Localization and in vitro binding studies suggest that the cytoplasmic/nuclear tobacco lectin can interact in situ with high-mannose and complex N-glycans

Lectins and lectibodies: potential promising antiviral agents

In nature, lectins are widely dispersed proteins that selectively recognize and bind to carbohydrates and glycoconjugates via reversible bonds at specific binding sites. Many viral diseases have been treated with lectins due to their wide range of structures, specificity for carbohydrates, and ability to bind carbohydrates. Through hemagglutination assays, these proteins can be detected interacting with various carbohydrates on the surface of cells and viral envelopes. This review discusses the most robust lectins and their rationally engineered versions, such as lectibodies, as antiviral proteins. Fusion of lectin and antibody’s crystallizable fragment (Fc) of immunoglobulin G (IgG) produces a molecule called a “lectibody” that can act as a carbohydrate-targeting antibody. Lectibodies can not only bind to the surface glycoproteins via their lectins and neutralize and clear viruses or infected cells by viruses but also perform Fc-mediated antibody effector functions. These functions include complement-dependent cytotoxicity (CDC), antibody-dependent cell-mediated cytotoxicity (ADCC), and antibody-dependent cell-mediated phagocytosis (ADCP). In addition to entering host cells, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike protein S1 binds to angiotensin-converting enzyme 2 (ACE2) and downregulates it and type I interferons in a way that may lead to lung disease. The SARS-CoV-2 spike protein S1 and human immunodeficiency virus (HIV) envelope are heavily glycosylated, which could make them a major target for developing vaccines, diagnostic tests, and therapeutic drugs. Lectibodies can lead to neutralization and clearance of viruses and cells infected by viruses by binding to glycans located on the envelope surface (e.g., the heavily glycosylated SARS-CoV-2 spike protein).

Identification of N-linked Glycoproteins in Silkworm Serum Using Con A Lectin Affinity Chromatography and Mass Spectrometry

Glycosylation is one of the most common post-translational modifications to occur during protein biosynthesis, but remains poorly understood in insects. In this study, we collected serum proteins from two silkworm developmental stages, namely day 7 of the fifth instar larval stage and day 2 of the pupal stage. Results of SDS-PAGE and periodic acid-Schiff staining revealed that most serum proteins with high abundance were putative glycoproteins. LC-MS/MS identified 149 larval and 303 pupal serum proteins in the Con A lectin-enriched fractions. GO analysis revealed that many serum proteins were involved in the proteolysis and carbohydrate metabolic process. 82 N-linked glycoproteins with at least one glycosylation site were identified. N-Linked glycosylation occurred at the sequon, Asn-X-Ser/Thr, and the proportions of Ser and Thr glycosylation at the hydroxy position were found 39.6% and 60.3%, respectively. The N-glycan structures found in serum glycoproteins were mainly Man2FucGlcNAc2 (67.9%). Since storage protein 1 and transferrin had a relatively high abundance in the serum and could be significantly enriched by Con A lectin, their glycosylation was analyzed in detail. Glycoside hydrases, serine proteases and serpins were found to form three interacting glycoprotein networks using the website STRING. This study provides important clues for the understanding of the function of N-linked glycosylation in metabolism, immunity, and metamorphosis.

Man-Specific Lectins from Plants, Fungi, Algae and Cyanobacteria, as Potential Blockers for SARS-CoV, MERS-CoV and SARS-CoV-2 (COVID-19) Coronaviruses: Biomedical Perspectives

Betacoronaviruses, responsible for the “Severe Acute Respiratory Syndrome” (SARS) and the “Middle East Respiratory Syndrome” (MERS), use the spikes protruding from the virion envelope to attach and subsequently infect the host cells. The coronavirus spike (S) proteins contain receptor binding domains (RBD), allowing the specific recognition of either the dipeptidyl peptidase CD23 (MERS-CoV) or the angiotensin-converting enzyme ACE2 (SARS-Cov, SARS-CoV-2) host cell receptors. The heavily glycosylated S protein includes both complex and high-mannose type N-glycans that are well exposed at the surface of the spikes. A detailed analysis of the carbohydrate-binding specificity of mannose-binding lectins from plants, algae, fungi, and bacteria, revealed that, depending on their origin, they preferentially recognize either complex type N-glycans, or high-mannose type N-glycans. Since both complex and high-mannose glycans substantially decorate the S proteins, mannose-specific lectins are potentially useful glycan probes for targeting the SARS-CoV, MERS-CoV, and SARS-CoV-2 virions. Mannose-binding legume lectins, like pea lectin, and monocot mannose-binding lectins, like snowdrop lectin or the algal lectin griffithsin, which specifically recognize complex N-glycans and high-mannose glycans, respectively, are particularly adapted for targeting coronaviruses. The biomedical prospects of targeting coronaviruses with mannose-specific lectins are wide-ranging including detection, immobilization, prevention, and control of coronavirus infection.

BanLec-eGFP Chimera as a Tool for Evaluation of Lectin Binding to High-Mannose Glycans on Microorganisms

Fluorescently labeled lectins are useful tools for in vivo and in vitro studies of the structure and function of tissues and various pathogens such as viruses, bacteria, and fungi. For the evaluation of high-mannose glycans present on various glycoproteins, a three-dimensional (3D) model of the chimera was designed from the crystal structures of recombinant banana lectin (BanLec, Protein Data Bank entry (PDB): 5EXG) and an enhanced green fluorescent protein (eGFP, PDB 4EUL) by applying molecular modeling and molecular mechanics and expressed in Escherichia coli. BanLec-eGFP, produced as a soluble cytosolic protein of about 42 kDa, revealed β-sheets (41%) as the predominant secondary structures, with the emission peak maximum detected at 509 nm (excitation wavelength 488 nm). More than 65% of the primary structure was confirmed by mass spectrometry. Competitive BanLec-eGFP binding to high mannose glycans of the influenza vaccine (Vaxigrip®) was shown in a fluorescence-linked lectin sorbent assay (FLLSA) with monosaccharides (mannose and glucose) and wild type BanLec and H84T BanLec mutant. BanLec-eGFP exhibited binding to mannose residues on different strains of Salmonella in flow cytometry, with especially pronounced binding to a Salmonella Typhi clinical isolate. BanLec-eGFP can be a useful tool for screening high-mannose glycosylation sites on different microorganisms.

