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 ($L_{\mathrm{X}\mathrm{R}}$ phase), before condensing into a bundled phase of parallel, close-packed rods. This behavior is generic for alkali earth divalent ions ${\mathrm{M}\mathrm{g}}^{2+}$, ${\mathrm{C}\mathrm{a}}^{2+}$, ${\mathrm{S}\mathrm{r}}^{2+}$, and ${\mathrm{B}\mathrm{a}}^{2+}$, and the structural transitions are achieved without any architecture-specific actin-binding linker proteins.