Biosynthesis and immunolocalization of Lewis a-containing N-glycans in the plant cell

A century journey of organelles research in the plant endomembrane system

We are entering an exciting century in the study of the plant organelles in the endomembrane system. Over the past century, especially within the past 50 years, tremendous advancements have been made in the complex plant cell to generate a much clearer and informative picture of plant organelles, including the molecular/morphological features, dynamic/spatial behavior, and physiological functions. Importantly, all these discoveries and achievements in the identification and characterization of organelles in the endomembrane system would not have been possible without: (1) the innovations and timely applications of various state-of-art cell biology tools and technologies for organelle biology research; (2) the continuous efforts in developing and characterizing new organelle markers by the plant biology community; and (3) the landmark studies on the identification and characterization of the elusive organelles. While molecular aspects and results for individual organelles have been extensively reviewed, the development of the techniques for organelle research in plant cell biology is less appreciated. As one of the ASPB Centennial Reviews on "organelle biology," here we aim to take a journey across a century of organelle biology research in plants by highlighting the important tools (or landmark technologies) and key scientists that contributed to visualize organelles. We then highlight the landmark studies leading to the identification and characterization of individual organelles in the plant endomembrane systems.

Crystal structure of mango α1,3/α1,4-fucosyltransferase elucidates unique elements that regulate Lewis A-dominant oligosaccharide assembly

In various organisms, α1,3/α1,4-fucosyltransferases (CAZy GT10 family enzymes) mediate the assembly of type I (Galβ1,3GlcNAc) and/or type II (Galβ1,4GlcNAc)-based Lewis structures that are widely distributed in glycoconjugates. Unlike enzymes of other species, plant orthologues show little fucosyltransferase activity for type II-based glycans and predominantly catalyze the assembly of the Lewis A structure [Galβ1,3(Fucα1,4)GlcNAc] on the type I disaccharide unit of their substrates. However, the structural basis underlying this unique substrate selectivity remains elusive. In this study, we investigated the structure-function relationship of MiFUT13A, a mango α1,3/α1,4-fucosyltransferase. The prepared MiFUT13A displayed distinct α1,4-fucosyltransferase activity. Consistent with the enzymatic properties of this molecule, X-ray crystallography revealed that this enzyme has a typical GT-B fold-type structure containing a set of residues that are responsible for its SN2-like catalysis. Site-directed mutagenesis and molecular docking analyses proposed a rational binding mechanism for type I oligosaccharides. Within the catalytic cleft, the pocket surrounding Trp121 serves as a binding site, anchoring the non-reducing terminal β1,3-galactose that belongs to the type I disaccharide unit. Furthermore, Glu177 was postulated to function as a general base catalyst through its interaction with the 4-hydroxy group of the acceptor N-acetylglucosamine residue. Adjacent residues, specifically Thr120, Thr157 and Asp175 were speculated to assist in binding of the reducing terminal residues. Intriguingly, these structural elements were not fully conserved in mammalian orthologue which also shows predominant α1,4-fucosyltransferase activity. In conclusion, we have proposed that MiFUT13A generates the Lewis A structure on type I glycans through a distinct mechanism, divergent from that of mammalian enzymes.

Glycan Complexity and Heterogeneity of Glycoproteins in Somatic Extracts and Secretome of the Infective Stage of the Helminth Fasciola hepatica

Fasciola hepatica is a global helminth parasite of humans and their livestock. The invasive stage of the parasite, the newly excysted juvenile (NEJs), relies on glycosylated excreted-secreted (ES) products and surface/somatic molecules to interact with host cells and tissues and to evade the host's immune responses, such as disarming complement and shedding bound antibody. While -omics technologies have generated extensive databases of NEJs' proteins and their expression, detailed knowledge of the glycosylation of proteins is still lacking. Here, we employed glycan, glycopeptide, and proteomic analyses to determine the glycan profile of proteins within the NEJs' somatic (Som) and ES extracts. These analyses characterized 123 NEJ glycoproteins, 71 of which are secreted proteins, and allowed us to map 356 glycopeptides and their associated 1690 N-glycan and 37 O-glycan forms to their respective proteins. We discovered abundant micro-heterogeneity in the glycosylation of individual glycosites and between different sites of multi-glycosylated proteins. The global heterogeneity across NEJs' glycoproteome was refined to 53 N-glycan and 16 O-glycan structures, ranging from highly truncated paucimannosidic structures to complex glycans carrying multiple phosphorylcholine (PC) residues, and included various unassigned structures due to unique linkages, particularly in pentosylated O-glycans. Such exclusive glycans decorate some well-known secreted molecules involved in host invasion, including cathepsin B and L peptidases, and a variety of membrane-bound glycoproteins, suggesting that they participate in host interactions. Our findings show that F. hepatica NEJs generate exceptional protein variability via glycosylation, suggesting that their molecular portfolio that communicates with the host is far more complex than previously anticipated by transcriptomic and proteomic analyses. This study opens many avenues to understand the glycan biology of F. hepatica throughout its life-stages, as well as other helminth parasites, and allows us to probe the glycosylation of individual NEJs proteins in the search for innovative diagnostics and vaccines against fascioliasis.

β1,3-galactosyltransferase on chromosome 6 is essential for the formation of Lewis a structure on N-glycan in Oryza sativa

β1,3-galactose is the component of outer-chain elongation of complex N-glycans that, together with α1,4-fucose, forms Lewis a structures in plants. Previous studies have revealed that N-glycan maturation is mediated by sequential attachment of β1,3-galactose and α1,4-fucose by individual β1,3-galactosyltransferase (GalT) and α1,4-fucosyltransferase (1,4-FucT), respectively. Although GalT from several species has been studied, little information about GalT from rice is available. I therefore characterized three GalT candidate genes on different chromosomes in Oryza sativa. Seeds of rice lines that had T-DNA insertions in regions corresponding to individual putative GalT genes were obtained from a Rice Functional Genomic Express Database and plants grown until maturity. Homozygotes were selected from the next generation by genotyping PCR, and used for callus induction. Callus extracts of two independent T-DNA mutant rice which have T-DNA insertions at the same gene on chromosome 6 but in different exons showed highly reduced band intensity on a western blots using an anti-Lewis a antibody. Cell extracts and cultured media from suspension culture of the one of these mutant rice were further analysed by N-glycan profiling using matrix-associated laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF). Identified N-glycan species containing β1,3-galactose from both cell extracts and cultured media of knock-out mutant were less than 0.5% of total N-glycans while that of WT cells were 9.8% and 49.1%, respectively. This suggests that GalT located on rice chromosome 6 plays a major role in N-glycan galactosylation, and mutations within it lead to blockage of Lewis a epitope formation.

