Thin filament proteins actin interactions

The cycle has four major steps (Fig. 5-33). In step (l), ATP binds to myosin and a cleft in the myosin molecule opens, disrupting the actin-myosin interaction so that the bound actin is released. ATP is then hydrolyzed in step (2), causing a conformational change in the protein to a high-energy state that moves the myosin head and changes its orientation in relation to the actin thin filament. Myosin then binds weakly to an F-actin subunit... 

Apart from the phosphorylation theory, other regulatory mechanisms have also been suggested for smooth muscle contraction. A thin-filament protein that has been proposed as a regulatory component is caldesmon [102], Purified caldesmon is a potent inhibitor of actin-tropomyosin interaction with myosin. The mechanisms by which calcium removes this inhibition are controversial. Furthermore, phosphorylation of caldesmon by a caldesmon kinase in vitro has also been implicated in this... 

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]. 

The interaction between actin and myosin, like that between all proteins and ligands, involves weak bonds. When ATP is not bound to myosin, a face on the myosin head group binds tightly to actin (Fig. 5-33). When ATP binds to myosin and is hydrolyzed to ADP and phosphate, a coordinated and cyclic series of conformational changes occurs in which myosin releases the F-actin subunit and binds another subunit farther along the thin filament. 

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. 

A family of actin-binding proteins that exist in various isoforms. As with other protein isoforms or isoenzymes, the expression of the isoforms is tissue-specific. The interaction of calponin with actin inhibits the actomyosin Mg-ATRase activity. See Winder, S. and Walsh, M., Inhibition of the actinomyosin MgATRase by chicken gizzard calponin. Prog. Clin. Biol. Res. 327, 141-148, 1990 Winder, S.J., Sutherland, C., and Walsh, M.R., Biochemical and functional characterization of smooth muscle calponin, Adv. Exp. Med. Biol. 304, 37-51, 1991 Winder, S.J. and Walsh, M.R., Calponin thin filament-linked regulation of smooth muscle contraction. Cell Signal. 5,677-686,1993 el-Mezgueldi, M., Calponin, Int. J. Biochem. 

The answer is b. (Murray, pp 48-62. Scriver, pp 3-45. Sack, pp 1-3. Wilson, pp 101-120.) Two kinds of interacting protein filaments are found in skeletal muscle. Thick filaments 15 nm in diameter contain primarily myosin. Thin filaments 7 nm in diameter are composed of actin, troponin, and tropomyosin. The thick and thin filaments slide past one another during muscle contraction. Myosin is an ATPase that binds to thin filaments during contraction, ot-actinin can be found in the Z line. 

Schematic diagram of the organization of skeletal muscle thin filament, showing the position of tropo-myosin and the troponin complex on the actin filament. The binding of Ca " to TnC, the calcium-binding subunit of the troponin complex, removes Tnl, the inhibitory subunit, from actin and thus permits an interaction with a specialized protein, myosin, on neighboring thick muscle filaments (not shown). An ATP-driven conformation change in the myosin head group makes the thick and thin filaments move relative to one another, so that muscle contraction occurs.

Intermediate filaments are a family of structures intermediate in size between the thin actin filaments and the fat microtubules. Intermediate filaments are formed by coiled-coil interactions between intermediate filament proteins. 

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