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

Binding of Orysata lectin induces an immune response in insect cells

In mammals, plant lectinshave been shown to possess immunomodulatory properties, acting in both the innate and adaptive immune system to modulate the production of mediators of the immune response, ultimately improving host defences. At present, knowledge of immunomodulatory effects of plant lectins in insects is scarce. Treatment of insect cells with the Orysa sativa lectin, Orysata, was previously reported to induce cell aggregation, mimicking the immune process of encapsulation. In this project we investigated the potential immunomodulatory effects of this mannose-binding lectin using Drosophila melanogaster S2 cells. Identification of the Orysata binding partners on the surface of S2 cells through a pull-down assay and proteomic analysis revealed 221 putative interactors, several of which were immunity-related proteins. Subsequent qPCR analysis revealed the upregulation of Toll- and immune deficiency (IMD)-regulated antimicrobial peptides (Drs, Mtk, AttA, and Dpt) and signal transducers (Rel and Hid) belonging to the IMD pathway. In addition, the iron-binding protein Transferrin 3 was identified as a putative interactor for Orysata, and treatment of S2 cells with Orysata was shown to reduce the intracellular iron concentration. All together, we believe these results offer a new perspective on the effects by which plant lectins influence insect cells and contribute to the study of their immunomodulatory properties.

Legume Lectins with Different Specificities as Potential Glycan Probes for Pathogenic Enveloped Viruses

Pathogenic enveloped viruses are covered with a glycan shield that provides a dual function: the glycan structures contribute to virus protection as well as host cell recognition. The three classical types of N-glycans, in particular complex glycans, high-mannose glycans, and hybrid glycans, together with some O-glycans, participate in the glycan shield of the Ebola virus, influenza virus, human cytomegalovirus, herpes virus, human immunodeficiency virus, Lassa virus, and MERS-CoV, SARS-CoV, and SARS-CoV-2, which are responsible for respiratory syndromes. The glycans are linked to glycoproteins that occur as metastable prefusion glycoproteins on the surface of infectious virions such as gp120 of HIV, hemagglutinin of influenza, or spike proteins of beta-coronaviruses. Plant lectins with different carbohydrate-binding specificities and, especially, mannose-specific lectins from the Vicieae tribe, such as pea lectin and lentil lectin, can be used as glycan probes for targeting the glycan shield because of their specific interaction with the α1,6-fucosylated core Man3GlcNAc2, which predominantly occurs in complex and hybrid glycans. Other plant lectins with Neu5Ac specificity or GalNAc/T/Tn specificity can also serve as potential glycan probes for the often sialylated complex glycans and truncated O-glycans, respectively, which are abundantly distributed in the glycan shield of enveloped viruses. The biomedical and therapeutical potential of plant lectins as antiviral drugs is discussed.

Glycosylation reduces the glycan-independent immunomodulatory effect of recombinant Orysata lectin in Drosophila S2 cells

Several plant lectins, or carbohydrate-binding proteins, interact with glycan moieties on the surface of immune cells, thereby influencing the immune response of these cells. Orysata, a mannose-binding lectin from rice, has been reported to exert immunomodulatory activities on insect cells. While the natural lectin is non-glycosylated, recombinant Orysata produced in the yeast Pichia pastoris (YOry) is modified with a hyper-mannosylated N-glycan. Since it is unclear whether this glycosylation can affect the YOry activity, non-glycosylated rOrysata was produced in Escherichia coli (BOry). In a comparative analysis, both recombinant Orysata proteins were tested for their carbohydrate specificity on a glycan array, followed by the investigation of the carbohydrate-dependent agglutination of red blood cells (RBCs) and the carbohydrate-independent immune responses in Drosophila melanogaster S2 cells. Although YOry and BOry showed a similar carbohydrate-binding profiles, lower concentration of BOry were sufficient for the agglutination of RBCs and BOry induced stronger immune responses in S2 cells. The data are discussed in relation to different hypotheses explaining the weaker responses of glycosylated YOry. In conclusion, these observations contribute to the understanding how post-translational modification can affect protein function, and provide guidance in the selection of the proper expression system for the recombinant production of lectins.

