Crystal structure of a dimeric mannose-specific agglutinin from garlic: quaternary association and carbohydrate specificity

Immunological characterization and function analysis of L-type lectin from spotted knifejaw, Oplegnathus punctatus

Lily-type lectin (LTL) plays significant roles in innate immune response against pathogen infection. LTL in animals and plants has received widespread attention. In the present study, an LTL (OppLTL) was identified from spotted knifejaw Oplegnathus punctatus. The OppLTL encoded a typical Ca²⁺-dependent carbohydrate-binding protein containing a CRD domain. The qRT-PCR showed that it was mainly expressed in the gill and was significantly upregulated after Vibrio anguillarum challenge. The agglutination analysis showed that the recombinant OppLTL could bind and agglutinate Gram-negative and Gram-positive bacteria in a Ca²⁺-dependent manner. However, the binding activity was different. Meanwhile, the recombinant OppLTL could hemagglutinate mammalian and teleost erythrocytes. Subcellular localization revealed that OppLTL was mainly detected in the cytoplasm of HEK293T cells. The dual-luciferase analysis revealed that OppLTL could inhibit the activity of the NF-κB signal pathway in HEK293T cells after OppLTL overexpression. These findings collectively demonstrated that OppLTL could be involved in host innate immune response and defense against bacterial infection in spotted knifejaw.

Investigations on the Biological Activity of Allium sativum Agglutinin (ASA) Isolated from Garlic

Background:

Garlic (Allium sativum) from the family Amaryllidaceae is widely used in culinary and is reported to have potential anticancer, anti-diabetic, antimicrobial, and cardioprotective activities. Allium sativum agglutinin (ASA) is a bulb-type lectin (BTL) domaincontaining lectin isolated from garlic and has been studied for its various biological functions. Previous studies have reported the anti-cancer effects of ASA on histiocytic lymphoma (U937), promyelocytic leukemia (HL60), and oral cancer (KB).

Methods:

In this study, we have purified and characterized ASA and evaluated it for its anticancer effects on other cancer cell lines. MTT assay and FACS analysis was done to corroborate the anticancer findings against cervical (HeLa) and lung cancer (A549) cell lines.

Results:

IC50 value of 37 μg/ml in HeLa and a weak activity (26.4 ± 1.9% cellular inhibition at 100μg/ml treatment) in A549 were found in the MTT assay. FACS analysis further corroborated these findings and showed the apoptotic effects of ASA in these cell lines.

Conclusion:

Anticancer activity for members of bulb-type lectin (BTL) domain-containing lectins has been widely reported, and we hope that our study forms a basis for the development of ASA as a therapeutic agent.

Internalization of Garlic-Derived Nanovesicles on Liver Cells is Triggered by Interaction With CD98

The mechanism of how plant-derived nanovesicles are uptaken by cells remains unknown. In this study, the garlic-derived nanovesicles (GDVs) were isolated and digested with trypsin to remove all surface proteins. Digested GDVs showed less uptake compared to undigested GDVs, confirming that the surface proteins played a role in the endocytosis. On the cell side (HepG2), interestingly, blocking the CD98 receptors significantly reduced the uptake of GDVs. During the cellular internalization of GDVs, we observed that some surface proteins of GDVs were co-localized with CD98. A total lysate of the GDV surface showed a high presence of a mannose-specific binding protein, II lectin. Blocking GDV II lectin (using mannose preincubation) highly reduced the GDV internalization, which supports that direct interaction between II lectin and CD98 plays an important role in internalization. The GDVs also exhibited in vitro anti-inflammatory effect by downregulating proinflammatory factors on the HepG2 cells. This work contributes to understanding a part of the GDV internalization process and the cellular anti-inflammatory effects of garlic.

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.

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.

Three novel B-type mannose-specific lectins of Cynoglossus semilaevis possess varied antibacterial activities against Gram-negative and Gram-positive bacteria

Lectins are a group of sugar-binding proteins that are important factors of the innate immune system. In this study, we examined, in a comparative manner, the expression and function of three Bulb-type (B-type) mannose-specific lectins (named CsBML1, CsBML2, and CsBML3) from tongue sole. All three lectins possess three repeats of the conserved mannose binding motif QXDXNXVXY. Expression of CsBML1, CsBML2, and CsBML3 was most abundant in liver and upregulated by bacterial infection. Recombinant (r) CsBML1, CsBML2, and CsBML3 bound to a wide arrange of bacteria in a dose-dependent manner and with different affinities. All three lectins displayed mannose-specific and calcium-dependent agglutinating capacities but differed in agglutinating profiles. rCsBML1 and rCsBML2, but not rCsBML3, killed target bacteria in vitro and inhibited bacterial dissemination in fish tissues in vivo. These results indicate for the first time that in teleost, different members of B-type mannose-specific lectins likely play different roles in antibacterial immunity.

Deciphering the mode of action of a mutant Allium sativum Leaf Agglutinin (mASAL), a potent antifungal protein on Rhizoctonia solani

Background

Mutant Allium sativum leaf agglutinin (mASAL) is a potent, biosafe, antifungal protein that exhibits fungicidal activity against different phytopathogenic fungi, including Rhizoctonia solani.

