Fig. 7. Model of the actin compartments defined by tropomyosin in the cell. Three compartments are shown. Green, the dynamic leading edge compartment of the lamellipodium contains short actin filaments that are severed by cofilin. Here the Arp2/3 complex induces branch formation on newly polymerized actin filaments to give highly branched actin filaments (see top box). Tropomyosin is absent. Red: the second compartment is the base of the lamellipodium. It contains longer, unbranched actin filaments as well as tropomyosin bound to actin, free cofilin and free Arp2/3 complex. Tropomyosin stabilizes actin filaments and protects them from cofilin severing and Arp2/3 complex-induced branch formation (see bottom box). White: the third compartment is the main body of the cell that contains stress fibers stabilized by tropomyosin, as in the red compartment, and other actin binding proteins. The main body of the cell also contains free cofilin and Arp 2/3 complex.