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General Comment |
A novel human b1,3-N-acetylgalactosaminyltransferase which synthesizes a unique carbohydrate structure, GalNAcb1-3GlcNAc
Hiruma T,et al found a novel human gene (GenBankTM accession number BC029564) that possesses b3-glycosyltransferase motifs by a Blast search. The full-length open reading frame consists of 500 amino acids and encodes a typical type II membrane protein. This enzyme had a domain containing b1,3-glycosyltransferase motifs which are widely conserved in the b1,3-galactosyltransferase and b1,3-N-acetylglucosaminyltransferase families. The putative catalytic domain was expressed in human embryonic kidney 293T cells as a soluble protein. Its N-acetylgalactosaminyltransferase activity was observed when N-acetylglucosamine (GlcNAc) b1-O-benzyl was used as an acceptor substrate. The enzyme product was determined to have a b1,3-linkage by NMR spectroscopic analysis, therefore we named it b1,3-N-acetylgalactosaminyltransferase-II (b3GalNAc-T2). The acceptor substrate specificity of b3GalNAc-T2 was examined using various oligosaccharide substrates. GlcNAcb1-3GalNAca1-O-para-nitrophenyl (core 2-pNP) was the best acceptor substrate for b3GalNAc-T2, followed by GlcNAcb1-4GlcNAc-b1-O-benzyl and GlcNAcb1-6GalNAca1-O-paranitrophenyl (core 6-pNP), among oligosaccharide substrates we examined. Quantitative real time PCR analysis revealed that the b3GalNAc-T2 transcript was restricted in its distribution, mainly to the testis, adipose, skeletal muscle and ovary. Its putative orthologous gene, mb3GalNAc-T2 was also found in a database of mouse expressed sequence tags. In situ hybridization analysis with mouse testis showed that the transcripts are expressed in germ line cells. b3GalNAc-T2 efficiently transferred GalNAc to N-glycans of fatal calf fetuin which was treated with neuraminidase and b-galactosidase. However, it showed no activity toward any kinds of glycolipid examined. Although the GalNAcb1-3GlcNAcb1-R structure has not been reported in humans and other mammals, this is the first report of a novel human glycosyltransferase producing an unknown structure, GalNAcb1-3GlcNAc-R, on N- and O-glycans.
NCBI Summary:
This gene is a member of the beta-1,3-galactosyltransferase (beta3GalT) gene family. This family encodes type II membrane-bound glycoproteins with diverse enzymatic functions using different donor substrates (UDP-galactose and UDP-N-acetylglucosamine) and different acceptor sugars (N-acetylglucosamine, galactose, N-acetylgalactosamine). The beta3GalT genes are distantly related to the Drosophila Brainiac gene and have the protein coding sequence contained in a single exon. The beta3GalT proteins also contain conserved sequences not found in the beta4GalT or alpha3GalT proteins. The carbohydrate chains synthesized by these enzymes are designated as type 1, whereas beta4GalT enzymes synthesize type 2 carbohydrate chains. The ratio of type 1:type 2 chains changes during embryogenesis. By sequence similarity, the beta3GalT genes fall into at least two groups: beta3GalT4 and 4 other beta3GalT genes (beta3GalT1-3, beta3GalT5). The encoded protein of this gene does not use N-acetylglucosamine as an acceptor sugar at all. Four transcript variants have been described which differ in both the 5' and 3' UTR sequences. All transcript variants encode an identical protein.
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