Role of Protein Glycosylation in Host-Pathogen Interaction

Host-pathogen interactions are fundamental to our understanding of infectious diseases. Protein glycosylation is one kind of common post-translational modification, forming glycoproteins and modulating numerous important biological processes. It also occurs in host-pathogen interaction, affecting host resistance or pathogen virulence often because glycans regulate protein conformation, activity, and stability, etc. This review summarizes various roles of different glycoproteins during the interaction, which include: host glycoproteins prevent pathogens as barriers; pathogen glycoproteins promote pathogens to attack host proteins as weapons; pathogens glycosylate proteins of the host to enhance virulence; and hosts sense pathogen glycoproteins to induce resistance. In addition, this review also intends to summarize the roles of lectin (a class of protein entangled with glycoprotein) in host-pathogen interactions, including bacterial adhesins, viral lectins or host lectins. Although these studies show the importance of protein glycosylation in host-pathogen interaction, much remains to be discovered about the interaction mechanism.

Sugar-Recognizing Ubiquitin Ligases: Action Mechanisms and Physiology

F-box proteins, the substrate recognition subunits of SKP1–CUL1–F-box protein (SCF) E3 ubiquitin ligase complexes, play crucial roles in various cellular events mediated by ubiquitination. Several sugar-recognizing F-box proteins exist in both mammalian and plant cells. Although glycoproteins generally reside outside of cells, or in organelles of the secretory pathway, these lectin-type F-box proteins reside in the nucleocytoplasmic compartment. Mammalian sugar-recognizing F-box proteins commonly bind to the innermost position of N-glycans through a unique small hydrophobic pocket in their loops. Two cytosolic F-box proteins, Fbs1 and Fbs2, recognize high-mannose glycans synthesized in the ER, and SCF^Fbs1 and SCF^Fbs2 ubiquitinate excess unassembled or misfolded glycoproteins in the ERAD pathway by recognizing the innermost glycans, which serve as signals for aberrant proteins. On the other hand, endomembrane-bound Fbs3 recognizes complex glycans as well as high-mannose glycans, and SCF^Fbs3 ubiquitinates exposed glycoproteins in damaged lysosomes fated for elimination by selective autophagy. Plants express stress-inducible lectin-type F-box proteins recognizing a wider range of N- and O-glycans, suggesting that the roles of mammalian and plant lectin-type F-box proteins have diverged over the course of evolution to recognize species-specific targets with distinct functions. These sugar-recognizing F-box proteins interpret glycans in the cytosol as markers of unwanted proteins and organelles, and degrade them via the proteasome or autophagy.

Overview of the Structure⁻Function Relationships of Mannose-Specific Lectins from Plants, Algae and Fungi

To date, a number of mannose-binding lectins have been isolated and characterized from plants and fungi. These proteins are composed of different structural scaffold structures which harbor a single or multiple carbohydrate-binding sites involved in the specific recognition of mannose-containing glycans. Generally, the mannose-binding site consists of a small, central, carbohydrate-binding pocket responsible for the "broad sugar-binding specificity" toward a single mannose molecule, surrounded by a more extended binding area responsible for the specific recognition of larger mannose-containing N-glycan chains. Accordingly, the mannose-binding specificity of the so-called mannose-binding lectins towards complex mannose-containing N-glycans depends largely on the topography of their mannose-binding site(s). This structure⁻function relationship introduces a high degree of specificity in the apparently homogeneous group of mannose-binding lectins, with respect to the specific recognition of high-mannose and complex N-glycans. Because of the high specificity towards mannose these lectins are valuable tools for deciphering and characterizing the complex mannose-containing glycans that decorate both normal and transformed cells, e.g., the altered high-mannose N-glycans that often occur at the surface of various cancer cells.

Signaling through plant lectins: modulation of plant immunity and beyond

Lectins constitute an abundant group of proteins that are present throughout the plant kingdom. Only recently, genome-wide screenings have unraveled the multitude of different lectin sequences within one plant species. It appears that plants employ a plurality of lectins, though relatively few lectins have already been studied and functionally characterized. Therefore, it is very likely that the full potential of lectin genes in plants is underrated. This review summarizes the knowledge of plasma membrane-bound lectins in different biological processes (such as recognition of pathogen-derived molecules and symbiosis) and illustrates the significance of soluble intracellular lectins and how they can contribute to plant signaling. Altogether, the family of plant lectins is highly complex with an enormous diversity in biochemical properties and activities.

Histology-guided high-resolution AP-SMALDI mass spectrometry imaging of wheat-Fusarium graminearum interaction at the root-shoot junction

BACKGROUND

Fungal pathogens like Fusarium graminearum can cause severe yield losses and mycotoxin contamination of food and feed worldwide. We recently showed its ability to systemically colonize wheat via root infection. However, the molecular response of wheat to Fusarium root rot (FRR) infection and systemic spread is still unknown. As a molecular camera, mass spectrometry (MS) imaging combines label-free and multiplex metabolite profiling with histopathology.

RESULTS

Atmospheric-pressure (AP)-SMALDI-MS imaging was combined with optical microscopy to study wheat-F. graminearum interaction at the root-shoot junction, which is a crucial line of defense against a pathogen that can invade all distal plant parts. To scope the functional, temporal and local aspects of FRR disease spread, metabolic changes were simultaneous visualized in diseased and healthy stem bases of the resistant cultivar Florence-Aurore at 10, 14 and 21 days after root inoculation. Histological information was used to identify disease relevant tissues and to assist the interpretation of molecular images. Detected mycotoxin compounds secreted by F. graminearum showed a route of stem infection that was consistent with observations made by microscopy. The outer epidermis and vasculature of leaf sheath were, at different disease stages, identified as prominent sites of pathogen migration and wheat protection. Wheat metabolites mapped to these relatively small tissues indicated cell wall strengthening and antifungal activity as direct defenses as well as conservation in the wheat reactions to F. graminearum diseases that affect different plant organs.

CONCLUSIONS

AP-SMALDI-MS imaging at high spatial resolution is a versatile technique that can be applied to basic and applied aspects of agricultural research. Combining the technology with optical microscopy was found to be a powerful tool to gain in-depth information on almost unknown crop disease. Moreover, the approach allowed studying metabolism at the host-pathogen interface. The results provide important hints to an understanding of the complex spatio-temporal organization of plant resistance. Defense-on-demand responses to pathogen ingress were found, which provide opportunities for future research towards an improved resistance that does not negatively impact yield development in the field by saving plant resources and, moreover, may control different Fusarium diseases.

