title: Against the odds? De novo structure determination of a pilin with two cysteine residues by sulfur SAD.
abstract: Exploiting the anomalous signal of the intrinsic S atoms to phase a protein structure is advantageous, as ideally only a single well diffracting native crystal is required. However, sulfur is a weak anomalous scatterer at the typical wavelengths used for X-ray diffraction experiments, and therefore sulfur SAD data sets need to be recorded with a high multiplicity. In this study, the structure of a small pilin protein was determined by sulfur SAD despite several obstacles such as a low anomalous signal (a theoretical Bijvoet ratio of 0.9% at a wavelength of 1.8 Å), radiation damage-induced reduction of the cysteines and a multiplicity of only 5.5. The anomalous signal was improved by merging three data sets from different volumes of a single crystal, yielding a multiplicity of 17.5, and a sodium ion was added to the substructure of anomalous scatterers. In general, all data sets were balanced around the threshold values for a successful phasing strategy. In addition, a collection of statistics on structures from the PDB that were solved by sulfur SAD are presented and compared with the data. Looking at the quality indicator R(anom)/R(p.i.m.), an inconsistency in the documentation of the anomalous R factor is noted and reported.
authors.affiliation: Department of Molecular Biology and Genetics, Aarhus University, Gustav Wieds Vej 10C, DK-8000 Aarhus C, Denmark.
authors.forename: Manuela
authors.initials: M
authors.lastname: Gorgel
authors.affiliation: Department of Molecular Biology and Genetics, Aarhus University, Gustav Wieds Vej 10C, DK-8000 Aarhus C, Denmark.
authors.forename: Andreas
authors.initials: A
authors.lastname: Bøggild
authors.affiliation: Department of Molecular Biology and Genetics, Aarhus University, Gustav Wieds Vej 10C, DK-8000 Aarhus C, Denmark.
authors.forename: Jakob Jensen
authors.initials: JJ
authors.lastname: Ulstrup
authors.affiliation: Macromolecular Crystallography (HZB-MX), Helmholtz Zentrum Berlin für Materialien und Energie, Albert-Einstein-Strasse 15, D-12489 Berlin, Germany.
authors.forename: Manfred S
authors.initials: MS
authors.lastname: Weiss
authors.affiliation: Macromolecular Crystallography (HZB-MX), Helmholtz Zentrum Berlin für Materialien und Energie, Albert-Einstein-Strasse 15, D-12489 Berlin, Germany.
authors.forename: Uwe
authors.initials: U
authors.lastname: Müller
authors.affiliation: Department of Molecular Biology and Genetics, Aarhus University, Gustav Wieds Vej 10C, DK-8000 Aarhus C, Denmark.
authors.forename: Poul
authors.initials: P
authors.lastname: Nissen
authors.affiliation: Department of Molecular Biology and Genetics, Aarhus University, Gustav Wieds Vej 10C, DK-8000 Aarhus C, Denmark.
authors.forename: Thomas
authors.initials: T
authors.lastname: Boesen
mesh_terms: D003460:Crystallization; D018360:Crystallography, X-Ray; D003545:Cysteine / Q000737:chemistry*; D030562:Databases, Protein; D036781:Fimbriae Proteins / Q000737:chemistry*; D008958:Models, Molecular; D011487:Protein Conformation; D013455:Sulfur / Q000737:chemistry*; D014961:X-Ray Diffraction / Q000379:methods*
chemical_list: D036781:Fimbriae Proteins; D013455:Sulfur; D003545:Cysteine
keywords: S-SAD; de novo structure determination; pilin