Expression cloning of cDNA encoding a human beta-1,3-N-acetylglucosaminyltransferase that is essential for poly-N-acetyllactosamine synthesis

Synthesis of poly-N-acetyllactosamine in core 2 branched O-glycans. The requirement of novel beta-1,4-galactosyltransferase IV and beta-1,3-n-acetylglucosaminyltransferase

Poly-N-acetyllactosamine is a unique carbohydrate composed of N-acetyllactosamine repeats and provides the backbone structure for additional modifications such as sialyl Lex. Poly-N-acetyllactosamines in mucin-type O-glycans can be formed in core 2 branched oligosaccharides, which are synthesized by core 2 beta-1,6-N-acetylglucosaminyltransferase. Using a beta-1, 4-galactosyltransferase (beta4Gal-TI) present in milk and the recently cloned beta-1,3-N-acetylglucosaminyltransferase, the formation of poly-N-acetyllactosamine was found to be extremely inefficient starting from a core 2 branched oligosaccharide, GlcNAcbeta1-->6(Galbeta1-->3)GalNAcalpha-->R. Since the majority of synthesized oligosaccharides contained N-acetylglucosamine at the nonreducing ends, galactosylation was judged to be inefficient, prompting us to test novel members of the beta4Gal-T gene family for this synthesis. Using various synthetic acceptors and recombinant beta4Gal-Ts, beta4Gal-TIV was found to be most efficient in the addition of a single galactose residue to GlcNAcbeta1-->6(Galbeta1-->3)GalNAcalpha-->R. Moreover, beta4Gal-TIV, together with beta-1,3-N-acetylglucosaminyltransferase, was capable of synthesizing poly-N-acetyllactosamine in core 2 branched oligosaccharides. On the other hand, beta4Gal-TI was found to be most efficient for poly-N-acetyllactosamine synthesis in N-glycans. In contrast to beta4Gal-TI, the efficiency of beta4Gal-TIV decreased dramatically as the acceptors contained more N-acetyllactosamine repeats, consistent with the fact that core 2 branched O-glycans contain fewer and shorter poly-N-acetyllactosamines than N-glycans in many cells. These results, as a whole, indicate that beta4Gal-TIV is responsible for poly-N-acetyllactosamine synthesis in core 2 branched O-glycans.

A novel second isoenzyme of the human UDP-N-acetylglucosamine:alpha1,3-D-mannoside beta1,4-N-acetylglucosaminyltransferase family: cDNA cloning, expression, and chromosomal assignment

We isolated a novel cDNA encoding a second isoenzyme of UDP-N-acetylglucosamine:alpha1,3-D-mannoside beta1,4-N-acetylglucosaminyltransferase (GnT-IV; EC 2.4.1.145). The nucleotide and deduced amino acid sequences of the cDNA were homologous to those of the previously cloned human GnT-IV cDNA (63% and 62% identity, respectively). The new cDNA is also confirmed to express GnT-IV activity, suggesting that two isoenzymes of human GnT-IV exist. Although genomic Southern analysis suggested that both genes exist in many mammalian species and the chicken, northern analysis revealed that both genes are expressed in different ways in human tissues. This is the first report concerning the gene family of an N-acetylglucosaminyltransferase in mammals.

The centrally acting beta1,6N-acetylglucosaminyltransferase (GlcNAc to gal). Functional expression, purification, and acceptor specificity of a human enzyme involved in midchain branching of linear poly-N-acetyllactosamines