Nictaba Homologs from Arabidopsis thaliana Are Involved in Plant Stress Responses

Plants are constantly exposed to a wide range of environmental stresses, but evolved complicated adaptive and defense mechanisms which allow them to survive in unfavorable conditions. These mechanisms protect and defend plants by using different immune receptors located either at the cell surface or in the cytoplasmic compartment. Lectins or carbohydrate-binding proteins are widespread in the plant kingdom and constitute an important part of these immune receptors. In the past years, lectin research has focused on the stress-inducible lectins. The Nicotiana tabacum agglutinin, abbreviated as Nictaba, served as a model for one family of stress-related lectins. Here we focus on three non-chimeric Nictaba homologs from Arabidopsis thaliana, referred to as AN3, AN4, and AN5. Confocal microscopy of ArathNictaba enhanced green fluorescent protein (EGFP) fusion constructs transiently expressed in N. benthamiana or stably expressed in A. thaliana yielded fluorescence for AN4 and AN5 in the nucleus and the cytoplasm of the plant cell, while fluorescence for AN3 was only detected in the cytoplasm. RT-qPCR analysis revealed low expression for all three ArathNictabas in different tissues throughout plant development. Stress application altered the expression levels, but all three ArathNictabas showed a different expression pattern. Pseudomonas syringae infection experiments with AN4 and AN5 overexpression lines demonstrated a significantly higher tolerance of several transgenic lines to P. syringae compared to wild type plants. Finally, AN4 was shown to interact with two enzymes involved in plant defense, namely TGG1 and BGLU23. Taken together, our data suggest that the ArathNictabas represent stress-regulated proteins with a possible role in plant stress responses. On the long term this research can contribute to the development of more stress-resistant plants.

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.

Overexpression of Nictaba-Like Lectin Genes from Glycine max Confers Tolerance toward Pseudomonas syringae Infection, Aphid Infestation and Salt Stress in Transgenic Arabidopsis Plants

Plants have evolved a sophisticated immune system that allows them to recognize invading pathogens by specialized receptors. Carbohydrate-binding proteins or lectins are part of this immune system and especially the lectins that reside in the nucleocytoplasmic compartment are known to be implicated in biotic and abiotic stress responses. The class of Nictaba-like lectins (NLL) groups all proteins with homology to the tobacco (Nicotiana tabacum) lectin, known as a stress-inducible lectin. Here we focus on two Nictaba homologs from soybean (Glycine max), referred to as GmNLL1 and GmNLL2. Confocal laser scanning microscopy of fusion constructs with the green fluorescent protein either transiently expressed in Nicotiana benthamiana leaves or stably transformed in tobacco BY-2 suspension cells revealed a nucleocytoplasmic localization for the GmNLLs under study. RT-qPCR analysis of the transcript levels for the Nictaba-like lectins in soybean demonstrated that the genes are expressed in several tissues throughout the development of the plant. Furthermore, it was shown that salt treatment, Phytophthora sojae infection and Aphis glycines infestation trigger the expression of particular NLL genes. Stress experiments with Arabidopsis lines overexpressing the NLLs from soybean yielded an enhanced tolerance of the plant toward bacterial infection (Pseudomonas syringae), insect infestation (Myzus persicae) and salinity. Our data showed a better performance of the transgenic lines compared to wild type plants, indicating that the NLLs from soybean are implicated in the stress response. These data can help to further elucidate the physiological importance of the Nictaba-like lectins from soybean, which can ultimately lead to the design of crop plants with a better tolerance to changing environmental conditions.

Insights Into Triticum aestivum Seedling Root Rot Caused by Fusarium graminearum

Fusarium graminearum is one of the most common and potent fungal pathogens of wheat (Triticum aestivum), known for causing devastating spike infections and grain yield damage. F. graminearum is a typical soil-borne pathogen that builds up during consecutive cereal cropping. Speculation on systemic colonization of cereals by F. graminearum root infection have long existed but have not been proven. We have assessed the Fusarium root rot disease macroscopically in a diverse set of 12 wheat genotypes and microscopically in a comparative study of two genotypes with diverging responses. Here, we show a 'new' aspect of the F. graminearum life cycle, i.e., the head blight fungus uses a unique root-infection strategy with an initial stage typical for root pathogens and a later stage typical for spike infection. Root colonization negatively affects seedling development and leads to systemic plant invasion by tissue-adapted fungal strategies. Another major outcome is the identification of partial resistance to root rot. Disease severity assessments and histological examinations both demonstrated three distinct disease phases that, however, proceeded differently in resistant and susceptible genotypes. Soil-borne inoculum and root infection are considered significant components of the F. graminearum life cycle with important implications for the development of new strategies of resistance breeding and disease control.

A new set of ESTs from chickpea (Cicer arietinum L.) embryo reveals two novel F-box genes, CarF-box_PP2 and CarF-box_LysM, with potential roles in seed development

Considering the economic importance of chickpea (C. arietinum L.) seeds, it is important to understand the mechanisms underlying seed development for which a cDNA library was constructed from 6 day old chickpea embryos. A total of 8,186 ESTs were obtained from which 4,048 high quality ESTs were assembled into 1,480 unigenes that majorly encoded genes involved in various metabolic and regulatory pathways. Of these, 95 ESTs were found to be involved in ubiquitination related protein degradation pathways and 12 ESTs coded specifically for putative F-box proteins. Differential transcript accumulation of these putative F-box genes was observed in chickpea tissues as evidenced by quantitative real-time PCR. Further, to explore the role of F-box proteins in chickpea seed development, two F-box genes were selected for molecular characterization. These were named as CarF-box_PP2 and CarF-box_LysM depending on their C-terminal domains, PP2 and LysM, respectively. Their highly conserved structures led us to predict their target substrates. Subcellular localization experiment revealed that CarF-box_PP2 was localized in the cytoplasm and CarF-box_LysM was localized in the nucleus. We demonstrated their physical interactions with SKP1 protein, which validated that they function as F-box proteins in the formation of SCF complexes. Sequence analysis of their promoter regions revealed certain seed specific cis-acting elements that may be regulating their preferential transcript accumulation in the seed. Overall, the study helped in expanding the EST database of chickpea, which was further used to identify two novel F-box genes having a potential role in seed development.

