Fitting atomic models into electron-microscopy maps

Combining X-ray crystallographically determined atomic structures of compon ent domains or subunits with cryoelectron microscopic three-dimensional ima ges at around 22 Angstrom resolution can produce structural information tha t is accurate to about 2.2 Angstrom resolution. In an initial step, it is n ecessary to determine accurately the absolute scale and absolute hand of th e 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, si tes of glycosylation, the position of which helps to rt the component struc tures into the EM density maps. Examples are given from the analysis of alp haviruses, rhinovirus-receptor interactions and poliovirus-receptor interac tions.