Methods

The effect of mASAL on the morphology of R.solani was monitored primarily by scanning electron and light microscopic techniques. Besides different fluorescent probes were used for monitoring various intracellular changes associated with mASAL treatment like change in mitochondrial membrane potential (MMP), intracellular accumulation of reactive oxygen species (ROS) and induction of programmed cell death (PCD). In addition ligand blot followed by LC-MS/MS analyses were performed to detect the putative interactors of mASAL.

Results

Knowledge on the mode of function for any new protein is a prerequisite for its biotechnological application. Detailed morphological analysis of mASAL treated R. solani hyphae using different microscopic techniques revealed a detrimental effect of mASAL on both the cell wall and the plasma membrane. Moreover, exposure to mASAL caused the loss of mitochondrial membrane potential (MMP) and the subsequent intracellular accumulation of reactive oxygen species (ROS) in the target organism. In conjunction with this observation, evidence of the induction of programmed cell death (PCD) was also noted in the mASAL treated R. solani hyphae.

Furthermore, we investigated its interacting partners from R. solani. Using ligand blots followed by liquid chromatography tandem mass spectrometry (LC-MS/MS) analyses, we identified different binding partners including Actin, HSP70, ATPase and 14-3-3 protein.

Conclusions

Taken together, the present study provides insight into the probable mode of action of the antifungal protein, mASAL on R. solani which could be exploited in future biotechnological applications.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-015-0549-7) contains supplementary material, which is available to authorized users.

Jacalin-carbohydrate interactions: distortion of the ligand molecule as a determinant of affinity

Jacalin is among the most thoroughly studied lectins. Its carbohydrate-binding site has also been well characterized. It has been postulated that the lower affinity of β-galactosides for jacalin compared with α-galactosides is caused by steric interactions of the substituents in the former with the protein. This issue has been explored energetically and structurally using different appropriate carbohydrate complexes of jacalin. It turns out that the earlier postulation is not correct. The interactions of the substituent with the binding site remain essentially the same irrespective of the anomeric nature of the substitution. This is achieved through a distortion of the sugar ring in β-galactosides. The difference in energy, and therefore in affinity, is caused by a distortion of the sugar ring in β-galactosides. The elucidation of this unprecedented distortion of the ligand as a strategy for modulating affinity is of general interest. The crystal structures also provide a rationale for the relative affinities of the different carbohydrate ligands for jacalin.

Lectin-like bacteriocins from Pseudomonas spp. utilise D-rhamnose containing lipopolysaccharide as a cellular receptor

Lectin-like bacteriocins consist of tandem monocot mannose-binding domains and display a genus-specific killing activity. Here we show that pyocin L1, a novel member of this family from Pseudomonas aeruginosa, targets susceptible strains of this species through recognition of the common polysaccharide antigen (CPA) of P. aeruginosa lipopolysaccharide that is predominantly a homopolymer of d-rhamnose. Structural and biophysical analyses show that recognition of CPA occurs through the C-terminal carbohydrate-binding domain of pyocin L1 and that this interaction is a prerequisite for bactericidal activity. Further to this, we show that the previously described lectin-like bacteriocin putidacin L1 shows a similar carbohydrate-binding specificity, indicating that oligosaccharides containing d-rhamnose and not d-mannose, as was previously thought, are the physiologically relevant ligands for this group of bacteriocins. The widespread inclusion of d-rhamnose in the lipopolysaccharide of members of the genus Pseudomonas explains the unusual genus-specific activity of the lectin-like bacteriocins.

Due to rapidly increasing rates of antibiotic resistance observed among Gram-negative pathogens, such as Pseudomonas aeruginosa, there is an urgent requirement for novel approaches to the treatment of bacterial infections. Lectin-like bacteriocins are highly potent protein antibiotics that display an unusual ability to kill a select group of bacteria within a specific genus. In this work, we show how the lectin-like protein antibiotic, pyocin L1, can kill Pseudomonas aeruginosa with extraordinary potency through specific binding to the common polysaccharide antigen (CPA) of P. aeruginosa lipopolysaccharide. The CPA is predominantly a homopolymer of the sugar d-rhamnose that although generally rare in nature is found frequently as a component of the lipopolysaccharide of members of the genus Pseudomonas. The targeting of d-rhamnose containing polysaccharides by pyocin L1 and a related lectin-like protein antibiotic, putidacin L1, explains the unusual genus- specific killing activity of the lectin-like bacteriocins. As we learn more about the link between changes to the microbiome and a range of chronic diseases there is a growing realisation that the ability to target specific bacterial pathogens while maintaining the normal gut flora is a desirable property for next generation antibiotics.

