Transcription of the beta-galactoside alpha2,6-sialyltransferase gene (SIAT1) in B-lymphocytes: cell type-specific expression correlates with presence of the divergent 5'-untranslated sequence

CCCTC-binding factor: the specific transcription factor of β-galactoside α-2,6-sialyltransferase 1 that upregulates the sialylation of anti-citrullinated protein antibodies in rheumatoid arthritis

OBJECTIVE

Sialylation of the crystallizable fragment (Fc) of ACPAs, which is catalysed by β-galactoside α-2,6-sialyltransferase 1 (ST6GAL1) could attenuate inflammation of RA. In this study, we screened the transcription factor of ST6GAL1 and elucidated the mechanism of transcriptionally upregulating sialylation of ACPAs in B cells to explore its role in the progression of RA.

METHODS

Transcription factors interacting with the P2 promoter of ST6GAL1 were screened by DNA pull-down and liquid chromatography with tandem mass spectrometry (LC-MS/MS), and verified by chromatin immunoprecipitation (ChIP), dual luciferase reporter assay and electrophoretic mobility shift assay (EMSA). The function of the CCCTC-binding factor (CTCF) on the expression of ST6GAL1 and the inflammatory effect of ACPAs were verified by knocking down and overexpressing CTCF in B cells. The CIA model was constructed from B cell-specific CTCF knockout mice to explore the effect of CTCF on arthritis progression.

RESULTS

We observed that the levels of ST6GAL1 and ACPAs sialylation decreased in the serum of RA patients and were negatively correlated with DAS28 scores. Subsequently, CTCF was screened and verified as the transcription factor interacting with the P2 promoter of ST6GAL1, which enhances the sialylation of ACPAs, thus weakening the inflammatory activity of ACPAs. Furthermore, the above results were also verified in the CIA model constructed from B cell-specific CTCF knockout mice.

CONCLUSION

CCCTC-binding factor is the specific transcription factor of β-galactoside α-2,6-sialyltransferase 1 in B cells that upregulates the sialylation of ACPAs in RA and attenuates the disease progression.

Recombinant Sialyltransferase Infusion Mitigates Infection-Driven Acute Lung Inflammation

Inappropriate inflammation exacerbates a vast array of chronic and acute conditions with severe health risks. In certain situations, such as acute sepsis, traditional therapies may be inadequate in preventing severe organ damage or death. We have previously shown cell surface glycan modification by the circulating sialyltransferase ST6Gal-1 regulates de novo inflammatory cell production via a novel extrinsic glycosylation pathway. Here, we show that therapeutic administration of recombinant, bioactive ST6Gal-1 (rST6G) mitigates acute inflammation in a murine model mimicking acute exacerbations experienced by patients with chronic obstructive pulmonary disease (COPD). In addition to suppressing proximal neutrophil recruitment at onset of infection-mediated inflammation, rST6G also muted local cytokine production. Histologically, exposure with NTHI, a bacterium associated with COPD exacerbations, in rST6G-treated animals revealed consistent and pronounced reduction of pulmonary inflammation, characterized by smaller inflammatory cuffs around bronchovascular bundles, and fewer inflammatory cells within alveolar walls, alveolar spaces, and on pleural surfaces. Taken together, the data advance the idea that manipulating circulatory ST6Gal-1 levels has potential in managing inflammatory conditions by leveraging the combined approaches of controlling new inflammatory cell production and dampening the inflammation mediator cascade.

Treatment response in Kawasaki disease is associated with sialylation levels of endogenous but not therapeutic intravenous immunoglobulin G

Objectives

Although intravenous immunoglobulin (IVIG) is highly effective in Kawasaki disease (KD), mechanisms are not understood and 10-20% of patients are treatment-resistant, manifesting a higher rate of coronary artery aneurysms. Murine models suggest that α2-6-linked sialic acid (α2-6Sia) content of IVIG is critical for suppressing inflammation. However, pro-inflammatory states also up-regulate endogenous levels of β-galactoside:α2-6 sialyltransferase-I (ST6Gal-I), the enzyme that catalyzes addition of α2-6Sias to N-glycans. We asked whether IVIG failures correlated with levels of α2-6Sia on infused IVIG or on the patient’s own endogenous IgG.

