Legume lectin family, the 'natural mutants of the quaternary state', provide insights into the relationship between protein stability and oligomerization

Protein fraction from Sesbania virgata (Cav.) Pers. seeds exhibit antioxidant and antifungal activities

Sesbania virgata (Cav.) Pers. seeds are protein sources with health and environmental benefits. In this research, proteins with lectin activity were identified in a protein fraction from S. virgata seeds (PFLA), as well its antioxidant and antimicrobial potentials, in addition to cytotoxic effects. To obtain PFLA, seed flour was homogenized in Glycine-NaOH (100 mM; pH 9.0; NaCl 150 mM) and precipitated in ammonium sulfate. PFLA concentrates bioactive lectins (32 HU/mL, 480 HU/gFa, 18.862 HU/mgP) and essential amino acids (13.36 g/100g protein). PFLA exerts antioxidant activity, acting as a promising metal chelating agent (~77% of activity). Analyzes of cell culture assay results suggest that antioxidant activity of PFLA may be associated with the recruitment of essential molecules to prevent the metabolic impairment of cells exposed to oxidative stress. PFLA (256 - 512 µg/mL) also exhibits antifungal activity, inhibiting the growth of Aspergillus flavus, Candida albicans, Candida tropicalis and Penicillium citrinum. Cytotoxic analysis indicates a tendency of low interference in the proliferation of 3T3 and HepG2 cells in the range of PFLA concentrations with biological activity. These findings support the notion that PFLA is a promising adjuvant to be applied in current policies on the management of metal ion chelation and fungal infections.

Immune Responses Induced by Recombinant Bacillus subtilis Expressing the PEDV Spike Protein Targeted at Microfold Cells

Bacillus subtilis (B. subtilis), a probiotic bacterium and feeding additive, is widely used for heterologous antigen expression and protective immunisation. Porcine epidemic diarrhoea virus (PEDV) invades swine via mucosal tissue. To enhance the mucosal immune response to PEDV, we modified B. subtilis to express a PEDV antigen and used it as a mucosal vaccine delivery system. Initially, we constructed a recombinant B. subtilis strain (B.s-RCL) that expressed the PEDV spike protein and L-Lectin-β-GF, with the goal of inducing mucosal secretory immunoglobulin A (sIgA) and anti-PEDV serum immunoglobulin G (IgG) production, as well as to increase the number of microfold cells (M cells). Following the oral administration of B.s-RCL to mice, the small intestinal PEDV-specific sIgA expression levels significantly increased, as well as the increased number of B.s-RCL adhered to M cells. Moreover, we found that mice administered B.s-RCL exhibited markedly higher percentages of CD4+ and CD8+ T cells in the mesenteric lymph nodes and spleen compared to the control mice. Furthermore, we found that intestinal mucosa sIgA and serum anti-PEDV IgG levels were higher in mice orally immunised with B.s-RCL, suggesting that the mice could be more resistant to PEDV. In this study, we developed a novel oral vaccine to prevent porcine diarrhoea epidemics.

Fluorinated Carbohydrates as Lectin Ligands: Simultaneous Screening of a Monosaccharide Library and Chemical Mapping by 19F NMR Spectroscopy

Molecular recognition of carbohydrates is a key step in essential biological processes. Carbohydrate receptors can distinguish monosaccharides even if they only differ in a single aspect of the orientation of the hydroxyl groups or harbor subtle chemical modifications. Hydroxyl-by-fluorine substitution has proven its merits for chemically mapping the importance of hydroxyl groups in carbohydrate-receptor interactions. 19F NMR spectroscopy could thus be adapted to allow contact mapping together with screening in compound mixtures. Using a library of fluorinated glucose (Glc), mannose (Man), and galactose (Gal) derived by systematically exchanging every hydroxyl group by a fluorine atom, we developed a strategy combining chemical mapping and 19F NMR T2 filtering-based screening. By testing this strategy on the proof-of-principle level with a library of 13 fluorinated monosaccharides to a set of three carbohydrate receptors of diverse origin, i.e. the human macrophage galactose-type lectin, a plant lectin, Pisum sativum agglutinin, and the bacterial Gal-/Glc-binding protein from Escherichia coli, it became possible to simultaneously define their monosaccharide selectivity and identify the essential hydroxyls for interaction.