Live imaging of companion cells and sieve elements in Arabidopsis leaves

The phloem is a complex tissue composed of highly specialized cells with unique subcellular structures and a compact organization that is challenging to study in vivo at cellular resolution. We used confocal scanning laser microscopy and subcellular fluorescent markers in companion cells and sieve elements, for live imaging of the phloem in Arabidopsis leaves. This approach provided a simple framework for identifying phloem cell types unambiguously. It highlighted the compactness of the meshed network of organelles within companion cells. By contrast, within the sieve elements, unknown bodies were observed in association with the PP2-A1:GFP, GFP:RTM1 and RTM2:GFP markers at the cell periphery. The phloem lectin PP2-A1:GFP marker was found in the parietal ground matrix. Its location differed from that of the P-protein filaments, which were visualized with SEOR1:GFP and SEOR2:GFP. PP2-A1:GFP surrounded two types of bodies, one of which was identified as mitochondria. This location suggested that it was embedded within the sieve element clamps, specific structures that may fix the organelles to each another or to the plasma membrane in the sieve tubes. GFP:RTM1 was associated with a class of larger bodies, potentially corresponding to plastids. PP2-A1:GFP was soluble in the cytosol of immature sieve elements. The changes in its subcellular localization during differentiation provide an in vivo blueprint for monitoring this process. The subcellular features obtained with these companion cell and sieve element markers can be used as landmarks for exploring the organization and dynamics of phloem cells in vivo.

NICTABA and UDA, two GlcNAc-binding lectins with unique antiviral activity profiles

OBJECTIVES

This study aimed to assess the antiviral properties of a unique lectin (NICTABA) produced by the tobacco plant, Nicotiana tabacum.

METHODS

Cellular assays were used to investigate the antiviral activity of NICTABA and Urtica dioica agglutinin (UDA). Surface plasmon resonance (SPR) studies were performed to study the sugar specificity and the interactions of both lectins with the envelope glycoproteins of HIV-1.

RESULTS

The N-acetyl-d-glucosamine (GlcNAc)-binding lectins exhibited broad-spectrum activity against several families of enveloped viruses including influenza A/B, Dengue virus type 2, herpes simplex virus types 1 and 2 and HIV-1/2. The IC50 of NICTABA for various HIV-1 strains, clinical isolates and HIV-2 assessed in PBMCs ranged from 5 to 30 nM. Furthermore, NICTABA inhibited syncytium formation between persistently HIV-1-infected T cells and uninfected CD4+ T lymphocytes and prevented DC-SIGN-mediated HIV-1 transmission to CD4+ target T lymphocytes. However, unlike many other antiviral carbohydrate-binding agents (CBAs) described so far, NICTABA did not block HIV-1 capture to DC-SIGN+ cells and it did not interfere with the binding of the human monoclonal antibody 2G12 to gp120. SPR studies with HIV-1 envelope glycoproteins showed that the affinity of NICTABA for gp120 and gp41 was in the low nanomolar range. The specific binding of NICTABA to gp120 could be prevented in the presence of a GlcNAc trimer, but not in the presence of mannose trimers. NICTABA displayed no antiviral activity against non-enveloped viruses.

CONCLUSIONS

Since CBAs possess a high genetic barrier for the development of viral resistance and NICTABA shows a broad antiviral activity profile, this CBA may qualify as a potential antiviral candidate with a pleiotropic mode of action aimed at targeting the entry of enveloped viruses.

Insights into animal and plant lectins with antimicrobial activities

Lectins are multivalent proteins with the ability to recognize and bind diverse carbohydrate structures. The glyco -binding and diverse molecular structures observed in these protein classes make them a large and heterogeneous group with a wide range of biological activities in microorganisms, animals and plants. Lectins from plants and animals are commonly used in direct defense against pathogens and in immune regulation. This review focuses on sources of animal and plant lectins, describing their functional classification and tridimensional structures, relating these properties with biotechnological purposes, including antimicrobial activities. In summary, this work focuses on structural-functional elucidation of diverse lectin groups, shedding some light on host-pathogen interactions; it also examines their emergence as biotechnological tools through gene manipulation and development of new drugs.

HIV-1 and its resistance to peptidic carbohydrate-binding agents (CBAs): an overview

The glycoproteins on the surfaces of enveloped viruses, such as HIV, can be considered as a unique target for antiviral therapy. Different carbohydrate-binding agents (CBAs) target specific glycans present on viral glycoproteins of enveloped viruses. It has been shown that long-term CBA pressure in vitro can result in mutant HIV-1 isolates with several N-linked glycan deletions on gp120. These studies demonstrated that mainly high-mannose type glycans are deleted. However, interestingly, N241, N262 and N356 on gp120 have never been found to be affected after prolonged CBA exposure. Here, we review the mutation and (cross)-resistance profiles of eleven specific generated CBA-resistant HIV-1 strains. We observed that the broad-neutralizing anti-carbohydrate binding mAb 2G12 became completely inactive against all the generated CBA-resistant HIV-1 clade B isolates. In addition, all of the CBAs discussed in this review, with the exception of NICTABA, interfered with the binding of 2G12 mAb to gp120 expressed on HIV-1-infected T cells. The cross-resistance profiles of mutant HIV-1 strains are varying from increased susceptibility to very high resistance levels, even among different classes of CBAs with dissimilar sugar specificities or binding moieties [e.g., α(1,3), α(1,2), α(1,6)]. Recent studies demonstrated promising results in non-topical formulations (e.g., intranasally or subcutaneously), highlighting their potential for prevention (microbicides) and antiviral therapy.

In vivo interaction between the tobacco lectin and the core histone proteins

Nictaba, a lectin accumulating in tobacco (Nicotiana tabacum) leaves treated with jasmonate, is considered to act as a signaling protein in the stress physiology of the plant. Immunolocalization studies revealed that Nictaba has a nucleocytoplasmic localization. In previous research, histones were identified as primary interaction partners for Nictaba. Here, the interaction between Nictaba and tobacco histones was scrutinized in vivo. Localization studies, performed in stably transformed Nicotiana benthamiana plants, confirmed the nucleocytoplasmic localization of the lectin and colocalization with the presumed binding partners in the nucleus. Furthermore, bimolecular fluorescence complementation (BiFC) assays confirmed the interaction in vivo. Since BiFC signals were also observed for a Nictaba mutant incapable of binding sugar moieties, this interaction may be mediated by alternative binding sites. The interaction of Nictaba with core histones possibly reflects a role of this stress inducible lectin in gene regulation or chromatin remodeling.

Lectin domains at the frontiers of plant defense

Plants are under constant attack from pathogens and herbivorous insects. To protect and defend themselves, plants evolved a multi-layered surveillance system, known as the innate immune system. Plants sense their encounters upon perception of conserved microbial structures and damage-associated patterns using cell-surface and intracellular immune receptors. Plant lectins and proteins with one or more lectin domains represent a major part of these receptors. The whole group of plant lectins comprises an elaborate collection of proteins capable of recognizing and interacting with specific carbohydrate structures, either originating from the invading organisms or from damaged plant cell wall structures. Due to the vast diversity in protein structures, carbohydrate recognition domains and glycan binding specificities, plant lectins constitute a very diverse protein superfamily. In the last decade, new types of nucleocytoplasmic plant lectins have been identified and characterized, in particular lectins expressed inside the nucleus and the cytoplasm of plant cells often as part of a specific plant response upon exposure to different stress factors or changing environmental conditions. In this review, we provide an overview on plant lectin motifs used in the constant battle against pathogens and predators during plant defenses.

