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This Article Full Text Full Text (PDF) All Versions of this Article: 553/2/457    most recent jphysiol.2003.053694v1 Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Lu, X. Articles by Kawai, M. Search for Related Content PubMed PubMed Citation Articles by Lu, X. Articles by Kawai, M.

Effects of tropomyosin internal deletion 23Tm on isometric tension and the cross-bridge kinetics in bovine myocardium

Xiaoying Lu, Larry S. Tobacman* and Masataka Kawai

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 23Tm, which lacks residues 47-123 (regions 2-3). The effects of phosphate, MgATP, MgADP and Ca²⁺ were studied in Tm-reconstituted myocardium and 23Tm-reconstituted myocardium at pH 7.00 and 25 °C. In 23Tm, both isometric tension and stiffness were about 40 % of the control. The Hill factor with 23Tm, deduced from the pCa-tension plot, was 1.4 times that of the control, but the Ca²⁺ sensitivity was the same. Sinusoidal analysis indicated that the cross-bridge number in force-generating states was not decreased with 23Tm. We conclude that the thin filament cooperativity is increased with 23Tm, presumably because of the increased density of the Ca²⁺-binding sites. We further conclude that tension per cross-bridge is 40 % of control and stiffness per cross-bridge is 40 % of control in 23Tm. 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 23Tm, the interface may not have a perfect stereospecific match so that the tension- and stiffness-generating capacity is greatly diminished.

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