Comparing the Conformational Stability of Pyruvate Kinase in the Gas Phase and in Solution

Collision induced unfolding (CIU) is increasingly used to characterize protein complexes in the gas phase and is often employed to detect ligand binding-induced conformational stabilization. However, the extent to which gas-phase conformational stabilities measured by CIU reflect analogous parameters in solution is not yet clear, particularly for systems where conformational and protein complex stability are modulated by point mutation. Here, we compare CIU-derived relative stabilities of four point mutants of the homotetramer pyruvate kinase to solution stabilities measured by differential scanning fluorimetry (DSF) and solution conformational dynamics measured by time-resolved electrospray ionization hydrogen-deuterium exchange (TRESI-HDX). Our results demonstrate that both destabilization of the tetrameric state and generally reduced conformational stability of the monomer in solution are well correlated to lower onset energies for specific unfolding transitions observed in CIU. However, this correlation not fully retained when comparing CIU to HDX data, where the latter measurement is strongly impacted by conformational dynamics within the tetramer.

Dalbergieae lectins: A review of lectins from species of a primitive Papilionoideae (leguminous) tribe

Lectins are (glyco)proteins capable of reversibly binding to specific carbohydrates, thus having various functions and applications. Plant lectins are the best studied, and the Leguminoseae family is highlighted in a number of published works, especially species of the Papilionoideae subfamily. Dalbergieae is one of the tribes in this subfamily comprising 49 genera and over 1300 species. From this tribe, about 26 lectins were studied, among which we can highlight the Arachis hypogaea lectin, widely used in cancer studies. Dalbergieae lectins demonstrate various carbohydrate specificities and biological activities including anti-inflammatory, vasorelaxant, nociceptive, antibacterial, antiviral among others. Structurally, these lectins are quite similar in their three-dimensional folding but present significant differences in oligomerization patterns and in the conservation of carbohydrate-recognition domain. Despite the existence of structural data from some lectins, only sparse literature has reported on this tribe's diversity, not to mention the range of biological effects, determined through specific assays. Therefore, this work will review the most important studies on Dalbergieae lectins and their potential biomedical applications.

The potent anti-cancer activity of Dioclea lasiocarpa lectin

The lectin DLasiL was isolated from seeds of the Dioclea lasiocarpa collected from the northeast coast of Brazil and characterized for the first time by mass spectrometry, DNA sequencing, inductively coupled plasma-mass spectrometry, electron paramagnetic resonance, and fluorescence spectroscopy. The structure of DLasiL lectin obtained by homology modelling suggested strong conservation of the dinuclear Ca/Mn and sugar-binding sites, and dependence of the solvent accessibility of tryptophan-88 on the oligomerisation state of the protein. DLasiL showed highly potent (low nanomolar) antiproliferative activity against several human carcinoma cell lines including A2780 (ovarian), A549 (lung), MCF-7 (breast) and PC3 (prostate), and was as, or more, potent than the lectins ConBr (Canavalia brasiliensis), ConM (Canavalia maritima) and DSclerL (Dioclea sclerocarpa) against A2780 and PC3 cells. Interestingly, DLasiL lectin caused a G2/M arrest in A2780 cells after 24h exposure, activating caspase 9 and delaying the on-set of apoptosis. Confocal microscopy showed that fluorescently-labelled DLasiL localized around the nuclei of A2780 cells at lectin doses of 0.5-2× IC₅₀ and gave rise to enlarged nuclei and spreading of the cells at high doses. These data reveal the interesting antiproliferative activity of DLasiL lectin, and suggest that further investigations to explore the potential of DLasiL as a new anticancer agent are warranted.

Biological roles of glycans

Simple and complex carbohydrates (glycans) have long been known to play major metabolic, structural and physical roles in biological systems. Targeted microbial binding to host glycans has also been studied for decades. But such biological roles can only explain some of the remarkable complexity and organismal diversity of glycans in nature. Reviewing the subject about two decades ago, one could find very few clear-cut instances of glycan-recognition-specific biological roles of glycans that were of intrinsic value to the organism expressing them. In striking contrast there is now a profusion of examples, such that this updated review cannot be comprehensive. Instead, a historical overview is presented, broad principles outlined and a few examples cited, representing diverse types of roles, mediated by various glycan classes, in different evolutionary lineages. What remains unchanged is the fact that while all theories regarding biological roles of glycans are supported by compelling evidence, exceptions to each can be found. In retrospect, this is not surprising. Complex and diverse glycans appear to be ubiquitous to all cells in nature, and essential to all life forms. Thus, >3 billion years of evolution consistently generated organisms that use these molecules for many key biological roles, even while sometimes coopting them for minor functions. In this respect, glycans are no different from other major macromolecular building blocks of life (nucleic acids, proteins and lipids), simply more rapidly evolving and complex. It is time for the diverse functional roles of glycans to be fully incorporated into the mainstream of biological sciences.