Methods

We quantified levels of α2-6Sia in infused IVIG and endogenous IgG from 10 IVIG-responsive and 10 resistant KD subjects using multiple approaches. Transcript levels of ST6GAL1, in patient whole blood and B cell lines were evaluated by RT-PCR. Plasma soluble (s)ST6Gal-I levels were measured by ELISA.

Results

There was no consistent difference in median sialylation levels of infused IVIG between groups. However, α2-6Sia levels in endogenous IgG, ST6GAL1 transcript levels, and ST6Gal-I protein in serum from IVIG-resistant KD subjects were lower than in responsive subjects at both pre-treatment and one-year time points (p <0.001, respectively).

Conclusions

Our data indicate sialylation levels of therapeutic IVIG are unrelated to treatment response in KD. Rather, lower sialylation of endogenous IgG and lower blood levels of ST6GALI mRNA and ST6Gal-I enzyme predict therapy resistance. These differences were stable over time, suggesting a genetic basis. Because IVIG-resistance increases risk of coronary artery aneurysms, our findings have important implications for the identification and treatment of such individuals.

The effect of the CCR5-delta32 deletion on global gene expression considering immune response and inflammation

Background

The natural function of the C-C chemokine receptor type 5 (CCR5) is poorly understood. A 32 base pair deletion in the CCR5 gene (CCR5-delta32) located on chromosome 3 results in a non-functional protein. It is supposed that this deletion causes an alteration in T-cell response to inflammation. For example, the presence of the CCR5-delta32 allele in recipients of allografts constitutes as an independent and protective factor associated with a decreased risk of graft-versus-host disease (GVHD) and graft rejection. However, the mechanism of this beneficial effect of the deletion regarding GVHD is unknown. In this survey we searched for a CCR5-delta32 associated regulation of critical genes involved in the immune response and the development of GVHD.

Methods

We examined CD34+ hematopoietic progenitor cells derived from bone marrow samples from 19 healthy volunteers for the CCR5-delta32 deletion with a genomic PCR using primers flanking the site of the deletion.

Results

12 individuals were found to be homozygous for CCR5 WT and 7 carried the CCR5-delta32 deletion heterozygously. Global gene expression analysis led to the identification of 11 differentially regulated genes. Six of them are connected with mechanisms of immune response and control: LRG1, CXCR2, CCRL2, CD6, CD7, WD repeat domain, and CD30L.

Conclusions

Our data indicate that the CCR5-delta32 mutation may be associated with differential gene expression. Some of these genes are critical for immune response, in the case of CD30L probably protective in terms of GVHD.

Molecular phylogeny and functional genomics of beta-galactoside alpha2,6-sialyltransferases that explain ubiquitous expression of st6gal1 gene in amniotes

Sialyltransferases are key enzymes in the biosynthesis of sialoglycoconjugates that catalyze the transfer of sialic residue from its activated form to an oligosaccharidic acceptor. β-Galactoside α2,6-sialyltransferases ST6Gal I and ST6Gal II are the two unique members of the ST6Gal family described in higher vertebrates. The availability of genome sequences enabled the identification of more distantly related invertebrates' st6gal gene sequences and allowed us to propose a scenario of their evolution. Using a phylogenomic approach, we present further evidence of an accelerated evolution of the st6gal1 genes both in their genomic regulatory sequences and in their coding sequence in reptiles, birds, and mammals known as amniotes, whereas st6gal2 genes conserve an ancestral profile of expression throughout vertebrate evolution.

