Lectin structures: classification based on the 3-D structures

Tandem-repeat lectins: structural and functional insights

Multivalency in lectins plays a pivotal role in influencing glycan cross-linking, thereby affecting lectin functionality. This multivalency can be achieved through oligomerization, the presence of tandemly repeated carbohydrate recognition domains, or a combination of both. Unlike lectins that rely on multiple factors for the oligomerization of identical monomers, tandem-repeat lectins inherently possess multivalency, independent of this complex process. The repeat domains, although not identical, display slightly distinct specificities within a predetermined geometry, enhancing specificity, affinity, avidity and even oligomerization. Despite the recognition of this structural characteristic in recently discovered lectins by numerous studies, a unified criterion to define tandem-repeat lectins is still necessary. We suggest defining them multivalent lectins with intrachain tandem repeats corresponding to carbohydrate recognition domains, independent of oligomerization. This systematic review examines the folding and phyletic diversity of tandem-repeat lectins and refers to relevant literature. Our study categorizes all lectins with tandemly repeated carbohydrate recognition domains into nine distinct folding classes associated with specific biological functions. Our findings provide a comprehensive description and analysis of tandem-repeat lectins in terms of their functions and structural features. Our exploration of phyletic and functional diversity has revealed previously undocumented tandem-repeat lectins. We propose research directions aimed at enhancing our understanding of the origins of tandem-repeat lectin and fostering the development of medical and biotechnological applications, notably in the design of artificial sugars and neolectins.

Construction of L-type lectin displaying Saccharomyces cerevisiae for Vibrio parahaemolyticus agglutination

The utilization of Aga1P anchor protein in the display system for expressing heterologous proteins on the surface of Saccharomyces cerevisiae has been shown to be an ideal approach. This system has the ability to improve the expression of target proteins beyond the cell surface, resulting in increased activity and stability of the expression system. Recent studies have demonstrated that a new L-type lectin from Litopenaeus vannamei (LvLTLC1) has been found to possess the capability of agglutinating Vibrio parahaemolyticus, a pathogen responsible for causing acute hepatopancreatic necrosis disease (AHPND) in shrimp. In this study, LvLTLC1 protein was designed to be expressed on the surface of S. cerevisiae via Aga1P anchor. The expression of LvLTLC1 protein on the surface of S. cerevisiae::pYIP-LvLTLC1-Aga1P was confirmed through the use of analytical techniques including SDS-PAGE, dot blot, and fluorescent immunoassay with LvLTC1-specific antibody. Subsequently, the newly generated yeast strain was evaluated for its ability to agglutinate V. parahaemolyticus and A. hydrophila. The obtained results indicated that S. cerevisiae expressing LvLTLC1 protein on its surface had the ability to agglutinate both AHPND-causing V. parahaemolyticus and A. hydrophila. This newly generated yeast strain could be served as a feed supplement for controlling bacteria in general and AHPND in particular.

In situ characterization of the agglutination of lectins via cross-linking of carbohydrates by time-resolved measurement of forward static light scattering

We present real-time observations of a structurally variable process for cross-linking agglutination between multivalent lectins and glycoclusters using a small-angle forward static light scattering (F-SLS) technique. In this study, a cross-linking agglutination reaction was carried out using a tetravalent Neu5Acα2,6LacNAc-glycocluster and Sambucus sieboldiana agglutinin (SSA). The scattering intensity of time-resolved F-SLS increased with formation of the Neu5Acα2,6LacNAc-glycocluster-SSA cross-linked complex. Using this approach, fine sequential cross-linking agglutination between glycoclusters and lectins was observed in real-time. The rate of increase in the intensity of time-resolved F-SLS increased with the concentration of sialo-glycoclusters and SSA. Structural analysis based on the fractal dimension using time-resolved F-SLS patterns revealed that the density of the aggregates changed with progression of the cross-linking reaction until equilibrium was reached. This is the first report to evaluate the cross-linking agglutination reaction between glycoclusters and lectins and analysis of the subsequent structure of the obtained aggregates using time-resolved measurements of F-SLS.

