The galactoside 2-α-L-fucosyltransferase FUT1 from Arabidopsis thaliana: crystallization and experimental MAD phasing

The plant cell wall is a complex network of polysaccharides made up of cellulose, hemicelluloses and pectins. Xyloglucan (XyG), which is the main hemicellulosic component of dicotyledonous plants, has attracted much attention for its role in plant development and for its many industrial applications. The XyG-specific fucosyltransferase (FUT1) adds a fucose residue from GDP-fucose to the 2-O position of the terminal galactosyl residues on XyG side chains. Recombinant FUT1 from Arabidopsis thaliana was crystallized in two different crystal forms, with the best diffracting crystals (up to 1.95 Å resolution) belonging to the monoclinic space group P21, with unit-cell parameters a = 87.6, b = 84.5, c = 150.3 Å, β = 96.3°. Ab initio phases were determined using a two-wavelength anomalous dispersion experiment on a tantalum bromide-derivatized crystal with data collected at the rising and descending inflection points of the Ta white line. An interpretable electron-density map was obtained after elaborate density modification. Model completion and structural analysis are currently under way.

Characterization and role of fucose mutarotase in mammalian cells

L-Fucose for mammalian glycosylation contains an anomeric carbon atom generating alpha- and beta-L-fucoses. Based on sequence comparison of mouse and human homologs with the prokaryotic fucose mutarotases (FucU) characterized previously, we investigated their function in mammalian cells. By nuclear magnetic resonance (NMR) measurement with saturation difference analysis, the purified mammalian mutarotases were demonstrated to be involved in an interconversion between the two anomeric forms with comparable efficiency as that of the Escherichia coli FucU. The mouse gene was widely expressed in various tissues and cell lines, including kidney, liver, and pancreas, although expression was marginal in muscle and testis. By generating stably expressed cell lines for mutarotase genes in HepG2, it was shown that fucose incorporations into cellular proteins were increased as demonstrated by an incorporation of radiolabeled fucose into the cells. Furthermore, intracellular levels of GDP-L-fucose, measured with high performance liquid chromatography (HPLC), were enhanced by an overproduction of cellular mutarotase, which was reversed by gene silencing of mutarotase based on RNA interference. The results suggest that the mammalian mutarotase is functional in facilitated incorporation of fucose through the salvage pathway.

Reconstitution in vitro of the GDP-fucose biosynthetic pathways of Caenorhabditis elegans and Drosophila melanogaster

The deoxyhexose sugar fucose has an important fine-tuning role in regulating the functions of glycoconjugates in disease and development in mammals. The two genetic model organisms Caenorhabditis elegans and Drosophila melanogaster also express a range of fucosylated glycans, and the nematode particularly has a number of novel forms. For the synthesis of such glycans, the formation of GDP-fucose, which is generated from GDP-mannose in three steps catalysed by two enzymes, is required. By homology we have identified and cloned cDNAs encoding these two proteins, GDP-mannose dehydratase (GMD; EC 4.2.1.47) and GDP-keto-6-deoxymannose 3,5-epimerase/4-reductase (GER or FX protein; EC 1.1.1.271), from both Caenorhabditis and Drosophila. Whereas the nematode has two genes encoding forms of GMD (gmd-1 and gmd-2) and one GER-encoding gene (ger-1), the insect has, like mammalian species, only one homologue of each (gmd and gmer). This compares to the presence of two forms of both enzymes in Arabidopsis thaliana. All corresponding cDNAs from Caenorhabditis and Drosophila, as well as the previously uncharacterized Arabidopsis GER2, were separately expressed, and the encoded proteins found to have the predicted activity. The biochemical characterization of these enzymes is complementary to strategies aimed at manipulating the expression of fucosylated glycans in these organisms.

Effect of the manganese ion on human alpha3/4 fucosyltransferase III activity

The effect of manganese and other divalent cations on the activity of a soluble recombinant form of human alpha3/4 fucosyltransferase III (SFT3) expressed in Spodoptera frugiperda (Sf9) insect cells was studied. SFT3 was active in the absence of divalent cations with an optimum pH of 4.5. In the absence of Mn2+ increasing the pH from 4.5 to 7.0 caused a decrease in the affinity of SFT3 for the acceptor Galbeta3GlcNAcO(CH2)3NHCO(CH2)5NH-biotin, as monitored by the 4-fold increase in the apparent KM value (0.9 to 3.3 mM). At pH 7.0, the addition of Mn2+ activated the enzyme and caused an increase in the affinity of SFT3 for the acceptor, as monitored by the 5-fold decrease of the apparent KM value (3.3 to 0.7 mM). In solution, a complex between GDP-Fuc donor and the divalent cation Mn2+ was observed by electrospray ionization mass spectrometry, in a 1:1 stoichiometry. These results indicated that Mn2+ bound the enzyme and increased its affinity for the acceptor; one possible functional role of manganese in catalysis could be as an electrophilic catalyst, co-ordinating the negative charges of the phosphate groups of the GDP-Fuc donor and promoting Fuc transfer. At low pH values such role would be played by the proton.

