Convergent and divergent mechanisms of sugar recognition across kingdoms

Secondary Sites of the C-type Lectin-Like Fold

C-type lectins are a large superfamily of proteins involved in a multitude of biological processes. In particular, their involvement in immunity and homeostasis has rendered them attractive targets for diverse therapeutic interventions. They share a characteristic C-type lectin-like domain whose adaptability enables them to bind a broad spectrum of ligands beyond the originally defined canonical Ca2+-dependent carbohydrate binding. Together with variable domain architecture and high-level conformational plasticity, this enables C-type lectins to meet diverse functional demands. Secondary sites provide another layer of regulation and are often intricately linked to functional diversity. Located remote from the canonical primary binding site, secondary sites can accommodate ligands with other physicochemical properties and alter protein dynamics, thus enhancing selectivity and enabling fine-tuning of the biological response. In this review, we outline the structural determinants allowing C-type lectins to perform a large variety of tasks and to accommodate the ligands associated with it. Using the six well-characterized Ca2+-dependent and Ca2+-independent C-type lectin receptors DC-SIGN, langerin, MGL, dectin-1, CLEC-2 and NKG2D as examples, we focus on the characteristics of non-canonical interactions and secondary sites and their potential use in drug discovery endeavors.

Plant cell wall-mediated disease resistance: Current understanding and future perspectives

Beyond their function as structural barriers, plant cell walls are essential elements for the adaptation of plants to environmental conditions. Cell walls are dynamic structures whose composition and integrity can be altered in response to environmental challenges and developmental cues. These wall changes are perceived by plant sensors/receptors to trigger adaptative responses during development and upon stress perception. Plant cell wall damage caused by pathogen infection, wounding, or other stresses leads to the release of wall molecules, such as carbohydrates (glycans), that function as damage-associated molecular patterns (DAMPs). DAMPs are perceived by the extracellular ectodomains (ECDs) of pattern recognition receptors (PRRs) to activate pattern-triggered immunity (PTI) and disease resistance. Similarly, glycans released from the walls and extracellular layers of microorganisms interacting with plants are recognized as microbe-associated molecular patterns (MAMPs) by specific ECD-PRRs triggering PTI responses. The number of oligosaccharides DAMPs/MAMPs identified that are perceived by plants has increased in recent years. However, the structural mechanisms underlying glycan recognition by plant PRRs remain limited. Currently, this knowledge is mainly focused on receptors of the LysM-PRR family, which are involved in the perception of various molecules, such as chitooligosaccharides from fungi and lipo-chitooligosaccharides (i.e., Nod/MYC factors from bacteria and mycorrhiza, respectively) that trigger differential physiological responses. Nevertheless, additional families of plant PRRs have recently been implicated in oligosaccharide/polysaccharide recognition. These include receptor kinases (RKs) with leucine-rich repeat and Malectin domains in their ECDs (LRR-MAL RKs), Catharanthus roseus RECEPTOR-LIKE KINASE 1-LIKE group (CrRLK1L) with Malectin-like domains in their ECDs, as well as wall-associated kinases, lectin-RKs, and LRR-extensins. The characterization of structural basis of glycans recognition by these new plant receptors will shed light on their similarities with those of mammalians involved in glycan perception. The gained knowledge holds the potential to facilitate the development of sustainable, glycan-based crop protection solutions.

Fungal Cell Wall-Associated Effectors: Sensing, Integration, Suppression, and Protection

The cell wall (CW) of plant-interacting fungi, as the direct interface with host plants, plays a crucial role in fungal development. A number of secreted proteins are directly associated with the fungal CW, either through covalent or non-covalent interactions, and serve a range of important functions. In the context of plant-fungal interactions many are important for fungal development in the host environment and may therefore be considered fungal CW-associated effectors (CWAEs). Key CWAE functions include integrating chemical/physical signals to direct hyphal growth, interfering with plant immunity, and providing protection against plant defenses. In recent years, a diverse range of mechanisms have been reported that underpin their roles, with some CWAEs harboring conserved motifs or functional domains, while others are reported to have novel features. As such, the current understanding regarding fungal CWAEs is systematically presented here from the perspective of their biological functions in plant-fungal interactions. An overview of the fungal CW architecture and the mechanisms by which proteins are secreted, modified, and incorporated into the CW is first presented to provide context for their biological roles. Some CWAE functions are reported across a broad range of pathosystems or symbiotic/mutualistic associations. Prominent are the chitin interacting-effectors that facilitate fungal CW modification, protection, or suppression of host immune responses. However, several alternative functions are now reported and are presented and discussed. CWAEs can play diverse roles, some possibly unique to fungal lineages and others conserved across a broad range of plant-interacting fungi. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Elucidating the glycan-binding specificity and structure of Cucumis melo agglutinin, a new R-type lectin

Plant lectins have garnered attention for their roles as laboratory probes and potential therapeutics. Here, we report the discovery and characterization of Cucumis melo agglutinin (CMA1), a new R-type lectin from melon. Our findings reveal CMA1's unique glycan-binding profile, mechanistically explained by its 3D structure, augmenting our understanding of R-type lectins. We expressed CMA1 recombinantly and assessed its binding specificity using multiple glycan arrays, covering 1,046 unique sequences. This resulted in a complex binding profile, strongly preferring C2-substituted, beta-linked galactose (both GalNAc and Fuca1-2Gal), which we contrasted with the established R-type lectin Ricinus communis agglutinin 1 (RCA1). We also report binding of specific glycosaminoglycan subtypes and a general enhancement of binding by sulfation. Further validation using agglutination, thermal shift assays, and surface plasmon resonance confirmed and quantified this binding specificity in solution. Finally, we solved the high-resolution structure of the CMA1 N-terminal domain using X-ray crystallography, supporting our functional findings at the molecular level. Our study provides a comprehensive understanding of CMA1, laying the groundwork for further exploration of its biological and therapeutic potential.

An overview of microbial enzymatic approaches for pectin degradation

Pectin, a complex natural macromolecule present in primary cell walls, exhibits high structural diversity. Pectin is composed of a main chain, which contains a high amount of partly methyl-esterified galacturonic acid (GalA), and numerous types of side chains that contain almost 17 different monosaccharides and over 20 different linkages. Due to this peculiar structure, pectin exhibits special physicochemical properties and a variety of bioactivities. For example, pectin exhibits strong bioactivity only in a low molecular weight range. Many different degrading enzymes, including hydrolases, lyases and esterases, are needed to depolymerize pectin due to its structural complexity. Pectin degradation involves polygalacturonases/rhamnogalacturonases and pectate/pectin lyases, which attack the linkages in the backbone via hydrolytic and β-elimination modes, respectively. Pectin methyl/acetyl esterases involved in the de-esterification of pectin also play crucial roles. Many α-L-rhamnohydrolases, unsaturated rhamnogalacturonyl hydrolases, arabinanases and galactanases also contribute to heterogeneous pectin degradation. Although numerous microbial pectin-degrading enzymes have been described, the mechanisms involved in the coordinated degradation of pectin through these enzymes remain unclear. In recent years, the degradation of pectin by Bacteroides has received increasing attention, as Bacteroides species contain a unique genetic structure, polysaccharide utilization loci (PULs). The specific PULs of pectin degradation in Bacteroides species are a new field to study pectin metabolism in gut microbiota. This paper reviews the scientific information available on pectin structural characteristics, pectin-degrading enzymes, and PULs for the specific degradation of pectin.