Purification, cDNA cloning and recombinant protein expression of a phloem lectin-like anti-insect defense protein BPLP from the phloem exudate of the wax gourd, Benincasa hispida

Latex and other exudates in plants contain various proteins that are thought to play important defensive roles against herbivorous insects and pathogens. Herein, the defensive effects of phloem exudates against the Eri silkworm, Samia ricini (Saturniidae, Lepidoptera) in several cucurbitaceous plants were investigated. It was found that phloem exudates are responsible for the defensive activities of cucurbitaceous plants, such as the wax gourd Benincasa hispida and Cucumis melo, especially in B. hispida, whose leaves showed the strongest growth-inhibitory activity of all the cucurbitaceous plants tested. A 35 kDa proteinaceous growth-inhibitory factor against insects designated BPLP (B. hispida Phloem Lectin-like Protein) was next isolated and purified from the B. hispida exudate, using anion exchange and gel filtration chromatography. A very low concentration (70 μg/g) of BPLP significantly inhibited growth of S. ricini larvae. The full-length cDNA (1076 bp) encoding BPLP was cloned and its nucleotide sequence was determined. The deduced amino acid sequence of BPLP had 51% identity with a cucurbitaceous phloem lectin (phloem protein 2, PP2), and showed binding specificity to oligomers of N-acetylglucosamine. Some features of BPLP indicated that it does not have a cysteine residue and it is composed of two repeats of similar sequences, suggesting that BPLP is distinct from PP2. Recombinant BPLP, obtained by expressing the cDNA in Escherichia coli, showed both chitin-binding lectin activity and growth-inhibitory activity against S. ricini larvae. The present study thus provides experimental evidence that phloem exudates of Cucurbitaceae plants, analogous to plant latex, play defensive roles against insect herbivores, especially against chewing insects, and contain defensive substances toxic to them.

Expression analysis of jasmonate-responsive lectins in plants

The Nicotiana tabacum lectin, also designated Nictaba, is a nucleocytoplasmic carbohydrate-binding protein produced in tobacco leaves after application of specific jasmonates and upon insect herbivory. Here, we describe different techniques by which lectin production can be induced through exogenous jasmonate application on tobacco plants. Furthermore, we elaborate on the assays to detect Nictaba expression at RNA and protein levels as well as on the agglutination assays to identify the lectin activity.

Promiscuity of the euonymus carbohydrate-binding domain

Plants synthesize small amounts of carbohydrate-binding proteins on exposure to stress. For example, on exposure to drought, high salt, wounding and by treatment with some plant hormones or by pathogen attack. In contrast to the 'classical' plant lectins that are mostly located in the vacuolar compartment, this new class of inducible lectins is present in the cytoplasm and in the nucleus. Taking into account that any physiological role of plant lectins most likely relies on their specific carbohydrate-binding activity and specificity, the discovery of these stress-related lectins provides strong evidence for the importance of protein-carbohydrate-interactions in plant cells. Hitherto, six families of such nucleocytoplasmic lectins have been identified in plants. This review will focus on the nucleocytoplasmic lectins with one or more Euonymus lectin (EUL) domain(s). The carbohydrate-binding specificity of EUL proteins from a monocot, a dicot and a lower plant has been compared. Furthermore, modeling of the different EUL domains revealed a similar ß-trefoil fold consisting of three bundles of ß-sheet organized around a pseudo three-fold symmetry axis. Despite the sequence similarity and the conserved amino acids in the binding site, glycan array analyses showed that the EUL domain has a promiscuous carbohydrate-binding site capable of accommodating high mannose N-glycans, blood group B related structures and galactosylated epitopes.

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.

Plant lectins as defense proteins against phytophagous insects

One of the most important direct defense responses in plants against the attack by phytophagous insects is the production of insecticidal peptides or proteins. One particular class of entomotoxic proteins present in many plant species is the group of carbohydrate-binding proteins or lectins. During the last decade a lot of progress was made in the study of a few lectins that are expressed in response to herbivory by phytophagous insects and the insecticidal properties of plant lectins in general. This review gives an overview of lectins with high potential for the use in pest control strategies based on their activity towards pest insects. In addition, potential target sites for lectins inside the insect and the mode of action are discussed. In addition, the effect of plant lectins on non-target organisms such as beneficial insects as well as on human/animal consumers is discussed. It can be concluded that some insecticidal lectins are useful tools that can contribute to the development of integrated pest management strategies with minimal effect(s) on non-target organisms.

Jasmonate response of the Nicotiana tabacum agglutinin promoter in Arabidopsis thaliana

NICTABA is a carbohydrate-binding protein (also called lectin) that is expressed in several Nicotiana species after treatment with jasmonates and insect herbivory. Analyses with tobacco lines overexpressing the NICTABA gene as well as lines with reduced lectin expression have shown the entomotoxic effect of NICTABA against Lepidopteran larvae, suggesting a role of the lectin in plant defense. Until now, little is known with respect to the upstream regulatory mechanisms that are controlling the expression of inducible plant lectins. Using Arabidopsis thaliana plants stably expressing a promoter-β-glucuronidase (GUS) fusion construct, it was shown that jasmonate treatment influenced the NICTABA promoter activity. A strong GUS staining pattern was detected in very young tissues (the apical and root meristems, the cotyledons and the first true leaves), but the promoter activity decreased when plants were getting older. NICTABA was also expressed at low concentrations in tobacco roots and expression levels increased after cold treatment. The data presented confirm a jasmonate-dependent response of the promoter sequence of the tobacco lectin gene in Arabidopsis. These new jasmonate-responsive tobacco promoter sequences can be used as new tools in the study of jasmonate signalling related to plant development and defense.