Structural analysis of Centrolobium tomentosum seed lectin with inflammatory activity

A glycosylated lectin (CTL) with specificity for mannose and glucose has been detected and purified from seeds of Centrolobium tomentosum, a legume plant from Dalbergieae tribe. It was isolated by mannose-sepharose affinity chromatography. The primary structure was determined by tandem mass spectrometry and consists of 245 amino acids, similar to other Dalbergieae lectins. CTL structures were solved from two crystal forms, a monoclinic and a tetragonal, diffracted at 2.25 and 1.9 Å, respectively. The carbohydrate recognition domain (CRD), metal-binding site and glycosylation site were characterized, and the structural basis for mannose/glucose-binding was elucidated. The lectin adopts the canonical dimeric organization of legume lectins. CTL showed acute inflammatory effect in paw edema model. The protein was subjected to ligand screening (dimannosides and trimannoside) by molecular docking, and interactions were compared with similar lectins possessing the same ligand specificity. This is the first crystal structure of mannose/glucose native seed lectin with proinflammatory activity isolated from the Centrolobium genus.

Birth of plant proteomics in India: a new horizon

In the post-genomic era, proteomics is acknowledged as the next frontier for biological research. Although India has a long and distinguished tradition in protein research, the initiation of proteomics studies was a new horizon. Protein research witnessed enormous progress in protein separation, high-resolution refinements, biochemical identification of the proteins, protein-protein interaction, and structure-function analysis. Plant proteomics research, in India, began its journey on investigation of the proteome profiling, complexity analysis, protein trafficking, and biochemical modeling. The research article by Bhushan et al. in 2006 marked the birth of the plant proteomics research in India. Since then plant proteomics studies expanded progressively and are now being carried out in various institutions spread across the country. The compilation presented here seeks to trace the history of development in the area during the past decade based on publications till date. In this review, we emphasize on outcomes of the field providing prospects on proteomic pathway analyses. Finally, we discuss the connotation of strategies and the potential that would provide the framework of plant proteome research.

BIOLOGICAL SIGNIFICANCE

The past decades have seen rapidly growing number of sequenced plant genomes and associated genomic resources. To keep pace with this increasing body of data, India is in the provisional phase of proteomics research to develop a comparative hub for plant proteomes and protein families, but it requires a strong impetus from intellectuals, entrepreneurs, and government agencies. Here, we aim to provide an overview of past, present and future of Indian plant proteomics, which would serve as an evaluation platform for those seeking to incorporate proteomics into their research programs. This article is part of a Special Issue entitled: Proteomics in India.

Structural insights into SraP-mediated Staphylococcus aureus adhesion to host cells

Staphylococcus aureus, a Gram-positive bacterium causes a number of devastating human diseases, such as infective endocarditis, osteomyelitis, septic arthritis and sepsis. S. aureus SraP, a surface-exposed serine-rich repeat glycoprotein (SRRP), is required for the pathogenesis of human infective endocarditis via its ligand-binding region (BR) adhering to human platelets. It remains unclear how SraP interacts with human host. Here we report the 2.05 Å crystal structure of the BR of SraP, revealing an extended rod-like architecture of four discrete modules. The N-terminal legume lectin-like module specifically binds to N-acetylneuraminic acid. The second module adopts a β-grasp fold similar to Ig-binding proteins, whereas the last two tandem repetitive modules resemble eukaryotic cadherins but differ in calcium coordination pattern. Under the conditions tested, small-angle X-ray scattering and molecular dynamic simulation indicated that the three C-terminal modules function as a relatively rigid stem to extend the N-terminal lectin module outwards. Structure-guided mutagenesis analyses, in addition to a recently identified trisaccharide ligand of SraP, enabled us to elucidate that SraP binding to sialylated receptors promotes S. aureus adhesion to and invasion into host epithelial cells. Our findings have thus provided novel structural and functional insights into the SraP-mediated host-pathogen interaction of S. aureus.

Structural studies of an anti-inflammatory lectin from Canavalia boliviana seeds in complex with dimannosides

Plant lectins, especially those purified from species of the Leguminosae family, represent the best-studied group of carbohydrate-binding proteins. Lectins purified from seeds of the Diocleinae subtribe exhibit a high degree of sequence identity notwithstanding that they show very distinct biological activities. Two main factors have been related to this feature: variance in key residues influencing the carbohydrate-binding site geometry and differences in the pH-dependent oligomeric state profile. In this work, we have isolated a lectin from Canavalia boliviana (Cbol) and solved its x-ray crystal structure in the unbound form and in complex with the carbohydrates Man(α1-3)Man(α1-O)Me, Man(α1-4)Man(α1-O)Me and 5-bromo-4-chloro-3-indolyl-α-D-mannose. We evaluated its oligomerization profile at different pH values using Small Angle X-ray Scattering and compared it to that of Concanavalin A. Based on predicted pKa-shifts of amino acids in the subunit interfaces we devised a model for the dimer-tetramer equilibrium phenomena of these proteins. Additionally, we demonstrated Cbol anti-inflammatory properties and further characterized them using in vivo and in vitro models.

