Effects of the cardiomyopathy type mutations of troponin on the ATPase activity and Ca sensitization in the myofibrils

Matsumoto, Fumiko; Maeda, Kayo*; Nitanai, Yasushi*; Oda, Toshiro*; Maeda, Yuichiro*; Fujiwara, Satoru

Familial hypertrophic cardiomyopathy (HMC) has been reported to be caused by mutations in a regulatory protein, troponin (Tn). HMC is characterized by functional aberration on the force-pCa relationship. Only a few cardiomyopathy-causing mutations have been mapped on the coiled-coil region (IT-arm) in the Tn core domain. Here we focus on two mutations in the IT-arm, E244D and K247R of TnT. Whereas E244D has been reported to show an increase of the maximum level of ATPase activity, the functional consequence of K247R mutation has not been analyzed. In order to understand how these mutations cause functional aberrations, we measured ATPase activity of myofibrils containing various mutants of these residues (E244; D, M, A, K and K247; R, E, A). The maximum level of ATPase activity was found to increase in K247R, without changing the Ca ion sensitivity, as found in E244D. A close inspection of the crystal structure showed that the side chains of E244 and K247 form the triplet with that of E110 of TnI on the outside of the hydrophobic core of the coiled-coil. This triplet is thus likely to introduce flexibility into the IT-arm at this position. The mutations at the residues 244 and 247 could alter this flexibility. The results obtained here suggested that the proper flexibility of the IT-arm is important for the correct function of the myofibrils. The relationship between the IT-arm flexibility and functional aberration in the myofibril will be discussed.