β-Propeller blades as ancestral peptides in protein evolution

Proteins of the β-propeller fold are ubiquitous in nature and widely used as structural scaffolds for ligand binding and enzymatic activity. This fold comprises between four and twelve four-stranded β-meanders, the so called blades that are arranged circularly around a central funnel-shaped pore. Despite the large size range of β-propellers, their blades frequently show sequence similarity indicative of a common ancestry and it has been proposed that the majority of β-propellers arose divergently by amplification and diversification of an ancestral blade. Given the structural versatility of β-propellers and the hypothesis that the first folded proteins evolved from a simpler set of peptides, we investigated whether this blade may have given rise to other folds as well. Using sequence comparisons, we identified proteins of four other folds as potential homologs of β-propellers: the luminal domain of inositol-requiring enzyme 1 (IRE1-LD), type II β-prisms, β-pinwheels, and WW domains. Because, with increasing evolutionary distance and decreasing sequence length, the statistical significance of sequence comparisons becomes progressively harder to distinguish from the background of convergent similarities, we complemented our analyses with a new method that evaluates possible homology based on the correlation between sequence and structure similarity. Our results indicate a homologous relationship of IRE1-LD and type II β-prisms with β-propellers, and an analogous one for β-pinwheels and WW domains. Whereas IRE1-LD most likely originated by fold-changing mutations from a fully formed PQQ motif β-propeller, type II β-prisms originated by amplification and differentiation of a single blade, possibly also of the PQQ type. We conclude that both β-propellers and type II β-prisms arose by independent amplification of a blade-sized fragment, which represents a remnant of an ancient peptide world.

Fish lily type lectin-1 contains β-prism architecture: immunological characterization

In this study we report a full-length lily type lectin-1 (CsLTL-1) identified from striped murrel, Channa striatus. CsLTL-1 was identified from the established C. striatus cDNA library using GS-FLX™ genome sequencing technology and was found to contain 354 nucleotide base pairs and its open reading frame (ORF) encodes a 118 amino acid residue. CsLTL-1 mRNA is predominately expressed in the gills and is up-regulated upon infection with fungus (Aphanomyces invadans) and bacteria (Aeromonas hydrophila). Hemagglutination studies with recombinant CsLTL-1 show that, at 4μg/ml agglutinates occurs in a calcium independent manner and is inhibited in the presence of d-mannose (50mM) and d-glucose (100mM). The CsLTL-1 sequence was completely characterized using various bioinformatics tools. CsLTL-1 peptide contains a mannose binding site at 30-99 along with its specific motif of β-prism architecture. The phylogenetic analysis showed that CsLTL-1 clustered together with LTL-1 from Oplegnathus fasciatus. CsLTL-1 protein 3D structure was predicted by I-Tasser program and the model was evaluated using Ramachanran plot analysis. The secondary structure analysis of CsLTL-1 reveals that the protein contains 23% β-sheets and 77% coils. The overall results showed that CsLTL-1 is an important immune gene involved in the recognition and elimination of pathogens in murrels.

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.

Monomeric banana lectin at acidic pH overrules conformational stability of its native dimeric form

Banana lectin (BL) is a homodimeric protein categorized among jacalin-related family of lectins. The effect of acidic pH was examined on conformational stability of BL by using circular dichroism, intrinsic fluorescence, 1-anilino-8-napthalene sulfonate (ANS) binding, size exclusion chromatography (SEC) and dynamic light scattering (DLS). During acid denaturation of BL, the monomerization of native dimeric protein was found at pH 2.0. The elution profile from SEC showed two different peaks (59.65 ml & 87.98 ml) at pH 2.0 while single peak (61.45 ml) at pH 7.4. The hydrodynamic radii (R h) of native BL was 2.9 nm while at pH 2.0 two species were found with R h of 1.7 and 3.7 nm. Furthermore at, pH 2.0 the secondary structures of BL remained unaltered while tertiary structure was significantly disrupted with the exposure of hydrophobic clusters confirming the existence of molten globule like state. The unfolding of BL with different subunit status was further evaluated by urea and temperature mediated denaturation to check their stability. As inferred from high Cm and ΔG values, the monomeric form of BL offers more resistance towards chemical denaturation than the native dimeric form. Besides, dimeric BL exhibited a Tm of 77°C while no loss in secondary structures was observed in monomers even up to 95°C. To the best of our knowledge, this is the first report on monomeric subunit of lectins showing more stability against denaturants than its native dimeric state.

Pinellia ternata agglutinin expression in chloroplasts confers broad spectrum resistance against aphid, whitefly, Lepidopteran insects, bacterial and viral pathogens

Broad spectrum protection against different insects and pathogens requires multigene engineering. However, such broad spectrum protection against biotic stress is provided by a single protein in some medicinal plants. Therefore, tobacco chloroplasts were transformed with the agglutinin gene from Pinellia ternata (pta), a widely cultivated Chinese medicinal herb. Pinellia ternata agglutinin (PTA) was expressed up to 9.2% of total soluble protein in mature leaves. Purified PTA showed similar hemagglutination activity as snowdrop lectin. Artificial diet with purified PTA from transplastomic plants showed marked and broad insecticidal activity. In planta bioassays conducted with T0 or T1 generation PTA lines showed that the growth of aphid Myzus persicae (Sulzer) was reduced by 89%-92% when compared with untransformed (UT) plants. Similarly, the larval survival and total population of whitefly (Bemisia tabaci) on transplastomic lines were reduced by 91%-93% when compared with UT plants. This is indeed the first report of lectin controlling whitefly infestation. When transplastomic PTA leaves were fed to corn earworm (Helicoverpa zea), tobacco budworm (Heliothis virescens) or the beet armyworm (spodoptera exigua), 100% mortality was observed against all these three insects. In planta bioassays revealed Erwinia population to be 10,000-fold higher in control than in PTA lines. Similar results were observed with tobacco mosaic virus (TMV) challenge. Therefore, broad spectrum resistance to homopteran (sap-sucking), Lepidopteran insects as well as anti-bacterial or anti-viral activity observed in PTA lines provides a new option to engineer protection against biotic stress by hyper-expression of an unique protein that is naturally present in a medicinal plant.

