Regulatory Proteins Troponin and Tropomyosin

Study of the molecular biology of calcium regulation of muscle contraction was initiated by the discovery of a new protein factor sensitizing actomyosin to calcium ions (Ebashi, 1963 Ebashi and Ebashi, 1964). This protein factor was called native tropomyosin, because of its similarity in amino acid composition to tropomyosin, which had been discovered earlier (Bailey, 1946, 1948). It was soon found that this factor is a complex of tropomyosin and a new globular protein, termed troponin (Ebashi and Kodama, 1965 Ebashi et al., 1968). Thus four proteins, i.e., myosin, actin, troponin, and tropomyosin, are involved in calcium-regulated physiological muscle contraction (Ebashi et al., 1968, 1969 Ebashi and Endo, 1968). The contractile interaction between myosin and actin is depressed by troponin and tropomyosin in the absence of calcium ions. When calcium ion acts on troponin, this depression is removed and the contractile interaction is then activated (Figs. 1 and 2). 

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The regulatory proteins troponin and tropomyosin 1118 Table 19-1 Some Actin-Binding Proteins... 

Both the thick and thin filaments contain other proteins. For example, the thick filament contains titin (molecular weight about 3,000,000) and the thin filament contains nebulin (although not in cardiac muscle), and the regulatory proteins troponin (molecular weight about 33,000) and tropomyosin (molecular weight about 70,000). Nebulin and titin are thought to be ruler proteins, that is, they determine the overall length of the thin and the thick filament, respectively. The... 

The contractile proteins of the myofibril include the regulatory proteins troponin. The troponins are a complex of three protein subunits, troponin C (the calcium-binding component), troponin I (the inhibitory component), and troponin T (the tropomyosin-binding component).The subunits exist in a number of isoforms. The distribution of these iso-forms varies between cardiac muscle and slow and fast twitch skeletal muscle. Only two major isoforms of troponin C are found in human heart and skeletal muscle. These are characteristic of slow and fast twitch skeletal muscle. The heart isoform is identical with the slow twitch skeletal muscle isoform. Isoforms of cardiac-specific troponin T (cTnT) and cardiac-specific troponin I (cTnl) also have been identified and are the products of unique genes.Troponin is localized primarily in the myofibrils (94% to 97%), with a-smaller cytoplasmic fraction (3% to 6%). 

The major regulatory proteins located on an actin filament are troponin and tropomyosin, each occupying 5% of the total myofibrillar proteins. Both proteins confer calcium sensitivity on the ATP-actin-myosin interactions (see Section II). There are minor regulatory proteins that modify the fine structures of myosin and actin filaments and also of Z lines. 

In addition to the major proteins of striated muscle (myosin, actin, tropomyosin, and the troponins), numerous other proteins play important roles in the maintenance of muscle structure and the regulation of muscle contraction. Myosin and actin together account for 65% of the total muscle protein, and tropomyosin and the troponins each contribute an additional 5% (Table 17.1). The other regulatory and structural proteins thus comprise approximately 25% of the myofibrillar protein. The regulatory proteins can be classified as either myosin-associated proteins or actin-associated proteins. 

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]. 

A cytoskeletal regulatory protein that attaches laterally to actin filament without resulting in filament cross-linking and/or bundling. These include troponin, tropomyosin, calponin, tropomodulin, adducin, caldesmon, and hisactophilin. See also ABM-1 ABM-2 Sequences in Actin-Based Motors Actin-Based Bacterial Motility Actin Assembly Kinetics... 

The muscle sarcomere contains the principal contractile proteins myosin and actin (Fig. 3A to C), which on their own can produce force and movement, together with a number of cytoskeletal and regulatory proteins. The latter include titin, C-protein (MyBP-C), tropomyosin, troponin, a-actinin, myomesin, M-protein, and so on. Some of these help to organize the myosin and actin filaments in the sarcomere, some to define the filament lengths and structure, some to regulate activity, and some to modulate the actin-myosin interaction when the muscle is active. 

The contraction of ascidian smooth muscle was found to be regulated through the troponin-tropomyosin system. But the action of troponin components was different from that of troponin of vertebrate striated muscles (Endo and Obinata, 1981). In this system, the inhibitory action of troponin I (MW 24,000) is less remarkable compared with vertebrate skeletal troponin I, and troponin C (MW 18,000) does not neutralize the inhibition by troponin I. But upon further addition of troponin T (MW 33,000) in the concomitant presence of all three components and tropomyosin, the contractile interaction of myosin and actin is activated. In this case, the action of troponin T has some similarity with that of the above-mentioned cardiac troponin T hybridized with skeletal troponin C-I. Since actomyosin, without these regulatory proteins, is inhibited regardless of Ca concentration, Ca " and troponin-tropomyosin are activators for contraction of actomyosin in ascidian smooth muscle. In this respect, the type of Ca + regulation of ascidian smooth muscle is the same as that for vertebrate smooth muscles which do not contain troponin (Ebashi, 1980). 

Thin filaments contain nebulin and two regulatory proteins called tropomyosin and troponin (Figure 21-5). Tropomyosin (M.W. 68,000) is a coiled-coil cz-helical... 

Troponin is a heterotrimeric protein involved in the regulation of striated and cardiac muscle contraction. Most troponin in the cell is bound to the actin-tropomyosin complex in the muscle fibril. The three subunits of troponin consist of troponin-C, troponin-T, and troponin-l, each with a specific function in the regulatory process. Troponin-T and troponin-l exist as different isoforms in cardiac and skeletal muscle (sequences with a different amino acid composition), thus allowing the development of specific antibodies against each form. As a consequence, either cardiac troponin-T or cardiac troponin-l may be rapidly measured in blood samples by immunoassay with a good degree of specificity. 

Thin filaments (8 nm diam.) consist of polymerized Actin (see) (30(M00 actin molecules per 1.0 pm length), each chain being accompanied by threadlike tropomyosin molecules and, in striated muscle, globular molecules of troponin. In smooth muscle troponin is replaced by caldesmon. The actin and myosin are responsible for muscle contraction, while the tropomyosin and troponin or caldesmon are regulatory proteins... 

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