Myofibril proteins

Sato, R., Katayama, S., Sawabe,T., and Saeki, H. 2003. Stability and emulsion-forming ability of water-soluble fish myofibriller protein prepared by conjugation with alginate oligosaccharide. /. Agr. Food Chem., 57,4376. 

Table II. Size and Shape of Fish Myofibril Proteins" Sedimenta- Intrinsic tion Viscosity ...

The myofibril is comprised of relatively few proteins compared with the 200 or more different proteins that exist in a muscle cell. Table V lists the known myofibril proteins of vertebrate skeletal muscle and the approximate amount of each protein in the myofibril. Note that myosin and actin comprise up to 75% of the myofibrillar proteins and together are necessary and sufficient for in vitro contraction. Contraction of... 

Hatate, H., Numata, Y. and Kochi, M. (1990) Synergistic effect of sardine myofibril protein hydrolyzates with antioxidants. Nippon Suisan Gakkaishi 56, 1011. 

Two cytoskeletal proteins, tltln (also known as connectm) and nebulm, account for 15% of the total protein in the myofibril. Together these proteins form a flexible filamentous network that surrounds the myofibrils. Titin is an elastic protein and can stretch under tension. Its discovery and characteriza-... 

FIGURE 17.21 A drawing of the arrangement of the elastic protein titin in the skeletal mnscle sarcomere. Titin filaments originate at the periphery of the M band and extend along the myosin filaments to the Z lines. These titin filaments produce the passive tension existing in myofibrils that have been stretched so that the thick and thin filaments no longer overlap and cannot interact. (Adapted from Ohtsuki, ., Maruyama, K, and Ebashi,. S ., 1986. Advances ia Protein Chemisti y 38 1—67.)... 

Muscle contraction is initiated by a signal from a motor nerve. This triggers an action potential, which is propagated along the muscle plasma membrane to the T-tubule system and the sarcotubular reticulum, where a sudden large electrically excited release of Ca " into the cytosol occurs. Accessory proteins closely associated with actin (troponins T, I, and C) together with tropomyosin mediate the Ca -dependent motor command within the sarcomere. Other accessory proteins (titin, nebulin, myomesin, etc.) serve to provide the myofibril with both stability... 

When myofibrils are examined by electron microscopy, it appears that each one is constructed of two types of longimdinal filaments. One type, the thick filament, confined to the A band, contains chiefly the protein myosin. These filaments are about 16 run in diameter and arranged in cross-section as a hexagonal array (Figure 49-2, center right-hand cross-section). 

The muscle is a highly organized tissue, built up of individual cells known as fibres, which are held together by connective tissue. Each muscle fibre consists of a high number of single strands of myofibrils. The myofibrils are again comprised of myofilaments. The myofilaments are divided into thin and thick filaments, which mainly contain two filamentary proteins, actin and myosin, respectively. The myofibrils occupy approximately 80% of the muscle cell volume, and the majority of the water, which makes up about 75% of the muscle, is located in the spaces between thin and thick filaments. A schematic drawing of muscle structure is shown in Fig. 1. 

The anatomical unit of muscle is an elongated cell called a fibre. Each individual fibre cell consists of myofibrils which are bundles of contractile protein filaments composed of actin and myosin (Figure 7.1). Differences in structure indicate that muscles have evolved to perform particular functions. Although the structure of fibres, myofibrils and filaments of actin and myosin, is similar in all muscle types, their arrangement, action and control allow identification of three tissue types ... 

Contractile proteins which form the myofibrils are of two types myosin ( thick filaments each approximately 12 nm in diameter and 1.5 (im long) and actin ( thin filaments 6nm diameter and 1 (Am in length). These two proteins are found not only in muscle cells but widely throughout tissues being part of the cytoskeleton of all cell types. Filamentous actin (F-actin) is a polymer composed of two entwined chains each composed of globular actin (G-actin) monomers. Skeletal muscle F-actin has associated with it two accessory proteins, tropomyosin and troponin complex which are not found in smooth muscle, and which act to regulate the contraction cycle (Figure 7.1). 

Fig. 2. Macroscopic and microscopic structure of muscle (a) Entire muscle and its cross-section with fatty septa, (b) Fascicle with several muscle fibres (cells). A layer of fat along the fascicle is indicated, (c) Striated myofibre corresponding with one single muscle cell containing several nuclei. The lengths of a myofibre can be several tens of centimetres, (d) Myofibril inside a myocyte. It is one contractile element and contains actin and myosin and further proteins important for the muscular function, (e) Electron myograph of human skeletal muscle showing the band structure caused by the contractile myofilaments in the sarcomeres. One nucleus (Nu) and small glycogen granules (arrow, size <0.1 pm) are indicated.

Actin and myosin are the two principal muscle proteins in the myofibrils (Fig. 2d). They are arranged in a cylindrical structure, usually with six thin actin strands surrounding a thicker myosin fibril. The myosin fibril has... 

In the cells of this tissue, which are known as fibres, the two major proteins, actin and myosin, are orgaiused to form myofibrils. These are structural rods that can contract (Figure 1.11). This enables muscle cells to shorten, which provides for movement and locomotion (Figure 1.12). There are three types ... 

In 1987 the gene responsible for muscular dystrophy was identified, leading to the isolation of a protein, known as dystrophin, which is either totally absent in Duchenne, or partially absent in the Becker type. The protein is located on the inside of the plasma membrane of all muscles (and some neurones). Although its precise function is not known, the mutant form results in structural abnormalities of the plasma member which results in degradation of myofibrils, but the hnk between the abnormalities of the membrane and degradation is not known. One theory is that it leads to an increase in the activity of a Ca " ion channel in the membrane and, therefore, a marked increase in the Ca ion concentration in the cytosol. This chronic elevation results in the activation of calpain, which leads to protein breakdown and the degeneration within the fibre (Chapter 13). 

Each fibre contains an array of parallel myofibrils each consisting of overlapping thick and thin filaments that form repeating units (sarcomeres) along the length of the fibre (Figure 13.5). The thick filaments are composed almost entirely of the protein myosin, whereas the thin filaments contain actin as well as troponin and tropomyosin. 

Myofibrils contain four major proteins myosin, actin, tropomyosin and troponin. 

Myosin is quantitatively the most important protein in the myofibrils, representing 65% of the total. It is shaped like a golf club (bottom right). The molecule is a hexamer consisting of two identical heavy chains (2 X 223 kDa) and four light chains (each about 20 kDa). Each of the two heavy chains has a globular head at its amino end, which extends into a tail about 150 nm long in which the two chains are intertwined to form a superhelix. The small subunits are attached in the head area. Myosin is present as a bundle of several hundred stacked molecules in the form of a thick myosin filament. The head portion of the molecule acts as an ATPase, the activity of which is modulated by the small subunits. 

In the resting state, the Ca "" level in the sarcoplasm is very low (less than 10 M). By contrast, the sarcoplasmic reticulum (SR), which corresponds to the ER, contains Ca "" ions at a concentration of about 10 M. The SR is a branched organelle that surrounds the myofibrils like a net stocking inside the muscle fibers (illustrated at the top using the example of a heart muscle cell). The high Ca "" level in the SR is maintained by Ca ""-transporting ATPases (see p. 220). In addition, the SR also contains calsequestrin, a protein (55 kDa) that is able to bind numerous Ca "" -ions via acidic amino acid residues. 

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