Plant Toxic Proteins: Their Biological Activities, Mechanism of Action and Removal Strategies

Plants evolve to synthesize various natural metabolites to protect themselves against threats, such as insects, predators, microorganisms, and environmental conditions (such as temperature, pH, humidity, salt, and drought). Plant-derived toxic proteins are often secondary metabolites generated by plants. These proteins, including ribosome-inactivating proteins, lectins, protease inhibitors, α-amylase inhibitors, canatoxin-like proteins and ureases, arcelins, antimicrobial peptides, and pore-forming toxins, are found in different plant parts, such as the roots, tubers, stems, fruits, buds, and foliage. Several investigations have been conducted to explore the potential applications of these plant proteins by analyzing their toxic effects and modes of action. In biomedical applications, such as crop protection, drug development, cancer therapy, and genetic engineering, toxic plant proteins have been utilized as potentially useful instruments due to their biological activities. However, these noxious metabolites can be detrimental to human health and cause problems when consumed in high amounts. This review focuses on different plant toxic proteins, their biological activities, and their mechanisms of action. Furthermore, possible usage and removal strategies for these proteins are discussed.

A Useful Guide to Lectin Binding: Machine-Learning Directed Annotation of 57 Unique Lectin Specificities

Glycans are critical to every facet of biology and medicine, from viral infections to embryogenesis. Tools to study glycans are rapidly evolving; however, the majority of our knowledge is deeply dependent on binding by glycan binding proteins (e.g., lectins). The specificities of lectins, which are often naturally isolated proteins, have not been well-defined, making it difficult to leverage their full potential for glycan analysis. Herein, we use a combination of machine learning algorithms and expert annotation to define lectin specificity for this important probe set. Our analysis uses comprehensive glycan microarray analysis of commercially available lectins we obtained using version 5.0 of the Consortium for Functional Glycomics glycan microarray (CFGv5). This data set was made public in 2011. We report the creation of this data set and its use in large-scale evaluation of lectin-glycan binding behaviors. Our motif analysis was performed by integrating 68 manually defined glycan features with systematic probing of computational rules for significant binding motifs using mono- and disaccharides and linkages. Combining machine learning with manual annotation, we create a detailed interpretation of glycan-binding specificity for 57 unique lectins, categorized by their major binding motifs: mannose, complex-type N-glycan, O-glycan, fucose, sialic acid and sulfate, GlcNAc and chitin, Gal and LacNAc, and GalNAc. Our work provides fresh insights into the complex binding features of commercially available lectins in current use, providing a critical guide to these important reagents.

Quantitative evaluation of glycan-binding specificity of recombinant concanavalin A produced in lettuce (Lactuca sativa)

Concanavalin A (ConA), a mannose (Man)-specific leguminous lectin isolated from the jack bean (Canavalia ensiformis) seed extracts, was discovered over a century ago. Although ConA has been extensively applied in various life science research, recombinant mature ConA expression has not been fully established. Here, we aimed to produce recombinant ConA (rConA) in lettuce (Lactuca sativa) using an Agrobacterium tumefaciens-mediated transient expression system. rConA could be produced as a fully active form from soluble fractions of lettuce leaves and purified by affinity chromatography. From 12 g wet weight of lettuce leaves, 0.9 mg rConA could be purified. The glycan-binding properties of rConA were then compared with that of the native ConA isolated from jack bean using glycoconjugate microarray and frontal affinity chromatography. rConA demonstrated a glycan-binding specificity similar to nConA. Both molecules bound to N-glycans containing a terminal Man residue. Consistent with previous reports, terminal Manα1-6Man was found to be an essential unit for the high-affinity binding of rConA and nConA, while bisecting GlcNAc diminished the binding of rConA and nConA to Manα1-6Man-terminated N-glycans. These results demonstrate that the fully active rConA could be produced using the A. tumefaciens-mediated transient expression system and used as a recombinant substitute for nConA.

A review on the antimicrobial properties of lectins

Lectins are biologically versatile biomolecules with remarkable antimicrobial effects, notably against bacteria, fungi and protozoa, in addition to modulating host immunity. For this, the lectins bind to carbohydrates on the surface of the pathogen, which can cause damage to the cell wall and prevent the attachment of microorganisms to host cells. Thus, this study intends to review the biological activities of lectins, with an emphasis on antimicrobial activity. Lectins of plant stood out for its antimicrobial effects, demonstrating that they act against a variety of strains, where in vitro were able to inhibit their development and affect their morphology. In vivo, they modulated host immunity, signaling and activating defense cells. Some of these lectins were capable to modulate the action of antibiotics, indicating their potential to minimize the antibiotic resistance. The results suggest that lectins have antimicrobial activity with potential to be used in drug development.

