Functional impact of TnT mutations responsible for Hypertrophic Cardiomyopathy on Tn-exchanged single myofibrils

Hypertrophic Cardiomyopathy (HCM) is an autosomal dominant cardiac disease resulting from mutations in genes encoding contractile proteins, including troponin (Tn; the complex of TnT, TnI and TnC). From measurements of ATPase activity of myofibrils, into which TnT mutants were exchanged, we showed that two HCM-associated mutations of TnT, E244D and K247R, increase the maximum ATPase activity without any change in Ca2þ sensitivity (Matsumoto et al., BSJ 2007, 1P150). It is, however, still not clear if the mutations have direct effects on the mechanical properties of muscle because ATPase activity and tension development are not necessarily coupled. We thus examined the effects of the mutations on maximal isometric tension and kinetics of Tn-exchanged myofibrils. Measurements were made in single myofibrils activated and relaxed by a fast solution switching device, with a dead time < 10ms (Tesi et al., Biophys. J., 2002, 83, 2142). Maximal isometric tension values of the mutant TnT exchanged myofibrils were significantly higher (E244D) or tended to be higher (K247R) than those of the wild type TnT exchanged myofibrils while Ca2þ sensitivity was unchanged. These results indicate that the effects of the mutations on myofibril mechanics are directly coupled with the effects on the ATPase activity. Measurements of the rate constants of force generation showed that the mutant TnT exchanged myofibrils had similar kinetics as the wild type TnT exchanged myofibrils. The result suggests that kinetics of cross bridge transitions are not influenced by these mutants.