The Rho‐GTPase Rac1 stimulates actin remodelling at the cell periphery by relaying signals to Scar/WAVE proteins leading to activation of Arp2/3‐mediated actin polymerization. Scar/WAVE proteins do not interact with Rac1 directly, but instead assemble into multiprotein complexes, which was shown to regulate their activity in vitro . However, little information is available on how these complexes function in vivo . Here we show that the specifically Rac1‐associated protein‐1 (Sra‐1) and Nck‐associated protein 1 (Nap1) interact with WAVE2 and Abi‐1 (e3B1) in resting cells or upon Rac activation. Consistently, Sra‐1, Nap1, WAVE2 and Abi‐1 translocated to the tips of membrane protrusions after microinjection of constitutively active Rac. Moreover, removal of Sra‐1 or Nap1 by RNA interference abrogated the formation of Rac‐dependent lamellipodia induced by growth factor stimulation or aluminium fluoride treatment. Finally, microinjection of an activated Rac failed to restore lamellipodia protrusion in cells lacking either protein. Thus, Sra‐1 and Nap1 are constitutive and essential components of a WAVE2‐ and Abi‐1‐containing complex linking Rac to site‐directed actin assembly.

Cell migration is initiated by lamellipodia—membrane‐enclosed sheets of cytoplasm containing densely packed actin filament networks. Although the molecular details of network turnover remain obscure, recent work points towards key roles in filament nucleation for Arp2/3 complex and its activator WAVE complex. Here, we combine fluorescence recovery after photobleaching (FRAP) of different lamellipodial components with a new method of data analysis to shed light on the dynamics of actin assembly/disassembly. We show that Arp2/3 complex is incorporated into the network exclusively at the lamellipodium tip, like actin, at sites coincident with WAVE complex accumulation. Capping protein likewise showed a turnover similar to actin and Arp2/3 complex, but was confined to the tip. In contrast, cortactin—another prominent Arp2/3 complex regulator—and ADF/cofilin—previously implicated in driving both filament nucleation and disassembly—were rapidly exchanged throughout the lamellipodium. These results suggest that Arp2/3‐ and WAVE complex‐driven actin filament nucleation at the lamellipodium tip is uncoupled from the activities of both cortactin and cofilin. Network turnover is additionally regulated by the spatially segregated activities of capping protein at the tip and cofilin throughout the mesh.

Cell migration is an essential process, both in unicellular organisms such as amoeba and as individual or collective motility in highly developed multicellular organisms like mammals. It is controlled by a variety of activities combining protrusive and contractile forces, normally generated by actin filaments. Here, we summarize actin filament assembly and turnover processes, and how respective biochemical activities translate into different protrusion types engaged in migration. These actin-based plasma membrane protrusions include actin-related protein 2/3 complex-dependent structures such as lamellipodia and membrane ruffles, filopodia as well as plasma membrane blebs. We also address observed antagonisms between these protrusion types, and propose a model – also inspired by previous literature – in which a complex balance between specific Rho GTPase signaling pathways dictates the protrusion mechanism employed by cells. Furthermore, we revisit published work regarding the fascinating antagonism between Rac and Rho GTPases, and how this intricate signaling network can define cell behavior and modes of migration. Finally, we discuss how the assembly of actin filament networks can feed back onto their regulators, as exemplified for the lamellipodial factor WAVE regulatory complex, tightly controlling accumulation of this complex at specific subcellular locations as well as its turnover.