Figure 243

Polymerization of actin filaments. Actin filaments are polarized structures. Their fast-growing end is referred to as the plus (+) or barbed end; the slow-growing end is referred to as the minus (-) or pointed end. The dynamic process of actin polymerization requires energy in the form of an ATP molecule that is hydrolyzed to ADP after a G-actin molecule is incorporated into the filament.

(j) actin bound to adp r IQ) actin bound to atp pi 1-

• Actin-capping proteins block further addition of actin molecules by binding to the free end of an actin filament. An example is tropomodulin, which can be isolated from skeletal and cardiac muscle cells. Tropomodulin binds to the free end of actin myofilaments, regulating the length of the filaments in a sarcomere.

• Actin cross-linking proteins are responsible for cross-linking actin filaments with each other. An example of such proteins can be found in the cytoskeleton of erythrocytes. Several proteins, such as spectrin, adductin, protein 4.1, or protein 4.9, are involved in cross-linking actin filaments (see Fig. 9.4, page 219).

• Actin motor proteins belong to the myosin family, which hydrolyzes ATP to provide the energy for movement along the actin filament from the minus end to the plus end. Some cells, such as muscle cells, are characterized by the size, amount, and nature of the filaments and actin-rnotor proteins they contain. There are two types of filaments (myofilaments) present in muscle cells: 6- to 8-nm actin filaments (called thin filaments) (Fig. 2.44) and 15-nm filaments (called thick filaments) of myosin II, which is the predominant protein in muscle cells. Myosin II is a double-headed molecule with an elongated rod-like tail. The specific structural and functional relationships among actin, myosin, and other ABPs in muscle contraction are discussed in Chapter 10 (Muscle Tissue).

In addition to myosin II, nonmuscle cells contain myosin I, a protein with a single globular domain and short tail that attaches to other molecules or organelles. Extensive studies have revealed the presence of a variety of other nonmuscle myosin isoforms that are responsible for motor functions in many specialized cells, such as melanocytes,

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