Unusual sugar specificity of banana lectin from Musa paradisiaca and its probable evolutionary origin. Crystallographic and modelling studies

High resolution functional analysis and community structure of photogranules

Photogranules are spherical aggregates formed of complex phototrophic ecosystems with potential for “aeration-free” wastewater treatment. Photogranules from a sequencing batch reactor were investigated by fluorescence microscopy, 16S/18S rRNA gene amplicon sequencing, microsensors, and stable- and radioisotope incubations to determine the granules’ composition, nutrient distribution, and light, carbon, and nitrogen budgets. The photogranules were biologically and chemically stratified, with filamentous cyanobacteria arranged in discrete layers and forming a scaffold to which other organisms were attached. Oxygen, nitrate, and light gradients were also detectable. Photosynthetic activity and nitrification were both predominantly restricted to the outer 500 µm, but while photosynthesis was relatively insensitive to the oxygen and nutrient (ammonium, phosphate, acetate) concentrations tested, nitrification was highly sensitive. Oxygen was cycled internally, with oxygen produced through photosynthesis rapidly consumed by aerobic respiration and nitrification. Oxygen production and consumption were well balanced. Similarly, nitrogen was cycled through paired nitrification and denitrification, and carbon was exchanged through photosynthesis and respiration. Our findings highlight that photogranules are complete, complex ecosystems with multiple linked nutrient cycles and will aid engineering decisions in photogranular wastewater treatment.

N2 -fixation can sustain wastewater treatment performance of photogranules under nitrogen-limiting conditions

Wastewater characteristics can vary significantly, and in some municipal wastewaters the N:P ratio is as low as 5 resulting in nitrogen-limiting conditions. In this study, the microbial community, function, and morphology of photogranules under nitrogen-replete (N+) and limiting (N-) conditions was assessed in sequencing batch reactors. Photogranules under N- condition were nitrogen deprived 2/3 of a batch cycle duration. Surprisingly, this nitrogen limitation had no adverse effect on biomass productivity. Moreover, phosphorus and chemical oxygen demand removal were similar to their removal under N+ conditions. Although performance was similar, the difference in granule morphology was obvious. While N+ photogranules were dense and structurally confined, N- photogranules showed loose structures with occasional voids. Microbial community analysis revealed high abundance of cyanobacteria capable of N₂ -fixation. These were higher at N- (38%) than N+ (29%) treatments, showing that photogranules could adjust and maintain treatment performance and high biomass productivity by means of N₂ -fixation.

Structure and Carbohydrate Recognition by the Nonmitogenic Lectin Horcolin

Lectins are sugar-binding proteins that have shown considerable promise as antiviral agents because of their ability to interact with envelope glycoproteins present on the surface of viruses such as HIV-1. However, their therapeutic potential has been compromised by their mitogenicity that stimulates uncontrolled division of T-lymphocytes. Horcolin, a member of the jacalin family of lectins, tightly binds the HIV-1 envelope glycoprotein gp120 and neutralizes HIV-1 particles but is nonmitogenic. In this report, we combine X-ray crystallography and NMR spectroscopy to obtain atomic-resolution insights into the structure of horcolin and the molecular basis for its carbohydrate recognition. Each protomer of the horcolin dimer adopts a canonical β-prism I fold with three Greek key motifs and carries two carbohydrate-binding sites. The carbohydrate molecule binds in a negatively charged pocket and is stabilized by backbone and side chain hydrogen bonds to conserved residues in the ligand-binding loop. NMR titrations reveal a two-site binding mode and equilibrium dissociation constants for the two binding sites determined from two-dimensional (2D) lineshape modeling are 4-fold different. Single-binding-site variants of horcolin confirm the dichotomy in binding sites and suggest that there is allosteric communication between the two sites. An analysis of the horcolin structure shows a network of hydrogen bonds linking the two carbohydrate-binding sites directly and through a secondary binding site, and this coupling between the two sites is expected to assume importance in the interaction of horcolin with high-mannose glycans found on viral envelope glycoproteins.

