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Effects of tropomyosin internal deletion Δ23Tm on isometric tension and the cross‐bridge kinetics in bovine myocardium
Author(s) -
Lu Xiaoying,
Tobacman Larry S.,
Kawai Masataka
Publication year - 2003
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.2003.053694
Subject(s) - tropomyosin , actin , myosin , isometric exercise , cooperativity , biophysics , protein filament , chemistry , myosin head , troponin , crossbridge , tension (geology) , gelsolin , crystallography , biochemistry , myosin light chain kinase , biology , compression (physics) , physics , thermodynamics , physiology , psychiatry , myocardial infarction , psychology
Tropomyosin (Tm) spans seven actin monomers and contains seven quasi‐repeating, loosely similar regions, 1–7. Deletion of regions 2–3 decreases the in vitro sliding speed of synthetic filaments of actin‐Tm‐Troponin (Tn), and weakens Tm binding to the actin‐myosin subfragment 1 (S1) complex (acto‐S1). The thin filament was selectively removed from bovine myocardium by gelsolin, and the actin filament was reconstituted, followed by further reconstitution with Tm and Tn. In this reconstitution, full‐length Tm (control) was compared with Tm internal deletion mutant Δ23Tm, which lacks residues 47–123 (regions 2–3). The effects of phosphate, MgATP, MgADP and Ca 2+ were studied in Tm‐reconstituted myocardium and Δ23Tm‐reconstituted myocardium at pH 7.00 and 25 °C. In Δ23Tm, both isometric tension and stiffness were about 40 % of the control. The Hill factor with Δ23Tm, deduced from the pCa‐tension plot, was 1.4 times that of the control, but the Ca 2+ sensitivity was the same. Sinusoidal analysis indicated that the cross‐bridge number in force‐generating states was not decreased with Δ23Tm. We conclude that the thin filament cooperativity is increased with Δ23Tm, presumably because of the increased density of the Ca 2+ ‐binding sites. We further conclude that tension per cross‐bridge is 40 % of control and stiffness per cross‐bridge is 40 % of control in Δ23Tm. These results are consistent with the idea that Tm modifies the actin‐myosin interface so as to increase the stereospecific interaction between moieties of actin and myosin. In Δ23Tm, the interface may not have a perfect stereospecific match so that the tension‐ and stiffness‐generating capacity is greatly diminished.