The Regulation of Actomyosin ATPase in Cardiac Muscle by the N-Terminal Extension of Cardiac Troponin T

Abstract

Cardiac troponin T (cTnT) and cardiac troponin I (cTnI) both have an amino-terminal variable region that plays a role in modifying the overall protein conformation and functions in the calcium-dependent regulation of cardiac muscle function. Previous studies have showed that in comparison with wild type controls, transgenic mouse hearts over-expressing cTnT lacking the N-terminal variable region (cTnT-ND) had an increased rate of relaxation. In contrast, transgenic mouse heart over-expressing cTnI lacking the N-terminal extension (cTnI-ND) had lower sensitivity to calcium activation of ATPase, resulting enhanced ventricular relaxation and Frank-Starling response. Recently, we demonstrated that the second order mant-dATP binding rate of cardiac myofibrils containing cTnI-ND was three-fold as fast as that of wild type myofibrils in low [Ca2+]. The ADP dissociation rate of cTnI-ND myofibrils was positively dependent on calcium concentrations, while the wild type controls were not significantly affected. These results from native cardiac myofibrils under physiological conditions indicate that the N-terminal extension of cTnI plays a role in the calcium regulation of the kinetics of actomyosin ATPase. In the present study, we employed the same techniques in order to investigate which step(s) of the ATPase cycle is regulated by the N-terminal variable region of cTnT. mant-ATP binding, mant-ADP/ADP dissociation, and phosphate releasing rate of cTnT-ND myofibrils are examined and the results will provide novel information dissecting the function of the two troponin subunits and their posttranslational modifications.