Muscle thick filaments

Myosin II is in the same subfamily as the myosins in muscle thick filaments and it forms large, two-headed myosins with two light chains per heavy chain. Although myosin II is abundantly expressed in brain, little is known about its function in the nervous system. In other nonmuscle cells, myosin II has been implicated in many types of cellular contractility and may serve a similar function in developing neurons. However, myosin II remains abundant in the mature nervous system, where examples of cell contractility are less common. 

Chew, M. W., and Squire, J. M. (1995). Packing of alpha-helical coiled-coil myosin rods invertebrate muscle thick filaments./. Struct. Biol. 115, 233-249. 

Trinick, J. A. (1981). End-filaments A new structural element of vertebrate skeletal muscle thick filaments. / Mol. Biol. 151, 309-314. 

Organization of myosin in striated-muscle thick filaments. Filament formation begins with tail-to-tail (antiparallel) binding of myosin molecules, with subsequent parallel binding of myosin molecules to the ends of the initial nucleus, leaving the central clear zone. There are approximately 500 myosin molecules per striated thick filament. 

The answer is b. (Murray, pp 48-62. Scriver, pp 3-45. Sack, pp 1-3. Wilson, pp 101-120.) Two kinds of interacting protein filaments are found in skeletal muscle. Thick filaments 15 nm in diameter contain primarily myosin. Thin filaments 7 nm in diameter are composed of actin, troponin, and tropomyosin. The thick and thin filaments slide past one another during muscle contraction. Myosin is an ATPase that binds to thin filaments during contraction, ot-actinin can be found in the Z line. 

In passive muscle, thick filaments exist in stable suspension at the center of the sarcomere, which contributes to the muscle s passive elastic properties [32]. 

Proteins can be broadly classified into fibrous and globular. Many fibrous proteins serve a stmctural role (11). CC-Keratin has been described. Fibroin, the primary protein in silk, has -sheets packed one on top of another. CoUagen, found in connective tissue, has a triple-hehcal stmcture. Other fibrous proteins have a motile function. Skeletal muscle fibers are made up of thick filaments consisting of the protein myosin, and thin filaments consisting of actin, troponin, and tropomyosin. Muscle contraction is achieved when these filaments sHde past each other. Microtubules and flagellin are proteins responsible for the motion of ciUa and bacterial dageUa. 

FIGURE 17.12 Electron micrograph of a skeletal muscle myofibril (in longitndinal section). The length of one sarcomere is indicated, as are the A and I bands, the H zone, the M disk, and the Z lines. Cross-sections from the H zone show a hexagonal array of thick filaments, whereas the I band cross-section shows a hexagonal array of thin filaments. (Photo courtesy of Hugh Huxley, Brandeis University)... 

Smooth muscles, as the name implies, do not contain sarcomeres. In fact, it was initially difficult to demonstrate the presence of thick filaments in smooth muscle, although their presence is now well-established. On the other hand, it is very difficult to demonstrate thick filaments in highly motile cells, such as macrophages and neutrophils, and this may reflect the necessity to rapidly form and redistribute cytoskeletal elements during migration. Thick filaments in smooth muscles appear to be considerably longer than those in striated muscles. They run diagonally in smooth muscle cells and attach to the membrane at structures known as dense bodies. Thus, there is a cork-screw effect when smooth muscles contract (Warshaw etal., 1987). 

Both the thick and thin filaments contain other proteins. For example, the thick filament contains titin (molecular weight about 3,000,000) and the thin filament contains nebulin (although not in cardiac muscle), and the regulatory proteins troponin (molecular weight about 33,000) and tropomyosin (molecular weight about 70,000). Nebulin and titin are thought to be ruler proteins, that is, they determine the overall length of the thin and the thick filament, respectively. The... 

Figure 8. (Continued). As described above, the packing of myosin molecules into the thick filament is such that a layer of heads is seen every 14.3 nm, and this reflection is thought to derive from this packing. Off the meridian the 42.9 nm myosin based layer line is shown. This arises from the helical pitch of the thick filament, due to the way in which the myosin molecules pack into the filament. The helical pitch is 42.9 nm. c) Meridional reflections from actin. Actin based layer lines can be seen at 35.5 nm, 5.9 nm and 5.1 nm (1st, 6th, and 7th layer lines)and they all arise from the various helical repeats along the thin filament. Only the 35.5 nm layer line is shown here.The 5.9 nm and 5.1 nm layer lines arise from the monomeric repeat. The 35.5 nm layer line arises from the long pitch helical repeat and is roughly equivalent to seven actin monomers. A meridional spot at 2.8 nm can also be seen, d) The equatorial reflections, 1,0 and 1,1 which arise from the spacings between crystal planes seen in cross section of muscle.

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