Processive Acceleration of Actin Barbed End Assembly by N-WASP

Actin-based cell motility plays crucial roles throughout the lifetime of an organism. The dynamic rearrangement of the actin cytoskeleton triggers a plethora of cellular processes including cellular migration. Neural Wiskott Aldrich syndrome protein (N-WASP) is involved in transduction of signals from receptors on the cell surface to the actin cytoskeleton. N-WASP activated actin polymerization drives extension of invadopodia and podosomes into the basement layer. In addition to activating Arp2/3 complex, N-WASP binds actin filament barbed ends, and both N-WASP and barbed ends are tightly clustered in these invasive structures.

We used nanofibers coated with N-WASP WWCA domains as model cell surfaces and single actin filament imaging to determine how clustered N-WASP affects Arp2/3-independent barbed end assembly. Individual barbed ends captured by WWCA domains of N-WASP grew at or below their diffusion limited assembly rate. At high filament densities, overlapping filaments formed buckles between their nanofiber tethers and myosin attachment points. These buckles grew 3.4-fold faster than the diffusion-limited rate of unattached barbed ends. N-WASP constructs with and without the native poly-proline (PP) region showed similar rate enhancements. Increasing polycationic Mg2+ or Spermine to enhance filament bundling increased the frequency of filament buckle formation, consistent with a requirement of accelerated assembly on barbed end bundling.

Our preliminary data shows that tethered N-WASP construct containing one WH2 domain does not generate processive bundles or filament loops leading us to believe that tandem WH2 is required for processivity. We propose that this novel N-WASP assembly activity provides an Arp2/3-independent force that drives nascent filament bundles into the basement layer during cell invasion. Discovery of this bundle mediated unique pathway involved in invasion and metastasis will provide new targets for therapeutic development.