Banana Lectin from Musa paradisiaca Is Mitogenic for Cow and Pig PBMC via IL-2 Pathway and ELF1

The aim of the study was to gain deeper insights in the potential of polyclonal stimulation of PBMC with banana lectin (BanLec) from Musa paradisiaca. BanLec induced a marked proliferative response in cow and pig PBMC, but was strongest in pigs, where it induced an even higher proliferation rate than Concanavalin A. Molecular processes associated with respective responses in porcine PBMC were examined with differential proteome analyses. Discovery proteomic experiments was applied to BanLec stimulated PBMC and cellular and secretome responses were analyzed with label free LC-MS/MS. In PBMC, 3955 proteins were identified. After polyclonal stimulation with BanLec, 459 proteins showed significantly changed abundance in PBMC. In respective PBMC secretomes, 2867 proteins were identified with 231 differentially expressed candidates as reaction to BanLec stimulation. The transcription factor “E74 like ETS transcription factor 1 (ELF1)” was solely enriched in BanLec stimulated PBMC. BanLec induced secretion of several immune regulators, amongst them positive regulators of activated T cell proliferation and Jak-STAT signaling pathway. Top changed immune proteins were CD226, CD27, IFNG, IL18, IL2, CXCL10, LAT, ICOS, IL2RA, LAG3, and CD300C. BanLec stimulates PBMC of cows and pigs polyclonally and induces IL2 pathway and further proinflammatory cytokines. Proteomics data are available via ProteomeXchange with identifier PXD027505.

Mevo lectin specificity towards high-mannose structures with terminal αMan(1,2)αMan residues and its implication to inhibition of the entry of Mycobacterium tuberculosis into macrophages

Mannose-binding lectins can specifically recognize and bind complex glycan structures on pathogens and have potential as anti-viral and anti-bacterial agents. We previously reported the structure of a lectin from an archaeal species, Mevo lectin, which has specificity towards terminal α1,2 linked manno-oligosaccharides. Mycobacterium tuberculosis (M. tuberculosis) expresses mannosylated structures including, lipoarabinomannan (ManLAM) on its surface and exploits C-type lectins to gain entry into the host cells. ManLAM structure has mannose capping with terminal αMan(1,2)αMan residues and is important for recognition by innate immune cells. Here, we aim to address the specificity of Mevo lectin towards high-mannose type glycans with terminal αMan(1,2)αMan residues and its effect on M. tuberculosis internalization by macrophages. ITC studies demonstrated that Mevo lectin shows preferential binding towards manno-oligosaccharides with terminal αMan(1,2)αMan structures, and showed a strong affinity for ManLAM, whereas it binds weakly to Mycobacterium smegmatis (M. smegmatis) lipoarabinomannan (MsmLAM), which displays relatively fewer and shorter mannosyl caps. Crystal structure of Mevo lectin complexed with a Man7D1 revealed the multivalent cross-linking interaction, which explains avidity-based high affinity for these ligands when compared to previously studied manno-oligosaccharides lacking the specific termini. Functional studies suggest that M. tuberculosis internalization by the macrophage was impaired by binding of Mevo lectin to ManLAM present on the surface of M. tuberculosis. Selectivity shown by Mevo lectin towards glycans with terminal αMan(1,2)αMan structures, and its ability to compromise the internalization of M. tuberculosis in vitro, underscore the potential utility of Mevo lectin as a research tool to study host-pathogen interactions.

