Z-discs

Thin actin filaments are pulled inwards by myosin heads so shortening the fibre. The Z disc is an anchorage point for actin... 

Figure 1.12 Diagrammatic interpretation of contraction in a myo-fibril of skeletal muscle. The diagram shows a single sarcomere, the basic contractile unit, limited at each end by a Z-disc. Muscle fibres are packed with hundreds of parallel myofibrils, each of which consists of many, often thousands, of sarcomeres arranged end to end. Contraction is the conseguence of the thin actin filaments being pulled over the thick filaments to increase the region of overlap and telescope the sarcomere.

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.

The C-protein (thick filaments), myomesin (M-line protein), and a-actinin (Z-line protein)110113114 each provide 2% of the protein in the myofibril. Less than 1% each of 11 or more other proteins may also be present within the sarcomere.86115 Several of these, including the cytoskeletal proteins desmin and vimentin, and synemin surround the Z-discs.116/116a... 

A characteristic is the appearance of "nemaline bodies" or thickened Z-discs containing Z-disc proteins. Some hereditary diseases involve nonmuscle myosins. Among these is Usher syndrome, the commonest cause of deaf-blindness. The disease, which results from a defect in the myosin VIA gene, typically causes impairment of hearing and retinitis pigmentosa (Chapter 23).hh... 

Hijikata, T., Murakami, T., Imamura, M., Fujimaki, N., and Ishikawa, H. (1999). Plectin is a linker of intermediate filaments to Z-discs in skeletal muscle fibers. /. Cell Sci. 112, 867-876. 

Price, M., and Sanger, J. W. (1979). Intermediate filaments connect Z-discs in adult chicken muscle. / Exp. Zool. 208, 263-269. 

Faulkner, G., Pallavicini, A., Comelli, A., Salamon, M., Bortoletto, G., Ievolella, C., Trevisan, S., Kojic, S., Dalla Vecchia, F., Laveder, P., Valle, G., and Lanffanchi, G. (2000). FATZ, a filamin-, actinin-, and telethonin-binding protein of the Z-disc of skeletal muscle./. Biol. Chem. 275, 41234—11242. 

Millevoi, S., Trombitas, K., Kolmerer, b., Kostin, S., Schaper,J., Pelin, K., Granzier, H., and Labeit, S. (1998). Characterization of nebulette and nebulin and emerging concepts of their roles for vertebrate Z-discs./. Mol. Biol. 282, 111-123. 

Mues, A., van der Ven, P. F., Young, P., Furst, D. O., and Gautel, M. (1998). Two immunoglobulin-like domains of the Z-disc portion of titin interact in a conformation-dependent way with telethonin. FEBSLett. 428, 111-114. 

Gregorio, C. C., Trombitas, K., and Centner, T. (1998). The NH2 terminus of titin spans the Z-disc Its interaction with a novel 19-kD ligand (T-cap) is required for sarcomeric integrity./. Cell. Biol. 143, 1013-1027. 

Knoll, R., Hoshijima, M., and Hoffman, H. M. (2002). The cardiac mechanical stretch sensor machinery involves a Z disc complex that is defective in a subset of human dilated cardiomyopathy. Cell 111, 943-955. 

Sorimachi, H., Freiburg, A., and Kolmerer, B. (1997). Tissue-specific expression and alpha-actinin binding properties of the Z-disc titin Implications for the nature of vertebrate Z-discs./. Mol. Biol. 270, 688-695. 

Des Intermediate filament Z discs KO Desmin- related myopathy... 

The thick filament system, which comprises myosin protein, connected from the M-line to the Z-disc by titin (connectin), and myosin-binding protein C, which binds at one end to the thick filament and at the other to actin. 

Actin filaments and titin molecules are cross-linked in the Z-disc via the Z-line protein a-actinin. 

Proteins of the M-line and Z-disc. The M-line region contains the structural protein myomesin, which binds to both titin and myosin and holds the two together. Fast skeletal and cardiac fibers also confain another M-protein, which may bridge between myosin filaments. Both the C-terminal region of nebulin and the N termini of pairs of titin molecules meet in the Z-disc, where they are bound into a lattice containing a-actinin 4 and other proteins (Fig. 

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