Strategies and Tactics for the Development of Selective Glycan-Binding Proteins

The influences of glycans impact all biological processes, disease states, and pathogenic interactions. Glycan-binding proteins (GBPs), such as lectins, are decisive tools for interrogating glycan structure and function because of their ease of use and ability to selectively bind defined carbohydrate epitopes and glycosidic linkages. GBP reagents are prominent tools for basic research, clinical diagnostics, therapeutics, and biotechnological applications. However, the study of glycans is hindered by the lack of specific and selective protein reagents to cover the massive diversity of carbohydrate structures that exist in nature. In addition, existing GBP reagents often suffer from low affinity or broad specificity, complicating data interpretation. There have been numerous efforts to expand the GBP toolkit beyond those identified from natural sources through protein engineering, to improve the properties of existing GBPs or to engineer novel specificities and potential applications. This review details the current scope of proteins that bind carbohydrates and the engineering methods that have been applied to enhance the affinity, selectivity, and specificity of binders.

Imitating evolution's tinkering by protein engineering reveals extension of human galectin-7 activity

Wild-type lectins have distinct types of modular design. As a step to explain the physiological importance of their special status, hypothesis-driven protein engineering is used to generate variants. Concerning adhesion/growth-regulatory galectins, non-covalently associated homodimers are commonly encountered in vertebrates. The homodimeric galectin-7 (Gal-7) is a multifunctional context-dependent modulator. Since the possibility of conversion from the homodimer to hybrids with other galectin domains, i.e. from Gal-1 and Gal-3, has recently been discovered, we designed Gal-7-based constructs, i.e. stable (covalently linked) homo- and heterodimers. They were produced and purified by affinity chromatography, and the sugar-binding activity of each lectin unit proven by calorimetry. Inspection of profiles of binding of labeled galectins to an array-like platform with various cell types, i.e. sections of murine epididymis and jejunum, and impact on neuroblastoma cell proliferation revealed no major difference between natural and artificial (stable) homodimers. When analyzing heterodimers, acquisition of altered properties was seen. Remarkably, binding properties and activity as effector can depend on the order of arrangement of lectin domains (from N- to C-termini) and on the linker length. After dissociation of the homodimer, the Gal-7 domain can build new functionally active hybrids with other partners. This study provides a clear direction for research on defining the full range of Gal-7 functionality and offers the perspective of testing applications for engineered heterodimers.

Proteome-wide prediction of bacterial carbohydrate-binding proteins as a tool for understanding commensal and pathogen colonisation of the vaginal microbiome

Bacteria use carbohydrate-binding proteins (CBPs), such as lectins and carbohydrate-binding modules (CBMs), to anchor to specific sugars on host surfaces. CBPs in the gut microbiome are well studied, but their roles in the vagina microbiome and involvement in sexually transmitted infections, cervical cancer and preterm birth are largely unknown. We established a classification system for lectins and designed Hidden Markov Model (HMM) profiles for data mining of bacterial genomes, resulting in identification of >100,000 predicted bacterial lectins available at unilectin.eu/bacteria. Genome screening of 90 isolates from 21 vaginal bacterial species shows that those associated with infection and inflammation produce a larger CBPs repertoire, thus enabling them to potentially bind a wider array of glycans in the vagina. Both the number of predicted bacterial CBPs and their specificities correlated with pathogenicity. This study provides new insights into potential mechanisms of colonisation by commensals and potential pathogens of the reproductive tract that underpin health and disease states.

LectomeXplore, an update of UniLectin for the discovery of carbohydrate-binding proteins based on a new lectin classification

Lectins are non-covalent glycan-binding proteins mediating cellular interactions but their annotation in newly sequenced organisms is lacking. The limited size of functional domains and the low level of sequence similarity challenge usual bioinformatics tools. The identification of lectin domains in proteomes requires the manual curation of sequence alignments based on structural folds. A new lectin classification is proposed. It is built on three levels: (i) 35 lectin domain folds, (ii) 109 classes of lectins sharing at least 20% sequence similarity and (iii) 350 families of lectins sharing at least 70% sequence similarity. This information is compiled in the UniLectin platform that includes the previously described UniLectin3D database of curated lectin 3D structures. Since its first release, UniLectin3D has been updated with 485 additional 3D structures. The database is now complemented by two additional modules: PropLec containing predicted β-propeller lectins and LectomeXplore including predicted lectins from sequences of the NBCI-nr and UniProt for every curated lectin class. UniLectin is accessible at https://www.unilectin.eu/.

