An Arabidopsis thaliana cDNA complements the N-acetylglucosaminyltransferase I deficiency of CHO Lec1 cells

Gel-Free/Label-Free Proteomic Analysis of Endoplasmic Reticulum Proteins in Soybean Root Tips under Flooding and Drought Stresses

Soybean is a widely cultivated crop; however, it is sensitive to flooding and drought stresses. The adverse environmental cues cause the endoplasmic reticulum (ER) stress due to accumulation of unfolded or misfolded proteins. To investigate the mechanisms in response to flooding and drought stresses, ER proteomics was performed in soybean root tips. The enzyme activity of NADH cytochrome c reductase was two-fold higher in the ER than other fractions, indicating that the ER was isolated with high purity. Protein abundance of ribosomal proteins was decreased under both stresses compared to control condition; however, the percentage of increased ribosomes was two-fold higher in flooding compared to drought. The ER proteins related to protein glycosylation and signaling were in response to both stresses. Compared to control condition, calnexin was decreased under both stresses; however, protein disulfide isomerase-like proteins and heat shock proteins were markedly decreased under flooding and drought conditions, respectively. Furthermore, fewer glycoproteins and higher levels of cytosolic calcium were identified under both stresses compared to control condition. These results suggest that reduced accumulation of glycoproteins in response to both stresses might be due to dysfunction of protein folding through calnexin/calreticulin cycle. Additionally, the increased cytosolic calcium levels induced by flooding and drought stresses might disturb the ER environment for proper protein folding in soybean root tips.

Plant protein glycosylation

Protein glycosylation is an essential co- and post-translational modification of secretory and membrane proteins in all eukaryotes. The initial steps of N-glycosylation and N-glycan processing are highly conserved between plants, mammals and yeast. In contrast, late N-glycan maturation steps in the Golgi differ significantly in plants giving rise to complex N-glycans with β1,2-linked xylose, core α1,3-linked fucose and Lewis A-type structures. While the essential role of N-glycan modifications on distinct mammalian glycoproteins is already well documented, we have only begun to decipher the biological function of this ubiquitous protein modification in different plant species. In this review, I focus on the biosynthesis and function of different protein N-linked glycans in plants. Special emphasis is given on glycan-mediated quality control processes in the ER and on the biological role of characteristic complex N-glycan structures.

Cell wall O-glycoproteins and N-glycoproteins: aspects of biosynthesis and function

Cell wall O-glycoproteins and N-glycoproteins are two types of glycomolecules whose glycans are structurally complex. They are both assembled and modified within the endomembrane system, i.e., the endoplasmic reticulum (ER) and the Golgi apparatus, before their transport to their final locations within or outside the cell. In contrast to extensins (EXTs), the O-glycan chains of arabinogalactan proteins (AGPs) are highly heterogeneous consisting mostly of (i) a short oligo-arabinoside chain of three to four residues, and (ii) a larger β-1,3-linked galactan backbone with β-1,6-linked side chains containing galactose, arabinose and, often, fucose, rhamnose, or glucuronic acid. The fine structure of arabinogalactan chains varies between, and within plant species, and is important for the functional activities of the glycoproteins. With regards to N-glycans, ER-synthesizing events are highly conserved in all eukaryotes studied so far since they are essential for efficient protein folding. In contrast, evolutionary adaptation of N-glycan processing in the Golgi apparatus has given rise to a variety of organism-specific complex structures. Therefore, plant complex-type N-glycans contain specific glyco-epitopes such as core β,2-xylose, core α1,3-fucose residues, and Lewis(a) substitutions on the terminal position of the antenna. Like O-glycans, N-glycans of proteins are essential for their stability and function. Mutants affected in the glycan metabolic pathways have provided valuable information on the role of N-/O-glycoproteins in the control of growth, morphogenesis and adaptation to biotic and abiotic stresses. With regards to O-glycoproteins, only EXTs and AGPs are considered herein. The biosynthesis of these glycoproteins and functional aspects are presented and discussed in this review.