Expression analysis of the nucleocytoplasmic lectin 'Orysata' from rice in Pichia pastoris

The Oryza sativa lectin, abbreviated Orysata, is a mannose-specific, jacalin-related lectin expressed in rice plants after exposure to certain stress conditions. Expression of a fusion construct containing the rice lectin sequence linked to enhanced green fluorescent protein in Bright Yellow 2 tobacco cells revealed that Orysata is located in the nucleus and the cytoplasm of the plant cell, indicating that it belongs to the class of nucleocytoplasmic jacalin-related lectins. Since the expression level of Orysata in rice tissues is very low the lectin was expressed in the methylotrophic yeast Pichia pastoris with the Saccharomyces α-factor sequence to direct the recombinant protein into the secretory pathway and express the protein into the medium. Approximately 12 mg of recombinant lectin was purified per liter medium. SDS/PAGE and western blot analysis showed that the recombinant lectin exists in two molecular forms. Far western blot analysis revealed that the 23 kDa lectin polypeptide contains an N-glycan which is absent in the 18.5 kDa polypeptide. Characterization of the glycans present in the recombinant Orysata revealed high-mannose structures, Man9-11 glycans being the most abundant. Glycan array analysis showed that Orysata interacts with high-mannose as well as with more complex N-glycan structures. Orysata has potent anti-human immunodeficiency virus and anti-respiratory syncytial virus activity in cell culture compared with other jacalin-related lectins.

Synergistic in vitro anti-HIV type 1 activity of tenofovir with carbohydrate-binding agents (CBAs)

Tenofovir, a well-known and highly prescribed anti-HIV-1 drug for the treatment of HIV/AIDS infections, has recently also shown its effectiveness as a potential microbicide drug in the prevention of HIV transmission. Here, we evaluated the combination of tenofovir with various members of the class of carbohydrate-binding agents (CBAs) targeting the glycans on the viral envelope gp120 for their anti-HIV efficacy. The tenofovir/CBA combinations predominantly showed synergistic antiviral activity using the median effect principle. These findings illustrate that combination of tenofovir with CBAs may increase the antiviral potency of the individual drugs and reducing the risk on potential side-effects.

Interaction of the tobacco lectin with histone proteins

The tobacco (Nicotiana tabacum) agglutinin or Nictaba is a member of a novel class of plant lectins residing in the nucleus and the cytoplasm of tobacco cells. Since tobacco lectin expression is only observed after the plant has been subjected to stress situations such as jasmonate treatment or insect attack, Nictaba is believed to act as a signaling protein involved in the stress physiology of the plant. In this paper, a nuclear proteomics approach was followed to identify the binding partners for Nictaba in the nucleus and the cytoplasm of tobacco cv Xanthi cells. Using lectin affinity chromatography and pull-down assays, it was shown that Nictaba interacts primarily with histone proteins. Binding of Nictaba with histone H2B was confirmed in vitro using affinity chromatography of purified calf thymus histone proteins on a Nictaba column. Elution of Nictaba-interacting histone proteins was achieved with 1 m N-acetylglucosamine (GlcNAc). Moreover, mass spectrometry analyses indicated that the Nictaba-interacting histone proteins are modified by O-GlcNAc. Since the lectin-histone interaction was shown to be carbohydrate dependent, it is proposed that Nictaba might fulfill a signaling role in response to stress by interacting with O-GlcNAcylated proteins in the plant cell nucleus.

Lectins as tools to select for glycosylated proteins

Glycosylation has been recognized as one of the most important modifications on proteins. The interactions between proteins and glycans are known to play an important role in many biological processes. Lectins are carbohydrate-binding proteins that can specifically interact with and select for carbohydrate structures. The technique of lectin affinity chromatography takes advantage of this specific interaction and enables the selection and purification of glycoproteins with carbohydrate structures complementary to the lectin-binding site. Depending on the carbohydrate specificity of the lectin glycoprotein fractions enriched for example, high mannose or complex N-glycans or O-glycans can be obtained. Afterward both the protein part and the glycan part can be analyzed in more detail allowing the identification of the interacting partners and the type of glycans involved.

Intermolecular interaction studies using small volumes

We present the use of 1-mm room-temperature probe technology to perform intermolecular interaction studies using chemical shift perturbation methods and saturation transfer difference (STD) spectroscopy using small sample volumes. The use of a small sample volume (5-10 µl) allows for an alternative titration protocol where individual samples are prepared for each titration point, rather than the usual protocol used for a 5-mm probe setup where the ligand is added consecutively to the solution containing the protein or host of interest. This allows for considerable economy in the consumption and cost of the protein and ligand amounts required for interaction studies. For titration experiments, the use of the 1-mm setup consumes less than 10% of the ligand amount required using a 5-mm setup. This is especially significant when complex ligands that are only available in limited quantities, typically because they are obtained from natural sources or through elaborate synthesis efforts, need to be investigated. While the use of smaller volumes does increase the measuring time, we demonstrate that the use of commercial small volume probes allows the study of interactions that would otherwise be impossible to address by NMR.

Mutational analysis of the carbohydrate binding activity of the tobacco lectin

At present the three-dimensional structure of the tobacco lectin, further referred to as Nictaba, and its carbohydrate-binding site are unresolved. In this paper, we propose a three-dimensional model for the Nictaba domain based on the homology between Nictaba and the carbohydrate-binding module 22 of Clostridium thermocellum Xyn10B. The suggested model nicely fits with results from circular dichroism experiments, indicating that Nictaba consists mainly of β-sheet. In addition, the previously identified nuclear localization signal is located at the top of the protein as a part of a protruding loop. Judging from this model and sequence alignments with closely related proteins, conserved glutamic acid and tryptophan residues in the Nictaba sequence were selected for mutational analysis. The mutant DNA sequences as well as the original Nictaba sequence have been expressed in Pichia pastoris and the recombinant proteins were purified from the culture medium. Subsequently, the recombinant proteins were characterized and their carbohydrate binding properties analyzed with glycan array technology. It was shown that mutation of glutamic acid residues in the C-terminal half of the protein did not alter the carbohydrate-binding activity of the lectin. In contrast, mutation of tryptophan residues in the N-terminal half of the Nictaba domain resulted in a complete loss of carbohydrate binding activity. These results suggest that tryptophan residues play an important role in the carbohydrate binding site of Nictaba.

Rapid affinity-purification and physicochemical characterization of pumpkin (Cucurbita maxima) phloem exudate lectin

The chito-oligosaccharide-specific lectin from pumpkin (Cucurbita maxima) phloem exudate has been purified to homogeneity by affinity chromatography on chitin. After SDS/PAGE in the presence of 2-mercaptoethanol, the pumpkin phloem lectin yielded a single band corresponding to a molecular mass of 23.7 kDa, whereas ESI-MS (electrospray ionization MS) gave the molecular masses of the subunit as 24645 Da. Analysis of the CD spectrum of the protein indicated that the secondary structure of the lectin consists of 9.7% α-helix, 35.8% β-sheet, 22.5% β-turn and 32.3% unordered structure. Saccharide binding did not significantly affect the secondary and tertiary structures of the protein. The haemagglutinating activity of pumpkin phloem lectin was mostly unaffected in the temperature range 4–70 °C, but a sharp decrease was seen between 75 and 85 °C. Differential scanning calorimetric and CD spectroscopic studies suggest that the lectin undergoes a co-operative thermal unfolding process centred at approx. 81.5 °C, indicating that it is a relatively stable protein.

