Phase Behavior of F-actin

To better understand the close spatial proximity of F-actin (filamentous actin) bundles to other structures comprised of F-actin in cellular environments, we have measured the phase boundary between F-actin and F-actin bundles as a function of spermine concentration. To do this, we first grew actin filaments by adding MgCl$_{2}$ to G-actin (globular actin). F-actin was then incubated with spermine (a low-binding-energy linker and actin-bundling factor) overnight, and then the samples were spun at low speeds to separate bundles from unbundled F-actin. The relative amounts of actin in the pellet and supernatant were determined via gel electrophoresis, yielding a description of the bundling transition as a function of actin and spermine concentrations. With this approach, we are constructing a phase diagram for the F-actin/spermine system. Surprisingly, the dependence of bundle formation on actin concentration is small to non-existent. At the actin concentrations we studied (4.5, 9, 18 and 36$\mu $M), actin tends to form bundles at the same spermine concentration. This observation calls for the evaluation of the effect of the ambient Mg$^{2+}$ in solution (added to polymerize actin) on actin bundling by spermine.