Binding of actin filaments to charged lipid monolayers: Film balance experiments combined with neutron reflectivity

The binding of polymerised actin - a prototype of semi-flexible macromolecule - to lipid monolayers is studied by neutron reflectivity to deduce the average thickness, the interfacial roughness and the polymer volume fraction of the adsorbed film. Electrostatic interaction between actin filaments (F-actin) and the lipid monolayer is mediated through a cat ionic lipid (1,2-dimyristoyl-3-trimethylammonium-propane, DMTAP). The adsorbed F-actin forms a monolayer with an average thickness of 69 to 84 Å, depending on the ionic strength of the buffer and surface charge density of the monolayer. The volume fraction of F-actin in the adsorbed layer can be as high as 0.29. The thickness and high volume fraction of the actin layer suggest that actin filaments lie flat on the surface and form nematic ordering. The binding-unbinding equilibrium of F-actin is controlled by the ionic strength and exhibits a strong hysteresis. In contrast to the results obtained for filamentous actin, monomeric actin (G-actin) shows no detectable binding to the positively charged lipid layers.