Ride to cell wall: Arabidopsis XTH11, XTH29 and XTH33 exhibit different secretion pathways and responses to heat and drought stress

The xyloglucan endotransglucosylase/hydrolases (XTHs) are enzymes involved in cell wall assembly and growth regulation, cleaving and re-joining hemicellulose chains in the xyloglucan-cellulose network. Here, in a homologous system, we compare the secretion patterns of XTH11, XTH33 and XTH29, three members of the Arabidopsis thaliana XTH family, selected for the presence (XTH11 and XTH33) or absence (XTH29) of a signal peptide, and the presence of a transmembrane domain (XTH33). We show that XTH11 and XTH33 reached, respectively, the cell wall and plasma membrane through a conventional protein secretion (CPS) pathway, whereas XTH29 moves towards the apoplast following an unconventional protein secretion (UPS) mediated by exocyst-positive organelles (EXPOs). All XTHs share a common C-terminal functional domain (XET-C) that, for XTH29 and a restricted number of other XTHs (27, 28 and 30), continues with an extraterminal region (ETR) of 45 amino acids. We suggest that this region is necessary for the correct cell wall targeting of XTH29, as the ETR-truncated protein never reaches its final destination and is not recruited by EXPOs. Furthermore, quantitative real-time polymerase chain reaction analyses performed on 4-week-old Arabidopsis seedlings exposed to drought and heat stress suggest a different involvement of the three XTHs in cell wall remodeling under abiotic stress, evidencing stress-, organ- and time-dependent variations in the expression levels. Significantly, XTH29, codifying the only XTH that follows a UPS pathway, is highly upregulated with respect to XTH11 and XTH33, which code for CPS-secreted proteins.

Cell wall modification by the xyloglucan endotransglucosylase/hydrolase XTH19 influences freezing tolerance after cold and sub-zero acclimation

Freezing triggers extracellular ice formation leading to cell dehydration and deformation during a freeze-thaw cycle. Many plant species increase their freezing tolerance during exposure to low, non-freezing temperatures, a process termed cold acclimation. In addition, exposure to mild freezing temperatures after cold acclimation evokes a further increase in freezing tolerance (sub-zero acclimation). Previous transcriptome and proteome analyses indicate that cell wall remodelling may be particularly important for sub-zero acclimation. In the present study, we used a combination of immunohistochemical, chemical and spectroscopic analyses to characterize the cell walls of Arabidopsis thaliana and characterized a mutant in the XTH19 gene, encoding a xyloglucan endotransglucosylase/hydrolase (XTH). The mutant showed reduced freezing tolerance after both cold and sub-zero acclimation, compared to the Col-0 wild type, which was associated with differences in cell wall composition and structure. Most strikingly, immunohistochemistry in combination with 3D reconstruction of centres of rosette indicated that epitopes of the xyloglucan-specific antibody LM25 were highly abundant in the vasculature of Col-0 plants after sub-zero acclimation but absent in the XTH19 mutant. Taken together, our data shed new light on the potential roles of cell wall remodelling for the increased freezing tolerance observed after low temperature acclimation.

Plant Xyloglucan Xyloglucosyl Transferases and the Cell Wall Structure: Subtle but Significant

Plant xyloglucan xyloglucosyl transferases or xyloglucan endo-transglycosylases (XET; EC 2.4.1.207) catalogued in the glycoside hydrolase family 16 constitute cell wall-modifying enzymes that play a fundamental role in the cell wall expansion and re-modelling. Over the past thirty years, it has been established that XET enzymes catalyse homo-transglycosylation reactions with xyloglucan (XG)-derived substrates and hetero-transglycosylation reactions with neutral and charged donor and acceptor substrates other than XG-derived. This broad specificity in XET isoforms is credited to a high degree of structural and catalytic plasticity that has evolved ubiquitously in algal, moss, fern, basic Angiosperm, monocot, and eudicot enzymes. These XET isoforms constitute gene families that are differentially expressed in tissues in time- and space-dependent manners during plant growth and development, and in response to biotic and abiotic stresses. Here, we discuss the current state of knowledge of broad specific plant XET enzymes and how their inherently carbohydrate-based transglycosylation reactions tightly link with structural diversity that underlies the complexity of plant cell walls and their mechanics. Based on this knowledge, we conclude that multi- or poly-specific XET enzymes are widespread in plants to allow for modifications of the cell wall structure in muro, a feature that implements the multifaceted roles in plant cells.

The brassinosteroid-responsive xyloglucan endotransglucosylase/hydrolase 19 (XTH19) and XTH23 genes are involved in lateral root development under salt stress in Arabidopsis

Lateral roots (LRs) are the main component of the root system architecture in Arabidopsis. The plasticity of LR development has an important role in improving plant survival in response to the external environment. Previous studies have revealed a number of genetic pathways that control plant growth in response to environmental stimuli. Here, we find that the xyloglucan endotransglucosylase 19 (XTH19) and XTH23 genes are involved in LR development under salt stress. The density of LRs was decreased in the xth23 single mutant, which was also more sensitive to salt than the wild type, and the xth19xth23 double mutant exhibited additive downregulated LR initiation and salt sensitivity compared with the single mutant. On the contrary, constitutive overexpression of XTH19 or XTH23 caused increased LR densities. Furthermore, XTH19 and XTH23 were induced by salt via the key brassinosteroid signaling pathway transcription factor BES1. In addition, we found that 35S::BES1 increased salt tolerance and the phenotype of xth19xth23 & 35S::BES1 was partially complementary to the wild-type level. In vivo and in vitro assays demonstrated that BES1 acts directly upstream of XTH19 and XTH23 to control their expression. Overall, our results revealed that XTH19 and XTH23 are involved in LR development via the BES1-dependent pathway, and contribute to LR adaptation to salt.

Bump-and-Hole Engineering Identifies Specific Substrates of Glycosyltransferases in Living Cells

Studying posttranslational modifications classically relies on experimental strategies that oversimplify the complex biosynthetic machineries of living cells. Protein glycosylation contributes to essential biological processes, but correlating glycan structure, underlying protein, and disease-relevant biosynthetic regulation is currently elusive. Here, we engineer living cells to tag glycans with editable chemical functionalities while providing information on biosynthesis, physiological context, and glycan fine structure. We introduce a non-natural substrate biosynthetic pathway and use engineered glycosyltransferases to incorporate chemically tagged sugars into the cell surface glycome of the living cell. We apply the strategy to a particularly redundant yet disease-relevant human glycosyltransferase family, the polypeptide N-acetylgalactosaminyl transferases. This approach bestows a gain-of-chemical-functionality modification on cells, where the products of individual glycosyltransferases can be selectively characterized or manipulated to understand glycan contribution to major physiological processes.