Arginine/lysine residues in the cytoplasmic tail promote ER export of plant glycosylation enzymes

Plant N-glycan processing enzymes are arranged along the early secretory pathway, forming an assembly line to facilitate the step-by-step modification of oligosaccharides on glycoproteins. Thus, these enzymes provide excellent tools to study signals and mechanisms, promoting their localization and retention in the endoplasmic reticulum (ER) and Golgi apparatus. Herein, we focused on a detailed investigation of amino acid sequence motifs present in their short cytoplasmic tails in respect to ER export. Using site-directed mutagenesis, we determined that single arginine/lysine residues within the cytoplasmic tail are sufficient to promote rapid Golgi targeting of Golgi-resident N-acetylglucosaminyltransferase I (GnTI) and alpha-mannosidase II (GMII). Furthermore, we reveal that an intact ER export motif is essential for proper in vivo function of GnTI. Coexpression studies with Sar1p provided evidence for COPII-dependent transport of GnTI to the Golgi. Our data provide evidence that efficient ER export of Golgi-resident plant N-glycan processing enzymes occurs through a selective mechanism based on recognition of single basic amino acids present in their cytoplasmic tails.

Functional UDP-xylose transport across the endoplasmic reticulum/Golgi membrane in a Chinese hamster ovary cell mutant defective in UDP-xylose Synthase

In mammals, xylose is found as the first sugar residue of the tetrasaccharide GlcAbeta1-3Galbeta1-3Galbeta1-4Xylbeta1-O-Ser, initiating the formation of the glycosaminoglycans heparin/heparan sulfate and chondroitin/dermatan sulfate. It is also found in the trisaccharide Xylalpha1-3Xylalpha1-3Glcbeta1-O-Ser on epidermal growth factor repeats of proteins, such as Notch. UDP-xylose synthase (UXS), which catalyzes the formation of the UDP-xylose substrate for the different xylosyltransferases through decarboxylation of UDP-glucuronic acid, resides in the endoplasmic reticulum and/or Golgi lumen. Since xylosylation takes place in these organelles, no obvious requirement exists for membrane transport of UDP-xylose. However, UDP-xylose transport across isolated Golgi membranes has been documented, and we recently succeeded with the cloning of a human UDP-xylose transporter (SLC25B4). Here we provide new evidence for a functional role of UDP-xylose transport by characterization of a new Chinese hamster ovary cell mutant, designated pgsI-208, that lacks UXS activity. The mutant fails to initiate glycosaminoglycan synthesis and is not capable of xylosylating Notch. Complementation was achieved by expression of a cytoplasmic variant of UXS, which proves the existence of a functional Golgi UDP-xylose transporter. A approximately 200 fold increase of UDP-glucuronic acid occurred in pgsI-208 cells, demonstrating a lack of UDP-xylose-mediated control of the cytoplasmically localized UDP-glucose dehydrogenase in the mutant. The data presented in this study suggest the bidirectional transport of UDP-xylose across endoplasmic reticulum/Golgi membranes and its role in controlling homeostasis of UDP-glucuronic acid and UDP-xylose production.

A CMP-sialic acid transporter cloned from Arabidopsis thaliana

Sialylation of glycans is ubiquitous in vertebrates, but was believed to be absent in plants, arthropods, and fungi. However, recently evidence has been provided for the presence of sialic acid in these evolutionary clades. In addition, homologs of mammalian genes involved in sialylation can be found in the genomes of these taxa and for some Drosophila enzymes, involvement in sialic acid metabolism has been documented. In plant genomes, homologs of sialyltransferase genes have been identified, but there activity could not be confirmed. Several mammalian cell lines exist with defects in the sialylation pathway. One of these is the Chinese hamster ovary cell line Lec2, deficient in CMP-sialic acid transport to the Golgi lumen. These mutants provide the possibility to clone genes by functional complementation. Using expression cloning, we have identified an Arabidopsis thaliana nucleotide sugar transporter that is able to complement the CMP-sialic acid transport deficiency of Lec2 cells. The isolated gene (At5g41760) is a member of the triose-phosphate/nucleotide sugar transporter gene family. Recombinant expression of the gene in yeast and testing in vitro confirmed its ability to transport CMP-sialic acid.

