In a wide variety of cellular settings, from organelle transport to muscle contraction, Ca2+ binding to members of the EF hand family of proteins controls the interaction between actin and different myosins that are responsible for generating movement. In vertebrate skeletal and cardiac muscle the Ca(2+)-binding protein troponin C (TnC) is one subunit of the ternary troponin complex which, through its association with actin and tropomyosin on the thin filament, inhibits the actomyosin interaction at submicromolar Ca2+ concentrations and stimulates the interaction at micromolar Ca2+ concentrations. Because TnC does not interact directly with actin or tropomyosin, the Ca(2+)-binding signal must be transmitted to the thin filament via the other two troponin subunits: troponin I (TnI), the inhibitory subunit, and troponin T (TnT), the tropomyosin-binding subunit. Thus, the troponin complex is a Ca(2+)-sensitive molecular switch and the structures of and interactions between its components have been of great i...