The lectin Orysata induces phosphatase-mediated and carbohydrate-independent aggregation of insect cells

Lectins, or carbohydrate-binding proteins, can cause agglutination of particular cells. This process is mediated by the interaction of the carbohydrate-binding domain with sugar structures on the cell surface, and this binding can be inhibited by pre-incubation of the lectin with its specific sugars. However, when incubated with insect cells, Orysata, a mannose-binding lectin from rice, caused aggregation of the cells, independent from carbohydrate binding activity. This phenomenon was observed for multiple insect cell lines, confirming the robustness of this phenotype. While the carbohydrate-dependent agglutination of red blood cells happens within minutes, the carbohydrate-independent aggregation of insect cells requires longer incubation times. Further analysis with the galactose-binding lectins SSA and Jacalin, validated the robustness of this lectin-induced, carbohydrate-independent aggregation in different insect cell lines. Since proteomic analysis revealed no changes in the proteome after treatment with the lectins, this cell aggregation is likely caused by the (in) activation or re-organization of the existing surface proteins. The use of inhibitors of phosphorylation and dephosphorylation, staurosporine (STS) and a phosphatase inhibitor (PPI) cocktail, pointed to dephosphorylation as a key mechanism in the lectin-induced, carbohydrate-independent aggregation of insect cells. Similar to contact inhibition, cell proliferation in cell aggregates was decreased. Analysis of the marker for cell proliferation, cyclin E, confirmed that aggregated cells enter a quiescent state. The current data offer a new perspective on the mechanism by which lectins execute their activities, specifically through lectin-induced phosphatase-mediated cell aggregation and proliferation inhibition, independent from their carbohydrate-binding activity.

The barley lectin, horcolin, binds high-mannose glycans in a multivalent fashion, enabling high-affinity, specific inhibition of cellular HIV infection

N-Linked glycans are critical to the infection cycle of HIV, and most neutralizing antibodies target the high-mannose glycans found on the surface envelope glycoprotein-120 (gp120). Carbohydrate-binding proteins, particularly mannose-binding lectins, have also been shown to bind these glycans. Despite their therapeutic potency, their ability to cause lymphocyte proliferation limits their application. In this study, we report one such lectin named horcolin (Hordeum vulgare lectin), seen to lack mitogenicity owing to the divergence in the residues at its carbohydrate-binding sites, which makes it a promising candidate for exploration as an anti-HIV agent. Extensive isothermal titration calorimetry experiments reveal that the lectin was sensitive to the length and branching of mannooligosaccharides and thereby the total valency. Modeling and simulation studies demonstrate two distinct modes of binding, a monovalent binding to shorter saccharides and a bivalent mode for higher glycans, involving simultaneous interactions of multiple glycan arms with the primary carbohydrate-binding sites. This multivalent mode of binding was further strengthened by interactions of core mannosyl residues with a secondary conserved site on the protein, leading to an exponential increase in affinity. Finally, we confirmed the interaction of horcolin with recombinant gp120 and gp140 with high affinity and inhibition of HIV infection at nanomolar concentrations without mitogenicity.

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.

Colletotrichum higginsianum extracellular LysM proteins play dual roles in appressorial function and suppression of chitin-triggered plant immunity

The genome of the hemibiotrophic anthracnose fungus, Colletotrichum higginsianum, encodes a large repertoire of candidate-secreted effectors containing LysM domains, but the role of such proteins in the pathogenicity of any Colletotrichum species is unknown. Here, we characterized the function of two effectors, ChELP1 and ChELP2, which are transcriptionally activated during the initial intracellular biotrophic phase of infection. Using immunocytochemistry, we found that ChELP2 is concentrated on the surface of bulbous biotrophic hyphae at the interface with living host cells but is absent from filamentous necrotrophic hyphae. We show that recombinant ChELP1 and ChELP2 bind chitin and chitin oligomers in vitro with high affinity and specificity and that both proteins suppress the chitin-triggered activation of two immune-related plant mitogen-activated protein kinases in the host Arabidopsis. Using RNAi-mediated gene silencing, we found that ChELP1 and ChELP2 are essential for fungal virulence and appressorium-mediated penetration of both Arabidopsis epidermal cells and cellophane membranes in vitro. The findings suggest a dual role for these LysM proteins as effectors for suppressing chitin-triggered immunity and as proteins required for appressorium function.

