Skeletal muscle tropomyosin

Proteins can be broadly classified into fibrous and globular. Many fibrous proteins serve a stmctural role (11). CC-Keratin has been described. Fibroin, the primary protein in silk, has -sheets packed one on top of another. CoUagen, found in connective tissue, has a triple-hehcal stmcture. Other fibrous proteins have a motile function. Skeletal muscle fibers are made up of thick filaments consisting of the protein myosin, and thin filaments consisting of actin, troponin, and tropomyosin. Muscle contraction is achieved when these filaments sHde past each other. Microtubules and flagellin are proteins responsible for the motion of ciUa and bacterial dageUa. 

Thus, Ca " controls skeletal muscle contraction and relaxation by an allosteric mechanism mediated by TpC, Tpl, TpT, tropomyosin, and F-actin. 

The general picture of muscle contraction in the heart resembles that of skeletal muscle. Cardiac muscle, like skeletal muscle, is striated and uses the actin-myosin-tropomyosin-troponin system described above. Unlike skeletal muscle, cardiac muscle exhibits intrinsic rhyth-micity, and individual myocytes communicate with each other because of its syncytial nature. The T tubular system is more developed in cardiac muscle, whereas the sarcoplasmic reticulum is less extensive and consequently the intracellular supply of Ca for contraction is less. Cardiac muscle thus relies on extracellular Ca for contraction if isolated cardiac muscle is deprived of Ca, it ceases to beat within approximately 1 minute, whereas skeletal muscle can continue to contract without an extraceUular source of Ca +. Cyclic AMP plays a more prominent role in cardiac than in skeletal muscle. It modulates intracellular levels of Ca through the activation of protein kinases these enzymes phosphorylate various transport proteins in the sarcolemma and sarcoplasmic reticulum and also in the troponin-tropomyosin regulatory complex, affecting intracellular levels of Ca or responses to it. There is a rough correlation between the phosphorylation of Tpl and the increased contraction of cardiac muscle induced by catecholamines. This may account for the inotropic effects (increased contractility) of P-adrenergic compounds on the heart. Some differences among skeletal, cardiac, and smooth muscle are summarized in... 

Smooth muscles have molecular structures similar to those in striated muscle, but the sarcomeres are not aligned so as to generate the striated appearance. Smooth muscles contain a-actinin and tropomyosin molecules, as do skeletal muscles. They do not have the troponin system, and the fight chains of smooth muscle myosin molecules differ from those of striated muscle myosin. Regulation of smooth muscle contraction is myosin-based, unlike striated muscle, which is actin-based. However, like striated muscle, smooth muscle contraction is regulated by Ca. ... 

When smooth muscle myosin is bound to F-actin in the absence of other muscle proteins such as tropomyosin, there is no detectable ATPase activity. This absence of activity is quite unlike the situation described for striated muscle myosin and F-actin, which has abundant ATPase activity. Smooth muscle myosin contains fight chains that prevent the binding of the myosin head to F-actin they must be phosphorylated before they allow F-actin to activate myosin ATPase. The ATPase activity then attained hydrolyzes ATP about tenfold more slowly than the corresponding activity in skeletal muscle. The phosphate on the myosin fight chains may form a chelate with the Ca bound to the tropomyosin-TpC-actin complex, leading to an increased rate of formation of cross-bridges between the myosin heads and actin. The phosphorylation of fight chains initiates the attachment-detachment contraction cycle of smooth muscle. 

The myofibrils of skeletal muscle contain thick and thin filaments. The thick filaments contain myosin. The thin filaments contain actin, tropomyosin, and the troponin complex (troponins T, I, and C). 

In skeletal muscle, calcium binds to troponin and causes the repositioning of tropomyosin. As a result, the myosin-binding sites on the actin become uncovered and crossbridge cycling takes place. Although an increase in cytosolic calcium is also needed in smooth muscle, its role in the mechanism of contraction is very different. Three major steps are involved in smooth muscle contraction ... 

Zot, A. S. and Potter, J. D. Structural aspects of troponin-tropomyosin regulation of skeletal muscle contraction. Annu. Rev. Biophys. Biophys. Chem. 16 535-560,1987. 

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). 

Tropomyosin is a fibrous molecule which twists around the F-actin strands. The troponin (Tn) complex is composed of three proteins Tnl (I = inhibitory) which prevents myosin binding to actin in the resting muscle, TnT which binds tropomyosin and TnC (C for calcium-binding). Cardiac muscle troponins are different from those of skeletal muscle and are designated cTnl, cTnT and cTnC. 