Functional expression of Escherichia coli enzymes synthesizing GDP-L-fucose from inherent GDP-D-mannose in Saccharomyces cerevisiae

Fucosylation of glycans on glycoproteins and -lipids requires the enzymatic activity of relevant fucosyltransferases and GDP-L-fucose as the donor. Due to the biological importance of fucosylated glycans, a readily accessible source of GDP-L-fucose would be required. Here we describe the construction of a stable recombinant S.cerevisiae strain expressing the E.coli genes gmd and wcaG encoding the two enzymes, GDP-mannose-4,6-dehydratase (GMD) and GDP-4-keto-6-deoxy-D-mannose-3,5-epimerase/4-reductase (GMER(FX)) respectively, needed to convert GDP-mannose to GDP-fucose via the de novo pathway. Taking advantage of the rich inherent cytosolic GDP-mannose pool in S.cerevisiae cells we could easily produce 0.2 mg/l of GDP-L-fucose with this recombinant yeast strain without addition of any external GDP-mannose. The GDP-L-fucose product could be used as the fucose donor for alpha1,3fucosyltransferase to synthesize sialyl Lewis x (sLex), a glycan crucial for the selectin-dependent leukocyte traffic.

A strategy for the solution-phase parallel synthesis of N-(pyrrolidinylmethyl)hydroxamic acids

Both five- and six-membered iminocyclitols have proven to be useful transition-state analogue inhibitors of glycosidases. They also mimic the transition-state sugar moiety of the nucleoside phosphate sugar in glycosyltransferase-catalyzed reactions. Described here is the development of a general strategy toward the parallel synthesis of a five-membered iminocyclitol linked to a hydroxamic acid group designed to mimic the transition state of GDP-fucose complexed with Mn(II) in fucosyltransferase reactions. The iminocyclitol 8 containing a protected hydroxylamine unit was prepared from D-mannitol. The hydroxamic acid moiety was introduced via the reaction of 8 with various acid chlorides. The strategy is generally applicable to the construction of libraries for identification of glycosyltransferase inhibitors.

A high-yield, enzymatic synthesis of GDP-D-[3H]arabinose and GDP-L-[3H]fucose

For assays involving glycosyltransferases or transporters, several GDP-sugars are either commercially unavailable or expensive. We describe an enzymatic synthesis of GDP-d-[3H]arabinosep and GDP-l-[3H]fucose that yields 66-95% nucleotide-sugar from the appropriate radiolabeled sugar in less than 30 min. The coupled reaction requires Mg2+, ATP, and GTP along with the appropriate radioactive monosaccharide, sugar-1-kinase, and pyrophosphorylase. The latter two activities are present in a cytosolic fraction of Crithidia fasciculata, which is easily grown at room temperature in simple culture medium without serum or added CO2. Addition of commercial yeast inorganic pyrophosphatase shifts the equilibrium of the pyrophosphorylase reaction toward nucleotide-sugar formation. To verify that these nucleotide-sugars are biologically active, we tested their ability to serve as substrates for glycosyltransferases. GDP-l-[3H]fucose functions as the donor substrate for recombinant human fucosyltransferase V, and GDP-d-[3H]arabinosep serves as the donor substrate for the arabinosyltransferase activities present in Leishmania major microsomes.

The nodulation gene nolK of Azorhizobium caulinodans is involved in the formation of GDP-fucose from GDP-mannose

The nolK gene of Azorhizobium caulinodans is essential for the incorporation of a fucosyl group in Nod factors. A NAD(P)-binding site is present in the NolK amino acid sequence and the gene is homologous to Escherichia coli genes, presumably involved in GDP-fucose synthesis. Protein extracts of A. caulinodans, overexpressing nolK, have an enzyme activity that synthesizes GDP-fucose from GDP-mannose. nolK most probably encodes a 4-reductase performing the last step in GDP-fucose synthesis. Wild-type A. caulinodans produces a population of fucosylated and non-fucosylated molecules but the nolK-overexpressing strain produces only fucosylated Nod factors. Thus, the production of activated fucosyl donors is a rate-limiting step in Nod factor fucosylation.

A continuous spectrophotometric assay for glycosyltransferases

This paper describes a continuous spectrophotometric assay for glycosyltransferases. In this assay, a nucleotide diphosphate is coupled to NADH oxidation via pyruvate kinase and lactate dehydrogenase. The nucleotide diphosphate is produced either directly during the glycosyltransferase mediated reaction, or indirectly by the production of a nucleotide monophosphate during the glycosyltransferase mediated reaction, and subsequent conversion of the nucleotide monophosphate to nucleotide diphosphate using nucleoside monophosphate kinase. Using this assay, kinetic parameters for fucosyl-, sialyl-, and N-acetylglucosaminyltransferases were determined. The assay not only allows continual monitoring of the enzymatic reaction, but is rapid and allows the processing of 96 samples at once since it is performed in 96-well microtiter plates. In addition, the procedure provides a means of monitoring the activity of these enzymes using sugar-nucleotide donor analogs, where radiochemical procedures cannot be used.

Large-scale synthesis of beta-L-fucopyranosyl phosphate and the preparation of GDP-beta-L-fucose

A practical 15-mmol large-scale synthesis of beta-L-fucopyranosyl dicyclohexylammonium phosphate from L-fucose in 63% overall yield was developed. The synthesis took advantage of a neighboring Bz-2 group participating in a Koenigs-Knorr-like glycosylation. The sugar phosphate was transformed into the activated sugar nucleoside, guanosine diphosphate beta-L-fucopyranose, on a gram scale.