Actin thin filament structure

As in the case of the myosin head, knowledge of actin filament structure, or thin filament structure as it is termed when tropomyosin and troponin are present, also progressed rapidly when the structure of the globular actin (G-actin) monomer was determined by protein crystallography in... 

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. 

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. 

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... 

Tropomyosin and troponin are proteins located in the thin filaments, and together with Ca2+, they regulate the interaction of actin and myosin (Fig. 43-3) [5]. Tropomyosin is an a-helical protein consisting of two polypeptide chains its structure is similar to that of the rod portion of myosin. Troponin is a complex of three proteins. If the tropomyosin-troponin complex is present, actin cannot stimulate the ATPase activity of myosin unless the concentration of free Ca2+ increases substantially, while a system consisting solely of purified actin and myosin does not exhibit any Ca2+ dependence. Thus, the actin-myosin interaction is controlled by Ca2+ in the presence of the regulatory troponin-tropomyosin complex [6]. 

Actin filaments are thought to exist in a double-stranded, right-hand helix with 14 subunits (per strand) per complete turn (Fig. 4.4), and a crossover distance of 38 nm. This strings of beads appearance is 70 A in diameter and thought to represent the structure of thin filaments. As the new fila-... 

The above processes describe how the growth and depolymerisation of actin filaments thin filaments) is controlled. However, actin filaments are assembled into filamentous networks, and these three-dimensional structures are themselves controlled and also stabilised by a number of proteins ... 

F-actin structure 338 thin filaments 369 Actinidin 618 a-Actinin 370,405 Actinomycetes 6 Activation... 

In the second frame (b), strong bridges form between actin and myosin. This is followed by a structural alteration in the myosin molecules and an effective translocation of the thick filament relative to the thin filament in (c). During this process the ADP is released. After the translocation step, the bridge structure is broken by the binding of ATP, which is rapidly hydrolyzed to ADP and Pj. Each thick filament has about 500 myosin heads, and each head cycles about five times per second in the course of a rapid contraction. 

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