Structural analysis of monosaccharide recognition by rat liver mannose-binding protein

Structural analysis of carbohydrate binding by the macrophage mannose receptor CD206

The human mannose receptor expressed on macrophages and hepatic endothelial cells scavenges released lysosomal enzymes, glycopeptide fragments of collagen, and pathogenic microorganisms and thus reduces damage following tissue injury. The receptor binds mannose, fucose, or N-acetylglucosamine (GlcNAc) residues on these targets. C-type carbohydrate-recognition domain 4 (CRD4) of the receptor contains the site for Ca2+-dependent interaction with sugars. To investigate the details of CRD4 binding, glycan array screening was used to identify oligosaccharide ligands. The strongest signals were for glycans that contain either Manα1-2Man constituents or fucose in various linkages. The mechanisms of binding to monosaccharides and oligosaccharide substructures present in many of these ligands were examined in multiple crystal structures of CRD4. Binding of mannose residues to CRD4 results primarily from interaction of the equatorial 3- and 4-OH groups with a conserved principal Ca2+ common to almost all sugar-binding C-type CRDs. In the Manα1-2Man complex, supplementary interactions with the reducing mannose residue explain the enhanced affinity for this disaccharide. Bound GlcNAc also interacts with the principal Ca2+ through equatorial 3- and 4-OH groups, whereas fucose residues can bind in several orientations, through either the 2- and 3-OH groups or the 3- and 4-OH groups. Secondary contacts with additional sugars in fucose-containing oligosaccharides, such as the Lewis-a trisaccharide, provide enhanced affinity for these glycans. These results explain many of the biologically important interactions of the mannose receptor with both mammalian glycoproteins and microbes such as yeast and suggest additional classes of ligands that have not been previously identified.

Molecular Recognition in C-Type Lectins: The Cases of DC-SIGN, Langerin, MGL, and L-Sectin

Carbohydrates play a pivotal role in intercellular communication processes. In particular, glycan antigens are key for sustaining homeostasis, helping leukocytes to distinguish damaged tissues and invading pathogens from healthy tissues. From a structural perspective, this cross‐talk is fairly complex, and multiple membrane proteins guide these recognition processes, including lectins and Toll‐like receptors. Since the beginning of this century, lectins have become potential targets for therapeutics for controlling and/or avoiding the progression of pathologies derived from an incorrect immune outcome, including infectious processes, cancer, or autoimmune diseases. Therefore, a detailed knowledge of these receptors is mandatory for the development of specific treatments. In this review, we summarize the current knowledge about four key C‐type lectins whose importance has been steadily growing in recent years, focusing in particular on how glycan recognition takes place at the molecular level, but also looking at recent progresses in the quest for therapeutics.

Characterization and functional study of Galectin3 from Japanese flounder (Paralichthys olivaceus)

Galectins belong to the β-galactoside binding protein family and participate in both innate and acquired immunity. In this study, we described the molecular characteristics of Galectin3 gene from Japanese flounder (Paralichthys olivaceus), designed as PoGalectin3. Its open reading frame was 1128 bp, encoding a protein composed of 375 amino acids. PoGalectin3 belongs to chimeric galactose agglutinin, which contains a C-terminal carbohydrate recognition domain (CRD) (L₂₅₀-P₃₇₂), and its N-terminal is rich in proline (P) and glycine (G). Multiple sequence alignment and phylogenetic tree showed that PoGalectin3 was conservative in different aquatic animals. Tissue distribution confirmed that PoGalectin3 showed significantly highest expression in brain, moderate expression in liver, intestine and muscle. PoGalectin3 was significantly increased post infection with Edwardsiella tarda from intestine tissue of P. olivaceus. In order to investigate the binding ability of PoGalectin3 to pathogen-associated molecular patterns, the recombinant PoGalectin3 protein (rPoGalectin3) was successfully expressed and purified, and an Enzyme linked immunosorbent assay (ELISA) experiment was performed. ELISA refers to the qualitative and quantitative detection method of immune response by combining soluble antigen or antibody with solid-phase carrier. It was confirmed that rPoGalectin3 exhibited high affinity to lipopolysaccharide and peptidoglycan. The rPoGalectin3 also exhibited a concentration dependent binding capacity with Gram-positive bacteria (Bacillus pumilus, Bacillus subtilis, Bacillus cereus) and Gram-negative bacteria (Aeromonas salmonicida, E. tarda, Vibrio vulnificus). In addition, the results of microbial agglutination experiment showed that rPoGalectin3 could agglutinate Gram-positive bacteria (B. pumilus, B. subtilis) and Gram-negative bacteria (A. salmonicida, E. tarda) in the presence of Ca²⁺. In conclusion, this research laid an important foundation for the specific function analysis of PoGalectin3, which provide theoretical basis for the prevention and control of aquatic diseases.

Collectins: Innate Immune Pattern Recognition Molecules

Collectins are collagen-containing C-type (calcium-dependent) lectins which are important pathogen pattern recognising innate immune molecules. Their primary structure is characterised by an N-terminal, triple-helical collagenous region made up of Gly-X-Y repeats, an a-helical coiled-coil trimerising neck region, and a C-terminal C-type lectin or carbohydrate recognition domain (CRD). Further oligomerisation of this primary structure can give rise to more complex and multimeric structures that can be seen under electron microscope. Collectins can be found in serum as well as in a range of tissues at the mucosal surfaces. Mannanbinding lectin can activate the complement system while other members of the collectin family are extremely versatile in recognising a diverse range of pathogens via their CRDs and bring about effector functions designed at the clearance of invading pathogens. These mechanisms include opsonisation, enhancement of phagocytosis, triggering superoxidative burst and nitric oxide production. Collectins can also potentiate the adaptive immune response via antigen presenting cells such as macrophages and dendritic cells through modulation of cytokines and chemokines, thus they can act as a link between innate and adaptive immunity. This chapter describes the structure-function relationships of collectins, their diverse functions, and their interaction with viruses, bacteria, fungi and parasites.

Mammalian lectin arrays for screening host-microbe interactions

Many members of the C-type lectin family of glycan-binding receptors have been ascribed roles in the recognition of microorganisms and serve as key receptors in the innate immune response to pathogens. Other mammalian receptors have become targets through which pathogens enter target cells. These receptor roles have often been documented with binding studies involving individual pairs of receptors and microorganisms. To provide a systematic overview of interactions between microbes and the large complement of C-type lectins, here we developed a lectin array and suitable protocols for labeling of microbes that could be used to probe this array. The array contains C-type lectins from cow, chosen as a model organism of agricultural interest for which the relevant pathogen–receptor interactions have not been previously investigated in detail. Screening with yeast cells and various strains of both Gram-positive and -negative bacteria revealed distinct binding patterns, which in some cases could be explained by binding to lipopolysaccharides or capsular polysaccharides, but in other cases they suggested the presence of novel glycan targets on many of the microorganisms. These results are consistent with interactions previously ascribed to the receptors, but they also highlight binding to additional sugar targets that have not previously been recognized. Our findings indicate that mammalian lectin arrays represent unique discovery tools for identifying both novel ligands and new receptor functions.

