Neutron spin echo study on elastic properties of phospholipid membranes interacting with pore-forming peptides

Abstract

Cell membranes, which consist of phospholipid bilayers, play important roles in cells as barriers for maintaining concentrations and matrices to host membrane proteins. During cellular processes such as endo- and exo-cytosis, cell fission and fusion, the cell membranes undergo various morphological changes which are mainly governed by the interplay between protein and lipid membranes. A number of mechanisms have been proposed for protein-induced membrane deformations, including insertion of amphipathic helices, direct or indirect scafolding, and oligomerization of membrane proteins which change membrane curvature. However, it is not well exploited how the elastic properties of membranes, which play a key role in membrane deformation, are affected by the protein-membrane interactions. Therefore, to understand cell functions related protein-membrane interactions, in terms of the elastic properties of the membrane, is an important step toward elucidating the mechanisms of protein functions in membranes. A well-known example of protein-membrane interaction is the activity of antimicrobial peptides. Such peptides associate with a lipid bilayer in two distinct ways. At low peptide to lipid molar ratio P/L, the peptides adsorb horizontally to the surface of membrane and above a threshold concentration P/$L^{*}$, the peptides begin to insert into the membrane, forming trans-membrane pores. Here, we report, for the first time, the thermal fuctuation and elasticity of dioleoyl phosphocholine large unilamellar vesicle membranes interacting with pore-forming peptides, melittin, which were measured by in-situ neutron spin echo spectroscopy (NSE). The relaxation behavior of the intermediate dynamic structure factors of the membrane at different P/L can be divided into three regions, resulting from characteristic changes of the effective bending modulus $\tilde{\kappa}$ ($\tilde{\kappa}$ = $\kappa+d^{2}\it{k}$ is the bending modulus, $\it{d}$ is the height of the neutral surf...