Fig. 1. Electron microscopy of thin filaments.

A. Negatively stained F-actin. B. Model for F-actin based on EM. C. Negatively stained thin filament showing tropomyosin strands (arrows) (from Moody et al., 1990). D. 3D reconstruction of thin filaments based on negative stain data. Top, projection of 3D reconstruction down long pitch helices, showing two strands of actin subunits, each with an outer (Ao) and inner (Ai) domain (from Vibert et al., 1997, with permission). At low Ca²⁺, tropomyosin binds to the outer and in high Ca²⁺ to the inner domain. Bottom, surface views of reconstructions (grey) with fitting of actin atomic structure (yellow ribbon) and tropomyosin (red and green ribbons), showing leftward shift in tropomyosin from low Ca²⁺ (red) to high Ca²⁺ (green) (from Mun et al., 2014). E. Cryo-EM of F-actin with direct detector, showing (top) excellent contrast and specimen preservation, even without stain, and (bottom) 3D reconstruction at ~ 5.5 Å resolution, revealing actin secondary structure. Scale bars: A, C, 200 Å; D, 50 Å; E, 500 Å (top), 20 Å (bottom).