Fig. 3.

Coulombic interaction energy landscape of actin-tropomyosin. Shown are tropomyosin positions on F-actin (surface with alternating gray and blue colors). a the Li et al. (2011) tropomyosin model (red). b the Li et al. (2011) model in red and a tropomyosin model in green that corresponds to energy minimum A’ in d. c the Li et al. (2011) model (orange) shifted helically by one actin unit relative to its position in a. Note that the models shown here cover a large portion of the actin surface accessible to tropomyosin. d Each point in the energy landscape on this map gives the electrostatic energy of interaction between F-actin and one tropomyosin coiled-coil, which has been longitudinally and azimuthally repositioned while showing the same interacting-face to F-actin (see “Methods” section). The lower right hand and upper left hand corners of the graph were not sampled, since ridges on F-actin in those locations would result in severe steric clashes with the tropomyosin cable. Positions at the energy minima of the landscape are labeled A₁, A’ and A₂ (and with white circles and crosses) and correspond to the structures shown in a–c, respectively. The asterisk marked M indicates the position of the M-state model of Behrmann et al. (2012) for comparison. The energies [in kcal/mol] are given relative to the minimum energy position