Striated muscle sarcomeres

Electron micrograph of a striated muscle sarcomere showing the appearance of filamentous structures when cross-sectioned at the locations illustrated below. (Electron micrograph courtesy of Dr. Hugh Huxley, Brandeis University.)... 

Other M. p. are filamin, M, 250,000, which binds ac-tin vinculin, 130,000, which is part of the Z line, and titin (or connectin), M, 250,000, found in cardiac and skeletal muscle. The giant, single molecule of 11-tin forms a filament extending from the M-line to the Z-line in the striated muscle sarcomere. In smooth muscle, a-actinin, vinculin and filamin anchor the thin filaments to the cell membrane. [R.M.Bagby in Newman Stephens (eds.) Biochemistry of Smooth Muscle (CRC Press, Boca Raton, 1983) pp.1-84 R.M.Dowben J.W.Shay (eds.) Cell and Muscle Motility Vol.4 (Plenum, New York, 1983) S.B.Marston C.W.J. Smith J. Muscle Res Cell Motil. 6 (1985) 669-708 K.Wang in Cell and Muscle Motility (ed. J.W.Shay) Vol.6 (Plenum, New York, 1985) pp. 315-369 J.-P.Jin J. Biol. Chem. 270 (1995) 6908-6916 B.J.Agnew etal. J. Biol. Chem. 27 (1995) 17582-17587 A.S.Rovner etal. J. Biol. Chem. 270 (1995) 30260-30263]... 

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). 

Smooth muscles have molecular structures similar to those in striated muscle, but the sarcomeres are not aligned so as to generate the striated appearance. Smooth muscles contain a-actinin and tropomyosin molecules, as do skeletal muscles. They do not have the troponin system, and the fight chains of smooth muscle myosin molecules differ from those of striated muscle myosin. Regulation of smooth muscle contraction is myosin-based, unlike striated muscle, which is actin-based. However, like striated muscle, smooth muscle contraction is regulated by Ca. ... 

S100A1 is the most abundant S100 protein found in striated muscle and predominates in myocardial tissue (Kato and Kimura, 1985). Besides its cytoplasmic occurrence, S100A1 was reported in these cells to associate with the sarcolemma, sarcoplasmatic reticulum (SR), contractile filaments, intercalated discs, outer mitochondrial membrane and other intracellular membrane stmctures (Arcuri et al., 2002 Donato et al., 1989 Haimoto and Kato, 1988 Sorci et al., 1999). However, the exact location of S100A1 on the contractile elements of the sarcomere is still controversial (Maco et al., 2001 Zimmer, 1991). 

The A-band lattices in different kinds of striated muscles have distinct arrangements. As shown in Fig. 3 and reproduced in simpler form in Fig. 10A and B, vertebrate striated muscle A-bands have actin filaments at the trigonal points of the hexagonal myosin filament array. As discussed prevously, this array also occurs in two types, the simple lattice and superlattice. The ratio of actin filaments to myosin filaments in each unit cell is 2 1. In both cases the center-to-center distance between adjacent myosin filaments is 70 A, but this varies as a function of overlap, becoming smaller as the sarcomere lengthens, giving an almost constant volume to the sarcomere (April et al, 1971). 

The vertebrate striated muscle Z-band is a cross-linking structure that links actin filaments of opposite polarity in successive sarcomeres along a myofibril. One of the curious things about it is that, unlike the A-band,... 

Fig. 14. Stereo pairs of the transverse structure (A) and the axial structure (B) of a 3D model relating successive half sarcomeres in vertebrate-striated muscles. In both images, the wide blue and brown cylinders represent actin filaments, the gray cross-links...

Wang, K. (1985). Sarcomere-associated cytoskeletal lattices in striated muscle. Review and hypothesis. Cell Muscle Motil. 6, 315-369. 

Each cell within vertebrate striated muscle contains within its sarcoplasm many parallel myofibrils which in turn are made up of repeating sarcomere units. Within the sarcomere are the alternating dark A band and light I band, in the middle of which are the H zone and Z line, respectively. A myofibril contains two types of filaments the thick filaments consisting of myosin which are present only in the A band, and the thin filaments consisting of actin, tropomyosin and troponin. When muscle contracts, the thick and thin filaments slide over one another, shortening the length of the sarcomere. 

In recent years the biological importance of the cytoskeletal structures—microtubules, intermediate filaments, and microfilaments (actin)—has attracted the attention of many cell biologists. In striated muscle, myosin and actin filaments together with Z lines are the main cytoskeletal structures to form sarcomeres of the myofibril. However, myosin and actin are contractile proteins, and some of the proteins constituting the Z line are classified as actin-associated proteins. Therefore, cell membrane attachment proteins, intermediate filaments, and some other structural proteins are described in this section. There has not been any report on muscle microtubules, although their presence is shown in some electron micrographs of sectioned samples. 

As the name implies, smooth muscle lacks the highly ordered sarcomere structure of striated muscle, having thick and thin filaments in less orderly arrays with relatively less myosin (one fifth as much) than in striated muscle. Smooth muscle thin filaments have tropomyosin but generally lack troponin. Myosin in smooth muscle is found in monomeric form as well as small thick filaments, and phosphorylation is almost essential for condensation of monomeric myosin into filaments. Thus, the amount of myosin available to cross-bridge with actin may be physiologically adjustable. Like other myosin II types, smooth muscle myosin is a hexamer, and several isoforms of the heavy chains and both light chains are known. The SM-1 isoform (M.W. 204,000) has an unusually long COOH-... 

Desmin is a cytoplasmic IFP that is characteristically found in muscle cells and in the neoplasms associated with them. In smooth muscle cells, it is seen with cytoplasmic dense bodies and subplasmalemmal dense plaques in striated muscle, desmin filaments are linked to sarcomeric Z disks. 

Immunostaining studies of cells isolated from the contracting areas within the EBs confirmed the presence of cardiac-specific proteins (MHC, sarcomeric a-actinin, des-min, cTnf, ANP). These studies also demonstrated the presence of early-cardiac morphology with a typical early-striated staining pattern. The cells, however, did not exhibit immunoreactivify with anti-nebuhn monoclonal antibodies (mAbs), a specific skeletal muscle sarcomeric protein shown to be expressed early in skeletal myoblast differentiation. 

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