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.

Allergenicity assessment of Allium sativum leaf agglutinin, a potential candidate protein for developing sap sucking insect resistant food crops

Background

Mannose-binding Allium sativum leaf agglutinin (ASAL) is highly antinutritional and toxic to various phloem-feeding hemipteran insects. ASAL has been expressed in a number of agriculturally important crops to develop resistance against those insects. Awareness of the safety aspect of ASAL is absolutely essential for developing ASAL transgenic plants.

Methodology/Principal Findings

Following the guidelines framed by the Food and Agriculture Organization/World Health Organization, the source of the gene, its sequence homology with potent allergens, clinical tests on mammalian systems, and the pepsin resistance and thermostability of the protein were considered to address the issue. No significant homology to the ASAL sequence was detected when compared to known allergenic proteins. The ELISA of blood sera collected from known allergy patients also failed to show significant evidence of cross-reactivity. In vitro and in vivo assays both indicated the digestibility of ASAL in the presence of pepsin in a minimum time period.

Conclusions/Significance

With these experiments, we concluded that ASAL does not possess any apparent features of an allergen. This is the first report regarding the monitoring of the allergenicity of any mannose-binding monocot lectin having insecticidal efficacy against hemipteran insects.

Crystal structure of a β-prism II lectin from Remusatia vivipara

The crystal structure of a β-prism II (BP2) fold lectin from Remusatia vivipara, a plant of traditional medicinal value, has been determined at a resolution of 2.4 Å. This lectin (RVL, Remusatia vivipara lectin) is a dimer with each protomer having two distinct BP2 domains without a linker between them. It belongs to the "monocot mannose-binding" lectin family, which consists of proteins of high sequence and structural similarity. Though the overall tertiary structure is similar to that of lectins from snowdrop bulbs and garlic, crucial differences in the mannose-binding regions and oligomerization were observed. Unlike most of the other structurally known proteins in this family, only one of the three carbohydrate recognition sites (CRSs) per BP2 domain is found to be conserved. RVL does not recognize simple mannose moieties. RVL binds to only N-linked complex glycans like those present on the gp120 envelope glycoprotein of HIV and mannosylated blood proteins like fetuin, but not to simple mannose moieties. The molecular basis for these features and their possible functional implications to understand the different levels of carbohydrate affinities in this structural family have been investigated through structure analysis, modeling and binding studies. Apart from being the first structure of a lectin to be reported from the Araceae/Arum family, this protein also displays a novel mode of oligomerization among BP2 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.

Functional alteration of a dimeric insecticidal lectin to a monomeric antifungal protein correlated to its oligomeric status

BACKGROUND

Allium sativum leaf agglutinin (ASAL) is a 25-kDa homodimeric, insecticidal, mannose binding lectin whose subunits are assembled by the C-terminal exchange process. An attempt was made to convert dimeric ASAL into a monomeric form to correlate the relevance of quaternary association of subunits and their functional specificity. Using SWISS-MODEL program a stable monomer was designed by altering five amino acid residues near the C-terminus of ASAL.

METHODOLOGY/PRINCIPAL FINDINGS

By introduction of 5 site-specific mutations (-DNSNN-), a β turn was incorporated between the 11(th) and 12(th) β strands of subunits of ASAL, resulting in a stable monomeric mutant ASAL (mASAL). mASAL was cloned and subsequently purified from a pMAL-c2X system. CD spectroscopic analysis confirmed the conservation of secondary structure in mASAL. Mannose binding assay confirmed that molecular mannose binds efficiently to both mASAL and ASAL. In contrast to ASAL, the hemagglutination activity of purified mASAL against rabbit erythrocytes was lost. An artificial diet bioassay of Lipaphis erysimi with mASAL displayed an insignificant level of insecticidal activity compared to ASAL. Fascinatingly, mASAL exhibited strong antifungal activity against the pathogenic fungi Fusarium oxysporum, Rhizoctonia solani and Alternaria brassicicola in a disc diffusion assay. A propidium iodide uptake assay suggested that the inhibitory activity of mASAL might be associated with the alteration of the membrane permeability of the fungus. Furthermore, a ligand blot assay of the membrane subproteome of R. solani with mASAL detected a glycoprotein receptor having interaction with mASAL.

CONCLUSIONS/SIGNIFICANCE

Conversion of ASAL into a stable monomer resulted in antifungal activity. From an evolutionary aspect, these data implied that variable quaternary organization of lectins might be the outcome of defense-related adaptations to diverse situations in plants. Incorporation of mASAL into agronomically-important crops could be an alternative method to protect them from dramatic yield losses from pathogenic fungi in an effective manner.

