High-resolution spot-scan electron microscopy of microcrystals of an alpha-helical coiled-coil protein.

We describe the electron microscopy of a crystalline assembly of an alpha-helical coiled-coil protein extracted from the ootheca of the praying mantis. Electron diffraction patterns of unstained crystals show crystal lattice sampling of the coiled-coil molecular transform to a resolution beyond 1.5 A. Using a "spot-scan" method of electron imaging, micrographs of unstained crystals have been obtained that visibly diffract laser light from crystal spacings as small as 4.3 A. A projection map was calculated to 4 A using electron diffraction amplitudes and phases from computer-processed images. The projection map clearly shows modulations in density arising from the 5.1 A alpha-helical repeat, the first time this type of modulation has been revealed by electron microscopy. The crystals have p2 plane group symmetry with a = 92.4 A, b = 150.7 A, y = 92.4 degrees. Examination of tilted specimens shows that c is approximately 18 A, indicating that the unit cell is only one molecule thick. A preliminary interpretation shows tightly packed molecules some 400 A long lying with their long axes in the plane of the projection. The molecules have a coiled-coil configuration for most of their length. The possible modes of packing of the molecules in three dimensions are discussed.