Conformations of the regulatory domain of cardiac troponin C examined by residual dipolar couplings

Conformations of the regulatory domain of cardiac troponin C (cNTnC) were s tudied by means of residual dipolar couplings measured from samples dissolv ed in dilute liquid crystals. Changes in the main chain HN residual dipolar couplings revealed a conformational change in cNTnC due to the complexatio n with the second binding region (amino acids 148-163) of cardiac troponin I (cTnI). Formation of the complex is accompanied with a molecular realignm ent in the liquid crystal. The residual dipolar couplings measured for apo- cNTnC and the complex with TnI were in agreement with the values computed f rom the corresponding closed and open solution structures, whereas for the calcium-loaded conformation the correlation and quality factor were only mo dest. Ca2+-cNTnC may be subject to conformational exchange. The data suppor t the model that cardiac troponin C functions as a calcium-dependent open-c losed switch, such as the skeletal troponin C.