Structural insights into the molecular mechanism of Escherichia coli SdiA, a quorum-sensing receptor.

Escherichia coli SdiA is a quorum-sensing (QS) receptor that responds to autoinducers produced by other bacterial species to control cell division and virulence. Crystal structures reveal that E. coli SdiA, which is composed of an N-terminal ligand-binding domain and a C-terminal DNA-binding domain (DBD), forms a symmetrical dimer. Although each domain shows structural similarity to other QS receptors, SdiA differs from them in the relative orientation of the two domains, suggesting that its ligand-binding and DNA-binding functions are independent. Consistently, in DNA gel-shift assays the binding affinity of SdiA for the ftsQP2 promoter appeared to be insensitive to the presence of autoinducers. These results suggest that autoinducers increase the functionality of SdiA by enhancing the protein stability rather than by directly affecting the DNA-binding affinity. Structural analyses of the ligand-binding pocket showed that SdiA cannot accommodate ligands with long acyl chains, which was corroborated by isothermal titration calorimetry and thermal stability analyses. The formation of an intersubunit disulfide bond that might be relevant to modulation of the DNA-binding activity was predicted from the proximal position of two Cys residues in the DBDs of dimeric SdiA. It was confirmed that the binding affinity of SdiA for the uvrY promoter was reduced under oxidizing conditions, which suggested the possibility of regulation of SdiA by multiple independent signals such as quorum-sensing inducers and the oxidation state of the cell.

Structure of inorganic pyrophosphatase from Helicobacter pylori.

Inorganic pyrophosphatase (PPase) is a ubiquitous cytosolic enzyme which catalyzes the hydrolysis of inorganic pyrophosphate (PPi) to orthophosphate (Pi). The crystal structure of inorganic pyrophosphatase from Helicobacter pylori (H-PPase) has been solved by MAD and refined to an R factor of 20.6% at 2.6 A resolution. The crystallographic asymmetric unit contains a homohexameric H-PPase arranged as a dimer of trimers. While most of the structural elements of PPases are highly conserved in H-PPase, some unique structural features are localized in the flexible loops near the active site, suggesting that the structural flexibility of these loops is required for the catalytic efficiency of PPase.

A poxvirus protein forms a complex with left-handed Z-DNA: crystal structure of a Yatapoxvirus Zalpha bound to DNA.

A conserved feature of poxviruses is a protein, well characterized as E3L in vaccinia virus, that confers IFN resistance on the virus. This protein comprises two domains, an N-terminal Z-DNA-binding protein domain (Zalpha) and a C-terminal double-stranded RNA-binding domain. Both are required for pathogenicity of vaccinia virus in mice infected by intracranial injection. Here, we describe the crystal structure of the Zalpha domain from the E3L-like protein of Yaba-like disease virus, a Yatapoxvirus, in a complex with Z-DNA, solved at a 2.0-A resolution. The DNA contacting surface of Yaba-like disease virus Zalpha(E3L) closely resembles that of other structurally defined members of the Zalpha family, although some variability exists in the beta-hairpin region. In contrast to the Z-DNA-contacting surface, the nonbinding surface of members of the Zalpha family are unrelated; this surface may effect protein-specific interactions. The presence of the conserved and tailored Z-DNA-binding surface, which interacts specifically with the zigzag backbone and syn base diagnostic of the Z-form, reinforces the importance to poxvirus infection of the ability of this protein to recognize the Z-conformation.

Crystallization and preliminary X-ray crystallographic study of UDP-glucose pyrophosphorylase (UGPase) from Helicobacter pylori.

UDP-glucose pyrophosphorylase (UGPase) catalyzes the synthesis of UDP-glucose, an essential metabolite in all living organisms. An X-ray crystallographic study of UGPase from Helicobacter pylori has been performed in order to elucidate its role in the regulation of this important metabolic pathway. UGPase was crystallized from 0.1 M sodium acetate trihydrate pH 4.6, 2.0 M ammonium sulfate and 0.1 M guanidine-HCl. According to diffraction data collected at a resolution of 2.9 A using a synchrotron-radiation source, the crystal belongs to the orthorhombic space group P2(1)2(1)2(1), with unit-cell parameters a = 91.47, b = 98.61, c = 245.70 A, alpha = beta = gamma = 90.0 degrees.

Purification, crystallization and preliminary X-ray studies of ClpX from Helicobacter pylori.

ClpX, a member of the HSP (heat-shock protein) 100 family, functions as a molecular chaperone and is a regulatory subunit of the ClpXP protease. To understand the chaperone and regulatory mechanisms of ClpX, Helicobacter pylori ClpX has been overexpressed in Escherichia coli and crystallized at 295 K using (NH(4))(2)HPO(4) as precipitant. X-ray diffraction data have been collected to 2.6 A resolution using a synchrotron-radiation source. The crystals belong to the hexagonal space group P6(5) or P6(1), with unit-cell parameters a = b = 78.52 (04), c = 131.51 (09) A, alpha = beta = 90, gamma = 120 degrees. The crystallographic asymmetric unit contains one molecule of ClpX, with a corresponding V(M) of 2.78 A(3) Da(-1) and a solvent content of 55.8%.