Bump-and-Hole Engineering Identifies Specific Substrates of Glycosyltransferases in Living Cells

Studying posttranslational modifications classically relies on experimental strategies that oversimplify the complex biosynthetic machineries of living cells. Protein glycosylation contributes to essential biological processes, but correlating glycan structure, underlying protein, and disease-relevant biosynthetic regulation is currently elusive. Here, we engineer living cells to tag glycans with editable chemical functionalities while providing information on biosynthesis, physiological context, and glycan fine structure. We introduce a non-natural substrate biosynthetic pathway and use engineered glycosyltransferases to incorporate chemically tagged sugars into the cell surface glycome of the living cell. We apply the strategy to a particularly redundant yet disease-relevant human glycosyltransferase family, the polypeptide N-acetylgalactosaminyl transferases. This approach bestows a gain-of-chemical-functionality modification on cells, where the products of individual glycosyltransferases can be selectively characterized or manipulated to understand glycan contribution to major physiological processes.

The UGTome: The expanding diversity of UDP glycosyltransferases and its impact on small molecule metabolism

The UDP glycosyltransferase (UGT) superfamily of enzymes is responsible for the metabolism and clearance of thousands of lipophilic chemicals including drugs, toxins and endogenous signaling molecules. They provide a protective interface between the organism and its chemical-rich environment, as well as controlling critical signaling pathways to maintain healthy tissue function. UGTs are associated with drug responses and interactions, as well as a wide range of diseases including cancer. The human genome contains 22 UGT genes; however as befitting their exceptionally diverse substrate ranges and biological activities, the output of these UGT genes is functionally diversified by multiple processes including alternative splicing, post-translational modification, homo- and hetero-oligomerization, and interactions with other proteins. All UGT genes are subject to extensive alternative splicing generating variant/truncated UGT proteins with altered functions including the capacity to dominantly modulate/inhibit cognate full-length forms. Heterotypic oligomerization of different UGTs can alter kinetic properties relative to monotypic complexes, and potentially produce novel substrate specificities. Moreover, the recently profiled interactions of UGTs with non-UGT proteins may facilitate coordination between different metabolic processes, as well as providing opportunities for UGTs to engage in novel 'moonlighting' functions. Herein we provide a detailed and comprehensive review of all known modes of UGT functional diversification and propose a UGTome model to describe the resulting expansion of metabolic capacity and its potential to modulate drug/xenobiotic responses and cell behaviours in normal and disease contexts.

Diverse biological effects of glycosyltransferase genes from Tartary buckwheat

BACKGROUND

Tartary buckwheat (Fagopyrum tataricum) is an edible cereal crop whose sprouts have been marketed and commercialized for their higher levels of anti-oxidants, including rutin and anthocyanin. UDP-glucose flavonoid glycosyltransferases (UFGTs) play an important role in the biosynthesis of flavonoids in plants. So far, few studies are available on UFGT genes that may play a role in tartary buckwheat flavonoids biosynthesis. Here, we report on the identification and functional characterization of seven UFGTs from tartary buckwheat that are potentially involved in flavonoid biosynthesis (and have varying effects on plant growth and development when overexpressed in Arabidopsis thaliana.) RESULTS: Phylogenetic analysis indicated that the potential function of the seven FtUFGT proteins, FtUFGT6, FtUFGT7, FtUFGT8, FtUFGT9, FtUFGT15, FtUFGT40, and FtUFGT41, could be divided into three Arabidopsis thaliana functional subgroups that are involved in flavonoid biosynthesis of and anthocyanin accumulation. A significant positive correlation between FtUFGT8 and FtUFGT15 expression and anthocyanin accumulation capacity was observed in the tartary buckwheat seedlings after cold stress. Overexpression in Arabidopsis thaliana showed that FtUFGT8, FtUFGT15, and FtUFGT41 significantly increased the anthocyanin content in transgenic plants. Unexpectedly, overexpression of FtUFGT6, while not leading to enhanced anthocyanin accumulation, significantly enhanced the growth yield of transgenic plants. When wild-type plants have only cotyledons, most of the transgenic plants of FtUFGT6 had grown true leaves. Moreover, the growth speed of the oxFtUFGT6 transgenic plant root was also significantly faster than that of the wild type. At later growth, FtUFGT6 transgenic plants showed larger leaves, earlier twitching times and more tillers than wild type, whereas FtUFGT15 showed opposite results.

CONCLUSIONS

Seven FtUFGTs were isolated from tartary buckwheat. FtUFGT8, FtUFGT15, and FtUFGT41 can significantly increase the accumulation of total anthocyanins in transgenic plants. Furthermore, overexpression of FtUFGT6 increased the overall yield of Arabidopsis transgenic plants at all growth stages. However, FtUFGT15 shows the opposite trend at later growth stage and delays the growth speed of plants. These results suggested that the biological function of FtUFGT genes in tartary buckwheat is diverse.

Molecular insights of a xyloglucan endo-transglycosylase/hydrolase of radiata pine (PrXTH1) expressed in response to inclination: Kinetics and computational study

Xyloglucan endotransglycosylase/hydrolases (XTH) may have endotransglycosylase (XET) and/or hydrolase (XEH) activities. Previous studies confirmed XET activity for PrXTH1 protein from radiata pine. XTHs could interact with many hemicellulose substrates, but the favorite substrate of PrXTH1 is still unknown. The prediction of union type and energy stability of the complexes formed between PrXTH1 and different substrates (XXXGXXXG, XXFGXXFG, XLFGXLFG and cellulose) were determined using bioinformatics tools. Molecular Docking, Molecular Dynamics, MM-GBSA and Electrostatic Potential Calculations were employed to predict the binding modes, free energies of interaction and the distribution of electrostatic charge. The results suggest that the enzyme formed more stable complexes with hemicellulose substrates than cellulose, and the best ligand was the xyloglucan XLFGXLFG (free energy of -58.83 ± 0.8 kcal mol^-1). During molecular dynamics trajectories, hemicellulose fibers showed greater stability than cellulose. Aditionally, the kinetic properties of PrXTH1 enzyme were determined. The recombinant protein was active and showed an optimal pH 5.0 and optimal temperature of 37 °C. A Km value of 20.9 mM was determined for xyloglucan oligomer. PrXTH1 is able to interact with different xyloglycans structures but no activity was observed for cellulose as substrate, remodeling cell wall structure in response to inclination.

Ethylene promotes cadmium-induced root growth inhibition through EIN3 controlled XTH33 and LSU1 expression in Arabidopsis

Cadmium (Cd) stress is one of the most serious heavy metal stresses limiting plant growth and development. However, the molecular mechanisms underlying Cd-induced root growth inhibition remain unclear. Here, we found that ethylene signalling positively regulates Cd-induced root growth inhibition. Arabidopsis seedlings pretreated with the ethylene precursor 1-aminocyclopropane-1-carboxylic acid exhibited enhanced Cd-induced root growth inhibition, whereas the addition of the ethylene biosynthesis inhibitor aminoethoxyvinyl glycine decreased Cd-induced root growth inhibition. Consistently, ethylene-insensitive mutants, such as ein4-1, ein3-1 eil1-1 double mutant, and EBF1ox, displayed an increased tolerance to Cd. Furthermore, we also observed that Cd inhibited EIN3 protein degradation, a process that was regulated by ethylene signalling. Genetic and biochemical analyses showed that EIN3 enhanced root growth inhibition under Cd stress through direct binding to the promoters and regulating the expression of XTH33 and LSU1, which encode key regulators of cell wall extension and sulfur metabolic process, respectively. Collectively, our study demonstrates that ethylene plays a positive role in Cd-regulated root growth inhibition through EIN3-mediated transcriptional regulation of XTH33 and LSU1 and provides a molecular framework for the integration of environmental signals and intrinsic regulators in modulating plant root growth.

