Sarcomeres actin filaments

The light bands comprise actin filaments that are attached to disks at each end of the sarcomeres. (Courtesy of Roger Craig.)... 

Within each sarcomere the relative sliding of thick and thin filaments is brought about by "cross-bridges," parts of the myosin molecules that stick out from the myosin filaments and interact cyclically with the thin actin filaments, transporting them hy a kind of rowing action. During this process, the hydrolysis of ATP to ADP and phosphate couples the conformational... 

Because there are no sarcomeres in smooth muscle, there are no Z lines. Instead, the actin filaments are attached to dense bodies. These structures, which contain the same protein as Z lines, are positioned throughout the cytoplasm of the smooth muscle cell as well as attached to the internal surface of the plasma membrane. Myosin filaments are associated with the actin filaments, forming contractile bundles oriented in a diagonal manner. This arrangement forms a diamond-shaped lattice of contractile elements throughout the cytoplasm. Consequently, the interaction of actin and myosin during contraction causes the cell to become shorter and wider. 

You may be asked to draw a diagram of the sarcomere. It is made up of actin and myosin filaments, as shown below. The thick myosin filaments contain many crossbridges, which, when activated, bind to the thin actin filaments. Tropomyosin molecules (containing troponin) run alongside the actin filaments and play an important role in excitation-contraction coupling. 

Z line The junction between neighbouring actin filaments that forms the border between sarcomeres. It has a Z-shaped appearance on the diagram. M line The middle zone of the sarcomere, formed from the junction between neighbouring myosin filaments. There are no cross-bridges in this region. 

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.

Figure 19-6 (A) The structure of a typical sarcomere of skeletal muscle. The longitudinal section depicted corresponds to that of the electron micrograph, Fig. 19-7A. The titin molecules in their probable positions are colored green. The heads of only a fraction of the myosin molecules are shown protruding toward the thin actin filaments with which they interact.

The muscle sarcomere contains the principal contractile proteins myosin and actin (Fig. 3A to C), which on their own can produce force and movement, together with a number of cytoskeletal and regulatory proteins. The latter include titin, C-protein (MyBP-C), tropomyosin, troponin, a-actinin, myomesin, M-protein, and so on. Some of these help to organize the myosin and actin filaments in the sarcomere, some to define the filament lengths and structure, some to regulate activity, and some to modulate the actin-myosin interaction when the muscle is active. 

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. 11. The structure of a-actinin and the two vertebrate Z-band lattices. (A) The ubiquitous protein a-actinin is an anti-parallel homodimer. Each 100 KDa monomer comprises four central spectrin repeats (SI to S4) an EF-hand domain and two calponin homology domains (CH) at the N-terminus. The EF-hand domains bind calcium in non-muscle cells. One a-actinin molecule binds two actin filaments via the calponin homology domains. a-Actinin binds titin via EF-hand domains. (B, C) The Z-band is the site where actin filaments from adjacent sarcomeres overlap in a tetragonal lattice and are crosslinked by a-actinin molecules. The polarity and origin of the actin filaments is indicated by U (up) and D (down). The appearance of the Z-band in cross-section is typically basketweave-like (B) or small square-like (G). The appearance is reported to transform between the two appearances depending on the state of the muscle.

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...

In thinking about X-ray diffraction from this assembly, a number of the sarcomere components contribute to the observed patterns in ways that have been the subject of detailed analysis. In the A-band, these include the myosin filament backbone, where the coiled-coil a-helical myosin rods pack together, the myosin head arrays in the bridge regions of the myosin filaments, the non-myosin A-band proteins titin and C-protein (MyBP-C), and the A-band parts of the actin filaments. Very little has been seen in X-ray patterns so far that appears to be related to the M-band, probably... 

In active or rigor muscle, heads that do not overlap actin filaments become disordered (Cantino et al, 2002 Padron and Craig, 1989). They therefore contribute little to the observed low-angle X-ray diffraction patterns. This population increases with increasing sarcomere length (reduced filament overlap). 

---