The second bovine beta-galactoside-alpha2,6-sialyltransferase (ST6Gal II): genomic organization and stimulation of its in vitro expression by IL-6 in bovine mammary epithelial cells

We have cloned a cDNA sequence encoding the second bovine beta-galactoside-alpha2,6-sialyltransferase whose sequence shares more than 75% of identity with hST6Gal II cDNA coding sequence. The bovine gene, located on BTA 11, spans over 50 kbp with five exons (E1-E5) containing the 1488 bp open reading frame and a 5'-untranslated exon (E0). The gene expression pattern reveals a specific tissue distribution (brain, lungs, spleen, salivary, and mammary glands) compared to ST6Gal I which is ubiquitously expressed. We identified for bovine ST6Gal II three kinds of transcripts which differ by their 5'-untranslated regions. Among them, two transcripts are brain specific whereas the third one is found in all of the tissues expressing the gene. Two pFlag-bST6Gal II vector constructions were separately transfected in COS-1 cells in order to express either membrane-bound or soluble active forms of ST6Gal II. Enzymatic assays with these two forms indicated that the enzyme used the LacdiNAc structure (GalNAcbeta1,4GlcNAc) as a better acceptor substrate than the Type II (Galbeta1-4GlcNAc) disaccharide. Moreover, the enzyme's efficiency is improved when the acceptor substrate is provided as a free oligosaccharide rather than as a protein-bound oligosaccharide. In order to investigate the potential role of ST6Gal II during the acute phase of inflammation, we used primary cultures of bovine mammary epithelial cells which were stimulated with pro-inflammatory cytokines. It appears that the ST6Gal II gene was upregulated in cells stimulated by IL-6. This result suggested that alpha2,6-sialylation mediated by this gene could contribute to organism's response to infections.

Expression of the human CMP-NeuAc:GM3 alpha2,8-sialyltransferase (GD3 synthase) gene through the NF-kappaB activation in human melanoma SK-MEL-2 cells

To elucidate the mechanism underlying the regulation of human GD3 synthase gene expression in human melanoma SK-MEL-2 cells, we identified the promoter region of the human GD3 synthase gene. The 5'-rapid amplification of cDNA end (5'-RACE) using mRNA prepared from SK-MEL-2 cells revealed the presence of multiple transcription start sites of human GD3 synthase gene. Promoter analyses of the 5'-flanking region of the human GD3 synthase gene using luciferase gene reporter system showed the strong promoter activity in SK-MEL-2 cells. Deletion study revealed that the region as the core promoter from -1146 to -646 (A of the translational start ATG as position +1) was indispensable for endogenous expression of human GD3 synthase gene. This region lacks apparent TATA and CAAT boxes but contains putative binding sites for transcription factors c-Ets-1, CREB, AP-1 and NF-kappaB. Electrophoretic mobility shift assays using specific competitors, chromatin immunoprecipitation assay and site-directed mutagenesis demonstrated that only NF-kappaB element in this region is required for the promoter activity in SK-MEL-2 cells. These results indicate that NF-kappaB plays an essential role in the transcriptional activity of human GD3 synthase gene essential for GD3 synthesis in SK-MEL-2 cells.

Transcriptional regulation of the human GD3 synthase gene expression in Fas-induced Jurkat T cells: a critical role of transcription factor NF-kappaB in regulated expression

The transcriptional regulation mechanisms involved in the up-regulation of Fas-induced GD3 synthase gene have not yet been elucidated. 5'-Rapid amplification of cDNA end (5'-RACE) using mRNA prepared from Fas-induced Jurkat T cells revealed the presence of multiple transcription start sites of human GD3 synthase gene, and the 5'-end analysis of the longest of its product showed that transcription started from 650 nucleotides upstream of the translational initiation site. Promoter analyses of the 5'-flanking region of the human GD3 synthase gene using luciferase gene reporter system showed strong promoter activity in Fas-induced Jurkat T cells. Deletion study revealed that the region from -1146 to -646 (A of the translational start ATG as position +1) was indispensable for the Fas response. This region lacks apparent TATA and CAAT boxes but contains putative binding sites for transcription factors c-Ets-1, cAMP-responsive element-binding (CREB) protein, activating protein 1 (AP-1), and NF-kappaB. Base-substitution experiment showed that only the NF-kappaB-binding site of putative binding sites is required for the maximal expression induced by Fas. Both DNase I footprint and electrophoretic mobility shift assays with the nuclear extract of Fas-induced Jurkat T cells revealed that NF-kappaB was bound specifically to the probe being mediated by its binding site in the promoter sequence. Taken together, these results indicate that NF-kappaB plays an essential role in the transcriptional activity of human GD3 synthase gene in Fas-induced Jurkat T cells. In addition, the translocation of NF-kappaB-binding protein to nucleus by Fas activation is also crucial for the increased expression of the GD3 synthase gene in Fas-activated Jurkat T cells.

