Conformational variation of calcium-bound troponin C

Troponin is the Ca super(2+)-binding regulatory protein in mammalian skeletal and cardiac muscles. Together with tropomyosin, troponin regulates the interaction between myosin crossbridges and actin in response to rising and falling levels of intracellular Ca super(2+) (10 super(-5) to 10 super(-7) M). The troponin complex has three subunits: troponin C (TnC) which binds Ca super(2+) specifically, troponin I (TnI), and troponin T (TnT). When TnC binds 4 Ca super(2+) ions, it undergoes a conformational transition which is transmitted to TnI, causing TnI to release its inhibition of the actinmyosin interaction via TnT and tropomyosin. We present here the crystal structure of 4-Ca super(2+) rabbit TnC in a new crystal form. A detailed comparison with other calcium-saturated structures shows differences that help describe the dynamics of TnC and its range of conformations.