Ca²⁺ sensitization and potentiation of the maximum level of myofibrillar ATPase activity caused by mutations of troponin T found in familial hypertrophic cardiomyopathy

Human wild-type cardiac troponin T, I, C and five troponin T mutants (I79N, R92Q, F110I, E244D, and R278C) causing familial hypertrophic cardiomyopathy were expressed in Escherichia coli, and then were purified and incorporated into rabbit cardiac myofibrils using a troponin exchange technique. The Ca²⁺-sensitive ATPase activity of these myofibrillar preparations was measured in order to examine the functional consequences of these troponin mutations. An I79N troponin T mutation was found to cause a definite increase in Ca²⁺ sensitivity of the myofibrillar ATPase activity without inducing any significant change in the maximum level of ATPase activity. A detailed analysis indicated the inhibitory action of troponin I to be impaired by the I79N troponin T mutation. Two more troponin T mutations (R92Q and R278C) were also found to have a Ca²⁺-sensitizing effect without inducing any change in maximum ATPase activity. Two other troponin T mutations (F110I and E244D) had no Ca²⁺-sensitizing effects on the ATPase activity, but remarkably potentiated the maximum level of ATPase activity. These findings indicate that hypertrophic cardiomyopathy-linked troponin T mutations have at least two different effects on the Ca²⁺-sensitive ATPase activity, Ca²⁺-sensitization and potentiation of the maximum level of the ATPase activity.