Biochemical analyses of multiple endoxylanases from the rumen bacterium Ruminococcus albus 8 and their synergistic activities with accessory hemicellulose-degrading enzymes

Ruminococcus albus 8 is a ruminal bacterium capable of metabolizing hemicellulose and cellulose, the major components of the plant cell wall. The enzymes that allow this bacterium to capture energy from the two polysaccharides, therefore, have potential application in plant cell wall depolymerization, a process critical to biofuel production. For this purpose, a partial genome sequence of R. albus 8 was generated. The genomic data depicted a bacterium endowed with multiple forms of plant cell wall-degrading enzymes. The endoxylanases of R. albus 8 exhibited diverse modular architectures, including incorporation of a catalytic module, a carbohydrate binding module, and a carbohydrate esterase module in a single polypeptide. The accessory enzymes of xylan degradation were a β-xylosidase, an α-l-arabinofuranosidase, and an α-glucuronidase. We hypothesized that due to the chemical complexity of the hemicellulose encountered in the rumen, the bacterium uses multiple endoxylanases, with subtle differences in substrate specificities, to attack the substrate, while the accessory enzymes hydrolyze the products to simple sugars for metabolism. To test this hypothesis, the genes encoding the predicted endoxylanases were expressed, and the proteins were biochemically characterized either alone or in combination with accessory enzymes. The different endoxylanase families exhibited different patterns of product release, with the family 11 endoxylanases releasing more products in synergy with the accessory enzymes from the more complex substrates. Aside from the insights into hemicellulose degradation by R. albus 8, this report should enhance our knowledge on designing effective enzyme cocktails for release of fermentable sugars in the biofuel industry.     

The endocannabinoid system controls key epileptogenic circuits in the hippocampus

Balanced control of neuronal activity is central in maintaining function and viability of neuronal circuits. The endocannabinoid system tightly controls neuronal excitability. Here, we show that endocannabinoids directly target hippocampal glutamatergic neurons to provide protection against acute epileptiform seizures in mice. Functional CB1 cannabinoid receptors are present on glutamatergic terminals of the hippocampal formation, colocalizing with vesicular glutamate transporter 1 (VGluT1). Conditional deletion of the CB1 gene either in cortical glutamatergic neurons or in forebrain GABAergic neurons, as well as virally induced deletion of the CB1 gene in the hippocampus, demonstrate that the presence of CB1 receptors in glutamatergic hippocampal neurons is both necessary and sufficient to provide substantial endogenous protection against kainic acid (KA)-induced seizures. The direct endocannabinoid-mediated control of hippocampal glutamatergic neurotransmission may constitute a promising therapeutic target for the treatment of disorders associated with excessive excitatory neuronal activity.     

Structural characterization of the RNase E S1 domain and identification of its oligonucleotide-binding and dimerization interfaces

S1 domains occur in four of the major enzymes of mRNA decay in Escherichia coli: RNase E, PNPase, RNase II, and RNase G. Here, we report the structure of the S1 domain of RNase E, determined by both X-ray crystallography and NMR spectroscopy. The RNase E S1 domain adopts an OB-fold, very similar to that found with PNPase and the major cold shock proteins, in which flexible loops are appended to a well-ordered five-stranded beta-barrel core. Within the crystal lattice, the protein forms a dimer stabilized primarily by intermolecular hydrophobic packing. Consistent with this observation, light-scattering, chemical crosslinking, and NMR spectroscopic measurements confirm that the isolated RNase E S1 domain undergoes a specific monomer-dimer equilibrium in solution with a K(D) value in the millimolar range. The substitution of glycine 66 with serine dramatically destabilizes the folded structure of this domain, thereby providing an explanation for the temperature-sensitive phenotype associated with this mutation in full-length RNase E. Based on amide chemical shift perturbation mapping, the binding surface for a single-stranded DNA dodecamer (K(D)=160(+/-40)microM) was identified as a groove of positive electrostatic potential containing several exposed aromatic side-chains. This surface, which corresponds to the conserved ligand-binding cleft found in numerous OB-fold proteins, lies distal to the dimerization interface, such that two independent oligonucleotide-binding sites can exist in the dimeric form of the RNase E S1 domain. Based on these data, we propose that the S1 domain serves a dual role of dimerization to aid in the formation of the tetrameric quaternary structure of RNase E as described by Callaghan et al. in 2003 and of substrate binding to facilitate RNA hydrolysis by the adjacent catalytic domains within this multimeric enzyme.     

