Interaction electrostatic energy landscape for tropomyosin. In previous landscapes, tropomyosin was truncated by 20–25 residues at the termini to avoid complications from twisting in the overlap. Here, the overlap was included in the calculations using constraints to maintain the tropomyosin structure as if part of a continuous cable on actin. The electrostatic energy at each grid point between tropomyosin and actin was calculated and graphed on a grid of 2.5° × 2.5 Å, making a total of 512 grid points. The grid was tilted along the x axis to mimic the actin helical symmetry. Isolines are plotted from −2,700 to −600 kcal mol⁻¹ in increments of 300 kcal mol⁻¹. Note that because the long axis helical repeat of F-actin consists of an ∼55-Å longitudinal translation coupled with an ∼26° azimuthal rotation of actin subunits, the area of the plot spanning longitude −35 to −10 Å will be very similar to the area of the plot spanning 20–45 Å since these points start from almost identical structures. Reference lines show the position of tropomyosin in the open M state, closed C state, and blocked B state and are indicated by the green, yellow, and red lines, respectively. The position of Pro333 on actin is shown as a white line.