Structural Aspects of Troponin and Tropomyosin

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 

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. 

The fine axial orientation of troponin T was then demonstrated using the antibodies against chymotryptic troponin T subfragments, i.e., troponin T1 (MW 18,700) and troponin T2 (MW 11,900) of chicken skeletal muscle (Ohtsuki, 1979b). Anti-troponin T2 stained the same position as that stained by the antibodies against troponin I and C, whereas antitroponin T1 stained a position 13 nm toward the Z line from that stained by anti-troponin T2. Thus each member of the striation pair formed by anti-troponin T was proved to be derived from different regions in the molecule. 

If the positions of the antibodies are taken to represent the overall position of troponin components in this case, the troponin T molecule occupies the region of tropomyosin over the width of 13 nm including the edge of the Z-band side. Troponins C, I, and T2 would form a triangle complex in the plane either perpendicular or parallel to the filament axis. The other extreme edge of the troponin T molecule would be available for the antibody against troponin Ti. For this explanation to apply, the shape of the troponin molecule would be far from globular. 

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