**5.2.2 Protrusion formation**

Inherent polarity drives the formation of membrane protrusions, and the organization of filaments depends on the type of protrusion. Actin filaments form a branching dendritic network in lamellipodia, but form long parallel bundles in filopodia (Pollard, Blanchoin et al. 2000). The dendritic organization of lamelipodia that provides a tight brush-like structure, formed via the actin-nucleating activity of the actin-related proteins 2/3 (Arp2/3) protein complex (Urban, Jacob et al.). Rac stimulates new actin polymerization by acting on Arp2/3 complexes, which binds to pre-existing filaments (Campellone and Welch). Activation of Arp2/3 complexes by Rac is carried out through its target IRSp53. Upon activation, IRSp53 interacts with WAVE through its SH3 domain, it then binds to and activates Arp2/3 complexes (Chesarone and Goode 2009). It has also been reported that IRSp53 binds to Cdc42 through a separate domain (Miki, Yamaguchi et al. 2000). So, IRSp53 can serve as a direct link between Cdc42 and Rac, which explains how Cdc42 induces Rac involvement in lamellipodium formation. Furthermore, IRSp53 can bind to a Rho target, Dial, which might underlie the capability of Rho to facilitate lamellipodium extension (Cox and Huttenlocher 1998; Fujiwara, Mammoto et al. 2000).
