The x-ray structure of chicken skeletal muscle troponin C (TnC), the Ca ²⁺ -binding subunit of the troponin complex, shows that the protein is about 70 angstroms long with an unusual dumbbell shape. The carboxyl and amino domains are separated by a single long α helix of about nine turns. Only the two high-affinity Ca ²⁺ -Mg ²⁺ sites of the COOH-domain are occupied by metal ions resulting in conformational differences between the COOH- and NH ₂ -domains. These differences are probably important in the triggering of muscle contraction by TnC. Also the structure of TnC is relevant in understanding the function of other calcium-regulated proteins, in particular that of calmodulin because of its strong similarity in amino acid sequence.