Coexistence of a Two-States Organization for a Cell-Penetrating Peptide in Lipid Bilayer

Primary amphipathic cell-penetrating peptides transport cargoes across cell membranes with high efficiency and low lytic activity. These primary amphipathic peptides were previously shown to form aggregates or supramolecular structures in mixed lipid-peptide monolayers, but their behavior in lipid bilayers remains to be characterized. Using atomic force microscopy, we have examined the interactions of P ( α) , a primary amphipathic cell-penetrating peptide which remains α-helical whatever the environment, with dipalmitoylphosphatidylcholine (DPPC) bilayers. Addition of P ( α) at concentrations up to 5 mol % markedly modified the supported bilayers topography. Long and thin filaments lying flat at the membrane surface coexisted with deeply embedded peptides which induced a local thinning of the bilayer. On the other hand, addition of P ( α) only exerted very limited effects on the corresponding liposome’s bilayer physical state, as estimated from differential scanning calorimetry and diphenylhexatriene fluorescence anisotropy experiments. The use of a gel-fluid phase separated supported bilayers made of a dioleoylphosphatidylcholine/dipalmitoylphosphatidylcholine mixture confirmed both the existence of long filaments, which at low peptide concentration were preferentially localized in the fluid phase domains and the membrane disorganizing effects of 5 mol % P ( α) . The simultaneous two-states organization of P ( α) , at the membrane surface and deeply embedded in the bilayer, may be involved in the transmembrane carrier function of this primary amphipathic peptide.