Above is a graphical representation of an actual ActA-coated latex sphere undergoing the transition from a bead (red circle) symmetrically coated with actin (pseudocolor) to a bead being propelled by an actin comet tail.  To watch a video of this process, click here (150k).  As you can see, the bead is initially subject to random fluctuations in actin density around the bead.  Analysis shows that unidirectional escape of the bead from the cloud is immediately preceded by significant actin density on the opposite side of the sphere.

Using a few basic assumptions about the polymerization of actin at the surface of the bead, computer modelling can illustrate how this transition from symmetry to asymmetry can occur.  A virtual "bead" is positioned in the exact center of a cloud of fixed actin filaments.  The filaments are then subjected to rounds of growth or depolymerization, with a new lowest-energy position of the bead calculated each iteration.  Just as seen above, the virtual bead eventually escapes the virtual cloud and acquires unidirectional motility.

Somewhat surprisingly, an increase in the off-rate of monomers from filaments increases the likelihood of motility acquisition.  This suggests that dynamic fluctuation, rather than brute force, is a key player in this process.  Further, statistical analysis of the filament activities during the transition from non-moving to moving beads demonstrates that polymerization-based force is highly cooperative.  The presence of the bead couples the dynamics of different filaments, resulting in a complex collective system of interacting Brownian ratchets that exhibits an emergent symmetry-breaking behavior.