Lamellar Phase of Stacked Two-Dimensional Rafts of Actin Filaments

We examined liquid crystalline phases of the cytoskeletal polyelectrolyte filamentous (F-)actin in the presence of multivalent counterions. As a function of increasing ion concentration, the F-actin rods in either an isotropic or a nematic phase will transform into a new and unexpected lamellar phase of cross-linked rafts (LXR phase), before condensing into a bundled phase of parallel, close-packed rods. This behavior is generic for alkali earth divalent ions Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺, and the structural transitions are achieved without any architecture-specific actin-binding linker proteins.