Tropomyosin subunits

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... 

The amino acid sequence of the /8-tropomyosin subunit is mostly the same as that of a-tropomyosin, with the difference being 39 residues. Most residues were replaced by the same family of amino acid residues, with the exception that Ser-229 and His-276 in the a-subunit are replaced by Glu-229 and Asn-276 in the /8-subunit. Hence, /3-tropomyosin is a little more acidic than a-tropomyosin (Mak et ai, 1980). 

Single pair FRET has also been used to image macromolecular interactions at the single molecule level. FRET takes place only if the donor and acceptor fluorophores are very near to each other (distance, <10 nm). FRET is the most suitable method for detecting the complex formation between two molecules, whereas observation of co-locahzation of two molecules is sometimes difficult to interpret because of the diffraction Hmit. Ishii et aL (1999) measured FRET between single Cy3 and Cy5 fluorophores conjugated separately to a-tropomyosin subunits [14]. When the two subunits of a-tropomyosin attached to the different fluorophores formed a homodimer, the two fluorophores were close enough... 

Tropomyosin is a long thin rodlike protein (42x2 nm) of about 30 kDa, composed of two nonidentical subunits. It associates with actin in a ratio of about... 

Figure 49-3. Schematic representation of the thin fiiament, showing the spatiai configuration of its three major protein components actin, myosin, and tropomyosin. The upper panei shows individual molecules of G-actin. The middle panel shows actin monomers assembled into F-actin. Individual molecules of tropomyosin (two strands wound around one another) and of troponin (made up of its three subunits) are also shown. The lower panel shows the assembled thin filament, consisting of F-actin, tropomyosin, and the three subunits of troponin (TpC, Tpl, andTpT).

In addition to actin and myosin, other proteins are found in the two sets of filaments. Tropomyosin and a complex of three subunits collectively called troponin are present in the thin filaments and play an important role in the regulation of muscle contraction. Although the proteins constituting the M and the Z bands have not been fully characterized, they include a-actinin and desmin as well as the enzyme creatine kinase, together with other proteins. A continuous elastic network of proteins, such as connectin, surround the actin and myosin filaments, providing muscle with a parallel passive elastic element. Actin forms the backbone of the thin filaments [4]. The thin... 

Movement. The interaction between actin and myosin is responsible for muscle contraction and cell movement (see p.332). Myosin (right), with a length of over 150 nm, is among the largest proteins there are. Actin filaments (F-actin) arise due to the polymerization of relatively small protein subunits (G-actin). Along with other proteins, tropomyosin, which is associated with F-actin, controls contraction. 

Troponin s role in the thin filament of vertebrate striated muscles is primarily that of regulation. The three subunits of this complex form what has been described as a Ca2+-sensitive latch that fixes tropomyosin s position on the actin helix in the off state of contraction (Lehman et al., 2001). One subunit of the complex, troponin T (TnT), maintains an invariant linkage to tropomyosin, and another, troponin I (Tnl), a variable linkage to actin. The third subunit, troponin (TnC) is the Ca2+sensor of the complex and indeed of the myofibril itself. The latch is opened or closed depending on the level of Ca2+. Correspondingly, a series of conformational changes takes place in the entire complex and in the thin... 

Contraction in skeletal and cardiac muscles is triggered by the binding of Ca2+ to the troponin C (TnC) subunit (Fig. 5) of the troponin complex. The other two subunits are troponin I (Tnl), which binds to F-actin and inhibits actomyosin ATPase (for review, see Perry, 1999), and troponin T (TnT), which binds to tropomyosin, linking it to the rest of the troponin... 

Jancso, A., and Graceffa, P. (1991). Smooth muscle tropomyosin coiled-coil dimers. Subunit composition, assembly, and end-to-end interaction. J. Biol. Chem. 266, 5891-5897. 

The diffraction pattern from troponin is very different. Here, there is one troponin complex for each tropomyosin molecule, but the end-to-end repeat along the tropomyosin strands is about 385 A It is longer than the actin filament crossover repeat of just over 357 A in vertebrate muscles (Fig. 11A and B). Because much of the troponin complex is globular, unlike tropomyosin, it shows very marked discontinuous density every 385 A along each strand of the actin filament, with the troponins in opposite strands axially shifted by the actin monomer subunit translation h of... 

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. 

An enhancement of ATPase action comes through the phosphorylation of myosin light chains (MW 18,000). The phosphorylation is achieved because the high cellular [Ca2+] activates myosin kinase, an enzyme that contains calmodulin, a Ca2+-binding subunit. Phosphorylation of myosin is absolutely required for smooth muscle contraction, though not for the contraction of skeletal or cardiac muscle, because smooth muscle has no troponin. Thus, whereas contraction and relaxation in skeletal and cardiac muscle are achieved principally via the action of Ca2+ on troponin, in smooth muscle they must depend solely on the Ca2+-dependent phosphorylation of myosin. In skeletal and cardiac muscle, once the stimulus to the sarcolemma is removed, [Ca2+] in sarcoplasm drops rapidly back to 10 7 or 10 8 M via various Ca2+ pump mechanisms present in the sarcoplasmic reticulum, and tropomyosin can once again interfere with the myosin-actin interaction. 

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%). 

Tropomyosin is a coiled-coil dimer consisting of identical 35 kDa subunits. It binds to the grooves of the actin filament spanning approximately seven actin subunits and serves to stiffen and straighten the filaments. The protein binds F-actin cooperatively, meaning that once one molecule binds, the entire length of the filament is likely to bind tropomyosin in its grooves. 

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