Biochemical complementation of chalcone synthase mutants defines a role for flavonols in functional pollen

Chalcone synthase catalyzes the initial step of that branch of the phenylpropanoid pathway that leads to flavonoids. A lack of chalcone synthase activity has a pleiotropic effect in maize and petunia mutants: pollen fertility as well as flavonoid synthesis is disrupted. Both maize and petunia mutants are self-sterile due to a failure to produce a functional pollen tube. The finding that the mutant pollen is partially functional on wild-type stigmas led to the isolation and identification of kaempferol as a pollen germination-inducing constituent in wild-type petunia stigma extracts. We show that adding micromolar quantities of kaempferol to the germination medium or to the stigma at pollination is sufficient to restore normal pollen germination and tube growth in vitro and full seed set in vivo. Further we show that the rescue ability resides in particular structural features of a single class of compounds, the flavonol aglycones. This finding identifies another constituent of plant reproduction and suggests that addition or removal of the flavonol signal during pollen germination and tube growth provides a feasible way to control plant fertility.

Imaging membrane protein helical wheels

Resonance patterns have been observed in 2D solid-state NMR spectra of the transmembrane segment of M2 protein from Influenza A virus in oriented samples reflecting the helical wheel of this alpha-helix. The center of this pattern uniquely defines the helical tilt with respect to the bilayer normal without a need for resonance assignments. The distribution of resonances from amino acid specific labels around the "PISA wheel" defines the rotational orientation of the helix and yields preliminary site-specific assignments. With assignments high-resolution structural detail, such as differences in tilt and rotational orientation along the helical axis leading to an assessment of helical coiling, can be obtained.

Structure of Tetrahymena GCN5 bound to coenzyme A and a histone H3 peptide

Gene activation is a highly regulated process that requires the coordinated action of proteins to relieve chromatin repression and to promote transcriptional activation. Nuclear histone acetyltransferase (HAT) enzymes provide a mechanistic link between chromatin destabilization and gene activation by acetylating the epsilon-amino group of specific lysine residues within the aminoterminal tails of core histones to facilitate access to DNA by transcriptional activators. Here we report the high-resolution crystal structure of the HAT domain of Tetrahymena GCN5 (tGCN5) bound with both its physiologically relevant ligands, coenzyme A (CoA) and a histone H3 peptide, and the structures of nascent tGCN5 and a tGCN5/acetyl-CoA complex. Our structural data reveal histone-binding specificity for a random-coil structure containing a G-K-X-P recognition sequence, and show that CoA is essential for reorienting the enzyme for histone binding. Catalysis appears to involve water-mediated proton extraction from the substrate lysine by a glutamic acid general base and a backbone amide that stabilizes the transition-state reaction intermediate. Comparison with related N-acetyltransferases indicates a conserved structural framework for CoA binding and catalysis, and structural variability in regions associated with substrate-specific binding.

Electrostatic stress in catalysis: structure and mechanism of the enzyme orotidine monophosphate decarboxylase

Orotidine 5'-monophosphate decarboxylase catalyzes the conversion of orotidine 5'-monophosphate to uridine 5'-monophosphate, the last step in biosynthesis of pyrimidine nucleotides. As part of a Structural Genomics Initiative, the crystal structures of the ligand-free and the6-azauridine 5'-monophosphate-complexed forms have been determined at 1.8 and 1.5 A, respectively. The protein assumes a TIM-barrel fold with one side of the barrel closed off and the other side binding the inhibitor. A unique array of alternating charges (Lys-Asp-Lys-Asp) in the active site prompted us to apply quantum mechanical and molecular dynamics calculations to analyze the relative contributions of ground state destabilization and transition state stabilization to catalysis. The remarkable catalytic power of orotidine 5'-monophosphate decarboxylase is almost exclusively achieved via destabilization of the reactive part of the substrate, which is compensated for by strong binding of the phosphate and ribose groups. The computational results are consistent with a catalytic mechanism that is characterized by Jencks's Circe effect.

