Specific sequences determine the stability and cooperativity of folding of the C-terminal half of tropomyosin

Tropomyosin is a flexible 410 Å coiled-coil protein in which the relative stabilities of specific regions may be important for its proper function in the control of muscle contraction. In addition, tropomyosin can be used as a simple model of natural occurrence to understand the inter- and intramolecular interactions that govern the stability of coiled-coils. We have produced eight recombinant tropomyosin fragments (Tm_{143-284(50HW)}, Tm_{189-284(5OHW)}, Tm_189-284, TM_{220-284(5OHW)}, TM_220-284, Tm_143-235, Tm_167-260, and Tm_143-260) and one synthetic peptide (Ac-Tm_215-235) to investigate the relative conformational stability of different regions derived from the C-terminal region of the protein, which is known to interact with the troponin complex. Analytical ultracentrifugation experiments show that the fragments that include the last 24 residues of the molecule (Tm_{143-284(5OHw)}, Tm_{189-284(5OHW)}, TM_{220-284(5OHW)}, Tm_220-284) are completely dimerized at 10 μM dimer (50 mM phosphate, 100 mM NaCl, 1.0 mM dithiothreitol, and 0.5 mM EDTA, 10 °C), whereas fragments that lack the native C terminus (Tm_143-235,Tm_167-260, and Tm_143-260) are in a monomer-dimer equilibrium under these conditions. The presence of trifluoroethanol resulted in a reduction in the [θ]₂₂₂/[θ]₂₀₈ circular dichroism ratio in all of the fragments and induced stable trimer formation only in those containing residues 261-284. Urea denaturation monitored by circular dichroism and fluorescence revealed that residues 261-284 of tropomyosin are very important for the stability of the C-terminal half of the molecule as a whole. Furthermore, the absence of this region greatly increases the cooperativity of ureainduced unfolding. Temperature and urea denaturation experiments show that Tm_143-235 is less stable than other fragments of the same size. We have identified a number of factors that may contribute to this particular instability, including an interhelix repulsion between g and e′ positions of the heptad repeat, a charged residue at the hydrophobic coiled-coil interface, and a greater fraction of β-branched residues located at d positions.