Investigation of the N-Glycosylation of the SARS-CoV-2 S Protein Contained in VLPs Produced in Nicotiana benthamiana

The emergence of the SARS-CoV-2 coronavirus pandemic in China in late 2019 led to the fast development of efficient therapeutics. Of the major structural proteins encoded by the SARS-CoV-2 genome, the SPIKE (S) protein has attracted considerable research interest because of the central role it plays in virus entry into host cells. Therefore, to date, most immunization strategies aim at inducing neutralizing antibodies against the surface viral S protein. The SARS-CoV-2 S protein is heavily glycosylated with 22 predicted N-glycosylation consensus sites as well as numerous mucin-type O-glycosylation sites. As a consequence, O- and N-glycosylations of this viral protein have received particular attention. Glycans N-linked to the S protein are mainly exposed at the surface and form a shield-masking specific epitope to escape the virus antigenic recognition. In this work, the N-glycosylation status of the S protein within virus-like particles (VLPs) produced in Nicotiana benthamiana (N. benthamiana) was investigated using a glycoproteomic approach. We show that 20 among the 22 predicted N-glycosylation sites are dominated by complex plant N-glycans and one carries oligomannoses. This suggests that the SARS-CoV-2 S protein produced in N. benthamiana adopts an overall 3D structure similar to that of recombinant homologues produced in mammalian cells.

Effect of fruit maturation on N-glycosylation of plant-derived native and recombinant miraculin

Miracle fruit, Synsepalum dulcificum, produces a unique taste-modifying protein, miraculin (MIR), which has an attractive potential for commercial application as a novel low-calorie sweetener. To establish a stable supply system for MIR, a previous study established a platform for recombinant MIR (rMIR) production in tomato plants and demonstrated that native miraculin from miracle fruit (nMIR) and rMIR were almost identical in their protein modifications with N-glycan. However, neither N-glycosylation nor the influence of fruit maturation on the structural changes of N-glycan have been fully characterized in detail. Here, with a focus on N-glycosylation and the contribution of fruit maturation to N-glycan, we reanalyzed the N-glycosylation of the natural maturation stages of nMIR and rMIR, and then compared the N-glycan structures on MIRs prepared from the fruit at two different maturation stages. The detailed peptide mapping and N-glycosylation analysis of MIRs provided evidence that MIRs have variants, which were derived mainly from the differences in the N-glycan structure in nMIR and the N-glycosylation in rMIR and not from the cleavage of the peptide backbone. N-Glycan analysis of MIRs from the maturation stage of fruits demonstrated that N-glycan structures were similar among nMIRs and rMIRs at every maturation stage. These results indicated that the heterogeneously expressed rMIRs had the same characteristics in post-translational modifications, especially N-glycosylation and N-glycan structures, throughout the maturation stages. This study demonstrated the potential of recombinant protein expressed in tomato plants and paves the way for the commercial use of rMIR.

Unexpected Arabinosylation after Humanization of Plant Protein N-Glycosylation

As biopharmaceuticals, recombinant proteins have become indispensable tools in medicine. An increasing demand, not only in quantity but also in diversity, drives the constant development and improvement of production platforms. The N-glycosylation pattern on biopharmaceuticals plays an important role in activity, serum half-life and immunogenicity. Therefore, production platforms with tailored protein N-glycosylation are of great interest. Plant-based systems have already demonstrated their potential to produce pharmaceutically relevant recombinant proteins, although their N-glycan patterns differ from those in humans. Plants have shown great plasticity towards the manipulation of their glycosylation machinery, and some have already been glyco-engineered in order to avoid the attachment of plant-typical, putatively immunogenic sugar residues. This resulted in complex-type N-glycans with a core structure identical to the human one. Compared to humans, plants lack the ability to elongate these N-glycans with β1,4-linked galactoses and terminal sialic acids. However, these modifications, which require the activity of several mammalian enzymes, have already been achieved for Nicotiana benthamiana and the moss Physcomitrella. Here, we present the first step towards sialylation of recombinant glycoproteins in Physcomitrella, human β1,4-linked terminal N-glycan galactosylation, which was achieved by the introduction of a chimeric β1,4-galactosyltransferase (FTGT). This chimeric enzyme consists of the moss α1,4-fucosyltransferase transmembrane domain, fused to the catalytic domain of the human β1,4-galactosyltransferase. Stable FTGT expression led to the desired β1,4-galactosylation. However, additional pentoses of unknown identity were also observed. The nature of these pentoses was subsequently determined by Western blot and enzymatic digestion followed by mass spectrometric analysis and resulted in their identification as α-linked arabinoses. Since a pentosylation of β1,4-galactosylated N-glycans was reported earlier, e.g., on recombinant human erythropoietin produced in glyco-engineered Nicotiana tabacum, this phenomenon is of a more general importance for plant-based production platforms. Arabinoses, which are absent in humans, may prevent the full humanization of plant-derived products. Therefore, the identification of these pentoses as arabinoses is important as it creates the basis for their abolishment to ensure the production of safe biopharmaceuticals in plant-based systems.

Towards understanding the extensive diversity of protein N-glycan structures in eukaryotes

N‐glycosylation is an important post‐translational modification of proteins that has been highly conserved during evolution and is found in Eukaryota, Bacteria and Archaea. In eukaryotes, N‐glycan processing is sequential, involving multiple specific steps within the secretory pathway as proteins travel through the endoplasmic reticulum and the Golgi apparatus. In this review, we first summarize the different steps of the N‐glycan processing and further describe recent findings regarding the diversity of N‐glycan structures in eukaryotic clades. This comparison allows us to explore the different regulation mechanisms of N‐glycan processing among eukaryotic clades. Recent findings regarding the regulation of protein N‐glycosylation are highlighted, especially the regulation of the biosynthesis of complex‐type N‐glycans through manganese and calcium homeostasis and the specific role of transmembrane protein 165 (TMEM165) for which homologous sequences have been identified in several eukaryotic clades. Further research will be required to characterize the function of TMEM165 homologous sequences in different eukaryotic clades.

Impact of Specific N-Glycan Modifications on the Use of Plant-Produced SARS-CoV-2 Antigens in Serological Assays

The receptor binding domain (RBD) of the SARS-CoV-2 spike protein plays a key role in the virus-host cell interaction, and viral infection. The RBD is a major target for neutralizing antibodies, whilst recombinant RBD is commonly used as an antigen in serological assays. Such assays are essential tools to gain control over the pandemic and detect the extent and durability of an immune response in infected or vaccinated populations. Transient expression in plants can contribute to the fast production of viral antigens, which are required by industry in high amounts. Whilst plant-produced RBDs are glycosylated, N-glycan modifications in plants differ from humans. This can give rise to the formation of carbohydrate epitopes that can be recognized by anti-carbohydrate antibodies present in human sera. For the performance of serological tests using plant-produced recombinant viral antigens, such cross-reactive carbohydrate determinants (CCDs) could result in false positives. Here, we transiently expressed an RBD variant in wild-type and glycoengineered Nicotiana benthamiana leaves and characterized the impact of different plant-specific N-glycans on RBD reactivity in serological assays. While the overall performance of the different RBD glycoforms was comparable to each other and to a human cell line produced RBD, there was a higher tendency toward false positive results with sera containing allergy-related CCD-antibodies when an RBD carrying β1,2-xylose and core α1,3-fucose was used. These rare events could be further minimized by pre-incubating sera from allergic individuals with a CCD-inhibitor. Thereby, false positive signals obtained from anti-CCD antibodies, could be reduced by 90%, on average.

