High Resolution Structure of a Type Iv Pilin from the Metal Reducing Bacterium Shewanella Oneidensis

BACKGROUND: Type IV pili are widely expressed among Gram-negative bacteria, where they are involved in biofilm formation, serve in the transfer of DNA, motility and in the bacterial attachment to various surfaces. Type IV pili in Shewanella oneidensis are also supposed to play an important role in extracellular electron transfer by the attachment to sediments containing electron acceptors and potentially forming conductive nanowires. RESULTS: The potential nanowire type IV pilin Pil(Bac1) from S. oneidensis was characterized by a combination of complementary structural methods and the atomic structure was determined at a resolution of 1.67 Å by X-ray crystallography. Pil(Bac1) consists of one long N-terminal α-helix packed against four antiparallel β-strands, thus revealing the core fold of type IV pilins. In the crystal, Pil(Bac1) forms a parallel dimer with a sodium ion bound to one of the monomers. Interestingly, our Pil(Bac1) crystal structure reveals two unusual features compared to other type IVa pilins: an unusual position of the disulfide bridge and a straight α-helical section, which usually exhibits a pronounced kink. This straight helix leads to a distinct packing in a filament model of Pil(Bac1) based on an EM model of a Neisseria pilus. CONCLUSIONS: In this study we have described the first structure of a pilin from Shewanella oneidensis. The structure possesses features of the common type IV pilin core, but also exhibits significant variations in the α-helical part and the D-region. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/s12900-015-0031-7) contains supplementary material, which is available to authorized users

Against the odds? De novo structure determination of a pilin with two cysteine residues by sulfur SAD

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 A , 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 Ranom Rp.i.m., an inconsistency in the documentation of the anomalous R factor is noted and reporte