Actin thin filaments

Actin thin filaments consist of actin, tropomyosin, and the troponins in a 7 1 1 ratio (Figure 17.15). Each tropomyosin molecule spans seven actin molecules, lying along the thin filament groove, between pairs of actin monomers. 

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). 

Myosin Thick Filaments Slide along Actin Thin Filaments... 

The cycle has four major steps (Fig. 5-33). In step (l), ATP binds to myosin and a cleft in the myosin molecule opens, disrupting the actin-myosin interaction so that the bound actin is released. ATP is then hydrolyzed in step (2), causing a conformational change in the protein to a high-energy state that moves the myosin head and changes its orientation in relation to the actin thin filament. Myosin then binds weakly to an F-actin subunit... 

Moore, P. B., Huxley, H. E., and De Rosier, D. J. (1970). Three-dimensional reconstruction of F-actin, thin filaments and decorated thin filaments. /. Mol. Biol. 50, 279-295. 

Fig. 3. Schematic diagram showing the interaction of the myosin thick filaments and the actin thin filaments during skeletal muscle contraction.

Fig. 4. Mechanism for the generation of force in muscle as an S1 head of a myosin thick filament interacts with an actin thin filament.

Answer Binding of ATP to myosin triggers dissociation of myosin from the actin thin filament. In the absence of ATP, actin and myosin bind tightly to each other. 

The less dense region of the A band, also known as the H zone (not shown in Fig. 5-29b), is the region in which the myosin thick filaments do not overlap the actin thin filaments. When the sarcomere contracts (see Fig. 5-29c), the H zone and the I band decrease in width. 

B. The myosin thick filament expands, while the actin thin filament contracts to achieve maximal force of contraction. 

Actin (thin) filaments and myosin (thick) filaments myosin heads form bridges during contraction filaments are stable in sarcomeres and stereocilia, but often labile in cytoplasm. 

Schematic the interaction of myosin and actin. (a) In the absence of Ca ", tropomyosin prevents the binding of myosin heads to actin. (b) When the [Ca ] rises, Cd binds to a subunit of troponin, which causes the tropomyosin to shift slightly into the groove of the actin thin filament. The shift in position of tropomyosin allows the myosin heads to bind to actin. Lowering of the [Ca " ] results in reversal of these events.

In skeletal muscle cells, actin thin filaments and myosin thick filaments are organized Into highly ordered structures, called sarcomeres (see Figure 19-22). The (+) end of the thin filaments Is attached to the Z disk, the demarcation between adjacent sarcomeres. 

Actomyosin structure. The actin thin filament is shown as the gray line on the right. One monomer of myosin is shown in purple, with the thick vertical part representing part of the thick filament. For clarity, the other monomer of the dimer, and additional myosin molecules in the thick filament, are not shown. The myosin head domain is shown interacting with the thin filament. The ATP binding site is shown as an indentation... 

In contrast to actin thin filaments, the 50 or so different types of intermediate filaments are each composed of a different protein having the same general structure described above (Table 10.2). Some of the intermediate filaments, such as the nuclear lamins, are common to all cell types. These filaments provide a lattice-like support network attached to the inner nuclear membrane. Other intermediate filaments are specific for types of cells (e.g., epithelial cells have cytokeratins, and neurons have neurofilaments). These provide an internal network that helps to support the shape and resilience of the cell. 

Actin Thin filament 42 kDa >6 including 3a, iP and 2y 6-11 genes gene duplication Contraction... 

FIGURE 22.3 Contractile apparatus in cells, (a) A sarcomere is ahighly organized contractile unit in muscle cells that uses actin thin filaments and myosin thick filaments to shorten the length of a muscle, (b) A stress fiber is a loosely organized contractile structure in nonmuscle cells that is essential for migration and contraction. 

From the perspective of the consilient mechanism, the assembly of filaments as required for muscle contraction and the necessary movement of components within the cell involves hydrophobic association/dissociation between composite subunits. The actin thin filament of... 

The initial event in the contraction of skeletal muscles is generally assumed to be the binding of calcium ions to troponin C, one of three subunits in the protein complex, troponin, on the actin thin filament. Troponin C (MW 18,000) contains 46 carboxylic groups out of a total of 159 amino acids (26). Based on primary structure analogies between parvalbumin, the crystal structure of which is known, and troponin C (TnC), Kretsinger and Barry have predicted the location and general structure of four... 

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