Functional Diversity of Novel Lectins with Unique Structural Features in Marine Animals

Due to their remarkable structural diversity, glycans play important roles as recognition molecules on cell surfaces of living organisms. Carbohydrates exist in numerous isomeric forms and can adopt diverse structures through various branching patterns. Despite their relatively small molecular weights, they exhibit extensive structural diversity. On the other hand, lectins, also known as carbohydrate-binding proteins, not only recognize and bind to the diverse structures of glycans but also induce various biological reactions based on structural differences. Initially discovered as hemagglutinins in plant seeds, lectins have been found to play significant roles in cell recognition processes in higher vertebrates. However, our understanding of lectins in marine animals, particularly marine invertebrates, remains limited. Recent studies have revealed that marine animals possess novel lectins with unique structures and glycan recognition mechanisms not observed in known lectins. Of particular interest is their role as pattern recognition molecules in the innate immune system, where they recognize the glycan structures of pathogens. Furthermore, lectins serve as toxins for self-defense against foreign enemies. Recent discoveries have identified various pore-forming proteins containing lectin domains in fish venoms and skins. These proteins utilize lectin domains to bind target cells, triggering oligomerization and pore formation in the cell membrane. These findings have spurred research into the new functions of lectins and lectin domains. In this review, we present recent findings on the diverse structures and functions of lectins in marine animals.

Glycomimetics for the inhibition and modulation of lectins

Carbohydrates are essential mediators of many processes in health and disease. They regulate self-/non-self- discrimination, are key elements of cellular communication, cancer, infection and inflammation, and determine protein folding, function and life-times. Moreover, they are integral to the cellular envelope for microorganisms and participate in biofilm formation. These diverse functions of carbohydrates are mediated by carbohydrate-binding proteins, lectins, and the more the knowledge about the biology of these proteins is advancing, the more interfering with carbohydrate recognition becomes a viable option for the development of novel therapeutics. In this respect, small molecules mimicking this recognition process become more and more available either as tools for fostering our basic understanding of glycobiology or as therapeutics. In this review, we outline the general design principles of glycomimetic inhibitors (Section 2). This section is then followed by highlighting three approaches to interfere with lectin function, i.e. with carbohydrate-derived glycomimetics (Section 3.1), novel glycomimetic scaffolds (Section 3.2) and allosteric modulators (Section 3.3). We summarize recent advances in design and application of glycomimetics for various classes of lectins of mammalian, viral and bacterial origin. Besides highlighting design principles in general, we showcase defined cases in which glycomimetics have been advanced to clinical trials or marketed. Additionally, emerging applications of glycomimetics for targeted protein degradation and targeted delivery purposes are reviewed in Section 4.

MS-based glycomics and glycoproteomics methods enabling isomeric characterization

Glycosylation is one of the most significant and abundant posttranslational modifications in mammalian cells. It mediates a wide range of biofunctions, including cell adhesion, cell communication, immune cell trafficking, and protein stability. Also, aberrant glycosylation has been associated with various diseases such as diabetes, Alzheimer's disease, inflammation, immune deficiencies, congenital disorders, and cancers. The alterations in the distributions of glycan and glycopeptide isomers are involved in the development and progression of several human diseases. However, the microheterogeneity of glycosylation brings a great challenge to glycomic and glycoproteomic analysis, including the characterization of isomers. Over several decades, different methods and approaches have been developed to facilitate the characterization of glycan and glycopeptide isomers. Mass spectrometry (MS) has been a powerful tool utilized for glycomic and glycoproteomic isomeric analysis due to its high sensitivity and rich structural information using different fragmentation techniques. However, a comprehensive characterization of glycan and glycopeptide isomers remains a challenge when utilizing MS alone. Therefore, various separation methods, including liquid chromatography, capillary electrophoresis, and ion mobility, were developed to resolve glycan and glycopeptide isomers before MS. These separation techniques were coupled to MS for a better identification and quantitation of glycan and glycopeptide isomers. Additionally, bioinformatic tools are essential for the automated processing of glycan and glycopeptide isomeric data to facilitate isomeric studies in biological cohorts. Here in this review, we discuss commonly employed MS-based techniques, separation hyphenated MS methods, and software, facilitating the separation, identification, and quantitation of glycan and glycopeptide isomers.

LectinOracle: A Generalizable Deep Learning Model for Lectin-Glycan Binding Prediction

Ranging from bacterial cell adhesion over viral cell entry to human innate immunity, glycan-binding proteins or lectins are abound in nature. Widely used as staining and characterization reagents in cell biology and crucial for understanding the interactions in biological systems, lectins are a focal point of study in glycobiology. Yet the sheer breadth and depth of specificity for diverse oligosaccharide motifs has made studying lectins a largely piecemeal approach, with few options to generalize. Here, LectinOracle, a model combining transformer-based representations for proteins and graph convolutional neural networks for glycans to predict their interaction, is presented. Using a curated data set of 564,647 unique protein-glycan interactions, it is shown that LectinOracle predictions agree with literature-annotated specificities for a wide range of lectins. Using a range of specialized glycan arrays, it is shown that LectinOracle predictions generalize to new glycans and lectins, with qualitative and quantitative agreement with experimental data. It is further demonstrated that LectinOracle can be used to improve lectin classification, accelerate lectin directed evolution, predict epidemiological outcomes in the context of influenza virus, and analyze whole lectomes in host-microbe interactions. It is envisioned that the herein presented platform will advance both the study of lectins and their role in (glyco)biology.

Finding the sweet spot: glycosylation mediated regulation of intestinal inflammation

Glycans are essential cellular components that facilitate a range of critical functions important for tissue development and mucosal homeostasis. Furthermore, specific alterations in glycosylation represent important diagnostic hallmarks of cancer that contribute to tumor cell dissociation, invasion, and metastasis. However, much less is known about how glycosylation contributes to the pathobiology of inflammatory mucosal diseases. Here we will review how epithelial and immune cell glycosylation regulates gut homeostasis and how inflammation-driven changes in glycosylation contribute to intestinal pathobiology.

A novel protein fusion partner, carbohydrate-binding module family 66, to enhance heterologous protein expression in Escherichia coli

Background

Proteins with novel functions or advanced activities developed by various protein engineering techniques must have sufficient solubility to retain their bioactivity. However, inactive protein aggregates are frequently produced during heterologous protein expression in Escherichia coli. To prevent the formation of inclusion bodies, fusion tag technology has been commonly employed, owing to its good performance in soluble expression of target proteins, ease of application, and purification feasibility. Thus, researchers have continuously developed novel fusion tags to expand the expression capacity of high-value proteins in E. coli.