Toxoplasma gondii Recruits Factor H and C4b-Binding Protein to Mediate Resistance to Serum Killing and Promote Parasite Persistence in vivo

Regulating complement is an important step in the establishment of infection by microbial pathogens. Toxoplasma gondii actively resists complement-mediated killing in non-immune human serum (NHS) by inactivating C3b, however the precise molecular basis is unknown. Here, a flow cytometry-based C3b binding assay demonstrated that Type II strains had significantly higher levels of surface-bound C3b than Type I strains. However, both strains efficiently inactivated C3b and were equally resistant to serum killing, suggesting that resistance is not strain-dependent. Toxoplasma activated both the lectin (LP) and alternative (AP) pathways, and the deposition of C3b was both strain and lectin-dependent. A flow cytometry-based lectin binding assay identified strain-specific differences in the level and heterogeneity of surface glycans detected. Specifically, increased lectin-binding by Type II strains correlated with higher levels of the LP recognition receptor mannose binding lectin (MBL). Western blot analyses demonstrated that Toxoplasma recruits both classical pathway (CP) and LP regulator C4b-binding proteins (C4BP) and AP regulator Factor H (FH) to the parasite surface to inactivate bound C3b-iC3b and C3dg and limit formation of the C5b-9 attack complex. Blocking FH and C4BP contributed to increased C5b-9 formation in vitro. However, parasite susceptibility in vitro was only impacted when FH was blocked, indicating that down regulation of the alternative pathway by FH may be more critical for parasite resistance. Infection of C3 deficient mice led to uncontrolled parasite growth, acute mortality, and reduced antibody production, indicating that both the presence of C3, and the ability of the parasite to inactivate C3, was protective. Taken together, our results establish that Toxoplasma regulation of the complement system renders mice resistant to acute infection by limiting parasite proliferation in vivo, but susceptible to chronic infection, with all mice developing transmissible cysts to maintain its life cycle.

Atomic-resolution crystal structures of the immune protein conglutinin from cow reveal specific interactions of its binding site with N-acetylglucosamine

Bovine conglutinin is an immune protein that is involved in host resistance to microbes and parasites and interacts with complement component iC3b, agglutinates erythrocytes, and neutralizes influenza A virus. Here, we determined the high-resolution (0.97–1.46 Å) crystal structures with and without bound ligand of a recombinant fragment of conglutinin's C-terminal carbohydrate-recognition domain (CRD). The structures disclosed that the high-affinity ligand N-acetyl-d-glucosamine (GlcNAc) binds in the collectin CRD calcium site by interacting with the O3′ and O4′ hydroxyls alongside additional specific interactions of the N-acetyl group oxygen and nitrogen with Lys-343 and Asp-320, respectively. These residues, unique to conglutinin and differing both in sequence and in location from those in other collectins, result in specific, high-affinity binding for GlcNAc. The binding pocket flanking residue Val-339, unlike the equivalent Arg-343 in the homologous human surfactant protein D, is sufficiently small to allow conglutinin Lys-343 access to the bound ligand, whereas Asp-320 lies in an extended loop proximal to the ligand-binding site and bounded at both ends by conserved residues that coordinate to both calcium and ligand. This loop becomes ordered on ligand binding. The electron density revealed both α and β anomers of GlcNAc, consistent with the added α/βGlcNAc mixture. Crystals soaked with α1–2 mannobiose, a putative component of iC3b, reported to bind to conglutinin, failed to reveal bound ligand, suggesting a requirement for presentation of mannobiose as part of an extended physiological ligand. These results reveal a highly specific GlcNAc-binding pocket in conglutinin and a novel collectin mode of carbohydrate recognition.

Functional Characterization of YdjH, a Sugar Kinase of Unknown Specificity in Escherichia coli K12

The ydj gene cluster is annotated to catalyze the catabolism of an unknown carbohydrate. Previously, YdjI, a class II aldolase, was shown to catalyze the retro-aldol cleavage of l-glycero-l-galacto-octuluronate-1-phosphate into DHAP and l-arabinuronate. In this report, the functional characterization of YdjH is presented. YdjH catalyzes the phosphorylation of 2-keto-monosaccharides at the C1 hydroxyl group with a substrate profile significantly more stringent than that of YdjI. Similar to YdjI, YdjH shows a strong preference for higher-order monosaccharides (seven to nine carbons) with a carboxylate terminus. The best substrate was determined to be l-glycero-l-galacto-octuluronate, yielding l-glycero-l-galacto-octuluronate-1-phosphate with a k_cat of 16 s^-1 and a k_cat/K_m of 2.1 × 10⁴ M^-1 s^-1. This is apparently the first reported example of kinase activity with eight-carbon monosaccharides. Two crystal structures of YdjH were previously determined to 2.15 and 1.8 Å resolution (Protein Data Bank entries 3H49 and 3IN1 ). We present an analysis of the active site layout and use computational docking to identify potential key residues in the binding of l-glycero-l-galacto-octuluronate.

Unraveling Sugar Binding Modes to DC-SIGN by Employing Fluorinated Carbohydrates

A fluorine nuclear magnetic resonance (¹⁹F-NMR)-based method is employed to assess the binding preferences and interaction details of a library of synthetic fluorinated monosaccharides towards dendritic cell-specific intercellular adhesion molecule 3-grabbing non-integrin (DC-SIGN), a lectin of biomedical interest, which is involved in different viral infections, including HIV and Ebola, and is able to recognize a variety of self- and non-self-glycans. The strategy employed allows not only screening of a mixture of compounds, but also obtaining valuable information on the specific sugar–protein interactions. The analysis of the data demonstrates that monosaccharides Fuc, Man, Glc, and Gal are able to bind DC-SIGN, although with decreasing affinity. Moreover, a new binding mode between Man moieties and DC-SIGN, which might have biological implications, is also detected for the first time. The combination of the ¹⁹F with standard proton saturation transfer difference (¹H-STD-NMR) data, assisted by molecular dynamics (MD) simulations, permits us to successfully define this new binding epitope, where Man coordinates a Ca²⁺ ion of the lectin carbohydrate recognition domain (CRD) through the axial OH-2 and equatorial OH-3 groups, thus mimicking the Fuc/DC-SIGN binding architecture.

Structural Aspects of Carbohydrate Recognition Mechanisms of C-Type Lectins

Carbohydrate recognition is an essential function occurring in all living organisms. Lectins are carbohydrate-binding proteins and are classified into several families. In mammals, Ca²⁺-dependent C-type lectins, such as β-galactoside-binding galectin and sialic acid-binding siglec, play crucial roles in the immune response and homeostasis. C-type lectins are abundant and diverse in animals. Their immunological activities include lymphocyte homing, pathogen recognition, and clearance of apoptotic bodies. C-type lectin domains are composed of 110-130 amino acid residues with highly conserved structural folds. Remarkably, individual lectins can accept a wide variety of sugar ligands and can distinguish subtle structural differences in closely related ligands. In addition, several C-type lectin-like proteins specifically bind to carbohydrate ligands in Ca²⁺-independent ways. The accumulated 3D structural evidence clarifies the unexpected structural versatility of C-type lectins underlying the variety of ligand binding modes. In this issue, we focus on the structural aspects of carbohydrate recognition mechanisms of C-type lectins and C-type lectin-like proteins.

