Fascin and VASP team up for increased rigidity of actin comets

The actin cytoskeleton is the cellular scaffolding that primarily determines the shape and mechanical properties of cell protrusions. We examined the combined effect of two key actin-binding proteins, fascin and VASP, on the mechanical properties of actin comets, test-tube mimics of cell protrusions. We find that the effect of the two molecules together is more than the sum of the individual effects, indicating synergy between the two proteins for increased actin network rigidity.

HFSP Young Investigator Grant holders Julie Plastino and Laurent Kreplak and colleagues
authored on Mon, 19 March 2012

Migration is key to many cellular processes such as embryonic development and tumor cell metastasis. Cell movement is driven by the polymerization of the cytoskeletal protein actin into filaments that push out the front of the cell, forming flat protrusions called lamellipodia or finger-like needles, called filopodia.  The goal of our research is to provide an integrated picture of how the biochemical activities of cytoskeletal proteins affect individual filament mechanics and overall network properties to generate protrusions and movement.  We use an in vitro system where the actin polymerization found in cell protrusions is reproduced in a controlled manner on the surface of a bead.  These actin tails or “comets” are then deformed with an Atomic Force Microscopy (AFM) tip. From the force versus deformation curve, we can extract the Young’s modulus, which is the elastic constant describing the stiffness of a material. This set-up allows us to vary the amount of different actin-binding proteins in the actin comet, and precisely measure the effect on actin network mechanical properties.

Figure: AFM imaging of an actin comet tail.  The bead is in the lower right corner of the image.  The actin tail is compact, but some ordered structure is evident, and individual actin filaments are visible on the substrate.

We used this approach to test how actin mechanics are affected by the protein VASP, a member of the Ena/VASP protein family, implicated in the formation of dynamic actin structures in the cell, including filopodia.  We found that VASP modestly increased the Young’s modulus (rigidity) of actin comets in a dose-dependent manner, as did fascin, an actin-bundling protein also important for filopodia structure. However when these two proteins were present together in the actin comet system, we found that, at a dose where each factor had no effect individually, the two together drastically increased the Young’s modulus of the actin comet. This mutual amplification may be important in vivo, where the combined action of fascin and VASP at low physiological concentrations renders filopodia stiff and enables them to penetrate into the extracellular matrix surrounding the moving cell.

References

(1) Fascin and VASP synergistically increase the Young’s modulus of actin comet tails.Suei, S., J. Plastino, and L. Kreplak. 2012. J. Struct. Biol. 177:40-45.

(2) The mechanical role of VASP in an Arp2/3-complex-based motility assay.Suei, S., R. Seyan, P. Noguera, J. Manzi, J. Plastino, and L. Kreplak. 2011. J. Mol. Biol. 413:573-583.

Pubmed link ref (1)

Pubmed link ref (2)