Stimulation of IFN-γ production by garlic lectin in mouse spleen cells: involvement of IL-12 via activation of p38 MAPK and ERK in macrophages

Several lectins, present in beans and edible plant products, have immuno-potentiating and anti-tumor activities. We here report the effects of garlic lectin purified from garlic bulbs on the production of cytokines such as interleukin-12 (IL-12) and interferon-γ (IFN-γ) in the mouse. Garlic lectin induced IFN-γ production in spleen cells in a bell-shaped time (24-60 h)- and concentration (0.25-2.0 mg/ml)-dependent manner. The maximal enhancement was observed at 36 h with 0.5 mg/ml of garlic lectin. The stimulatory effect of garlic lectin on IFN-γ production was completely inhibited by both actinomycin D and cycloheximide, an inhibitor of ribosomal protein synthesis and DNA-dependent RNA polymerase, respectively, and was associated with an increase in IFN-γ mRNA level. Garlic lectin also induced IL-12 production in mouse peritoneal macrophages in a concentration (0.25-1.0 mg/ml)- and bell-shaped time (3-24 h)-dependent manner. The lectin increased the phosphorylation of extracellular signal-regulated kinases (ERK) and p38 mitogen-activated protein kinase (p38 MAPK) in macrophages. Furthermore, specific pharmacological inhibitors of ERK kinase (U0126) and p38 MAPK (SB203580) also suppressed the production of IL-12 induced by garlic lectin. The present findings suggest that garlic lectin induces IL-12 production via activation of p38 MAPK and ERK in mouse macrophages, which, in turn, stimulates IFN-γ production through an increase in IFN-γ mRNA in the spleen cells.

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.

Molecular modeling and NMR studies of benzyl substituted mannosyl trisaccharide binding to two mannose-specific lectins: Allium sativam agglutinin I and Concanavalin A

The interaction of trimannoside, α-benzyl 3, 6-di-O-(α-D-mannopyranosyl)-α-D-mannopyranoside, 1 with ASAI (Allium sativam agglutinin I, garlic lectin) was studied to reveal the conformational preferences of this ligand in bound-state and detailed binding mode at atomic level. The binding phenomenon was then compared with another well-known mannose-binding lectin, ConA (Concanavalin A). Structural studies of the ligand in free state were done using NMR spectroscopy and Molecular Dynamics simulations. It is found that the substituted-trimannoside can undergo conformational transitions in solution, with one major and one minor conformation per glycosidic linkage (α 1→3 and α 1→6). On the other hand in the bound-state only one of the two major conformations was significantly populated. The role of phenyl ring in the binding process was explored. An extended binding site was observed for the trimannoside in ASAI utilizing the aromatic substituent, which is not seen in ConA. Binding data from difference absorption spectroscopy supported this fact that the binding of benzyl-substituted ligand is tighter with ASAI than ConA. © 2010 Wiley Periodicals, Inc. Biopolymers 93: 952-967, 2010.

Identity of the immunomodulatory proteins from garlic (Allium sativum) with the major garlic lectins or agglutinins

Garlic (Allium sativum), an important medicinal spice, displays a plethora of biological effects including immunomodulation. Although some immunomodulatory proteins from garlic have been described, their identities are still unknown. The present study was envisaged to isolate immunomodulatory proteins from raw garlic, and examine their effects on certain cells of the immune system (lymphocytes, mast cells, and basophils) in relation to mitogenicity and hypersensitivity. Three protein components of approximately 13 kD (QR-1, QR-2, and QR-3 in the ratio 7:28:1) were separated by Q-Sepharose chromatography of 30 kD ultrafiltrate of raw garlic extract. All the 3 proteins exhibited mitogenic activity towards human peripheral blood lymphocytes, murine splenocytes and thymocytes. The mitogenicity of QR-2 was the highest among the three immunomodulatory proteins. QR-1 and QR-2 displayed hemagglutination and mannose-binding activities; QR-3 showed only mannose-binding activity. Immunoreactivity of rabbit anti-QR-1 and anti-QR-2 polyclonal antisera showed specificity for their respective antigens as well as mutual cross-reactivity; QR-3 was better recognized by anti-QR-2 (82%) than by anti-QR-1 (55%). QR-2 induced a 2-fold higher histamine release in vitro from leukocytes of atopic subjects compared to that of non-atopic subjects. In all functional studies, QR-2 was more potent compared to QR-1. Taken together, all these results indicate that the two major proteins QR-2 and QR-1 present in a ratio of 4:1 in raw garlic contribute to garlic's immunomodulatory activity, and their characteristics are markedly similar to the abundant Allium sativum agglutinins (ASA) I and II, respectively.

