Figure 105

Sarcomeres in different functional stages. In the resting state (middle), interdigitation of thin (actin) and thick (myosin) filaments is not complete; the H and I bands are relatively wide. In the contracted state (bottom), the interdigitation of the thin and thick filaments is increased according to the degree of contraction. In the stretched state (top), the thin and thick filaments do not interact; the H and I bands are very wide. The length of the A band always remains the same and corresponds to the length of the thick filaments; the lengths of the H and I bands change, again in proportion to the degree of sarcomere relaxation or contraction.

Myosin II, a 510-kDa protein, is composed of two polypeptide heavy chains (222 kDa each) and four light chains. Light chains are of two types (18 kDa and 22 kDa), and one molecule of each type is present in association with each myosin head. The phosphorylation by myosin light chain kinase of one of the two types of myosin light chains initiates contraction in smooth muscles (see page 267). Each heavy chain has a small globular head that projects at an approximately right angle at one end of the long rod-shaped molecule. This globular head has two specific binding sites, one for ATP and one for actin. It also demonstrates ATPase and motor activity. The myosin molecules aggregate tail to tail to form the thick filaments; the rod-shaped segments overlap, so that the globular heads project from the thick filament. The "bare" zone in the middle of the filament, i.e., the portion of the filament that does not have globular projections, is the H band. The projecting globular heads of the myosin molecules form cross-bridges between the thick and thin filaments on either side of the H band (see Fig. 10.5).

actin filaments are polar; all G-actin molecules are oriented in the same direction. The plus end of each filament is bound to the Z line by a-actinin; the minus end extends toward the M line. Each G-actin molecule of the thin filament has a binding site for myosin.

Tropomyosin is a 64-kDa protein that also consists of a double helix of two polypeptides. It forms filaments that run in the groove between the F-actin molecules in the thin filament. In resting muscle, tropomyosin and the troponins mask the myosin-binding site on the actin molecule.

Troponin actually consists of a complex of three globular subunits. Each tropomyosin molecule contains one troponin complex. Troponin-C (TnC) is the smallest subunit of the troponin complex (18 kDa). It binds Ca2+, the essential step in the initiation of contraction (see below). Tro-ponin-T (TnT), a 30-kDa subunit, binds to tropomyosin, anchoring the troponin complex. Troponin-I (Tnl), also a 30-kDa subunit, binds to actin, thus inhibiting actin-myosin interaction.

Accessory proteins maintain precise alignment of thin and thick filaments

To maintain efficiency and speed of muscle contraction, both thin and thick filaments in each myofibril must be aligned precisely and kept at an optimal distance from one another. Proteins known as accessory proteins are essential in regulating the spacing, attachment, and alignment of the myofilaments. These structural protein components of skeletal muscle fibrils constitute less than 25% of the total protein of the muscle fiber. They include (Fig. 10.6)

• Titin, a large (2500-kDa) protein, forms an elastic lattice that anchors thick filaments in the Z lines. Two spring-

tropomyosin molecules troponin complex

end tropomyosin molecules troponin complex thick filament

thick filament

18-kDa light chain 22-kDa light chain actin-

binding site ATP-

binding site head' ~ "actin-binding site myosin molecule

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