A study by Blake et al reveals that the formation and extension of filopodia depend on the interaction between two proteins: Ena/VASP and TOCA-1.
These interactions are density-dependent; In vitro, increasing densities of TOCA-1 enhances Ena/VASP protein binding. In vivo, the authors found that filopodia protrusion correlates with accumulation of TOCA-1, coincident with recruitment of Ena.