Geometry of the C-/N-Terminal Connection of Tropomyosin on F-Actin Based on Molecular Dynamics Simulations of the Thin Filament

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The binding of tropomyosin molecules to actin filaments is extremely weak and only is effective because of cooperative interactions resulting from the head-to-tail polymerization of tropomyosin on thin filaments. Experiments show that interfering with the head/tail interactions prevents actin-binding (Johnson and Smillie, 1977; Heald and Hitchcock-DeGregori, 1988). Crystal- and NMR-structures of mixtures of head/tail-fragments stabilized with various foreign adducts, yet unconstrained by interactions with F-actin, have suggested that head-to-tail linkage results from a splayed C-terminal coiled-coil region enveloping a more compact N-terminus to form a four-helix nexus with varying degree of overlap. However, it is possible that structures observed previously may have been influenced by the adducts and the absence of F-actin. In fact, N-/C-terminal sequences vary greatly between tropomyosin isoforms and are not strictly tetra-coiled-coiled sequences, complicating interpretation. Here, based on static and dynamic models of αα-striated muscle tropomyosin on F-actin, we derive the overlap and the relative pseudo-rotation angle (which determines the face of tropomyosin which is seen by F-actin) between the C- and N-terminal ends. Our recently described atomic model of tropomyosin on F-actin (Li et al., 2011) suggests that, when constrained by interaction with the F-actin surface, the tropomyosin ends overlap by five amino acids, slightly shorter than in the adduct-structures. MD simulations provide a measure of the variability in end-to-end distance of tropomyosin on F-actin (and therefore of the possible overlap length) and in the degree of pseudo-rotation between N- and C-tropomyosin termini (and therefore C-/N-nexus geometry). These simulations demonstrate local C-terminal coiled-coil splaying, relatively short overlap (7.9 ± 2.2A, i.e. ∼four to seven residues) and predict a rotation of 17.5 ± 13° at the end of one tropomyosin relative to the beginning of the next.