Ablation of CD22 in ligand-deficient mice restores B cell receptor signaling

CD22 is a negative regulator of B cell signaling, an activity modulated by its interaction with glycan ligands containing alpha2-6-linked sialic acids. B cells deficient in the enzyme (ST6Gal I) that forms the CD22 ligand show suppressed BCR signaling. Here we report that mice deficient in both CD22 and its ligand (Cd22-/- St6gal1-/- mice) showed restored B cell receptor (BCR) signaling, suggesting that the suppressed signaling of St6gal1-/- cells is mediated through CD22. Coincident with suppressed BCR signaling, B cells lacking ST6Gal I showed a net redistribution of the BCR to clathrin-rich microdomains containing most of the CD22, resulting in a twofold increase in the localization of CD22 together with the BCR. These studies suggest an important function for the CD22-ligand interaction in regulating BCR signaling and microdomain localization.

Masking of CD22 by cis ligands does not prevent redistribution of CD22 to sites of cell contact

CD22, a negative regulator of B cell signaling, is a member of the siglec family that binds to alpha2-6-linked sialic acids on glycoproteins. Previous reports demonstrated that binding of multivalent sialoside probes to CD22 is blocked, or "masked," by endogenous (cis) ligands, unless they are first destroyed by sialidase treatment. These results suggest that cis ligands on B cells make CD22 functionally unavailable for binding to ligands in trans. Through immunofluorescence microscopy, however, we observed that CD22 on resting B cells redistributes to the site of contact with other B or T lymphocytes. Redistribution is mediated by interaction with trans ligands on the opposing cell because it does not occur with ligand-deficient lymphocytes from ST6GalI-null mice. Surprisingly, CD45, proposed as both a cis and trans ligand of CD22, was not required for redistribution to sites of cell contact, given that redistribution of CD22 was independent of CD45 and was observed with lymphocytes from CD45-deficient mice. Furthermore, CD45 is not required for CD22 masking as similar levels of masking were observed in the WT and null mice. Comparison of the widely used sialoside-polyacrylamide probe with a sialoside-streptavidin probe revealed that the latter bound a subset of B cells without sialidase treatment, suggesting that cis ligands differentially impacted the binding of these two probes in trans. The combined results suggest that equilibrium binding to cis ligands does not preclude binding of CD22 to ligands in trans, and allows for its redistribution to sites of contact between lymphocytes.

C5a-induced gene expression in human umbilical vein endothelial cells

The endothelium plays a critical role in the inflammatory process. The complement activation product, C5a, is known to have proinflammatory effects on the endothelium, but the molecular mechanisms remain unclear. We have used cDNA microarray analysis to assess gene expression in human umbilical vein endothelial cells (HUVECs) that were stimulated with human C5a in vitro. Chip analyses were confirmed by reverse transcriptase-polymerase chain reaction and by Western blot analysis. Gene activation responses were remarkably similar to gene expression patterns of HUVECs stimulated with human tumor necrosis factor-alpha or bacterial lipopolysaccharide. HUVECs stimulated with C5a showed progressive increases in gene expression for cell adhesion molecules (eg, E-selectin, ICAM-1, VCAM-1), cytokines/chemokines, and related receptors (eg, VEGFC, IL-6, IL-18R). Surprisingly, HUVECs showed little evidence for up-regulation of complement-related genes. There were transient increases in gene expression associated with broad functional activities. The three agonists used also caused down-regulation of genes that regulate angiogenesis and drug metabolism. With a single exception, C5a caused little evidence of activation of complement-related genes. These studies indicate that endothelial cells respond robustly to C5a by activation of genes related to progressive expression of cell adherence molecules, and cytokines and chemokines in a manner similar to responses induced by tumor necrosis factor-alpha and lipopolysaccharide.