Production of Human Acid-Alpha Glucosidase With a Paucimannose Structure by Glycoengineered Arabidopsis Cell Culture

Plant cell cultures have emerged as a promising platform for the production of biopharmaceutics due to their cost-effectiveness, safety, ability to control the cultivation, and secrete products into culture medium. However, the use of this platform is hindered by the generation of plant-specific N-glycans, the inability to produce essential N-glycans for cellular delivery of biopharmaceutics, and low productivity. In this study, an alternative acid-alpha glucosidase (GAA) for enzyme replacement therapy of Pompe disease was produced in a glycoengineered Arabidopsis alg3 cell culture. The N-glycan composition of the GAA consisted of a predominantly paucimannosidic structure, Man3GlcNAc2 (M3), without the plant-specific N-glycans. Supplementing the culture medium with NaCl to a final concentration of 50 mM successfully increased GAA production by 3.8-fold. GAA from an NaCl-supplemented culture showed a similar N-glycan profile, indicating that the NaCl supplementation did not affect N-glycosylation. The results of this study highlight the feasibility of using a glycoengineered plant cell culture to produce recombinant proteins for which M3 or mannose receptor-mediated delivery is desired.

Complex-Type N-Glycans Influence the Root Hair Landscape of Arabidopsis Seedlings by Altering the Auxin Output

Roots supply plants with nutrients and water, besides anchoring them in the soil. The primary root with its lateral roots constitutes the central skeleton of the root system. In particular, root hairs increase the root surface, which is critical for optimizing uptake efficiency. During root-cell growth and development, many proteins that are components of, e.g., the cell wall and plasma membrane are constitutively transported through the secretory system and become posttranslationally modified. Here, the best-studied posttranslational modification is protein N-glycosylation. While alterations in the attachment/modification of N-glycans within the ER lumen results in severe developmental defects, the impact of Golgi-localized complex N-glycan modification, particularly on root development, has not been studied in detail. We report that impairment of complex-type N-glycosylation results in a differential response to synthetic phytohormones with earlier and increased root-hair elongation. Application of either the cytokinin BAP, the auxin NAA, or the ethylene precursor ACC revealed an interaction of auxin with complex N-glycosylation during root-hair development. Especially in gntI mutant seedlings, the early block of complex N-glycan formation resulted in an increased auxin sensitivity. RNA-seq experiments suggest that gntI roots have permanently elevated nutrient-, hypoxia-, and defense-stress responses, which might be a consequence of the altered auxin responsiveness.

Lewis A Glycans Are Present on Proteins Involved in Cell Wall Biosynthesis and Appear Evolutionarily Conserved Among Natural Arabidopsis thaliana Accessions

N-glycosylation is a highly abundant protein modification present in all domains of life. Terminal sugar residues on complex-type N-glycans mediate various crucial biological processes in mammals such as cell-cell recognition or protein-ligand interactions. In plants, the Lewis A trisaccharide constitutes the only known outer-chain elongation of complex N-glycans. Lewis A containing complex N-glycans appear evolutionary conserved, having been identified in all plant species analyzed so far. Despite their ubiquitous occurrence, the biological function of this complex N-glycan modification is currently unknown. Here, we report the identification of Lewis A bearing glycoproteins from three different plant species: Arabidopsis thaliana, Nicotiana benthamiana, and Oryza sativa. Affinity purification via the JIM84 antibody, directed against Lewis A structures on complex plant N-glycans, was used to enrich Lewis A bearing glycoproteins, which were subsequently identified via nano-LC-MS. Selected identified proteins were recombinantly expressed and the presence of Lewis A confirmed via immunoblotting and site-specific N-glycan analysis. While the proteins identified in O. sativa are associated with diverse functions, proteins from A. thaliana and N. benthamiana are mainly involved in cell wall biosynthesis. However, a Lewis A-deficient mutant line of A. thaliana showed no change in abundance of cell wall constituents such as cellulose or lignin. Furthermore, we investigated the presence of Lewis A structures in selected accessions from the 1001 genome database containing amino acid variations in the enzymes required for Lewis A biosynthesis. Besides one relict line showing no detectable levels of Lewis A, the modification was present in all other tested accessions. The data provided here comprises the so far first attempt at identifying Lewis A bearing glycoproteins across different species and will help to shed more light on the role of Lewis A structures in plants.

Protein Glycosylation Investigated by Mass Spectrometry: An Overview

The protein glycosylation is a post-translational modification of crucial importance for its involvement in molecular recognition, protein trafficking, regulation, and inflammation. Indeed, abnormalities in protein glycosylation are correlated with several disease states such as cancer, inflammatory diseases, and congenial disorders. The understanding of cellular mechanisms through the elucidation of glycan composition encourages researchers to find analytical solutions for their detection. Actually, the multiplicity and diversity of glycan structures bond to the proteins, the variations in polarity of the individual saccharide residues, and the poor ionization efficiencies make their detection much trickier than other kinds of biopolymers. An overview of the most prominent techniques based on mass spectrometry (MS) for protein glycosylation (glycoproteomics) studies is here presented. The tricks and pre-treatments of samples are discussed as a crucial step prodromal to the MS analysis to improve the glycan ionization efficiency. Therefore, the different instrumental MS mode is also explored for the qualitative and quantitative analysis of glycopeptides and the glycans structural composition, thus contributing to the elucidation of biological mechanisms.

Stable Protein Sialylation in Physcomitrella

Recombinantly produced proteins are indispensable tools for medical applications. Since the majority of them are glycoproteins, their N-glycosylation profiles are major determinants for their activity, structural properties and safety. For therapeutical applications, a glycosylation pattern adapted to product and treatment requirements is advantageous. Physcomitrium patens (Physcomitrella, moss) is able to perform highly homogeneous complex-type N-glycosylation. Additionally, it has been glyco-engineered to eliminate plant-specific sugar residues by knock-out of the β1,2-xylosyltransferase and α1,3-fucosyltransferase genes (Δxt/ft). Furthermore, Physcomitrella meets wide-ranging biopharmaceutical requirements such as GMP compliance, product safety, scalability and outstanding possibilities for precise genome engineering. However, all plants, in contrast to mammals, lack the capability to perform N-glycan sialylation. Since sialic acids are a common terminal modification on human N-glycans, the property to perform N-glycan sialylation is highly desired within the plant-based biopharmaceutical sector. In this study, we present the successful achievement of protein N-glycan sialylation in stably transformed Physcomitrella. The sialylation ability was achieved in a Δxt/ft moss line by stable expression of seven mammalian coding sequences combined with targeted organelle-specific localization of the encoded enzymes responsible for the generation of β1,4-galactosylated acceptor N-glycans as well as the synthesis, activation, transport and transfer of sialic acid. Production of free (Neu5Ac) and activated (CMP-Neu5Ac) sialic acid was proven. The glycosidic anchor for the attachment of terminal sialic acid was generated by the introduction of a chimeric human β1,4-galactosyltransferase gene under the simultaneous knock-out of the gene encoding the endogenous β1,3-galactosyltransferase. Functional complex-type N-glycan sialylation was confirmed via mass spectrometric analysis of a stably co-expressed recombinant human protein.

