane tension, membrane potential, and lipid composition on MAP-induced formation of nanopores and their stability. Finally, we describe our perspectives on future GUV-based studies of MAP-induced nanopore formation.