Mechanism of pathogen recognition by human dectin-2

Dectin-2, a C-type lectin on macrophages and other cells of the innate immune system, functions in response to pathogens, particularly fungi. The carbohydrate-recognition domain (CRD) in dectin-2 is linked to a transmembrane sequence that interacts with the common Fc receptor γ subunit to initiate immune signaling. The molecular mechanism by which dectin-2 selectively binds to pathogens has been investigated by characterizing the CRD expressed in a bacterial system. Competition binding studies indicated that the CRD binds to monosaccharides with modest affinity and that affinity was greatly enhanced for mannose-linked α1–2 or α1–4 to a second mannose residue. Glycan array analysis confirmed selective binding of the CRD to glycans that contain Manα1–2Man epitopes. Crystals of the CRD in complex with a mammalian-type high-mannose Man₉GlcNAc₂ oligosaccharide exhibited interaction with Manα1–2Man on two different termini of the glycan, with the reducing-end mannose residue ligated to Ca²⁺ in a primary binding site and the nonreducing terminal mannose residue occupying an adjacent secondary site. Comparison of the binding sites in DC-SIGN and langerin, two other pathogen-binding receptors of the innate immune system, revealed why these two binding sites accommodate only terminal Manα1–2Man structures, whereas dectin-2 can bind Manα1–2Man in internal positions in mannans and other polysaccharides. The specificity and geometry of the dectin-2-binding site provide the molecular mechanism for binding of dectin-2 to fungal mannans and also to bacterial lipopolysaccharides, capsular polysaccharides, and lipoarabinomannans that contain the Manα1–2Man disaccharide unit.

Pathogen recognition of a novel C-type lectin from Marsupenaeus japonicus reveals the divergent sugar-binding specificity of QAP motif

C-type lectins (CTLs) are calcium-dependent carbohydrate-binding proteins known to assist the innate immune system as pattern recognition receptors (PRRs). The binding specificity of CTLs lies in the motif of their carbohydrate recognition domain (CRD), the tripeptide motifs EPN and QPD bind to mannose and galactose, respectively. However, variants of these motifs were discovered including a QAP sequence reported in shrimp believed to have the same carbohydrate specificity as QPD. Here, we characterized a novel C-type lectin (MjGCTL) possessing a CRD with a QAP motif. The recombinant MjGCTL has a calcium-dependent agglutinating capability against both Gram-negative and Gram-positive bacteria, and its sugar specificity did not involve either mannose or galactose. In an encapsulation assay, agarose beads coated with rMjGCTL were immediately encapsulated from 0 h followed by melanization at 4 h post-incubation with hemocytes. These results confirm that MjGCTL functions as a classical CTL. The structure of QAP motif and carbohydrate-specificity of rMjGCTL was found to be different to both EPN and QPD, suggesting that QAP is a new motif. Furthermore, MjGCTL acts as a PRR binding to hemocytes to activate their adherent state and initiate encapsulation.

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.

Plasma-based proteomics reveals immune response, complement and coagulation cascades pathway shifts in heat-stressed lactating dairy cows

Heat stress (HS) has an enormous economic impact on the dairy industry. In recent years, many researchers have investigated changes in the gene expression and metabolomics profiles in dairy cows caused by HS. However, the proteomics profiles of heat-stressed dairy cows have not yet been completely elucidated. We compared plasma proteomics from HS-free and heat-stressed dairy cows using an iTRAQ labeling approach. After the depletion of high abundant proteins in the plasma, 1472 proteins were identified. Of these, 85 proteins were differentially abundant in cows exposed to HS relative to HS-free. Database searches combined with GO and KEGG pathway enrichment analyses revealed that many components of the complement and coagulation cascades were altered in heat-stressed cows compared with HS-free cows. Of these, many factors in the complement system (including complement components C1, C3, C5, C6, C7, C8, and C9, complement factor B, and factor H) were down-regulated by HS, while components of the coagulation system (including coagulation factors, vitamin K-dependent proteins, and fibrinogens) were up-regulated by HS. In conclusion, our results indicate that HS decreases plasma levels of complement system proteins, suggesting that immune function is impaired in dairy cows exposed to HS.

BIOLOGICAL SIGNIFICANCE

Though many aspects of heat stress (HS) have been extensively researched, relatively little is known about the proteomics profile changes that occur during heat exposure. In this work, we employed a proteomics approach to investigate differential abundance of plasma proteins in HS-free and heat-stressed dairy cows. Database searches combined with GO and KEGG pathway enrichment analyses revealed that HS resulted in a decrease in complement components, suggesting that heat-stressed dairy cows have impaired immune function. In addition, through integrative analyses of proteomics and previous metabolomics, we showed enhanced glycolysis, lipid metabolic pathway shifts, and nitrogen repartitioning in dairy cows exposed to HS. Our findings expand our current knowledge on the effects of HS on plasma proteomics in dairy cows and offer a new perspective for future research.

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.

Emerging roles of protein mannosylation in inflammation and infection

Proteins are frequently modified by complex carbohydrates (glycans) that play central roles in maintaining the structural and functional integrity of cells and tissues in humans and lower organisms. Mannose forms an essential building block of protein glycosylation, and its functional involvement as components of larger and diverse α-mannosidic glycoepitopes in important intra- and intercellular glycoimmunological processes is gaining recognition. With a focus on the mannose-rich asparagine (N-linked) glycosylation type, this review summarises the increasing volume of literature covering human and non-human protein mannosylation, including their structures, biosynthesis and spatiotemporal expression. The review also covers their known interactions with specialised host and microbial mannose-recognising C-type lectin receptors (mrCLRs) and antibodies (mrAbs) during inflammation and pathogen infection. Advances in molecular mapping technologies have recently revealed novel immuno-centric mannose-terminating truncated N-glycans, termed paucimannosylation, on human proteins. The cellular presentation of α-mannosidic glycoepitopes on N-glycoproteins appears tightly regulated; α-mannose determinants are relative rare glycoepitopes in physiological extracellular environments, but may be actively secreted or leaked from cells to transmit potent signals when required. Simultaneously, our understanding of the molecular basis on the recognition of mannosidic epitopes by mrCLRs including DC-SIGN, mannose receptor, mannose binding lectin and mrAb is rapidly advancing, together with the functional implications of these interactions in facilitating an effective immune response during physiological and pathophysiological conditions. Ultimately, deciphering these complex mannose-based receptor-ligand interactions at the detailed molecular level will significantly advance our understanding of immunological disorders and infectious diseases, promoting the development of future therapeutics to improve patient clinical outcomes.

