The Conserved C-terminal End Segment of Troponin T Is A New Tropomyosin-Binding Site Modulating the Kinetics of Cardiac Muscle

Evolved from duplication of a troponin I (TnI)-like ancestor gene, troponin T (TnT) and TnI are two subunits of the troponin complex that regulates the contraction and relaxation of striated muscles. Proteolytic deletion of the evolutionarily added N-terminal variable region of TnT in adaptation to acute myocardial stress restores a conformation like that of the C-terminal end segment of TnI, a tropomyosin (Tm)-binding and inhibitory structure, with an effect on reducing the contractile velocity of cardiac muscle to prolong ejection time and sustain the stroke volume. To investigate the underlying mechanism of this adaptive conditional function of TnT that is known to have two Tm-binding sites, our study localized a conformationally modulated new Tm-binding site in the highly conserved 14 amino acid C-terminal end segment of TnT. Localized surface plasmon resonance data showed that a hypertrophic cardiomyopathy mutation R278C within the C-terminal end segment of TnT alters the tropomyosin binding with direct responses to physiological concentrations of Ca2+. The functions are retained in the form of free peptide with an inhibitory regulatory effect on the contractile and relaxation kinetics of skinned cardiac muscle. In addition to revealing the underlying mechanisms of cTnT-ND adaptation and cardiac TnT C-terminal myopathic mutations, the new findings provide novel insights into the structure-function relationship of TnT in the kinetics of striated muscle contraction and relaxation with broad physiological and pathophysiological implications.