Fitting atomic models into electron-microscopy maps

Combining X-ray crystallographically determined atomic structures of component domains or subunits with cryoelectron microscopic three-dimensional images at around 22 Angstrom resolution can produce structural information that is accurate to about 2.2 Angstrom resolution. In an initial step, it is necessary to determine accurately the absolute scale and absolute hand of the cryo-electron microscopy map, the former of which can be off by up to 5%. It is also necessary to determine the relative height of density by using a suitable scaling function. Difference maps can identify, for instance, sites of glycosylation, the position of which helps to rt the component structures into the EM density maps. Examples are given from the analysis of alphaviruses, rhinovirus-receptor interactions and poliovirus-receptor interactions.