X-Ray Crystallographic Structure Determination of Large Asymmetric Macromolecular Assemblies

Abstract

X-ray crystallography is currently the only technique available that permits high-resolution structure determination of extremely large macromolecular complexes. An increasing number of large structures and macromolecular assemblies have been determined by crystallographic methods. Difference Fourier analysis is powerful in identifying anomalous scatterers or heavy atoms. However, such analysis requires some phase information, and often low-resolution phases are adequate. To determine such low-resolution phases, electron microscopy three-dimensional (3D) reconstructions of the particle should be used. This technique was pioneered in the structure determination of crystals of the tomato bushy stunt virus for which 3D maps derived from electron micrographs of negatively stained virus particles were employed. In that case, the crystallographic symmetry unambiguously determined the orientation and the position of the virus particle in the unit cell, and molecular replacement procedures were not necessary. There has been a dramatic increase in the success of crystallography in solving structures of large asymmetric subunits. The major contributors to this success are the improved quality and flux of synchrotron beam lines that allow data to be measured at the highest possible accuracy.