Coupled diffusion of peripherally bound peptides along the outer and inner membrane leaflets.

Transmembrane signaling implies that peripheral protein binding to one leaflet be detected by the opposite leaflet. Therefore, protein recruitment into preexisting cholesterol and sphingolipid rich platforms may be required. However, no clear molecular picture has evolved about how these rafts in both leaflets are connected. By using planar lipid bilayers, we show that the peripheral binding of a charged molecule (poly-lysine, PLL) is detected at the other side of the bilayer without involvement of raft lipids. The diffusion coefficient, D(P), of PLL differed by a factor of radical2 when PLL absorbed to one or to both leaflets of planar membranes. Fluorescence correlation spectroscopy showed that the changes of the lipid diffusion coefficient, D(M), were even more pronounced. Although D(M) remained larger than D(P) on PLL binding to the first membrane leaflet, D(M) dropped to D(P) on PLL binding to both leaflets, which indicated that the lipids sandwiched between two PLL molecules had formed a nanodomain. Due to its small area of approximately 20 nm(2) membrane electrostriction or leaflet interaction at bilayer midplane can only make a small contribution to interleaflet coupling. The tendency of the system to maximize the area where the membrane is free to undulate seems to be more important. As a spot with increased bending stiffness, the PLL bound patch in one leaflet attracts a stiffening additive on the other leaflet. That is to say, instead of suppressing undulations in two spots, two opposing PLL molecules migrate along a membrane at matching positions and suppress these undulations in a single spot. The gain in undulation energy is larger than the energy required for the alignment of two small PLL domains in opposite leafs and their coordinated diffusion. We propose that this type of mechanical interaction between two membrane separated ligands generally contributes to transmembrane signaling.