Characterization of the rat A2A adenosine receptor gene: a 4.8-kb promoter-proximal DNA fragment confers selective expression in the central nervous system

We isolated and characterized a 4.8-kb 5' flanking region of the rat A2A adenosine receptor (A2A-R) gene in the present study. Promoter activity was observed with this DNA fragment in PC12 cells and C6 cells which contain endogenous A2A-Rs. A fusion fragment consisting of the 4.8-kb promoter-proximal DNA fragment of the A2A-R gene, and the coding region of lacZ was utilized to produce mice harbouring the fusion gene. In three independent founder lines, proteins and transcripts of the transgene were found in many areas of the central nervous system (CNS), but not in three peripheral tissues examined. Double immunohistochemical analyses revealed that the transgene was coexpressed with endogenous A2A-R and proper neuronal markers in the brain. Specifically, the transgene in the striatum was found in the enkephalin-containing GABAergic neurons and in the cholinergic neurons as was found for the endogenous A2A-R. However, a selectively enriched striatal expression of the transgene was not found as was observed for the endogenous A2A-R. Collectively, the 4.8-kb promoter-proximal DNA fragment of the rat A2A-R gene contains important element(s) to direct its expression in the CNS where functional A2A-R are found, but were not sufficient to confer the highly concentrated expression of the striatal A2A-R. Furthermore, expressions of A2A-R and the transgene were found in both neurons and astrocytes, suggesting that adenosine might mediate its function through A2A-R in both cell types.

Human-specific regulation of alpha 2-6-linked sialic acids

Many microbial pathogens and toxins recognize animal cells via cell surface sialic acids (Sias) that are alpha 2-3- or alpha 2-8-linked to the underlying glycan chain. Human influenza A/B viruses are unusual in preferring alpha 2-6-linked Sias, undergoing a switch from alpha 2-3 linkage preference during adaptation from animals to humans. This correlates with the expression of alpha 2-6-linked Sias on ciliated human airway epithelial target cells and of alpha 2-3-linked Sias on secreted soluble airway mucins, which are unable to inhibit virus binding. Given several known differences in Sia biology between humans and apes, we asked whether this pattern of airway epithelial Sia linkages is also human-specific. Indeed, we show that since the last common ancestor with apes, humans underwent a concerted bidirectional switch in alpha 2-6-linked Sia expression between airway epithelial cell surfaces and secreted mucins. This can explain why the chimpanzee appears relatively resistant to experimental infection with human Influenza viruses. Other tissues showed additional examples of human-specific increases or decreases in alpha 2-6-linked Sia expression and only one example of a change specific to certain great apes. Furthermore, while human and great ape leukocytes both express alpha 2-6-linked Sias, only human erythrocytes have markedly up-regulated expression. These cell type-specific changes in alpha 2-6-Sia expression during human evolution represent another example of a human-specific change in Sia biology. Because the data set involves multiple great apes, we can also conclude that Sia linkage expression patterns can be conserved during millions of years of evolution within some vertebrate taxa while undergoing sudden major changes in other closely related ones.

