Interaction at the Membrane Midplane Mediates Interleaflet Coupling

Abstract

Transmembrane signaling implies that peripheral protein binding to one leaflet be detected by the opposite leaflet. Even without involvement of raft lipids we showed (Horner et. al. Biophys. J. 2009) that, peripheral binding of charged molecules (poly-lysine, PLL) to planar lipid bilayers is detected at the other side. Addition of PLL to both sides of the membrane sandwiched lipids between two PLL molecules, indicating a formed nanodomain. As the 2dim surface tension between the monolayers is thought to be of comparable size as the line tension in one of the monolayers, interactions at the membrane midplane don't explain the existence of small nano domains. We tested this assumption by measuring the individual lipid mobility's in both leaflets of a free standing planar lipid bilayers and developed a model. The adsorption of a polypeptide decreased lipid mobility of the adjacent (lower) monolayer. This was revealed by fluorescence correlation spectroscopy (FCS). Depending on the size of the polypeptide, lipid diffusion was either slower or faster than that of the polypeptide. Although only one lipid type was used in both monolayers, lipid mobility's were not equal. The polypeptide decelerated lipid movement in the distant monolayer to a much smaller degree than it did affect lipid mobility in the adjacent monolayer. Based on these observations we propose a model which suggests that the coupling between monolayers is due to friction at the membrane midplane. The interlayer friction coefficient seems to be much larger than the in-layer friction coefficient which explains the existence of small nano-domains.