Pectin: cell biology and prospects for functional analysis

Pectin is a major component of primary cell walls of all land plants and encompasses a range of galacturonic acid-rich polysaccharides. Three major pectic polysaccharides (homogalacturonan, rhamnogalacturonan-I and rhamnogalacturonan-II) are thought to occur in all primary cell walls. This review surveys what is known about the structure and function of these pectin domains. The high degree of structural complexity and heterogeneity of the pectic matrix is produced both during biosynthesis in the endomembrane system and as a result of the action of an array of wall-based pectin-modifying enzymes. Recent developments in analytical techniques and in the generation of anti-pectin probes have begun to place the structural complexity of pectin in cell biological and developmental contexts. The in muro de-methyl-esterification of homogalacturonan by pectin methyl esterases is emerging as a key process for the local modulation of matrix properties. Rhamnogalacturonan-I comprises a highly diverse population of spatially and developmentally regulated polymers, whereas rhamnogalacturonan-II appears to be a highly conserved and stable pectic domain. Current knowledge of biosynthetic enzymes, plant and microbial pectinases and the interactions of pectin with other cell wall components and the impact of molecular genetic approaches are reviewed in terms of the functional analysis of pectic polysaccharides in plant growth and development.     

Central vasopressin in the modulation of catecholamine release in conscious rats

Neurons containing arginine vasopressin (AVP) have been shown to project from the paraventricular nucleus of the hypothalamus to the nucleus tractus solitarius (NTS) in the medulla. We investigated whether AVP acts in brain stem regions to influence sympathoadrenal outflow. Cannulae were implanted into the fourth ventricle of rats 7 days prior to the experiment. The effects of intracerebroventricular (icv) injections of AVP, the vehicle, and AVP antagonist, d(CH2)5Tyr(Me)AVP, on mean arterial pressure (MAP) and plasma noradrenaline (NA) and adrenaline (A) levels were determined in conscious unrestrained rats. Injections of AVP (icv, 23 and 73 ng/kg) but not the vehicle increased MAP and plasma NA and A levels. In contrast, iv injection of AVP increased MAP but decreased plasma concentrations of A and NA. The pressor response to icv injection of AVP was abolished by prior icv injection of AVP antagonist. Injection of AVP antagonist (icv, 0.5 and 1.5 microgram/kg) had no effect on MAP or plasma NA or A levels. These results show that centrally injected AVP activates sympathoadrenal outflow, possibly via an inhibition of baroreceptor reflexes. Since centrally administered AVP antagonist did not influence MAP or plasma NA or A levels, it appears that endogenously released AVP does not have a tonic influence on central cardiovascular reflex system in conscious, unrestrained rats.     

Heat treatment of bovine alpha-lactalbumin results in partially folded, disulfide bond shuffled states with enhanced surface activity

Prolonged heating of holo bovine alpha-lactalbumin (BLA) at 80 degrees C in pH 7 phosphate buffer in the absence of a thiol initiator improves the surface activity of the protein at the air:water interface, as determined by surface tension measurements. Samples after 30, 60, and 120 min of heating were analyzed on cooling to room temperature. Size-exclusion chromatography shows sample heterogeneity that increases with the length of heating. After 120 min of heating monomeric, dimeric, and oligomeric forms of BLA are present, with aggregates formed from disulfide bond linked hydrolyzed protein fragments. NMR characterization at pH 7 in the presence of Ca2+ of the monomer species isolated from the sample heated for 120 min showed that it consisted of a mixture of refolded native protein and partially folded protein and that the partially folded protein species had spectral characteristics similar to those of the pH 2 molten globule state of the protein. Circular dichroism spectroscopy showed that the non-native species had approximately 40% of the alpha-helical content of the native state, but lacked persistent tertiary interactions. Proteomic analysis using thermolysin digestion of three predominant non-native monomeric forms isolated by high-pressure liquid chromatography indicated the presence of disulfide shuffled isomers, containing the non-native 61-73 disulfide bond. These partially folded, disulfide shuffled species are largely responsible for the pronounced improvement in surface activity of the protein on heating.