Pectin RG-I rhamnosyltransferases represent a novel plant-specific glycosyltransferase family

Pectin is one of the three key cell wall polysaccharides in land plants and consists of three major structural domains: homogalacturonan, rhamnogalacturonan I (RG-I) and RG-II. Although the glycosyltransferase required for the synthesis of the homogalacturonan and RG-II backbone was identified a decade ago, those for the synthesis of the RG-I backbone, which consists of the repeating disaccharide unit [→2)-α-L-Rha-(1 → 4)-α-D-GalUA-(1→], have remained unknown. Here, we report the identification and characterization of Arabidopsis RG-I:rhamnosyltransferases (RRTs), which transfer the rhamnose residue from UDP-β-L-rhamnose to RG-I oligosaccharides. RRT1, which is one of the four Arabidopsis RRTs, is a single-spanning transmembrane protein, localized to the Golgi apparatus. RRT1 was highly expressed during formation of the seed coat mucilage, which is a specialized cell wall with abundant RG-I. Loss-of-function mutation in RRT1 caused a reduction in the level of RG-I in the seed coat mucilage. The RRTs belong to a novel glycosyltransferase family, now designated GT106. This is a large plant-specific family, and glycosyltransferases in this family seem to have plant-specific roles, such as biosynthesis of plant cell wall polysaccharides.

A mechanism for bistability in glycosylation

Glycosyltransferases are a class of enzymes that catalyse the posttranslational modification of proteins to produce a large number of glycoconjugate acceptors from a limited number of nucleotide-sugar donors. The products of one glycosyltransferase can be the substrates of several other enzymes, causing a combinatorial explosion in the number of possible glycan products. The kinetic behaviour of systems where multiple acceptor substrates compete for a single enzyme is presented, and the case in which high concentrations of an acceptor substrate are inhibitory as a result of abortive complex formation, is shown to result in non-Michaelian kinetics that can lead to bistability in an open system. A kinetic mechanism is proposed that is consistent with the available experimental evidence and provides a possible explanation for conflicting observations on the β-1,4-galactosyltransferases. Abrupt switching between steady states in networks of glycosyltransferase-catalysed reactions may account for the observed changes in glycosyl-epitopes in cancer cells.

Activity of xyloglucan endotransglucosylases/hydrolases suggests a role during host invasion by the parasitic plant Cuscuta reflexa

The parasitic vines of the genus Cuscuta form haustoria that grow into other plants and connect with their vascular system, thus allowing the parasite to feed on its host. A major obstacle that meets the infection organ as it penetrates the host tissue is the rigid plant cell wall. In the present study, we examined the activity of xyloglucan endotransglucosylases/hydrolases (XTHs) during the host-invasive growth of the haustorium. The level of xyloglucan endotransglucosylation (XET) activity was found to peak at the penetrating stage of Cuscuta reflexa on its host Pelargonium zonale. In vivo colocalization of XET activity and donor substrate demonstrated XET activity at the border between host and parasite. A test for secretion of XET-active enzymes from haustoria of C. reflexa corroborated this and further indicated that the xyloglucan-modifying enzymes originated from the parasite. A known inhibitor of XET, Coomassie Brilliant Blue R250, was shown to reduce the level of XET in penetrating haustoria of C. reflexa. Moreover, the coating of P. zonale petioles with the inhibitor compound lowered the number of successful haustorial invasions of this otherwise compatible host plant. The presented data indicate that the activity of Cuscuta XTHs at the host-parasite interface is essential to penetration of host plant tissue.

Xyloglucan endo-transglycosylase/hydrolase (XET/H) gene is expressed during the seed germination in Podophyllum hexandrum: a high altitude Himalayan plant

Xyloglucan endo-transglycosylase/hydrolase ( Ph XET/H) regulates Podophyllum seed germination via GA mediated up-accumulation of Ph XET protein and subsequent endosperm weakening. Xyloglucan endo-transglycosylase/hydrolase (XET/H) belong to glycosyl hydrolase family 16, which play an important role in endosperm weakening and embryonic expansion during seed germination. Podophyllum hexandrum is a high altitude medicinal plant exploited for its etoposides which are potential anticancer compounds. During seed germination in Podophyllum, accumulation of XET/H transcripts was recorded. This data confirmed its possible role in determining the fate of seed for germination. Full length cDNA of a membrane bound XET/H (here onwards PhXET) was cloned from the germinating seeds of Podophyllum. Analysis of nucleotide sequence revealed PhXET with an open reading frame of 720 bp encoding a protein of 239 amino acids with a molecular mass of 28 kDa and pI of 7.58. In silico structure prediction of PhXET showed homology with that of Populus tremula (1UN1). PhXET was predicted to have a potential GPI-anchor domain and was located in plasma membrane. It was found that the exogenously applied phytohormones (GA and ABA) regulate the expression of PhXET. The obtained data showed that the PhXET regulates seed germination in Podophyllum by supplementing its activity along with other endosperm weakening and embryo expansion genes.

Two sides of the same coin: Xyloglucan endotransglucosylases/hydrolases in host infection by the parasitic plant Cuscuta

The holoparasitic angiosperm Cuscuta develops haustoria that enable it to feed on other plants. Recent findings corroborate the long-standing theory that cell wall modifications are required in order for the parasite to successfully infect a host, and further suggest that changes to xyloglucan through the activity of xyloglucan endotransglucosylases/hydrolases (XTHs) are essential. On the other hand, XTH expression was also detected in resistant tomato upon an attack by Cuscuta, which suggests that both host and parasite use these enzymes in their "arms race." Here, we summarize existing data on the cell wall-modifying activities of XTHs during parasitization and present a model suggesting how XTHs might function to make the host's resources accessible to Cuscuta.

The Polymerization of Aeromonas hydrophila AH-3 O-Antigen LPS: Concerted Action of WecP and Wzy

The repeat units of heteropolymeric O antigen are synthesized at the cytosolic side of the inner bacterial membrane via the Wzx/Wzy-dependent assembly pathway. After being translocated across the membrane by Wzx, each repeat unit is polymerized by Wzy to form a glycan chain. In this study, we demonstrate the need of the corresponding enzyme transferring the initial HexNAc to undecaprenol phosphate (lipid carrier), together with the corresponding O-antigen polymerase (Wzy), to produce the Aeromonas hydrophila O:34-antigen. We suggest, the concerted action of WecA or P enzyme (UDP-HexNAc: polyprenol-P HexNAc-1-P transferase) and Wzy is involved in the mechanism responsible for the A. hydrophila O-antigen polymerization.