Results

A novel fusion tag comprising carbohydrate-binding module 66 (CBM66) was developed for the soluble expression of heterologous proteins in E. coli. The target protein solubilization capacity of the CBM66 tag was verified using seven proteins that are poorly expressed or form inclusion bodies in E. coli: four human-derived signaling polypeptides and three microbial enzymes. Compared to native proteins, CBM66-fused proteins exhibited improved solubility and high production titer. The protein-solubilizing effect of the CBM66 tag was compared with that of two commercial tags, maltose-binding protein and glutathione-S-transferase, using poly(ethylene terephthalate) hydrolase (PETase) as a model protein; CBM66 fusion resulted in a 3.7-fold higher expression amount of soluble PETase (approximately 370 mg/L) compared to fusion with the other commercial tags. The intact PETase was purified from the fusion protein upon serial treatment with enterokinase and affinity chromatography using levan-agarose resin. The bioactivity of the three proteins assessed was maintained even when the CBM66 tag was fused.

Conclusions

The use of the CBM66 tag to improve soluble protein expression facilitates the easy and economic production of high-value proteins in E. coli.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-021-01725-w.

Structural and functional insights into the first Bacillus thuringiensis vegetative insecticidal protein of the Vpb4 fold, active against western corn rootworm

The western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte, is a major maize pest in the United States causing significant economic loss. The emergence of field-evolved resistant WCR to Bacillus thuringiensis (Bt) traits has prompted the need to discover and deploy new insecticidal proteins in transgenic maize. In the current study we determined the crystal structure and mode of action (MOA) of the Vpb4Da2 protein (formerly known as Vip4Da2) from Bt, the first identified insecticidal Vpb4 protein with commercial level control against WCR. The Vpb4Da2 structure exhibits a six-domain architecture mainly comprised of antiparallel β-sheets organized into β-sandwich layers. The amino-terminal domains 1-3 of the protein share structural homology with the protective antigen (PA) PA14 domain and encompass a long β-pore forming loop as in the clostridial binary-toxB module. Domains 5 and 6 at the carboxyl-terminal half of Vpb4Da2 are unique as this extension is not observed in PA or any other structurally-related protein other than Vpb4 homologs. These unique Vpb4 domains adopt the topologies of carbohydrate-binding modules known to participate in receptor-recognition. Functional assessment of Vpb4Da2 suggests that domains 4-6 comprise the WCR receptor binding region and are key in conferring the observed insecticidal activity against WCR. The current structural analysis was complemented by in vitro and in vivo characterizations, including immuno-histochemistry, demonstrating that Vpb4Da2 follows a MOA that is consistent with well-characterized 3-domain Bt insecticidal proteins despite significant structural differences.

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.

Structural Basis of Ligand Selectivity by a Bacterial Adhesin Lectin Involved in Multispecies Biofilm Formation

Bacterial adhesins are key virulence factors that are essential for the pathogen-host interaction and biofilm formation that cause most infections. Many of the adhesin-driven cell-cell interactions are mediated by lectins.

Carbohydrate recognition by lectins governs critical host-microbe interactions. MpPA14 (Marinomonas primoryensis PA14 domain) lectin is a domain of a 1.5-MDa adhesin responsible for a symbiotic bacterium-diatom interaction in Antarctica. Here, we show that MpPA14 binds various monosaccharides, with l-fucose and N-acetylglucosamine being the strongest ligands (dissociation constant [K_d], ∼150 μM). High-resolution structures of MpPA14 with 15 different sugars bound elucidated the molecular basis for the lectin’s apparent binding promiscuity but underlying selectivity. MpPA14 mediates strong Ca²⁺-dependent interactions with the 3,4-diols of l-fucopyranose and glucopyranoses, and it binds other sugars via their specific minor isomers. Thus, MpPA14 only binds polysaccharides like branched glucans and fucoidans with these free end groups. Consistent with our findings, adhesion of MpPA14 to diatom cells was selectively blocked by l-fucose, but not by N-acetyl galactosamine. The MpPA14 lectin homolog present in a Vibrio cholerae adhesin was produced and was shown to have the same sugar binding preferences as MpPA14. The pathogen’s lectin was unable to effectively bind the diatom in the presence of fucose, thus demonstrating the antiadhesion strategy of blocking infection via ligand-based antagonists.

Draft genome of the glucose tolerant β-glucosidase producing rare Aspergillus unguis reveals complete cellulolytic machinery with multiple beta-glucosidase genes

Draft genome sequence of the glucose tolerant beta glucosidase (GT-BGL) producing rare fungus Aspergillus unguis NII 08,123 was generated through Next Generation Sequencing (NGS). The genome size of the fungus was estimated to be 37.1 Mb. A total of 3116 contigs were assembled using SPades, and 15,161 proteins were predicted using AUGUSTUS 3.1. Among them, 13,850 proteins were annotated using UniProt. Distribution of CAZyme genes specifically those encoding lignocellulose degrading enzymes were analyzed and compared with those from the industrial cellulase producer Trichoderma reesei in view of the huge differences in detectable enzyme activities between the fungi, despite the ability of A. unguis to grow on lignocellulose as sole carbon source. Full length gene sequence of the inducible GT-BGL could be identified through tracing back from peptide mass fingerprint. A total of 403 CAZymes were predicted from the genome, which includes 232 glycoside hydrolases (GHs), 12 carbohydrate esterases (CEs), 109 glycosyl transferases (GTs), 15 polysaccharide lyases (PLs), and 35 genes with auxiliary activities (AAs). The high level of zinc finger motif containing transcription factors could possibly hint a tight regulation of the cellulolytic machinery, which may also explain the low cellulase activities even when a complete repertoire of cellulase degrading enzyme genes are present in the fungus.

Distinct protein architectures mediate species-specific beta-glucan binding and metabolism in the human gut microbiota

Complex glycans that evade our digestive system are major nutrients that feed the human gut microbiota (HGM). The prevalence of Bacteroidetes in the HGM of populations worldwide is engendered by the evolution of polysaccharide utilization loci (PULs), which encode concerted protein systems to utilize the myriad complex glycans in our diets. Despite their crucial roles in glycan recognition and transport, cell-surface glycan-binding proteins (SGBPs) remained understudied cogs in the PUL machinery. Here, we report the structural and biochemical characterization of a suite of SGBP-A and SGBP-B structures from three syntenic β(1,3)-glucan utilization loci (1,3GULs) from Bacteroides thetaiotaomicron (Bt), Bacteroides uniformis (Bu), and B. fluxus (Bf), which have varying specificities for distinct β-glucans. Ligand complexes provide definitive insight into β(1,3)-glucan selectivity in the HGM, including structural features enabling dual β(1,3)-glucan/mixed-linkage β(1,3)/β(1,4)-glucan-binding capability in some orthologs. The tertiary structural conservation of SusD-like SGBPs-A is juxtaposed with the diverse architectures and binding modes of the SGBPs-B. Specifically, the structures of the trimodular BtSGBP-B and BuSGBP-B revealed a tandem repeat of carbohydrate-binding module-like domains connected by long linkers. In contrast, BfSGBP-B comprises a bimodular architecture with a distinct β-barrel domain at the C terminus that bears a shallow binding canyon. The molecular insights obtained here contribute to our fundamental understanding of HGM function, which in turn may inform tailored microbial intervention therapies.