Purification, biochemical/biophysical characterization and chitooligosaccharide binding to BGL24, a new PP2-type phloem exudate lectin from bottle gourd (Lagenaria siceraria)

Phloem Protein 2 (PP2), highly abundant in the sieve elements of plants, plays a significant role in wound sealing and anti-pathogenic responses. In this study, we report the purification and characterization of a new PP2-type lectin, BGL24 from the phloem exudate of bottle gourd (Lagenaria siceraria). BGL24 is a homodimer with a subunit mass of ~24 kDa and exhibits high specificity for chitooligosaccharides. The isoelectric point of BGL24 was estimated from zeta potential measurements as 5.95. Partial amino acid sequence obtained by mass spectrometric studies indicated that BGL24 exhibits extensive homology with other PP2-type phloem exudate lectins. CD spectroscopic measurements revealed that the lectin contains predominantly β-sheets, with low α-helical content. CD spectroscopic and DSC studies showed that BGL24 exhibits high thermal stability with an unfolding temperature of ~82 °C, and that its secondary structure is essentially unaltered between pH 3.0 and 8.0. Fluorescence titrations employing 4-methylumbelliferyl-β-D-N,N',N″-triacetylchitotrioside as an indicator ligand revealed that the association constants for BGL24-chitooligosaccharide interaction increase considerably when the ligand size is increased from chitotriose to chitotetraose, whereas only marginal increase was observed for chitopentaose and chitohexaose. BGL24 exhibited moderate cytotoxicity against MDA-MB-231 breast cancer cells, whereas its effect on normal splenocytes was marginal.

Structural insights into the fungi-nematodes interaction mediated by fucose-specific lectin AofleA from Arthrobotrys oligospora

Fungal lectin can bind specific carbohydrate structures of the host and work in recognition and adhesion or as a toxic factor. AofleA, as a fucose-specific lectin from widely studied nematode predatory fungus Arthrobotrys oligospora, possibly plays a key role in the event of capturing nematodes, but the mechanism remains unknown. Here we report the crystal structure of AofleA, which exists as a homodimer with each subunit folds as a six-bladed β-propeller. Our structural and biological results revealed that three of the six putative binding sites of AofleA had fucose-binding abilities. In addition, we found that AofleA could bind to the pharynx and intestine of the nematode in a fucose-binding-dependent manner. Our results facilitate the understanding of the mechanism that fucose-specific lectin mediates fungi-nematodes interaction, and provide structural information for the development of potential applications of AofleA.

A systematic analysis of protein-carbohydrate interactions in the Protein Data Bank

Protein-carbohydrate interactions underlie essential biological processes. Elucidating the mechanism of protein-carbohydrate recognition is a prerequisite for modeling and optimizing protein-carbohydrate interactions, which will help in discovery of carbohydrate-derived therapeutics. In this work, we present a survey of a curated database consisting of 6,402 protein-carbohydrate complexes in the Protein Data Bank (PDB). We performed an all-against-all comparison of a subset of nonredundant binding sites, and the result indicates that the interaction pattern similarity is not completely relevant to the binding site structural similarity. Investigation of both binding site and ligand promiscuities reveals that the geometry of chemical feature points is more important than local backbone structure in determining protein-carbohydrate interactions. A further analysis on the frequency and geometry of atomic interactions shows that carbohydrate functional groups are not equally involved in binding interactions. Finally, we discuss the usefulness of protein-carbohydrate complexes in the PDB with acknowledgement that the carbohydrates in many structures are incomplete.

How galectins have become multifunctional proteins

Having identified glycans of cellular glycoconjugates as versatile molecular messages, their recognition by sugar receptors (lectins) is a fundamental mechanism within the flow of biological information. This type of molecular interplay is increasingly revealed to be involved in a wide range of (patho)physiological processes. To do so, it is a vital prerequisite that a lectin (and its expression) can develop more than a single skill, that is the general ability to bind glycans. By studying the example of vertebrate galectins as a model, a total of five relevant characteristics is disclosed: i) access to intra- and extracellular sites, ii) fine-tuned gene regulation (with evidence for co-regulation of counterreceptors) including the existence of variants due to alternative splicing or single nucleotide polymorphisms, iii) specificity to distinct glycans from the glycome with different molecular meaning, iv) binding capacity also to peptide motifs at different sites on the protein and v) diversity of modular architecture. They combine to endow these lectins with the capacity to serve as multi-purpose tools. Underscoring the arising broad-scale significance of tissue lectins, their numbers in terms of known families and group members have steadily grown by respective research that therefore unveiled a well-stocked toolbox. The generation of a network of (ga)lectins by evolutionary diversification affords the opportunity for additive/synergistic or antagonistic interplay in situ, an emerging aspect of (ga)lectin functionality. It warrants close scrutiny. The realization of the enormous potential of combinatorial permutations using the five listed features gives further efforts to understand the rules of functional glycomics/lectinomics a clear direction.

