Preliminary electron crystallographic analysis of ice-embedded tropomyosin crystals.

Electron images and diffraction patterns of ice-embedded tropomyosin crystalline sheets have been recorded at 100 and 400 kV. Optical diffractograms from the images indicated an elongated, centered unit cell with a = 799.2 +/- 10.6 A, b = 55.1 +/- 3.5 A. Evaluation of the phases in the computed Fourier transforms up to 7 A resolution revealed the presence of symmetry axes consistent with two-dimensional space group cmm. Electron diffraction patterns show diffuse arcs and discrete sampling at a resolution of 5.1 A, arising from the alpha-helical coiled-coil features of the molecule. These results demonstrate that tropomyosin thin sheets are highly ordered and suggest that retrieval of its high-resolution three-dimensional structure may be feasible by electron crystallography.