Cryptogein-induced transcriptional reprogramming in tobacco is light dependent

The fungal elicitor cryptogein triggers a light-dependent hypersensitive response in tobacco (Nicotiana tabacum). To assess the effect of light on this nonhost resistance in more detail, we studied various aspects of the response under dark and light conditions using the tobacco-cryptogein experimental system. Here, we show that light drastically alters the plant's transcriptional response to cryptogein, notably by dampening the induction of genes involved in multiple processes, such as ethylene biosynthesis, secondary metabolism, and glutathione turnover. Furthermore, chlorophyll fluorescence measurements demonstrated that quantum yield and functioning of the light-harvesting antennae decreased simultaneously, indicating that photoinhibition underlies the observed decreased photosynthesis and that photooxidative damage might be involved in the establishment of the altered response. Analysis of the isomer distribution of hydroxy fatty acids illustrated that, in the light, lipid peroxidation was predominantly due to the production of singlet oxygen. Differences in (reduced) glutathione concentrations and the rapid development of symptoms in the light when cryptogein was coinfiltrated with glutathione biosynthesis inhibitors suggest that glutathione might become a limiting factor during the cryptogein-induced hypersensitive response in the dark and that this response might be modified by an increased antioxidant availability in the light.

Xyloglucan endotransglucosylase/hydrolase (XTH) overexpression affects growth and cell wall mechanics in etiolated Arabidopsis hypocotyls

Growth and biomechanics of etiolated hypocotyls from Arabidopsis thaliana lines overexpressing xyloglucan endotransglucosylase/hydrolase AtXTH18, AtXTH19, AtXTH20, and PttXET16-34 were studied. Overexpression of AtXTH18, AtXTH19, and AtXTH20 stimulated growth of hypocotyls, while PttXET16-34 overexpression did not show this effect. In vitro extension of frozen/thawed hypocotyls measured by a constant-load extensiometer started from a high-amplitude initial deformation followed by a slow time-dependent creep. Creep of growing XTH-overexpressing (OE) hypocotyls was more linear in time compared with the wild type at pH 5.0, reflecting their higher potential for long-term extension. XTH-OE plants deposited 65-84% more cell wall material per hypocotyl cross-sectional area than wild-type plants. As a result, their wall stress under each external load was lower than in the wild-type. Growing XTH-OE hypocotyls had higher values of initial deformation·stress(-1) compared with the wild type. Plotting creep rates for each line under different loads against the respective wall stress values gave straight lines. Their slopes and intercepts with the abscissa correspond to ϕ (in vitro cell wall extensibility) and y (in vitro cell wall yield threshold) values characterizing cell wall material properties. The wall material in XTH-OE lines was more pliant than in the wild type due to lower y values. In contrast, the acid-induced wall extension in vitro resulted from increasing ϕ values. Thus, three factors contributed to the XTH-OE-stimulated growth in Arabidopsis hypocotyls: their more linear creep, higher values of initial deformation·stress(-1), and lower y values.

Loss of LARGE2 disrupts functional glycosylation of α-dystroglycan in prostate cancer

Dystroglycan (DG) is a cell surface receptor for extracellular matrix proteins and is involved in cell polarity, matrix organization, and mechanical stability of tissues. Previous studies documented loss of DG protein expression and glycosylation in a variety of cancer types, but the underlying mechanisms and the functional consequences with respect to cancer progression remain unclear. Here, we show that the level of expression of the βDG subunit as well as the glycosylation status of the αDG subunit inversely correlate with the Gleason scores of prostate cancers; furthermore, we show that the functional glycosylation of αDG is substantially reduced in prostate cancer metastases. Additionally, we demonstrate that LARGE2 (GYLTL1B), a gene not previously implicated in cancer, regulates functional αDG glycosylation in prostate cancer cell lines; knockdown of LARGE2 resulted in hypoglycosylation of αDG and loss of its ability to bind laminin-111 while overexpression restored ligand binding and diminished growth and migration of an aggressive prostate cancer cell line. Finally, our analysis of LARGE2 expression in human cancer specimens reveals that LARGE2 is significantly down-regulated in the context of prostate cancer, and that its reduction correlates with disease progression. Our results describe a novel molecular mechanism to account for the commonly observed hypoglycosylation of αDG in prostate cancer.

N-glycans in cell survival and death: cross-talk between glycosyltransferases

Asparagine-linked (N-linked) protein glycosylation is one of the most important protein modifications. N-glycans with "high mannose", "hybrid", or "complex" type sugar chains participate in a multitude of cellular processes. These include cell-cell/cell-matrix/receptor-ligand interaction, cell signaling/growth and differentiation, to name a few. Many diseases such as disorders of blood clotting, congenital disorder of glycosylation, diseases of blood vessels, cancer, neo-vascularization, i.e., angiogenesis essential for breast and other solid tumor progression and metastasis are associated with N-glycan expression. Biosynthesis of N-glycans requires multiple steps and multiple cellular compartments. Following transcription and translation the proteins migrate to the endoplasmic reticulum (ER) lumen to acquire glycan chain(s) with a defined glycoform, i.e., a tetradecasaccharide. These are further modified, i.e., edited in ER lumen and in Golgi prior to moving to their respective destinations. The tetradecasaccharide is pre-assembled on a poly-isoprenoid lipid called dolichol, and becomes an essential component of the supply chain. Therefore, dolichol cycle synthesizing the lipid-linked oligosaccharide (LLO) is a hallmark for all N-linked glycoproteins. It is expected that there is a great deal of cross-talk between the participating glycosyltransferases and any missed step would express defective N-glycans that could have fatal consequences. The positive impact of the structurally altered N-glycans could lead to discovery of an N-glycan signature for a disease and/or help developing glycotherapeutic treating cancer or other human diseases. The purpose of this review is to identify the gaps of N-glycan biology and help developing appropriate technology for biomedical applications. This article is part of a Special Issue entitled Glycoproteomics.

[Biosynthesis mechanisms of bacterial polysaccharide--a review]

Bacterial polysaccharides play important roles both in medicine and industry. Due to the development of bacterial genome sequencing, many gene clusters related to the biosynthesis of bacterial polysaccharide have been found, aligned and analyzed. Despite of their complex composition and structures, different bacterial polysaccharides are biosynthesized via similar pathways. This review discussed the research development of the biosynthetic mechanism of different bacterial polysaccharides, with emphasis on the glycosyltransferases and polymerases involved in the biosynthetic pathway.

Differential axial requirements for lunatic fringe and Hes7 transcription during mouse somitogenesis

Vertebrate segmentation is regulated by the “segmentation clock”, which drives cyclic expression of several genes in the caudal presomitic mesoderm (PSM). One such gene is Lunatic fringe (Lfng), which encodes a modifier of Notch signalling, and which is also expressed in a stripe at the cranial end of the PSM, adjacent to the newly forming somite border. We have investigated the functional requirements for these modes of Lfng expression during somitogenesis by generating mice in which Lfng is expressed in the cranial stripe but strongly reduced in the caudal PSM, and find that requirements for Lfng activity alter during axial growth. Formation of cervical, thoracic and lumbar somites/vertebrae, but not sacral and adjacent tail somites/vertebrae, depends on caudal, cyclic Lfng expression. Indeed, the sacral region segments normally in the complete absence of Lfng and shows a reduced requirement for another oscillating gene, Hes7, indicating that the architecture of the clock alters as segmentation progresses. We present evidence that Lfng controls dorsal-ventral axis specification in the tail, and also suggest that Lfng controls the expression or activity of a long-range signal that regulates axial extension.