The History of Carbohydrates in Type I Allergy

Although first described decades ago, the relevance of carbohydrate specific antibodies as mediators of type I allergy had not been recognized until recently. Previously, allergen specific IgE antibodies binding to carbohydrate epitopes were considered to demonstrate a clinically irrelevant cross-reactivity. However, this changed following the discovery of type I allergies specifically mediated by oligosaccharide structures. Especially the emerging understanding of red meat allergy characterized by IgE directed to the oligosaccharide alpha-gal showed that carbohydrate-mediated reactions can result in life threatening systemic anaphylaxis which in contrast to former assumptions proves a high clinical relevance of some carbohydrate allergens. Within the scope of this review article, we illustrate the historical development of carbohydrate-allergen-research, reaching from only diagnostically relevant crossreactive-carbohydrate-determinants to clinically important antigens mediating type I allergy. Focusing on clinical and immunological features of the alpha-gal syndrome, we highlight the discovery of oligosaccharides as potentially highly immunogenic antigens and mediators of type I allergy, report what is known about the route of sensitization and the immunological mechanisms involved in sensitization and elicitation phase of allergic responses as well as currently available diagnostic and therapeutic tools. Finally, we briefly report on carbohydrates being involved in type I allergies different from alpha-gal.

Structural and functional diversity of neutrophil glycosylation in innate immunity and related disorders

The granulated neutrophils are abundant innate immune cells that utilize bioactive glycoproteins packed in cytosolic granules to fight pathogenic infections, but the neutrophil glycobiology remains poorly understood. Facilitated by technological advances in glycoimmunology, systems glycobiology and glycoanalytics, a considerable body of literature reporting on novel aspects of neutrophil glycosylation has accumulated. Herein, we summarize the building knowledge of the structural and functional diversity displayed by N- and O-linked glycoproteins spatiotemporally expressed and sequentially brought-into-action across the diverse neutrophil life stages during bone marrow maturation, movements to, from and within the blood circulation and microbicidal processes at the inflammatory sites in peripheral tissues. It transpires that neutrophils abundantly decorate their granule glycoproteins including neutrophil elastase, myeloperoxidase and cathepsin G with peculiar glyco-signatures not commonly reported in other areas of human glycobiology such as hyper-truncated chitobiose core- and paucimannosidic-type N-glycans and monoantennary complex-type N-glycans. Sialyl Lewis^x, Lewis^x, poly-N-acetyllactosamine extensions and core 1-/2-type O-glycans are also common neutrophil glyco-signatures. Granule-specific glycosylation is another fascinating yet not fully understood feature of neutrophils. Recent literature suggests that unconventional biosynthetic pathways and functions underpin these prominent neutrophil-associated glyco-phenotypes. The impact of glycosylation on key neutrophil effector functions including extravasation, degranulation, phagocytosis and formation of neutrophil extracellular traps during normal physiological conditions and in innate immune-related diseases is discussed. We also highlight new technologies that are expected to further advance neutrophil glycobiology and briefly discuss the untapped diagnostic and therapeutic potential of neutrophil glycosylation that could open avenues to combat the increasingly prevalent innate immune disorders.

Allostery in C-type lectins

C-type lectins are the largest and most diverse family of mammalian carbohydrate-binding proteins. They share a common protein fold, which provides the unifying basis for calcium-mediated carbohydrate recognition. Their involvement in a multitude of biological functions is remarkable. Here, we review the variety of tasks these lectins are involved in alongside with the structural demands on the overall protein architecture. Subtle changes of the protein structure are implemented to cope with such diverse functional requirements. The presence of a high level of structural dynamics over a broad palette of time scales is paired with the presence of secondary binding sites and allosteric coordination of remote sites and renders this lectin fold a highly adaptable scaffold.

Ultrahigh performance liquid chromatography methods facilitate the development of glucose-responsive insulin therapeutics

Insulin oligosaccharide conjugates hold promise as potential glucose-responsive insulins (GRIs), which can improve the therapeutic index of insulins and mitigate the risk of hypoglycemia. A key challenge for the analytical development of such molecules is finding an efficient method to characterize the purity and impurities of conjugated insulins. Using the S-Matrix Fusion QbD-ultrahigh performance liquid chromatography (UHPLC) integrated system, we were able to quickly screen and develop two short UHPLC methods. These methods were used to support process development, clinical batch drug substance (DS) release, and stability studies of MK-2640, an insulin oligosaccharide conjugate. Both methods used a Waters CSH C18 column, with a shallow gradient of acetonitrile to aqueous mobile phase containing 25 mM sodium perchlorate and 0.05% perchloric acid. The 10-min run time method was well suited for process development and monitoring as it was able to separate the main product, MK-2640, six oligosaccharide-substituted recombinant human insulin (RHI) impurities, A21 deamidated MK-2640, and the starting material RHI. The 13-min run time method provided improved separation of the major impurities and demonstrated good chromatographic reproducibility on different instruments or using columns from different lots of stationary phase, which made it ideal for the final DS release. Validation of the 13-min method demonstrated great linearity for both the MK-2640 main peak and its related impurities, low limit of detection (0.02%), and limit of quantitation (0.05%). The high specificity of the method allowed the separation of the degradation products from main peak, thus makes it suitable for stability monitoring. The major impurities in the DS were characterized by two-dimensional liquid chromatography-mass spectrometry (2D-LC-MS).

The lectin domain containing proteins with mucosal immunity and digestive functions in oyster Crassostrea gigas

Lectins are carbohydrate-binding proteins with lectin domains, which are extensively studied for their numerous roles in biological recognition. However, the lectin domain containing proteins (LDCPs) chimerized with other non-lectin domains have not received sufficient attention. In the present study, a genome-wide survey of LDCPs in oyster Crassostrea gigas was conducted, and an expansive 640 LDCPs derived from ten lectin domains were identified and functionally explored. In these LDCPs, a total of 282 kinds of domains were predicted, and 90% of the LDCPs contained more than one kind of domain. The lectin domains were frequently fused with non-lectin domains, such as epidermal growth factor domain and peptidase related domains, which supplied LDCPs with more diversity in structures and functions. The C-type lectin domains were the most abundant domains in LDCPs, and they were largely co-existed with non-lectin domains of complement activation-related domains (such as CUB domain and PAN-1 domain) but relative independence with other lectin domains. Furthermore, the C-type lectin domain containing proteins (CTLPs) found to mainly act as pattern immune recognition receptors and were highly expressed in mucosal tissues (digestive gland, male gonad and labial palp) to provide mucosal immune protections. The Concanavalin A-like lectin domains were the second richest domains in LDCPs, and they were mostly constructed into chimeric proteins with epidermal growth factor domain and peptidase related domains. The Concanavalin A-like lectin domain containing proteins (CALPs) were significantly enriched with peptidase activities and mainly expressed in digestive tissues. All the results suggested the mucosal immunity and digestive functions of oyster LDCPs, which provided a fresh idea about the functions of invertebrate lectin family.

