STRUCTURAL STABILITY OF SHORT SUBSEQUENCES OF THE TROPOMYOSIN CHAIN

The native tropomyosin molecule is a parallel, registered, alpha-helic al coiled coil made from two 284-residue chains. Long excised subseque nces (greater than or equal to 95 residues) form the same structure wi th comparable thermal stability. Here, we investigate local stability using shorter subsequences (20-50 residues) that are chemically synthe sized 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 labil ity of such secondary structure as does form. This behavior is in star k contrast to extant data on leucine-zipper peptides and short ''desig ned'' synthetic peptides, many of which have high alpha-helix content and form highly stable coiled coils. The existence of short coiled coi ls calls into question the older idea that short subsequences of a pro tein 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 a lpha-helical protein is sufficiently weak as to require not only dimer ization, but macromolecular amplification in order to attain its nativ e conformation in common benign media near neutral pH. (C) 1995 John W iley & Sons, Inc.