[Deletion mutation of rbfCxoo, encoding a putative glycosyltransferase in Xanthomonas oryzae pv. oryzae leads to enhanced virulence expression]

OBJECTIVE

To better understand the structure and biological function of rbfCxoo, a gene with the putative function in lipopolysaccharide O-antigen synthesis in Xanthomonas oryzae pv. oryzae (Xoo), the causal pathogen of bacterial blight of rice.

METHODS

The molecular identification and function analysis of rbfCxoo were performed through gene cloning, sequencing and deletion analysis.

RESULTS

The sequence of rbfCxoo cloned from the genomic DNA of wild-type PXO99(A) was the same as that of the sequenced strain KACC10331. There were glycosyltransferase domains (Glycos_transf_2) at N and C terminal of RbfCxoo respectively. The deletion mutant delta rbfCxoo generated through a double crossover recombination and validated by PCR assay displayed the reduced flagellin glycosylation and no change in lipopolysaccharide O-antigen synthesis compared with PXO99(A). Moreover, no significant changes in flagellar mobility, biofilm formation and production of extracellular cellulase and xylanase in vitro were observed in delta rbfCxoo compared to PXO99(A). Most importantly, the deletion mutation of rbfCxoo resulted in enhanced virulence and gene expression.

CONCLUSION

RbfCxoo might be related to flagellin glycosylation and virulence expression in Xoo.

Substrate specificities of family 1 UGTs gained by domain swapping

Family 1 glycosyltransferases are a group of enzymes known to embrace a large range of different substrates. This study devises a method to enhance the range of substrates even further by combining domains from different glycosyltransferases to gain improved substrate specificity and catalytic efficiency. Chimeric glycosyltransferases were made by combining domains from seven different family 1 glycosyltransferases, UGT71C1, UGT71C2, UGT71E1, UGT85C1, UGT85B1, UGT88B1 and UGT94B1. Twenty different chimeric glycosyltransferases were formed of which twelve were shown to be catalytically active. The chimeric enzymes of Arabidopsis thaliana UGT71C1 and UGT71C2 showed major changes in acceptor substrate specificity and were able to glycosylate etoposide significantly better than the parental UGT71C1 and UGT71C2 enzymes, with K(cat) and efficiency coefficients 3.0 and 2.6 times higher, respectively. Chimeric glycosyltransferases of UGT71C1 combined with Stevia rebaudiana UGT71E1, also afforded enzymes with high catalytic efficiency, even though the two enzymes only display 38% amino acid sequence identity. These chimeras show a significantly altered regiospecificity towards especially trans-resveratrol, enabling the production of trans-resveratrol-beta-4'-O-glucoside (resveratroloside). The study demonstrates that it is possible to obtain improved catalytic properties by combining domains from both closely as well as more distantly related glycosyltransferases. The substrate specificity gained by the chimeras is difficult to predict because factors determining the acceptor specificity reside in the N- terminal as well as the C-terminal domains.

A xyloglucan endotransglucosylase/hydrolase involves in growth of primary root and alters the deposition of cellulose in Arabidopsis

Xyloglucan endotransglucosylase/hydrolases (XTHs) are a class of enzymes that mediate the construction and restructure of the cellulose/xyloglucan framework by splitting and reconnecting xyloglucan molecule cross-linking among cellulose microfibrils. Remodification of cellulose microfibrils within cell-wall matrices is realized to be one of the most critical steps in the regulation of cells expansion in plants. Thirty-three XTH genes have been found in Arabidopsis thaliana but their roles remain unclear. AtXTH21 (At2g18800), an Arabidopsis XTH gene that mainly expresses in root and flower, exhibits different expression profiles from other XTH members under hormone treatment. We examined loss-of-function mutants using T-DNA insertion lines and overexpression lines and found that the AtXTH21 gene played a principal role in the growth of the primary roots by altering the deposition of cellulose and the elongation of cell wall.

Characterization of IsaA and SceD, two putative lytic transglycosylases of Staphylococcus aureus

Bacterial cell wall peptidoglycan is a dynamic structure requiring hydrolysis to allow cell wall growth and division. Staphylococcus aureus has many known and putative peptidoglycan hydrolases, including two likely lytic transglycosylases. These two proteins, IsaA and SceD, were both found to have autolytic activity. Regulatory studies showed that the isaA and sceD genes are partially mutually compensatory and that the production of SceD is upregulated in an isaA mutant. The expression of sceD is also greatly upregulated by the presence of NaCl. Several regulators of isaA and sceD expression were identified. Inactivation of sceD resulted in impaired cell separation, as shown by light microscopy, and "clumping" of bacterial cultures. An isaA sceD mutant is attenuated for virulence, while SceD is essential for nasal colonization in cotton rats, thus demonstrating the importance of cell wall dynamics in host-pathogen interactions.

The cyclohedron test for finding periodic genes in time course expression studies

The problem of finding periodically expressed genes from time course microarray experiments is at the center of numerous efforts to identify the molecular components of biological clocks. We present a new approach to this problem based on the cyclohedron test, which is a rank test inspired by recent advances in algebraic combinatorics. The test has the advantage of being robust to measurement errors, and can be used to ascertain the significance of top-ranked genes. We apply the test to recently published measurements of gene expression during mouse somitogenesis and find 32 genes that collectively are significant. Among these are previously identified periodic genes involved in the Notch/FGF and Wnt signaling pathways, as well as novel candidate genes that may play a role in regulating the segmentation clock. These results confirm that there are an abundance of exceptionally periodic genes expressed during somitogenesis. The emphasis of this paper is on the statistics and combinatorics that underlie the cyclohedron test and its implementation within a multiple testing framework.

Genetic basis for the biosynthesis of methylglucose lipopolysaccharides in Mycobacterium tuberculosis

Mycobacteria produce two unusual polymethylated polysaccharides, the 6-O-methylglucosyl-containing lipopolysaccharides (MGLP) and the 3-O-methylmannose polysaccharides, which have been shown to regulate fatty acid biosynthesis in vitro. A cluster of genes dedicated to the synthesis of MGLP was identified in Mycobacterium tuberculosis and Mycobacterium smegmatis. Overexpression of the putative glycosyltransferase gene Rv3032 in M. smegmatis greatly stimulated MGLP production, whereas the targeted disruption of Rv3032 in M. tuberculosis and that of the putative methyltransferase gene MSMEG2349 in M. smegmatis resulted in a dramatic reduction in the amounts of MGLP synthesized and in the accumulation of precursors of these molecules. Disruption of Rv3032 also led to a significant decrease in the glycogen content of the tubercle bacillus, indicating that the product of this gene is likely to be involved in the elongation of more than one alpha-(1-->4)-glucan in this bacterium. Results thus suggest that Rv3032 encodes the alpha-(1-->4)-glucosyltransferase responsible for the elongation of MGLP, whereas MSMEG2349 encodes the O-methyltransferase required for the 6-O-methylation of these compounds.

