Ab initio phasing of high-symmetry macromolecular complexes: Successful phasing of authentic poliovirus data to 3.0 angstrom resolution

A genetic algorithm-based computational method for the nb initio phasing of diffraction data from crystals of symmetric macromolecular structures, suc h as icosahedral viruses, has been implemented and applied to authentic dat a from the P1/Mahoney strain of poliovirus. Using only single-wavelength na tive diffraction data, the method is shown to be able to generate correct p hases, and thus electron density, to 3.0 Angstrom resolution. Beginning wit h no advance knowledge of the shape of the virus and only approximate knowl edge of its size, the method uses a genetic algorithm to determine coarse, low-resolution (here, 20.5 Angstrom) models of the virus that obey the know n non-crystallographic symmetry (NCS) constraints. The best scoring of thes e models are subjected to refinement and NCS-averaging, with subsequent pha se extension to high resolution (3.0 Angstrom). Initial difficulties in pha se extension were overcome by measuring and including all low-resolution te rms in the transform. With the low-resolution data included, the method was successful in generating essentially correct phases and electron density t o 6.0 Angstrom in every one of ten trials from different models identified by the genetic algorithm. Retrospective analysis revealed that these correc t high-resolution solutions converged from a range of significantly differe nt low-resolution phase sets (average differences of 59.7 degrees below 24 Angstrom). This method represents an efficient way to determine phases for icosahedral viruses, and has the advantage of producing phases free from mo del bias. It is expected that the method can be extended to other protein s ystems with high NCS. (C) 2001 Academic Press.