Plumieribetin, a fish lectin homologous to mannose-binding B-type lectins, inhibits the collagen-binding alpha1beta1 integrin

Recently, a few fish proteins have been described with a high homology to B-type lectins of monocotyledonous plants. Because of their mannose binding activity, they have been ascribed a role in innate immunity. By screening various fish venoms for their integrin inhibitory activity, we isolated a homologous protein from the fin stings and skin mucus of the scorpionfish (Scorpaena plumieri). This protein inhibits alpha1beta1 integrin binding to basement membrane collagen IV. By protein chemical and spectroscopic means, we demonstrated that this fish protein, called plumieribetin, is a homotetramer and contains a high content of anti-parallel beta strands, similar to the mannose-binding monocot B-lectins. It lacks both N-linked glycoconjugates and common O-glycan motifs. Despite its B-lectin-like structure, plumieribetin binds to alpha1beta1 integrin irrespective of N-glycosylation, suggesting a direct protein-protein interaction. This interaction is independent of divalent cations. On the cellular level, plumieribetin failed to completely detach hepatocarcinoma HepG2 cells and primary arterial smooth muscle cells from the collagen IV fragment CB3. However, plumieribetin weakened the cell-collagen contacts, reduced cell spreading, and altered the actin cytoskeleton, after the compensating alpha2beta1 integrin was blocked. The integrin inhibiting effect of plumieribetin adds a new function to the B-lectin family, which is known for pathogen defense.

Differential expression of surface glycoconjugates on Entamoeba histolytica and Entamoeba dispar

The human large intestine can harbor two morphologically similar amoebae; the invasive Entamoeba histolytica and the non-invasive Entamoeba dispar. Whereas E. histolytica can produce intestinal and extra-intestinal lesions, E. dispar is present in non-symptomatic carriers. Although biochemical, genetic and proteomic studies have identified clear differences between these Entamoebae, it has become clear that several molecules, once assumed to be involved in tissue destruction, exist in both the virulent and the avirulent species. As surface molecules may play a role in invasion and could therefore determine which amoebae are invasive, we analyzed the glycoconjugate composition of E. histolytica and E. dispar using lectins. There was a significant difference between E. histolytica and E. dispar in the expression of glycoconjugates containing d-mannose and N-acetyl-alpha-D-galactosamine residues, but not between virulent and avirulent strains of E. histolytica. N-glycoconjugates with terminal alpha (1-3)-linked mannose residues participate in the adhesion and subsequent cytotoxicity of E. histolytica to cultured hamster hepatocytes. One of them probably is the Gal/GalNAc lectin.

Neoculin, a taste-modifying sweet protein, accumulates in ripening fruits of cultivated Curculigo latifolia

Neoculin is a sweet protein with a taste-modifying activity of converting sourness to sweetness. It occurs in the fruits of Curculigo latifolia, a wild plant found in tropical Asia. We successfully cultivated the plant and evaluated the production of neoculin. The neoculin content of the fruit was high for 10 weeks after flowering, following which the yield decreased gradually. The optimal period for harvesting the fruits with sensory activity coincided with this 10-week peak period during which the amount of neoculin was 1-3mg in the whole fruit and 1.3mg/g of pulp. Immunohistochemical staining showed that neoculin occurred in the whole fruit, especially at the basal portion. Although it is known that neoculin comprises an acidic subunit (NAS) with an N-glycosylated moiety and a basic subunit (NBS), protein gel blot analysis revealed the presence of a non-glycosylated NAS species. This suggests the presence of multiple NAS-NBS heterodimers in our cultivar.

Structural modules for receptor dimerization in the S-locus receptor kinase extracellular domain

The highly polymorphic S-locus receptor kinase (SRK) is the stigma determinant of specificity in the self-incompatibility response of the Brassicaceae. SRK spans the plasma membrane of stigma epidermal cells, and it is activated in an allele-specific manner on binding of its extracellular region (eSRK) to its cognate pollen coat-localized S-locus cysteine-rich (SCR) ligand. SRK, like several other receptor kinases, forms dimers in the absence of ligand. To identify domains in SRK that mediate ligand-independent dimerization, we assayed eSRK for self-interaction in yeast. We show that SRK dimerization is mediated by two regions in eSRK, primarily by a C-terminal region inferred by homology modeling/fold recognition techniques to assume a PAN_APPLE-like structure, and secondarily by a region containing a signature sequence of the S-domain gene family, which might assume an EGF-like structure. We also show that eSRK exhibits a marked preference for homodimerization over heterodimerization with other eSRK variants and that this preference is mediated by a small, highly variable region within the PAN_APPLE domain. Thus, the extensive polymorphism exhibited by the eSRK not only determines differential affinity toward the SCR ligand, as has been assumed thus far, but also underlies a previously unrecognized allelic specificity in SRK dimerization. We propose that preference for SRK homodimerization explains the codominance exhibited by a majority of SRKs in the typically heterozygous stigmas of self-incompatible plants, whereas an increased propensity for heterodimerization combined with reduced affinity of heterodimers for cognate SCRs might underlie the dominant-recessive or mutual weakening relationships exhibited by some SRK allelic pairs.

