Antimicrobial and cell-penetrating peptides induce lipid vesicle fusion by folding and aggregation

According to their distinct biological functions, membrane-active peptides are generally classified as antimicrobial (AMP), cell-penetrating (CPP), or fusion peptides (FP). The former two classes are known to have some structural and physicochemical similarities, but fusogenic peptides tend to have rather different features and sequences. Nevertheless, we found that many CPPs and some AMPs exhibit a pronounced fusogenic activity, as measured by a lipid mixing assay with vesicles composed of typical eukaryotic lipids. Compared to the HIV fusion peptide (FP23) as a representative standard, all designer-made peptides showed much higher lipid-mixing activities (MSI-103, MAP, transportan, penetratin, Pep1). Native sequences, on the other hand, were less fusogenic (magainin 2, PGLa, gramicidin S), and pre-aggregated ones were inactive (alamethicin, SAP). The peptide structures were characterized by circular dichroism before and after interacting with the lipid vesicles. A striking correlation between the extent of conformational change and the respective fusion activities was found for the series of peptides investigated here. At the same time, the CD data show that lipid mixing can be triggered by any type of conformation acquired upon binding, whether α-helical, β-stranded, or other. These observations suggest that lipid vesicle fusion can simply be driven by the energy released upon membrane binding, peptide folding, and possibly further aggregation. This comparative study of AMPs, CPPs, and FPs emphasizes the multifunctional aspects of membrane-active peptides, and it suggests that the origin of a peptide (native sequence or designer-made) may be more relevant to define its functional range than any given name

Membrane association landscape of myelin basic protein portrays formation of the myelin major dense line

Abstract Compact myelin comprises most of the dry weight of myelin, and its insulative nature is the basis for saltatory conduction of nerve impulses. The major dense line (MDL) is a 3-nm compartment between two cytoplasmic leaflets of stacked myelin membranes, mostly occupied by a myelin basic protein (MBP) phase. MBP is an abundant myelin protein involved in demyelinating diseases, such as multiple sclerosis. The association of MBP with lipid membranes has been studied for decades, but the MBP-driven formation of the MDL remains elusive at the biomolecular level. We employed complementary biophysical methods, including atomic force microscopy, cryo-electron microscopy, and neutron scattering, to investigate the formation of membrane stacks all the way from MBP binding onto a single membrane leaflet to the organisation of a stable MDL. Our results support the formation of an amorphous protein phase of MBP between two membrane bilayers and provide a molecular model for MDL formation during myelination, which is of importance when understanding myelin assembly and demyelinating conditions