High-resolution structure of the nitrile reductase QueF combined with molecular simulations provide insight into enzyme mechanism.

Here, we report the 1.53-Å crystal structure of the enzyme 7-cyano-7-deazaguanine reductase (QueF) from Vibrio cholerae, which is responsible for the complete reduction of a nitrile (CN) bond to a primary amine (H(2)C-NH(2)). At present, this is the only example of a biological pathway that includes reduction of a nitrile bond, establishing QueF as particularly noteworthy. The structure of the QueF monomer resembles two connected ferrodoxin-like domains that assemble into dimers. Ligands identified in the crystal structure suggest the likely binding conformation of the native substrates NADPH and 7-cyano-7-deazaguanine. We also report on a series of numerical simulations that have shed light on the mechanism by which this enzyme affects the transfer of four protons (and electrons) to the 7-cyano-7-deazaguanine substrate. In particular, the simulations suggest that the initial step of the catalytic process is the formation of a covalent adduct with the residue Cys194, in agreement with previous studies. The crystal structure also suggests that two conserved residues (His233 and Asp102) play an important role in the delivery of a fourth proton to the substrate.

The mannitol operon repressor MtlR belongs to a new class of transcription regulators in bacteria.

Many bacteria express phosphoenolpyruvate-dependent phosphotransferase systems (PTS). The mannitol-specific PTS catalyze the uptake and phosphorylation of d-mannitol. The uptake system comprises several genes encoded in the single operon. The expression of the mannitol operon is regulated by a proposed transcriptional factor, mannitol operon repressor (MtlR) that was first studied in Escherichia coli. Here we report the first crystal structures of MtlR from Vibrio parahemeolyticus (Vp-MtlR) and its homolog YggD protein from Shigella flexneri (Sf-YggD). MtlR and YggD belong to the same protein family (Pfam05068). Although Vp-MtlR and Sf-YggD share low sequence identity (22%), their overall structures are very similar, representing a novel all alpha-helical fold, and indicate similar function. However, their lack of any known DNA-binding structural motifs and their unfavorable electrostatic properties imply that MtlR/YggD are unlikely to bind a specific DNA operator directly as proposed earlier. This structural observation is further corroborated by in vitro DNA-binding studies of E. coli MtlR (Ec-MtlR), which detected no interaction of Ec-MtlR with the well characterized mannitol operator/promoter region. Therefore, MtlR/YggD belongs to a new class of transcription factors in bacteria that may regulate gene expression indirectly as a part of a larger transcriptional complex.

Structure and electrostatic property of cytoplasmic domain of ZntB transporter.

ZntB is the distant homolog of CorA Mg(2+) transporter within the metal ion transporter superfamily. It was early reported that the ZntB from Salmonella typhimurium facilitated efflux of Zn(2+) and Cd(2+), but not Mg(2+). Here, we report the 1.90 A crystal structure of the intracellular domain of ZntB from Vibrio parahemolyticus. The domain forms a funnel-shaped homopentamer that is similar to the full-length CorA from Thermatoga maritima, but differs from two previously reported dimeric structures of truncated CorA intracellular domains. However, no Zn(2+) or Cd(2+) binding sites were identified in the high-resolution structure. Instead, 25 well-defined Cl(-) ions were observed and some of these binding sites are highly conserved within the ZntB family. Continuum electrostatics calculations suggest that the central pore of the funnel is highly attractive for cations, especially divalents. The presence of the bound Cl(-) ions increases the stability of cations along the pore suggesting they could be important in enhancing cation transport.

Crystal structures of the effector-binding domain of repressor Central glycolytic gene Regulator from Bacillus subtilis reveal ligand-induced structural changes upon binding of several glycolytic intermediates.

Expression of genes in the gapA operon encoding five enzymes for triose phosphate interconversion in Bacillus subtilis is negatively regulated by the Central glycolytic genes Regulator (CggR). CggR belongs to the large SorC/DeoR family of prokaryotic transcriptional regulators, characterized by an N-terminal DNA-binding domain and a large C-terminal effector-binding domain. When no glucose is present in growth media, CggR binds to its target DNA sequence and blocks the transcription of genes in the gapA operon. In the presence of glucose, binding of the known effector molecule fructose-1,6-bisphosphate abolishes this interaction. We have identified dihydroxyacetone phosphate, glucose-6-phosphate and fructose-6-phosphate as additional CggR ligands that can bind to the effector-binding site. Crystal structures of C-CggR, the C-terminal effector-binding domain of CggR, both unliganded as well as in complex with the four ligands at resolutions between 1.65 and 1.80 A reveal unique ligand-specific structural changes in the binding site that affect the dimer interface. Binding affinities of these ligands were determined by isothermal titration calorimetry. Chemical cross-linking shows that CggR oligomerization is mediated through its effector-binding domain, and that binding of the different ligands differentially affects the distribution of oligomers. Electrophoretic mobility shift assays (EMSAs) confirmed a destabilizing effect of fructose-1,6-bisphosphate on the CggR/DNA complex, and also showed similar effects for dihydroxyacetone phosphate. Our results suggest that CggR stability and function may be modulated by various effectors in a complex fashion.

Crystal structure and putative function of small Toprim domain-containing protein from Bacillus stearothermophilus.

The crystal structure of the Midwest Center for Structural Genomics target APC35832, a 14.7-kDa cytosolic protein from Bacillus stearothermophilus, has been determined at 1.3 A resolution by the single anomalous diffraction method from a mercury soaked crystal. The APC35832 protein is a representative of large group of bacterial and archeal proteins entirely consisting of the Toprim (topoisomerase-primase) domain. This domain is found in the catalytic centers of many enzymes catalyzing phosphodiester bond formation or cleavage, but the function of small Toprim domain proteins remains unknown. Consistent with the sequence analysis, the APC35832 structure shows a conserved Toprim fold, with a central 4-stranded parallel beta-sheet surrounded by four alpha-helixes. Comparison of the APC35832 structure with its closest structural homolog, the catalytic core of bacteriophage T7 primase, revealed structural conservation of a metal binding site and isothermal titration calorimetry indicates that APC35832 binds Mg2+ with a sub-millimolar dissociation constant (K(d)). The APC35832-Mg2+ complex structure was determined at 1.65 A and reveals the role of conserved acidic residues in Mg2+ ion coordination. The structural similarities to other Toprim domain containing proteins and potential function and substrates of APC35832 are discussed in this article.

Primary structure-based function characterization of BRCT domain replicates in BRCA1.

BRCA1 is a large protein that exhibits a multiplicity of functions in its apparent role in DNA repair. Certain mutations of BRCA1 are known to have exceptionally high penetrance with respect to familial breast and ovarian cancers. The structures of the N-terminus and C-terminus of the protein have been determined. The C-terminus unit consists of two alpha-beta-alpha domains designated BRCT. We predicated two homologous BRCT regions in the BRCA1 internal region, and subsequently produced and purified these protein domains. Both recombinant domains show significant self-association capabilities as well as a preferential tendency to interact with each other. These results suggest a possible regulatory mechanism for BRCA1 function. We have demonstrated p53-binding activity by an additional region, and confirmed previous results showing that two regions of BRCA1 protein bind p53 in vitro. Based on sequence analysis, we predict five p53-binding sites. Our comparison of binding by wild-type and mutant domains indicates the sequence specificity of BRCA1-p53 interaction.