Thin filament proteins

The characteristic stoichiometry of tropomyosin binding, in which one molecule of tropomyosin interacts with approximately 7 actin monomers of F-actin, is the same in skeletal and smooth muscles (Matsumura 

35-40 actins per total cell actin (Lehman et al., 1993). The molar ratio of calponin tropomyosin actin in vivo appears to be closer to 1 1 7 in a variety of smooth muscles (Takahashi etal., 1986 Lehman, 1991 Barany and Barany, 1993). 

TABLE I Compstrison of the Amino-Terminal Amino Acids in Smooth and Nonmuscle Actins  

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Szent-Gyorgyi further showed that the viscosity of an actomyosin solution was lowered by the addition of ATP, indicating that ATP decreases myosin s affinity for actin. Kinetic studies demonstrated that myosin ATPase activity was increased substantially by actin. (For this reason, Szent-Gyorgyi gave the name actin to the thin filament protein.) The ATPase turnover number of pure myosin is 0.05/sec. In the presence of actin, however, the turnover number increases to about 10/sec, a number more like that of intact muscle fibers. 

Tao, T., Lamkin, M., and Schemer, C. (1984) Studies on the proximity relationships between thin filament proteins using benzophenone-4-maleimide as a site-specific photoreactive crosslinker. Biophys. J. 45,261. 

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... 

The key step in LC isolation is the preparation of pure smooth muscle myosin (Hasegawa et al., 1988). This is usually executed by homogenizing the muscle mince with salt solutions (60-100 mM ionic strength) to remove the cytoplasmic proteins and to a certain extent the thin filament proteins. A crude actomyosin is then extracted form the well-washed muscle residue with salt solutions containing ATP and the actin component is removed by ultracentrifugation. The LCs are... 

Collateral Regulation Role for Thin Filament Proteins... 

The second aspect of the force-RLC phosphorylation relation that suggests additional regulation is illustrated by alterations in the slope of the relation (Fig. 1, lower right). Alterations in isometric force at fixed values of RLC phosphorylation are probably the strongest evidence for the in vivo operation of thin filament regulation. Smooth muscle contains two thin filament proteins, caldesmon and calponin, that inhibit actin-activated MgATPase activity of phosphorylated myosin. This inhibitory activity is reversed by the binding of Ca +ZCaM or by phosphorylation, and thus CD and CP may modulate the RLC phosphorylation... 

Actin isoform specific localization of thin filament proteins... 

Cohen DM, Murphy RA (1979) Cellular thin filament protein contents and force generation in porcine arteries and veins. Circ Res 45 661-665 Cohen P (1989) The structure and regulation of protein phosphatases. Annu Rev Biochem 58 453-508... 

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