The polyelectrolyte nature of F-actin and the mechanism of actin bundle formation.

Polymerized (F-)actin is induced to form bundles by a number of polycations including divalent metal ions, Co(NH3)63+, and basic polypeptides. The general features of bundle formation are largely independent of the specific structure of the bundling agent used. A threshold concentration of polycation is required to form lateral aggregates of actin filaments. The threshold concentration varies strongly with the valence of the cation and increases with the ionic strength of the solution. Polyanions such as nucleoside phosphates or oligomers of acidic amino acids disaggregate actin bundles into single filaments. These features are similar to the phenomenon of DNA condensation and can be explained analogously by polyelectrolyte theories. Similar results were found when F-actin was bundled by the peptide corresponding to the actin binding site of myristoylated alanine-rich protein kinase C substrate protein (MARCKS) or by smooth muscle calponin, suggesting that a broad class of actin bundling factors may function in a common manner. Physiologic concentrations of both small ions and large proteins can induce actin interfilament association independent of a requirement for specific binding sites.