Glucosylated Forms Research Articles

Purification, Cloning, and Expression of a Pathogen Inducible UDP-glucose:Salicylic Acid Glucosyltransferase from Tobacco

Salicylic acid (SA) plays an important role in plant disease resistance. Inoculation of tobacco leaves with incompatible pathogens triggers the biosynthesis of SA which accumulates primarily as the SA 2-O-beta-D-glucoside (SAG) and glucosyl salicylate (GS). The tobacco UDP-glucose:salicylic acid glucosyltransferase (SA GTase) capable of forming both SAG and GS was purified, characterized, and partially sequenced. It has an apparent molecular mass of 48 kDa, a pH optimum of 7.0, and an isoelectric point at pH 4.4. UDP-glucose was the sole sugar donor for the enzyme. However, SA and several phenolics served as glucose acceptors. The apparent K(m) values for UDP-glucose and SA were 0.27 and 1-2 mM, respectively. Zn(2+) and UDP inhibited its activity. The corresponding cDNA clone which encoded a protein of 459 amino acids was isolated from an SA-induced tobacco cDNA library and overexpressed in Escherichia coli. The recombinant protein catalyzed the formation of SAG and GS, and exhibited a broad specificity to simple phenolics, similar to that of the purified enzyme. Northern blot analysis showed that the SA GTase mRNA was induced both by SA and incompatible pathogens. The rapid induction timing of the mRNA by SA indicates that it belongs to the early SA response genes.

Non-injured maize contains several 1,4-benzoxazin-3-one related compounds but only as glucoconjugates

DIMBOA (2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one) and its derivatives are known to participate in the resistance of the Poaceae to several pests. A more efficient analytical method for the determination of these compounds is reported, employing high performance liquid chromatography coupled with UV and atmospheric pressure chemical ionisation-tandem mass spectrometric detection. The results showed that in non-injured 5 or 15-day-old plants only the glucosylated forms of DIMBOA and related compounds were present: aglycones were only detected when enzymatic deglucosylation occurred during sample work-up. Maize plants contain seven glucosides related to DIMBOA belonging to three classes: hydroxamates, lactams and N-O-methylated hydroxamate derivatives. Rarely reported dimethoxylated compounds and 8-methoxy-N-O-methylated DIMBOA are now reported in the Poaceae, the latter for the first time. Their presence and relative concentrations in 5 and 15-day-old maize roots and aerial parts are discussed. The non-hydroxamic N-O-methylated DIMBOA is present in the plant in significant quantities which exceed all others in 15-day-old maize roots. After enzymatic degradation, deglucosylation products from hydroxamates and lactams are observed. The N-O-methylated deglucosylated compounds are not observed as they are chemically degraded, most probably to benzoxazolinone derivatives. The results reported here provide a new view on the DIMBOA-related compounds present in maize plants, and the implications of these findings are discussed. Copyright (C) 1999 John Whey & Sons, Ltd.

Structure of the N-linked oligosaccharides of MHC class I molecules from cells deficient in the antigenic peptide transporter. Implications for the site of peptide association.

Class I molecules are N-linked glycoproteins encoded by the MHC. They carry cytosolic protein-derived peptides to the cell surface, displaying them to enable immune surveillance of cellular processes. Peptides are delivered to class I molecules by the transporter associated with Ag processing (TAP). Peptide association is known to occur before exposure of class I molecules to the medial Golgi-processing enzyme alpha-mannosidase II, but there is limited information regarding the location or timing of peptide binding within the earlier regions of the endoplasmic reticulum (ER)-Golgi pathway. A reported association of newly synthesized class I molecules with the ER chaperonin calnexin raises the possibility of persistence of the monoglycosylated N-linked oligosaccharide (NLO) Glc1Man8GlcNAc2, known to be recognized by this lectin. To explore these matters, we determined the structure of the NLOs on the subset of newly synthesized class I molecules awaiting the loading of peptide. We pulse-labeled murine MHC H-2Db class I molecules in RMA/S cells, which lack one of the TAP subunits, causing the great majority of the molecules to be retained for prolonged periods in an early secretory compartment, awaiting peptide binding. MHC molecules pulse-labeled with [3H]glucosamine were isolated, the NLOs specifically released and structurally analyzed by a variety of techniques. Within the chosen window of biosynthetic time, most Db molecules from parental RMA cells carried mature NLOs of the biantennary complex-type, with one to two sialic acid residues. In RMA/S cells, such chains were in the minority, the majority consisting of the precursor forms Man8GlcNAc2 and Man9GlcNAc2. No glucosylated forms were detected, nor were the later processing intermediates Man5-7GlcNAc2 or GlcNAc1Man4-5GlcNAc2. Thus, most Db molecules in TAP-deficient cells are retained in an early compartment of the secretory pathway, before the point of first access to the Golgi alpha-mannosidase I, which trims alpha 1-2 linked mannose residues, but beyond the point where the alpha 1-3-linked glucose residue is finally removed by the ER glucosidase II. Thus, structural analysis of NLOs on class I molecules within a defined biosynthetic window has established a biochemical measure of the timing of peptide association.

