Characterization of the L29Q hypertrophic cardiomyopathy mutation in cardiac troponin C by paramagnetic relaxation enhancement nuclear magnetic resonance

The key events in regulating muscle contraction involve the troponin (Tn) heterotrimeric protein complex in which the binding to and release of Ca²⁺ from the highly conserved troponin C (TnC) subunit trigger a series of structural changes within Tn, and the other thin filament proteins, to result in contraction. In the heart, the control of contraction and relaxation events can be altered by many single-point mutations that may result in cardiomyopathy and sometimes sudden cardiac death. Here we have examined the structural effects of one hypertrophic cardiomyopathy mutation, L29Q, on Ca²⁺-induced structural transitions within whole TnC. This mutation is of particular interest as several physiological and structural studies have indicated that the response of TnC to Ca²⁺ binding is altered in the presence of the L29Q mutation, but the structural nature of these changes continues to be debated. In addition, little is known about the effect of this mutation in the Ca²⁺ free state. Here we have used paramagnetic relaxation enhancement nuclear magnetic resonance (PRE-NMR) to assess the structural effects arising from the L29Q mutation. PRE-NMR distances obtained from a nitroxide spin-label at Cys84 showed that the L29Q mutation perturbs the structure of the TnC N-domain in the presence and absence of Ca²⁺, with a more "open" TnC N-domain observed in the apo form. In addition, binding of Ca²⁺ to the TnC-L29Q construct triggers a change in the orientation between the two domains of TnC. Together, these structural perturbations, revealed by PRE-NMR, provide insight into the pathogenesis of this mutation.