title: 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.
abstract: 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 A wavelength, where the Bijvoet ratio is approximately 1.2%. Data collected to d(min) = 2.5 A from a crystal of form I, which has a diffraction limit of d(min) = 1.5 A and a solvent content of approximately 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, approximately 72%, and a much lower diffraction limit than form I and at 1.77 A wavelength yielded data only to d(min) = 2.7 A. 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 A, an excellent quality interpretable electron-density map. This was then improved by phase extension to the d(min) = 2.35 A 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.
authors.affiliation: Macromolecular Crystallography, European Synchrotron Radiation Facility, BP 220, F-38043 Grenoble CEDEX, France.
authors.forename: Elena
authors.initials: E
authors.lastname: Micossi
authors.forename: William N
authors.initials: WN
authors.lastname: Hunter
authors.forename: Gordon A
authors.initials: GA
authors.lastname: Leonard
mesh_terms: D000818:Animals; D016831:Crithidia fasciculata / Q000201:enzymology; D018360:Crystallography, X-Ray; D011487:Protein Conformation; D012542:Scattering, Radiation; D013879:Thioredoxins / Q000737:chemistry*
chemical_list: C114110:tryparedoxin; D013879:Thioredoxins