A pseudo-cell based approach to efficient crystallographic refinement of viruses

Strategies have been developed for the inexpensive refinement of atomic models of viruses and of other highly symmetric structures. These methods, which have been used in the refinement of several strains of poliovirus, focus on an arbitrary-sized parallelepiped (termed the 'protomer' box) containing a single complete averaged copy of the structural motif which forms the protein capsid, together with the fragments of other symmetry-related copies of the motif which are located in its immediate neighborhood. The Fourier transform of the protomer box provides reference structure factors for stereochemically restrained crystallographic refinement of the atomic model parameters. The phases of the reference structure factors are based on the averaged map, and are not permitted to change during the refinement. It is demonstrated that models refined using the protomer box methods do not differ significantly from models refined by more expensive full-cell calculations.