Protein assemblies are recruited to the tips of lamellipodia and filopodia that regulate and drive the polymerisation of actin for protrusion. For reviews see Stradal et al., 2004; Chesarone and Goode, 2009; Takenawa and Suetsugu, 2007; Derivery and Gautreau, 2010.
Live cell microscopy with GFP-tagged proteins has shown that members of this complex are specifically targeted to lamellipodia at the onset of protrusion (Hahne et al., 2001; Nakagawa et al., 2003; Rottner et al., 1999; Stradal et al., 2001). The following movies show some members of the tip complex in lamellipodia and filopodia: mouse melanoma cells were transfected with the proteins indicated, tagged with GFP:
The WAVE complex comprises in mammals Scar/WAVE and Abi, Sra1/PIR121, Nap1 (or haematopoietic Hem1) and Brick1/HSPC300. The role of WAVE is to activate the Arp2/3 complex to initiate actin branching for the generation of actin networks in lamellipodia. Actin branching can be reproduced in vitro by mixing actin with an active domain of the WAVE or WASP protein and the Arp2/3 complex (Amann and Pollard, 2001):
Branching of actin in vitro by WAVE and the Arp2/3 complex observed using TIRF microscopy. Courtesy of Kai Schlueter and Theresia Stradal (unpublished).
As shown by the Welch group, the Arp2/3 complex is localized over the whole lamellipodium network. Photobleach experiments show that the Aprp2/3 complex treadmills with actin in the lamellipodium:
Fluorescence recovery after photobleaching (FRAP) in the lamellipodium of a B16 melanoma cell expressing Actin-RFP and ArpC5-GFP. Note that the recovery of the Arp label parallels the recovery of actin (Lai et al., 2008). As shown by the Svitkina group, the Arp complex is absent from the core of actin bundles (microspikes) that transverse the lamellipodium. Video courtesy of Malgozata Szcozodrak and Klemens Rottner (unpublished).
VASP is a member of the Ena/VASP family of proteins that act as processive elongators of actin filaments (Breitsprecher et al., 2011), as shown in the two figures below:
Plastic beads coated with VASP promote the growth of actin filaments at the bead surface by tethering actin filaments and feeding monomers into the plus ends. From Breitsprecher et al., 2008.
Shows again the localization of VASP at protruding lamellipodia and filopodia tips in a fish fibroblast expressing VASP-GFP and Actin-RFP.
Schematic illustration of some of the proteins and protein complexes engaged in initiating and organising lamellipodia and filopodia. (Small et al., 2002)
- Amann, K.J., Pollard T.D. (2001). The Arp2/3 complex nucleates actin filament branches from the sides of pre-existing filaments. Nat Cell Biol. 3:306–10.
- Breitsprecher, D., Kiesewetter, A.K., Linkner, J., Urbanke, C., Resch, G.P., Small, J.V., Faix, J. (2008). Clustering of VASP actively drives processive, WH2 domain-mediated actin filament elongation. EMBO J. 2008 27(22): 2943 — 2954.
- Breitsprecher, D., Kiesewetter, A.K., Linkner, J., Vinzenz, M., Stradal, T.E., Small, J.V., Curth, U., Dickinson, R.B., Faix J. (2011). Molecular mechanism of Ena/VASP-mediated actin-filament elongation. EMBO J. 30:456–67.
- Chesarone, M., A., Goode, B., L. (2009). Actin nucleation and elongation factors: mechanisms and interplay. Curr. Opin. Cell. Biol. 21, 28–37.
- Derivery, E. and Gautreau, A. (2010). Generation of branched actin networks: assembly and regulation of the N-WASP and WAVE molecular machines. Bioessays. 32, 119–131.
- Hahne, P., Sechi, A., Benesch, S. and Small, J.V. (2001). Scar/WAVE is localised at the tips of protruding lamellipodia in living cells. FEBS Lett. 492, 215–220.
- Lai, F.P.L., Szczodrak, M., Block, J., Mannherz, H.G., Small, J.V., Stradal, T.E.B., Dunn, G.A. and Rottner G.A.K. (2008). Arp2/3 complex interactions and actin network turnover in lamellipodia. EMBO J. 27, 982 – 992.
- Nakagawa, H., Miki, H. Nozumi, M. Takenawa, T. Miyamoto, S. Wehland, J. and Small, J.V. (2003). IRSp53 is co-localised with WAVE2 at the tips of protruding lamellipodia and filopodia independently of Mena. J Cell Sci. 116, 2577–83.
- Rottner, K., Behrendt, B., Small, JV, Wehland, J. (1999). VASP dynamics during lamellipodia protrusion. Nat. Cell. Biol. 1, 321–322.
- Small, JV., Stradal, T., Vignal, E. and Rottner, K. (2002). The lamellipodium: where motility begins. Trends Cell Biol., 12, 112–120.
- Stradal, T., Courtney, K.D., Rottner, K., Hahne, P., Small, J.V. and Pendergast, A.M. (2001). The Abl-interactor (Abi) proteins localize to sites of actin polymerization at the tips of lamellipodia and filopodia. Curr. Biol. 11, 891–895.
- Stradal, T. E., Rottner, K., Disanza, A., Confalonieri, S., Innocenti, M., Scita, G. (2004). Regulation of actin dynamics by WASP and WAVE family proteins. Trends Cell. Biol. 14, 303–311.
- Takenawa, T., Suetsugu, S. (2007). The WASP-WAVE protein network: connecting the membrane to the cytoskeleton. Nat. Rev. Mol. Cell. Biol. 8, 37–48.