Molecular characterization of two Arabidopsis thaliana glycosyltransferase mutants, rra1 and rra2, which have a reduced residual arabinose content in a polymer tightly associated with the cellulosic wall residue

Two putative glycosyltransferases in Arabidopsis thaliana, designated reduced residual arabinose-1 and -2 (RRA1 and RRA2), are characterized at the molecular level. Both genes are classified in CAZy GT-family-77 and are phylogenetically related to putative glycosyltranferases of Chlamydomonas reinhardtii. The expression pattern of the two genes was analyzed by semi-quantitative RT-PCR using mRNA extracted from various organs of bolting Arabidopsis thaliana plants. In addition, promoter::gusA analysis of transgenic Arabidopsis thaliana containing a fusion between either the RRA-1 or -2 promoter fragment and the gusA reporter gene showed that whereas the RRA1 promoter was primarily active in the apical meristem, the expression pattern of the RRA2 promoter was more diverse but also highly active in the meristematic region. In addition, T-DNA mutant insertion lines of both RRA-1 and -2, were identified and characterized at the molecular and biochemical level. Monosaccharide compositional analyses of cell wall material isolated from the meristematic region showed a ca. 20% reduction in the arabinose content in the insoluble/undigested cell wall residue after enzymatic removal of xyloglucan and pectic polysaccharides. These data indicate that both RRA-1 and -2 play a role in the arabinosylation of cell wall component(s).

Clostridium difficile toxin A-induced apoptosis is p53-independent but depends on glucosylation of Rho GTPases

Clostridium difficile toxin A (TcdA) is one of two homologous glucosyltransferases that mono-glucosylate Rho GTPases. HT29 cells were challenged with wild-type and mutant TcdA to investigate the mechanism by which apoptosis is induced. The TcdA-induced re-organization of the actin cytoskeleton led to an increased number of cells within the G2/M phase. Depolymerization of the actin filaments with subsequent G2/M arrest, however, was not causative for apoptosis, as shown in a comparative study using latrunculin B. The activation of caspase-3, -8, and -9 strictly depended on the glucosylation of Rho GTPases. Apoptosis measured by flow cytometry was completely abolished by a pan-caspase inhibitor (z-VAD-fmk). Interestingly, cleavage of procaspase-3 and Bid was not inhibited by z-VAD-fmk, but was inhibited by the calpain/cathepsin inhibitor ALLM. Cleavage of procaspase-8 was susceptible to inhibition by z-VAD-fmk and to the caspase-3 inhibitor Ac-DMQD-CHO, indicating a contribution to the activation of caspase-3 in an amplifying manner. Although TcdA induced mitochondrial damage and cytochrome c release, p53 was not activated or up-regulated. A p53-independent apoptotic effect was also checked by treatment of HCT 116 p53(-/-) cells. In summary, TcdA-induced apoptosis in HT29 cells depends on glucosylation of Rho GTPases leading to activation of cathepsins and caspase-3.

Engineering the glycosylation of natural products in actinomycetes

Bioactive natural products are frequently glycosylated with saccharide chains of different length, in which the sugars contribute to specific interactions with the biological target. Combinatorial biosynthesis approaches are being used in antibiotic-producing actinomycetes to generate derivatives with novel sugars in their architecture. Recent advances in this area indicate that glycosyltransferases involved in the biosynthesis of natural products have substrate flexibility regarding the sugar donor but also, less frequently, with respect to the aglycon acceptor. Therefore, the possibility exists of altering the glycosylation pattern of natural products, thus enabling an increase in the structural diversity of natural products.

Regulation of Integrin Functions by N-glycans

Integrins are cell surface transmembrane glycoproteins that function as adhesion receptors in cell-ECM interactions and link matrix proteins to the cytoskeleton. Integrins play an important role in cytoskeleton organization and in the transduction of intracellular signals, regulating various processes such as proliferation, differentiation, apoptosis, and cell migration. Although integrin-mediated adhesion is based on the binding of alpha and beta subunits to a defined peptide sequence, the strength of this binding is modulated by various factors including the status of glycosylation of integrin. Glycosylation reactions are catalyzed by the catalytic action of glycosyltransferases, such as N-acetylglucosaminyltransferase III, V and alpha1,6 fucosyltransferase, etc., which catalyze the formation of glycosidic bonds. In this talk we will briefly overview the N-glycan structures of integrins, such as alpha3beta1 and alpha5beta1, and their related functions arising from recent studies, which provide insight into some long-standing questions concerning N-glycosylation functions.

Towards understanding the functional role of the glycosyltransferases involved in the biosynthesis of Moraxella catarrhalis lipooligosaccharide

The glycosyltransferase enzymes (Lgts) responsible for the biosynthesis of the lipooligosaccharide-derived oligosaccharide structures from Moraxella catarrhalis have been investigated. This upper respiratory tract pathogen is responsible for a spectrum of illnesses, including otitis media (middle ear infection) in children, and contributes to exacerbations of chronic obstructive pulmonary disease in elderly patients. To investigate the function of the glycosyltransferase enzymes involved in the biosynthesis of lipooligosaccharide of M. catarrhalis and to gain some insight into the mechanism of serotype specificity for this microorganism, mutant strains of M. catarrhalis were produced. Examination by NMR and MS of the oligosaccharide structures produced by double-mutant strains (2951lgt1/4Delta and 2951lgt5/4Delta) and a single-mutant strain (2951lgt2Delta) of the bacterium has allowed us to propose a model for the serotype-specific expression of lipooligosaccharide in M. catarrhalis. According to this model, the presence/absence of Lgt4 and the Lgt2 allele determines the lipooligosaccharide structure produced by a strain. Furthermore, it is concluded that Lgt4 functions as an N-acetylglucosylamine transferase responsible for the addition of an alpha-D-GlcNAc (1-->2) glycosidic linkage to the (1-->4) branch, and also that there is competition between the glycosyltransferases Lgt1 and Lgt4. That is, in the presence of an active Lgt4, GlcNAc is preferentially added to the (1-->4) chain of the growing oligosaccharide, instead of Glc. In serotype B strains, which lack Lgt4, Lgt1 adds a Glc at this position. This implies that active Lgt4 has a much higher affinity/specificity for the beta-(1-->4)-linked Glc on the (1-->4) branch than does Lgt1.

Identification of the glycosyl transferase required for synthesis of the principal glycolipid characteristic of heterocysts of Anabaena sp. strain PCC 7120

Nitrogenase is oxygen-labile. Cyanobacterial heterocysts can fix N(2) in an oxic milieu because their interior is micro-oxic, for which the glycolipid layer of the heterocyst envelope is required. ORF all5341 of the Anabaena sp. genome predicts a glycosyl transferase. An insertional mutant of all5341 synthesized only a nonglycosylated form of heterocyst envelope glycolipid, and lacked a glycolipid layer. All5341 appears to be the transferase required to glycosylate the glycolipid aglycone.

