Visualizing filamentous actin on lipid bilayers by atomic force microscopyin solution

The surface structure of actin filaments (F-actin) was visualized at high r esolution, by atomic force microscopy (AFM) in aqueous solution, in large p aracrystals prepared on positively charged lipid monolayers. The increased stability of these closely packed specimens allowed us to show that both th e long pitch (38 nm) and the monomer (5.8 nm) can be directly reserved by A FM in the contact mode. The right-handed helical surface, distinguishable i n high resolution images, was compared with reconstructed models based on e lectron microscopy. The height of the rafts, a measure of the actin filamen t diameter, was 10 +/- 1 nm, whereas the smaller inter-filament distance, 8 +/- 1 nm, was consistent with interdigitation of the filaments. The 10 +/- 1 nm F-actin diameter is in good agreement with the results of fibre X-ray diffraction. As such specimens are relatively easy to prepare without spec ialized equipment, this method may allow the study of the thin filaments in which F-actin-associated proteins are also present.