When two into one won't go: fitting in the presence of steric hindrance and partial occupancy

Combining structural data from cryo-electron microscopy (cryo-EM) and X-ray crystallography to give pseudo-atomic models of large molecular complexes has proved particularly suitable for studying viruses and viral complexes. Several groups are developing programs to fit X-ray data to EM data. These programs are in general tailored to particular problems with regard to size , symmetry, number of rigid bodies, resolution etc. Here, two approaches ar e described to fitting X-ray data to EM data in the presence of steric inte rference and their relative merits and limitations are indicated. These fit ting techniques are applied to the case of the rotavirus double-layered par ticle (DLP) in complex with antibodies which inhibit the transcription of m RNA by the DLP. This is a particularly good test case, as the cryo-electron microscopy map of the DLP-Fab complex, the X-ray structure of the viral pr otein (VP6) and also that of the VP6-Fab complex are available. The estimat ion of partial occupancy is also considered.