Thin filament model

The sliding filament model describes the mechanism involved in muscle contraction. In this model, sarcomeres become shorter when the thin and thick filaments slide alongside each other and telescope together, with ATP being consumed. During contraction, the following reaction cycle is repeated several times ... 

Figure 5.15 Model of a thin filament in muscle fibers TnC, TnT, and Tnl are all different forms of the protein troponin. (Adapted, with modifications, from Stryer, 1975.)...

The sliding-filament model of muscle contraction. During contraction, the thick and thin filaments slide past each other so that the overall length of the sarcomere becomes shorter. 

A series of striking observations, beginning in the early 1960s, led to the formulation of the so-called steric-blocking model, which has been the major paradigm for the mechanism of thin filament activation (reviewed by Cohen and Vibert, 1987). 

Tobacman, L. S., and Butters, C. A. (2000). A new model of cooperative myosin-thin filament binding. / Biol. Chem. 275, 27587-27593. 

Vibert, P.J., Craig, R., and Lehman, W. (1997). Steric-model for activation of muscle thin filaments. J. Molec. Biol. 266, 8-14. 

Actin and tubulin are two important cellular components that are involved in cell shape and movement. Actin is present in all mammalian cells and is involved in cellular transport and phagocytosis (eating of extracellular materials), provides rigidity to cell membranes, and when bonded to tropomyosin and troponin, forms the thin filaments of muscle. Thbulin is the subunit from which microtubules are self-assembled. Microtubules are most commonly known for their role in cell division. The mechanisms of self-assembly of these macromolecules have been well studied and are important models of biological assembly processes. Below we examine each of these processes. 

The bis-urea thin filaments can be very long in non-polar solvents such as 1,3,5-trimethylbenzene. Consequently, these solutions show a high viscosity r]/r]Q = 8 at a concentration C = 0.04 molL and at T = 20 °C) and a high concentration dependence of the viscosity (ri/rio C ) [43]. As in the case of UPy based supramolecular polymers, the value of this exponent is in agreement with Cates s model for reversibly breakable polymers [26,27]. However, the solutions are not viscoelastic, even at concentrations well above the overlap concentration [43]. Consequently, the relaxation of entanglements, probably by chain scission, must be fast (r < 0.01 s). 

Figure 34.14. Sliding-Filament Model. Muscle contraction depends on the motion of thin fdaments (blue) relative to thick fdaments (red). [After H. E. Huxley. The mechanism of muscular contraction. Copyright 1965 hy Scientific American, Inc. All rights reserved.]... 

These considerations led us to propose the first model of the thin filament as a complex of troponin-tropomyosin-actin (Ebashi et al., 1969) (Fig. 9A). In this model, two head-to-tail filaments of tropomyosin run almost in register along the grooves of actin double strands. Troponin attaches to a specific region of each tropomyosin and thus is... 

Fig. 9. Structural models of thin filaments. (A) The first model (B) the refined model. For detailed explanations, see text.

The model has been refined by two lines of study (Ohtsuki, 1974). The first refinement was made by analysis of the troponin-tropomyosin relationship in the paracrystalline structure (discussed in Section II,E,2). The analysis has confirmed that troponin lies approximately two-thirds of the molecular length (i.e., 27 nm) from one end of a filamentous tropomyosin molecule of 40-nm length. Another refinement was based on consideration of the arrangement of actin molecules in the thin filament. Corresponding molecules in two long-pitched strands of actin in the filament are shifted relative to each other by a distance of half the... 

Based on the above considerations, the refined model of the thin filament is presented in Fig. 9B (Ohtsuki, 1974 Ebashi, 1980). In this second model, the position of end-to-end bonding of tropomyosins is indicated, and the two troponin-tropomyosin filaments in the grooves of actin double strands are shifted by half the actin size relative to each other. The shift between two troponin-tropomyosin filaments has been verified by X-ray diffraction studies on invertebrate striated muscles (Wray et al., 1978 Maeda et al., 1979 Namba et al., 1980). The fact that the distance between the top of the thin filament and the position of the nearest troponin is 27 nm (i.e., two-thirds of the troponin period length) (Ohtsuki, 1974) indicates that the top of the filament is situated at the left in the model of the thin filament of Fig. 9B. 

Model of a thin filament. Two tropomyosin filaments, each composed of two subfilaments, wind around the actin chain. They block the binding of the globular heads of myosin molecules (in thick filaments) to the actin molecules. Troponin consists of three polypeptides and binds to both actin and tropomyosin. 

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