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

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

Structural and related studies on Mevo lectin from Methanococcus voltae A3: the first thorough characterization of an archeal lectin and its interactions

Crystallographic and solution studies of Mevo lectin and its complexes, the first effort of its kind on an archeal lectin, reveal a structure similar to β-prism I fold lectins from plant and animal sources, but with a quaternary association involving a ring structure with seven-fold symmetry. Each subunit in the heptamer carries one sugar binding site on the first Greek key motif. The oligomeric interface is primarily made up of a parallel β-sheet involving a strand of Greek key I of one subunit and Greek key ΙΙΙ from a neighboring subunit. The crystal structures of the complexes of the lectin with mannose, αMan(1,2)αMan, αMan(1,3)αMan, a mannotriose and a mannopentose revealed a primary binding site similar to that found in other mannose specific β-prism I fold lectins. The complex with αMan(1,3)αMan provides an interesting case in which a few subunits have the reducing end at the primary binding site, while the majority have the nonreducing end at the primary binding site. The structures of complexes involving the trisaccharide and the pentasaccharide exhibit cross-linking among heptameric molecules. The observed arrangements may be relevant to the multivalency of the lectin. Phylogenetic analysis of amino acid sequences indicates that Mevo lectin is closer to β-prism I fold animal lectins than with those of plant origin. The results presented here reinforce the conclusion regarding the existence of lectins in all three domains of life. It would also appear that lectins evolved to the present form before the three domains diverged.

Antiviral activity of native banana lectin against bovine viral diarrhea virus and bovine alphaherpesvirus type 1

Bovine viral diarrhea virus (BVDV) and bovine alphaherpesvirus type 1 (BoHV-1) are responsible for major economic losses of livestock worldwide, making their eradication an important objective of veterinary research. Vaccines against these infectious agents are commercially available but have some limitations due to the specific features of these viral agents. The development of new antiviral drugs is therefore essential. Native banana lectin (BanLec) is a lectin isolated from banana fruit (Musa acuminata) and has a high affinity for mannose glycans found in several viral envelopes. The inhibitory properties of this lectin against several viruses has already been demonstrated. The aim of this work was therefore to test the antiviral and virucidal activities of BanLec against BVDV-1 and BoHV-1. Its antiviral activity was assessed by measuring the viral titer and viability of susceptible Madin-Darby Bovine Kidney cells (MDBK) treated with BanLec before and after viral infection. The virucidal properties of BanLec were determined by preincubation of the lectin with the viruses, followed by measurement of the viral load in exposed cells. Treatment with 25 μg/mL BanLec resulted in high levels of inhibition against BVDV-1 (99.98%) and BoHV-1 (99.68%) without affecting cell viability, demonstrating promising potential as an antiviral agent.

ELLSA based profiling of surface glycosylation in microorganisms reveals that ß-glucan rich yeasts' surfaces are selectively recognized with recombinant banana lectin

The surface of microorganisms is covered with polysaccharide structures which are in immediate contact with receptor structures on host's cells and antibodies. The interaction between microorganisms and their host is dependent on surface glycosylation and in this study we have tested the interaction of plant lectins with different microorganisms. Enzyme-linked lectin sorbent assay - ELLSA was used to test the binding of recombinant Musa acuminata lectin - BL to 27 selected microorganisms and 7 other lectins were used for comparison: Soy bean agglutinin - SBA, Lens culinaris lectin - LCA, Wheat germ agglutinin - WGA, RCA₁₂₀ - Ricinus communis agglutinin, Con A - from Canavalia ensiformis, Sambucus nigra agglutinin - SNA I and Maackia amurensis agglutinin - MAA. The goal was to define the microorganisms' surface glycosylation by means of interaction with the selected plant lectins and to make a comparison with BL. Among the tested lectins most selective binding was observed for RCA₁₂₀ which preferentially bound Lactobacillus casei DG. Recombinant banana lectin showed specific binding to all of the tested fungal species. The binding of BL to Candida albicans was further tested with fluorescence microscopy and flow cytometry and it was concluded that this lectin can differentiate ß-glucan rich surfaces. The binding of BL to S. boulardii could be inhibited with ß-glucan from yeast with IC₅₀ 1.81 μg mL^-1 and to P. roqueforti with 1.10 μg mL^-1. This unique specificity of BL could be exploited for screening purposes and potentially for the detection of ß-glucan in solutions.

