An internal domain of beta-tropomyosin increases myofilament Ca(2+) sensitivity.
Tropomyosin (TM) is involved in Ca(2+)-mediated muscle contraction and relaxation in the heart. Striated muscle alpha-TM is the major isoform expressed in the heart. The expression of striated muscle beta-TM in the murine myocardium results in a decreased rate of relaxation and increased myofilament Ca(2+) sensitivity. Replacing the carboxyl terminus (amino acids 258-284) of alpha-TM with beta-TM (a troponin T-binding region) results in decreased rates of contraction and relaxation in the heart and decreased myofilament Ca(2+) sensitivity. We hypothesized that the putative internal troponin T-binding domain (amino acids 175-190) of beta-TM may be responsible for the increased myofilament Ca(2+) sensitivity observed when the entire beta-TM is expressed in the heart. To test this hypothesis, we generated transgenic mice that expressed chimeric TM containing beta-TM amino acids 175-190 in the backbone of alpha-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 Ca(2+) 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 Ca(2+) 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 Ca(2+).
Jagatheesan
Ganapathy
G
Department of Molecular Genetics, Biochemistry, and Microbiology, University of Cincinnati College of Medicine, Cincinnati, OH 45267-0524, USA.
Rajan
Sudarsan
S
Schulz
Emily M
EM
Ahmed
Rafeeq P H
RP
Petrashevskaya
Natalia
N
Schwartz
Arnold
A
Boivin
Greg P
GP
Arteaga
Grace M
GM
Wang
Tao
T
Wang
Yi-Gang
YG
Ashraf
Muhammad
M
Liggett
Stephen B
SB
Lorenz
John
J
Solaro
R John
RJ
Wieczorek
David F
DF
D008841:Actin Cytoskeleton / Q000187:drug effects*; D000318:Adrenergic beta-Agonists / Q000494:pharmacology; D019943:Amino Acid Substitution; D000818:Animals; D015139:Blotting, Southern; D002118:Calcium / Q000494:pharmacology*; D002316:Cardiotonic Agents / Q000494:pharmacology; D002942:Circular Dichroism; D004452:Echocardiography; D066298:In Vitro Techniques; D007525:Isoelectric Focusing; D007545:Isoproterenol / Q000494:pharmacology; D051379:Mice; D009154:Mutation / Q000502:physiology; D009186:Myelin Sheath / Q000502:physiology; D009200:Myocardial Contraction / Q000187:drug effects; D011489:Protein Denaturation; D012313:RNA / Q000096:biosynthesis / Q000235:genetics; D011994:Recombinant Proteins / Q000494:pharmacology; D012518:Sarcomeres / Q000187:drug effects; D014335:Tropomyosin / Q000737:chemistry / Q000235:genetics / Q000494:pharmacology*
D000318:Adrenergic beta-Agonists; D002316:Cardiotonic Agents; D011994:Recombinant Proteins; D014335:Tropomyosin; D012313:RNA; D007545:Isoproterenol; D002118:Calcium