Subcellular locations of potential cell wall proteins in plants: predictors, databases and cross-referencing

The cell wall is the most striking feature that distinguishes plant cells from animal cells. It plays an essential role in cell shape, stability, growth and protection. Despite being present in small amounts, cell wall proteins (CWPs) are crucial components of the cell wall. The cell wall proteome generally consists of sensu stricto CWPs, apoplast proteins and extracellular secreted proteins. Currently, there is a need for the bioinformatics analysis of a tremendous number of protein sequences that have been generated from genomic, transcriptomic and proteomics research. Compared with intracellular proteins, the location prediction of CWPs is challenging because many aspects of these proteins have not been experimentally characterized, and there are no CWP-trained, specific predictors available. By introducing the biological relevance (particularly molecular aspects) of the cell wall and CWPs, we critically evaluated the accuracy of 16 state-of-the-art predictors and classical predictors for the prediction of CWPs using an independent database of Arabidopsis and rice proteins. All experimentally verified CWPs and non-CWPs were retrieved from the UniProt Knowledgebase. Based on the evaluation, we currently recommend the predictors mGOASVM, HybridGO-Loc and FUEL-mLoc for CWPs. Furthermore, we outlined the public databases that can be used to cross-reference the subcellular location of CWPs. We illustrate a flowchart of the subcellular location prediction and a cross-reference of possible CWPs. Finally, we discuss challenges and perspectives in the bioinformatics analysis of CWPs. It is hoped that this article will provide practical guidance regarding CWPs for nonspecialists and provide insight for bioinformatics experts to develop computational tools for CWPs.

Automated glycan assembly of Lewis type I and II oligosaccharide antigens

Human blood group related glycan antigens are fucosylated (neo-)lactoseries oligosaccharides that play crucial roles in pathogenic processes. Lewis type-II-chain antigens mark the surface of cancer cells, but are also mediators of bacterial infections. To investigate the biological roles of Lewis type glycans a host of synthetic approaches has been developed. Here, we illustrate how automated glycan assembly (AGA) using a set of six monosaccharide building blocks provides quick access to a series of more than ten defined Lewis type-I and type-II antigens, including Lex, Ley, Lea, Leb and KH-1. Glycans with up to three α-fucose branches were assembled following a strictly linear approach and obtained in excellent stereoselectivity and purity.

Plant-Made Antibodies: Properties and Therapeutic Applications

BACKGROUND

A cost-effective plant platform for therapeutic monoclonal antibody production is both flexible and scalable. Plant cells have mechanisms for protein synthesis and posttranslational modification, including glycosylation, similar to those in animal cells. However, plants produce less complex and diverse Asn-attached glycans compared to animal cells and contain plant-specific residues. Nevertheless, plant-made antibodies (PMAbs) could be advantageous compared to those produced in animal cells due to the absence of a risk of contamination from nucleic acids or proteins of animal origin.

OBJECTIVE

In this review, the various platforms of PMAbs production are described, and the widely used transient expression system based on Agrobacterium-mediated delivery of genetic material into plant cells is discussed in detail.

RESULTS

We examined the features of and approaches to humanizing the Asn-linked glycan of PMAbs. The prospects for PMAbs in the prevention and treatment of human infectious diseases have been illustrated by promising results with PMAbs against human immunodeficiency virus, rotavirus infection, human respiratory syncytial virus, rabies, anthrax and Ebola virus. The pre-clinical and clinical trials of PMAbs against different types of cancer, including lymphoma and breast cancer, are addressed.

CONCLUSION

PMAb biosafety assessments in patients suggest that it has no side effects, although this does not completely remove concerns about the potential immunogenicity of some plant glycans in humans. Several PMAbs at various developmental stages have been proposed. Promise for the clinical use of PMAbs is aimed at the treatment of viral and bacterial infections as well as in anti-cancer treatment.

The Scope, Functions, and Dynamics of Posttranslational Protein Modifications

Assessing posttranslational modification (PTM) patterns within protein molecules and reading their functional implications present grand challenges for plant biology. We combine four perspectives on PTMs and their roles by considering five classes of PTMs as examples of the broader context of PTMs. These include modifications of the N terminus, glycosylation, phosphorylation, oxidation, and N-terminal and protein modifiers linked to protein degradation. We consider the spatial distribution of PTMs, the subcellular distribution of modifying enzymes, and their targets throughout the cell, and we outline the complexity of compartmentation in understanding of PTM function. We also consider PTMs temporally in the context of the lifetime of a protein molecule and the need for different PTMs for assembly, localization, function, and degradation. Finally, we consider the combined action of PTMs on the same proteins, their interactions, and the challenge ahead of integrating PTMs into an understanding of protein function in plants.

User-friendly extraction and multistage tandem mass spectrometry based analysis of lipid-linked oligosaccharides in microalgae

BACKGROUND

Protein N-glycosylation is initiated within the endoplasmic reticulum through the synthesis of a lipid-linked oligosaccharides (LLO) precursor. This precursor is then transferred en bloc on neo-synthesized proteins through the action of the oligosaccharyltransferase giving birth to glycoproteins. The N-linked glycans bore by the glycoproteins are then processed into oligomannosides prior to the exit of the glycoproteins from the endoplasmic reticulum and its entrance into the Golgi apparatus. In this compartment, the N-linked glycans are further maturated in complex type N-glycans. This process has been well studied in a lot of eukaryotes including higher plants. In contrast, little information regarding the LLO precursor and synthesis of N-linked glycans is available in microalgae.

METHODS

In this report, a user-friendly extraction method combining microsomal enrichment and solvent extractions followed by purification steps is described. This strategy is aiming to extract LLO precursor from microalgae. Then, the oligosaccharide moiety released from the extracted LLO were analyzed by multistage tandem mass spectrometry in two models of microalgae namely the green microalgae, Chlamydomonas reinhardtii and the diatom, Phaeodactylum tricornutum.

RESULTS

The validity of the developed method was confirmed by the analysis of the oligosaccharide structures released from the LLO of two xylosyltransferase mutants of C. reinhardtii confirming that this green microalga synthesizes a linear Glc3Man5GlcNAc2 identical to the one of the wild-type cells. In contrast, the analysis of the oligosaccharide released from the LLO of the diatom P. tricornutum demonstrated for the first time a Glc2Man9GlcNAc2 structure.

CONCLUSION

The method described in this article allows the fast, non-radioactive and reliable multistage tandem mass spectrometry characterization of oligosaccharides released from LLO of microalgae including the ones belonging to the Phaeodactylaceae and Chlorophyceae classes, respectively. The method is fully adaptable for extracting and characterizing the LLO oligosaccharide moiety from microalgae belonging to other phyla.

Degradation pathway of plant complex-type N-glycans: identification and characterization of a key α1,3-fucosidase from glycoside hydrolase family 29

Plant complex-type N-glycans are characterized by the presence of α1,3-linked fucose towards the proximal N-acetylglucosamine residue and β1,2-linked xylose towards the β-mannose residue. These glycans are ultimately degraded by the activity of several glycoside hydrolases. However, the degradation pathway of plant complex-type N-glycans has not been entirely elucidated because the gene encoding α1,3-fucosidase, a glycoside hydrolase acting on plant complex-type N-glycans, has not yet been identified, and its substrate specificity remains to be determined. In the present study, we found that AtFUC1 (an Arabidopsis GH29 α-fucosidase) is an α1,3-fucosidase acting on plant complex-type N-glycans. This fucosidase has been known to act on α1,4-fucoside linkage in the Lewis A epitope of plant complex-type N-glycans. We found that this glycoside hydrolase specifically acted on GlcNAcβ1-4(Fucα1-3)GlcNAc, a degradation product of plant complex-type N-glycans, by sequential actions of vacuolar α-mannosidase, β1,2-xylosidase, and endo-β-mannosidase. The AtFUC1-deficient mutant showed no distinct phenotypic plant growth features; however, it accumulated GlcNAcβ1-4(Fucα1-3)GlcNAc, a substrate of AtFUC1. These results showed that AtFUC1 is an α1,3-fucosidase acting on plant complex-type N-glycans and elucidated the degradation pathway of plant complex-type N-glycans.