Mannose-Specific Lectins from Marine Algae: Diverse Structural Scaffolds Associated to Common Virucidal and Anti-Cancer Properties

To date, a number of mannose-specific lectins have been isolated and characterized from seaweeds, especially from red algae. In fact, man-specific seaweed lectins consist of different structural scaffolds harboring a single or a few carbohydrate-binding sites which specifically recognize mannose-containing glycans. Depending on the structural scaffold, man-specific seaweed lectins belong to five distinct structurally-related lectin families, namely (1) the griffithsin lectin family (β-prism I scaffold); (2) the Oscillatoria agardhii agglutinin homolog (OAAH) lectin family (β-barrel scaffold); (3) the legume lectin-like lectin family (β-sandwich scaffold); (4) the Galanthus nivalis agglutinin (GNA)-like lectin family (β-prism II scaffold); and, (5) the MFP2-like lectin family (MFP2-like scaffold). Another algal lectin from Ulva pertusa, has been inferred to the methanol dehydrogenase related lectin family, because it displays a rather different GlcNAc-specificity. In spite of these structural discrepancies, all members from the five lectin families share a common ability to specifically recognize man-containing glycans and, especially, high-mannose type glycans. Because of their mannose-binding specificity, these lectins have been used as valuable tools for deciphering and characterizing the complex mannose-containing glycans from the glycocalyx covering both normal and transformed cells, and as diagnostic tools and therapeutic drugs that specifically recognize the altered high-mannose N-glycans occurring at the surface of various cancer cells. In addition to these anti-cancer properties, man-specific seaweed lectins have been widely used as potent human immunodeficiency virus (HIV-1)-inactivating proteins, due to their capacity to specifically interact with the envelope glycoprotein gp120 and prevent the virion infectivity of HIV-1 towards the host CD4+ T-lymphocyte cells in vitro.

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

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

Biochemical and structural characterization of a mannose binding jacalin-related lectin with two-sugar binding sites from pineapple (Ananas comosus) stem

A mannose binding jacalin-related lectin from Ananas comosus stem (AcmJRL) was purified and biochemically characterized. This lectin is homogeneous according to native, SDS-PAGE and N-terminal sequencing and the theoretical molecular mass was confirmed by ESI-Q-TOF-MS. AcmJRL was found homodimeric in solution by size-exclusion chromatography. Rat erythrocytes are agglutinated by AcmJRL while no agglutination activity is detected against rabbit and sheep erythrocytes. Hemagglutination activity was found more strongly inhibited by mannooligomannosides than by D-mannose. The carbohydrate-binding specificity of AcmJRL was determined in some detail by isothermal titration calorimetry. All sugars tested were found to bind with low affinity to AcmJRL, with K_a values in the mM range. In agreement with hemagglutination assays, the affinity increased from D-mannose to di-, tri- and penta-mannooligosaccharides. Moreover, the X-ray crystal structure of AcmJRL was obtained in an apo form as well as in complex with D-mannose and methyl-α-D-mannopyranoside, revealing two carbohydrate-binding sites per monomer similar to the banana lectin BanLec. The absence of a wall separating the two binding sites, the conformation of β7β8 loop and the hemagglutinating activity are reminiscent of the BanLec His84Thr mutant, which presents a strong anti-HIV activity in absence of mitogenic activity.

Antiviral lectins: Selective inhibitors of viral entry

Many natural lectins have been reported to have antiviral activity. As some of these have been put forward as potential development candidates for preventing or treating viral infections, we have set out in this review to survey the literature on antiviral lectins. The review groups lectins by structural class and class of source organism we also detail their carbohydrate specificity and their reported antiviral activities. The review concludes with a brief discussion of several of the pertinent hurdles that heterologous proteins must clear to be useful clinical candidates and cites examples where such studies have been reported for antiviral lectins. Though the clearest path currently being followed is the use of antiviral lectins as anti-HIV microbicides via topical mucosal administration, some investigators have also found systemic efficacy against acute infections following subcutaneous administration.

