Self-Assembly of Artificial Actin Filaments

Actin filaments (F-actins) are long, double-stranded, helical biopolymers that make up the cytoskeleton along with microtubules and intermediate filaments. In order to better understand the self-assembly process of actin filaments, a coarse-grained model to recreate their geometry was developed, which was motivated by the wedge model of microtubule self-assembly. The model monomer has the shape of a bent, twisted rod with binding sites on its lateral and top/bottom surfaces. The longitudinal binding through the binding sites on the top/bottom surfaces of the rod allows the formation of strands, while the lateral binding between the rods enables the strands to adhere laterally to form double-stranded helical filaments. Such a design captures the assembly behavior of G-actins into F-actins. With molecular dynamics simulations, we explored the self-assembly of these bent-rod monomers. A variety of assembled phases were observed when the strengths of the binding interactions between monomers were varied. Our simulations indicate that only a narrow range of binding strengths yields double-stranded helical filaments resembling F-actins. Furthermore, the structure of the assembled filaments is much more robust and resistant to fluctuations and defects when the strength of the longitudinal binding interaction is stronger than that of the lateral binding between monomers. Finally, double-stranded filaments are found to be much more stable structurally than single-stranded ones. Our results reveal fresh insights into the fact that actin filaments are predominantly double-stranded.