A family of actin-related proteins (Arps), exhibiting 50 percent sequence similarity with actin, has been identified in many eukaryotic organisms. One group of Arps, a complex of seven proteins called Arp2/3, stimulates actin assembly in vivo. (Another Arp group that is associated with micro-tubules and a microtubule motor protein is discussed in the next chapter.) Isolated from cell extracts on the basis of its ability to bind profilin, the Arp2/3 complex binds at 70° to the side of an actin filament to nucleate a daughter filament. The combination of mother and daughter filaments creates a branched network in which Arp2/3 is located at the branch points (Figure 19-12). As a result, the newly created ends of filaments elongate and create the force to push the membrane forward.
M FIGURE 19-12 Branched actin filaments with Arp2/3 at the branch points. An extensive network of actin filaments fills the cytoplasm at the leading edge of a keratinocyte. Within selected areas of the network, highly branched filaments (green) are seen. At each branch point lies the Arp2/3 complex. [From T M. Svitkina and G. G. Borisy, 1999, J. Cell Biol. 145:1009; courtesy of T M. Svitkina and G. G. Borisy.]
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