Adaptation of the "in-gel release method" to N-glycome analysis of low-milligram amounts of material

Protein N-glycosylation is a post-translational modification which plays numerous crucial physiological roles. The N-glycan pattern varies depending on the species organs, tissues and even cell types and their respective physiological states. Obtaining enough starting material from a particular cell type or tissue for N-glycan purification by conventional methods can, in certain cases, be very difficult. Previously, a sensitive technique, the "in-gel release method" that allows the determination of N-glycans attached to a protein isolated by SDS-PAGE, has been developed in this and other laboratories. Here, we describe the adaptation of this method to obtain information on the N-glycome from minute amounts of tissue. The starting material, ranging from less than a milligram to a few milligrams of fresh tissue, is directly ground in Laemmli sample buffer and subject briefly to discontinuous Tris-glycine-SDS-PAGE. The Coomassie-stained band containing the majority of the proteins is subject to the "in-gel release method". The developed technique was used to analyze N-glycan patterns of different samples from Caenorhabditis elegans, Drosophila melanogaster, Spodoptera frugiperda, Trichoplusia ni, Nicotiana benthamiana, Arabidopsis thaliana, and Mus musculus. Furthermore, the technique was used to determine the effects of transient small-scale RNAi-mediated knock-down of a glycosylation-related gene in Drosophila Schneider 2 cell line.

Biosynthesis of truncated N-linked oligosaccharides results from non-orthologous hexosaminidase-mediated mechanisms in nematodes, plants, and insects

In many invertebrates and plants, the N-glycosylation profile is dominated by truncated paucimannosidic N-glycans, i.e. glycans consisting of a simple trimannosylchitobiosyl core often modified by core fucose residues. Even though they lack antennal N-acetylglucosamine residues, the biosynthesis of these glycans requires the sequential action of GlcNAc transferase I, Golgi mannosidase II, and, finally, beta-N-acetylglucosaminidases. In Drosophila, the recently characterized enzyme encoded by the fused lobes (fdl) gene specifically removes the non-reducing N-acetylglucosamine residue from the alpha1,3-antenna of N-glycans. In the present study, we examined the products of five beta-N-acetylhexosaminidase genes from Caenorhabditis elegans (hex-1 to hex-5, corresponding to reading frames T14F9.3, C14C11.3, Y39A1C.4, Y51F10.5, and Y70D2A.2) in addition to three from Arabidopsis thaliana (AtHEX1, AtHEX2, and AtHEX3, corresponding to reading frames At1g65590, At3g55260, and At1g05590). Based on homology, the Caenorhabditis HEX-1 and all three Arabidopsis enzymes are members of the same sub-family as the aforementioned Drosophila fused lobes enzyme but either act as chitotriosidases or non-specifically remove N-acetylglucosamine from both N-glycan antennae. The other four Caenorhabditis enzymes are members of a distinct sub-family; nevertheless, two of these enzymes displayed the same alpha1,3-antennal specificity as the fused lobes enzyme. Furthermore, a deletion of part of the Caenorhabditis hex-2 gene drastically reduces the native N-glycan-specific hexosaminidase activity in mutant worm extracts and results in a shift in the N-glycan profile, which is a demonstration of its in vivo enzymatic relevance. Based on these data, it is hypothesized that the genetic origin of paucimannosidic glycans in nematodes, plants, and insects involves highly divergent members of the same hexosaminidase gene family.

An antibody produced in tobacco expressing a hybrid beta-1,4-galactosyltransferase is essentially devoid of plant carbohydrate epitopes

N-glycosylation of a mAb may have a major impact on its therapeutic merits. Here, we demonstrate that expression of a hybrid enzyme (called xylGalT), consisting of the N-terminal domain of Arabidopsis thaliana xylosyltransferase and the catalytic domain of human beta-1,4-galactosyltransferase I (GalT), in tobacco causes a sharp reduction of N-glycans with potentially immunogenic core-bound xylose (Xyl) and fucose (Fuc) residues as shown by Western blot and MALDI-TOF MS analysis. A radioallergosorbent test inhibition assay with proteins purified from leaves of WT and these transgenic tobacco plants using sera from allergic patients suggests a significant reduction of potential immunogenicity of xylGalT proteins. A mAb purified from leaves of plants expressing xylGalT displayed an N-glycan profile that featured high levels of galactose, undetectable xylose, and a trace of fucose. Hence, a transgenic plant expressing the hybrid GalT might yield more effective and safer monoclonals for therapeutic purposes than WT plants and even transgenic plants expressing the unchanged GalT.

