Actin Filaments

Contractile activity in muscle cells results primarily from an interaction between two proteins: actin and myosin. Actin is present in muscle as a thin (5–7 nm in diameter) filament composed of globular subunits organized into a double-stranded helix (Figure 2–36). Structural and biochemical studies reveal that there are several types of actin and that this protein is present in all cells.

Figure 2-36

The cytosolic actin filament. Actin dimers are added to the plus (+) end and removed at the minus (–) end, dynamically lengthening or shortening the filament, as required by the cell. (Redrawn and reproduced, with permission, from Junqueira LC, Carneiro J: Biologia Celular e Molecular, 6th ed. Editora Guanabara, 1997.)

Within cells, microfilaments can be organized in many forms.

1. In skeletal muscle, they assume a paracrystalline array integrated with thick (16-nm) myosin filaments.

2. In most cells, actin filaments form a thin sheath just beneath the plasmalemma, called the cell cortex. These filaments appear to be associated with membrane activities such as endocytosis, exocytosis, and cell migratory activity.

3. Actin filaments are intimately associated with several cytoplasmic organelles, vesicles, and granules. The filaments are believed to play a role in moving and shifting cytoplasmic components (cytoplasmic streaming).

4. Actin filaments are associated with myosin and form a "purse-string" ring of filaments whose constriction results in the cleavage of mitotic cells.

5. In most cells, actin filaments are found scattered in what appears to be an unorganized fashion within the cytoplasm (Figure 2–31).

Although actin filaments in muscle cells are structurally stable, in nonmuscle cells they readily dissociate and reassemble. Actin filament polymerization appears to be under the direct control of minute changes in Ca²⁺ and cyclic AMP levels. A large number of actin-binding proteins have been demonstrated in a wide variety of cells, and much current research is focused on how these proteins regulate the state of polymerization and lateral aggregation of actin filaments. Their importance can be deduced from the fact that only about half the cell's actin is in the form of filaments.

Presumably, most actin filament-related activities depend upon the interaction of myosin with actin. (The structure and activity of the thick myosin filaments are described in the section on muscle tissues.)

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