The Tetrameric Plant Lectin BanLec Neutralizes HIV through Bidentate Binding to Specific Viral Glycans

Select lectins have powerful anti-viral properties that effectively neutralize HIV-1 by targeting the dense glycan shield on the virus. Here, we reveal the mechanism by which one of the most potent lectins, BanLec, achieves its inhibition. We identify that BanLec recognizes a subset of high-mannose glycans via bidentate interactions spanning the two binding sites present on each BanLec monomer that were previously considered separate carbohydrate recognition domains. We show that both sites are required for high-affinity glycan binding and virus neutralization. Unexpectedly we find that BanLec adopts a tetrameric stoichiometry in solution whereby the glycan-binding sites are positioned to optimally target glycosylated viral spikes. The tetrameric architecture, together with bidentate binding to individual glycans, leads to layers of multivalency that drive viral neutralization through enhanced avidity effects. These structural insights will prove useful in engineering successful lectin therapeutics targeting the dense glycan shield of HIV.

Why Do Membranes of Some Unhealthy Cells Adopt a Cubic Architecture?

Nonlamellar lipid arrangements, including cubosomes, appear in unhealthy cells, e.g., when they are subject to stress, starvation, or viral infection. The bioactivity of cubosomes-nanoscale particles exhibiting bicontinuous cubic structures-versus more common vesicles is an unexplored area due to lack of suitable model systems. Here, glycodendrimercubosomes (GDCs)-sugar-presenting cubosomes assembled from Janus glycodendrimers by simple injection into buffer-are proposed as mimics of biological cubic membranes. The bicontinuous cubic GDC architecture has been demonstrated by electron tomography. The stability of these GDCs in buffer enabled studies on lectin-dependent agglutination, revealing significant differences compared with the vesicular glycodendrimersome (GDS) counterpart. In particular, GDCs showed an increased activity toward concanavalin A, as well as an increased sensitivity and selectivity toward two variants of banana lectins, a wild-type and a genetically modified variant, which is not exhibited by GDSs. These results suggest that cells may adapt under unhealthy conditions by undergoing a transformation from lamellar to cubic membranes as a method of defense.

Effect of linkage on the location of reducing and nonreducing sugars bound to jacalin

The crystal structures of jacalin complexed with Gal α-(1,4) Gal and Gal α-(1,3) Gal β-(1,4) Gal have been determined with the primary objective of exploring the effect of linkage on the location of reducing and non-reducing sugars in the extended binding site of the lectin, an issue which has not been studied thoroughly. Contrary to the earlier surmise based on simple steric considerations, the two structures demonstrate that α-linked sugars can bind to jacalin with nonreducing sugar at the primary binding site. This is made possible substantially on account of the hitherto underestimated plasticity of a non-polar region of the extended binding site. Modeling studies involving conformational search and energy minimization, along with available crystallographic and thermodynamic data, indicate a strong preference for complexation with Gal β-(1,3) Gal with the reducing Gal at the primary site, followed by that with Gal α-(1,3) Gal, with the reducing or non-reducing Gal located at the primary binding site. This observation is in consonance with the facility of jacalin to bind mucin type O-glycans containing T-antigen core. © 2016 IUBMB Life, 68(12):971-979, 2016.