Mapping the Arabidopsis organelle proteome

A challenging task in the study of the secretory pathway is the identification and localization of new proteins to increase our understanding of the functions of different organelles. Previous proteomic studies of the endomembrane system have been hindered by contaminating proteins, making it impossible to assign proteins to organelles. Here we have used the localization of organelle proteins by the isotope tagging technique in conjunction with isotope tags for relative and absolute quantitation and 2D liquid chromatography for the simultaneous assignment of proteins to multiple subcellular compartments. With this approach, the density gradient distributions of 689 proteins from Arabidopsis thaliana were determined, enabling confident and simultaneous localization of 527 proteins to the endoplasmic reticulum, Golgi apparatus, vacuolar membrane, plasma membrane, or mitochondria and plastids. This parallel analysis of endomembrane components has enabled protein steady-state distributions to be determined. Consequently, genuine organelle residents have been distinguished from contaminating proteins and proteins in transit through the secretory pathway.

Molecular basis of N-acetylglucosaminyltransferase I deficiency in Arabidopsis thaliana plants lacking complex N-glycans

GnTI (N-acetylglucosaminyltransferase I) is a Golgi-resident enzyme essential for the processing of high-mannose to hybrid and complex N-glycans. The Arabidopsis thaliana cgl mutant lacks GnTI activity and as a consequence accumulates oligomannosidic structures. Molecular cloning of cgl GnTI cDNA revealed a point mutation, which causes a critical amino acid substitution (Asp144-->Asn), thereby creating an additional N-glycosylation site. Heterologous expression of cgl GnTI in insect cells confirmed its lack of activity and the use of the N-glycosylation site. Remarkably, introduction of the Asp144-->Asn mutation into rabbit GnTI, which does not result in the formation of a new N-glycosylation site, led to a protein with strongly reduced, but still detectable enzymic activity. Expression of Asn144 rabbit GnTI in cgl plants could partially restore complex N-glycan formation. These results indicate that the complete deficiency of GnTI activity in cgl plants is mainly due to the additional N-glycan, which appears to interfere with the proper folding of the enzyme.

Localization of Organelle Proteins by Isotope Tagging (LOPIT)

We describe a proteomics method for determining the subcellular localization of membrane proteins. Organelles are partially separated using centrifugation through self-generating density gradients. Proteins from each organelle co-fractionate and therefore exhibit similar distributions in the gradient. Protein distributions can be determined through a series of pair-wise comparisons of gradient fractions, using cleavable ICAT to enable relative quantitation of protein levels by MS. The localization of novel proteins is determined using multivariate data analysis techniques to match their distributions to those of proteins that are known to reside in specific organelles. Using this approach, we have simultaneously demonstrated the localization of membrane proteins in both the endoplasmic reticulum and the Golgi apparatus in Arabidopsis. Localization of organelle proteins by isotope tagging is a new tool for high-throughput protein localization, which is applicable to a wide range of research areas such as the study of organelle function and protein trafficking.