Efficacy of nisin in treatment of clinical mastitis in lactating dairy cows

Nisin is an antimicrobial polypeptide produced by Lactococcus lactis and is believed nontoxic to humans. The objective of this study was to evaluate a nisin-based formulation for the treatment of bovine clinical mastitis in lactating dairy cattle. A total of 92 cows with 107 clinically mastitic quarters were randomly assigned to nisin- (48 cows with 51 quarters) and gentamicin (GM)-treated (44 cows with 56 quarters) groups. In the nisin-treated group, cows received an intramammary infusion of nisin at a dose of 2,500,000 IU; in the GM-treated group, intramammary infusion of GM was administered at a dose of 0.8 g. Results indicated that nisin offered a clinical cure rate similar to GM (90.2 vs. 91.1%) and no difference in bacteriological cure rate than GM-treated group (60.8 vs. 44.6%, respectively). Proportion of the quarters with milk somatic cell counts <500,000 cells/mL was not different in the nisin-treated group (50.0 and 47.8%) compared with the GM-treated group (33.3 and 37.3%) 1 and 2 wk after treatment. Of 17 Staphylococcus aureus isolates, 82.5% were resistant to penicillin, and 35.3% to GM, but none of them to nisin. Nisin therapy eliminated 54.5% (6 of 11) of S. aureus IMI, whereas GM eliminated 33.3% (2 of 6). Nisin in milk (4.5 +/- 0.8 IU/mL) was detected only at 12 h following intramammary infusion, which was much lower than the upper limit (500 mg/mL) allowed as preservative in milk by the China authority. Because of its efficacy in the treatment of bovine clinical mastitis, especially resistant Staph. aureus-caused IMI, as well as its safety in humans, nisin deserves further study to clarify its effects on mastitis caused by different mastitis pathogens on a larger scale.

Structures of SAP-1 bound to DNA targets from the E74 and c-fos promoters: insights into DNA sequence discrimination by Ets proteins

SAP-1 is a member of the Ets transcription factors and cooperates with SRF protein to activate transcription of the c-fos protooncogene. The crystal structures of the conserved ETS domain of SAP-1 bound to DNA sequences from the E74 and c-fos promoters reveal that a set of conserved residues contact a GGA core DNA sequence. Discrimination for sequences outside this core is mediated by DNA contacts from conserved and nonconserved protein residues and sequence-dependent DNA structural properties characteristic of A-form DNA structure. Comparison with the related PU.1/DNA and GABPalpha/beta/DNA complexes provides general insights into DNA discrimination between Ets proteins. Modeling studies of a SAP-1/SRF/DNA complex suggest that SRF may modulate SAP-1 binding to DNA by interacting with its ETS domain.

Structure of the elk-1-DNA complex reveals how DNA-distal residues affect ETS domain recognition of DNA

SAP-1 and Elk-1 are members of a large group of eukaryotic transcription factors that contain a conserved ETS DNA binding domain and that cooperate with the serum response factor (SRF) to activate transcription of the c-fos protooncogene. Despite the high degree of sequence similarity, which includes an identical amino acid sequence for the DNA recognition helix within the ETS domain of these proteins, they exhibit different DNA binding properties. Here we report the 2.1 ¿ crystal structure of the ETS domain of Elk-1 bound to a high affinity E74 DNA (E74DNA) site and compare it to a SAP-1-E74DNA complex. This comparison reveals that the differential DNA binding properties of these proteins are mediated by non-conserved residues distal to the DNA binding surface that function to orient conserved residues in the DNA recognition helix for protein-specific DNA contacts. As a result, nearly one-third of the interactions between the protein recognition helix and the DNA are different between the SAP-1 and Elk-1 DNA complexes. Taken together, these studies reveal a novel mechanism for the modulation of DNA binding specificity within a conserved DNA binding domain, and have implications for how highly homologous ETS proteins exhibit differential DNA-binding properties.

New insight on the origin of the unusual acidity of Meldrum's acid from ab initio and combined QM/MM simulation study

Ab initio molecular orbital and combined QM/MM Monte Carlo simulations have been carried out to investigate the origin of the unusually high acidity of Meldrum's acid. Traditionally, the high acidity of Meldrum's acid relative to that of methyl malonate has been attributed to an additive effect due to the presence of two E esters in the dilactone system. However, the present study reveals that there is significant nonadditive effect that also makes major contributions. This results from preferential stabilization of the enolate anion over that of Meldrum's acid due to anomeric stereoelectronic interactions. To investigate solvent effects on the acidity in aqueous solution, the relative acidities of Z and E conformers of methyl acetate have been determined in combined ab initio QM/MM simulations. There is significant solvent effect on the conformational equilibria for both the neutral ester and its enolate anion in water, leading to stabilization of the E stereoisomer. However, the computed solvent effect of 4.4 kcal/mol in favor of the E isomer of methyl acetate is largely offset by the favorable solvation of 3.4 kcal/mol for the E conformer of the enolate anion. This leads to an enhanced acidity of 3.4 kcal/mol for the (E)-methyl acetate in water over the Z conformer. In Meldrum's acid, it is the preferential stabilization of the enolate anion due to anomeric effects coupled with the intrinsically higher acidity of the E conformation of ester that is responsible for its high acidity.