Structural studies on a non-toxic homologue of type II RIPs from bitter gourd: Molecular basis of non-toxicity, conformational selection and glycan structure

The structures of nine independent crystals of bitter gourd seed lectin (BGSL), a non-toxic homologue of type II RIPs, and its sugar complexes have been determined. The four-chain, two-fold symmetric, protein is made up of two identical two-chain modules, each consisting of a catalytic chain and a lectin chain, connected by a disulphide bridge. The lectin chain is made up of two domains. Each domain carries a carbohydrate binding site in type II RIPs of known structure. BGSL has a sugar binding site only on one domain, thus impairing its interaction at the cell surface. The adenine binding site in the catalytic chain is defective. Thus, defects in sugar binding as well as adenine binding appear to contribute to the non-toxicity of the lectin. The plasticity of the molecule is mainly caused by the presence of two possible well defined conformations of a surface loop in the lectin chain. One of them is chosen in the sugar complexes, in a case of conformational selection, as the chosen conformation facilitates an additional interaction with the sugar, involving an arginyl residue in the loop. The N-glycosylation of the lectin involves a plant-specific glycan while that in toxic type II RIPs of known structure involves a glycan which is animal as well as plant specific.

Two carbohydrate recognizing domains from Cycas revoluta leaf lectin show the distinct sugar-binding specificity-A unique mannooligosaccharide recognition by N-terminal domain

Cycas revoluta leaf lectin (CRLL) of mannose-recognizing jacalin-related lectin (mJRL) has two tandem repeated carbohydrate recognition domains, and shows the characteristic sugar-binding specificity toward high mannose-glycans, compared with other mJRLs. We expressed the N-terminal domain and C-terminal domain (CRLL-N and CRLL-C) separately, to determine the fine sugar-binding specificity of each domain, using frontal affinity chromatography, glycan array and equilibrium dialysis. The specificity of CRLL toward high mannose was basically derived from CRLL-N, whereas CRLL-C had affinity for α1-6 extended mono-antennary complex-type glycans. Notably, the affinity of CRLL-N was most potent to one of three Man 8 glycans and Man 9 glycan, whereas the affinity of CRLL-C decreased with the increase in the number of extended α1-2 linked mannose residue. The recognition of the Man 8 glycans by CRLL-N has not been found for other mannose recognizing lectins. Glycan array reflected these specificities of the two domains. Furthermore, it was revealed by equilibrium dialysis method that the each domain had two sugar-binding sites, similar with Banlec, banana mannose-binding Jacalin-related lectin.

Engineering a therapeutic lectin by uncoupling mitogenicity from antiviral activity

A key effector route of the Sugar Code involves lectins that exert crucial regulatory controls by targeting distinct cellular glycans. We demonstrate that a single amino-acid substitution in a banana lectin, replacing histidine 84 with a threonine, significantly reduces its mitogenicity, while preserving its broad-spectrum antiviral potency. X-ray crystallography, NMR spectroscopy, and glycocluster assays reveal that loss of mitogenicity is strongly correlated with loss of pi-pi stacking between aromatic amino acids H84 and Y83, which removes a wall separating two carbohydrate binding sites, thus diminishing multivalent interactions. On the other hand, monovalent interactions and antiviral activity are preserved by retaining other wild-type conformational features and possibly through unique contacts involving the T84 side chain. Through such fine-tuning, target selection and downstream effects of a lectin can be modulated so as to knock down one activity, while preserving another, thus providing tools for therapeutics and for understanding the Sugar Code.

Banana lectin: a brief review

Lectins are a group of proteins of non-immune origin that recognize and bind to carbohydrates without modifying them. Banana is the common name for both herbaceous plants of the genus Musa and for the fruit they produce. They are indeed a promising source for many medicinal applications. Banana lectins have the potential for inhibiting HIV-1 reverse transcriptase activity, suppressing cancer cell proliferation and stimulating macrophage activities. Nevertheless, compared to other plant lectins, there is relatively little information in the literature on banana lectins, particularly with respect to their structure and biological functions. Herein we focus our review on the structure, functions and exploitable properties of banana lectins.

