Table of Contents
Journal of Biophysics
Volume 2009, Article ID 380967, 17 pages
Research Article

Tropomyosin Period 3 Is Essential for Enhancement of Isometric Tension in Thin Filament-Reconstituted Bovine Myocardium

1Department of Anatomy and Cell Biology, The University of Iowa, Iowa City, IA 52242, USA
2Department of Neuroscience and Cell Biology, Robert Wood Johnson Medical School, Piscataway, NJ 08854, USA

Received 23 March 2009; Revised 29 May 2009; Accepted 5 July 2009

Academic Editor: P. Bryant Chase

Copyright © 2009 Masataka Kawai et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


Tropomyosin (Tm) consists of 7 quasiequivalent repeats known as “periods,” and its specific function may be associated with these periods. To test the hypothesis that either period 2 or 3 promotes force generation by inducing a positive allosteric effect on actin, we reconstituted the thin filament with mutant Tm in which either period 2 (2Tm) or period 3 (3Tm) was deleted. We then studied: isometric tension, stiffness, 6 kinetic constants, and the pCa-tension relationship. N-terminal acetylation of Tm did not cause any differences. The isometric tension in 2Tm remained unchanged, and was reduced to 60% in 3Tm. Although the kinetic constants underwent small changes, the occupancy of strongly attached cross-bridges was not much different. The Hill factor (cooperativity) did not differ significantly between 2Tm (1.79 0.19) and the control (1.73 0.21), or 3Tm (1.35 0.22) and the control. In contrast, p decreased slightly in 2Tm (5.11 0.07), and increased significantly in 3Tm (5.57 0.09) compared to the control (5.28 0.04). These results demonstrate that, when ions are present at physiological concentrations in the muscle fiber system, period 3 (but not period 2) is essential for the positive allosteric effect that enhances the interaction between actin and myosin, and increases isometric force of each cross-bridge.