A genetic and structural analysis of the N-glycosylation capabilities

The recent draft sequencing of the rice (Oryza sativa) genome has enabled a genetic analysis of the glycosylation capabilities of an agroeconomically important group of plants, the monocotyledons. In this study, we have not only identified genes putatively encoding enzymes involved in N-glycosylation, but have examined by MALDI-TOF MS the structures of the N-glycans of rice and other monocotyledons (maize, wheat and dates; Zea mays, Triticum aestivum and Phoenix dactylifera); these data show that within the plant kingdom the types of N-glycans found are very similar between monocotyledons, dicotyledons and gymnosperms. Subsequently, we constructed expression vectors for the key enzymes forming plant-typical structures in rice, N-acetylglucosaminyltransferase I (GlcNAc-TI; EC 2.4.1.101), core alpha1,3-fucosyltransferase (FucTA; EC 2.4.1.214) and beta1,2-xylosyltransferase (EC 2.4.2.38) and successfully expressed them in Pichia pastoris. Rice GlcNAc-TI, FucTA and xylosyltransferase are therefore the first monocotyledon glycosyltransferases involved in N-glycan biosynthesis to be characterised in a recombinant form.

Unaltered complex N-glycan profiles in Nicotiana benthamiana despite drastic reduction of beta1,2- N -acetylglucosaminyltransferase I activity

UDP-GlcNAc:alpha3-D-mannoside beta1,2- N -acetylglucosaminyltransferase I (GnTI; EC 2.4.1.101) is a Golgi-resident glycosyltransferase that is essential for the processing of oligomannose to hybrid and complex N-glycans in higher eukaryotes. The cDNA of Nicotiana tabacum GnTI has been cloned and characterised previously. To assess the influence of GnTI expression levels on the formation of complex N-glycans we used posttranscriptional gene silencing to knock down the expression of GnTI in the tobacco related species Nicotiana benthamiana. 143 independent transgenic plants containing GnTI constructs in either sense or antisense orientation were generated. 23 lines were selected for measurement of GnTI activity and 10 lines thereof showed a reduction of more than 85% in in vitro assays as compared to wildtype plants. GnTI reduction was stably inherited and did not interfere with the viability of the transformants. Noteworthy one line, 34S/2, exhibited a residual GnTI activity below the detection limit. beta1,2- N -acetylglucosaminyltransferase II (GnTII), an enzyme which acts further downstream in the N-glycosylation pathway, as well as other control enzymes (alpha-mannosidase, beta- N -acetylglucosaminidase) were not affected indicating the specific downregulation of GnTI. Remarkably, immunoblots and mass spectrometric N-glycan profiling revealed no significant changes of the total N-glycan comparable to wildtype plants.

The role of the GlcNAc(beta)1,2Man(alpha)- moiety in mammalian development. Null mutations of the genes encoding UDP-N-acetylglucosamine:alpha-3-D-mannoside beta-1,2-N-acetylglucosaminyltransferase I and UDP-N-acetylglucosamine:alpha-D-mannoside beta-1,2-N-acetylglucosaminyltransferase I.2 cause embryonic lethality and congenital muscular dystrophy in mice and men, respectively

The GlcNAc(beta)1,2Man(alpha)- moiety can be synthesized by at least two mammalian glycosyltransferases, UDP-GlcNAc:alpha-3-D-mannoside beta1,2-N-acetylglucosaminyltransferase I (GnT I, EC 2.4.1.101) and UDP-GlcNAc:alpha-D-mannoside beta1,2-N-acetylglucosaminyltransferase I.2 (GnT I.2). GnT I adds a GlcNAc residue in beta1,2 glycosidic linkage to the Man(alpha)1,3 arm of the N-glycan core to initiate the biosynthesis of hybrid and complex N-glycans. GnT I.2 can add GlcNAc in beta1,2 linkage to any alpha-linked terminal Man residue but has a strong preference for the Man(alpha)1-O-Thr- moiety which occurs in alpha-dystroglycan and other O-mannosylated glycoproteins. Mouse embryos lacking a functional GnT I gene (MgatI) were unable to synthesize complex N-glycans and none survived past 10.5 days after fertilization. The embryos showed multisystemic defects in various morphogenic processes such as neural tube formation, vascularization and the determination of left-right body plan asymmetry. Six human patients with muscle-eye-brain disease (MEB) were recently shown to have point mutations in the gene encoding GnT I.2 (MGATI.2). MEB is an autosomal recessive disease characterized by congenital muscular dystrophy, ocular abnormalities, brain malformations and other multisystemic defects. Both the MGATI.2 gene and MEB disease have been mapped to chromosome 1p32-p34. At least one of the biochemical sites affected by the MGATI.2 mutations is probably the interaction between laminin in the extracellular matrix and the peripheral membrane glycoprotein alpha-dystroglycan since this interaction is believed to require the presence of the sialyl(alpha)2,3Gal(beta)1,4GlcNAc(beta)1,2Man(alpha)1-O-Ser/Thr moiety on alpha-dystroglycan. It can be concluded that the GlcNAc(beta)1,2Man(alpha)- moiety is important for mammalian development due to an essential role in two distinct biochemical pathways.