The sequence and structure of snake gourd (Trichosanthes anguina) seed lectin, a three-chain nontoxic homologue of type II RIPs

The sequence and structure of snake gourd seed lectin (SGSL), a nontoxic homologue of type II ribosome-inactivating proteins (RIPs), have been determined by mass spectrometry and X-ray crystallography, respectively. As in type II RIPs, the molecule consists of a lectin chain made up of two β-trefoil domains. The catalytic chain, which is connected through a disulfide bridge to the lectin chain in type II RIPs, is cleaved into two in SGSL. However, the integrity of the three-dimensional structure of the catalytic component of the molecule is preserved. This is the first time that a three-chain RIP or RIP homologue has been observed. A thorough examination of the sequence and structure of the protein and of its interactions with the bound methyl-α-galactose indicate that the nontoxicity of SGSL results from a combination of changes in the catalytic and the carbohydrate-binding sites. Detailed analyses of the sequences of type II RIPs of known structure and their homologues with unknown structure provide valuable insights into the evolution of this class of proteins. They also indicate some variability in carbohydrate-binding sites, which appears to contribute to the different levels of toxicity exhibited by lectins from various sources.

Structural diversity based on variability in quaternary association. A case study involving eubacterial and related SSBs

Eubacterial and related single-stranded DNA-binding proteins (SSBs) exhibit considerable variability in their quaternary association in spite of their having the same tertiary fold. The variability involves differences in the orientation of dimers in the tetrameric molecule (or of two-domain subunits in the dimeric molecule) and that of monomers in each dimer. The presence of an additional strand in mycobacterial and related SSBs, which clamps the dimers together, is a major determinant of the mode of quaternary association in them. The variability in quaternary structure has implications to the stability of the protein and possibly to its mode of DNA binding.

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

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

Cloning, expression, purification, crystallization and preliminary X-ray studies of the mannose-binding lectin domain of MSMEG_3662 from Mycobacterium smegmatis

The mannose-binding lectin domain of MSMEG_3662 from Mycobacterium smegmatis has been cloned, expressed, purified and crystallized and the crystals have been characterized using X-ray diffraction. The Matthews coefficient suggests the possibility of two lectin domains in the triclinic cell. The amino-acid sequence of the domain indicates structural similarity to well characterized β-prism II fold lectins.

Monomerization of viral entry inhibitor griffithsin elucidates the relationship between multivalent binding to carbohydrates and anti-HIV activity

Mutations were introduced to the domain-swapped homodimer of the antiviral lectin griffithsin (GRFT). Whereas several single and double mutants remained dimeric, insertion of either two or four amino acids at the dimerization interface resulted in a monomeric form of the protein (mGRFT). Monomeric character of the modified proteins was confirmed by sedimentation equilibrium ultracentrifugation and by their high resolution X-ray crystal structures, whereas their binding to carbohydrates was assessed by isothermal titration calorimetry. Cell-based antiviral activity assays utilizing different variants of mGRFT indicated that the monomeric form of the lectin had greatly reduced activity against HIV-1, suggesting that the antiviral activity of GRFT stems from crosslinking and aggregation of viral particles via multivalent interactions between GRFT and oligosaccharides present on HIV envelope glycoproteins. Atomic resolution crystal structure of a complex between mGRFT and nonamannoside revealed that a single mGRFT molecule binds to two different nonamannoside molecules through all three carbohydrate-binding sites present on the monomer.

Crystal structures of human secretory proteins ZG16p and ZG16b reveal a Jacalin-related β-prism fold

ZG16p is a secretory protein that mediates condensation-sorting of pancreatic enzymes to the zymogen granule membrane in pancreatic acinar cells. ZG16p interacts with glycosaminoglycans and the binding is considered to be important for condensation-sorting of pancreatic enzymes. ZG16b/PAUF, a paralog of ZG16p, has recently been found to play a role in gene regulation and cancer metastasis. However, the detailed functions of ZG16p and ZG16b remain to be clarified. Here, in order to obtain insights into structure-function relationships, we conducted crystallographic studies of human ZG16p lectin as well as its paralog, ZG16b, and determined their crystal structures at 1.65 and 2.75 Å resolution, respectively. ZG16p has a Jacalin-related β-prism fold, the first to be reported among mammalian lectins. The putative sugar-binding site of ZG16p is occupied by a glycerol molecule, mimicking the mannose bound to Jacalin-related mannose-binding-type plant lectins such as Banlec. ZG16b also has a β-prism fold, but some amino acid residues of the putative sugar-binding site differ from those of the mannose-type binding site suggesting altered preference. A positively charged patch, which may bind sulfated groups of the glycosaminoglycans, is located around the putative sugar-binding site of ZG16p and ZG16b. Taken together, we suggest that the sugar-binding site and the adjacent basic patch of ZG16p and ZG16b cooperatively form a functional glycosaminoglycan-binding site.