Nicotiana tabacum agglutinin is active against Lepidopteran pest insects

A jasmonate-inducible lectin called Nicotiana tabacum agglutinin or NICTABA was found in tobacco (Nicotiana tabacum cv Samsun) leaves. Since NICTABA expression is also induced after insect herbivory, a role in the defence response of tobacco was suggested. In this report, a detailed analysis was made of the entomotoxic properties of NICTABA using different transgenic approaches. First, purified NICTABA was shown to be strongly resistant to proteolytic degradation by enzymes present in the Lepidopteran midgut. To address the question of whether NICTABA is also active against Lepidopteran larvae, transgenic N. tabacum plants that silence endogenous NICTABA expression were constructed using RNA interference. Feeding experiments with these transgenic N. tabacum plants demonstrated that silencing of NICTABA expression enhances the larval performance of the generalist pest insect Spodoptera littoralis. In a second transgenic approach, NICTABA was ectopically expressed in the wild diploid tobacco Nicotiana attenuata, a species that lacks a functional NICTABA gene. When these transgenic N. attenuata plants were used in feeding experiments with S. littoralis larvae, a clear reduction in mass gain and significantly slower development were observed. In addition, feeding experiments with the Solanaceae specialist, Manduca sexta, provided further evidence that NICTABA exerts clear entomotoxic effects on Lepidopteran larvae.

Proteins with an Euonymus lectin-like domain are ubiquitous in Embryophyta

BACKGROUND

Cloning of the Euonymus lectin led to the discovery of a novel domain that also occurs in some stress-induced plant proteins. The distribution and the diversity of proteins with an Euonymus lectin (EUL) domain were investigated using detailed analysis of sequences in publicly accessible genome and transcriptome databases.

RESULTS

Comprehensive in silico analyses indicate that the recently identified Euonymus europaeus lectin domain represents a conserved structural unit of a novel family of putative carbohydrate-binding proteins, which will further be referred to as the Euonymus lectin (EUL) family. The EUL domain is widespread among plants. Analysis of retrieved sequences revealed that some sequences consist of a single EUL domain linked to an unrelated N-terminal domain whereas others comprise two in tandem arrayed EUL domains. A new classification system for these lectins is proposed based on the overall domain architecture. Evolutionary relationships among the sequences with EUL domains are discussed.

CONCLUSION

The identification of the EUL family provides the first evidence for the occurrence in terrestrial plants of a highly conserved plant specific domain. The widespread distribution of the EUL domain strikingly contrasts the more limited or even narrow distribution of most other lectin domains found in plants. The apparent omnipresence of the EUL domain is indicative for a universal role of this lectin domain in plants. Although there is unambiguous evidence that several EUL domains possess carbohydrate-binding activity further research is required to corroborate the carbohydrate-binding properties of different members of the EUL family.

Nucleocytoplasmic plant lectins

During the last decade it was unambiguously shown that plants synthesize minute amounts of carbohydrate-binding proteins upon exposure to stress situations like drought, high salt, hormone treatment, pathogen attack or insect herbivory. In contrast to the 'classical' plant lectins, which are typically found in storage vacuoles or in the extracellular compartment this new class of lectins is located in the cytoplasm and the nucleus. Based on these observations the concept was developed that lectin-mediated protein-carbohydrate interactions in the cytoplasm and the nucleus play an important role in the stress physiology of the plant cell. Hitherto, six families of nucleocytoplasmic lectins have been identified. This review gives an overview of our current knowledge on the occurrence of nucleocytoplasmic plant lectins. The carbohydrate-binding properties of these lectins and potential ligands in the nucleocytoplasmic compartment are discussed in view of the physiological role of the lectins in the plant cell.

Spodoptera littoralis-induced lectin expression in tobacco

The induced defense response in plants towards herbivores is mainly regulated by jasmonates and leads to the accumulation of so-called jasmonate-induced proteins. Recently, a jasmonate (JA) inducible lectin called Nicotiana tabacum agglutinin or NICTABA was discovered in tobacco (N. tabacum cv Samsun) leaves. Tobacco plants also accumulate the lectin after insect attack by caterpillars. To study the functional role of NICTABA, the accumulation of the JA precursor 12-oxophytodienoic acid (OPDA), JA as well as different JA metabolites were analyzed in tobacco leaves after herbivory by larvae of the cotton leafworm (Spodoptera littoralis) and correlated with NICTABA accumulation. It was shown that OPDA, JA as well as its methyl ester can trigger NICTABA accumulation. However, hydroxylation of JA and its subsequent sulfation and glucosylation results in inactive compounds that have lost the capacity to induce NICTABA gene expression. The expression profile of NICTABA after caterpillar feeding was recorded in local as well as in systemic leaves, and compared to the expression of several genes encoding defense proteins, and genes encoding a tobacco systemin and the allene oxide cyclase, an enzyme in JA biosynthesis. Furthermore, the accumulation of NICTABA was quantified after S. littoralis herbivory and immunofluorescence microscopy was used to study the localization of NICTABA in the tobacco leaf.

Phosphorylation modification of wheat lectin VER2 is associated with vernalization-induced O-GlcNAc signaling and intracellular motility

BACKGROUND

O-linked beta-N-acetylglucosamine (O-GlcNAc) modification of proteins mediates stress response and cellular motility in animal cells. The plant lectin concanavalin A can increase nuclear O-GlcNAc levels and decrease cytoplasmic O-GlcNAc levels in T lymphocytes. However, the functions of O-GlcNAc signaling in plants, as well as the relation between plant lectins and O-GlcNAc in response to environmental stimuli are largely undefined.