Biochemical and genetic characterization of the pathways for trehalose metabolism in Propionibacterium freudenreichii, and their role in stress response

Propionibacterium freudenreichii accumulates high levels of trehalose, especially in response to stress. The pathways for trehalose metabolism were characterized, and their roles in response to osmotic, oxidative and acid stress were studied. Two pathways were identified: the trehalose-6-phosphate synthase/phosphatase (OtsA-OtsB) pathway, and the trehalose synthase (TreS) pathway. The former was used for trehalose synthesis, whereas the latter is proposed to operate in trehalose degradation. The activities of OtsA, OtsB and TreS were detected in cell extracts; the corresponding genes were identified, and the recombinant proteins were characterized in detail. In crude extracts of P. freudenreichii, OtsA was specific for ADP-glucose, in contrast to the pure recombinant OtsA, which used UDP-, GDP- and TDP-glucose, in addition to ADP-glucose. Moreover, the substrate specificity of OtsA in cell extracts was lost during purification, and the recombinant OtsA became specific to ADP-glucose upon incubation with a dialysed cell extract. The level of OtsA was enhanced (approximately twofold) by osmotic, oxidative and acid stress, whereas the level of TreS remained constant, or it decreased, under identical stress conditions. Therefore, the OtsA-OtsB pathway plays an important role in the synthesis of trehalose in response to stress. It is most likely that trehalose degradation proceeds via TreS to yield maltose, which is subsequently catabolized via amylomaltase activity. Hydrolytic activities that are potentially involved in trehalose degradation (trehalase, trehalose phosphorylase, trehalose-6-phosphate phosphorylase and trehalose-6-phosphate hydrolase) were not present. The role of trehalose as a common response to three distinct stresses is discussed.

The Arabidopsis irregular xylem8 mutant is deficient in glucuronoxylan and homogalacturonan, which are essential for secondary cell wall integrity

The secondary cell wall in higher plants consists mainly of cellulose, lignin, and xylan and is the major component of biomass in many species. The Arabidopsis thaliana irregular xylem8 (irx8) mutant is dwarfed and has a significant reduction in secondary cell wall thickness. IRX8 belongs to a subgroup of glycosyltransferase family 8 called the GAUT1-related gene family, whose members include GAUT1, a homogalacturonan galacturonosyltransferase, and GAUT12 (IRX8). Here, we use comparative cell wall analyses to show that the irx8 mutant contains significantly reduced levels of xylan and homogalacturonan. Immunohistochemical analyses confirmed that the level of xylan was significantly reduced in the mutant. Structural fingerprinting of the cell wall polymers further revealed that irx8 is deficient in glucuronoxylan. To explore the biological function of IRX8, we crossed irx8 with irx1 (affecting cellulose synthase 8). The homozygous irx1 irx8 exhibited severely dwarfed phenotypes, suggesting that IRX8 is essential for cell wall integrity during cellulose deficiency. Taken together, the data presented show that IRX8 affects the level of glucuronoxylan and homogalacturonan in higher plants and that IRX8 provides an important link between the xylan polymer and the secondary cell wall matrix and directly affects secondary cell wall integrity.

The mshA gene encoding the glycosyltransferase of mycothiol biosynthesis is essential in Mycobacterium tuberculosis Erdman

Mycothiol is the major low-molecular-weight thiol found in actinomycetes, including Mycobacterium tuberculosis, and has important antioxidant and detoxification functions. Gene disruption studies have shown that mycothiol is essential for the growth of M. tuberculosis. Because of mycothiol's unique characteristics, inhibitors directed against mycothiol biosynthesis have potential as drugs against M. tuberculosis. Four genes have been identified in mycobacteria that are involved in the biosynthesis of mycothiol. Two genes, mshB and mshD, are not essential for growth of M. tuberculosis. Mutants in these genes produce significant amounts of mycothiol or closely related thiol compounds. A targeted gene disruption in the mshC gene is lethal for M. tuberculosis, indicating that MshC is essential for growth. The remaining gene, mshA, encodes for a glycosyltransferase. In the present study, we attempted to produce a directed knock-out of the mshA gene in M. tuberculosis Erdman but found that this was only possible when a second copy of mshA was first incorporated into the chromosome. Bacteria with only a single copy of mshA that grew after mutagenesis produced normal levels of mycothiol. We therefore conclude that the mshA gene, like the mshC gene, is essential for the growth of M. tuberculosis.

Lunatic Fringe Controls T Cell Differentiation through Modulating Notch Signaling

T cells differentiate from bone marrow-derived stem cells by expressing developmental stage-specific genes. We here searched arrays of genes that are highly expressed in mature CD4-CD8+ (CD8 single-positive (SP)) T cells but little in CD4+CD8+ (double-positive (DP)) cells by cDNA subtraction. Lunatic fringe (Lfng), a modulator of Notch signaling, was identified to be little expressed in DP cells and highly expressed in CD8SP T cell as well as in CD4-CD8- (double-negative (DN)) and mature CD4+CD8- (CD4SP) T cells. Thus, we examined whether such change of expression of Lfng plays a role in T cell development. We found that overexpression of Lfng in Jurkat T cells strengthened Notch signaling by reporter gene assay, indicating that Lfng is a positive regulator for Notch signaling in T cells. The enforced expression of Lfng in thymocytes enhanced the development of immature CD8SP cells but decreased mature CD4SP and CD8SP cells. In contrast, the down-regulation of Lfng in thymocytes suppressed DP cells development due to the defective transition from CD44+CD25- stage to subsequent stage in DN cells. The overexpression of Lfng in fetal liver-derived hemopoietic stem cells enhanced T cell development, whereas its down-regulation suppressed it. These results suggested that the physiological high expression of Lfng in DN cells contributes to enhance T cell differentiation through strengthening Notch signaling. Shutting down the expression of Lfng in DP cells may have a physiological role in promoting DP cells differentiation toward mature SP cells.

Cloning, disruption and protein secretory phenotype of theGAS1homologue ofPichia pastoris

The aim of the study was the identification, cloning and disruption of the GAS1 homologue of Pichia pastoris. Gas1p is a glycoprotein anchored to the outer layer of the plasma membrane through a glycosylphosphatidylinositol (GPI) anchor. Gas1p is a beta-1,3-glucanosyltransglycosylase (EC 2.4.1.-). This cross-linking enzyme highly affects the structure and permeability of the yeast cell wall. The gene coding for the GAS1 homologue of P. pastoris was cloned by PCR, and its functionality was proven in a Saccharomyces cerevisiae GAS1 null mutant. Based on the nucleotide sequence information of the P. pastoris GAS1 homologue, a disruption cassette was constructed for the knockout of the GAS1 in P. pastoris. The morphology of DeltaGAS1 P. pastoris was identical to that of S. cerevisiae GAS1 mutants. Finally, the impact of GAS1 disruption on secretion of three recombinant model proteins in P. pastoris, human trypsinogen, human serum albumin and Rhizopus oryzae lipase, was evaluated. While the disruption had no effect on the secretion of trypsinogen and albumin, the amount of lipase released from the cells was doubled.

