Transport protein particle (TRAPP) I is a multisubunit vesicle tethering factor composed of seven subunits involved in ER-to-Golgi trafficking. The functional mechanism of the complex and how the subunits interact to form a functional unit are unknown. Here, we have used a multidisciplinary approach that includes X-ray crystallography, electron microscopy, biochemistry, and yeast genetics to elucidate the architecture of TRAPP I. The complex is organized through lateral juxtaposition of the subunits into a flat and elongated particle. We have also localized the site of guanine nucleotide exchange activity to a highly conserved surface encompassing several subunits. We propose that TRAPP I attaches to Golgi membranes with its large flat surface containing many highly conserved residues and forms a platform for protein-protein interactions. This study provides the most comprehensive view of a multisubunit vesicle tethering complex to date, based on which a model for the function of this complex, involving Rab1-GTP and long, coiled-coil tethers, is presented.

The chain length distribution of complex polysaccharides present on the bacterial surface is determined by polysaccharide co-polymerases (PCPs) anchored in the inner membrane. We report crystal structures of the periplasmic domains of three PCPs that impart substantially different chain length distributions to surface polysaccharides. Despite very low sequence similarities, they have a common protomer structure with a long central alpha-helix extending 100 A into the periplasm. The protomers self-assemble into bell-shaped oligomers of variable sizes, with a large internal cavity. Electron microscopy shows that one of the full-length PCPs has a similar organization as that observed in the crystal for its periplasmic domain alone. Functional studies suggest that the top of the PCP oligomers is an important region for determining polysaccharide modal length. These structures provide a detailed view of components of the bacterial polysaccharide assembly machinery.

No abstract

Campylobacter jejuni is unusual among bacteria in possessing a eukaryotic-like system for N-linked protein glycosylation at Asn residues in sequons of the type Asp/Glu-Xaa-Asn-Xaa-Ser/Thr. However, little is known about the structural context of the glycosylated sequons, limiting the design of novel recombinant glycoproteins. To obtain more information on sequon structure, we have determined the crystal structure of the PEB3 (Cj0289c) dimer. PEB3 has the class II periplasmic-binding protein fold, with each monomer having two domains with a ligand-binding site containing citrate located between them, and overall resembles molybdate-and sulfate-binding proteins. The sequon around Asn90 is located within a surface-exposed loop joining two structural elements. The three key residues are well exposed on the surface; hence, they may be accessible to the PglB oligosaccharyltransferase in the folded state.

Received March 10, 2010; Revised Manuscript Received July 15, 2010 ABSTRACT: The crystal structure of the urease maturation protein UreE from Helicobacter pylori has been determined in its apo form at 2.1 Å resolution, bound to Cu 2þ at 2.7 Å resolution, and bound to Ni 2þ at 3.1 Å resolution. Apo UreE forms dimers, while the metal-bound enzymes are arranged as tetramers that consist of a dimer of dimers associated around the metal ion through coordination by His102 residues from each subunit of the tetramer. Comparison of independent subunits from different crystal forms indicates changes in the relative arrangement of the N-and C-terminal domains in response to metal binding. The improved ability of engineered versions of UreE containing hexahistidine sequences at either the N-terminal or C-terminal end to provide Ni 2þ for the final metal sink (urease) is eliminated in the H102A version. Therefore, the ability of the improved Ni 2þ -binding versions to deliver more nickel is likely an effect of an increased local concentration of metal ions that can rapidly replenish transferred ions bound to His102.