Myosins, along with kinesins and dyneins, are “motor proteins” that participate in a variety of functions, including muscular contraction, cytokinesis, endocytosis, transport of cell organelles, signal transduction, and sensory functions such as hearing and vision. Like railway engines, the motor proteins move along tracks; in the case of myosin the interactions are with actin filaments, whereas kinesins and dyneins move along microtubules. All utilize chemical energy released during ATP hydrolysis to perform mechanical work.

Cardiac myosin is a tadpole-shaped molecule made up of two heavy chains and four light chains (Fig. 4-1). The “tail,” which is a “coiled coil” made up of two α-helical heavy chains wound around each other, provides rigidity to the thick filaments. The heavy chains contain paired “heads” which, along with the light chains, make up the cross-bridges that project from the thick filaments (see Chapter 1). Interactions between the cross-bridges, which contain the ATPase site of myosin, and actin filaments release the chemical energy that powers contraction.

Two fragments, called meromyosins, are released from myosin by the proteolytic enzymes trypsin and chymotrypsin. The smaller fragment, light meromyosin, is derived from the tail of the molecule, while the larger heavy meromyosin includes the head, a small portion of the tail, and the light chains (Fig. 4-2). Further digestion of heavy meromyosin with papain, another proteolytic enzyme, removes the rest of the tail, leaving a globular protein called heavy meromyosin subfragment 1 that includes the paired heads of the myosin heavy chains and the four light chains. Myosin ATPase activity and the ability to interact with actin are present in heavy meromyosin and heavy meromyosin subfragment 1.

Myosin forms filaments in vitro in which the cross-bridges project away from the center of these aggregates (Fig. 4-3). These structures, which resemble the thick filaments of striated muscle, allow polarized interactions between the cross-bridges and the thin filaments that enter the A-band from each of the two adjoining I bands. In resting hearts, where the thick and thin filaments are detached, the cross-bridges are nearly perpendicular to the long axis of the muscle (Fig. 4-4). Following activation, the cross-bridges attach and detach from actin in a series of steps that, like the oars of a racing shell, draw the thin filaments toward the center of the sarcomere. Because muscle volume remains virtually constant during contraction, the lateral distance between the thick and thin filaments is modified by changes in sarcomere length. The effects of


these changes in lattice spacing on tension development and shortening velocity are minimized by “hinges” in the myosin molecule (Fig. 4-1) which allow the cross-bridges to maintain contact with the thin filaments at short sarcomere lengths by varying the extent to which the myosin heads extend from the thick filaments (Fig. 4-5).

Actin, a highly conserved protein found in all eukaryotic cells, received its name because of its ability to activate myosin ATPase activity. Actin can be stabilized in vitro as a monomer, called G-actin (G = globular), or as the filamentous F-actin polymer (F = fibrous). Both G- and F-actin contain nucleotide- and cation-binding sites. The bound nucleotide in G-actin is ATP while F-actin ordinarily contains bound ADP; in both, the bound cation can be either calcium or magnesium. Actin monomers, which are much smaller than myosin (Table 4-1), are ovoid globular proteins that are ∼55 á in diameter. The polymer, a macromolecular helix in which two chains of actin monomers are wound around one another like two strings of beads, makes up the backbone of the thin filaments (Fig. 4-6) and represents one of the three types of cytoskeletal filament described in Chapters 1 and 5.

Two actin isoforms are found in the human heart, α-cardiac actin and α-skeletal actin. Adult human ventricles contain mainly α-cardiac actin; α-skeletal actin, which is present in smaller amounts, is the fetal isoform.

Actomyosins reconstituted in vitro from purified actin and myosin can hydrolyze ATP and undergo physicochemical changes similar to those that occur during muscle contraction. However, these two-protein actomyosins are not regulated by calcium because they lack the regulatory proteins (see below).

Tropomyosin is a homodimer or heterodimer made up of two α-helical peptide chains, called α- and β-tropomyosin. Although this protein had no known function for the first 15 years after it was first purified, its structure was of interest to physical chemists because, like the tail of the myosin molecule, it is a rigid coiled-coil. Tropomyosin itself has no biological activity, but when incorporated into reconstituted actomyosins, this seemingly inert protein regulates the interactions between myosin and actin. These regulatory effects arise from shifts in the position of tropomyosin in the grooves between the two F-actin chains in the thin filament (Fig. 4-7). The activity of the cardiac contractile proteins is inhibited when p38-MAPK, a stress-activated mitogen-activated protein kinase (see Chapter 9), causes a regulatory serine in tropomyosin to be dephosphorylated.

The troponin complex includes three proteins (Table 4-1) that, along with tropomyosin, are found in the thin filaments (Figs. 4-8 and 4-9). Troponin I received its name because its most important effect is to inhibit actin–myosin interactions, troponin T binds the troponin complex to tropomyosin, and troponin C contains the calcium-binding sites that participate in excitation-contraction coupling and relaxation.