Structural features in the heptad substructure and longer range repeats of two-stranded alpha-fibrous proteins

Considerable sequence data have been collected from the intermediate filament proteins and other alpha-fibrous proteins including myosin, tropomyosin, paramyosin, desmoplakin and M-protein. The data show that there is a clear preference for some amino acids to occur in specific positions within the heptad substructure that characterizes the sequences which form the coiled-coil rod domain in this class of proteins. The results also indicate that although there are major similarities between the various proteins there are also key differences. In all cases, however, significant regularities in the linear disposition of the acidic and the basic residues in the coiled-coil segments can be related to modes of chain and molecular aggregation. In particular a clear trend has been observed which relates the mode of molecular aggregation to the number of interchain ionic interactions per heptad pair.