CD and (13)C(alpha)-NMR studies of folding equilibria in a two-stranded coiled coil formed by residues 190-254 of alpha-tropomyosin.
Synthesis and CD and (13)C(alpha)-NMR studies in a near-neutral saline buffer are reported for a 65-residue peptide ((190)Tm(254)) comprising residues 190-254 of the alpha-tropomyosin chain. CD on a version disulfide cross-linked via the N-terminal cysteine side chains indicates that this dimer is highly helical and melts near 48 degrees C. The CD is independent of peptide concentration, showing that association of (190)Tm(254) stops at the two-strand level. Similar studies on the reduced version show much lower helix content at low temperature, melting points below room temperature, and the expected concentration dependence. The observed melting temperature of the reduced peptide is far below (by 27 degrees C) that expected from an extant analysis of calorimetry data on parent tropomyosin that designates (190)Tm(254) as an independently melting "cooperative block." This disagreement and the pronounced nonadditivity seen when data for (190)Tm(254) are combined with extant data for other subsequences argue decisively against the concept of specific independently melting blocks within the tropomyosin chain. The data for (190)Tm(254) also serve to test recent ideas on the sequence determinants of structure and stability in coiled coils. Analysis shows that some ideas, such as the stabilizing effect of leucine in the d heptad position, find support, but others--such as the destabilizing effect of alanine in d, the dimer-disfavoring effect of beta-branching in d and its dimer-favoring effect in a, and the dimer-directing effect of asparagine in a--are more questionable in tropomyosin than in the leucine zipper coiled coils. (13)C(alpha)-NMR data at two labeled sites, L228(d) and V246(a), of (190)Tm(254) display well-separated resonances for folded and unfolded forms at each site, indicating that the transition is slow on the NMR time scale and thus demonstrating the possibility of obtaining thermodynamic and kinetic information on the transition at the residue level.
Emerson Holtzer
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Department of Chemistry, Washington University, Campus Box 1134, One Brookings Drive, St. Louis, MO 63130-4899, USA. holtzer@wuchem.wustl.edu
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Hogue Angeletti
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D000595:Amino Acid Sequence; D001704:Biopolymers / Q000737:chemistry; D002247:Carbon Isotopes; D002942:Circular Dichroism; D066298:In Vitro Techniques; D009682:Magnetic Resonance Spectroscopy; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000737:chemistry; D017510:Protein Folding; D013816:Thermodynamics; D014335:Tropomyosin / Q000737:chemistry*
D001704:Biopolymers; D002247:Carbon Isotopes; D010446:Peptide Fragments; D014335:Tropomyosin