Intrahelical Interactions in an α-Helical Coiled Coil Determine the Structural Stability of Tropomyosin.

Tropomyosin (Tpm) is a two-stranded parallel α-helical coiled-coil protein, and studying its structure is crucial for understanding the nature of coiled coils. Previously, we found that the N-terminal half of the human skeletal muscle α-Tpm (α-Tpm 140) was less structurally stable in the presence of phosphate ions than the coiled-coil protein carrier (CCPC) 140 variant with 18 mutated residues, in which all amino acid residues located at the interface between the two α-helices were completely conserved. A classical hypothesis explains that interhelical interactions stabilize the coiled-coil structure. In this study, we tested the hypothesis that the structural stability of Tpm and its variant is governed by the binding of multivalent ions that form a bridge between charged side chains located at positions b, c, and f of the heptad repeat on a single α-helical chain. We found that the structural stability of α-Tpm 140 and CCPC 140 markedly increased upon addition of divalent cations and divalent anions, respectively. We also clarified that the structural stability of the α-Tpm 140/CCPC 140 heteromeric coiled-coil molecule was governed by the stability of a less stable α-helical chain. These results demonstrated that the entire structural stability of Tpm is determined by the stability of a single α-helix. Our findings provide new insights into the study of the structure of coiled-coil proteins.