Figure 1.

Mechanistic models of filopodia initiation. Convergent elongation model (top): (1) Branched actin network is formed in lamellipodil by Arp2/3-mediated nucleation. (2) Elongation factors (Ena/VASP or formin) maintain continuous elongation of some barbed end. (3) Interaction between barbed ends, likely also mediated by Ena/VASP or formin, results in synchronized parallel elongation of several converged filaments. (4) Parallel filaments are cross-linked by fascin resulting in formation of an actin bundle. (5) Over time, the Arp2/3 complex that nucleated filopodial filaments dissociates leaving free pointed ends of splayed actin filament. Tip nucleation model (bottom): (1) Activated formin is clustered on the plasma membrane. (2) Formin cluster nucleates a bunch of actin filaments and maintain their elongation. (3) Elongating filaments are cross-linked by fascin to form an actin bundle.