De novo phasing of two crystal forms of tryparedoxin II using the anomalous scattering from S atoms: a combination of small signal and medium resolution reveals this to be a general tool for solving protein crystal structures

The de novo phasing of the structures of two crystal forms of tryparedoxin II from Crithidia fasciculata has been carried out using single-wavelength anomalous diffraction techniques exploiting only the small anomalous signal from the S atoms intrinsic to the native protein. Data were collected at 1.77 Å wavelength, where the Bijvoet ratio is approximately 1.2%. Data collected to d_min = 2.5 Å from a crystal of form I, which has a diffraction limit of d_min = 1.5 Å and a solvent content of ∼46%, produced readily interpretable electron-density maps. When these phases were extended to the resolution limit of the crystals, almost the entire model could be traced automatically. Crystals of form II have a much higher solvent content, ∼72%, and a much lower diffraction limit than form I and at 1.77 Å wavelength yielded data only to d_min = 2.7 Å. Despite the medium resolution of the data for this crystal form, it was possible both to determine the heavy-atom partial structure and then use it to produce, still at d_min = 2.7 Å, an excellent quality interpretable electron-density map. This was then improved by phase extension to the d_min = 2.35 Å diffraction limits of a different crystal for which data were collected on a more intense beamline. The success of this latter structure solution markedly increases the potential use in macromolecular crystal structure determination of the anomalous signal available from S atoms that occur naturally in proteins and, as is discussed, has significant implications for structure determination in the high-throughput era.