Vacuolar Targeting and Characterization of Recombinant Antibodies

Plant-based platforms are extensively use for the expression of recombinant proteins, including monoclonal antibodies (mAbs). Generally, immunoglobulins (Igs) are sorted to the apoplast, which is often afflicted with intense proteolysis. Here, we describe methods to transiently express mAbs sorted to central vacuole in Nicotiana benthamiana leaves and to characterize the obtained IgG. Central vacuole is an appropriate compartment for the efficient production of Abs, consequently vacuolar sorting should be considered as an alternative strategy to obtain high protein yields.

N-linked Glycan Micro-heterogeneity in Glycoproteins of Arabidopsis

N-glycosylation is one of the most common protein post-translational modifications in eukaryotes and has a relatively conserved core structure between fungi, animals and plants. In plants, the biosynthesis of N-glycans has been extensively studied with all the major biosynthetic enzymes characterized. However, few studies have applied advanced mass spectrometry to profile intact plant N-glycopeptides. In this study, we use hydrophilic enrichment, high-resolution tandem mass spectrometry with complementary and triggered fragmentation to profile Arabidopsis N-glycopeptides from microsomal membranes of aerial tissues. A total of 492 N-glycosites were identified from 324 Arabidopsis proteins with extensive N-glycan structural heterogeneity revealed through 1110 N-glycopeptides. To demonstrate the precision of the approach, we also profiled N-glycopeptides from the mutant (xylt) of β-1,2-xylosyltransferase, an enzyme in the N-glycan biosynthetic pathway. This analysis represents the most comprehensive and unbiased collection of Arabidopsis N-glycopeptides revealing an unsurpassed level of detail on the micro-heterogeneity present in N-glycoproteins of Arabidopsis. Data are available via ProteomeXchange with identifier PXD006270.

Molecular cloning and functional expression of Lewis type α1,3/α1,4-fucosyltransferase cDNAs from Mangifera indica L

In higher plants, complex type N-glycans contain characteristic carbohydrate moieties that are not found in mammals. In particular, the attachment of the Lewis a (Le^a) epitope is currently the only known outer chain elongation that is present in plant N-glycans. Such a modification is of great interest in terms of the biological function of complex type N-glycans in plant species. However, little is known regarding the exact molecular basis underlying their Le^a expression. In the present study, we cloned two novel Lewis type fucosyltransferases (MiFUT13) from mango fruit, Mangifera indica L., heterologously expressed the proteins and structurally and functionally characterized them. Using an HPLC-based assay, we demonstrated that the recombinant MiFUT13 proteins mediate the α1,4-fucosylation of acceptor tetrasaccharides with a strict preference for type I-based structure to type II. The results and other findings suggest that MiFUT13s are involved in the biosynthesis of Le^a containing glycoconjugates in mango fruits.

Golgi α1,4-fucosyltransferase of Arabidopsis thaliana partially localizes at the nuclear envelope

We analyzed plant-derived α1,4-fucosyltransferase (FucTc) homologs by reporter fusions and focused on representatives of the Brassicaceae and Solanaceae. Arabidopsis thaliana AtFucTc-green fluorescent protein (GFP) or tomato LeFucTc-GFP restored Lewis-a formation in a fuctc mutant, confirming functionality in the trans-Golgi. AtFucTc-GFP partly accumulated at the nuclear envelope (NE) not observed for other homologs or truncated AtFucTc lacking the N-terminus or catalytic domain. Analysis of At/LeFucTc-GFP swap constructs with exchanged cytosolic, transmembrane and stalk (CTS), or only the CT regions, revealed that sorting information resides in the membrane anchor. Other domains of AtFuctc also contribute, since amino-acid changes in the CT region strongly reduced but did not abolish NE localization. By contrast, two N-terminal GFP copies did, indicating localization at the inner nuclear membrane (INM). Tunicamycin treatment of AtFucTc-GFP abolished NE localization and enhanced overlap with an endosomal marker, suggesting involvement of N-glycosylation. Yet neither expression in protoplasts of Arabidopsis N-glycosylation mutants nor elimination of the N-glycosylation site in AtFucTc prevented perinuclear accumulation. Disruption of endoplasmic reticulum (ER)-to-Golgi transport by co-expression of Sar1(H74L) trapped tunicamycin-released AtFucTc-GFP in the ER, however, without NE localization. Since recovery after tunicamycin-washout required de novo-protein synthesis, our analyses suggest that AtFucTc localizes to the NE/INM due to interaction with an unknown (glyco)protein.

Plant glyco-biotechnology

Glycosylation is an important protein modification in all eukaryotes. Whereas the early asparagine-linked glycosylation (N-glycosylation) and N-glycan processing steps in the endoplasmic reticulum are conserved between mammals and plants, the maturation of complex N-glycans in the Golgi apparatus differs considerably. Due to a restricted number of Golgi-resident N-glycan processing enzymes and the absence of nucleotide sugars such as CMP-N-acetylneuraminic acid, plants produce only a limited repertoire of different N-glycan structures. Moreover, mammalian mucin-type O-glycosylation of serine or threonine residues has not been described in plants and the required machinery is not encoded in their genome which enables de novo build-up of the pathway. As a consequence, plants are very well-suited for the production of homogenous N- and O-glycans and are increasingly used for the production of recombinant glycoproteins with custom-made glycans that may result in the generation of biopharmaceuticals with improved therapeutic potential.

Expression of Lewis-a glycans on polymorphonuclear leukocytes augments function by increasing transmigration

PMN-expressed fucosylated glycans from the Lewis glycan family, including Lewis-x (Le^x) and sialyl Lewis-x (sLe^x), have previously been implicated in the regulation of important PMN functions, including selectin-mediated trafficking across vascular endothelium. Although glycans, such as Le^x and sLe^x, which are based on the type 2 sequence (Galβ1-4GlcNAc-R), are abundant on PMNs, the presence of type 1 Galβ1-3GlcNAc-R glycans required for PMN expression of the closely related stereoisomer of Le^x, termed Lewis-A (Le^a), has not, to our knowledge, been reported. Here, we show that Le^a is abundantly expressed by human PMNs and functionally regulates PMN migration. Using mAbs whose precise epitopes were determined using glycan array technology, Le^a function was probed using Le^a-selective mAbs and lectins, revealing increased PMN transmigration across model intestinal epithelia, which was independent of epithelial-expressed Le^a Analyses of glycan synthetic machinery in PMNs revealed expression of β1-3 galactosyltransferase and α1-4 fucosyltransferase, which are required for Le^a synthesis. Specificity of functional effects observed after ligation of Le^a was confirmed by failure of anti-Le^a mAbs to enhance migration using PMNs from individuals deficient in α1-4 fucosylation. These results demonstrate that Le^a is expressed on human PMNs, and its specific engagement enhances PMN migration responses. We propose that PMN Le^a represents a new target for modulating inflammation and regulating intestinal, innate immunity.

