We study the influence of the VASP protein on dynamics and mechanics of the actin networks that drive cell motility. Our experimental setup is an in vitro bead system that mimics cell movement. The beads are induced to form a network of actin on their surface that resembles a "comet" and propels the bead forward. The actin comet reproduces in many ways the actin network that pushes forward the front of a moving cell. Using mutant forms of VASP, we define which functional domains of VASP are necessary for its function in enhancement of motility. We show that VASP exercises its effect on actin polymerization via synergy with the Arp2/3 complex, an actin polymerization nucleator and an important component of branched actin networks in the cell. We study how this synergy is brought about by co-recruitment of VASP and the Arp2/3 complex by actin polymerization activators of the WASP/WAVE family. In collaboration, we also study how VASP affects the mechanical properties of actin networks. Via AFM nano-indentation of actin comets, we show that VASP increases actin network rigidity, and we corroborate this result by rheological measurements of the elasticity of pure actin networks in the presence of VASP, where we also gain insights into how VASP affects actin network architecture. Our results define the mechanism of action of VASP for enhancement of actin dynamics and cell motility, and shed light on why VASP is implicated in various pathologies, including cancer cell metastasis.