Galectin CvGal2 from the Eastern Oyster (Crassostrea virginica) Displays Unique Specificity for ABH Blood Group Oligosaccharides and Differentially Recognizes Sympatric Perkinsus Species

Galectins are highly conserved lectins that are key to multiple biological functions, including pathogen recognition and regulation of immune responses. We previously reported that CvGal1, a galectin expressed in phagocytic cells (hemocytes) of the eastern oyster (Crassostrea virginica), is hijacked by the parasite Perkinsus marinus to enter the host, where it causes systemic infection and death. Screening of an oyster hemocyte cDNA library revealed a novel galectin, which we designated CvGal2, with four tandemly arrayed carbohydrate recognition domains (CRDs). Phylogentic analysis of the CvGal2 CRDs suggests close relationships with homologous CRDs from CvGal1. Glycan array analysis, however, revealed that, unlike CvGal1 which preferentially binds to the blood group A tetrasaccharide, CvGal2 recognizes both blood group A and B tetrasaccharides and related structures, suggesting that CvGal2 has broader binding specificity. Furthermore, SPR analysis demonstrated significant differences in the binding kinetics of CvGal1 and CvGal2, and structural modeling revealed substantial differences in their interactions with the oligosaccharide ligands. CvGal2 is homogeneously distributed in the hemocyte cytoplasm, is released to the extracellular space, and binds to the hemocyte surface. CvGal2 binds to P. marinus trophozoites in a dose-dependent and β-galactoside-specific manner. Strikingly, negligible binding of CvGal2 was observed for Perkinsus chesapeaki, a sympatric parasite species mostly prevalent in the clams Mya arenaria and Macoma balthica. The differential recognition of Perkinsus species by the oyster galectins is consistent with their relative prevalence in oyster and clam species and supports their role in facilitating parasite entry and infectivity in a host-preferential manner.

Mannose-recognition mutant of the galactose/N-acetylgalactosamine-specific C-type lectin CEL-I engineered by site-directed mutagenesis

BACKGROUND

CEL-I is a galactose/N-acetylgalactosamine-specific C-type lectin isolated from the sea cucumber Cucumaria echinata. Its carbohydrate-binding site contains a QPD (Gln-Pro-Asp) motif, which is generally recognized as the galactose specificity-determining motif in the C-type lectins. In our previous study, replacement of the QPD motif by an EPN (Glu-Pro-Asn) motif led to a weak binding affinity for mannose. Therefore, we examined the effects of an additional mutation in the carbohydrate-binding site on the specificity of the lectin.

METHODS

Trp105 of EPN-CEL-I was replaced by a histidine residue using site-directed mutagenesis, and the binding affinity of the resulting mutant, EPNH-CEL-I, was examined by sugar-polyamidoamine dendrimer assay, isothermal titration calorimetry, and glycoconjugate microarray analysis. Tertiary structure of the EPNH-CEL-I/mannose complex was determined by X-ray crystallographic analysis.

RESULTS

Sugar-polyamidoamine dendrimer assay and glycoconjugate microarray analysis revealed a drastic change in the specificity of EPNH-CEL-I from galactose/N-acetylgalactosamine to mannose. The association constant of EPNH-CEL-I for mannose was determined to be 3.17×10(3) M(-1) at 25°C. Mannose specificity of EPNH-CEL-I was achieved by stabilization of the binding of mannose in a correct orientation, in which the EPN motif can form proper hydrogen bonds with 3- and 4-hydroxy groups of the bound mannose.

CONCLUSIONS

Specificity of CEL-I can be engineered by mutating a limited number of amino acid residues in addition to the QPD/EPN motifs.

GENERAL SIGNIFICANCE

Versatility of the C-type carbohydrate-recognition domain structure in the recognition of various carbohydrate chains could become a promising platform to develop novel molecular recognition proteins.

Manno-oligosaccharide-binding ability of mouse RegIV/GST-fusion protein evaluated by complex formation with the carbohydrate-containing polyamidoamine dendrimer

The carbohydrate-binding properties of the C-type lectin-like mouse RegIV and glutathione S-transferase-fusion protein (GST-mRegIV) were examined using carbohydrate-containing polyamidoamine dendrimers (PD). GST-mRegIV showed affinity for mannan- and manno-oligosaccharide containing PD. Binding was inhibited by manno-oligosaccharides but not by mannose or other tested carbohydrates, suggesting that the binding site may have an extended structure in contrast with typical C-type lectins.

Expression, purification, crystallization and preliminary X-ray diffraction analysis of a ribokinase from the thermohalophile Halothermothrix orenii

A ribokinase gene (rbk) from the anaerobic halothermophilic bacterium Halothermothrix orenii was cloned and overexpressed in Escherichia coli. The recombinant protein (Ho-Rbk) was purified using immobilized metal-ion affinity chromatography and crystals were obtained using the sitting-drop method. Diffraction data were collected to a resolution of 3.1 Å using synchrotron radiation. The crystals belonged to the orthorhombic space group P2(1)2(1)2(1), with unit-cell parameters a = 45.6, b = 61.1, c = 220.2, and contained two molecules per asymmetric unit. A molecular-replacement solution has been found and attempts are currently under way to build a model of the ribokinase. Efforts to improve crystal quality so that higher resolution data can be obtained are also being considered.

PDB-scale analysis of known and putative ligand-binding sites with structural sketches

Computational investigation of protein functions is one of the most urgent and demanding tasks in the field of structural bioinformatics. Exhaustive pairwise comparison of known and putative ligand-binding sites, across protein families and folds, is essential in elucidating the biological functions and evolutionary relationships of proteins. Given the vast amounts of data available now, existing 3D structural comparison methods are not adequate due to their computation time complexity. In this article, we propose a new bit string representation of binding sites called structural sketches, which is obtained by random projections of triplet descriptors. It allows us to use ultra-fast all-pair similarity search methods for strings with strictly controlled error rates. Exhaustive comparison of 1.2 million known and putative binding sites finished in ∼30 h on a single core to yield 88 million similar binding site pairs. Careful investigation of 3.5 million pairs verified by TM-align revealed several notable analogous sites across distinct protein families or folds. In particular, we succeeded in finding highly plausible functions of several pockets via strong structural analogies. These results indicate that our method is a promising tool for functional annotation of binding sites derived from structural genomics projects.

Comparative analysis reveals selective recognition of glycans by the dendritic cell receptors DC-SIGN and Langerin

DC-SIGN (dendritic cell-specific ICAM-3 grabbing non-integrin) and Langerin are homologous C-type lectins expressed as cell-surface receptors on different populations of dendritic cells (DCs). DC-SIGN interacts with glycan structures on HIV-1, facilitating virus survival, transmission and infection, whereas Langerin, which is characteristic of Langerhans cells (LCs), promotes HIV-1 uptake and degradation. Here we describe a comprehensive comparison of the glycan specificities of both proteins by probing a synthetic carbohydrate microarray comprising 275 sugar compounds using the bacterially produced and fluorescence-labeled, monomeric carbohydrate-recognition domains (CRDs) of DC-SIGN and Langerin. In this side-by-side study DC-SIGN was found to preferentially bind internal mannose residues of high-mannose-type saccharides and the fucose-containing blood-type antigens H, A, B, Le(a), Le(b) Le(x), Le(y), sialyl-Le(a) as well as sulfatated derivatives of Le(a) and Le(x). In contrast, Langerin appeared to recognize a different spectrum of compounds, especially those containing terminal mannose, terminal N-acetylglucosamine and 6-sulfogalactose residues, but also the blood-type antigens H, A and B. Of the Lewis antigens, only Le(b), Le(y), sialyl-Le(a) and the sialyl-Le(x) derivative with 6'-sulfatation at the galactose (sialyl-6SGal Le(x)) were weakly bound by Langerin. Notably, Ca(2+)-independent glycan-binding activity of Langerin could not be detected either by probing the glycan array or by isothermal titration calorimetry of the CRD with mannose and mannobiose. The precise knowledge of carbohydrate specificity of DC-SIGN and Langerin receptors resulting from our study may aid the future design of microbicides that specifically affect the DC-SIGN/HIV-1 interaction while not compromising the protective function of Langerin.