Glycan-binding F-box protein from Arabidopsis thaliana protects plants from Pseudomonas syringae infection

BACKGROUND

A small group of F-box proteins consisting of a conserved F-box domain linked to a domain homologous to the glycan-binding protein has been identified within the genome of Arabidopsis thaliana. Previously, the so-called F-box-Nictaba protein, encoded by the gene At2g02360, was shown to be a functional lectin which binds N-acetyllactosamine structures. Here, we present a detailed qRT-PCR expression analysis of F-box-Nictaba in Arabidopsis plants upon different stresses and hormone treatments.

RESULTS

Expression of the F-box-Nictaba gene was enhanced after plant treatment with salicylic acid and after plant infection with the virulent Pseudomonas syringae pv. tomato strain DC3000 (Pst DC3000). β-glucuronidase histochemical staining of transgenic Arabidopsis plants displayed preferential activity of the At2g02360 promoter in trichomes present on young rosette leaves. qRT-PCR analyses confirmed high expression of F-box-Nictaba in leaf trichomes. A. thaliana plants overexpressing the gene showed less disease symptoms after Pst DC3000 infection with reduced bacterial colonization compared to infected wild type and F-box-Nictaba knock-out plants.

CONCLUSIONS

Our data show that the Arabidopsis F-box-Nictaba gene is a stress-inducible gene responsive to SA, bacterial infection and heat stress, and is involved in salicylic acid related plant defense responses. This knowledge enriched our understanding of the physiological importance of F-box-Nictaba, and can be used to create plants with better performance in changing environmental conditions.

The Arabidopsis tonoplast is almost devoid of glycoproteins with complex N-glycans, unlike the rat lysosomal membrane

The distribution of the N-glycoproteome in integral membrane proteins of the vacuolar membrane (tonoplast) or the plasma membrane of Arabidopsis thaliana and, for further comparison, of the Rattus norvegicus lysosomal and plasma membranes, was analyzed. In silico analysis showed that potential N-glycosylation sites are much less frequent in tonoplast proteins. Biochemical analysis of Arabidopsis subcellular fractions with the lectin concanavalin A, which recognizes mainly unmodified N-glycans, or with antiserum against Golgi-modified N-glycans confirmed the in silico results and showed that, unlike the plant plasma membrane, the tonoplast is almost or totally devoid of N-glycoproteins with Golgi-modified glycans. Lysosomes share with vacuoles the hydrolytic functions and the position along the secretory pathway; however, our results indicate that their membranes had a divergent evolution. We propose that protection against the luminal hydrolases that are abundant in inner hydrolytic compartments, which seems to have been achieved in many lysosomal membrane proteins by extensive N-glycosylation of the luminal domains, has instead been obtained in the vast majority of tonoplast proteins by limiting the length of such domains.

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.

Comparative analysis of N-glycans in the ungerminated and germinated stages of Oryza sativa

All fundamental information such as signal transduction, metabolic control, infection, cell-to-cell signaling, and cell differentiation related to the growth of plants are preserved in germs. In preserving these information, glycans have a key role and are involved in the development and differentiation of organisms. Glycans which exist in rice germ are expected to have an important role in germination. In this study, we performed structural and correlation analysis of the N-glycans in rice germ before and after germination. Our results confirmed that the N-glycans in the ungerminated stage of the rice germ had low number of N-glycans consisting only of six kinds especially with high-mannose and paucimannose type N-glycans being 16.0% and 76.7%, respectively. On the other hand, after 48 hours germinated germ stage, there was an increase in the complex type N-glycans with the appearance of Lewis a structure, the most complex type and a decrease in paucimannose types. These results suggest that at least six kinds of N-glycans are utilized for long time preservation of rice seed, while the diversification of most complex types of N-glycans is produced an environment dependent for shoot formation of rice.

Review/N-glycans: The making of a varied toolbox

Asparagine (N)-linked protein glycosylation is one of the most crucial, prevalent, and complex co- and post-translational protein modifications. It plays a pivotal role in protein folding, quality control, and endoplasmic reticulum (ER)-associated degradation (ERAD) as well as in protein sorting, protein function, and in signal transduction. Furthermore, glycosylation modulates many important biological processes including growth, development, morphogenesis, and stress signaling processes. As a consequence, aberrant or altered N-glycosylation is often associated with reduced fitness, diseases, and disorders. The initial steps of N-glycan synthesis at the cytosolic side of the ER membrane and in the lumen of the ER are highly conserved. In contrast, the final N-glycan processing in the Golgi apparatus is organism-specific giving rise to a wide variety of carbohydrate structures. Despite our vast knowledge on N-glycans in yeast and mammals, the modus operandi of N-glycan signaling in plants is still largely unknown. This review will elaborate on the N-glycosylation biosynthesis pathway in plants but will also critically assess how N-glycans are involved in different signaling cascades, either active during normal development or upon abiotic and biotic stresses.

Subcellular Targeting of Proteins Involved in Modification of Plant N- and O-Glycosylation

Plants are attractive expression hosts for the production of recombinant glycoprotein therapeutics. The quality and efficiency of these biopharmaceuticals are very often influenced by the glycosylation profile. Consequently, approaches are needed that enable the production of recombinant glycoproteins with customized and homogenous N- and O-glycan structures. Here, we describe convenient tools that allow targeting and retention of glycan-modifying enzymes in the early secretory pathway of plants. These protocols can be used to fine-tune the subcellular localization of glycosyltransferases and glycosidases in plants and consequently to increase the homogeneity of glycosylation on recombinant glycoproteins.

Glucose Trimming ofN-Glycan in Endoplasmic Reticulum Is Indispensable for the Growth ofRaphanus sativusSeedling (kaiware radish)

Recently I found that glycosidase inhibitors such as castanospermine, deoxynojirimycin, swainsonine, 2-acetamindo 2,3-dideoxynojirimycin, and deoxymannojirimycin change the N-glycan structure of root glycoproteins, and that the glucosidase inhibitors castanospermine and deoxynojirimycin suppress the growth of Raphanus sativus seedlings (Mega, T., J. Biochem., 2004). The present study undertook to see whether the growth suppression is due to the inhibition of glucose trimming in endoplasmic reticulum (ER). The study, using three glucosidase inhibitors, castanospermine, N-methyl deoxynojirimycin, and deoxynojirimycin, upon the growth of R. sativus foliage leaf, made clear that glucose trimming is indispensable for plant growth, because the inhibition of glucose trimming correlated with leaf growth. On the other hand, processing inhibition in the Golgi apparatus by other glycosidase inhibitors had little effect on plant growth, although N-glycan processing was disrupted depending on inhibitor specificity. These results suggest that N-glycan processing after glucosidase processing is dispensable for plant growth and cell differentiation.

