Tropomyosin Levels in Smooth Muscles, Purification, and General Properties

TROPOMYOSIN LEVELS IN SMOOTH MUSCLES, PURIFICATION, AND GENERAL PROPERTIES 

Estimates of the levels of the major protein components of smooth and skeletal muscles have been compiled by Hartshorne (1987). The concentrations of myosin, actin, and TM for arterial and nonarterial smooth muscles are estimated to be approximately 56 xM, 1.6 mM, 0.27 mM and 56 jjlM, 0.87 mM, 0.15 mM, respectively. In skeletal muscle, the corresponding concentrations are approximately 0.18, 0.7, and 0.1 mM. Thus in smooth muscles the myosin concentration is significantly reduced, whereas the actin and TM concentrations are elevated relative to those of skeletal muscle. In the present context the important point is that the molar ratio ( 1.2 7) of TM to actin monomers is more than adequate to fully saturate the available TM binding sites on F-actin (one per seven actin monomers). 

TMs from various sources have very similar physical and solubility properties consistent with their highly asymmetric shape (Tsao etal., 1951 McCubbinetflZ., 1967) and high charge density of amino acid side chains. With an excess of acidic residues (Asp plus Glu = 28%) over basic amino acids (—19%), their isoelectric points are in the range of 4-5. As pointed out originally by Crick (1953), their content of nonpolar residues is consistent with the predictions of a coiled-coil structure. Avian and mammalian TMs are devoid of Pro and Trp. 

ISOFORM DIVERSITY OF SKELETAL AND SMOOTH MUSCLE TROPOMYOSINS 

The isoform diversity in striated muscles and their electrophoretic gel patterns are further complicated by partial phosphorylation of serine 283, the penultimate COOH-terminal residue (Ribolow and Barany, 1977 Mak etal., 1978 Montarras cfal., 1981,1982 Heeley cf al., 1982,1985). The extent of phosphorylation is highest for the a isoform in embryonic and neonatal tissue, dropping to much lower levels in adult muscle. Heeley et al. (1982,1989) and Heeley (1994) have compared the phosphorylated and nonphosphorylated forms of rabbit skeletal aa TM and observed significant differences 

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