Cooperative symmetry-breaking by actin polymerization in a model for cell motility.

Polymerizing networks of actin filaments are capable of exerting significant mechanical forces, used by eukaryotic cells and their prokaryotic pathogens to change shape or to move. Here we show that small beads coated uniformly with a protein that catalyses actin polymerization are initially surrounded by symmetrical clouds of actin filaments. This symmetry is broken spontaneously, after which the beads undergo directional motion. We have developed a stochastic theory, in which each actin filament is modelled as an elastic brownian ratchet, that quantitatively accounts for the observed emergent symmetry-breaking behaviour. Symmetry-breaking can only occur for polymers that have a significant subunit off-rate, such as the biopolymers actin and tubulin. central problem in cell biology lies in understanding how small-scale biochemical interactions generate large-scale organization and cellular structure. Most eukaryotic cells are structurally polarized, and the establishment and maintenance of their polarity depends on anisotropic organization of their cytoskeletal elements 1 . In multicellular organisms, cell polarity is often influenced by external signals, but many cell types are capable of spontaneously breaking symmetry and generating well-defined structural polarity in the absence of extrinsic spatial cues 2 . Actin is a major cytoskeletal protein of eukaryotic cells, which binds and hydrolyses ATP and self-assembles to form long helical filaments