Filaments, thin troponin arrangement

The myofibril also contains thin filaments of F-actin with tropomyosin dimers bound in the grooves and about one troponin complex per tropomyosin dimer. The thin filaments are also arranged in a bipolar fashion. All the pointed ends point away from each side of the Z-disk which contains a number of proteins including n-actinin. o-Actinin serves to bundle the actin filaments very close to their barbed ends and anchor them very firmly in the disk. 

The thin filament (about 7 nm in diameter) fies in the 1 band and extends into the A band but not into its H zone (Figure 49-2). Thin filaments contain the proteins actin, tropomyosin, and troponin (Figure 49-3). In the A band, the thin filaments are arranged around the thick (myosin) filament as a secondary hexagonal array. Each thin filament lies symmetrically between three thick filaments (Figure 49-2, center mid cross-... 

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... 

On separated thin filaments of rabbit skeletal muscle, anti-troponin T1 formed a relatively wide striation at the same position as in the case of chicken skeletal muscle. The Fab fragment of the antibody formed a narrow striation at a position corresponding to the top side of the filament of the wide striation formed by y-globulin (1. Ohtsuki, unpublished results). This is consistent with the latter explanation at least for the orientation of the antigenic regions in troponin Ti. A possible arrangement of troponin Ti and T2 regions in the thin filament is shown in Fig. 10. 

Fig. 10. Possible arrangement of troponin components in the thin filament. The thin filament is viewed obliquely from the Z line side toward the top of the filament of the far side (Ohtsuki, 1980).

Thin filaments are composed of a polymer of actin (called F-actin) arranged in a helix, tropomyosin (a fibrous protein that exists as elongated dimers lying along, or close to, the groove in the F-actin helix), and three small proteins called troponins I, C, and T. The presence of tropomyosin and the troponins... 

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