Fig. 2.

Difference density analysis. (A–C) Data from Ca²⁺-treated filaments, (D–F) data from Ca²⁺-free filaments. (A and D) Wire mesh representation of the single particle based reconstructions of the thin filament in the two states. An atomic F-actin model is docked into the reconstruction and each subunit is colour coded. The barbed end (Z-band end) of the actin filament is at the bottom of the figure. (B and E) Difference density maps calculated by subtracting the docked F-actin model (grey) from the single particle reconstructions. This leaves the density attributable to the regulatory proteins troponin and tropomyosin (both orange). (C and F) Difference density maps calculated by subtracting docked F-actin (grey) and tropomyosin (orange) models from the single particle reconstructions leaving density attributable only to troponin (blue).