Molecular Dynamics of Tropomyosin: Implications for the Assembly and Regulation of Thin Filaments

Abstract

The molecular switching mechanism governing skeletal and cardiac muscle contraction couples the binding of Ca2+ on troponin to the movement of tropomyosin (Tm) on actin filaments. By shifting position around thin filaments in response to changing Ca2+, Tm either blocks or exposes myosin-binding sites on actin, thereby regulating myosin-crossbridge cycling and consequently contraction. Tm lies over actin at a ∼39 angstrom radius with considerable water between the two surfaces. Lorenz et al. (1995) and later Poole et al. (2006) proposed that Tm has a distinctive coiled coiled-coil shape designed to match the contours of F-actin. This arrangement might facilitate binding of Tm on F-actin and movement between regulatory states. In contrast, others have suggested that Tm flexibility is needed for binding and regulatory movements. To understand transitions of Tm between regulatory states better, the structure and flexibility of Tm was assessed by Molecular Dynamics performed in implicit water. A full-length Tm atomic structure was constructed by fitting different crystal structures of Tm segments (PDBs: 2D3E, 1IC2, and 2B9C) to the coordinates of the Lorenz coiled coiled-coil model. The Tm stretches and the model fitted to each other very well. Tm showed delocalized but pronounced anisotropic bending during 11ns MD, with no evidence of localized kinking, suggesting that Tm lacks discrete domains that flex. A persistence length several times the length of Tm was calculated, indicating that the molecule is only semi-flexible. Although Tm bends away from its initial supercoiled shape, it revisits the contours of the Lorenz model multiple times during simulation, implying that Tm may assume this shape when binding to F-actin. The results indicate that Tm is flexible enough to coil around actin, yet stiff enough to act as a cooperative unit during regulatory movements.