Thick myofilaments

Thick myofilaments are made primarily of myosin. Myofibrillar myosin is one member of the multi-gene myosin II sub-family in the myosin super-family. Of the 15 types of myosin that have been described, at least 7 occur in mammalian species. Of these seven, three are fairly well characterized. Myosin 1 has a single heavy chain and one to three (usually one) light chain, which is calmodulin. Myosin V has a heavy chain dimer, and carries six light chains, which are usually calmodulin. Both types I and V are motors that carry organelles and vesicles along the actin filaments of the cytoskeleton and probably 

The head domain of myosin shown in its relation to the actin filament. The NH2-terminal end of the myosin heavy chain is in the globular head. The light chains bind to the neck region of the MHC. In this figure, the orientation of the myosin to the actin is that of the rigor bond, i.e., at the end of the power stroke. [From M. Irving and G. Piazzesi, Motions of myosin heads that drive muscle contraction. News Physiol. Sci. 12(6), 249-254 (December 1997).] 

There is also a designation based on the source of the MLC, with the subscripts f and s indicating the MLCs are from fast or slow muscle fibers, respectively. The relationship of one nomenclature to another is shown in Table 21-4. As explained under control of contraction, the regulatory light chain (rather than troponin) is the major regulatory protein under some circumstances in smooth muscle. 

Filament assembly depends on characteristics of the myosin tail, especially the LMM, in which much of the amino acid sequence exhibits a heptapeptide repeat. If the repeat is represented as ABCDEFG, residues A and D are hydrophobic and lie at points where the a-helices 

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. 

---

April, E. W., Brandt, P. W., and Elliott, G. F. (1971). The myofilament lattice Studies on isolated fibers. I. The constancy of the unit-cell volume with variation in sarcomere length in a lattice in which the thin-to-thick myofilament ratio is 6 1./. Cell Biol. 51, 72-82. 

Fig. 2.7. Cortical muscle element consisting of a contractile myofibril portion and a mycyton portion. Arrows indicate two of the thick myofilaments distributed among the more numerous thin myofilaments. (After Lumsden Hildreth, 1983.)...

Paramyosin, a major structural component of thick filaments. It occurs exclusively in invertebrate organisms, where it is widely distributed. Paramyosin is found in varying quantities in different muscles types ranging from smooth to cross-striated muscles. It interacts with the core proteins within the thick myofilaments as well as with the surrounding myosin components, thus stabilizing the thick myofilaments. It is not a component of the cytoskeleton [L. Winkelman, Comp. Biochem. Physiol. B 1976, 55, 391 P. R. Deitiker, H. F. Epstein, J. Cell Biol. 1993, 123, 303]. 

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. 

A fibre is packed with longitudinally arranged myofibrils that contract. This is the contractile unit of the fibre it is 1-3 pm thick in diameter. Each myofibril consists of about 1000-2000 filaments, which are known as myofilaments. 

Figure 13.5 Electron micrograph of part of a longitudinal section of a myofibril. Identification of components and the mechanism of contraction. When a muscle fibre is stimulated to contract, the actin and myosin filaments react by sliding past each other but with no change in length of either myofilament. The thick myosin strands in the A band are relatively stationary, whereas the thin actin filaments, which are attached to the Z discs, extend further into the A band and may eventually obliterate the H band. Because the thin filaments are attached to Z discs, the discs are drawn toward each other, so that the sarcomeres, the distance between the adjacent Z-discs, are compressed, the myofibril is shortened, and contraction of the muscle occurs. Contraction, therefore, is not due to a shortening of either the actin or the myosin filaments but is due to an increase in the overlap between the filaments. The force is generated by millions of cross-bridges interacting with actin filaments (Fig. 13.6). The electron micrograph was kindly provided by Professor D.S. Smith.

At the microscopic level, a single muscle cell has several hundred nuclei and a striped appearance derived from the pattern of myofilaments. Long, cylindrical muscle fibers are formed from several myoblasts in fetal development. Multiple nuclei are important in muscle cells because of the tremendous amount of activity in muscle. The myofilaments, actin and myosin, overlap one another in a very specific arrangement. Myosin is a thick pro-... 

The muscle cell membrane is termed the plasma-lemma. The cytoplasm of the muscle cell is filled with myofilaments, which form the myofibrils. Myofibrils are composed of sarcomeres, which consist of longitudinally directed thin and thick filaments and perpendicularly disposed z bands that are a-actin filaments. The myofibrils form the contractile apparatus of the muscle. 

Each myofibril consists of two different protein structures myofilaments. These are myosin thick (15 nm x 1.5 pm) and thin (7 nm x 1 pm) filaments made from actin, tropomyosin, and troponin. 

Muscle fibers contain long cylindrical myofibrils (typically 1-2 /xm in diameter) aligned longitudinally and consisting of interpenetrating arrays of thin myofilaments (6-7 nm in diameter) and thick filaments (15-16 nm in diameter). These structures are the contractile apparatus of the fiber (Figure 21-3). 

Muscle force generation is believed to arise from the formation of crossbridge bonds between thick and thin myofilaments within the basic building block of muscle, the sarcomere. These structures, in the nanometer to micrometer range, must be viewed by electron microscopy or x-ray diffraction, limiting study to fixed, dead material. Consequently, muscle contraction at the sarcomere level must be described by models that integrate metabolic and structural information. 

Asynchrony in bond formation and unequal numbers of bonds formed in each half sarcomere, as well as mechanical disturbances such as shortening and imposed length transients, cause movement of thick with respect to thin filaments. Since myofilament masses are taken into account these movements take the form of damped vibrations. Such vibrations occur with a spectrum of frequencies due to the distributed system properties. 

For the thick filaments the number of filaments is half of the number of the thin filaments but the radius is twice as large. Therefore both arms of the viscous drag couple will exert the same force on the two sets of the inter-digitated myofilaments and cause them to slide into each other. 

Muscle proteins proteins present in muscle and constituting 20 % of muscle tissue. The insoluble contractile proteins are organized in myofibrils which consist of organized arrays of thick and thin myofilaments Thick filaments (about 1.5 im long, 12-16 nm diam.) contain 200-400 molecules of the protein myosin (Fig.l). 

Fig. 331. Different interference colours of the myofilaments within the myocardiocytes of a rat (No. 299) 24 h after a subcutaneous injection of 30 mg isoproterenol per kg body weight. Fixed in Bouin s fluid, paraffin section 4 (oa in thickness, unstained, embedded in glycerol. Interference contrast FI lOOx/ 1.36/< /-...

In principle, microfibrils are composed by two types of myofilaments, i.e. the thick myosin filament, (j) = 100 A and L = 1.5 pm, and the thin actin filaments, ( ) = 50 A and L = 2.0 pm respectively. In cross section, under very high magnification, both A and I bands can be seen to be hexagonal networks. Figure 4.3. These networks are apparently ordered and fixed at the M- and Z-lines. In the region where the A and I bands overlap (sometimes known as the H band) the two hexagonal networks intermesh so that each myosin filament is surrounded by six actin filaments. These networks appear to be anchored to (and through) the cell membrane in two ways. At the ends of fibrils, special structures... 

---