Plant glyco-biotechnology on the way to synthetic biology

Plants are increasingly being used for the production of recombinant proteins. One reason is that plants are highly amenable to glycan engineering processes and allow the production of therapeutic proteins with increased efficacies due to optimized glycosylation profiles. Removal and insertion of glycosylation reactions by knock-out/knock-down approaches and introduction of glycosylation enzymes have paved the way for the humanization of the plant glycosylation pathway. The insertion of heterologous enzymes at exactly the right stage of the existing glycosylation pathway has turned out to be of utmost importance. To enable such precise targeting chimeric enzymes have been constructed. In this short review we will exemplify the importance of correct targeting of glycosyltransferases, we will give an overview of the targeting mechanism of glycosyltransferases, describe chimeric enzymes used in plant N-glycosylation engineering and illustrate how plant glycoengineering builds on the tools offered by synthetic biology to construct such chimeric enzymes.

Biological significance of complex N-glycans in plants and their impact on plant physiology

Asparagine (N)-linked protein glycosylation is a ubiquitous co- and post-translational modification which can alter the biological function of proteins and consequently affects the development, growth, and physiology of organisms. Despite an increasing knowledge of N-glycan biosynthesis and processing, we still understand very little about the biological function of individual N-glycan structures in plants. In particular, the N-glycan-processing steps mediated by Golgi-resident enzymes create a structurally diverse set of protein-linked carbohydrate structures. Some of these complex N-glycan modifications like the presence of β1,2-xylose, core α1,3-fucose or the Lewis a-epitope are characteristic for plants and are evolutionary highly conserved. In mammals, complex N-glycans are involved in different cellular processes including molecular recognition and signaling events. In contrast, the complex N-glycan function is still largely unknown in plants. Here, in this short review, I focus on important recent developments and discuss their implications for future research in plant glycobiology and plant biotechnology.

Receptor-mediated transport of vacuolar proteins: a critical analysis and a new model

In this article we challenge the widely accepted view that receptors for soluble vacuolar proteins (VSRs) bind to their ligands at the trans-Golgi network (TGN) and transport this cargo via clathrin-coated vesicles (CCV) to a multivesicular prevacuolar compartment. This notion, which we term the "classical model" for vacuolar protein sorting, further assumes that low pH in the prevacuolar compartment causes VSR-ligand dissociation, resulting in a retromer-mediated retrieval of the VSRs to the TGN. We have carefully evaluated the literature with respect to morphology and function of the compartments involved, localization of key components of the sorting machinery, and conclude that there is little direct evidence in its favour. Firstly, unlike mammalian cells where the sorting receptor for lysosomal hydrolases recognizes its ligand in the TGN, the available data suggests that in plants VSRs interact with vacuolar cargo ligands already in the endoplasmic reticulum. Secondly, the evidence supporting the packaging of VSR-ligand complexes into CCV at the TGN is not conclusive. Thirdly, the prevacuolar compartment appears to have a pH unsuitable for VSR-ligand dissociation and lacks the retromer core and the sorting nexins needed for VSR recycling. We present an alternative model for protein sorting in the TGN that draws attention to the much overlooked role of Ca(2+) in VSR-ligand interactions and which may possibly also be a factor in the sequestration of secretory proteins.

Plant species and organ influence the structure and subcellular localization of recombinant glycoproteins

Many plant-based systems have been developed as bioreactors to produce recombinant proteins. The choice of system for large-scale production depends on its intrinsic expression efficiency and its propensity for scale-up, post-harvest storage and downstream processing. Factors that must be considered include the anticipated production scale, the value and intended use of the product, the geographical production area, the proximity of processing facilities, intellectual property, safety and economics. It is also necessary to consider whether different species and organs affect the subcellular trafficking, structure and qualitative properties of recombinant proteins. In this article we discuss the subcellular localization and N-glycosylation of two commercially-relevant recombinant glycoproteins (Aspergillus niger phytase and anti-HIV antibody 2G12) produced in different plant species and organs. We augment existing data with novel results based on the expression of the same recombinant proteins in Arabidopsis and tobacco seeds, focusing on similarities and subtle differences in N-glycosylation that often reflect the subcellular trafficking route and final destination, as well as differences generated by unique enzyme activities in different species and tissues. We discuss the potential consequences of such modifications on the stability and activity of the recombinant glycoproteins.

Organization of the ER-Golgi interface for membrane traffic control

Coat protein complex I (COPI) and COPII are required for bidirectional membrane trafficking between the endoplasmic reticulum (ER) and the Golgi. While these core coat machineries and other transport factors are highly conserved across species, high-resolution imaging studies indicate that the organization of the ER-Golgi interface is varied in eukaryotic cells. Regulation of COPII assembly, in some cases to manage distinct cellular cargo, is emerging as one important component in determining this structure. Comparison of the ER-Golgi interface across different systems, particularly mammalian and plant cells, reveals fundamental elements and distinct organization of this interface. A better understanding of how these interfaces are regulated to meet varying cellular secretory demands should provide key insights into the mechanisms that control efficient trafficking of proteins and lipids through the secretory pathway.

Moss-based production of asialo-erythropoietin devoid of Lewis A and other plant-typical carbohydrate determinants

Protein therapeutics represent one of the most increasing areas in the pharmaceutical industry. Plants gain acceptance as attractive alternatives for high-quality and economical protein production. However, as the majority of biopharmaceuticals are glycoproteins, plant-specific N-glycosylation has to be taken into consideration. In Physcomitrella patens (moss), glyco-engineering is an applicable tool, and the removal of immunogenic core xylose and fucose residues was realized before. Here, we present the identification of the enzymes that are responsible for terminal glycosylation (α1,4 fucosylation and β1,3 galactosylation) on complex-type N-glycans in moss. The terminal trisaccharide consisting of α1,4 fucose and β1,3 galactose linked to N-acetylglucosamine forms the so-called Lewis A epitope. This epitope is rare on moss wild-type proteins, but was shown to be enriched on complex-type N-glycans of moss-produced recombinant human erythropoietin, while unknown from the native human protein. Via gene targeting of moss galactosyltransferase and fucosyltransferase genes, we identified the gene responsible for terminal glycosylation and were able to completely abolish the formation of Lewis A residues on the recombinant biopharmaceutical.

Biologically active, magnICON®-expressed EPO-Fc from stably transformed Nicotiana benthamiana plants presenting tetra-antennary N-glycan structures

In the past two decades plants have emerged as a valuable alternative for the production of pharmaceutical proteins. Since N-glycosylation influences functionality and stability of therapeutic proteins, the plant N-glycosylation pathway should be humanized. Here, we report the transient magnICON(®) expression of the erythropoietin fusion protein (EPO-Fc) in Nicotiana benthamiana plants that produce multi-antennary N-glycans without the plant-specific β1,2-xylose and α1,3-fucose residues in a stable manner (Nagels et al., 2011). The EPO-Fc fusion protein consists of EPO with a C-terminal-linked IgG-Fc domain and is used for pulmonary delivery of recombinant EPO to patients (Bitonti et al., 2004). Plant expressed EPO-Fc was quantified using a paramagnetic-particle chemiluminescent immunoassay and shown to be active in vitro via receptor binding experiments in HEK293T cells. Mass spectrometry-based N-glycan analysis confirmed the presence of multi-antennary N-glycans on plant-expressed EPO-Fc. The described research is the next step towards the development of a production platform for pharmaceutical proteins in plants.

