Caldesmon tropomyosin interactions

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... 

Ultrastructural studies of isolated chicken gizzard thin filaments localized caldesmon on the thin filament beside tropomyosin, arranged continuously along the axis of the actin double helix (Moody et al 1990, Vibert et al 1993, I hman et al 1997). In smooth muscle filaments derived from vascular or visceral tissue, the stoichiometry of caldesmon to tropomyosin and actin has been determined to be 1 2 14 (Lehman et al 1989, Marston 1990, Lehman et al 1993). Marston and Redwood (1991) proposed that each caldesmon molecule is placed in register with tropomyosin and extends for 78 nm, the length of two tropomyosin molecules. Each caldesmon molecule interacts with 14 actin monomers. This would result in a filament without radial symmetry such that different parts of the caldesmon molecule would appear on the same side of the actin filament. 

To account for activation of arterial smooth muscle independently of LC20 phosphorylation, attention has been focused on the possible roles of the thin filament-associated regulatory proteins, caldesmon and calponin. Both proteins have been localized in the actomyosin domain of the smooth muscle cell and both have been shown to inhibit actin-activated myosin ATPase by interacting with F-actin, tropomyosin, and/or myosin (Clark et al., 1986 Takahashi et al.,... 

Marston SB, Fraser ID, Huber PA (1994) Smooth muscle caldesmon controls the strong binding interaction between actin-tropomyosin and myosin. J Biol Chem 269 3210432109... 

Calponin [119] and caldesmon [120] are two thin filament associated proteins that bind to F-actin, tropomyosin and calmodulin. Interaction of 34 kDa calponin with F-actin and tropomyosin takes place in a Ca +-inde-pendent manner, whereas that with calmodulin is regulated in a Ca -de-pendent manner. The key role of calponin and caldesmon in SM is to down-regulate actomyosin ATPase activity in vitro [120,121]. Thus, they may participate in regulation of contractile performance. Despite their apparent functional similarity, sequence analysis indicates that calponin and caldesmon are not related proteins. They act by different mechanisms... 

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