In the present experiments the cDNA coding for a truncated form of the beta1,6N-acetylglucosaminyltransferase responsible for the conversion of linear to branched polylactosamines in human PA1 cells was expressed in Sf9 insect cells. The catalytic ectodomain of the enzyme was fused to glutathione S-transferase, allowing effective one-step purification of the glycosylated 67-74-kDa fusion protein. Typically a yield of 750 microg of the purified protein/liter of suspension culture was obtained. The purified recombinant protein catalyzed the transfer of GlcNAc from UDP-GlcNAc to the linear tetrasaccharide Galbeta1-4GlcNAcbeta1-3Galbeta1-4GlcNAc, converting the acceptor to the branched pentasaccharide Galbeta1-4GlcNAcbeta1-3(GlcNAcbeta1-6)Galbeta1-4 GlcNAc as shown by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, degradative experiments, and 1H NMR spectroscopy of the product. By contrast, the recombinant enzyme did not catalyze any reaction when incubated with UDP-GlcNAc and the trisaccharide GlcNAcbeta1-3Galbeta1-4GlcNAc. Accordingly, we call the recombinant beta1,6-GlcNAc transferase cIGnT6 to emphasize its action at central rather than peridistal galactose residues of linear polylactosamines in the biosynthesis of blood group I antigens. Taken together this in vitro expression of I-branching enzyme, in combination with the previously cloned enzymes, beta1,4galactosyltransferase and beta1, 3N-acetylglucosaminyltransferase, should allow the general synthesis of polylactosamines based totally on the use of recombinant enzymes.

Increased elongation of N-acetyllactosamine repeats in doubly glycosylated lysozyme with a particular spacing of the glycosylation sites

Lysozyme is an example of an extensively studied secretory enzyme. Glycosylated mutant human lysozyme has been used as a model in studies on the biosynthesis of N-acetyllactosamine repeats in N-linked oligosaccharides. We examined the biosynthesis of the repeats in two doubly glycosylated mutants and describe here a rapid purification and separation of singly and doubly glycosylated molecules. In one of the mutants, the elongation of the repeats is enhanced if the molecules are doubly glycosylated, but not if the carbohydrate is attached to either site individually. This enhancement is not seen in the other doubly glycosylated mutant. Since lysozyme is not structurally related to glycoproteins bearing carbohydrate with N-acetyllactosamine repeats, we propose that in multivalent substrates the synthesis of the repeats can be promoted by a proper spacing of the elongated carbohydrate antennae in addition to any role of the protein backbone.

Molecular cloning and expression of GDP-D-mannose-4,6-dehydratase, a key enzyme for fucose metabolism defective in Lec13 cells

Subsets of mammalian cell surface oligosaccharides contain specific fucosylated moieties expressed in lineage- and/or temporal-specific patterns. The functional significance of these fucosylated structures is incompletely defined, although there is evidence that subsets of them, represented by the sialyl Lex determinant, are important participants in leukocyte adhesion and trafficking processes. Genetic deletion of these fucosylated structures in the mouse has been a powerful tool to address functional questions about fucosylated glycans. However, successful use of such approaches can be problematic, given the substantial redundancy in the mammalian alpha-1,3-fucosyltransferase and alpha-1,2-fucosyltransferase gene families. To circumvent this problem, we have chosen to clone the genetic locus encoding a mammalian GDP-D-mannose-4,6-dehydratase (GMD). This enzyme generates GDP-mannose-4-keto-6-D-deoxymannose from GDP-mannose, which is then converted by the FX protein (GDP-4-keto-6-D-deoxymannose epimerase/GDP-4-keto-6-L-galactose reductase) to GDP-L-fucose. GMD is thus imperative for the synthesis of all fucosylated oligosaccharides. An expression cloning approach and the GMD-deficient CHO host cell line Lec13 were used to generate a population of cDNA molecules enriched in GMD cDNAs. This enriched plasmid population was then screened using a human expressed sequence tag (EST AA065072) with sequence similarity to an Arabidopsis thaliana GMD cDNA. This approach, together with 5'-rapid amplification of cDNA ends, yielded a human cDNA that complements the fucosylation defect in the Lec13 cell line. Northern blot analyses indicate that the GMD transcript is absent in Lec13 cells, confirming the genetic deficiency of this locus in these cells. By contrast, the transcript encoding the FX protein, which forms GDP-L-fucose from the ketosugar intermediate produced by GMD, is present in increased amounts in the Lec13 cells. These results suggest that metabolites generated in this pathway may participate in the transcriptional regulation of the FX protein and possibly the GMD protein. The results also suggest that the genomic structure encoding GMD in Lec13 cells likely has a defect different from a point mutation in the coding region.