Expression and Purification of Full-Length and Domain-Fragment Recombinant Pentraxin 3 (PTX3) Proteins from Mammalian and Bacterial Cells

Although cell-based protein expression systems enable us a certain amount of protein suitable for subsequent biological experiments to be obtained, aggregates of the protein of interest are sometimes encountered during the purification procedure. Pentraxin 3 (PTX3), a member of the pentraxin family that is classified as a carbohydrate-binding protein based on its structure, comprises one of the humoral arms of the pattern recognition receptors that play an important role in the innate immune response. PTX3 comprises two domains; an N-terminal domain and a C-terminal domain. The C-terminal domain containing pentraxin signature has similar biological functions as other pentraxins such as C-reactive protein (CRP) and serum amyloid-P component (SAP). On the other side, the N-terminal domain is specific to PTX3. A supply of the PTX3 protein in full length or partial fragments is thus essential for the elucidation of its biological functions. Here we describe the expression and purification of recombinant PTX3. An arginine-containing buffer is essential for the elution of bacterially expressed PTX3 N-terminal domain to minimize aggregation. This method allows high-yield purification of full-length or domain-fragment recombinant PTX3 proteins for biological study.

Structural Database for Lectins and the UniLectin Web Platform

The search for new biomolecules requires a clear understanding of biosynthesis and degradation pathways. This view applies to most metabolites as well as other molecule types such as glycans whose repertoire is still poorly characterized. Lectins are proteins that recognize specifically and interact noncovalently with glycans. This particular class of proteins is considered as playing a major role in biology. Glycan-binding is based on multivalence, which gives lectins a unique capacity to interact with surface glycans and significantly contribute to cell-cell recognition and interactions. Lectins have been studied for many years using multiple technologies and part of the resulting information is available online in databases. Unfortunately, the connectivity of these databases with the most popular omics databases (genomics, proteomics, and glycomics) remains limited. Moreover, lectin diversity is extended and requires setting out a flexible classification that remains compatible with new sequences and 3D structures that are continuously released. We have designed UniLectin as a new insight into the knowledge of lectins, their classification, and their biological role. This platform encompasses UniLectin3D, a curated database of lectin 3D structures that follows a periodically updated classification, a set of comparative and visualizing tools and gradually released modules dedicated to specific lectins predicted in sequence databases. The second module is PropLec, focused on β-propeller lectin prediction in all species based on five distinct family profiles. This chapter describes how UniLectin can be used to explore the diversity of lectins, their 3D structures, and associated functional information as well as to perform reliable predictions of β-propeller lectins.

Expression and Characterization of Hyaluronan-Binding Protein Involved in Hyaluronan Depolymerization: HYBID, Alias KIAA1199 and CEMIP

Hyaluronan (HA), a major component of the extracellular matrix in vertebrate tissues, provides structural and functional integrity to cells and organs. Biological functions of HA are dependent on the molecular size of HA and the interaction with a wide range of HA-binding proteins, i.e., hyaladherins. In this book chapter, we introduce hyaladherins and focus on HYBID (Hyaluronan-binding protein involved in hyaluronan depolymerization, alias KIAA1199 and CEMIP), which is one of the hyaladherins and plays a central role in HA degradation in human dermal and arthritic synovial fibroblasts. The protocols describe the preparation of the stable transfectants expressing HYBID, the assays of HYBID-mediated HA depolymerization, and the binding assay of HYBID to HA. These methods will be helpful to further study the HYBID-mediated biological activities and its relevance on HA degradation and turnover under various physiological and pathological conditions such as wound healing, ageing, arthritis, and cancer.

