Urge to merge erects filopodia

Crawling cells often produce both broad, flat lamellipodia and long, thin filopodia. Once formed, both structures appear to use an actin-treadmilling mechanism to extend themselves. During treadmilling, lamellipodia continuously nucleate new filaments, but filopodia continue to use the same filaments. So how does a cell form a filopodium in the first place? Svitkina et al., page 409, address this question with a detailed kinetic and structural analysis, presenting a novel model that should help to guide future work in the field.

Using GFP-tagged versions of the Arp2/3, VASP, and fascin proteins and innovative electron microscopy techniques, the authors examined the molecular dynamics of filopodium initiation in mouse melanoma cells. Rather than forming a distinct nucleation complex, Arp2/3 seems to lay the groundwork for filopodium birth by producing a normal lamellipodial actin array. Conical structures, termed...