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... 

Linke, W. A., Rudy, D. E., and Centner, T. (1999). I-band titin in cardiac muscle is a three-element molecular spring and is critical for maintaining thin filament structure./. Cell. Biol. 146, 631-644. 

A major site on TM has been identified between 142 and 227. There is also a suggestion of a weaker site in the N terminus of TM (11-127) (Watson et al., 1990). These results can be reconciled with the model of thin filament structure (Figs. 2 and 3). CD and TM are both extended and can make multiple binding contacts along their length. CD is double the length of TM, hence the strong site (142-227) in adjacent TMs can probably interact with domain 2 and the N terminus of domain 4, whereas the weaker site (11-127) could interact with domains 1 and 3. We have no evidence for the relative polarity of CD and TM Fig. 3 shows an... 

Figure 1. Muscle development. A skeletal muscle fiber is formed by the fusion of many single cells (myoblasts) into a multinucleated myotube. Myotubes then develop into the muscle fiber (see text). Sarcomeres form in longitudinal structures called myofibrils. The repeating structure of the sarcomere contains interdigitating thick and thin filaments.

Figure 3. Structure of a muscle sarcomere. In a polarizing microscope muscle appears to have dark (A) and light (I) bands. The l-band region only contains thin filaments. The A-band region contains both thick and thin filaments. One sarcomere is the distance between two Z-lines. In cross section, the hexagonal packing of the thick and thin filaments can be seen.

X-ray diffraction of live muscle (H.E. Huxley and Brown, 1967) showed the structure of the thick and thin filaments and how they changed when the muscle contracted, or was put into rigor (in rigor muscle, ATP is absent from the muscle. 

Internally, muscle fibers are highly organized. Each fiber contains numerous myofibrils — cylindrical structures that also lie parallel to the long axis of the muscle. The myofibrils are composed of thick filaments and thin filaments. It is the arrangement of these filaments that creates alternating light and dark bands observed microscopically along the muscle fiber. Thus, skeletal muscle is also referred to as striated muscle. 

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 ... 

Until the past decade, the cytoplasm was widely considered to be structurally unorganized with the main division of labor at the organellar level. Certainly, relatively little was known about the nature of the cyto-skeleton (with the notable exception of the mitotic apparatus and striated muscle), and the dynamics of cytoplasmic behavior were conceptualized vaguely in terms of sol-gel transitions without a sound molecular foundation. Substantial improvements in electron, light, and fluorescence microscopy, as well as the isolation of discrete protein components of the cytoskeleton, have led the way to a much better appreciation of the structural organization of the cytoplasm. Indeed, the lacelike network of thin filaments, intermediate filaments, and microtubules in nonmuscle cells is as familiar today as the organelles identified... 

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