METHODOLOGY/PRINCIPAL FINDINGS

We describe a jacalin-like lectin VER2 in wheat that shows N-acetylglucosamine and galactose specificity. Immunocytochemical localization showed VER2 expression induced predominantly at potential nuclear structures in shoot tips and young leaves and weakly in cytoplasm in response to vernalization. In contrast, under devernalization (continuous stimulation with a higher temperature after vernalization), VER2 signals appeared predominantly in cytoplasm. 2-D electrophoresis, together with western blot analysis, showed phosphorylation modification of VER2 under vernalization. Immunoblot assay with O-GlcNAc-specific antibody revealed that vernalization increased O-GlcNAc modification of proteins at the global level. An O-GlcNAc-modified protein co-immunoprecipitated with VER2 in vernalized wheat plants but not in devernalized materials. The dynamic of VER2 was observed in transgenic Arabidopsis overexpressing the VER2-GFP fusion protein. Overexpressed VER2 accelerated nuclear migration. Immunogold labeling and indirect immunofluoresence colocalization assay indicated that VER2-GFP was targeted to the secretory pathway.

CONCLUSIONS/SIGNIFICANCE

O-GlcNAc signaling is involved in the vernalization response in wheat, and phosphorylation is necessary for the lectin VER2 involving O-GlcNAc signaling during vernalization. Our findings open the way to studies of O-GlcNAc protein modification in response to environmental signals in plants.

Glycobiology in the cytosol: the bitter side of a sweet world

Progress in glycobiology has undergone explosive growth over the past decade with more of the researchers now realizing the importance of glycan chains in various inter- and intracellular processes. However, there is still an area of glycobiology awaiting exploration. This is especially the case for the field of "glycobiology in the cytosol" which remains rather poorly understood. Yet evidence is accumulating to demonstrate that the glycoconjugates and their recognition molecules (i.e. lectins) are often present in this subcellular compartment.

Do F-box proteins with a C-terminal domain homologous with the tobacco lectin play a role in protein degradation in plants?

Protein turnover is a key post-translational event that regulates numerous cellular processes. It enables cells to respond rapidly to intracellular signals and changing environmental conditions by adjusting the levels of pivotal proteins. A major proteolytic pathway involves the ubiquitination of target proteins and subsequent targeting to the 26S proteasome for degradation. Many F-box proteins play a determining role in the substrate specificity of this degradation pathway. In most cases, selective recognition of the target proteins relies on protein-protein interactions mediated by the C-terminal domain of the F-box proteins. In mammals, the occurrence of F-box proteins with a C-terminal SBD (sugar-binding domain) that specifically interacts with high-mannose N-glycans on target glycoproteins has been documented. The identification and characterization of these sugar-binding F-box proteins demonstrated that F-box proteins do not exclusively use protein-protein interactions but also protein-carbohydrate interactions in the Ub (ubiquitin)/proteasome pathway. Recently, putative sugar-binding F-box proteins have been identified in plants. Genome analyses in Arabidopsis and rice revealed the presence of F-box proteins with a C-terminal lectin-related domain homologous with Nictaba, a jasmonate-inducible lectin from tobacco that was shown to interact with the core structure of high-mannose and complex N-glycans. Owing to the high similarity in structure and specificity between Nictaba and the SBD of the mammalian Fbs proteins, a similar role for the plant F-box proteins with a Nictaba domain in nucleocytoplasmic protein degradation in plant cells is suggested.

Cell-free expression and functionality analysis of the tobacco lectin

The Nicotiana tabacum lectin, called Nictaba, is a nucleocytoplasmic plant lectin expressed in tobacco leaves after exogenous application of specific jasmonates and upon insect herbivory. Since the lectin concentrations are rather low, huge amounts of plant material are needed to purify milligram quantities of the protein. In addition, the purified lectin fractions are always contaminated with low molecular weight compounds such as phenols. In an attempt to improve and facilitate the purification of the tobacco lectin in reasonable amounts, an in vitro-coupled transcription/translation system based on an Escherichia coli lysate was used to express the lectin gene. Recombinant expression levels could be enhanced by an adapted codon usage. Recombinant lectin was purified, biochemically characterized and found to be biologically active. The biological activity of the recombinant lectin towards insect epithelial midgut cells was clearly demonstrated in a functional bio-assay and the internal cellular localization was analyzed using immunocytochemical techniques.

The jasmonate-induced expression of the Nicotiana tabacum leaf lectin

Previous experiments with tobacco (Nicotiana tabacum L. cv Samsun NN) plants revealed that jasmonic acid methyl ester (JAME) induces the expression of a cytoplasmic/nuclear lectin in leaf cells and provided the first evidence that jasmonates affect the expression of carbohydrate-binding proteins in plant cells. To corroborate the induced accumulation of relatively large amounts of a cytoplasmic/nuclear lectin, a detailed study was performed on the induction of the lectin in both intact tobacco plants and excised leaves. Experiments with different stress factors demonstrated that the lectin is exclusively induced by exogeneously applied jasmonic acid and JAME, and to a lesser extent by insect herbivory. The lectin concentration depends on leaf age and the position of the tissue in the leaf. JAME acts systemically in intact plants but very locally in excised leaves. Kinetic analyses indicated that the lectin is synthesized within 12 h exposure time to JAME, reaching a maximum after 60 h. After removal of JAME, the lectin progressively disappears from the leaf tissue. The JAME-induced accumulation of an abundant nuclear/cytoplasmic lectin is discussed in view of the possible role of this lectin in the plant.

Expression of the nucleocytoplasmic tobacco lectin in the yeast Pichia pastoris

The Nicotiana tabacum lectin, also called Nictaba, is a nucleocytoplasmic plant lectin expressed in tobacco leaves after exposure to jasmonates. Purification of the lectin from raw material is a time-consuming process, demanding large amounts of induced plant material. In addition, the lectin yield is low and purified lectin fractions are always contaminated with low molecular weight compounds such as phenols. In a way to improve and facilitate the purification of the tobacco lectin, we cloned the Nictaba gene in a vector optimized for protein expression in the methylotrophic yeast Pichia pastoris. In this report, we present data of the expression profile of recombinant Nictaba in the P. pastoris culture medium and in P. pastoris cells together with the purification strategy using ion exchange chromatography and affinity chromatography on a column with immobilized ovomucoid. Pichia transformants were estimated to express approximately 6mg of recombinant lectin per liter medium after a 72h culture. SDS-PAGE and Western blot analysis revealed that the recombinant lectin expressed in Pichia exists in two molecular forms. Edman degradation and mass spectrometry analysis confirmed the presence of at least two forms of recombinant lectin with molecular weights of 19,060 and 20,100Da, corresponding to lectin polypeptides similar to the fully processed Nictaba which is N-terminally blocked, and Nictaba extended at the N-terminus with the amino acids residues EAEAYVEFT due to incomplete processing of the alpha-factor mating sequence. Further characterisation of the recombinant lectin revealed agglutination and carbohydrate-binding properties similar to the native tobacco lectin.