Counterion-induced formation of regular actin bundle networks

Dominating the cytoskeletal contribution to cell mechanics and migration, actin and actin-crosslinker systems have attained much attention with regard to rheology and network architecture. In contrast to protein crosslinkers or molecular motors-which self-assemble and self-organize actin into networks, bundles or asters-simple multivalent ions rely on the polyelectrolyte nature of actin and should not imply any particular binding geometry. Using controlled counterion condensation as a model linker at comparably high F-actin densities we report an entirely novel state displaying regularly spaced networks of actin bundles connected by aster-like clusters. Liquid crystalline effects at high densities of long filaments were found to substantially alter the bundle network structures directly reflecting the pre-existing order of the actin filaments.