A Specific, Glycomimetic Langerin Ligand for Human Langerhans Cell Targeting

Langerhans cells are a subset of dendritic cells residing in the epidermis of the human skin. As such, they are key mediators of immune regulation and have emerged as prime targets for novel transcutaneous cancer vaccines. Importantly, the induction of protective T cell immunity by these vaccines requires the efficient and specific delivery of both tumor-associated antigens and adjuvants. Langerhans cells uniquely express Langerin (CD207), an endocytic C-type lectin receptor. Here, we report the discovery of a specific, glycomimetic Langerin ligand employing a heparin-inspired design strategy and structural characterization by NMR spectroscopy and molecular docking. The conjugation of this glycomimetic to liposomes enabled the specific and efficient targeting of Langerhans cells in the human skin. We further demonstrate the doxorubicin-mediated killing of a Langerin+ monocyte cell line, highlighting its therapeutic and diagnostic potential in Langerhans cell histiocytosis, caused by the abnormal proliferation of Langerin+ myeloid progenitor cells. Overall, our delivery platform provides superior versatility over antibody-based approaches and novel modalities to overcome current limitations of dendritic cell-targeted immuno- and chemotherapy.

Mammalian sugar-binding receptors: known functions and unexplored roles

Mammalian glycan-binding receptors, sometimes known as lectins, interact with glycans, the oligosaccharide portions of endogenous mammalian glycoproteins and glycolipids as well as sugars on the surfaces of microbes. These receptors guide glycoproteins out of and back into cells, facilitate communication between cells through both adhesion and signaling, and allow the innate immune system to respond quickly to viral, fungal, bacterial, and parasitic pathogens. For many of the roughly 100 glycan-binding receptors that are known in humans, there are good descriptions of what types of glycans they bind and how selectivity for these ligands is achieved at the molecular level. In some cases, there is also comprehensive evidence for the roles that the receptors play at the cellular and organismal levels. In addition to highlighting these well-understood paradigms for glycan-binding receptors, this review will suggest where gaps remain in our understanding of the physiological functions that they can serve.

Kin discrimination and outer membrane exchange in Myxococcus xanthus: A comparative analysis among natural isolates

Genetically similar cells of the soil bacterium Myxococcus xanthus cooperate at multiple social behaviours, including motility and multicellular development. Another social interaction in this species is outer membrane exchange (OME), a behaviour of unknown primary benefit in which cells displaying closely related variants of the outer membrane protein TraA transiently fuse and exchange membrane contents. Functionally incompatible TraA variants do not mediate OME, which led to the proposal that TraA incompatibilities determine patterns of intercellular cooperation in nature, but how this might occur remains unclear. Using natural isolates from a centimetre-scale patch of soil, we analyse patterns of TraA diversity and ask whether relatedness at TraA is causally related to patterns of kin discrimination in the form of both colony-merger incompatibilities (CMIs) and interstrain antagonisms. A large proportion of TraA functional diversity documented among global isolates is predicted to be contained within this cm-scale population. We find evidence of balancing selection on the highly variable PA14-portion of TraA and extensive transfer of traA alleles across genomic backgrounds. CMIs are shown to be common among strains identical at TraA, suggesting that CMIs are not generally caused by TraA dissimilarity. Finally, it has been proposed that interstrain antagonisms might be caused by OME-mediated toxin transfer. However, we predict that most strain pairs previously shown to exhibit strong antagonisms are incapable of OME due to TraA dissimilarity. Overall, our results suggest that most documented patterns of kin discrimination in a natural population of M. xanthus are not causally related to the TraA sequences of interactants.

Engineering Glucose Responsiveness Into Insulin

Insulin has a narrow therapeutic index, reflected in a small margin between a dose that achieves good glycemic control and one that causes hypoglycemia. Once injected, the clearance of exogenous insulin is invariant regardless of blood glucose, aggravating the potential to cause hypoglycemia. We sought to create a "smart" insulin, one that can alter insulin clearance and hence insulin action in response to blood glucose, mitigating risk for hypoglycemia. The approach added saccharide units to insulin to create insulin analogs with affinity for both the insulin receptor (IR) and mannose receptor C-type 1 (MR), which functions to clear endogenous mannosylated proteins, a principle used to endow insulin analogs with glucose responsivity. Iteration of these efforts culminated in the discovery of MK-2640, and its in vitro and in vivo preclinical properties are detailed in this report. In glucose clamp experiments conducted in healthy dogs, as plasma glucose was lowered stepwise from 280 mg/dL to 80 mg/dL, progressively more MK-2640 was cleared via MR, reducing by ∼30% its availability for binding to the IR. In dose escalations studies in diabetic minipigs, a higher therapeutic index for MK-2640 (threefold) was observed versus regular insulin (1.3-fold).

Characterization of isomeric glycan structures by LC-MS/MS

The characterization of glycosylation is critical for obtaining a comprehensive view of the regulation and functions of glycoproteins of interest. Due to the complex nature of oligosaccharides, stemming from variable compositions and linkages, and ion suppression effects, the chromatographic separation of glycans, including isomeric structures, is necessary for exhaustive characterization by MS. This review introduces the fundamental principles underlying the techniques in LC utilized by modern day glycomics researchers. Recent advances in porous graphitized carbon, reverse phase, ion exchange, and hydrophilic interaction LC utilized in conjunction with MS, for the characterization of protein glycosylation, are described with an emphasis on methods capable of resolving isomeric glycan structures.

Cytotoxic protein from the mushroom Coprinus comatus possesses a unique mode for glycan binding and specificity

Glycans possess significant chemical diversity; glycan binding proteins (GBPs) recognize specific glycans to translate their structures to functions in various physiological and pathological processes. Therefore, the discovery and characterization of novel GBPs and characterization of glycan-GBP interactions are significant to provide potential targets for therapeutic intervention of many diseases. Here, we report the biochemical, functional, and structural characterization of a 130-amino-acid protein, Y3, from the mushroom Coprinus comatus Biochemical studies of recombinant Y3 from a yeast expression system demonstrated the protein is a unique GBP. Additionally, we show that Y3 exhibits selective and potent cytotoxicity toward human T-cell leukemia Jurkat cells compared with a panel of cancer cell lines via inducing caspase-dependent apoptosis. Screening of a glycan array demonstrated GalNAcβ1-4(Fucα1-3)GlcNAc (LDNF) as a specific Y3-binding ligand. To provide a structural basis for function, the crystal structure was solved to a resolution of 1.2 Å, revealing a single-domain αβα-sandwich motif. Two monomers were dimerized to form a large 10-stranded, antiparallel β-sheet flanked by α-helices on each side, representing a unique oligomerization mode among GBPs. A large glycan binding pocket extends into the dimeric interface, and docking of LDNF identified key residues for glycan interactions. Disruption of residues predicted to be involved in LDNF/Y3 interactions resulted in the significant loss of binding to Jurkat T-cells and severely impaired their cytotoxicity. Collectively, these results demonstrate Y3 to be a GBP with selective cytotoxicity toward human T-cell leukemia cells and indicate its potential use in cancer diagnosis and treatment.

