Direct phasing of protein crystals with non-crystallographic symmetry

Abstract

An iterative projection algorithm proposed previously for direct phasing of high-solvent-content protein crystals is extended to include non-crystallographic symmetry (NCS) averaging. For proper NCS, when the NCS axis is positioned, the molecular envelope can be automatically rebuilt. For improper NCS, when the NCS axis and the translation vector are known, the molecular envelope can also be automatically reconstructed. Some structures with a solvent content of around 50% could be directly solved using this ab initio phasing method. Trial calculations are described to illustrate the methodology. Real diffraction data are used and the calculated phases are good for automatic model building. The refinement of approximate NCS parameters is discussed.