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Thin filament proteins

The thin filaments contain three different proteins—actin, tropomyosin, and troponin. The latter is actually a complex of three proteins.

Actin, which constitutes about 25 percent of the protein of myofilaments, is the major component of the thin filaments in muscle. An individual molecule of actin is a single protein chain coiled to form a roughly egg-shaped unit. Actin in this form, called globular actin or G-actin, has one calcium or magnesium ion and one molecule of ATP bound to it. Under the proper conditions G-actin is transformed into the fibrous form, or F-actin, that exists in the thin filament in muscle. When the G-to-F transformation takes place, the ATP bound to G-actin breaks down, releasing inorganic phosphate (Pi ) and leaving an adenosine diphosphate (ADP) molecule bound to each actin unit. Actin molecules repeat every 2.75 nanometres along the thinfilament. They give rise to a helical structure that can be viewed as a double or single helix. The apparent half-pitch is about 40 nanometres long. Actin is believed to be directly involved in the process of contraction since the cross bridges can become attached to it.

Tropomyosin is a rod-shaped molecule about 40 nanometres long with a molecular weight of about 70,000. Two strands of tropomyosin molecules run diametrically opposed along the actin filaments. Tropomyosin has a structure similar to that of the myosin tail, being a coiled unit of two protein chains. Each tropomyosin molecule is in contact with seven actin units.

Troponin is a complex of three different protein subunits. One troponin complex is bound to every tropomyosin molecule. A troponin molecule is located approximately every 40 nanometres along the filament. Troponin and tropomyosin are both involved in the regulation ofthe contraction and relaxation of muscles. One of the subunits (TnC) is the receptor for Ca2+ released from the sarcoplasmic reticulum on activation of the muscle. It is thought that calcium-binding then causes further structural changes in the interaction of actin, tropomyosin, and another troponin subunit (TnI) that lead to contraction by activating the actin-myosin interaction.

The exact structure of the Z line has not yet been completely established, but it is thought that four filaments extend from the end of each thin filament to four different thin filaments on the other side of the Z line. This type of arrangement would give the Z line a three-dimensional structure resembling a series of pyramid-like units. The Z line contains proteins that form links with actin and establish lateral connections among Z disks (?-actinin, desmin) and establish connections between the Z disks and the cell membrane (e.g., vinculin).

The fine M bridges are composed of proteins that have not yet been fully characterized. The M bridges run between sites on the thick filaments at which myosin molecules are joined tail to tail. It may be that the M bridges keep the filaments in register during contraction. A continuous network of filaments extending from Z disk to Z disk has also been identified; it contains some large proteins (titin, nebulin).

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