Thin filaments turning

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-... 

Analyses of the tropomyosin sequence have shown long-range periodicities of certain surface acidic and apolar residues that are likely to be recognition sites for actin. These features are discussed below in relation to their role in regulation (see Turning on the Thin Filament section). 

Each cell within vertebrate striated muscle contains within its sarcoplasm many parallel myofibrils which in turn are made up of repeating sarcomere units. Within the sarcomere are the alternating dark A band and light I band, in the middle of which are the H zone and Z line, respectively. A myofibril contains two types of filaments the thick filaments consisting of myosin which are present only in the A band, and the thin filaments consisting of actin, tropomyosin and troponin. When muscle contracts, the thick and thin filaments slide over one another, shortening the length of the sarcomere. 

The structure of the cell, the shape of the cell surface, and the arrangement of subcellular organelles is organized by three major protein components microtubules composed of tubulin, which move and position organelles and vesicles thin filaments composed of actin, which form a cytoskeleton, and intermediate filaments composed of different fibrous proteins. Actin and tubulin, which are involved in cell movement, are dynamic structures composed of continuously associating and dissociating globular subunits. Intermediate filaments, which play a structural role, are composed of stable fibrous proteins that turn over more slowly. 

The thin filaments, composed primarily of actin, are approximately 3 urn long and 8 nm in diameter. The filament is basically formed by a nonintegral double helix of globular actin monomers (M.W. 50,000 5.5 nm in diameter) which have a period of approximately 37 nm with approximately 13 actin monomers per turn per strand. The thin filaments originate at the Z line in the center of the I band and, while negatively charged, do not normally form a lattice except close to their attachment to the Z-line in which region the lattice is square. 

Whole muscles are composed of groups of muscle fibers, which vary from 1 to 400 mm in length and from 10 to 60 tm in diameter. Muscle fibers, in turn, are composed of groups of myofibrils (Fig. 6.2b), and each myofibril is a series of sarcomeres added end to end (Fig. 6.2c). The sarcomere is both the structural and functional unit of skeletal muscle. During contraction, the sarcomeres are shortened to about 70 percent of their uncontracted, resting length. Electron microscopy and biochemical analysis have shown that each sarcomere contains two types of filaments thick filaments, composed of myosin, and thin filaments, containing actin (Fig. 6.2d). Near the center of the sarcomere, thin filaments overlap with thick filaments to form the AI zone (Fig. 6.2c). 

We now turn to the transient thin filament equations for a spinline, as given in Appendix 7B. For simplicity, we will restrict ourselves to isothermal spinning of a Newtonian fluid in the absence of inertia, air drag, or gravity, in which case the relevant equations are... 

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