A Peptide-Lectin Fusion Strategy for Developing a Glycan Probe for Use in Various Assay Formats

While nucleic acid and protein analysis approaches continue to see significant breakthroughs, analytical strategies for glycan determination have by comparison seen slower technological advances. Here we provide a strategy for glycan probe development using an engineered lectin fusion that can be incorporated into various common pathology lab assay formats including Western blot and agglutination assays. In this proof of concept, we use the natural lectin, Pseudomonas fluorescens agglutinin (PFA), capable of binding core Man alpha(1-3)-Man alpha(1-6)-Man units, where this lectin has previously been shown to bind to the glycans presented by the gp120 coat protein of (HIV) Human Immunodeficiency Virus. In our strategy, we engineered the lectin to possess a fusion of the biotin mimetic tag equence of amino acids V-S-H-P-Q-A-P-F. With the glycan receptive PFA directly linked to the biotin mimic, we could facilitate a probe for various standard clinical assay formats by virtue of coupling to streptavidin-HRP (horseradish peroxidase) or streptavidin beads for Western blot and agglutination assays respectively. We found the PFA fusion retained low nanomolar affinity for gp120 by ELISA (Enzyme Linked Immunosorbent Assay) and microscale thermophoresis. This probe engineering strategy proved effective in the relevant assay formats that may now allow detection for the presence of glycans containing the core Man alpha(1-3)-Man alpha(1-6)-Man units recognized by PFA.

The sugar code: letters and vocabulary, writers, editors and readers and biosignificance of functional glycan-lectin pairing

Ubiquitous occurrence in Nature, abundant presence at strategically important places such as the cell surface and dynamic shifts in their profile by diverse molecular switches qualifies the glycans to serve as versatile biochemical signals. However, their exceptional structural complexity often prevents one noting how simple the rules of objective-driven assembly of glycan-encoded messages are. This review is intended to provide a tutorial for a broad readership. The principles of why carbohydrates meet all demands to be the coding section of an information transfer system, and this at unsurpassed high density, are explained. Despite appearing to be a random assortment of sugars and their substitutions, seemingly subtle structural variations in glycan chains by a sophisticated enzymatic machinery have emerged to account for their specific biological meaning. Acting as 'readers' of glycan-encoded information, carbohydrate-specific receptors (lectins) are a means to turn the glycans' potential to serve as signals into a multitude of (patho)physiologically relevant responses. Once the far-reaching significance of this type of functional pairing has become clear, the various modes of spatial presentation of glycans and of carbohydrate recognition domains in lectins can be explored and rationalized. These discoveries are continuously revealing the intricacies of mutually adaptable routes to achieve essential selectivity and specificity. Equipped with these insights, readers will gain a fundamental understanding why carbohydrates form the third alphabet of life, joining the ranks of nucleotides and amino acids, and will also become aware of the importance of cellular communication via glycan-lectin recognition.

Wheat Germ Agglutinin as a Potential Therapeutic Agent for Leukemia

Dietary lectins are carbohydrate-binding proteins found in food sources. We used a panel of seven dietary lectins to analyze cytotoxicity against hematological cancers. Wheat germ agglutinin (WGA), even at low doses, demonstrated maximum toxicity toward acute myeloid leukemia (AML) cells. Using AML cell lines, we show time- and dose-dependent killing by WGA. We also show that low doses of WGA kills primary patient AML cells, irrespective of subtype, with no significant toxicity to normal cells. WGA caused AML cell agglutination, but failed to agglutinate RBC's at this dose. WGA, primarily, binds to N-acetyl-D-glucosamine (GlcNAc) and is also reported to interact with sialic-acid-containing glycoconjugates and oligosaccharides. After neuraminidase pre-treatment, which catalyzes the hydrolysis of terminal sialic acid residues, AML cells were less sensitive to WGA-induced cell death. AML cells were also not sensitive to succinyl-WGA, which does not react with sialic acid. Incubation with LEL lectin, which recognizes GlcNAc or SNA, which binds preferentially to sialic acid attached to terminal galactose in α-2,6 and to a lesser degree α-2,3 linkage, did not alter AML cell viability. These data indicate that WGA-induced AML cell death is dependent on both GlcNAc binding and interaction with sialic acids. We did not observe any in vitro or in vivo toxicity of WGA toward normal cells at the concentrations tested. Finally, low doses of WGA injection demonstrated significant in vivo toxicity toward AML cells, using xenograft mouse model. Thus, WGA is a potential candidate for leukemia therapy.

