Effects of tropomyosin internal deletion Δ23Tm on isometric tension and the cross-bridge kinetics in bovine myocardium

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 Ca2+ 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 Ca2+ 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 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 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