Filaments, thin tropomyosin arrangement

Ultrastructural studies of isolated chicken gizzard thin filaments localized caldesmon on the thin filament beside tropomyosin, arranged continuously along the axis of the actin double helix (Moody et al 1990, Vibert et al 1993, I hman et al 1997). In smooth muscle filaments derived from vascular or visceral tissue, the stoichiometry of caldesmon to tropomyosin and actin has been determined to be 1 2 14 (Lehman et al 1989, Marston 1990, Lehman et al 1993). Marston and Redwood (1991) proposed that each caldesmon molecule is placed in register with tropomyosin and extends for 78 nm, the length of two tropomyosin molecules. Each caldesmon molecule interacts with 14 actin monomers. This would result in a filament without radial symmetry such that different parts of the caldesmon molecule would appear on the same side of the actin filament. 

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... 

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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