Lectin engineering: the possible and the actual

Lectins are a widespread group of sugar-binding proteins occurring in all types of organisms including animals, plants, bacteria, fungi and even viruses. According to a recent report, there are more than 50 lectin scaffolds (∼Pfam), for which three-dimensional structures are known and sugar-binding functions have been confirmed in the literature, which far exceeds our view in the twentieth century (Fujimoto et al. 2014 Methods Mol. Biol. 1200, 579-606 (doi:10.1007/978-1-4939-1292-6_46)). This fact suggests that new lectins will be discovered either by a conventional screening approach or just by chance. It is also expected that new lectin domains including those found in enzymes as carbohydrate-binding modules will be generated in the future through evolution, although this has never been attempted on an experimental level. Based on the current state of the art, various methods of lectin engineering are available, by which lectin specificity and/or stability of a known lectin scaffold can be improved. However, the above observation implies that any protein scaffold, including those that have never been described as lectins, may be modified to acquire a sugar-binding function. In this review, possible approaches to confer sugar-binding properties on synthetic proteins and peptides are described.

Mutagenesis Studies and Structure-function Relationships for GalNAc/Gal-Specific Lectin from the Sea Mussel Crenomytilus grayanus

The GalNAc/Gal-specific lectin from the sea mussel Crenomytilus grayanus (CGL) with anticancer activity represents а novel lectin family with β-trefoil fold. Earlier, the crystal structures of CGL complexes with globotriose, galactose and galactosamine, and mutagenesis studies have revealed that the lectin contained three carbohydrate-binding sites. The ability of CGL to recognize globotriose (Gb3) on the surface of breast cancer cells and bind mucin-type glycoproteins, which are often associated with oncogenic transformation, makes this compound to be perspective as a biosensor for cancer diagnostics. In this study, we describe results on in silico analysis of binding mechanisms of CGL to ligands (galactose, globotriose and mucin) and evaluate the individual contribution of the amino acid residues from carbohydrate-binding sites to CGL activity by site-directed mutagenesis. The alanine substitutions of His37, His129, Glu75, Asp127, His85, Asn27 and Asn119 affect the CGL mucin-binding activity, indicating their importance in the manifestation of lectin activity. It has been found that CGL affinity to ligands depends on their structure, which is determined by the number of hydrogen bonds in the CGL-ligand complexes. The obtained results should be helpful for understanding molecular machinery of CGL functioning and designing a synthetic analog of CGL with enhanced carbohydrate-binding properties.

Identification, Characterization, and X-ray Crystallographic Analysis of a Novel Type of Lectin AJLec from the Sea Anemone Anthopleura japonica

A novel galactose-specific lectin, AJLec (18.5 kDa), was isolated from the sea anemone, Anthopleura japonica. AJLec was characterized using the hemagglutination assay, isothermal titration calorimetry (ITC), and glycoconjugate microarray analysis and we found that AJLec has a specificity for galactose monomers and β-linked terminal galactose residues in complex carbohydrates, but not for N-acetylgalactosamine (GalNAc), which is commonly recognized by galactose-binding lectins. The primary structure of AJLec did not show homology with known lectins, and a crystal structural analysis also revealed a unique homodimeric structure. The crystal structure of AJLec complexed with lactose was solved by measuring the sulfur single-wavelength anomalous diffraction (S-SAD) phasing with an in-house Cu Kα source method. This analysis revealed that the galactose residue in lactose was recognized via its O2, O3, and O4 hydroxyl groups and ring oxygen by calcium coordination and two hydrogen bonds with residues in the carbohydrate-binding site, which demonstrated strict specificity for the β-linked terminal galactose in this lectin.