Ribosome Inactivating Proteins from Rosaceae

Ribosome-inactivating proteins (RIPs) are widespread among higher plants of different taxonomic orders. In this study, we report on the RIP sequences found in the genome/transcriptome of several important Rosaceae species, including many economically important edible fruits such as apple, pear, peach, apricot, and strawberry. All RIP domains from Rosaceae share high sequence similarity with conserved residues in the catalytic site and the carbohydrate binding sites. The genomes of Malus domestica and Pyrus communis contain both type 1 and type 2 RIP sequences, whereas for Prunus mume, Prunus persica, Pyrus bretschneideri, and Pyrus communis a complex set of type 1 RIP sequences was retrieved. Heterologous expression and purification of the type 1 as well as the type 2 RIP from apple allowed to characterize the biological activity of the proteins. Both RIPs from Malus domestica can inhibit protein synthesis. Furthermore, molecular modelling suggests that RIPs from Rosaceae possess three-dimensional structures that are highly similar to the model proteins and can bind to RIP substrates. Screening of the recombinant type 2 RIP from apple on a glycan array revealed that this type 2 RIP interacts with terminal sialic acid residues. Our data suggest that the RIPs from Rosaceae are biologically active proteins.

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.

Transcriptional profiling of the leaves of near-isogenic rice lines with contrasting drought tolerance at the reproductive stage in response to water deficit

Background

Drought tolerance is a complex quantitative trait that involves the coordination of a vast array of genes belonging to different pathways. To identify genes related to the drought-tolerance pathway in rice, we carried out gene-expression profiling of the leaves of near-isogenic lines (NILs) with similar genetic backgrounds and different set of QTLs but contrasting drought tolerance levels in response to long-term drought-stress treatments. This work will help differentiate mechanisms of tolerance in contrasting NILs and accelerate molecular breeding programs to improve drought tolerance in this crop.

Results

The two pairs of rice NILs, developed at the International Rice Research Institute, along with the drought-susceptible parent, IR64, showed distinct gene-expression profiles in leaves under different water-deficit (WD) treatments. Drought tolerance in the highly drought-tolerant NIL (DTN), IR77298-14-1-2-B-10, could be attributed to the up-regulation of genes with calcium ion binding, transferase, hydrolase and transcription factor activities, whereas in the moderate DTN, IR77298-5-6-B-18, genes with transporter, catalytic and structural molecule activities were up-regulated under WD. In IR77298-14-1-2-B-10, the induced genes were characterized by the presence of regulatory motifs in their promoters, including TGGTTAGTACC and ([CT]AAC[GT]G){2}, which are specific to the TFIIIA and Myb transcription factors, respectively. In IR77298-5-6-B-18, promoters containing a GCAC[AG][ACGT][AT]TCCC[AG]A[ACGT]G[CT] motif, common to MADS(AP1), HD-ZIP, AP2 and YABBY, were induced, suggesting that these factors may play key roles in the regulation of drought tolerance in these two DTNs under severe WD.

Conclusions

We report here that the two pairs of NILs with different levels of drought tolerance may elucidate potential mechanisms and pathways through transcriptome data from leaf tissue. The present study serves as a resource for marker discovery and provides detailed insight into the gene-expression profiles of rice leaves, including the main functional categories of drought-responsive genes and the genes that are involved in drought-tolerance mechanisms, to help breeders identify candidate genes (both up- and down-regulated) associated with drought tolerance and suitable targets for manipulating the drought-tolerance trait in rice.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-2335-1) contains supplementary material, which is available to authorized users.

Host and non-host roots in rice: cellular and molecular approaches reveal differential responses to arbuscular mycorrhizal fungi

Oryza sativa, a model plant for Arbuscular Mycorrhizal (AM) symbiosis, has both host and non-host roots. Large lateral (LLR) and fine lateral (FLR) roots display opposite responses: LLR support AM colonization, but FLR do not. Our research aimed to study the molecular, morphological and physiological aspects related to the non-host behavior of FLR. RNA-seq analysis revealed that LLR and FLR displayed divergent expression profiles, including changes in many metabolic pathways. Compared with LLR, FLR showed down-regulation of genes instrumental for AM establishment and gibberellin signaling, and a higher expression of nutrient transporters. Consistent with the transcriptomic data, FLR had higher phosphorus content. Light and electron microscopy demonstrated that, surprisingly, in the Selenio cultivar, FLR have a two-layered cortex, which is theoretically compatible with AM colonization. According to RNA-seq, a gibberellin inhibitor treatment increased anticlinal divisions leading to a higher number of cortex cells in FLR. We propose that some of the differentially regulated genes that lead to the anatomical and physiological properties of the two root types also function as genetic factors regulating fungal colonization. The rice root apparatus offers a unique tool to study AM symbiosis, allowing direct comparisons of host and non-host roots in the same individual plant.