Structural basis for langerin recognition of diverse pathogen and mammalian glycans through a single binding site

Langerin mediates the carbohydrate-dependent uptake of pathogens by Langerhans cells in the first step of antigen presentation to the adaptive immune system. Langerin binds to an unusually diverse number of endogenous and pathogenic cell surface carbohydrates, including mannose-containing O-specific polysaccharides derived from bacterial lipopolysaccharides identified here by probing a microarray of bacterial polysaccharides. Crystal structures of the carbohydrate-recognition domain from human langerin bound to a series of oligomannose compounds, the blood group B antigen, and a fragment of β-glucan reveal binding to mannose, fucose, and glucose residues by Ca²⁺ coordination of vicinal hydroxyl groups with similar stereochemistry. Oligomannose compounds bind through a single mannose residue, with no other mannose residues contacting the protein directly. There is no evidence for a second Ca²⁺-independent binding site. Likewise, a β-glucan fragment, Glcβ1–3Glcβ1–3Glc, binds to langerin through the interaction of a single glucose residue with the Ca²⁺ site. The fucose moiety of the blood group B trisaccharide Galα1–3(Fucα1–2)Gal also binds to the Ca²⁺ site, and selective binding to this glycan compared to other fucose-containing oligosaccharides results from additional favorable interactions of the nonreducing terminal galactose, as well as of the fucose residue. Surprisingly, the equatorial 3-OH group and the axial 4-OH group of the galactose residue in 6SO₄–Galβ1–4GlcNAc also coordinate Ca²⁺, a heretofore unobserved mode of galactose binding in a C-type carbohydrate-recognition domain bearing the Glu-Pro-Asn signature motif characteristic of mannose binding sites. Salt bridges between the sulfate group and two lysine residues appear to compensate for the nonoptimal binding of galactose at this site.

Survey of immune-related, mannose/fucose-binding C-type lectin receptors reveals widely divergent sugar-binding specificities

C-type lectins (CTLs) are proteins that contain one or more carbohydrate-recognition domains (CRDs) that require calcium for sugar binding and share high degree of sequence homology and tertiary structure. CTLs whose CRD contain EPN (Glu-Pro-Asn) tripeptide motifs have potential to bind mannose (Man), N-acetylglucosamine (GlcNAc), glucose (Glc) and l-fucose (Fuc), whereas those with QPD (Glu-Pro-Asp) tripeptide motifs bind galactose (Gal) and N-acetylgalactosamine (GalNAc). We report here for the first time a direct comparison of monosaccharide (and some di- and trisaccharides)-binding characteristics of 11 EPX-containing (X = N, S or D) immune-related CTLs using a competition assay and an enzyme-linked immunosorbent assay, and neoglycoproteins as ligand. The EPX CTLs studied are DC-SIGN, L-SIGN, mSIGNR1, human and mouse mannose receptors, Langerin, BDCA-2, DCIR, dectin-2, MCL and MINCLE. We found that: (1) they all bound Man and Fuc; (2) binding of Glc and GlcNAc varied considerably among these lectins, but was always less than Man and Fuc; (3) in general, Gal and GalNAc were not bound. However, dectin-2, DCIR and MINCLE showed ability to bind Gal/GalNAc; (4) DC-SIGN, L-SIGN, mSIGNR1 and Langerin showed enhanced binding of Manα2Man over Man, whereas all others showed no enhancement; (5) DC-SIGN bound Le(x) trisaccharide structure, which has terminal Gal and Fuc residues, more avidly than Fuc, whereas L-SIGN, mSIGNR1, DCIR and MINCLE bound Le(x) less avidly than Fuc. BDCA-2, dectin-2, Langerin, MCL and mannose receptor did not bind Le(x) at all.

Self-organizing fuzzy graphs for structure-based comparison of protein pockets

Patterns of receptor-ligand interaction can be conserved in functionally equivalent proteins even in the absence of sequence homology. Therefore, structural comparison of ligand-binding pockets and their pharmacophoric features allow for the characterization of so-called "orphan" proteins with known three-dimensional structure but unknown function, and predict ligand promiscuity of binding pockets. We present an algorithm for rapid pocket comparison (PoLiMorph), in which protein pockets are represented by self-organizing graphs that fill the volume of the cavity. Vertices in these three-dimensional frameworks contain information about the local ligand-receptor interaction potential coded by fuzzy property labels. For framework matching, we developed a fast heuristic based on the maximum dispersion problem, as an alternative to techniques utilizing clique detection or geometric hashing algorithms. A sophisticated scoring function was applied that incorporates knowledge about property distributions and ligand-receptor interaction patterns. In an all-against-all virtual screening experiment with 207 pocket frameworks extracted from a subset of PDBbind, PoLiMorph correctly assigned 81% of 69 distinct structural classes and demonstrated sustained ability to group pockets accommodating the same ligand chemotype. We determined a score threshold that indicates "true" pocket similarity with high reliability, which not only supports structure-based drug design but also allows for sequence-independent studies of the proteome.

Structural studies of langerin and Birbeck granule: a macromolecular organization model

Dendritic cells, a sentinel immunity cell lineage, include different cell subsets that express various C-type lectins. For example, epidermal Langerhans cells express langerin, and some dermal dendritic cells express DC-SIGN. Langerin is a crucial component of Birbeck granules, the Langerhans cell hallmark organelle, and may have a preventive role toward HIV, by its internalization into Birbeck granules. Since langerin carbohydrate recognition domain (CRD) is crucial for HIV interaction and Birbeck granule formation, we produced the CRD of human langerin and solved its structure at 1.5 A resolution. On this basis gp120 high-mannose oligosaccharide binding has been evaluated by molecular modeling. Hydrodynamic studies reveal a very elongated shape of recombinant langerin extracellular domain (ECD). A molecular model of the langerin ECD, integrating the CRD structure, has been generated and validated by comparison with hydrodynamic parameters. In parallel, Langerhans cells were isolated from human skin. From their analysis by electron microscopy and the langerin ECD model, an ultrastructural organization is proposed for Birbeck granules. To delineate the role of the different langerin domains in Birbeck granule formation, we generated truncated and mutated langerin constructs. After transfection into a fibroblastic cell line, we highlighted, in accordance with our model, the role of the CRD in the membrane zipping occurring in BG formation as well as some contribution of the cytoplasmic domain. Finally, we have shown that langerin ECD triggering with a specific mAb promotes global rearrangements of LC morphology. Our results open the way to the definition of a new membrane deformation mechanism.

Structural characterisation of ligand-binding determinants in human lung surfactant protein D: influence of Asp325

The crystal structures of a biologically and therapeutically active recombinant homotrimeric fragment of human lung surfactant protein D with a series of bound ligands have been determined. While the structures reveal various different binding modes, all utilise a similarly positioned pair of mannose-type O3' and O4' hydroxyls with no direct interaction between any non-terminal sugar and protein. The orientation, position, and interactions of the bound terminal sugar depend on the sugar itself, the presence and form of glycosidic linkage, and the environment in the crystal, which, via Asp325, places stereochemical and electronic constraints, different for the three different subunits in the homotrimer, on the ligand-binding site. As a direct consequence of this influence, the other binding-pocket flanking residue, Arg343, exhibits variable conformation and variable interactions with bound ligand and leaves open to question which orientation of terminal mannobiose, and of other terminal disaccharides, may be present in extended physiological ligands. The combined structural evidence shows that there is significant flexibility in recognition; that Asp325, in addition to Arg343, is an important determinant of ligand selectivity, recognition, and binding; and that differences in crystal contact interfaces exert, through Asp325, significant influence on preferred binding modes.

