Changes in the actin cytoskeleton are a fundamental part of the motile response of cells to extracellular ligands. Dynamic alterations in both the structural and biochemical properties of the actin network mediate changes in cell shape and migration (Cooper, 1991; Gips et al., 1994). This meshwork of microfilaments is responsible for many characteristic intracellular structures, including stress fibers, the cortical network, and contractile rings. Actin filament turnover, for example, is associated with the formation of microspikes and lamellipodia at the leading edge of motile cells. In migrating cells, the forward extension of the cellular lamellipodium, a fan-shaped structure filled with a dense, cross-linked meshwork of actin filaments, is driven by the polarized assembly of actin monomers. This assembly is not unique to the lamellipodium, however, because it shares features with other actin-based motile processes occurring beneath the plasma membrane of stimulated platelets, neutrophils, chemotactic amoebas, or even at the rear of the pathogenic bacterium Listeria monocytogenes during its propulsive movement through the host cell cytoplasm (Theriot et al., 1994).