Phylogenetic and specificity studies of two-domain GNA-related lectins: generation of multispecificity through domain duplication and divergent evolution

A re-investigation of the occurrence and taxonomic distribution of proteins built up of protomers consisting of two tandem arrayed domains equivalent to the GNA [Galanthus nivalis (snowdrop) agglutinin] revealed that these are widespread among monotyledonous plants. Phylogenetic analysis of the available sequences indicated that these proteins do not represent a monophylogenetic group but most probably result from multiple independent domain duplication/in tandem insertion events. To corroborate the relationship between inter-domain sequence divergence and the widening of specificity range, a detailed comparative analysis was made of the sequences and specificity of a set of two-domain GNA-related lectins. Glycan microarray analyses, frontal affinity chromatography and surface plasmon resonance measurements demonstrated that the two-domain GNA-related lectins acquired a marked diversity in carbohydrate-binding specificity that strikingly contrasts the canonical exclusive specificity of their single domain counterparts towards mannose. Moreover, it appears that most two-domain GNA-related lectins interact with both high mannose and complex N-glycans and that this dual specificity relies on the simultaneous presence of at least two different independently acting binding sites. The combined phylogenetic, specificity and structural data strongly suggest that plants used domain duplication followed by divergent evolution as a mechanism to generate multispecific lectins from a single mannose-binding domain. Taking into account that the shift in specificity of some binding sites from high mannose to complex type N-glycans implies that the two-domain GNA-related lectins are primarily directed against typical animal glycans, it is tempting to speculate that plants developed two-domain GNA-related lectins for defence purposes.

Two structures of alliinase from Alliium sativum L.: apo form and ternary complex with aminoacrylate reaction intermediate covalently bound to the PLP cofactor

Alliinase (alliin lyase EC 4.4.1.4), a PLP-dependent alpha, beta-eliminating lyase, constitutes one of the major protein components of garlic (Alliium sativum L.) bulbs. The enzyme is a homodimeric glycoprotein and catalyzes the conversion of a specific non-protein sulfur-containing amino acid alliin ((+S)-allyl-L-cysteine sulfoxide) to allicin (diallyl thiosulfinate, the well known biologically active component of freshly crushed garlic), pyruvate and ammonia. The enzyme was crystallized in the presence of (+S)-allyl-L-cysteine, forming dendrite-like monoclinic crystals. In addition, intentionally produced apo-enzyme was crystallized in tetragonal form. These structures of alliinase with associated glycans were resolved to 1.4 A and 1.61 A by molecular replacement. Branched hexasaccharide chains N-linked to Asn146 and trisaccharide chains N-linked to Asn328 are seen. The structure of hexasaccharide was found similar to "short chain complex vacuole type" oligosaccharide most commonly seen in plant glycoproteins. An unexpected state of the enzyme active site has been observed in the present structure. The electron density in the region of the cofactor made it possible to identify the cofactor moiety as aminoacrylate intermediate covalently bound to the PLP cofactor. It was found in the present structure to be stabilized by large number of interactions with surrounding protein residues. Moreover, the existence of the expected internal aldimine bond between the epsilon-amino group of Lys251 and the aldehyde of the PLP is ruled out on the basis of a distinct separation of electron density of Lys251. The structure of the active site cavity in the apo-form is nearly identical to that seen in the holo-form, with two sulfate ions, an acetate and several water molecules from crystallization conditions that replace and mimic the PLP cofactor.

Carbohydrate-binding site of a novel mannose-specific lectin from fugu (Takifugu rubripes) skin mucus

Pufflectin-s, identified in the skin mucus of the fugu Takifugu rubripes, is a novel mannose-specific lectin with similar structure to monocotyledonous plant lectins. In the present study, mutational analysis was used to reveal the mannose-binding sites of pufflectin-s. Putative binding sites were mutated as follows: binding site 1; rPL-D32E (Asp32-->Glu32), rPL-N34S (Asn34-->Ser34) and rPL-V36A (Val36-->Ala36) whereas binding site 2; rPL-D61E (Asp61-->Glu61), rPL-N63S (Asn63-->Ser63) and rPL-V65A (Val65-->Ala65). All recombinant proteins were expressed in Escherichia coli, purified with two chromatographic steps, and then subjected to mannose-binding assay by affinity chromatography. Recombinant wild-type pufflectin-s (rPL-wt) as well as three mutants with changes in binding site 2 could bind to mannose, in contrast to the three mutants with changes in binding site 1 in which mannose-binding activity was completely lost. These results clearly demonstrate that, at the least, binding site 1 is critical to mannose-binding activity in pufflectin-s.

The Distinct Binding Specificities Exhibited by Enterobacterial Type 1 Fimbriae Are Determined by Their Fimbrial Shafts

Type 1 fimbriae of enterobacteria are heteropolymeric organelles of adhesion composed of FimH, a mannose-binding lectin, and a shaft composed primarily of FimA. We compared the binding activities of recombinant clones expressing type 1 fimbriae from Escherichia coli, Klebsiella pneumoniae, and Salmonella typhimurium for gut and uroepithelial cells and for various soluble mannosylated proteins. Each fimbria was characterized by its capacity to bind particular epithelial cells and to aggregate mannoproteins. However, when each respective FimH subunit was cloned and expressed in the absence of its shaft as a fusion protein with MalE, each FimH bound a wide range of mannose-containing compounds. In addition, we found that expression of FimH on a heterologous fimbrial shaft, e.g. K. pneumoniae FimH on the E. coli fimbrial shaft or vice versa, altered the binding specificity of FimH such that it closely resembled that of the native heterologous type 1 fimbriae. Furthermore, attachment to and invasion of bladder epithelial cells, which were mediated much better by native E. coli type 1 fimbriae compared with native K. pneumoniae type 1 fimbriae, were found to be dependent on the background of the fimbrial shaft (E. coli versus K. pneumoniae) rather than the background of the FimH expressed. Thus, the distinct binding specificities of different enterobacterial type 1 fimbriae cannot be ascribed solely to the primary structure of their respective FimH subunits, but are also modulated by the fimbrial shaft on which each FimH subunit is presented, possibly through conformational constraints imposed on FimH by the fimbrial shaft. The capacity of type 1 fimbrial shafts to modulate the tissue tropism of different enterobacterial species represents a novel function for these highly organized structures.

