Tropomyosin: Amino Acid Sequence and Coiled-Coil Structure

Tropomyosin preparations of mol wt of about 70,000 daltons are known to dissociate in the presence of reducing agents and denaturing media to monomers of about 35,000 daltons (McCubbin et al., 1967; Holtzer et al., 1965; Woods, 1967, 1969; Weber and Osborne, 1969; Olander et al., 1967). The highly helical (>90%) polypeptide chains are believed to be arranged in a two-stranded coiled-coil (Crick, 1953; Cohen and Holmes, 1963) whose structure is stabilized by hydrophobic interactions between the two strands of α-helix. The relative unimportance of electrostatic interactions in the stabilization of the structure is indicated by the unusual stability of the molecule in acid and alkali (Lowey, 1965; Riddiford and Scheraga, 1962), even though the content of charged amino acid residues is remarkably high (Hodges and Smillie, 1970). In the structure proposed by Crick (1953), two α-helices are packed side-by-side in a "knobs-into-holes" manner. In order to interlock two helices...