J. X. Tang*, R. Oldenbourg^, T. Ito+, J. A. Käs~, B. Millman#, and P. A. Janmey*
*Brigham & Women's Hospital, Harvard Medical School, LMRC301, Boston, MA 02115. ^Marine Biological Lab, Woods Hole, MA 02543. +Kyoto University, Kyoto, Japan. ~Univ. of Texas at Austin, Austin, TX 78712. #Univ. of Guelph, Guelph, ON N1G 2W1.
Polymerized (F-)actin is induced to form bundles by a number of polycations including divalent metal ions, trivalent hexaminecobalt, and basic polypeptides. The general behavior of bundle formation is similar to the phenomenon of DNA condensation and can be explained analogously by polyelectrolyte theories. The polyelectrolyte nature of F-actin causes a class of nonspecific bindings by ligands carrying several net opposite charges. Such a bundling mechanism can be applied to a class of cationic actin bundling proteins including smooth muscle calponin, microtubule associated proteins tau and Map2c, and the MARCKS actin binding domain (aa 151-175). One direct consequence of this model of bundling is that neither dual binding sites nor dimerization of a protein with single binding site is required in order to bundle F-actin.
A different type of actin bundle forms when solutions become crowded by macromolecules. Nonspecific proteins and inert macromolecules such as polyethylene glycol facilitate the lateral aggregation of F-actin, and the formation of these aggregates depends on the solution ionic strength and the concentration of actin in a manner opposite to the dependence of counterion-induced bundles. In the absence of other macromolecules, F-actin at very high concentrations can form densely packed tactoidal domains, co-existing with a less dense but orientationally aligned phase. Low-angle X-ray diffraction through such actin gels containing numerous, randomly oriented granules shows a clear ring corresponding to a spacing ranging from 11 to 16 nm. The spacing corresponds to the interfilament packing of F-actin, and it decreases monotonically as a function of ionic strength. The latter property points to electrostatic repulsion as the dominant force between the neighboring actin filaments, but the overall entropic drive of the suspension, including the modulation of small ions, facilitates the active seggregation of actin into a two-phase system. Analyzing these distinct, yet intricately related effects may help define their potential roles in forming functional arrays of bundled actin filaments found in many cell types.