Structure of a tetrameric galectin from Cinachyrella sp. (ball sponge)

The galectins are a family of proteins that bind with highest affinity to N-acetyllactosamine disaccharides, which are common constituents of asparagine-linked complex glycans. They play important and diverse physiological roles, particularly in the immune system, and are thought to be critical metastatic agents for many types of cancer cells, including gliomas. A recent bioactivity-based screen of marine sponge (Cinachyrella sp.) extract identified an ancestral member of the galectin family based on its unexpected ability to positively modulate mammalian ionotropic glutamate receptor function. To gain insight into the mechanistic basis of this activity, the 2.1 Å resolution X-ray structure of one member of the family, galectin CchG-1, is reported. While the protomer exhibited structural similarity to mammalian prototype galectin, CchG-1 adopts a novel tetrameric arrangement in which a rigid toroidal-shaped 'donut' is stabilized in part by the packing of pairs of vicinal disulfide bonds. Twofold symmetry between binding-site pairs provides a basis for a model for interaction with ionotropic glutamate receptors.

A lectin from Platypodium elegans with unusual specificity and affinity for asymmetric complex N-glycans

Lectin activity with specificity for mannose and glucose has been detected in the seed of Platypodium elegans, a legume plant from the Dalbergieae tribe. The gene of Platypodium elegans lectin A has been cloned, and the resulting 261-amino acid protein belongs to the legume lectin family with similarity with Pterocarpus angolensis agglutinin from the same tribe. The recombinant lectin has been expressed in Escherichia coli and refolded from inclusion bodies. Analysis of specificity by glycan array evidenced a very unusual preference for complex type N-glycans with asymmetrical branches. A short branch consisting of one mannose residue is preferred on the 6-arm of the N-glycan, whereas extensions by GlcNAc, Gal, and NeuAc are favorable on the 3-arm. Affinities have been obtained by microcalorimetry using symmetrical and asymmetrical Asn-linked heptasaccharides prepared by the semi-synthetic method. Strong affinity with K(d) of 4.5 μm was obtained for both ligands. Crystal structures of Platypodium elegans lectin A complexed with branched trimannose and symmetrical complex-type Asn-linked heptasaccharide have been solved at 2.1 and 1.65 Å resolution, respectively. The lectin adopts the canonical dimeric organization of legume lectins. The trimannose bridges the binding sites of two neighboring dimers, resulting in the formation of infinite chains in the crystal. The Asn-linked heptasaccharide binds with the 6-arm in the primary binding site with extensive additional contacts on both arms. The GlcNAc on the 6-arm is bound in a constrained conformation that may rationalize the higher affinity observed on the glycan array for N-glycans with only a mannose on the 6-arm.

Effect of lectins from Diocleinae subtribe against oral Streptococci

Surface colonization is an essential step in biofilm development. The ability of oral pathogens to adhere to tooth surfaces is directly linked with the presence of specific molecules at the bacterial surface that can interact with enamel acquired pellicle ligands. In light of this, the aim of this study was to verify inhibitory and antibiofilm action of lectins from the Diocleinaesubtribe against Streptococcus mutans and Streptococcus oralis. The inhibitory action against planctonic cells was assessed using lectins from Canavaliaensi formis (ConA), Canavalia brasiliensis (ConBr), Canavalia maritima (ConM), Canavalia gladiata (CGL) and Canavalia boliviana (ConBol). ConBol, ConBr and ConM showed inhibitory activity on S. mutans growth. All lectins, except ConA, stimulated significantly the growth of S. oralis. To evaluate the effect on biofilm formation, clarified saliva was added to 96-well, flat-bottomed polystyrene plates, followed by the addition of solutions containing 100 or 200 µg/mL of the selected lectins. ConBol, ConM and ConA inhibited the S. mutans biofilms. No effects were found on S. oralis biofilms. Structure/function analysis were carried out using bioinformatics tools. The aperture and deepness of the CRD (Carbohydrate Recognition Domain) permit us to distinguish the two groups of Canavalia lectins in accordance to their actions against S. mutans and S. oralis. The results found provide a basis for encouraging the use of plant lectins as biotechnological tools in ecological control and prevention of caries disease.

