The stability of tropomyosin, a two-stranded coiled-coil protein, is primarily a function of the hydrophobicity of residues at the helix-helix interface.

The sequences of coiled coils are characterized by a repeating heptad of amino acids, abcdefg, in which the a and d residues are generally hydrophobic and form the interface between the two alpha-helices. In this study, rat and chicken alpha-tropomyosins (alpha-TMs) have been used as models to determine whether the effects of mutations on the stability of two-stranded coiled coils can be predicted by a simple algorithm. The thermal stabilities of three wild-type muscle alpha-TMs and nine chimeras, in which the second and/or sixth or ninth coding exons of one alpha-TM cDNA were replaced with exons from other alpha-TM cDNAs, with a sequence encoding the GCN4 leucine zipper or a random coil sequence, have been obtained using circular dichroism spectroscopy. Tropomyosin is almost completely helical along its entire length, but there is no correlation of the thermal stability of the alpha-TMs with the helical propensity of their component amino acids. The stability can be predicted (P = 0.90), however, by assigning a weight to every amino acid residue in each sequence, depending on its frequency of occurrence at the abcdef or g position in a data base of coiled-coil fibrous proteins, and summing all the weights. The correlation improves if only the residues at the a and d interface are counted (P = 0.94). The major factor modulating the thermal stability appears to be the hydrophobicity of the residues at the coiled-coil interface, since there is a high correlation (P = 0.91) of the TM values with the sum of the hydrophobic moments of the residues found at the a and d positions.