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 α-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 Å. 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 Å. A projection map was calculated to 4 Å using electron diffraction amplitudes and phases from computer-processed images. The projection map clearly shows modulations in density arising from the 5·1 Å α-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 Å, b = 150·7 Å, γ = 92·4°. Examination of tilted specimens shows that c is approximately 18 Å, indicating that the unit cell is only one molecule thick. A preliminary interpretation shows tightly packed molecules some 400 Å 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. © 1990 Academic Press Limited.