Another group of proteins can cap the ends of actin filaments but, unlike severing proteins, cannot break filaments to create new ends. One such protein, CapZ, binds the (+) ends of actin filaments independently of Ca2+ and prevents the addition or loss of actin subunits from the (+) end. Capping by this protein is inhibited by PIP2, suggesting that its activity is regulated by the same signaling pathways that control cofilin and profilin. Tropomodulin, which is unrelated to CapZ in sequence, caps the (—) ends of actin filaments. Its capping activity is enhanced in the presence of tropomyosin, which suggests that the two proteins function as a complex to stabilize a filament. An actin filament that is capped at both ends is effectively stabilized, undergoing neither addition nor loss of subunits. Such capped actin filaments are needed in places where the organization of the cytoskeleton is unchanging, as in a muscle sarcomere (Figure 19-11) or at the erythrocyte membrane.
▲ FIGURE 19-11 Diagram of sarcomere in skeletal muscle showing location of actin-capping proteins. CapZ (green) caps the (+) ends of actin thin filaments, which are located at the Z disk separating adjacent sarcomeres. Tropomodulin (yellow) caps the (—) ends of thin filaments, located toward the center of a sarcomere. The presence of these two proteins at opposite ends of a thin filament prevents actin subunits from dissociating during muscle contraction.
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