Purification, biochemical characterization, and bioactive properties of a lectin purified from the seeds of white tepary bean (phaseolus acutifolius variety latifolius)

The present work shows the characterization of Phaseolus acutifolius variety latifolius, on which little research has been published, and provides detailed information on the corresponding lectin. This protein was purified from a semi-domesticated line of white tepary beans from Sonora, Mexico, by precipitation of the aqueous extract with ammonium sulfate, followed by affinity chromatography on an immobilized fetuin matrix. MALDI TOF analysis of Phaseolus acutifolius agglutinin (PAA) showed that this lectin is composed of monomers with molecular weights ranging between 28 and 31 kDa. At high salt concentrations, PAA forms a dimer of 63 kDa, but at low salt concentrations, the subunits form a tetramer. Analysis of PAA on 2D-PAGE showed that there are mainly three types of subunits with isoelectric points of 4.2, 4.4, and 4.5. The partial sequence obtained by LC/MS/MS of tryptic fragments from the PAA subunits showed 90-100% identity with subunits from genus Phaseolus lectins in previous reports. The tepary bean lectin showed lower hemagglutination activity than Phaseolus vulgaris hemagglutinin (PHA-E) toward trypsinized human A and O type erythrocytes. The hemagglutination activity was inhibited by N-glycans from glycoproteins. Affinity chromatography with the immobilized PAA showed a high affinity to glycopeptides from thyroglobulin, which also has N-glycans with a high content of N-acetylglucosamine. PAA showed less mitogenic activity toward human lymphocytes than PHA-L and Con A. The cytotoxicity of PAA was determined by employing three clones of the 3T3 cell line, demonstrating variability among the clones as follows: T4 (DI₅₀ 51.5 µg/mL); J20 (DI₅₀ 275 µg/mL), and N5 (DI₅₀ 72.5 µg/mL).

Metamorphic proteins mediate evolutionary transitions of structure

The primary sequence of proteins usually dictates a single tertiary and quaternary structure. However, certain proteins undergo reversible backbone rearrangements. Such metamorphic proteins provide a means of facilitating the evolution of new folds and architectures. However, because natural folds emerged at the early stages of evolution, the potential role of metamorphic intermediates in mediating evolutionary transitions of structure remains largely unexplored. We evolved a set of new proteins based on approximately 100 amino acid fragments derived from tachylectin-2--a monomeric, 236 amino acids, five-bladed beta-propeller. Their structures reveal a unique pentameric assembly and novel beta-propeller structures. Although identical in sequence, the oligomeric subunits adopt two, or even three, different structures that together enable the pentameric assembly of two propellers connected via a small linker. Most of the subunits adopt a wild-type-like structure within individual five-bladed propellers. However, the bridging subunits exhibit domain swaps and asymmetric strand exchanges that allow them to complete the two propellers and connect them. Thus, the modular and metamorphic nature of these subunits enabled dramatic changes in tertiary and quaternary structure, while maintaining the lectin function. These oligomers therefore comprise putative intermediates via which beta-propellers can evolve from smaller elements. Our data also suggest that the ability of one sequence to equilibrate between different structures can be evolutionary optimized, thus facilitating the emergence of new structures.

Organization and dynamics of tryptophan residues in tetrameric and monomeric soybean agglutinin: studies by steady-state and time-resolved fluorescence, phosphorescence and chemical modification

We have investigated the organization and dynamics of tryptophan residues in tetrameric, monomeric and unfolded states of soybean agglutinin (SBA) by selective chemical modification, steady-state and time-resolved fluorescence, and phosphorescence. Oxidation with N-bromosuccinimide (NBS) modifies two tryptophans (Trp 60 and Trp 132) in tetramer, four (Trp 8, Trp 203 and previous two) in monomer, and all six (Trp 8, Trp 60, Trp 132, Trp 154, Trp 203 and Trp 226) in unfolded state. Utilizing wavelength-selective fluorescence approach, we have observed a red-edge excitation shift (REES) of 10 and 5 nm for tetramer and monomer, respectively. A more pronounced REES (21 nm) is observed after NBS oxidation. These results are supported by fluorescence anisotropy experiments. Acrylamide quenching shows the Stern-Volmer constant (K(SV)) for tetramer, monomer and unfolded SBA being 2.2, 5.0 and 14.6 M(-1), respectively. Time-resolved fluorescence studies exhibit biexponential decay with the mean lifetime increasing along tetramer (1.0 ns) to monomer (1.9 ns) to unfolded (3.6 ns). Phosphorescence studies at 77 K give more structured spectra, with two (0,0) bands at 408.6 (weak) and 413.2 nm for tetramer. However, a single (0,0) band appears at 411.8 and 407.2 nm for monomer and unfolded SBA, respectively. The exposure of hydrophobic surface in SBA monomer has been examined by 8-anilino-1-naphthalenesulfonate (ANS) binding, which shows approximately 20-fold increase in ANS fluorescence compared to that for tetramer. The mean lifetime of ANS also shows a large increase (12.0 ns) upon binding to monomer. These results may provide important insight into the role of tryptophans in the folding and association of SBA, and oligomeric proteins in general.