Enhanced expression of alpha 2,6-sialyltransferase ST6Gal I in cervical squamous cell carcinoma

OBJECTIVE

Increased messenger ribonucleic acid (mRNA) expression of beta-galactoside alpha 2,6-sialyltransferase I (ST6Gal I) is important in squamous cell carcinoma (SCC) of the cervix. In many tissues, ST6Gal I is transcriptionally regulated through the use of promoters that originate in the mRNA species that diverge in the 5'-untranslated regions. To clarify the roles of ST6Gal I mRNA species in cervical SCC, we investigated their expression, including a "constitutive" promoter (placental or Y + Z form), "hepatic" promoter (H form), and a specific lymphoblastic promoter (X form), in normal and SCC tissues of the cervix using real-time quantitative reverse-transcription polymerase chain reaction (RT-PCR).

METHODS

Expression of the ST6Gal I species was investigated in normal cervical tissue samples (n = 38) and FIGO IB1 cervical SCC samples (n = 38) by relatively quantitative real-time RT-PCR, using primers designed for amplification of a portion of the coding region common to all mRNA species or ones for amplification of the placental transcript, the hepatic transcript, or lymphoblastic transcript.

RESULTS

ST6Gal I mRNA expression was significantly increased in cancerous tissues compared to that in normal tissues (P = 0.004, Mann-Whitney U test; P < 0.001, paired t test). Expression of the Y + Z form did not appear to be affected by cancer transformation, since it was detected at comparable levels in normal and cancerous tissues (P = 0.986), but H form expression was significantly enhanced in cancerous tissues compared to that in normal tissues (P < 0.001, Mann-Whitney U test and paired t test). Surprisingly, the X form could be detected in some patients with and without cancer, but the detection rate was significantly higher in patients with cancer (86.8% vs 52.6%, respectively; P = 0.021, Fisher's exact test). Although the X transcript was detected at a low level compared to the H and Y + Z transcripts, its expression was also significantly enhanced in patients with cancers compared to those without cancers (P < 0.001, Mann-Whitney U test and paired t test).

CONCLUSIONS

An increased level of hepatic transcripts may be important in cancer transformation because the transcripts contribute to enhance ST6Gal I expression in cancerous tissues.

Transcriptional regulation of human Galbeta1,3GalNAc/Galbeta1, 4GlcNAc alpha2,3-sialyltransferase (hST3Gal IV) gene in testis and ovary cell lines

The mRNA expression of sialyltransferase genes is regulated in a cell-type-specific manner. The mRNAs of human Galbeta1, 3GalNAc/Galbeta1, 4 GlcNAc alpha2,3-sialyltransferase gene (hST3Gal IV) consist of six isoforms, type A1, A2, B1, B2, B3, and BX. These mRNAs are transcribed from different promoters, pA, pB1, pB2, pB3, and pBX, respectively. Type B mRNAs are expressed in several cells, whereas type A mRNAs are specifically expressed in testis, ovary, and placenta, suggesting that pA promoter activity is especially high in these tissues. We show herein germ-cell specific transcriptional regulation of the hST3Gal IV pA promoter. Using a luciferase assay, pA promoter activity is shown to be high in testis and ovary cell lines. We identified the enhancer region of the pA promoter, located at nt -520 to -420. These results suggest that this element plays a critical role in germ-cell specific regulation of the pA promoter. The results of site-directed mutagenesis suggest that AP2 and c-Ets sites in this region are involved in pA promoter activity, which in turn suggests that the hST3Gal IV gene is regulated in a tissue-restricted fashion at the level of transcription.

Constitutively unmasked CD22 on B cells of ST6Gal I knockout mice: novel sialoside probe for murine CD22