UDP-N-acetylglucosamine:alpha-3-D-mannoside beta-1,2-N-acetylglucosaminyltransferase I and UDP-N-acetylglucosamine:alpha-6-D-mannoside beta-1,2-N-acetylglucosaminyltransferase II in Caenorhabditis elegans

UDP-N-acetylglucosamine:alpha-3-D-mannoside beta-1,2-N-acetylglucosaminyltransferase I (GnT I) and UDP-N-acetylglucosamine:alpha-6-D-mannoside beta-1,2-N-acetylglucosaminyltransferase II (GnT II) are key enzymes in the synthesis of Asn-linked hybrid and complex glycans. We have cloned cDNAs from Caenorhabditis elegans for three genes homologous to mammalian GnT I (designated gly-12, gly-13 and gly-14) and one gene homologous to mammalian GnT II. All four cDNAs encode proteins which have the domain structure typical of previously cloned Golgi-type glycosyltransferases and show enzymatic activity (GnT I and GnT II, respectively) on expression in transgenic worms. We have isolated worm mutants lacking the three GnT I genes by the method of ultraviolet irradiation in the presence of trimethylpsoralen (TMP); null mutants for GnT II have not yet been obtained. The gly-12 and gly-14 mutants as well as the gly-14;gly-12 double mutant displayed wild-type phenotypes indicating that neither gly-12 nor gly-14 is necessary for worm development under standard laboratory conditions. This finding and other data indicate that the GLY-13 protein is the major functional GnT I in C. elegans. The mutation lacking the gly-13 gene is partially lethal and the few survivors display severe morphological and behavioral defects. We have shown that the observed phenotype co-segregates with the gly-13 deletion in genetic mapping experiments although a second mutation near the gly-13 gene cannot as yet be ruled out. Our data indicate that complex and hybrid N-glycans may play critical roles in the morphogenesis of C. elegans, as they have been shown to do in mice and men.

Plant members of the alpha1-->3/4-fucosyltransferase gene family encode an alpha1-->4-fucosyltransferase, potentially involved in Lewis(a) biosynthesis, and two core alpha1-->3-fucosyltransferases

Three putative alpha1-->3/4-fucosyltransferase (alpha1-->3/4-FucT) genes have been detected in the Arabidopsis thaliana genome. The products of two of these genes have been identified in vivo as core alpha1-->3-FucTs involved in N-glycosylation. An orthologue of the third gene was isolated from a Beta vulgaris cDNA library. The encoded enzyme efficiently fucosylates Galbeta1-->3GlcNAcbeta1-->3Galbeta1-->4Glc. Analysis of the product by 400 MHz (1)H-nuclear magnetic resonance spectroscopy showed that the product is alpha1-->4-fucosylated at the N-acetylglucosamine residue. In vitro, the recombinant B. vulgaris alpha1-->4-FucT acts efficiently only on neutral type 1 chain-based glycan structures. In plants the enzyme is expected to be involved in Lewis(a) formation on N-linked glycans.