Microglial Lectins in Health and Neurological Diseases

Microglia are the innate sentinels of the central nervous system (CNS) and are responsible for the homeostasis and immune defense of the CNS. Under the influence of the local environment and cell-cell interaction, microglia exhibit a multidimensional and context-dependent phenotypes that can be cytotoxic and neuroprotective. Recent studies suggest that microglia express multitudinous types of lectins, including galectins, Siglecs, mannose-binding lectins (MBLs) and other glycan binding proteins. Because most studies that examine lectins focus on the peripheral system, the functions of lectins have not been critically investigated in the CNS. In addition, the types of brain cells that contribute to the altered levels of lectins present in diseases are often unclear. In this review, we will discuss how galectins, Siglecs, selectins and MBLs contribute to the dynamic functions of microglia. The interacting ligands of these lectins are complex glycoconjugates, which consist of glycoproteins and glycolipids that are expressed on microglia or surrounding cells. The current understanding of the heterogeneity and functions of glycans in the brain is limited. Galectins are a group of pleotropic proteins that recognize both β-galactoside-containing glycans and non- β-galactoside-containing proteins. The function and regulation of galectins have been implicated in immunomodulation, neuroinflammation, apoptosis, phagocytosis and oxidative bursts. Most Siglecs are expressed at a low level on the plasma membrane and bind to sialic acid residues for immunosurveillance and cell-cell communication. Siglecs are classified based on their inhibitory and activatory downstream signaling properties. Inhibitory Siglecs negatively regulate microglia activation upon recognizing the intact sialic acid patterns and vice versa. MBLs are expressed upon infection in cytoplasm and can be secreted in order to recognize molecules containing terminal mannose as an innate immune defense machinery. Most importantly, multiple studies have reported dysregulation of lectins in neurological disorders. Here, we reviewed recent studies on microglial lectins and their functions in CNS health and disease, and suggest that these lectin families are novel, potent therapeutic targets for neurological diseases.

Identification, Characterization, and X-ray Crystallographic Analysis of a Novel Type of Mannose-Specific Lectin CGL1 from the Pacific Oyster Crassostrea gigas

A novel mannose-specific lectin, named CGL1 (15.5 kDa), was isolated from the oyster Crassostrea gigas. Characterization of CGL1 involved isothermal titration calorimetry (ITC), glycoconjugate microarray, and frontal affinity chromatography (FAC). This analysis revealed that CGL1 has strict specificity for the mannose monomer and for high mannose-type N-glycans (HMTGs). Primary structure of CGL1 did not show any homology with known lectins but did show homology with proteins of the natterin family. Crystal structure of the CGL1 revealed a unique homodimer in which each protomer was composed of 2 domains related by a pseudo two-fold axis. Complex structures of CGL1 with mannose molecules showed that residues have 8 hydrogen bond interactions with O1, O2, O3, O4, and O5 hydroxyl groups of mannose. The complex interactions that are not observed with other mannose-binding lectins revealed the structural basis for the strict specificity for mannose. These characteristics of CGL1 may be helpful as a research tool and for clinical applications.

NMR analysis on the sialic acid-binding mechanism of an R-type lectin mutant by natural evolution-mimicry

A sialic acid-binding lectin (SRC) was created from the C-terminal domain of an R-type N-acetyl lactosamine-binding lectin (EW29Ch) by natural evolution-mimicry. Here, we clarified its sialic acid-binding mechanism using NMR spectroscopy. The NMR analysis showed differences between conformations of the 6'-sialyllactose-bound SRC in the solution state and that in the crystal state, and differences between the internal motion of the loop region in subdomain γ in SRC and that of the corresponding region in EW29Ch. The NMR analysis thus provided useful information to explain the manner of binding to 6'-sialyllactose in solution, which the previous X-ray crystal structure analysis lacked.

The Lectin Frontier Database (LfDB), and data generation based on frontal affinity chromatography

Lectins are a large group of carbohydrate-binding proteins, having been shown to comprise at least 48 protein scaffolds or protein family entries. They occur ubiquitously in living organisms—from humans to microorganisms, including viruses—and while their functions are yet to be fully elucidated, their main underlying actions are thought to mediate cell-cell and cell-glycoconjugate interactions, which play important roles in an extensive range of biological processes. The basic feature of each lectin’s function resides in its specific sugar-binding properties. In this regard, it is beneficial for researchers to have access to fundamental information about the detailed oligosaccharide specificities of diverse lectins. In this review, the authors describe a publicly available lectin database named “Lectin frontier DataBase (LfDB)”, which undertakes the continuous publication and updating of comprehensive data for lectin-standard oligosaccharide interactions in terms of dissociation constants (K_d’s). For K_d determination, an advanced system of frontal affinity chromatography (FAC) is used, with which quantitative datasets of interactions between immobilized lectins and >100 fluorescently labeled standard glycans have been generated. The FAC system is unique in its clear principle, simple procedure and high sensitivity, with an increasing number (>67) of associated publications that attest to its reliability. Thus, LfDB, is expected to play an essential role in lectin research, not only in basic but also in applied fields of glycoscience.