The N-terminal Region of Troponin T is Essential for the Maximal Activation of Rat Cardiac Myofilaments

Troponin T (TnT) is an essential protein in the transduction of the Ca2+-binding signal that triggers striated muscle contraction. Functional diversity among various TnT isoforms found in cardiac and skeletal muscles has been correlated with the sequence heterogeneity at the amino (N-) and the carboxyl (C-) terminal regions. The most striking difference between cardiac TnT (cTnT) and skeletal TnT (sTnT) is that cTnT has an extended N-terminus, which is rich in negatively charged amino acids. To investigate the role of this region in cTnT, we deleted the first 76 amino acids in rat cTnT (cTnT77–289) by site-directed mutagenesis. We exchanged the native troponin complex in rat cardiac myofibrillar preparations and detergent skinned cardiac fiber bundles by treatment with excess cTnT or cTnT77–289. After reconstituting the cTnT77–289 containing myofibrils with cardiac troponin I–cardiac troponin C (cTnI–cTnC), the MgATPase activity was 70% of the cTnT treated myofibrils in the relaxed state and 83% of the cTnT treated myofibrils in the maximal Ca2+-activated state. These observations were supported by force measurements in which cTnT and cTnT77–289 were exchanged into skinned fiber bundles. Prior to reconstitution with cTnI–cTnC, the Ca2+-independent maximal force developed by the cTnT77–289 containing fiber was 45% of the force developed by the cTnT containing fiber. After reconstituting with cTnI–cTnC, the Ca2+-activated maximal force of the cTnT77–289 containing fiber was 62% of the force developed by the cTnT containing +cTnI–cTnC reconstituted fiber. In both assays, no significant changes in the normalized Ca2+-activity relation or in co-operativity were observed. Fluorescence experiments using pyrene-labeled Tm demonstrated that the binding of cTnT77–289 to Tm was 3–4 fold stronger than that of cTnT. Our results suggest that strong interactions between cTnT77–289 and Tm stabilize cardiac myofilaments in a sub-maximally activated state. Our findings also indicate that the N-terminus of cTnT is essential for maximal activation of cardiac myofilaments.