Tissues of the clawed frog Xenopus laevis contain two closely related forms of UDP-GlcNAc:alpha3-D-mannoside beta-1,2-N-acetylglucosaminyltransferase I

UDP-GlcNAc:alpha3-D-mannoside beta-1,2-N-acetylglucosaminyltransferase I (GnTI; EC 2.4.1.101) is a medial-Golgi enzyme that is essential for the processing of oligomannose to hybrid and complex N-glycans. On the basis of highly conserved sequences obtained from previously cloned mammalian GnTI genes, cDNAs for two closely related GnTI isoenzymes were isolated from a Xenopus laevis ovary cDNA library. As typical for glycosyltransferases, both proteins exhibit a type II transmembrane protein topology with a short N-terminal cytoplasmic tail (4 amino acids); a transmembrane domain of 22 residues; a stem region with a length of 81 (isoenzyme A) and 77 (isoenzyme B) amino acids, respectively; and a catalytic domain consisting of 341 residues. The two proteins differ not only in length but also at 13 (stem) and 18 (catalytic domain) positions, respectively. The overall identity of the catalytic domains of the X. laevis GnTI isoenzymes with their mammalian and plant orthologues ranges from 30% (Nicotiana tabacum) to 67% (humans). Isoenzymes A and B are encoded by two separate genes that were both found to be expressed in all tissues examined, albeit in varying amounts and ratios. On expression of the cDNAs in the baculovirus/insect cell system, both isoenzymes were found to exhibit enzymatic activity. Isoenzyme B is less efficiently folded in vivo and thus appears of lower physiological relevance than isoenzyme A. However, substitution of threonine at position 223 with alanine was sufficient to confer isoenzyme B with properties similar to those observed for isoenzyme A.

Genetic model organisms in the study of N-glycans

Recently the genomic sequences of three multicellular eukaryotes, Caenorhabditis elegans, Drosophila melanogaster and Arabidopsis thaliana, have been elucidated. A number of cDNAs encoding glycosyltransferases demonstrated to have a role in N-linked glycosylation have already been cloned from these organisms, e.g., GlcNAc transferases and alpha 1,3-fucosyltransferases. However, many more homologues of glycosyltransferases and other glycan modifying enzymes have been predicted by analysis of the genome sequences, but the predictions of full length open reading frames appear to be particularly poor in Caenorhabditis. The use of these organisms as models in glycobiology may be hampered since they all have N-linked glycosylation repertoires unlike those of mammals. Arabidopsis and Drosophila have glycosylation similar to that of other plants or insects, while our new data from MALDI-TOF analysis of PNGase A-released neutral N-glycans of Caenorhabditis indicate that there exists a range of pauci- and oligomannosidic structures, with up to four fucose residues and up to two O-methyl groups. With all these three 'genetic model organisms', however, much more work is required for a full understanding of their glycobiology.

Cloning and expression of cDNAs encoding alpha1,3-fucosyltransferase homologues from Arabidopsis thaliana

The core alpha1,3-fucosyltransferases are involved in the synthesis of glycans specific to plants and invertebrates which are known to be immunogenic and allergenic. We report the identification, isolation and characterisation of the cDNAs of three genes (FucTA, FucTB and FucTC) encoding proteins similar to alpha1,3-fucosyltransferases in Arabidopsis thaliana. Reverse transcription-polymerase chain reaction was used to amplify the full length coding sequence of FucTA. The FucTA gene, which consists of seven exons, encodes a presumptive protein of 501 amino acids showing an overall sequence identity of 66% to the protein encoded by the recently isolated mung bean Fuc-T C3 cDNA. FucTA was expressed in Pichia pastoris under the control of the AOX1 gene promoter. The soluble enzyme was found to catalyse the same reaction as mung bean core alpha1,3-fucosyltransferase as judged by analyses of the products by MALDI-TOF and high-performance liquid chromatography. The FucTB cDNA was isolated from a lambda-ZAP library, but the clone used an alternative splicing site between the second and third exon resulting in a premature stop codon. The FucTC gene encodes a protein with less than 40% identity to FucTA across 115 amino acids of a total of 401 amino acids and is a member of a new sub-family of plant alpha1,3/4-fucosyltransferase homologues.

Endomembranes and vesicle trafficking

Over the past year extensive analyses of the accumulated data on the structural and functional organisation of the endomembrane system and vesicular trafficking in higher plants have shown it to be far more complex than previously anticipated. The availability of molecular tools combined with new opportunities to visualise endomembrane dynamics in vivo will allow better understanding of the fundamental processes underlying the complexity of endomembrane behaviour and vesicular trafficking.