Immune response of anti-lectin Pjlec antibody in freshwater crab Paratelphusa jacquemontii

Sialic acid specific lectin Pjlec isolated from serum of the freshwater crab Paratelphusa jacquemontii served as an antigen for the production of immunoglobulin (Ig) in Balb/c mice sera. Enzyme-linked immunosorbent assay (ELISA) of mice anti-sera with Pjlec lectin affirmed the induction and production of antibody. Anti-Pjlec antibody was isolated from the antisera of mice by Protein A Sepharose affinity chromatography and checked for purity by immunoblot with lectin. Mass spectrometry (MS/MS) of papain digethe peptide sequence of antigen binding fragment (Fab) and fragment crystallizable (Fc). Coatingsted anti-Pjlec revealed of anti-Pjlec to the target cell, rabbit erythrocyte failed to enhance in vitro phagocytosis in the crab. However, inoculation of anti-Pjlec in the hemolymph of the crab elicited in vitro phagocytosis. Proteins in hemocyte lysate supernatant (HLS) were separated by electrophoresis failed to immunoblot with Pjlec or anti-Pjlec. Peptide sequences of trypsin digested lectin protein appeared homologous to deuterostome chordate. The protostome crab that lack the ability to synthesize sialic acid however bind to sialic acid a deuterostome sugar to suggest the complexity in innate immune system of invertebrates. The application of lectin and its antibody require further study on application of pathological conditions associated with alterations in sialylated cell surface.

Plant Lectins and Lectin Receptor-Like Kinases: How Do They Sense the Outside?

Lectins are fundamental to plant life and have important roles in cell-to-cell communication; development and defence strategies. At the cell surface; lectins are present both as soluble proteins (LecPs) and as chimeric proteins: lectins are then the extracellular domains of receptor-like kinases (LecRLKs) and receptor-like proteins (LecRLPs). In this review; we first describe the domain architectures of proteins harbouring G-type; L-type; LysM and malectin carbohydrate-binding domains. We then focus on the functions of LecPs; LecRLKs and LecRLPs referring to the biological processes they are involved in and to the ligands they recognize. Together; LecPs; LecRLKs and LecRLPs constitute versatile recognition systems at the cell surface contributing to the detection of symbionts and pathogens; and/or involved in monitoring of the cell wall structure and cell growth.

The Distribution of Lectins across the Phylum Nematoda: A Genome-Wide Search

Nematodes are a very diverse phylum that has adapted to nearly every ecosystem. They have developed specialized lifestyles, dividing the phylum into free-living, animal, and plant parasitic species. Their sheer abundance in numbers and presence in nearly every ecosystem make them the most prevalent animals on earth. In this research nematode-specific profiles were designed to retrieve predicted lectin-like domains from the sequence data of nematode genomes and transcriptomes. Lectins are carbohydrate-binding proteins that play numerous roles inside and outside the cell depending on their sugar specificity and associated protein domains. The sugar-binding properties of the retrieved lectin-like proteins were predicted in silico. Although most research has focused on C-type lectin-like, galectin-like, and calreticulin-like proteins in nematodes, we show that the lectin-like repertoire in nematodes is far more diverse. We focused on C-type lectins, which are abundantly present in all investigated nematode species, but seem to be far more abundant in free-living species. Although C-type lectin-like proteins are omnipresent in nematodes, we have shown that only a small part possesses the residues that are thought to be essential for carbohydrate binding. Curiously, hevein, a typical plant lectin domain not reported in animals before, was found in some nematode species.

Lectins: a primer for histochemists and cell biologists

An experimental observation on selecting binding partners underlies the introduction of the term 'lectin'. Agglutination of erythrocytes depending on their blood-group status revealed the presence of activities in plant extracts that act in an epitope-specific manner like antibodies. As it turned out, their binding partners on the cell surface are carbohydrates of glycoconjugates. By definition, lectins are glycan-specific (mono- or oligosaccharides presented by glycoconjugates or polysaccharides) receptors, distinguished from antibodies, from enzymes using carbohydrates as substrates and from transporters of free saccharides. They are ubiquitous in Nature and structurally widely diversified. More than a dozen types of folding pattern have evolved for proteins that bind glycans. Used as tool, this capacity facilitates versatile mapping of glycan presence so that plant/fungal and also animal/human lectins have found a broad spectrum of biomedical applications. The functional pairing with physiological counterreceptors is involved in a wide range of cellular activities from cell adhesion, glycoconjugate trafficking to growth regulation and lets lectins act as sensors/effectors in host defense.

Genome-wide identification and domain organization of lectin domains in cucumber

Lectins are ubiquitous proteins in plants and play important roles in a diverse set of biological processes, such as plant defense and cell signaling. Despite the availability of the Cucumis sativus L. genome sequence since 2009, little is known with respect to the occurrence of lectins in cucumber. In this study, a total of 146 putative lectin genes belonging to 10 different lectin families were identified and localized in the cucumber genome. Domain architecture analysis revealed that most of these lectin gene sequences contain multiple domains, where lectin domains are linked with other domains, as such creating chimeric lectin sequences encoding proteins with dual activities. This study provides an overview of lectin motifs in cucumber and will help to understand their potential biological role(s).

The Sus operon: a model system for starch uptake by the human gut Bacteroidetes

Resident bacteria in the densely populated human intestinal tract must efficiently compete for carbohydrate nutrition. The Bacteroidetes, a dominant bacterial phylum in the mammalian gut, encode a plethora of discrete polysaccharide utilization loci (PULs) that are selectively activated to facilitate glycan capture at the cell surface. The most well-studied PUL-encoded glycan-uptake system is the starch utilization system (Sus) of Bacteroides thetaiotaomicron. The Sus includes the requisite proteins for binding and degrading starch at the surface of the cell preceding oligosaccharide transport across the outer membrane for further depolymerization to glucose in the periplasm. All mammalian gut Bacteroidetes possess analogous Sus-like systems that target numerous diverse glycans. In this review, we discuss what is known about the eight Sus proteins of B. thetaiotaomicron that define the Sus-like paradigm of nutrient acquisition that is exclusive to the Gram-negative Bacteroidetes. We emphasize the well-characterized outer membrane proteins SusDEF and the α-amylase SusG, each of which have unique structural features that allow them to interact with starch on the cell surface. Despite the apparent redundancy in starch-binding sites among these proteins, each has a distinct role during starch catabolism. Additionally, we consider what is known about how these proteins dynamically interact and cooperate in the membrane and propose a model for the formation of the Sus outer membrane complex.

