Tropomyosin and Troponin

Tropomyosin (ca. 5% of the contractile proteins) is a highly elongated molecule (2 x 45 nm) with a molecular weight of about 68,000, and is assumed to be a double-stranded a-helix. Although each chain contains the same number of amino acids, their sequences differ in 39 positions. Tropomyosin contains no di-sulfide bridges and 

Troponin (ca. 5% of the contractile proteins) sits on the actin filaments (cf. Fig. 12.10) and controls the contact between the filaments of myosin and actin during muscle contraction by means of a Ca concentration-dependent change in conformation (cf. 12.3.2.1.5). It is a complex of three components, T, I, and C. Troponin T consists of a peptide chain with 259 amino acid residues and binds to tropomyosin. Troponin I (179 amino acid residues) binds to actin and inhibits various enzyme activities (ATPase). Troponin C (158 amino acid residues) binds Ca ions reversibly through a change in conformation. 

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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-... 

Tropomyosin and troponin are proteins located in the thin filaments, and together with Ca2+, they regulate the interaction of actin and myosin (Fig. 43-3) [5]. Tropomyosin is an a-helical protein consisting of two polypeptide chains its structure is similar to that of the rod portion of myosin. Troponin is a complex of three proteins. If the tropomyosin-troponin complex is present, actin cannot stimulate the ATPase activity of myosin unless the concentration of free Ca2+ increases substantially, while a system consisting solely of purified actin and myosin does not exhibit any Ca2+ dependence. Thus, the actin-myosin interaction is controlled by Ca2+ in the presence of the regulatory troponin-tropomyosin complex [6]. 

Contractile proteins which form the myofibrils are of two types myosin ( thick filaments each approximately 12 nm in diameter and 1.5 (im long) and actin ( thin filaments 6nm diameter and 1 (Am in length). These two proteins are found not only in muscle cells but widely throughout tissues being part of the cytoskeleton of all cell types. Filamentous actin (F-actin) is a polymer composed of two entwined chains each composed of globular actin (G-actin) monomers. Skeletal muscle F-actin has associated with it two accessory proteins, tropomyosin and troponin complex which are not found in smooth muscle, and which act to regulate the contraction cycle (Figure 7.1). 

Myofibrils contain four major proteins myosin, actin, tropomyosin and troponin. 

Actin (42 kDa) is the most important component of the thin filaments. It represents ca. 20-25% of the muscle proteins. F-actin is also an important component of the cytoskeleton (see p. 204). This filamentous polymer is held in equilibrium with its monomer, G-actin. The other protein components of muscle include tropomyosin and troponin. Tropomyosin... 

The interaction between actin and myosin must be regulated so that contraction occurs only in response to appropriate signals from the nervous system. The regulation is mediated by a complex of two proteins, tropomyosin and troponin. Tropomyosin binds to the thin filament, blocking the attachment sites for the myosin head groups. Troponin is a Ca2+-binding protein. 

Brown, J. H., and Cohen, C. (2005). Regulation of muscle contraction by tropomyosin and troponin How structure illuminates function. Adv. Protein Chem. 71, 121-159. 

As in the case of the myosin head, knowledge of actin filament structure, or thin filament structure as it is termed when tropomyosin and troponin are present, also progressed rapidly when the structure of the globular actin (G-actin) monomer was determined by protein crystallography in... 

REGULATION OF MUSCLE CONTRACTION BY TROPOMYOSIN AND TROPONIN HOW STRUCTURE ILLUMINATES FUNCTION... 

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. 

Actin and tubulin are two important cellular components that are involved in cell shape and movement. Actin is present in all mammalian cells and is involved in cellular transport and phagocytosis (eating of extracellular materials), provides rigidity to cell membranes, and when bonded to tropomyosin and troponin, forms the thin filaments of muscle. Thbulin is the subunit from which microtubules are self-assembled. Microtubules are most commonly known for their role in cell division. The mechanisms of self-assembly of these macromolecules have been well studied and are important models of biological assembly processes. Below we examine each of these processes. 

Figure 8.11 Aggregation of F-actin, tropomyosin and troponin to form the thin filaments of myofibrils. (Reproduced by permission from Ohtsuki I, Maruyama K, Ebashi S. Regulatory and cytoskeletal protein of vertebrate skeletal muscle. Adv Prot Chem 38 1— 60, 1986.)...

A myofibril consists of the thick filaments (myosin) and thin filaments (F-actin with tropomyosin and troponin). The muscle cell, extending the length of the muscle, is termed a muscle fiber and contains a number of myofibrils. 

During the initial rapid stage, it appears that both myosin and actin undergo denaturation, while during the second stage, tropomyosin and troponin undergo denaturation (90). 

Tropomyosin and troponin. Tropomyosin is apparently the most stable of the fish fibrillar proteins during frozen storage. It can be extracted long after actin and myosin become inextract-able however, it does denature gradually (90). 

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