Bridging the resolution gap between x-ray crystallography and electron microscopy
While x-ray crystallography provides atomic resolution structures of proteins and small viruses, electron microscopy can provide complementary structural information on larger assemblies. A significant computational challenge is faced in bridging the resolution gap between the two techniques. X-ray crystallographic data is collected in the range of 2-10 Å, while image reconstructions from electron micrographs are at a resolution of 25-35 Å. A further problem is that density derived from cryo-electron micrographs is distorted by the contrast transfer function of the microscope, whichaccentuates certain resolution bands.A novel combination of electron microscopy and x-ray crystallography has revealed the various structural components forming the capsid of human type 2 adenovirus. An image reconstruction of the intact virus (Fig. 1), derived from cryo-electron micrographs, was deconvolved with an approximate contrast transfer function to mitigate microscope distortions (Fig. 2). A model capsid was calculated from 240 copies of the crystallographic structure of the major capsid protein and filtered to the correct resolution (Fig. 3).