Combining 3D structure with glycan array data provides insight into the origin of glycan specificity

Defining how a glycan-binding protein (GBP) specifically selects its cognate glycan from among the ensemble of glycans within the cellular glycome is an area of intense study. Powerful insight into recognition mechanisms can be gained from 3D structures of GBPs complexed to glycans; however, such structures remain difficult to obtain experimentally. Here an automated 3D structure generation technique, called computational carbohydrate grafting, is combined with the wealth of specificity information available from glycan array screening. Integration of the array data with modeling and crystallography allows generation of putative co-complex structures that can be objectively assessed and iteratively altered until a high level of agreement with experiment is achieved. Given an accurate model of the co-complexes, grafting is also able to discern which binding determinants are active when multiple potential determinants are present within a glycan. In some cases, induced fit in the protein or glycan was necessary to explain the observed specificity, while in other examples a revised definition of the minimal binding determinants was required. When applied to a collection of 10 GBP-glycan complexes, for which crystallographic and array data have been reported, grafting provided a structural rationalization for the binding specificity of >90% of 1223 arrayed glycans. A webtool that enables researchers to perform computational carbohydrate grafting is available at www.glycam.org/gr (accessed 03 March 2016).

Coronavirus receptor switch explained from the stereochemistry of protein-carbohydrate interactions and a single mutation

Hemagglutinin-esterases (HEs) are bimodular envelope proteins of orthomyxoviruses, toroviruses, and coronaviruses with a carbohydrate-binding "lectin" domain appended to a receptor-destroying sialate-O-acetylesterase ("esterase"). In concert, these domains facilitate dynamic virion attachment to cell-surface sialoglycans. Most HEs (type I) target 9-O-acetylated sialic acids (9-O-Ac-Sias), but one group of coronaviruses switched to using 4-O-Ac-Sias instead (type II). This specificity shift required quasisynchronous adaptations in the Sia-binding sites of both lectin and esterase domains. Previously, a partially disordered crystal structure of a type II HE revealed how the shift in lectin ligand specificity was achieved. How the switch in esterase substrate specificity was realized remained unresolved, however. Here, we present a complete structure of a type II HE with a receptor analog in the catalytic site and identify the mutations underlying the 9-O- to 4-O-Ac-Sia substrate switch. We show that (i) common principles pertaining to the stereochemistry of protein-carbohydrate interactions were at the core of the transition in lectin ligand and esterase substrate specificity; (ii) in consequence, the switch in O-Ac-Sia specificity could be readily accomplished via convergent intramolecular coevolution with only modest architectural changes in lectin and esterase domains; and (iii) a single, inconspicuous Ala-to-Ser substitution in the catalytic site was key to the emergence of the type II HEs. Our findings provide fundamental insights into how proteins "see" sugars and how this affects protein and virus evolution.

Glycobiology simplified: diverse roles of glycan recognition in inflammation

Glycans and complementary glycan-binding proteins are essential components in the language of cell-cell interactions in immunity. The study of glycan function is the purview of glycobiology, which has often been presented as an unusually complex discipline. In fact, the human glycome, composed of all of its glycans, is built primarily from only 9 building blocks that are combined by enzymes (writers) with specific and limited biosynthetic capabilities into a tractable and increasingly accessible number of potential glycan patterns that are functionally read by several dozen human glycan-binding proteins (readers). Nowhere is the importance of glycan recognition better understood than in infection and immunity, and knowledge in this area has already led to glycan mimetic anti-infective and anti-inflammatory drugs. This review includes a brief tutorial on human glycobiology and a limited number of specific examples of glycan-binding protein-glycan interactions that initiate and regulate inflammation. Examples include representatives from different glycan-binding protein families, including the C-type lectins (E-selectin, P-selectin, dectin-1, and dectin-2), sialic acid-binding immunoglobulin-like lectins (sialic acid-binding immunoglobulin-like lectins 8 and 9), galectins (galectin-1, galectin-3, and galectin-9), as well as hyaluronic acid-binding proteins. As glycoscience technologies advance, opportunities for enhanced understanding of glycans and their roles in leukocyte cell biology provide increasing opportunities for discovery and therapeutic intervention.

Natural diversity of cellulases, xylanases, and chitinases in bacteria

BACKGROUND

Glycoside hydrolases (GH) targeting cellulose, xylan, and chitin are common in the bacterial genomes that have been sequenced. Little is known, however, about the architecture of multi-domain and multi-activity glycoside hydrolases. In these enzymes, combined catalytic domains act synergistically and thus display overall improved catalytic efficiency, making these proteins of high interest for the biofuel technology industry.

RESULTS

Here, we identify the domain organization in 40,946 proteins targeting cellulose, xylan, and chitin derived from 11,953 sequenced bacterial genomes. These bacteria are known to be capable, or to have the potential, to degrade polysaccharides, or are newly identified potential degraders (e.g., Actinospica, Hamadaea, Cystobacter, and Microbispora). Most of the proteins we identified contain a single catalytic domain that is frequently associated with an accessory non-catalytic domain. Regarding multi-domain proteins, we found that many bacterial strains have unique GH protein architectures and that the overall protein organization is not conserved across most genera. We identified 217 multi-activity proteins with at least two GH domains for cellulose, xylan, and chitin. Of these proteins, 211 have GH domains targeting similar or associated substrates (i.e., cellulose and xylan), whereas only six proteins target both cellulose and chitin. Fifty-two percent of multi-activity GHs are hetero-GHs. Finally, GH6, -10, -44 and -48 domains were mostly C-terminal; GH9, -11, -12, and -18 were mostly N-terminal; and GH5 domains were either N- or C-terminal.

CONCLUSION

We identified 40,946 multi-domain/multi-activity proteins targeting cellulase, chitinase, and xylanase in bacterial genomes and proposed new candidate lineages and protein architectures for carbohydrate processing that may play a role in biofuel production.

An intrinsic mechanism of secreted protein aging and turnover

The composition and functions of the secreted proteome are controlled by the life spans of different proteins. However, unlike intracellular protein fate, intrinsic factors determining secreted protein aging and turnover have not been identified and characterized. Almost all secreted proteins are posttranslationally modified with the covalent attachment of N-glycans. We have discovered an intrinsic mechanism of secreted protein aging and turnover linked to the stepwise elimination of saccharides attached to the termini of N-glycans. Endogenous glycosidases, including neuraminidase 1 (Neu1), neuraminidase 3 (Neu3), beta-galactosidase 1 (Glb1), and hexosaminidase B (HexB), possess hydrolytic activities that temporally remodel N-glycan structures, progressively exposing different saccharides with increased protein age. Subsequently, endocytic lectins with distinct binding specificities, including the Ashwell-Morell receptor, integrin αM, and macrophage mannose receptor, are engaged in N-glycan ligand recognition and the turnover of secreted proteins. Glycosidase inhibition and lectin deficiencies increased protein life spans and abundance, and the basal rate of N-glycan remodeling varied among distinct proteins, accounting for differences in their life spans. This intrinsic multifactorial mechanism of secreted protein aging and turnover contributes to health and the outcomes of disease.