Arabidopsis F-box protein containing a Nictaba-related lectin domain interacts with N-acetyllactosamine structures

The Arabidopsis thaliana genome contains a small group of bipartite F-box proteins, consisting of an N-terminal F-box domain and a C-terminal domain sharing sequence similarity with Nictaba, the jasmonate-induced glycan-binding protein (lectin) from tobacco. Based on the high sequence similarity between the C-terminal domain of these proteins and Nictaba, the hypothesis was put forward that the so-called F-box-Nictaba proteins possess carbohydrate-binding activity and accordingly can be considered functional homologs of the mammalian sugar-binding F-box or Fbs proteins which are involved in proteasomal degradation of glycoproteins. To obtain experimental evidence for the carbohydrate-binding activity and specificity of the A. thaliana F-box-Nictaba proteins, both the complete F-box-Nictaba sequence of one selected Arabidopsis F-box protein (in casu At2g02360) as well as the Nictaba-like domain only were expressed in Pichia pastoris and analyzed by affinity chromatography, agglutination assays and glycan micro-array binding assays. These results demonstrated that the C-terminal Nictaba-like domain provides the F-box-protein with a carbohydrate-binding activity that is specifically directed against N- and O-glycans containing N-acetyllactosamine (Galβ1-3GlcNAc and Galβ1-4GlcNAc) and poly-N-acetyllactosamine ([Galβ1-4GlcNAc]n) as well as Lewis A (Galβ1-3(Fucα1-4)GlcNAc), Lewis X (Galβ1-4(Fucα1-3)GlcNAc, Lewis Y (Fucα1-2Galβ1-4(Fucα1-3)GlcNAc) and blood type B (Galα1-3(Fucα1-2)Galβ1-3GlcNAc) motifs. Based on these findings one can reasonably conclude that at least the A. thaliana F-box-Nictaba protein encoded by At2g02360 can act as a carbohydrate-binding protein. The results from the glycan array assays revealed differences in sugar-binding specificity between the F-box protein and Nictaba, indicating that the same carbohydrate-binding motif can accommodate unrelated oligosaccharides.

The secreted plant N-glycoproteome and associated secretory pathways

N-Glycosylation is a common form of eukaryotic protein post-translational modification, and one that is particularly prevalent in plant cell wall proteins. Large scale and detailed characterization of N-glycoproteins therefore has considerable potential in better understanding the composition and functions of the cell wall proteome, as well as those proteins that reside in other compartments of the secretory pathway. While there have been numerous studies of mammalian and yeast N-glycoproteins, less is known about the population complexity, biosynthesis, structural variation, and trafficking of their plant counterparts. However, technical developments in the analysis of glycoproteins and the structures the glycans that they bear, as well as valuable comparative analyses with non-plant systems, are providing new insights into features that are common among eukaryotes and those that are specific to plants, some of which may reflect the unique nature of the plant cell wall. In this review we present an overview of the current knowledge of plant N-glycoprotein synthesis and trafficking, with particular reference to those that are cell wall localized.

Sub-compartmental organization of Golgi-resident N-glycan processing enzymes in plants

In all eukaryotes, the Golgi apparatus is the main site of protein glycosylation. It is widely accepted that the glycosidases and glycosyltransferases involved in N-glycan processing are found concentrated within the Golgi stack where they provide their function. This means that enzymes catalyzing early steps in the processing pathway are located mainly at the cis-side, whereas late-acting enzymes mostly locate to the trans-side of the stacks, creating a non-uniform distribution along the cis-trans axis of the Golgi. There is compelling evidence that the information for their sorting to specific Golgi cisternae depends on signals encoded in the proteins themselves as well as on the trafficking machinery that recognizes these signals and it is believed that cisternal sub-compartmentalization is achieved and maintained by a combination of retention and retrieval mechanisms. Yet, the signals, mechanism(s), and molecular factors involved are still unknown. Here, we address recent findings and summarize the current understanding of this fundamental process in plant cell biology.

Deletion of fucose residues in plant N-glycans by repression of the GDP-mannose 4,6-dehydratase gene using virus-induced gene silencing and RNA interference

Production of pharmaceutical glycoproteins in plants has many advantages in terms of safety and reduced costs. However, plant-produced glycoproteins have N-glycans with plant-specific sugar residues (core β-1,2-xylose and α-1,3-fucose) and a Lewis a (Le(a) ) epitope, i.e., Galβ(1-3)[Fucα(1-4)]GlcNAc. Because these sugar residues and glycan structures seemed to be immunogenic, several attempts have been made to delete them by repressing their respective glycosyltransferase genes. However, until date, such deletions have not been successful in completely eliminating the fucose residues. In this study, we simultaneously reduced the plant-specific core α-1,3-fucose and α-1,4-fucose residues in the Le(a) epitopes by repressing the Guanosine 5'-diphosphate (GDP)-D-mannose 4,6-dehydratase (GMD) gene, which is associated with GDP-L-fucose biosynthesis, in Nicotiana benthamiana plants. Repression of GMD was achieved using virus-induced gene silencing (VIGS) and RNA interference (RNAi). The proportion of fucose-free N-glycans found in total soluble protein from GMD gene-repressed plants increased by 80% and 95% following VIGS and RNAi, respectively, compared to wild-type plants. A small amount of putative galactose substitution in N-glycans from the NbGMD gene-repressed plants was observed, similar to what has been previously reported GMD-knockout Arabidopsis mutant. On the other hand, the recombinant mouse granulocyte-macrophage colony-stimulating factor (GM-CSF) with fucose-deleted N-glycans was successfully produced in NbGMD-RNAi transgenic N. benthamiana plants. Thus, repression of the GMD gene is thus very useful for deleting immunogenic total fucose residues and facilitating the production of pharmaceutical glycoproteins in plants.

Sequential depletion and acquisition of proteins during Golgi stack disassembly and reformation

Herein, we report the stepwise transport of multiple plant Golgi membrane markers during disassembly of the Golgi apparatus in tobacco leaf epidermal cells in response to the induced expression of the GTP-locked Sar1p or Brefeldin A (BFA), and reassembly on BFA washout. The distribution of fluorescent Golgi-resident N-glycan processing enzymes and matrix proteins (golgins) with specific cis-trans-Golgi sub-locations was followed by confocal microscopy during disassembly and reassembly. The first event during Golgi disassembly was the loss of trans-Golgi enzymes and golgins from Golgi membranes, followed by a sequential redistribution of medial and cis-Golgi enzymes into the endoplasmic reticulum (ER), whilst golgins were relocated to the ER or cytoplasm. This event was confirmed by fractionation and immuno-blotting. The sequential redistribution of Golgi components in a trans-cis sequence may highlight a novel retrograde trafficking pathway between the trans-Golgi and the ER in plants. Release of Golgi markers from the ER upon BFA washout occurred in the opposite sequence, with cis-matrix proteins labelling Golgi-like structures before cis/medial enzymes. Trans-enzyme location was preceded by trans-matrix proteins being recruited back to Golgi membranes. Our results show that Golgi disassembly and reassembly occur in a highly ordered fashion in plants.