Does phosphorylase kinase control glycogen biosynthesis in skeletal muscle?

Immunoblotting as well as enzyme assays demonstrate the presence of the self-glucosylating protein, glycogenin, in the protein-glycogen complex, in the sarcoplasmic reticulum and in phosphorylase kinase. In all three compartments glycogenin occurs in different, albeit, defined glucosylated forms, which upon deglucosylation are converted into a 42 kDa form. We suggest that phosphorylase kinase might have a dual function in glycogen biogenesis: firstly, control of glycogen degradation in the protein-glycogen complex via phosphorylation of glycogen phosphorylase b; secondly, regulation of glycogen biosynthesis on the sarcoplasmic reticular membranes via phosphorylation and thereby inhibition of glycogen synthase.

The O18 antigens (lipopolysaccharides) of Escherichia coli. Structural characterization of the O18A, O18A1, O18B and O18B1-specific polysaccharides.

The O-specific polysaccharide moieties (PS) of the O18A, O18A1, O18B, and O18B1 antigens (lipopolysaccharides, LPS) consist of L-rhamnose (Rha), N-acetyl-D-glucosamine, D-galactose, and D-glucose in different molar ratios. By using chemical fragmentation, methylation, as well as one- and two-dimensional NMR spectroscopy, the structures of these polysaccharides were found to be [formula: see text] In O18A-PS and O18A1-PS x = 2, whereas in O18B-PS and in O18B11-PS x = 3. In all four polysaccharides alpha-D-Galp (residue D) is substituted at O-3. This substituent L (residue E) is beta-D-GlcpNAc-(1 in O18A-PS and O18A1-PS and it is alpha-D-Glcp-(1 in O18B-PS and O18B1-PS. Whereas there is no further substituent on the main chain of the O18A and O18B polysaccharides, in O18A1-PS and O18B1-PS the alpha-D-GlcpNAc residue A is substituted with alpha-Glcp-(1 (residue F), which is linked to O-6 in O18A1-PS and to O-4 in O18B1-PS. These results show that the O18 antigen comprises a group of four related LPS (O18A and O18B, with their glucosylated forms O18A1 and O18B1). The results are discussed with respect to epitope definition and biochemical implications.

Occurrence of 2-(2-Hydroxy-4,7-dimethoxy-2H-1,4-benzoxazin-3-one)- β-ᴅ-glucopyranoside in Triticum aestivum Leaves and Its Conversion into 6-Methoxy-benzoxazolinone

Abstract Cyclic hydroxamic acids occurring in the Gramineae have been reported to be involved in the resistance of cereals to fungi and insects. Here we describe the occurrence, in wheat leaves, of 1,4- benzoxazinone derivatives and their glucosylated forms, a major representative of which was found to be 2-(2-hydroxy-4,7-dimethoxy-2H -1,4-benzoxazin-3-one)-β-ᴅ-glucopyranoside. Owing to its spontaneous conversion into 6-methoxy-benzoxazolinone the corresponding aglucone could not be isolated following enzymic cleavage. We propose a plausible mechanism for this reaction which also explains the higher stability of other members of the 1.4-benzoxazinone family.

Endogenous Cytokinins in Bougainvillea 'San Diego Red.' II. The Complex in Mature Leaves

Nine compounds that stimulated division of soybean callus cells could be tentatively identified in the mature leaves of Bougainvillea `San Diego Red.' These included phosphorylated and glucosylated forms of zeatin, zeatin riboside, and their dihydro derivatives. The cytokinin glucosides detected in mature leaves may not have been formed entirely in situ as a result of metabolism but may have originated partly from the roots.

Endogenous Cytokinins in Bougainvillea `San Diego Red'

Using chromatographic, chemical, and enzymic techniques, 11 compounds capable of stimulating the division of soybean callus cells were tentatively identified in the root sap of Bougainvillea ;San Diego Red.' These cytokinin-like compounds included phosphorylated and glucosylated forms of zeatin, ribosylzeatin, and their dihydro derivatives. In addition, isopentenyl adenosine and isopentenyl adenine were apparently also present. The occurrence of glucosylated derivatives in the root sap of plants has not been substantiated previously.