Mechanism of direct coupling between binding and induced structural change in regulatory calcium binding proteins.

The structural transition in troponin C induced by the binding of two calcium ions involves an "opening" of the structure, an event that triggers skeletal muscle contraction. We have solved the solution structure of a mutant (E41A) of the regulatory domain of skeletal troponin C wherein one bidentate ligand to the calcium in site I is missing. This structure remains "closed" upon calcium binding, indicating that the linkage between calcium binding and the induced conformational change has been broken. This provides a snapshot of skeletal troponin C between the off and on state and thereby valuable insight into the mechanism of regulation within skeletal TnC. Although several factors contribute to the triggering mechanism, the opening of the troponin C structure is ultimately dependent on one amino acid, Glu41. Insights into the structure of cardiac troponin C can also be derived from this skeletal mutant.