Submitted on March 28, 2008
Revised on May 5, 2009
Accepted on May 5, 2009
An internal domain of 
-tropomyosin increases myofilament calcium sensitivity
Ganapathy Jagatheesan, Sudarsan Rajan1, Emily M Schulz1, Rafeeq P.H. Ahmed, Natalia N Petrashevskaya2, Arnold Schwartz1, Greg P. Boivin1, Grace M Arteaga3, Tao Wang2, Yi-Gang Wang2, Muhammad Ashraf2, Stephen Bryant Liggett4, John N Lorenz1, R. John Solaro5, and David F Wieczorek1*
1 University of Cincinnati College of Medicine
2 University of Cincinnati
3 University of Illinois, Chicago College of Medicine
4 University of Maryland School of Medicine
5 University of Illinois at Chicago
* To whom correspondence should be addressed. E-mail: david.wieczorek{at}uc.edu.
Tropomyosin (TM) is involved in calcium mediated muscle contraction and relaxation in the heart. Striated muscle 
-TM is the major isoform expressed in the heart. Expression of striated muscle 
-TM in murine myocardium results in a decreased rate of relaxation and increased myofilament calcium sensitivity. Replacing the carboxyl terminus (amino acids 258-284) of -TM with -TM (a troponin-T [TnT] binding region) results in decreased rates of contraction and relaxation in the heart and decreased myofilament calcium sensitivity. We hypothesized that the putative internal TnT binding domain (amino acids 175-190) of -TM may be responsible for the increased myofilament calcium sensitivity observed when the entire -TM is expressed in the heart. To test this hypothesis, we generated transgenic mice that express a chimeric TM containing -TM amino acids 175-190 in the backbone of -TM (amino acids 1-174 and 191-284). These mice express from 16% - 57% chimeric TM, and do not develop cardiac hypertrophy or any other morphological changes. Physiological analysis shows these hearts exhibit decreased rates of contraction and relaxation and a positive response to isoproterenol. Skinned fiber bundle analyses show a significant increase in myofilament calcium sensitivity. Biophysical studies demonstrate that the exchanged amino acids do not influence the flexibility of the TM. This is the first study to demonstrate that a specific domain within TM can increase calcium sensitivity of the thin filament and affect sarcomeric performance. Further, these results enhance the understanding of why TM mutations associated with familial hypertrophic cardiomyopathy demonstrate increased myofilament sensitivity to calcium.