Characterization of a type D1A EUL-related lectin from rice expressed in Pichia pastoris

OrysaEULD1A is one of the five EUL genes in rice (Oryza sativa) encoding a putative carbohydrate-binding protein belonging to the family of Euonymus related lectins (EUL). The OrysaEULD1A sequence comprises two highly similar EUL domains (91% sequence similarity and 72% sequence identity) separated by a 23 amino acid linker sequence and preceded by a 19 amino acid N-terminal sequence. In the present study, the full-length protein OrysaEULD1A as well as its individual domains OrysaEULD1A domain 1 and 2 were expressed in Pichia pastoris. After purification of the recombinant proteins, their carbohydrate-binding specificity was analyzed and compared. Interestingly, all recombinant lectins showed clear specificity towards galactosylated structures. Furthermore, all recombinant proteins agglutinated red blood cells, indicating that the full-length protein OrysaEULD1A and its domains are true lectins. These results taken together with data previously reported for single-domain EUL proteins indicate that although the amino acids--responsible for the formation of the carbohydrate-binding site--are identical for all EUL proteins in rice, these lectins show different carbohydrate specificities. This promiscuity of the carbohydrate-binding site can be attributed to gene divergence.

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.

Transcriptional behavior of EUL-related rice lectins toward important abiotic and biotic stresses

The rice genome encodes several genes for putative carbohydrate-binding proteins belonging to the family of Euonymus related lectins (EULs). This lectin family was discovered recently and evidence shows that the expression of these proteins is subject to multiple environmental stresses. In this study, quantitative reverse transcription PCR (qRT-PCR) was conducted on rice seedlings exposed to various abiotic (150mM NaCl, 100mM mannitol, and 100μM abscisic acid (ABA)) and biotic (Xanthomonas oryzae pv. oryzae and Magnaporthe oryzae) stresses to compare the transcriptional behavior of the EULs and a known stress related lectin Orysata belonging to the family of jacalin-related lectins. All EUL transcripts were strongly up-regulated after ABA and NaCl treatments in the roots whereas the overall expression level was generally lower and more variable in the shoots. Moreover, all abiotic stresses induced Orysata in both tissues except for mannitol treatment which failed to show an effect in the roots. Orysata also strongly accumulated after X. oryzae pv. oryzae infection, as were various D-type EUL lectins. In contrast, some of the EUL proteins, including OrysaEULS3, OrysaEULD1A and OrysaEULD2, as well as Orysata were significantly down-regulated upon M. oryzae attack, suggesting fungal manipulation of these genes. Collectively, our results clearly show that rice expresses multiple carbohydrate-binding proteins in response to a wide variety of abiotic and biotic stress conditions. We hypothesize that the Euonymus related proteins fulfill a prominent role in sensing and responding to multiple environmental cues.

Orysata, a jacalin-related lectin from rice, could protect plants against biting-chewing and piercing-sucking insects

The present study reports the insecticidal activity of Orysata, a lectin from rice with mannose specificity, belonging to the family of jacalin-related lectins. The effect of Orysata was investigated against three important pest insects in agriculture: the beet armyworm Spodoptera exigua Hübner (Lepidoptera: Noctuidae), and two aphid pests: green peach aphid Myzus persicae Sulzer and pea aphid Acyrthosiphon pisum (Hemiptera: Aphidoidea). Bioassays with S. exigua and M. persicae were performed using detached leaves from transgenic tobacco lines overexpressing Orysata. The expression levels ranged between 38 and 71 μg/g FW, corresponding to 0.6-1.1% of total soluble protein. Intoxicated larval stages of S. exigua revealed significant mortality, reductions in larval weight gain and a retardation of development. Similarly, feeding on leaves expressing Orysata lowered the mortality of the green peach aphids significantly. When pea aphids were fed on an artificial diet supplemented with different amounts of recombinant Orysata, mortality was high at relatively low lectin concentrations; the estimated 50% lethal concentration being 79 μg/ml. In conclusion, our results demonstrated that the jacalin-related lectin Orysata possesses strong insecticidal activity, suggesting that it can be considered as a valuable candidate to be used as a control agent against both biting-chewing and piercing-sucking pest insects.