Crystal structure of Dioclea rostrata lectin: insights into understanding the pH-dependent dimer-tetramer equilibrium and the structural basis for carbohydrate recognition in Diocleinae lectins

The legume lectins from the subtribe Diocleinae, often referred to as concanavalin A-like lectins, are a typical example of highly similar proteins that show distinct biological activities. The pH-dependent oligomerization that some of these lectins undergo and the relative position of amino acids within the carbohydrate-binding site are factors that have been reported to contribute to these differences in the activities of Diocleinae lectins. In the present work, we determined the amino acid sequence and the crystal structure of the lectin of Dioclea rostrata seeds (DRL), with the aim of investigating the structural bases of the different behavior displayed by this lectin in comparison to other Diocleinae lectins and determining the reason for the distinct pH-dependent dimer-tetramer equilibrium. In addition, we discovered a novel multimeric arrangement for this lectin.

Insights into the structural basis of the pH-dependent dimer-tetramer equilibrium through crystallographic analysis of recombinant Diocleinae lectins

The structural ground underlying the pH-dependency of the dimer-tetramer transition of Diocleinae lectins was investigated by equilibrium sedimentation and X-ray crystal structure determination of wild-type and site-directed mutants of recombinant lectins. Synthetic genes coding for the full-length alpha-chains of the seed lectins of Dioclea guianensis (termed r-alphaDguia) and Dioclea grandiflora (termed r-alphaDGL) were designed and expressed in Escherichia coli. This pioneering approach, which will be described in detail in the present paper, yielded recombinant lectins displaying carbohydrate-binding activity, dimer-tetramer equilibria and crystal structures indistinguishable from their natural homologues. Conversion of the pH-stable tetrameric r-alphaDGL into a structure exhibiting pH-dependent dimer-tetramer transition was accomplished through mutations that abolished the interdimeric interactions at the central cavity of the tetrameric lectins. Both the central and the peripheral interacting regions bear structural information for formation of the canonical legume lectin tetramer. We hypothesize that the strength of the ionic contacts at these sites may be modulated by the pH, leading to dissociation of those lectin structures that are not locked into a pH-stable tetramer through interdimeric contacts networking the central cavity loops.

Structural analysis of Canavalia maritima and Canavalia gladiata lectins complexed with different dimannosides: New insights into the understanding of the structure–biological activity relationship in legume lectins

Plant lectins, especially those purified from species of the Leguminosae family, represent the best studied group of carbohydrate-binding proteins. The legume lectins from Diocleinae subtribe are highly similar proteins that present significant differences in the potency/efficacy of their biological activities. The structural studies of the interactions between lectins and sugars may clarify the origin of the distinct biological activities observed in this high similar class of proteins. In this way, this work presents a crystallographic study of the ConM and CGL (agglutinins from Canavalia maritima and Canavalia gladiata, respectively) in the following complexes: ConM/CGL:Man(alpha1-2)Man(alpha1-O)Me, ConM/CGL:Man(alpha1-3)Man(alpha1-O)Me and ConM/CGL:Man(alpha1-4)Man(alpha1-O)Me, which crystallized in different conditions and space group from the native proteins. The structures were solved by molecular replacement, presenting satisfactory values for R(factor) and R(free). Comparisons between ConM, CGL and ConA (Canavalia ensiformis lectin) binding mode with the dimannosides in subject, presented different interactions patterns, which may account for a structural explanation of the distincts biological properties observed in the lectins of Diocleinae subtribe.

Leveraging protein purification strategies in proteomics

The proteomic studies, although, tend to be analytical in nature, yet many strategies of preparative protein purification can be usefully employed in such studies. This review points out the importance of purification techniques which are capable of dealing with samples which are suspensions rather than clear solution, e.g. aqueous two phase partitioning, three phase partitioning, expanded bed chromatography, etc. The review also outlines the potential of non-chromatographic techniques in dealing with fractionation of proteomes. Separation protocols which can deal with post-translationally modified (PTM) proteins are also considered.

