The major proteins in muscle are actin and myosin. Actin and myosin are also found in many other kinds of cells besides muscles and are involved in several kinds of cellular and intracellular motions (e.g., cell motility and changes of cell shape).

Actin - Under physiological conditions, actin exists as a long, helical polymer (fibrous actin, or F-actin) of a globular protein monomer (G-actin). The structure of the G-actin monomer, shown in Figure 8.1, is a two-domain molecule with a mass of 42,000 Daltons. The binding of ATP by a G-actin monomer leads to polymerization (i.e., the formation of F-actin). The ATP is subsequently hydrolyzed, but the ADP is retained in the actin filament. Within F-actin filaments, the G-actin monomers are arranged in a two-strand helix. Because the subunits are asymmetric, each F-actin filament has two distinguishable ends called the plus end and the minus end. The polymerization reaction exhibits a preferred direction-the plus end grows much more rapidly than the minus end. Actin filaments carry sites on each subunit that can bind to myosin.

Myosin - The functional myosin molecule (Figure 8.3) is composed of six polypeptide chains: two identical heavy chains (M = 230,000) and two each of two kinds of light chains (M = 20,000) - together they form a complex of molecular weight 540,000. The heavy chains have long -helical tails and globular head domains. The -helical tails are interwound into a two-strand coiled coil and the light chains are bound to the globular head domains. Between each head domain and tail domain is a flexible stalk. The coiled-coil structure of the tails is reminiscent of the structure of -keratin (see Figure 6.11a).

The myosin molecule can be cleaved by proteases, as shown in Figure 8.4. The tail domain can be cleaved at a specific point by trypsin to yield fragments called light meromyosin and heavy meromyosin. Myosin exhibits aspects of both fibrous and globular proteins, and its functional domains play quite different roles. The tail domains have a pronounced tendency to aggregate, causing myosin molecules to form thick bipolar filaments. The head domains, with their attached light chains, are often called headpieces; they have a strong tendency to bind to actin.