Nuclear localization of G protein beta 5 and regulator of G protein signaling 7 in neurons and brain

The role that Gbeta(5) regulator of G protein signaling (RGS) complexes play in signal transduction in brain remains unknown. The subcellular localization of Gbeta(5) and RGS7 was examined in rat PC12 pheochromocytoma cells and mouse brain. Both nuclear and cytosolic localization of Gbeta(5) and RGS7 was evident in PC12 cells by immunocytochemical staining. Subcellular fractionation of PC12 cells demonstrated Gbeta(5) immunoreactivity in the membrane, cytosolic, and nuclear fractions. Analysis by limited proteolysis confirmed the identity of Gbeta(5) in the nuclear fraction. Subcellular fractionation of mouse brain demonstrated Gbeta(5) and RGS7 but not Ggamma(2/3) immunoreactivity in the nuclear fraction. RGS7 and Gbeta(5) were tightly complexed in the brain nuclear extract as evidenced by their coimmunoprecipitation with anti-RGS7 antibodies. Chimeric protein constructs containing green fluorescent protein fused to wild-type Gbeta(5) but not green fluorescent fusion proteins with Gbeta(1) or a mutant Gbeta(5) impaired in its ability to bind to RGS7 demonstrated nuclear localization in transfected PC12 cells. These findings suggest that Gbeta(5) undergoes nuclear translocation in neurons via an RGS-dependent mechanism. The novel intracellular distribution of Gbeta(5).RGS protein complexes suggests a potential role in neurons communicating between classical heterotrimeric G protein subunits and/or their effectors at the plasma membrane and the cell nucleus.

Chalcone Synthase and Flavonol Accumulation in Stigmas and Anthers of Petunia hybrida

Flavonol aglycones are required for pollen germination in petunia (Petunia hybrida L.). Mutant plants lacking chalcone synthase (CHS), which catalyzes the first committed step in flavonoid synthesis, do not accumulate flavonols and are self-sterile. The mutant pollen can be induced to germinate by supplementing it with kaempferol, a flavonol aglycone, either at the time of pollination or by addition to an in vitro germination system. Biochemical complementation occurs naturally when the mutant, flavonol-deficient pollen is crossed to wild-type, flavonoid-producing stigmas. We found that successful pollination depends on stigma maturity, indicating that flavonol aglycone accumulation may be developmentally regulated. Quantitative immunoblotting, in vitro and in vivo pollen germination, and high-performance liquid chromatographic analyses of stigma and anther extracts were used to determine the relationship between CHS levels and flavonol aglycone accumulation in developing petunia flowers. Although substantial levels of CHS were measured, we detected no flavonol aglycones in wild-type stigma or anther extracts. Instead, the occurrence of a conjugated form (flavonol glycoside) suggests that a mechanism may operate to convert glycosides to the active aglycone form.

Crystal structure of a ternary SAP-1/SRF/c-fos SRE DNA complex

Combinatorial DNA binding by proteins for promoter-specific gene activation is a common mode of DNA regulation in eukaryotic organisms, and occurs at the promoter of the c-fos proto-oncogene. The c-fos promoter contains a serum response element (SRE) that mediates ternary complex formation with the Ets proteins SAP-1 or Elk-1 and the MADS-box protein, serum response factor (SRF). Here, we report the crystal structure of a ternary SAP-1/SRF/c-fos SRE DNA complex containing the minimal DNA-binding domains of each protein. The structure of the complex reveals that the SAP-1 monomer and SRF dimer are bound on opposite faces of the DNA, and that the DNA recognition helix of SAP-1 makes direct contact with the DNA recognition helix of one of the two SRF subunits. These interactions facilitate an 82 degrees DNA bend around SRF and a modulation of protein-DNA contacts by each protein when compared to each of the binary DNA complexes. A comparison with a recently determined complex containing SRF, an idealized DNA site, and a SAP-1 fragment containing a SRF-interacting B-box region, shows a similar overall architecture but also shows important differences. Specifically, the comparison suggests that the B-box region of the Ets protein does not significantly influence DNA recognition by either of the proteins, and that the sequence of the DNA target effects the way in which the two proteins cooperate for DNA recognition. These studies have implications for how DNA-bound SRF may modulate the DNA-binding properties of other Ets proteins such as Elk-1, and for how other Ets proteins may modulate the DNA-binding properties of other DNA-bound accessory factors to facilitate promoter-specific transcriptional responses.

