Muscle thin filaments

Finally, there is at least one more molecular component of muscle thin filaments that should be mentioned. This is phalloidin, which does not... 

In summary, the special features of tropomyosin sequence, which has both periodic features for the binding of F-actin and aperiodic features for a number of functions including the binding of troponin, are necessary for its special linkages to the striated-muscle thin filament. 

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. 

Head, J. G., Ritchie, M. D., and Geeves, M. A. (1995). Characterization of the equilibrium between blocked and closed states of muscle thin filaments. Eur. J. Biochem. 227, 694-699. 

Holthauzen, L. M., Correa, F., and Farah, C. S. (2004). Ca2+-induced rolling of tropomyosin in muscle thin filaments The alpha- and beta-band hypothesis revisited. J. Biol. Chem. 279, 15204-15213. 

Lehman, W., Vibert, P., Uman, P., and Craig, R. (1995). Steric-blocking by tropomyosin visualized in relaxed vertebrate muscle thin filaments./ Mol. Biol. 251, 191-196. 

Popp, D., and Holmes, K. C. (1992). X-ray diffraction studies on oriented gels of vertebrate smooth muscle thin filaments. / Mol. Biol. 224, 65-76. 

Stewart, M. (2001). Structural basis for bending tropomyosin around actin in muscle thin filaments. Proc. Natl Acad. Sci. USA 98, 8165—8166. 

Vibert, P.J., Craig, R., and Lehman, W. (1997). Steric-model for activation of muscle thin filaments. J. Molec. Biol. 266, 8-14. 

As the name implies, smooth muscle lacks the highly ordered sarcomere structure of striated muscle, having thick and thin filaments in less orderly arrays with relatively less myosin (one fifth as much) than in striated muscle. Smooth muscle thin filaments have tropomyosin but generally lack troponin. Myosin in smooth muscle is found in monomeric form as well as small thick filaments, and phosphorylation is almost essential for condensation of monomeric myosin into filaments. Thus, the amount of myosin available to cross-bridge with actin may be physiologically adjustable. Like other myosin II types, smooth muscle myosin is a hexamer, and several isoforms of the heavy chains and both light chains are known. The SM-1 isoform (M.W. 204,000) has an unusually long COOH-... 

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.

Actin and the Structure of Smooth Muscle Thin Filaments... 

Electron Microscopy and Three-Dimensional Reconstruction of Smooth Muscle Thin Filament Images... 

The possible differences in on- and off-positions of tropomyosin in smooth and skeletal muscle thin filaments may relate to significant differences in enzymatic behavior of the two systems. The well-known observation that tropomyosin activates the smooth muscle actomyosin ATPase (Chacko et al., 1977) and also accelerates the motility of smooth muscle preparations in vitro (Shirinsky et al., 1992) should be taken into account when evaluating caldesmon function. Indeed, the degree of actomyosin ATPase potentiation by tropomyosin in the smooth muscle system is considerably greater than that in skeletal muscle preparations (Chacko et al., 1977 Sobieszek and Small, 1977 Chacko, 1981 Lehrer and Morris, 1984 Williams et al.,... 

In vitro experimentation has shown that caldesmon is an integral component of smooth muscle thin filaments and plays a central role in their Ca2+-dependent regulation. A mechanism analogous to that of troponin has been proposed this now requires extensive testing. The structure of the regulatory domain of caldesmon is not well defined and we look forward to being able to describe this in three dimensions and in combination with its physiological partners actin, tropomyosin, and calmodulin. 

We have already noted that smooth muscle thin filaments contain both CD and CP. Analysis of isolated smooth muscle thin filaments has revealed, on average, similar amounts of CP and CD with a composition, on a molar basis of actin TM CD CP, of 7 0.9-l 0.5-0.6 0.45-0.7 (Marston, 1991 Nishida et al.,... 

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