Lung surfactant proteins A and D as pattern recognition proteins

Lung surfactant proteins A and D belong to a group of soluble humoral pattern recognition receptors, called collectins, which modulate the immune response to microorganisms. They bind essential carbohydrate and lipid antigens found on the surface of microorganisms via low affinity C-type lectin domains and regulate the host's response by binding to immune cell surface receptors. They form multimeric structures that bind, agglutinate, opsonise and neutralize many different pathogenic microorganisms including bacteria, yeast, fungi and viruses. They modulate the uptake of these microorganisms by phagocytic cells as well as both the inflammatory and the adaptive immune responses. Recent data have also highlighted their involvement in clearance of apoptotic cells, hypersensitivity and a number of lung diseases.

Recombinant production and characterization of the carbohydrate recognition domain from Atlantic salmon C-type lectin receptor C (SCLRC)

The Atlantic salmon C-type lectin receptor C (SCLRC) locus encodes a potential oligomeric type II receptor. C-type lectins recognize carbohydrates in a Ca(2+)-dependent manner through structurally conserved, yet functionally diverse, C-type lectin-like domains (CTLDs). Many conserved amino acids in animal CTLDs are present in SCLRC, with the notable exception of an asparagine crucially involved in Ca(2+)- and carbohydrate-binding, which is tyrosine in SCLRC. SCLRC also contains six cysteines that form three disulfide bonds. Although SCLRC was originally identified as an up-regulated transcript responding to Aeromonas salmonicida infection, the biological role of this protein is still unknown. To study the structure and ligand binding properties of SCLRC, we created a homology model of the 17kDa CTLD and produced it as an affinity-tagged protein in the periplasm of Escherichia coli by co-expression of proteins that facilitate disulfide bond formation. The recombinant form of SCLRC was characterized by a protease protection assay, a solid-phase carbohydrate-binding assay, and frontal affinity chromatography. On the basis of this characterization, we classify SCLRC as a C-type lectin that binds to mannose and its derivatives.

Carbohydrate ligands of human C-reactive protein: binding of neoglycoproteins containing galactose-6-phosphate and galactose-terminated disaccharide

Binding of carbohydrate ligand by human C-reactive protein (CRP), in both native form and structurally deviated form (neoCRP or mCRP), was investigated using galactose-6-phosphate (Gal6P)- and Galbeta3GalNAc-containing bovine serum albumin (BSA) derivatives. To this end, we synthesized glycosides of Gal6P and Galbeta3GalNAc that can potentially generate a terminal aldehyde group. omega-Aldehydo glycosides were then conjugated to BSA via reductive amination. Using these neoglycoproteins, we showed that: (1) Gal6P-BSA and Galbeta3GalNAc-BSA bound to both forms of CRP, the former with or without calcium and the latter only in the absence of calcium; (2) phosphate-containing ligands can be bound with or without calcium, but the binding is much stronger in the presence of calcium than in the absence, underscoring the importance of direct coordination of phosphate to two calcium ions observed in the X-ray structure of phosphorylcholine (PC)-CRP complex; (3) cross-inhibition studies further corroborated the hypothesis that binding sites of PC and sugar are contiguous; (4) while PC-BSA bound to the native CRP over a wide pH range of 4.5 to 9, all the carbohydrate ligands and protamine-BSA (poly-cation-based ligand) exhibited optimal binding at around pH 6 to 6.5; and (5) ligand-binding conformation of mCRP appears to be more fragile than that of the native CRP in the acidic media (pH < 6).

A Galectin of Unique Domain Organization from Hemocytes of the Eastern Oyster (Crassostrea virginica) Is a Receptor for the Protistan Parasite Perkinsus marinus

Invertebrates display effective innate immune responses for defense against microbial infection. However, the protozoan parasite Perkinsus marinus causes Dermo disease in the eastern oyster Crassostrea virginica and is responsible for catastrophic damage to shellfisheries and the estuarine environment in North America. The infection mechanisms remain unclear, but it is likely that, while filter feeding, the healthy oysters ingest P. marinus trophozoites released to the water column by the infected neighboring individuals. Inside oyster hemocytes, trophozoites resist oxidative killing, proliferate, and spread throughout the host. However, the mechanism(s) for parasite entry into the hemocyte are unknown. In this study, we show that oyster hemocytes recognize P. marinus via a novel galectin (C. virginica galectin (CvGal)) of unique structure. The biological roles of galectins have only been partly elucidated, mostly encompassing embryogenesis and indirect roles in innate and adaptive immunity mediated by the binding to endogenous ligands. CvGal recognized a variety of potential microbial pathogens and unicellular algae, and preferentially, Perkinsus spp. trophozoites. Attachment and spreading of hemocytes to foreign surfaces induced localization of CvGal to the cell periphery, its secretion and binding to the plasma membrane. Exposure of hemocytes to Perkinsus spp. trophozoites enhanced this process further, and their phagocytosis could be partially inhibited by pretreatment of the hemocytes with anti-CvGal Abs. The evidence presented indicates that CvGal facilitates recognition of selected microbes and algae, thereby promoting phagocytosis of both potential infectious challenges and phytoplankton components, and that P. marinus subverts the host's immune/feeding recognition mechanism to passively gain entry into the hemocytes.

The C-type lectin L-SIGN differentially recognizes glycan antigens on egg glycosphingolipids and soluble egg glycoproteins from Schistosoma mansoni

Recognition of pathogen-derived carbohydrate constituents by antigen presenting cells is an important step in the induction of protective immunity. Here we investigated the interaction of L-SIGN (liver/lymph node specific ICAM-3-grabbing nonintegrin), a C-type lectin that functions as antigen receptor on human liver sinusoidal endothelial cells, with egg-derived glycan antigens of the parasitic trematode Schistosoma mansoni. Our data demonstrate that L-SIGN binds both schistosomal soluble egg antigens (SEA) and egg glycosphingolipids, and can mediate internalization of SEA by L-SIGN expressing cells. Binding and internalization of SEA was strongly reduced after treatment of SEA with endoglycosidase H, whereas defucosylation affected neither binding nor internalization. These data indicate that L-SIGN predominantly interacts with oligomannosidic N-glycans of SEA. In contrast, binding to egg glycosphingolipids was completely abolished after defucosylation. Our data show that L-SIGN binds to a glycosphingolipid fraction containing fucosylated species with compositions of Hex(1)HexNAc(5-7)dHex(3-6)Cer, as evidenced by mass spectrometry. The L-SIGN "gain of function" mutant Ser363Val, which binds fucosylated Lewis antigens, did not bind to this fucosylated egg glycosphingolipid fraction, suggesting that L-SIGN displays different modes in binding fucoses of egg glycosphingolipids and Lewis antigens, respectively. Molecular modeling studies indicate that the preferred binding mode of L-SIGN to the respective fucosylated egg glycosphingolipid oligosaccharides involves a Fucalpha1-3GalNAcbeta1-4(Fucalpha1-3)GlcNAc tetrasaccharide at the nonreducing end. In conclusion, our data indicate that L-SIGN recognizes both oligomannosidic N-glycans and multiply fucosylated carbohydrate motifs within Schistosoma egg antigens, which demonstrates that L-SIGN has a broad but specific glycan recognition profile.

