Fig. 1.

Pre-treatment of filaments. (A) Electron microscope image of a region of negatively-stained cardiac muscle thin filament exhibiting slight curvature and readily identifiable troponin densities. (B) Thin filament region shown in A after computational straightening. (C) Axial density profile of Fourier-filtered version of B in which spatial frequencies ≤200 and ≥500 Å were suppressed, allowing the identification and location of regular peaks arising from troponin. (D) Filament segments centred on the troponin density derived from B. (E) Helical reconstruction from the data used for angular assignment.