Stabilization of the long central helix of troponin C by intrahelical salt bridges between charged amino acid side chains.

The unusual dumbbell shape of troponin C is due to the presence of a long alpha-helix of nine turns that connects the amino- and carboxyl-terminal calcium-binding domains. The center of the long helix appears to be stabilized by several salt bridges. The long helix is also bent about 16 degrees at glycine-92. Calmodulin, which lacks the central glycine, also is predicted to be stabilized by salt bridges in the central helix. The presence of a proline residue in the center of the long helix of ascidian troponin C and the myosin regulatory light chains suggests that a sharper bend may occur in these molecules. The conservation of the bend and salt bridges in the related calcium-binding proteins suggests they may have an important biological function. The structure of troponin C suggests that intrahelix salt bridges between neighboring charged residues may be involved in the stabilization of protein secondary structure. The preponderance of potential salt bridges in other muscle proteins as well may be related to their elongated structures and their participation in the contractile process.