An internal domain of β-tropomyosin increases myofilament Ca ²⁺ sensitivity

Tropomyosin (TM) is involved in Ca²⁺-mediated muscle contraction and relaxation in the heart. Striated muscle α-TM is the major isoform expressed in the heart. The expression of striated muscle β-TM in the murine myocardium results in a decreased rate of relaxation and increased myofilament Ca²⁺ sensitivity. Replacing the carboxyl terminus (amino acids 258-284) of α-TM with β-TM (a troponin T-binding region) results in decreased rates of contraction and relaxation in the heart and decreased myofilament Ca²⁺ sensitivity. We hypothesized that the putative internal troponin T-binding domain (amino acids 175-190) of β-TM may be responsible for the increased myofilament Ca²⁺ sensitivity observed when the entire β-TM is expressed in the heart. To test this hypothesis, we generated transgenic mice that expressed chimeric TM containing β-TM amino acids 175-190 in the backbone of α-TM (amino acids 1-174 and 191-284). These mice expressed 16-57% chimeric TM and did not develop cardiac hypertrophy or any other morphological changes. Physiological analysis showed that these hearts exhibited decreased rates of contraction and relaxation and a positive response to isoproterenol. Skinned fiber bundle analyses showed a significant increase in myofilament Ca²⁺ sensitivity. Biophysical experiments demonstrated that the exchanged amino acids did not influence the flexibility of the TM. This is the first study to demonstrate that a specific domain within TM can increase the Ca²⁺ sensitivity of the thin filament and affect sarcomeric performance. Furthermore, these results enhance the understanding of why TM mutations associated with familial hypertrophic cardiomyopathy demonstrate increased myofilament sensitivity to Ca²⁺.