Flocking Of Actin Propelled Beads

Polymerizing actin filaments convert chemical energy into mechanical work, giving eukaryotic cells the ability to move and exert forces. This motility is controlled by a large family of actin associated regulatory proteins. In this project, we created dense suspensions of micron sized beads propelled by polymerizing actin filaments. We bound Arp2/3 activating proteins to micron sized polystyrene bead, creating a thick branched network of polymerizing actin at the bead surface that spontaneously break symmetry. This anisotropic polymerization generates a polar force that propels the bead forward while leaving behind an actin 'comet tail'. Confining the beads in quasi-2D geometries leads to a transition from a active gas to an active polar phase, where actin-propelled beads form finite-sized flocks. We show that confinement controls the steric interactions between colliding beads. We are investigating what other interactions between beads are required to generate such polar flocks.