The role of the mannose-binding lectin in innate immunity

The innate immune system, which includes mannose-binding lectin (MBL), recognizes a broad range of molecular patterns on a broad range of infectious agents and is able to distinguish them from self. MBL is a liver-derived serum protein and is secreted into the serum, where it can activate an immune response before the induction of antigen-specific immunity. Circumstantial evidence in human populations suggests that low serum levels of MBL predispose to infection. To analyze the role of MBL in vivo, we created MBL-null mice and challenged these mice with infection under various conditions. Our results suggest that MBL plays an important role as a first-line host defense against certain infectious agents. In addition, it is likely that MBL is a key regulator of inflammation beyond expected roles in the infection.

Scavenger receptor C-type lectin binds to the leukocyte cell surface glycan Lewis(x) by a novel mechanism

The scavenger receptor C-type lectin (SRCL) is unique in the family of class A scavenger receptors, because in addition to binding sites for oxidized lipoproteins it also contains a C-type carbohydrate-recognition domain (CRD) that interacts with specific glycans. Both human and mouse SRCL are highly specific for the Lewis(x) trisaccharide, which is commonly found on the surfaces of leukocytes and some tumor cells. Structural analysis of the CRD of mouse SRCL in complex with Lewis(x) and mutagenesis show the basis for this specificity. The interaction between mouse SRCL and Lewis(x) is analogous to the way that selectins and DC-SIGN bind to related fucosylated glycans, but the mechanism of the interaction is novel, because it is based on a primary galactose-binding site similar to the binding site in the asialoglycoprotein receptor. Crystals of the human receptor lacking bound calcium ions reveal an alternative conformation in which a glycan ligand would be released during receptor-mediated endocytosis.

Burn injury reveals altered phenotype in mannan-binding lectin-deficient mice

Burn injury destroys skin, the second largest innate immune organ in the body, and triggers chaotic immune and inflammatory responses. The pattern recognition molecule, mannan-binding lectin (MBL), plays an important role in the first-line host defense against infectious agents. MBL initiates the lectin complement pathway and acts as an opsonin. Recent studies suggest that MBL also modulates inflammatory responses. We report that local responses after burn in MBL null mice differ from those found in wild-type (WT) mice in the following important biological markers: spontaneous eschar separation, thinned epidermis and dermis, upregulation of soluble factors including cytokines, chemokines, cell adhesion molecules, a growth factor-binding protein, and matrix metalloproteinases. Mice lacking C1q, C4, or C3 did not show the lack of eschar separation seen in MBL null-burn phenotype. These findings implicate MBL as an important molecule in the maintenance of the homeostatic balance.

Multiple modes of binding enhance the affinity of DC-SIGN for high mannose N-linked glycans found on viral glycoproteins

The dendritic cell surface receptor DC-SIGN and the closely related endothelial cell receptor DC-SIGNR specifically recognize high mannose N-linked carbohydrates on viral pathogens. Previous studies have shown that these receptors bind the outer trimannose branch Manalpha1-3[Manalpha1-6]Manalpha present in high mannose structures. Although the trimannoside binds to DC-SIGN or DC-SIGNR more strongly than mannose, additional affinity enhancements are observed in the presence of one or more Manalpha1-2Manalpha moieties on the nonreducing termini of oligomannose structures. The molecular basis of this enhancement has been investigated by determining crystal structures of DC-SIGN bound to a synthetic six-mannose fragment of a high mannose N-linked oligosaccharide, Manalpha1-2Manalpha1-3[Manalpha1-2Manalpha1-6]Manalpha1-6Man and to the disaccharide Manalpha1-2Man. The structures reveal mixtures of two binding modes in each case. Each mode features typical C-type lectin binding at the principal Ca2+-binding site by one mannose residue. In addition, other sugar residues form contacts unique to each binding mode. These results suggest that the affinity enhancement displayed toward oligosaccharides decorated with the Manalpha1-2Manalpha structure is due in part to multiple binding modes at the primary Ca2+ site, which provide both additional contacts and a statistical (entropic) enhancement of binding.

R5- and X4-HIV-1 use differentially the endometrial epithelial cells HEC-1A to ensure their own spread: implication for mechanisms of sexual transmission

The mechanism of viral transmission across the mucosal barrier is poorly understood. Using the endometrial epithelium-derived cell line HEC-1A, we found that the cells are capable of sequestering large numbers of HIV-1 particles but are refractory to cell-free viral infection. The removal of heparan sulfate moieties of cell-surface proteoglycans (HSPG) from the apical pole of HEC-1A accounted for at least 60% of both R5- and X4-HIV-1 attachment, showing their important implication in viral attachment. HEC-1A cells also have the capacity to endocytose a weak proportion of the attached virus and pass it along to underlying cells. Fucose, N-acetylglucosamine and mannosylated-residues inhibited the transcytosis of some virus isolates, suggesting that mannose receptors can be implicated on the both R5- and X4-HIV-1 transcytosis. The inhibition of HIV transcytosis by blocking CCR5 mAb suggests the implication of specific interaction between the viral gp120 and sulfated moiety of syndecans during the transcytosis of mostly R5- and X4-HIV-1. At the basolateral pole of HEC-1A, HSPG sequestered X4- and not R5-HIV-1, highlighting the important role of HEC-1A as an X4 virus reservoir. The cell-free virus particles that have transcytosed could infect activated T cells but with a weaker efficiency than virus that had not transcytosed. The specific stimulation of HEC-1A by R5-HIV-1 increased the release of monocytes/chemokines-attracting chemokines (IL-8 and GR0) and proinflammatory cytokines (TNF-beta and IL-1alpha) that enhanced the production of virus by activated T cells. This study suggests that R5 and X4 viruses can differentially use epithelial cells to ensure their own spread.

Calcium is not required for immulectin-2 binding, but protects the protein from proteinase digestion

Mammalian C-type lectins are calcium-dependent carbohydrate-binding proteins. They serve as cell adhesion molecules in cell-cell interactions, or function as pattern-recognition receptors in innate immunity. Calcium is a direct ligand for carbohydrate binding in mammalian C-type lectins such as mannose-binding proteins and macrophage mannose receptor. In the tobacco hornworm Manduca sexta, a group of lectins named immulectins have been discovered. Each immulectin contains dual carbohydrate-recognition domains. Previously, we showed that immulectin-2 (IML-2) binds to a bacterial lipopolysaccharide, and agglutination of Escherichia coli cells by IML-2 is calcium dependent. In this study, we demonstrated that IML-2 bound to bacterial lipid A, smooth and rough mutants of lipopolysaccharide, lipoteichoic acid and peptidoglycan, as well as to fungal mannan and beta-1, 3-glucan (laminarin and curdlan). Binding of IML-2 to microbial components was calcium independent, and was increased by addition of spermine, a polyamine. In addition, plasma IML-2 bound to mannan-agarose independent of calcium. But trypsin digestion of IML-2 was inhibited in the presence of calcium. Our results suggest that calcium is not required for IML-2 binding but protects IML-2 from trypsin digestion.

