- In cells, actin polymerization at the plasma membrane is induced by the recruitment of proteins such as the Arp2/3 complex, and the zyxin/VASP complex. The physical mechanism of force generation by actin polymerization has been described theoretically using various approaches, but lacks support from experimental data. By the use of reconstituted motility medium, we find that the Wiskott Aldrich syndrome protein (WASP) subdomain, known as VCA, is sufficient to induce actin polymerization and movement when grafted on microspheres. Changes in the surface density of VCA protein or in the microsphere diameter markedly affect the velocity regime, shifting from a continuous to a jerky movement resembling that of the mutated 'hopping' Listeria. These results highlight how simple physical parameters such as surface geometry and protein density directly affect spatially controlled actin polymerization, and play a fundamental role in actin-dependent movement.