Crystal structures of Cratylia floribunda seed lectin at acidic and basic pHs. Insights into the structural basis of the pH-dependent dimer-tetramer transition

Structural determinants underlaying the pH-dependent dimer-tetramer transition of Diocleinae lectins were investigated from the structures of Cratylia floribunda seed lectin crystallized in conditions where it exist as a dimer (pH 4.6) or as a tetramer (pH 8.5). The acidic (aCFL) and the basic (bCFL) tetramers superimpose with overall r.m.s.d. of 0.53 A, though interdimer contacts are drastically reduced in aCFL, and the r.m.s.d. for the superposition of the 117-120 loops of aCFL vs. the bCFL tetramer is 1.29 A. Our data support the view that His51 plays a role in determining the conformation of the central cavity loops and that interdimer contacts involving ordered loop residues stabilize the canonical, pH-dependent tetramer. In the bCFL tetramer, hydrogen bonds between Asn118 and Thr120 of monomers A and D and residues Ser66, Ser108, Ser110, and Thr49 of the opposite monomer stabilize the canonical, pH-dependent tetrameric lectin structure. In CFL, Asn131 makes intradimer contacts with Asn122 and Ala123. In comparison, His131 in Dioclea grandiflora lectin establishes a network of interdimer interactions bridging the four central loops of the pH-independent tetramer. Our data provide new insights into the participation of specific amino acid residues in the mechanism of the quaternary association of Diocleinae lectins.

2,2,2-Trifluoroethanol-Induced structural change of peanut agglutinin at different pH: A comparative account

Peanut Agglutinin (PNA) is a legume lectin with a unique open quarternary structure. It is a homotetrameric protein, the monomeric subunit of which is made up of 3 beta sheets. The structural change in this protein has been induced by 2,2,2-trifluoroethanol (TFE) at two different pH. At neutral pH, PNA exists as a homotetramer, while at pH 2.5, it is known to dissociate to a dimer. The effect of TFE has been studied at both the pH by intrinsic tryptophan fluorescence, far and near UV Circular Dichroism, ANS binding and dynamic light scattering. At low pH, 15% TFE is found to induce a molten globule like state that shows maximum ANS binding. Increasing concentration of TFE increases alpha helical content and the compactness of the protein. The compact PNA at higher concentration of TFE is structurally different from the native structure. The effect of TFE at neutral pH on PNA is somewhat different from that observed at low pH. TFE does not induce molten globule like state at this pH. The detailed study of the structural change of PNA by TFE has been presented.

Tryptophan environment, secondary structure and thermal unfolding of the galactose-specific seed lectin from Dolichos lablab: Fluorescence and circular dichroism spectroscopic studies

Fluorescence and circular dichroism spectroscopic studies were carried out on the galactose-specific lectin from Dolichos lablab seeds (DLL-II). The microenvironment of the tryptophan residues in the lectin under native and denaturing conditions were investigated by quenching of the intrinsic fluorescence of the protein by a neutral quencher (acrylamide), an anionic quencher (iodide ion) and a cationic quencher (cesium ion). The results obtained indicate that the tryptophan residues of DLL-II are largely buried in the hydrophobic core of the protein matrix, with positively charged side chains residing close to at least some of the tryptophan residues under the experimental conditions. Analysis of the far UV CD spectrum of DLL-II revealed that the secondary structure of the lectin consists of 57% alpha-helix, 21% beta-sheet, 7% beta-turns and 15% unordered structures. Carbohydrate binding did not significantly alter the secondary and tertiary structures of the lectin. Thermal unfolding of DLL-II, investigated by monitoring CD signals, showed a sharp transition around 75 degrees C both in the far UV region (205 nm) and the near UV region (289 nm), which shifted to ca. 77-78 degrees C in the presence of 0.1 M methyl-beta-D-galactopyranoside, indicating that ligand binding leads to a moderate stabilization of the lectin structure.

Interplay between metal binding and cis/trans isomerization in legume lectins: structural and thermodynamic study of P. angolensis lectin

The interplay between metal binding, carbohydrate binding activity, stability and structure of the lectin from Pterocarpus angolensis was investigated. Removal of the metals leads to a more flexible form of the protein with significantly less conformational stability. Crystal structures of this metal-free form show significant structural rearrangements, although some structural features that allow the binding of sugars are retained. We propose that substitution of an asparagine residue at the start of the C-terminal beta-strand of the legume lectin monomer hinders the trans-isomerization of the cis-peptide bond upon demetallization and constitutes an intramolecular switch governing the isomer state of the non-proline bond and ultimately the lectin phenotype.