Characterization of an equine mannose-binding lectin and its roles in disease

The mannose-binding lectin (MBL), a pattern recognition serum protein, participates in the innate immune system of mammals as an opsonin. In humans, MBL plays a key role in first-line host defense against infection during the lag period prior to the development of a specific immune response. MBL also activates complement via the lectin pathway that requires a MBL-associated serine protease-2 (MASP-2). Homologues of human MBL (hMBL) have been identified in a variety of mammals, fish, and primitive animals such as ascidians. In this study, we report that equine MBL (eMBL) has properties that are similar to hMBL. In addition, we found low levels of MBL:MASP activity in sick horses compared to healthy horses. These results suggest that eMBL is involved in the immune response of the horse and that low MBL:MASP activity could be used to monitor immune function and clinical outcome.

The C-type lectin-like domain superfamily

The superfamily of proteins containing C-type lectin-like domains (CTLDs) is a large group of extracellular Metazoan proteins with diverse functions. The CTLD structure has a characteristic double-loop ('loop-in-a-loop') stabilized by two highly conserved disulfide bridges located at the bases of the loops, as well as a set of conserved hydrophobic and polar interactions. The second loop, called the long loop region, is structurally and evolutionarily flexible, and is involved in Ca2+-dependent carbohydrate binding and interaction with other ligands. This loop is completely absent in a subset of CTLDs, which we refer to as compact CTLDs; these include the Link/PTR domain and bacterial CTLDs. CTLD-containing proteins (CTLDcps) were originally classified into seven groups based on their overall domain structure. Analyses of the superfamily representation in several completely sequenced genomes have added 10 new groups to the classification, and shown that it is applicable only to vertebrate CTLDcps; despite the abundance of CTLDcps in the invertebrate genomes studied, the domain architectures of these proteins do not match those of the vertebrate groups. Ca2+-dependent carbohydrate binding is the most common CTLD function in vertebrates, and apparently the ancestral one, as suggested by the many humoral defense CTLDcps characterized in insects and other invertebrates. However, many CTLDs have evolved to specifically recognize protein, lipid and inorganic ligands, including the vertebrate clade-specific snake venoms, and fish antifreeze and bird egg-shell proteins. Recent studies highlight the functional versatility of this protein superfamily and the CTLD scaffold, and suggest further interesting discoveries have yet to be made.

Structural basis for the interaction between human milk oligosaccharides and the bacterial lectin PA-IIL of Pseudomonas aeruginosa

One of the mechanisms contributing to the protection by breast-feeding of the newborn against enteric diseases is related to the ability of human milk oligosaccharides to prevent the attachment of pathogenic bacteria to the duodenual epithelium. Indeed, a variety of fucosylated oligosaccharides, specific to human milk, form part of the innate immune system. In the present study, we demonstrate the specific blocking of PA-IIL, a fucose-binding lectin of the human pathogen Pseudomonas aeruginosa, by milk oligosaccharides. Two fucosylated epitopes, Lewis a and 3-fucosyl-lactose (Lewis x glucose analogue) bind to the lectin with dissociation constants of 2.2x10(-7) M and 3.6x10(-7) M respectively. Thermodynamic studies indicate that these interactions are dominated by enthalpy. The entropy contribution is slightly favourable when binding to fucose and to the highest-affinity ligand, Lewis a. The high-resolution X-ray structures of two complexes of PA-IIL with milk oligosaccharides allow the precise determination of the conformation of a trisaccharide and a pentasaccharide. The different types of interaction between the oligosaccharides and the protein involve not only hydrogen bonding, but also calcium- and water-bridged contacts, allowing a rationalization of the thermodynamic data. This study provides important structural information about compounds that could be of general application in new therapeutic strategies against bacterial infections.

Comparative genetics and innate immune functions of collagenous lectins in animals

Collagenous lectins such as mannan-binding lectins (MBLs), ficolins (FCNs), surfactant proteins A and D (SP-A, SP-D), conglutinin (CG), and related ruminant lectins are multimeric proteins with carbohydrate-binding domains aligned in a manner that facilitates binding to microbial surface polysaccharides. MBLs and FCNs are structurally related to C1q, but activate the lectin complement pathway via interaction with MBL-associated serine proteases (MASPs). MBLs, FCNs, and other collagenous lectins also bind to some host macromolecules and contribute to their removal. While there is evidence that some lectins and the lectin complement pathway are conserved in vertebrates, many differences in collagenous lectins have been observed among humans, rodents, and other vertebrates. For example, humans have only one MBL but three FCNs, whereas most other species express two FCNs and two MBLs. Bovidae express CG and other SP-D-related collectins that are not found in monogastric species. Some dysfunctions of human MBL are due to single nucleotide polymorphisms (SNPs) that affect its expression or structure and thereby increase susceptibility to some infections. Collagenous lectins have well-established roles in innate immunity to various microorganisms, so it is possible that some lectin genotypes or induced phenotypes influence resistance to some infectious or inflammatory diseases in animals.

Characterization of a binary tandem domain F-type lectin from striped bass (Morone saxatilis)

Among other functions, lectins play an important role in the innate immune response of vertebrates and invertebrates by recognizing exposed glycans on the surface of potential pathogens. Despite the typically weak interaction of lectin domains with their carbohydrate ligands, they usually achieve high avidity through oligomeric structures or by the presence of tandem carbohydrate-binding domains along the polypeptide. The recently described structure of the fucose-binding European eel agglutinin revealed a novel lectin fold (the "F-type" fold), which is shared with other carbohydrate-binding proteins and apparently unrelated proteins from prokaryotes to vertebrates, and a unique fucose-binding sequence motif. Here we described the biochemical and molecular characterization of a unique fucose-binding lectin (MsaFBP32) isolated from serum of the striped bass (Morone saxatilis), composed of two tandem domains that exhibit the eel carbohydrate recognition sequence motif, which we designate F-type. We also described a novel lectin family ("F-type") constituted by a large number of proteins exhibiting greater multiples of the F-type motif, either tandemly arrayed or in mosaic combinations with other domains, including a putative transmembrane receptor, that suggests an extensive functional diversification of this lectin family. Among the tandem lectins, MsaFBP32 and other tandem binary homologues appear unique in that although their N-terminal domain shows close similarity to the fucose recognition domain of the eel agglutinin, their C-terminal domain exhibits changes that potentially could confer a distinct specificity for fucosylated ligands. In contrast with the amniotes, in which the F-type lectins appear conspicuously absent, the widespread gene duplication in the teleost fish suggests these F-type lectins acquired increasing evolutionary value within this taxon.

The mannose-binding lectin: an infection susceptibility gene

A critical but unanswered question is what defines each individual’s pre-morbid susceptibility to infection? w e propose that individuals must have an “immune haplotype” that shapes their response to infectious agents. Infection is a balance between the intrinsic virulence of the infectious agent and the host defenses. Recent viral outbreaks of SARS and influenza serve to illustrate this point as these viruses cause severe disease in certain individuals, yet there are others in whom the same infectious challenge results in minimal symptoms. On the other hand it might be that those self same people who are resistance to one particular viral infection might be susceptible to other infection challenges. Similar rules can apply to susceptibility to bacterial infections.