Structural stability of short subsequences of the tropomyosin chain

The native tropomyosin molecule is a parallel, registered, alpha-helical coiled coil made from two 284-residue chains. Long excised subsequences (> or = 95 residues) form the same structure with comparable thermal stability. Here, we investigate local stability using shorter subsequences (20-50 residues) that are chemically synthesized or excised from various regions along the protein chain. Thermal unfolding studies of such shorter peptides by CD in the same solvent medium used in extant studies of the parent protein indicate very low helix content, almost no coiled-coil formation, and high thermal lability of such secondary structure as does form. This behavior is in stark contrast to extant data on leucine-zipper peptides and short "designed" synthetic peptides, many of which have high alpha-helix content and form highly stable coiled coils. The existence of short coiled coils calls into question the older idea that short subsequences of a protein have little structure. The present study supports the older view, at least in its application to tropomyosin. The intrinsic local alpha-helical propensity and helix-helix interaction in this prototypical alpha-helical protein is sufficiently weak as to require not only dimerization, but macro-molecular amplification in order to attain its native conformation in common benign media near neutral pH.