A miniature CSTR cascade for continuous flow of reactions containing solids

Continuous handling of solids creates challenges for realizing continuous production of pharmaceuticals and fine chemicals.

Decreased epithelial and plasma miR-181b-5p expression associates with airway eosinophilic inflammation in asthma

BACKGROUND

Airway eosinophilic inflammation is a pivotal feature of asthma. Epithelial cells play critical roles in airway eosinophilia. We hypothesized that epithelial microRNAs (miRNAs) are involved in airway eosinophilia.

OBJECTIVE

This study investigated the associations between epithelial and plasma miR-181b-5p and airway eosinophilic inflammation, and the possible mechanism by which miR-181b-5p participates in eosinophilic inflammation.

METHODS

Epithelial miRNAs expression was profiled by miRNA array in eight subjects with asthma and four healthy controls. Epithelial miR-181b-5p expression was confirmed by quantitative PCR in the subjects for array experiment and another cohort including 21 subjects with asthma and 10 controls. Plasma miR-181b-5p was determined by quantitative PCR in 72 subjects with asthma and 35 controls. Correlation assays between epithelial and plasma miR-181b-5p expression and airway eosinophilia were performed. The target of miR-181b-5p, SPP1, was predicted by online algorithms and verified in BEAS-2B cells. The role of miR-181b-5p in epithelial proinflammatory cytokine expression was examined in an in vitro system.

RESULTS

Epithelial miR-181b-5p expression was decreased in subjects with asthma. Epithelial miR-181b-5p levels were inversely correlated with sputum and bronchial submucosal eosinophilia. Plasma miR-181b-5p was decreased and correlated with epithelial miR-181b-5p in subjects with asthma. There was a strong inverse correlation between plasma miR-181b-5p and airway eosinophilia in subjects with asthma. Plasma miR-181b-5p was increased after inhaled corticosteroids treatment. We verified that SPP1 is a target of miR-181b-5p. In human bronchial epithelial cells, miR-181b-5p regulated IL-13-induced IL-1β and CCL11 expression by targeting SPP1. Dexamethasone restored IL-13-induced miR-181b-5p down-regulation and suppressed IL-13-induced SPP1, IL-1β and CCL11 expression.

CONCLUSIONS AND CLINICAL RELEVANCE

Epithelial and plasma miR-181b-5p are potential biomarkers for airway eosinophilia in asthma. MiR-181b-5p may participate in eosinophilic airway inflammation by regulating proinflammatory cytokines expression via targeting SPP1.

Continuous N-Hydroxyphthalimide (NHPI)-Mediated Electrochemical Aerobic Oxidation of Benzylic C-H Bonds

Electroorganic chemistry has emerged as an environmentally benign tool for synthetic chemists to achieve efficient transformations that are challenging with traditional reagent-based methods. Continuous flow chemistry brings pharmaceutical industry numerous advantages, but implementing electroorganic synthesis in flow is challenging, especially for electroorganic reactions with coupled electrode reactions and slow chemical reactions. We present a continuous electrolysis system engineered for N-hydroxyphthalimide (NHPI) mediated electrochemical aerobic oxidation of benzylic C-H bonds. First, a cation-exchange membrane prevents the crossover of the NHPI anion from anolyte to catholyte avoiding reductive decomposition of NHPI at the cathode, and enables the usage of a cost-effective reticulated vitreous carbon (RVC) cathode instead of a platinum electrode. Second, running the electrochemical flow cell with recycle streams accommodates the inherently slow kinetics of the chemical reaction without phthalimide-N-oxyl (PINO) radical self-decomposition at the anode, and allows the usage of gaseous oxygen as co-oxidant.