Recombinant production of plant lectins in microbial systems for biomedical application - the frutalin case study

Frutalin is a homotetrameric partly glycosylated α-D-galactose-binding lectin of biomedical interest from Artocarpus incisa (breadfruit) seeds, belonging to the jacalin-related lectins family. As other plant lectins, frutalin is a heterogeneous mixture of several isoforms possibly with distinct biological activities. The main problem of using such lectins as biomedical tools is that "batch-to-batch" variation in isoforms content may lead to inconstant results. The production of lectins by recombinant means has the advantage of obtaining high amounts of proteins with defined amino-acid sequences and more precise properties. In this mini review, we provide the strategies followed to produce two different forms of frutalin in two different microbial systems: Escherichia coli and Pichia pastoris. The processing and functional properties of the recombinant frutalin obtained from these hosts are compared to those of frutalin extracted from breadfruit. Emphasis is given particularly to recombinant frutalin produced in P. pastoris, which showed a remarkable capacity as biomarker of human prostate cancer and as apoptosis-inducer of cancer cells. Recombinant frutalin production opens perspectives for its development as a new tool in human medicine.

Vibrio cholerae cytolysin recognizes the heptasaccharide core of complex N-glycans with nanomolar affinity

Pathogens selectively target host cells using adhesion molecules and secreted virulence factors that may utilize protein, lipid, or carbohydrate ligands on the cell surface. The human intestinal pathogen Vibrio cholerae secretes a pore-forming toxin, V.cholerae cytolysin (VCC), which contains two domains that are structurally similar to known carbohydrate-binding proteins. These tandem domains are attached to the carboxy-terminus of the cytolytic domain and contain a β-trefoil fold and a β-prism fold. VCC has been shown to bind glycosylated proteins, and removal of the β-prism domain leads to a large decrease in lytic activity against rabbit erythrocytes. Despite these clues, the identity of the glycan receptors of VCC and the role of glycan binding in toxin activity remain unknown. To better understand this specificity, we used a combination of structural and functional approaches to characterize the carbohydrate-binding activity of the VCC toxin. We first probed the monosaccharide-binding activity of VCC and demonstrated that the toxin exhibits millimolar affinity for aldohexoses. To understand this specificity, we solved the crystal structure of the VCC β-prism domain bound to methyl-α-mannose. Next, we utilized a mammalian glycan screen to determine that the β-prism domain preferentially binds complex N-glycans with a heptasaccharide GlcNAc(4)Man(3) core (NGA2). Fluorescence anisotropy and surface plasmon resonance indicated an approximately 100-nM affinity of the β-prism domain for the heptasaccharide core. Our results suggest that carbohydrate-binding domains on the VCC toxin facilitate high-affinity targeting of mammalian cell membranes, which may contribute to the ability of VCC to lyse cells at picomolar concentrations.

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.

Expression analysis of a type S2 EUL-related lectin from rice in Pichia pastoris

Rice (Oryza sativa) expresses different putative carbohydrate-binding proteins belonging to the class of lectins containing an Euonymus lectin (EUL)-related domain, one of them being OrysaEULS2. The OrysaEULS2 sequence consists of a 56 amino acid N-terminal domain followed by the EUL sequence. In this paper the original sequence of the EUL domain of OrysaEULS2 and some mutant forms have been expressed in Pichia pastoris. Subsequently, the recombinant proteins were purified and their carbohydrate binding properties determined. Analysis of the original protein on the glycan array revealed interaction with mannose containing structures and to a lesser extent with glycans containing lactosamine related structures. It was shown that mutation of tryptophan residue 134 into leucine resulted in an almost complete loss of carbohydrate binding activity of OrysaEULS2. Our results show that the EUL domain in OrysaEULS2 interacts with glycan structures, and hence can be considered as a lectin. However, the binding of the protein with the array is much weaker than that of other EUL-related lectins. Furthermore, our results indicate that gene divergence within the family of EUL-related lectins lead to changes in carbohydrate binding specificity.

Lectin activity of the nucleocytoplasmic EUL protein from Arabidopsis thaliana

The Euonymus lectin (EUL) domain was recognized as the structural motif for a novel class of putative carbohydrate binding proteins. Confocal microscopy demonstrated that the lectin from Euonymus europaeus (EEA) as well as the EUL protein from Arabidopsis thaliana (ArathEULS3) are located in the nucleocytoplasmic compartment of the plant cell. ArathEULS3 as well as its EUL domain were successfully expressed in Pichia pastoris and purified. The EUL domain from Arabidopsis interacts with glycan structures containing Lewis Y, Lewis X and lactosamine, indicating that it can be considered a true lectin domain. Despite the high sequence identity between the EUL domains in EEA and ArathEULS3, both domains recognize different carbohydrate structures.