The interaction of CD22 with glycoprotein ligands bearing the Siaalpha2,6Gal-R sequence is believed to modulate its function as a regulator of B cell signaling. Although a commercial sialoside-polyacrylamide (PAA) probe, NeuAc- alpha2,6Gal-PAA, has facilitated studies on ligand binding by human CD22, murine CD22 binds instead with high affinity to NeuGcalpha2,6Gal-R. A multivalent probe with this sequence was constructed to facilitate investigations of ligand binding in CD22 function using genetically defined murine models. The probe is based on the sialoside-PAA platform, which is then biotinylated for easy detection. A series of sialoside probes were constructed with two different length linker arms between the sialoside and the backbone and three different sialoside to PAA molar ratios. The NeuGcalpha2,6Gal-PAA probe is specific for CD22: it binds to sialidase-treated B cells of wild-type mice but not B cells of CD22-null mice. Additionally, because the probe only binds to sialidase-treated wild-type cells, it confirms that CD22 is constitutively "masked" on most B cells from wild-type mice by binding to ligands in cis. In contrast, the probe bound equally well to native or sialidase-treated B cells from the immunocompromised ligand-deficient ST6Gal I knockout mice, demonstrating that CD22 is constitutively "unmasked" in these cells.

The human sialyltransferase family

The human genome encodes probably more than 20 different sialyltransferases involved in the biosynthesis of sialylated glycoproteins and glycolipids but to date only 15 different human sialyltransferase cDNAs have been cloned and characterized. Each of the sialyltransferase genes is differentially expressed in a tissue-, cell type-, and stage-specific manner to regulate the sialylation pattern of cells. These enzymes differ in their substrate specificity, tissue distribution and various biochemical parameters. However, enzymatic analysis conducted in vitro with recombinant enzyme revealed that one linkage can be synthesized by multiple enzymes. We present here an overview of these human genes and enzymes, the regulation of their occurrence and their involvement in several physiological and pathological processes.

The sialyl-alpha2,6-lactosaminyl-structure: biosynthesis and functional role

Sialylation represents one of the most frequently occurring terminations of the oligosaccharide chains of glycoproteins and glycolipids. Sialic acid is commonly found alpha2,3- or alpha2,6-linked to galactose (Gal), alpha2,6-linked to N-acetylgalactosamine (GalNAc) or alpha2,8-linked to another sialic acid. The biosynthesis of the various linkages is mediated by the different members of the sialyltransferase family. The addition of sialic acid in alpha2,6-linkage to the galactose residue of lactosamine (type 2 chains) is catalyzed by beta-galactoside alpha2,6-sialyltransferase (ST6Gal.I). Although expressed by a single gene, this enzyme shows a complex pattern of regulation which allows its tissue- and stage-specific modulation. The cognate oligosaccharide structure, NeuAcalpha2,6Galbeta1,4GlcNAc, is widely distributed among tissues and is involved in biological processes such as the regulation of the immune response and the progression of colon cancer. This review summarizes the current knowledge on the biochemistry of ST6Gal.I and on the functional role of the sialyl-alpha2,6-lactosaminyl structure.

Isolation and characterization of plant N-acetyl glucosaminyltransferase I (GntI) cDNA sequences. Functional analyses in the Arabidopsis cgl mutant and in antisense plants

We report on the isolation and characterization of full-length cDNA sequences coding for N-acetylglucosaminyltransferase I (GnTI) from potato (Solanum tuberosum L.), tobacco (Nicotiana tabacum L.), and Arabidopsis. The deduced polypeptide sequences show highest homology among the solanaceous species (93% identity between potato and tobacco compared with about 75% with Arabidopsis) but share only weak homology with human GnTI (35% identity). In contrast to the corresponding enzymes from animals, all plant GnTI sequences identified are characterized by a much shorter hydrophobic membrane anchor and contain one putative N-glycosylation site that is conserved in potato and tobacco, but differs in Arabidopsis. Southern-blot analyses revealed that GntI behaves as a single-copy gene. Northern-blot analyses showed that GntI-mRNA expression is largely constitutive. Arabidopsis cgl mutants deficient in GnTI activity also possess GntI mRNA, indicating that they result from point mutations. GntI-expression constructs were tested for the ability to relieve the GnTI block in protoplasts of the Arabidopsis cgl mutant and used to obtain transgenic potato and tobacco plants that display a substantial reduction of complex glycan patterns. The latter observation indicates that production of heterologous glycoproteins with little or no antigenic glycans can be achieved in whole plants, and not in just Arabidopsis, using antisense technology.