Bundling Transition of F-actin

Filament of F-actin is formed from a linear array of G-actin and exhibits the characteristics as a negatively charged stiff polyelectrolyte with the persistence length above 1 mum. In living cellular system, F-actins from thick bundles with the help of specific proteins as the mediator. It has been found that bundling of F-actin filaments is generated with various cationic species, such as Fe³⁺ , spermidine³⁺, cationic synthetic polypeptides. It has been regarded that these multivalent cations work as a paste to stick the like-charge filaments. In the past studies, the bundling phenomenon of F-actin has frequently been observed through the increase of the light scattering. There observations indicated the cooperative, continuous nature in the transition. In the present study, we performed the observation on the bundling phenomenon by use of fluorescent microscopy. We found that the bundling transition proceeds through the coexistence of single filaments and bundles, which corresponds to the continuous nature of the transition revealed from the measurement of the light scattering. From these results, it is concluded that the bundling transition is characterized as a first-order phase transition under the criterion of Landau´s symmetry argument