Recent insights into structures and functions of C-type lectins in the immune system

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Sugar-binding C-type carbohydrate-recognition domains fall in five structural groups.

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Structures for many of these domains, covering all of the groups, have been obtained.

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Not all human C-type lectins have clear orthologues in other mammals such as mice.

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Different mechanisms by which C-type lectins initiate signalling remain to be defined.

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Hetero-oligomeric receptors add to the complexity of overlapping specificities.

The majority of the C-type lectin-like domains in the human genome likely to bind sugars have been investigated structurally, although novel mechanisms of sugar binding are still being discovered. In the immune system, adhesion and endocytic receptors that bind endogenous mammalian glycans are often conserved, while pathogen-binding C-type lectins on cells of the innate immune system are more divergent. Lack of orthology between some human and mouse receptors, as well as overlapping specificities of many receptors and formation of receptor hetero-oligomers, can make it difficult to define the roles of individual receptors. There is good evidence that C-type lectins initiate signalling pathways in several different ways, but this function remains the least well understood from a mechanistic perspective.

Protein-carbohydrate interactions as part of plant defense and animal immunity

The immune system consists of a complex network of cells and molecules that interact with each other to initiate the host defense system. Many of these interactions involve specific carbohydrate structures and proteins that specifically recognize and bind them, in particular lectins. It is well established that lectin-carbohydrate interactions play a major role in the immune system, in that they mediate and regulate several interactions that are part of the immune response. Despite obvious differences between the immune system in animals and plants, there are also striking similarities. In both cases, lectins can play a role as pattern recognition receptors, recognizing the pathogens and initiating the stress response. Although plants do not possess an adaptive immune system, they are able to imprint a stress memory, a mechanism in which lectins can be involved. This review will focus on the role of lectins in the immune system of animals and plants.

Molecular basis of sugar recognition by collectin-K1 and the effects of mutations associated with 3MC syndrome

BACKGROUND

Collectin-K1 (CL-K1, or CL-11) is a multifunctional Ca(2+)-dependent lectin with roles in innate immunity, apoptosis and embryogenesis. It binds to carbohydrates on pathogens to activate the lectin pathway of complement and together with its associated serine protease MASP-3 serves as a guidance cue for neural crest development. High serum levels are associated with disseminated intravascular coagulation, where spontaneous clotting can lead to multiple organ failure. Autosomal mutations in the CL-K1 or MASP-3 genes cause a developmental disorder called 3MC (Carnevale, Mingarelli, Malpuech and Michels) syndrome, characterised by facial, genital, renal and limb abnormalities. One of these mutations (Gly(204)Ser in the CL-K1 gene) is associated with undetectable levels of protein in the serum of affected individuals.

RESULTS

In this study, we show that CL-K1 primarily targets a subset of high-mannose oligosaccharides present on both self- and non-self structures, and provide the structural basis for its ligand specificity. We also demonstrate that three disease-associated mutations prevent secretion of CL-K1 from mammalian cells, accounting for the protein deficiency observed in patients. Interestingly, none of the mutations prevent folding or oligomerization of recombinant fragments containing the mutations in vitro. Instead, they prevent Ca(2+) binding by the carbohydrate-recognition domains of CL-K1. We propose that failure to bind Ca(2+) during biosynthesis leads to structural defects that prevent secretion of CL-K1, thus providing a molecular explanation of the genetic disorder.

CONCLUSIONS

We have established the sugar specificity of CL-K1 and demonstrated that it targets high-mannose oligosaccharides on self- and non-self structures via an extended binding site which recognises the terminal two mannose residues of the carbohydrate ligand. We have also shown that mutations associated with a rare developmental disorder called 3MC syndrome prevent the secretion of CL-K1, probably as a result of structural defects caused by disruption of Ca(2+) binding during biosynthesis.

Expression of ligands for Siglec-8 and Siglec-9 in human airways and airway cells

BACKGROUND

Balanced activation and inhibition of the immune system ensures pathogen clearance while avoiding hyperinflammation. Siglecs, sialic acid-binding proteins found on subsets of immune cells, often inhibit inflammation: Siglec-8 on eosinophils and Siglec-9 on neutrophils engage sialoglycan ligands on airways to diminish ongoing inflammation. The identities of human siglec ligands and their expression during inflammation are largely unknown.

OBJECTIVE

The histologic distribution, expression, and molecular characteristics of siglec ligands were explored in healthy and inflamed human upper airways and in a cellular model of airway inflammation.

METHODS

Normal and chronically inflamed upper airway tissues were stained for siglec ligands. The ligands were extracted from normal and inflamed tissues and from human Calu-3 cells for quantitative analysis by means of siglec blotting and isolation by means of siglec capture.

RESULTS

Siglec-8 ligands were expressed on a subpopulation of submucosal gland cells of human inferior turbinate, whereas Siglec-9 ligands were expressed more broadly (submucosal glands, epithelium, and connective tissue); both were significantly upregulated in patients with chronic rhinosinusitis. Human airway (Calu-3) cells expressed Siglec-9 ligands on mucin 5B (MUC5B) under inflammatory control through the nuclear factor κB pathway, and MUC5B carried sialoglycan ligands of Siglec-9 on human upper airway tissue.

CONCLUSION

Inflammation results in upregulation of immune-inhibitory Siglec-8 and Siglec-9 sialoglycan ligands on human airways. Siglec-9 ligands are upregulated through the nuclear factor κB pathway, resulting in their enhanced expression on MUC5B. Siglec sialoglycan ligand expression in inflamed cells and tissues may contribute to the control of airway inflammation.

Glycans and glycan-binding proteins in immune regulation: A concise introduction to glycobiology for the allergist

Cells are endowed with a rich surface coat of glycans that are carried as glycoproteins and glycolipids on the outer leaflets of their plasma membranes and constitute a major molecular interface between cells and their environment. Each cell's glycome, the sum of its diverse glycan structures, comprises a distinct cellular signature defined by expression levels of the enzymes responsible for glycan biosynthesis. This signature can be read by complementary glycan-binding proteins (GBPs) that translate glycan recognition into function. Nowhere is this more evident than in the immune system, where glycans and GBPs are integral to pathogen recognition and control of inflammatory responses. Glycobiology, the study of glycan structures and their functions, increasingly provides insight into immunoregulatory mechanisms and thereby provides opportunities for therapeutic intervention. This review briefly examines the makeup of the human glycome and the GBPs that translate glycan recognition into function and provides examples of glycan recognition events that are responsible for immune system regulation to promote wider appreciation of this rapidly expanding area of research.