Hierarchical self-assembly of F-actin and cationic lipid complexes: Stacked three-layer tubule networks

We describe a distinct type of spontaneous hierarchical self-assembly of cy toskeletal filamentous actin (F-actin), a highly charged polyelectrolyte, a nd cationic Lipid membranes. On the mesoscopic length scale,confocal micros copy reveals ribbonlike tubule structures that connect to form a network of tubules on the macroscopic scale (more than 100 micrometers). Within the t ubules, on the 0.5- to 50-nanometer length scale, x-ray diffraction reveals an unusual structure consisting of osmotically swollen stacks of composite membranes with no direct analog in simple amphiphilic systems. The composi te membrane is composed of three layers, a lipid bilayer sandwiched between two layers of actin, and is reminiscent of multilayered bacterial cell wal ls that exist far from equilibrium. Electron microscopy reveals that the ac tin layer consists of Laterally locked F-actin filaments forming an anisotr opic two-dimensional tethered crystal that appears to be the origin of the tubule formation.