Figure 16–78 Actin filament nucleation and web formation by the Arp 2/3 complex in lamellipodia. (A) A keratocyte with actin filaments labeled in red by fluorescent phalloidin and the Arp 2/3 complex labeled in green with an antibody against one of its subunits. The Arp 2/3 complex is highly concentrated near the front of the lamellipodium, where actin nucleation is most active. (B) Electron micrograph of a platinum-shadowed replica of the leading edge of a keratocyte, showing the dense actin filament meshwork. The labels denote areas enlarged in (C). (C) Close-up views of the marked regions of the actin web at the leading edge shown in (B). Numerous branched filaments can be seen, with the characteristic 70° angle formed when the Arp 2/3 complex nucleates a new actin filament off the side of a preexisting filament (see Figure 16–16). (From T. Svitkina and G. Borisy,
J. Cell Biol. 145:1009–1026, 1999. With permission from the authors.)
Figure 16–77 Migratory keratocytes from a fish epidermis. (A) Light micrographs of a keratocyte in culture, taken about  15 seconds apart. This cell is moving at about 15 μm/min (Movie 16.13 and see Movie 1.1). (B) Keratocyte seen by scanning electron microscopy, showing  its broad, flat lamellipodium and small  cell body, including the nucleus, carried  up above the substratum at the rear.
(C) Distribution of cytoskeletal filaments in this cell. Actin filaments (red) fill the large lamellipodium and are responsible for the cell’s rapid movement. Microtubules (green) and intermediate filaments (blue) are restricted to the regions close to the nucleus. (A and B, courtesy of Juliet Lee.) 954 Chapter 16: The Cytoskeleton
Maintenance of unidirectional motion by lamellipodia is thought to require the cooperation and mechanical integration of several factors. Filament nucleation is localized at the leading edge, with new actin filament growth occurring primarily in that location to push the plasma membrane forward. Most filament depolymerization occurs at sites located well behind the leading edge. Because cofilin (see Figure 16–20) binds cooperatively and preferentially to actin filaments containing ADP-actin (the D form), the new T-form filaments generated at the leading edge should be resistant to depolymerization by cofilin (Figure 16–79). As the filaments age and ATP hydrolysis proceeds, cofilin can efficiently disassemble the older filaments. Thus, the delayed ATP hydrolysis by filamentous actin is thought to provide the basis for a mechanism that maintains an efficient, unidirectional treadmilling process in the lamellipodium (Figure 16–80); it also explains the intracellular movement of bacterial pathogens such as Listeria (see Figure 16–25).
Myosin Contraction and Cell Adhesion Allow Cells to Pull Themselves Forward
Forces generated by actin filament polymerization at the front of a migrating cell are transmitted to the underlying substratum to drive cell motion. For the leading
Figure 16–79 Cofilin in lamellipodia.