A Description of the Techniques and Application of Molecular Replacement Used to Determine the Structure of Polyoma Virus Capsid at 22.5 Å Resolution.

The electron density map of polyoma virus capsid crystals solved at 22.5 Å resolution by molecular replacement [Rayment, Baker, Caspar & Murakami (1982). Nature (London), 295, 110-115] shows that the 72 capsomeres that form the polyoma capsid are all pentamers. An extensive series of refinement calculations were undertaken to demonstrate the validity of this unexpected result. This report describes the details of the data collection, structure determination and the tests of the methods applied. The refinement calculations demonstrate that the refined phases are insensitive to the initial phasing model for a wide variety of models. They also show that it is vital to include in the refinement calculations interpolated values for the unrecorded data. A variety of tests demonstrate that the all-pentamer structure of the polyoma capsid is determined by the diffraction amplitudes and the self-consistent constraint that the 72 capsomeres are arranged with icosahedral symmetry within a well defined limiting envelope.