Molecular Dynamics of Tropomyosin Bound to F-Actin

Abstract

We have previously studied the structural mechanics of isolated tropomyosin molecules by electron microscopy and Molecular Dynamics protocols. We found that single tropomyosin molecules bend anisotropically without showing any signs of kinks or joints. We observed that, even in the absence of actin, tropomyosin, on average, has a shape matching the helical curvature of F-actin filaments. Moreover, isolated tropomyosin is semi-rigid with an apparent persistence length (an estimate of curvature) of 104 nm and dynamic persistence length (a measure of flexibility) of 423 nm (Li et al., 2010). In order to determine the likely position of tropomyosin on F-actin, we docked theMD structure of tropomyosin into EM reconstructions of F-actin-tropomyosin. The location found is the same as that computed for tropomyosin on F-actin following a search to optimize their intermolecular electrostatic contacts. We have initiated NAMD MD simulations of this structure of F-actin-tropomyosin at 300°K in explicit water including 150mM NaCl. Here tropomyosin fluctuates locally around the closed C-state position on F-actin, maintaining its initial electrostatic contacts as well as hydrophobic interactions with actin residue Pro333. Over the current 25 ns MD run, the apparent and dynamic persistence lengths of the tropomyosin on F-actin are 113 nm and 3318 nm, demonstrating that tropomyosin maintains a comparable degree of curvature either when bound to F-actin or when unbound, but that its flexibility is restricted by the presence of F-actin. Any azimuthal movement of tropomyosin on F-actin that is induced by troponin and/or myosin may require perturbation of the weak stabilizing interactions noted between tropomyosin and F-actin. We are continuing the current MD run and are also planning to run simulations at higher temperatures in attempting to detect transitions of tropomyosin to the open and blocked M- and B-states of the thin filament.