A lectin isolated from bananas is a potent inhibitor of HIV replication

BanLec is a jacalin-related lectin isolated from the fruit of bananas, Musa acuminata. This lectin binds to high mannose carbohydrate structures, including those found on viruses containing glycosylated envelope proteins such as human immunodeficiency virus type-1 (HIV-1). Therefore, we hypothesized that BanLec might inhibit HIV-1 through binding of the glycosylated HIV-1 envelope protein, gp120. We determined that BanLec inhibits primary and laboratory-adapted HIV-1 isolates of different tropisms and subtypes. BanLec possesses potent anti-HIV activity, with IC(50) values in the low nanomolar to picomolar range. The mechanism for BanLec-mediated antiviral activity was investigated by determining if this lectin can directly bind the HIV-1 envelope protein and block entry of the virus into the cell. An enzyme-linked immunosorbent assay confirmed direct binding of BanLec to gp120 and indicated that BanLec can recognize the high mannose structures that are recognized by the monoclonal antibody 2G12. Furthermore, BanLec is able to block HIV-1 cellular entry as indicated by temperature-sensitive viral entry studies and by the decreased levels of the strong-stop product of early reverse transcription seen in the presence of BanLec. Thus, our data indicate that BanLec inhibits HIV-1 infection by binding to the glycosylated viral envelope and blocking cellular entry. The relative anti-HIV activity of BanLec compared favorably to other anti-HIV lectins, such as snowdrop lectin and Griffithsin, and to T-20 and maraviroc, two anti-HIV drugs currently in clinical use. Based on these results, BanLec is a potential component for an anti-viral microbicide that could be used to prevent the sexual transmission of HIV-1.

A novel recombinantly produced banana lectin isoform is a valuable tool for glycoproteomics and a potent modulator of the proliferation response in CD3+, CD4+, and CD8+ populations of human PBMCs

Lectins as carbohydrate-binding proteins have been employed in various biological assays for the detection and characterization of glycan structures on glycoproteins, including clinical biomarkers in disease states. A mannose-specific banana lectin (BanLec) is unique in its specificity for internal alpha1,3 linkages as well as beta1,3 linkages at the reducing termini. The immunomodulatory potential of natural BanLec was recognized by a strong immunoglobulin G4 antibody response and T cell mitogen activity in humans. To explore its applicability in glycoproteomics and its modulatory potential, the gene of banana lectin was cloned, sequenced and a recombinant protein was produced in Escherichia coli. The obtained cDNA revealed a novel banana lectin isoform, with an open reading frame of 426 nucleotides, encoding a cytoplasmatic protein of 141 amino acids. The molecular mass of rBanLec determined by ESI FT-MS and N-terminal sequencing confirmed the cDNA at the protein level. The specificity of rBanLec for detection glycan structures was the same as for natural BanLec as examined with five protein extracts rich in glycoprotein content, as well as with horseradish peroxidase glycoprotein. Besides, the immunomodulatory potential of rBanLec and nBanLec were comparable as assessed by an inhibition assay and a human T cell proliferation assay where they induced a strong proliferation response in CD3+, CD4+, and CD8+ populations of human PBMCs. This recombinant BanLec is a useful reagent for glycoproteomics and lectin microarrays, with a potential for modulation of the immune response.