Symmetry Breaking: Dual-channel imaging of symmetry breaking in in vitro motility system assembled from purified protein components. ActA-coated 5µm polystyrene beads were first incubated in a mixture of Arp2/3, capping protein, and Alexa488-labeled actin, then transferred into an equivalent mixture containing TMR-labeled actin and visualized using wide-field illumination. The use of a trace amount of TMR-labeled actin—approximately 1 fluorophore for 20,000 actin monomers—produces fluorescent speckles, which can be used as fiducial marks to study network dynamics. This experiment shows that new actin growth on the surface of the bead pushes older layers outward, and the speckles can be tracked to confirm the presence of elastic strain in the shell before symmetry breaking (not shown).
