Key Concepts of Section 17.6

• In skeletal muscle, contractile myofibrils are composed of thousands of repeating units called sarcomeres. Each sarcomere consists of two interdigitating filament types: myosin thick filaments and actin thin filaments (see Figure 17-30).

• Skeletal muscle contraction involves the ATP-dependent sliding of myosin thick filaments along actin thin filaments to shorten the sarcomere and hence the myofibril (see Figure 17-31).

• The ends of the actin thin filaments in skeletal muscle are stabilized by CapZ at the (+) end and by tropomodulin at the (−) end. Two large proteins, nebulin associated with the thin filaments and titin with the thick filaments, also contribute to skeletal muscle organization.

• Skeletal muscle contraction is subject to thin-filament regulation. At low levels of free Ca²⁺, tropomyosin blocks the interaction of myosin and F-actin, and the muscle is relaxed. At elevated levels of free Ca²⁺, the troponin complex associated with tropomyosin binds Ca²⁺ and moves the tropomyosin to uncover the myosin-binding sites on actin, allowing contraction (see Figure 17-34).

• Smooth muscle and nonmuscle cells have contractile bundles of actin and myosin filaments with an organization similar to that in skeletal muscle but less well ordered.

• Contractile bundles are subject to thick-filament regulation. A myosin light chain is phosphorylated by myosin light-chain kinase, which activates myosin and hence induces contraction. The MLC kinase is activated by binding of Ca²⁺-calmodulin when the free Ca²⁺ concentration rises (see Figure 17-36).

• Inactive myosin V is activated by binding cargo, which it transports processively along actin filaments.