Figure 19.6.5.1 

Examples of macromolecules studied by cryo EM and 3D image reconstruction and the resulting 3D structures (bottom row) after cryo EM analysis. All micrographs (top row) are displayed at ∼170 000× magnification and all models at ∼1 200 000× magnification. (a) A single particle without symmetry. The micrograph shows 70S E. coli ribosomes complexed with mRNA and fMet-tRNA. The surface-shaded density map, made by averaging 73 000 ribosome images from 287 micrographs, has a resolution of 11.5 Å. The 50S and 30S subunits and the tRNA are coloured blue, yellow and green, respectively. The identity of many of the protein and RNA components is known and some RNA double helices are clearly recognizable by their major and minor grooves (e.g. helix 44 is shown in red). Courtesy of J. Frank (SUNY, Albany), using unpublished data from I. Gabashvili, R. Agrawal, C. Spahn, R. Grassucci, J. Frank & P. Penczek. (b) A single particle with symmetry. The micrograph shows hepatitis B virus cores. The 3D reconstruction, at a resolution of 7.4 Å, was computed from 6384 particle images taken from 34 micrographs. From Böttcher, Wynne & Crowther (1997). (c) A helical filament. The micrograph shows actin filaments decorated with myosin S1 heads containing the essential light chain. The 3D reconstruction, at a resolution of 30–35 Å, is a composite in which the differently coloured parts are derived from a series of difference maps that were superimposed on F-actin. The components include: F-actin (blue), myosin heavy-chain motor domain (orange), essential light chain (purple), regulatory light chain (white), tropomyosin (green) and myosin motor domain N-terminal beta-barrel (red). Courtesy of A. Lin, M. Whittaker & R. Milligan (Scripps Research Institute, La Jolla). (d) A 2D crystal: light-harvesting complex LHCII at 3.4 Å resolution. The model shows the protein backbone and the arrangement of chromophores in a number of trimeric subunits in the crystal lattice. In this example, image contrast is too low to see any hint of the structure without image processing (see also Fig. 19.6.4.2). Courtesy of W. Kühlbrandt (Max-Planck-Institute for Biophysics, Frankfurt).