Additional information <1> (<1> isozyme of calmodulin-dependent multifunctional protein kinase II in smooth-muscle [5] <1> caldesmon is not a substrate of smooth-muscle myosin light-chain kinase [3] <1> no substrates are bovine cardiac C-protein, bovine brain fodrin, rabbit skeletal muscle glycogen synthase, phosphorylase B, troponon (I -I- T -I- C), actin, tropomyosin, smooth-muscle actin, filamin, vinculin, cr-actinin, protamine or phosvitin [1]) [1-3]... 

ATP -I- tropomyosin <1> (<1> the phosphorylation site is a single serine-residue close to COOH-terminus, i.e Ser-283 [2] <1> a-tropomyosin subunit preferred over -tropomyosin subunit [1,2] <1> other poor substrates are )3-tropomyosin from chicken leg muscle, rabbit skeletal muscle... 

Tropomyosin is a two-stranded, o-helical coiled-coil molecule that aggregates head-to-tail with others to form long filamentous ropes. These lie in each of the two long period grooves of the actin microfilaments where, in vertebrate skeletal muscle, they play an important part in the Ca2+-mediated regulation of actin via troponin (a tropomyosin-associated protein). An important feature of tropomyosin is its 39.2-residue period— that is also quasi-halved (19.6 residues)—in the linear distribution of the acidic residues and, to a lesser extent, the apolar residues (McLachlan and Stewart, 1976 Parry, 1975). The number of residues in tropomyosin (284 residues), and the head-to-tail overlap (nine residues) that allows axial... 

Parry, D. A. D. (1975). Analysis of the primary sequence of o-tropomyosin from rabbit skeletal muscle./. Mol. Biol. 98, 519-535. 

Bacchiocchi, C., and Lehrer, S. S. (2002). Ca(2+) -induced movement of tropomyosin in skeletal muscle thin filaments observed by multi-site FRET. Biophys.J. 82,1524—1536. 

Parry, D. A. D. (1976). Movement of tropomyosin during regulation of vertebrate skeletal muscle A simple physical model. Biochem. Biophys. Res. Comm. 68, 323-328. 

Sano, K.-I., Maeda, K., Oda, T., and Maeda, Y. (2000). The effect of single residue substitutions of serine-283 on the strength of head-to-tail interaction and actin binding properties of rabbit skeletal muscle a-tropomyosin. /. Biochem. 127, 1095-1102. 

Tanokura, M., Tawada, Y., Ono, A., and Ohtsuki, I. (1983). Chymotryptic subffagments of troponin T from rabbit skeletal muscle. Interaction with tropomyosin, troponin I and troponin C./. Biochem. (Tokyo) 93, 331-337. 

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.)...

In skeletal muscle in the relaxed state, the sarcoplasm has a high Mg ATP2 -concent rat ion, but the concentration of calcium is below the threshold required for initiation of contraction. The myosin head, under resting conditions is unable to react with actin of the thin filaments because in the absence of calcium the tropomyosin molecule masks the myosin binding site on G-actin monomer or holds it in a conformation that is unreactive, through the action of TN-1 subunit of troponin. One tropomyosin molecule inhibits the myosin binding activity of seven G-actin monomers. 

The contractile proteins of the myofibril include three troponin regulatory proteins. The troponin complex includes 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 isoforms 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 cTnl also have been identified and are the products of unique genes. All cardiac troponins are localized primarily in the myofibrils (94%-97%), with a smaller cytoplasm fraction (3%-6%). 

Smooth muscle. Smooth muscle, in contrast with skeletal muscle, is not regulated by a tropomyosin - troponin mechanism. Instead, vertebrate smooth muscle contraction is controlled by the degree of phosphorylation of its light chains. Phosphorylation induces contraction, and dephosphorylation leads to relaxation. Like that of skeletal muscle, smooth muscle contraction is triggered by an increase in the cytoplasmic calcium ion level. Propose a mechanism for this action of calcium ion on the basis of your knowledge of other signal-transduction processes. 

This section represents an extension of our previous reviews (Ebashi and Endo, 1968 Ebashi et al, 1969 Ebashi, 1974a, 1980) and concerns mainly the interacting properties and arrangement of the three troponin components and tropomyosin in reference to calcium regulation. The protein source is rabbit skeletal muscle unless otherwise mentioned. 

---