Novel lectin-like bacteriocins of biocontrol strain Pseudomonas fluorescens Pf-5

Bacteriocin LlpA, produced by Pseudomonas sp. strain BW11M1, is a peculiar antibacterial protein due to its homology to mannose-binding lectins mostly found in monocots (A. H. A. Parret, G. Schoofs, P. Proost, and R. De Mot, J. Bacteriol. 185:897-908, 2003). Biocontrol strain Pseudomonas fluorescens Pf-5 contains two llpA-like genes, named llpA1(Pf-5) and llpA2(Pf-5). Recombinant Escherichia coli cells expressing llpA1(Pf-5) or llpA2(Pf-5) acquired bacteriocin activity and secreted a 31-kDa protein cross-reacting with LlpA(BW11M1) antibodies. Antibacterial activity of the recombinant proteins was evidenced by gel overlay assays. Analysis of the antimicrobial spectrum indicated that LlpA1(Pf-5) and LlpA2(Pf-5) are able to inhibit P. fluorescens strains, as well as the related mushroom pathogen Pseudomonas tolaasii. LlpA-type bacteriocins are characterized by a domain structure consisting of tandem monocot mannose-binding lectin (MMBL) domains. Molecular phylogeny of these MMBL domains suggests that the individual MMBL domains within an LlpA protein have evolved separately toward a specific, as yet unknown, function or, alternatively, were acquired from different ancestral sources. Our observations are consistent with earlier observations, which hinted that MMBL-like bacteriocins represent a new family of antibacterial proteins, probably with a novel mode of action.

Structural mechanism governing the quaternary organization of monocot mannose-binding lectin revealed by the novel monomeric structure of an orchid lectin

Two isoforms of an antifungal protein, gastrodianin, were isolated from two subspecies of the orchid Gastrodia elata, belonging to the protein superfamily of monocot mannose-specific lectins. In the context that all available structures in this superfamily are oligomers so far, the crystal structures of the orchid lectins, both at 2.0 A, revealed a novel monomeric structure. It resulted from the rearrangement of the C-terminal peptide inclusive of the 12th beta-strand, which changes from the "C-terminal exchange" into a "C-terminal self-assembly" mode. Thus, the overall tertiary scaffold is stabilized with an intramolecular beta-sheet instead of the hybrid observed on subunit/subunit interface in all known homologous dimeric or tetrameric lectins. In contrast to the constrained extended conformation with a cis peptide bond between residues 98 and 99 commonly occurring in oligomers, a beta-hairpin forms from position 97 to 101 with a normal trans peptide bond at the corresponding site in gastrodianin, which determines the topology of the C-terminal peptide and thereby its unique fold pattern. Sequence and structure comparison shows that residue replacement and insertion at the position where the beta-hairpin occurs in association with cis-trans inter-conversion of the specific peptide bond (97-98) are possibly responsible for such a radical structure switch between monomers and oligomers. Moreover, this seems to be a common melody controlling the quaternary states among bulb lectins through studies on sequence alignment. The observations revealed a structural mechanism by which the quaternary organization of monocot mannose binding lectins could be governed. The mutation experiment performed on maltose-binding protein-gastrodianin fusion protein followed by a few biochemical detections provides direct evidence to support this conclusion. Potential carbohydrate recognition sites and biological implications of the orchid lectin based on its monomeric state are also discussed in this paper.

Computational analysis of multivalency in lectins: structures of garlic lectin-oligosaccharide complexes and their aggregates

Multivalency in lectins is a phenomenon that has been discussed at considerable length. The structural basis for the role of multivalency in garlic lectin has been investigated here through computational studies. Biochemical studies have shown that the binding affinity of garlic lectin for high mannose oligosaccharides is orders of magnitude greater than that for mannose. Modeling and energy calculations clearly indicate that such increase in affinity cannot be accounted for by binding of these oligosaccharides at any of the six sites of a garlic lectin dimer. These studies also indicate that a given oligosaccharide cannot bind simultaneously to more than one binding site on a lectin dimer. The possibility of a given oligosaccharide simultaneously binding to and hence linking two or more lectin molecules was therefore explored. This study showed that trimannosides and higher oligomers can cross-link lectin dimers, amplifying the protein-oligosaccharide interactions severalfold, thus explaining the role of multivalency in enhancing affinity. A comprehensive exploration of all possible cross-links posed a formidable computational problem. Even a partial exploration involving a carefully chosen region of the conformational space clearly showed that a given dimer pair can be cross-linked not only by a single oligosaccharide molecule but also simultaneously by two oligosaccharides. The number of such possible double cross-links, including those forming interesting tetrameric structures, generally increases with the size of the oligosaccharide, correlating with the biochemical data. In addition to their immediate relevance to garlic lectin, these studies are of general interest in relation to lectin-oligosaccharide interactions.