UCSF

In eukaryotic cells, actin networks are built and dynamically reorganized during cell polarization and morphogenesis. The formation of different actin network architectures depends on whether filaments are allowed to elongate persistently or are capped shortly after nucleation. One class of proteins regulating the actin filament length distribution in cells are the barbed end associated proteins, which can terminate or accelerate actin polymerization. Depending on the activity and abundance of these proteins, cells construct actin networks that are either highly branched (lamellipodia) or a densely bundled (filopodia). Regulating the transition between these two types of actin networks, is the ubiquitously expressed Ena/VASP protein family. To determine how Ena/VASP proteins regulate actin assembly I developed a way of visualizing single VASP tetramers interacting with single actin filaments in vitro. Visualization of single VASP tetramers revealed several novel mechanisms controlling lateral F-actin interactions, barbed end association, and processivity. Providing an additional level of regulation, I identified a membrane tethered actin binding protein, Lamellipodin, that can modulate Ena/VASP barbed end processivity by simultaneously interacting with filamentous actin. Together these results provide a mechanistic framework for understanding how Ena/VASP proteins regulate actin polymerization and network architecture in vivo.