NAME: Jorge Navaza

AFFILIATION: IBS, France

CONTACT: jorge.navaza@ibs.fr

 

TITLE: "Combining Electron Microscopy and X-Ray Techniques"

ABSTRACT: Electron Microscopy is a fascinating technique by its apparent simplicity to study isolated biological systems, which are visualized in almost "in vitro" conditions. The observed systems are, in general, assemblages of macromolecular structures, for example multimeric molecules or the proteins and nucleic acids constituting a virus. Due to limitations of the technique, the produced images lack of resolution.

 

On the other hand, X-Ray crystallography may provide the diffraction data for these systems to almost atomic resolution, but the associated structure factors lack of phases. In the case of the very large complexes that are now crystallized, phasing by isomorphous replacement is difficult.

 

However, EM and X-Ray data may be combined to start the process of crystal structure determination. Indeed, an initial phasing model may be a low resolution EM reconstruction of the complex used as a probe in the Molecular Replacement method. Phases are then extended by density modification, i.e. solvent flattening and non-crystallographic symmetry averaging.

 

EM and X-Ray data may also be combined the other way: very often X-Ray crystallography determines the structures of the individual proteins that constitutes an assemblage. It is then possible to interpret the EM image in terms of atomic models. This is achieved by docking individual molecules into the EM image, a technique quite similar to MR.