Functional analysis of desVIII homologues involved in glycosylation of macrolide antibiotics by interspecies complementation

The DesVIII is an auxiliary protein which enhances the transfer of TDP-d-desosamine catalyzed by DesVII glycosyltransferase in the biosynthesis of macrolide antibiotics, neomethymycin, methymycin and pikromycin, in Streptomyces venezuelae ATCC 15439. Homologues of the desVIII gene are present in a number of aminosugar-containing antibiotic biosynthetic gene clusters including eryCII from the erythromycin producer Saccharopolyspora erythraea, oleP1 from the oleandomycin producer Streptomyces antibioticus, dnrQ from the doxorubicin producer Streptomyces peucetius, and tylMIII from the tylosin producer Streptomyces fradiae. In order to gain further insight into the function of these DesVIII homologues, interspecies complementation experiments were carried out by expressing each gene in a desVIII deletion mutant strain of S. venezuelae. Complementation by expressing EryCII, OleP1, and DnrQ in this mutant strain restored the production of glycosylated macrolides to an approximate level of 66%, 26% and 26%, respectively, compared to self-complementation by DesVIII. However, expression of TylMIII did not restore the antibiotic production. These results suggest that the DesVIII homologues (except for TylMIII) can functionally replace the native DesVIII for glycosylation to proceed in vivo and their functions are similar in acting as glycosyltransferase auxiliary proteins. The requirement of glycosyltransferase auxiliary protein seems to be more widespread in polyketide biosynthetic pathways than previously known.

Selectins and glycosyltransferases in leukocyte rolling in vivo

Leukocyte rolling is an important step for the successful recruitment of leukocytes into tissue and occurs predominantly in inflamed microvessels and in high endothelial venules of secondary lymphoid organs. Leukocyte rolling is mediated by a group of C-type lectins, termed selectins. Three different selectins have been identified - P-, E- and L-selectin - which recognize and bind to crucial carbohydrate determinants on selectin ligands. Among selectin ligands, P-selectin glycoprotein ligand-1 is the main inflammatory selectin ligand, showing binding to all three selectins under in vivo conditions. Functional relevant selectin ligands expressed on high endothelial venules of lymphoid tissue are less clearly defined at the protein level. However, high endothelial venule-expressed selectin ligands were instrumental in uncovering the crucial role of post-translational modifications for selectin ligand activity. Several glycosyltransferases, such as core 2 beta1,6-N-acetylglucosaminyltransferase-I, beta1,4-galactosyltransferases, alpha1,3-fucosyltransferases and alpha2,3-sialyltransferases have been described to participate in the synthesis of core 2 decorated O-glycan structures carrying the tetrasaccharide sialyl Lewis X, a carbohydrate determinant on selectin ligands with binding activity to all three selectins. In addition, modifications, such as carbohydrate or tyrosine sulfation, were also found to contribute to the synthesis of functional selectin ligands.

Enhanced biofilm formation and loss of capsule synthesis: deletion of a putative glycosyltransferase in Porphyromonas gingivalis

Periodontitis is a biofilm-mediated disease. Porphyromonas gingivalis is an obligate anaerobe consistently associated with severe manifestations of this disease. As an opportunistic pathogen, the ability to proliferate within and disseminate from subgingival biofilm (plaque) is central to its virulence. Here, we report the isolation of a P. gingivalis transposon insertion mutant altered in biofilm development and the reconstruction and characterization of this mutation in three different wild-type strains. The mutation responsible for the altered biofilm phenotype was in a gene with high sequence similarity ( approximately 61%) to a glycosyltransferase gene. The gene is located in a region of the chromosome that includes up to 16 genes predicted to be involved in the synthesis and transport of capsular polysaccharide. The phenotype of the reconstructed mutation in all three wild-type backgrounds is that of enhanced biofilm formation. In addition, in strain W83, a strain that is encapsulated, the glycosyltransferase mutation resulted in a loss of capsule. Further experiments showed that the W83 mutant strain was more hydrophobic and exhibited increased auto-aggregation. Our results indicate that we have identified a gene involved in capsular-polysaccharide synthesis in P. gingivalis and that the production of capsule prevented attachment and the initiation of in vitro biofilm formation on polystyrene microtiter plates.

CPS1, a homolog of the Streptococcus pneumoniae type 3 polysaccharide synthase gene, is important for the pathobiology of Cryptococcus neoformans

The polysaccharide capsule is known to be the major factor required for the virulence of Cryptococcus neoformans. We have cloned and characterized a gene, designated CPS1, that encodes a protein containing a glycosyltransferase moiety and shares similarity with the type 3 polysaccharide synthase encoded by the cap3B gene of Streptococcus pneumoniae. Cps1p also shares similarity with hyaluronan synthase of higher eukaryotes. Deletion of the CPS1 gene from a serotype D strain of C. neoformans resulted in a slight reduction of the capsule size as observed by using an India ink preparation. The growth at 37 degrees C was impaired, and the ability to associate with human brain endothelial cells in vitro was also significantly reduced by the deletion of CPS1. Using site-specific mutagenesis, we showed that the conserved glycosyltransferase domains are critical for the ability of the strain to grow at elevated temperatures. A hyaluronan enzyme-linked immunosorbent assay method demonstrated that CPS1 is important for the synthesis of hyaluronan or its related polysaccharides in C. neoformans. Comparisons between the wild-type and the cps1Delta strains, using three different transmission electron microscopic methods, indicated that the CPS1 gene product is involved in the composition or maintenance of an electron-dense layer between the outer cell wall and the capsule. These and the virulence studies in a mouse model suggested that the CPS1 gene is important in the pathobiology of C. neoformans.

Glycosylation of flavonoids with a glycosyltransferase from Bacillus cereus

Microbial glycosyltransferases can convert many small lipophilic compounds such as phenolics, terpenoids, cyanohydrins and alkaloids into glycons using uridine-diphosphate-activated sugars. The main chemical functions of glycosylation processes are stabilization, detoxification and solubilization of the substrates. The gene encoding the UDP-glycosyltransferase from Bacillus cereus, BcGT-1, was cloned by PCR and sequenced. BcGT-1 was expressed in Escherichia coli BL21 (DE3) with a his-tag and purified using a His-tag affinity column. BcGT-1 could use apigenin, genistein, kaempferol, luteolin, naringenin and quercetin as substrates and gave two reaction products. The enzyme preferentially glycosylated at the 3-hydroxyl group, but it could transfer a glucose group onto the 7-hydroxyl group when the 3-hydroxyl group was not available. The reaction products made by biotransformation of flavonoids with E. coli expressing BcGT-1 are similar to those produced with the purified recombinant enzyme. Thus, this work provides a method that might be useful for the biosynthesis of flavonoid glucosides and for the glycosylation of related compounds.

The myotomal diwanka (lh3) glycosyltransferase and type XVIII collagen are critical for motor growth cone migration

The initial migration of motor growth cones from the spinal cord into the periphery requires extrinsic cues, yet their identities are largely unknown. In zebrafish diwanka mutants, motor growth cones are motile but fail to pioneer into the periphery. Here, we report on the positional cloning of diwanka and show that it encodes LH3, a myotomally expressed multifunctional enzyme with lysyl hydroxylase and glycosyltransferase domains. Cloning, expression analysis, and ubiquitous overexpression of other LH family members reveals that only diwanka (lh3) possesses a critical role in growth cone migration. We show that this unique role depends critically on the LH3 glycosyltransferase domain, and provide compelling evidence that diwanka (lh3) acts through myotomal type XVIII collagen, a ligand for neural-receptor protein tyrosine phosphatases that guide motor axons. Together, our results provide the first genetic evidence that glycosyltransferase modifications of the ECM play a critical role during vertebrate motor axon migration.