Reduced hematopoietic toxicity, enhanced cellular uptake and altered pharmacokinetics of azidothymidine loaded galactosylated liposomes

In order to target liposomes to the lectin receptors present on macrophages, galactosylated liposomes were prepared and characterized in vitro. O-palmitoylgalactose (OPG) for liposomal coating was synthesized by esterification of galactose with palmitoyl chloride. The galactose binding Ricinus communis lectin was employed as a model system for the determination of in vitro ligand binding capacity. Cellular drug uptake studies were performed using alveolar macrophages. Hematological changes, bone marrow toxicity, plasma and tissue distribution study of free, uncoated plain liposomal and galactosylated liposomal encapsulated azidothymidine (AZT) were determined following a bolus intravenous injection in Sprague-Dawley rats. Lectin (R. communis) carbohydrate interaction has been utilized for the effective delivery of AZT entrapped in galactosylated vesicles. Aggregation of galactosylated liposomes increased as lectin concentration was increased from 5 to 30 microg/ml. Cellular uptake of galactosylated liposomal formulation was maximum. No hematological toxicity was observed even after 10 days in case of galactosylated vesicle entrapped AZT. This formulation maintained a significant level of AZT in tissues rich in galactose specific receptors and had a prolonged residence in the body resulting in enhanced half-life of AZT. Conclusively, galactosylated liposomes are the potential candidate for targeted drug delivery and are anticipated to be promising in the treatment of AIDS6.

Kinetic stability plays a dominant role in the denaturant-induced unfolding of Erythrina indica lectin

The urea-induced denaturation of dimeric Erythrina indica lectin (EIL) has been studied at pH 7.2 under equilibrium and kinetic conditions in the temperature range of 40-55 degrees C. The structure of EIL is largely unaffected in this temperature range in absence of denaturant, and also in 8 M urea after incubation for 24 h at ambient temperature. The equilibrium denaturation of EIL exhibits a monophasic unfolding transition from the native dimer to the unfolded monomer as monitored by fluorescence, far-UV CD, and size-exclusion FPLC. The thermodynamic parameters determined for the two-state unfolding equilibrium show that the free energy of unfolding (DeltaGu, aq) remains practically same between 40 and 55 degrees C, with a value of 11.8 +/- 0.6 kcal mol(-1) (monomer units). The unfolding kinetics of EIL describes a single exponential decay pattern, and the apparent rate constants determined at different temperatures indicate that the rate of the unfolding reaction increases several fold with increase in temperature. The presence of probe like external metal ions (Mn2+, Ca2+) does not influence the unfolding reaction thermodynamically or kinetically; however, the presence of EDTA affects only kinetics. The present results suggest that the ability of EIL to preserve the structural integrity against the highly denaturing conditions is linked primarily to its kinetic stability, and the synergic action of heat and denaturant is involved in the unfolding of the protein.

Structural basis of carbohydrate recognition by a Man(alpha1-2)Man-specific lectin from Bowringia milbraedii

The crystal structure of the seed lectin from the tropical legume Bowringia milbraedii was determined in complex with the disaccharide ligand Man(alpha1-2)Man. In solution, the protein exhibits a dynamic dimer-tetramer equilibrium, consistent with the concanavalin A-type tetramer observed in the crystal. Contacts between the tetramers are mediated almost exclusively through the carbohydrate ligand, resulting in a crystal lattice virtually identical to that of the concanavalin-A:Man(alpha1-2)Man complex, even though both proteins have less than 50% sequence identity. The disaccharide binds exclusively in a "downstream" binding mode, with the non-reducing mannose occupying the monosaccharide-binding site. The reducing mannose is bound in a predominantly polar subsite involving Tyr131, Gln218, and Tyr219.

Insights into the quaternary association of proteins through structure graphs: a case study of lectins

The unique three-dimensional structure of both monomeric and oligomeric proteins is encoded in their sequence. The biological functions of proteins are dependent on their tertiary and quaternary structures, and hence it is important to understand the determinants of quaternary association in proteins. Although a large number of investigations have been carried out in this direction, the underlying principles of protein oligomerization are yet to be completely understood. Recently, new insights into this problem have been gained from the analysis of structure graphs of proteins belonging to the legume lectin family. The legume lectins are an interesting family of proteins with very similar tertiary structures but varied quaternary structures. Hence they have become a very good model with which to analyse the role of primary structures in determining the modes of quaternary association. The present review summarizes the results of a legume lectin study as well as those obtained from a similar analysis carried out here on the animal lectins, namely galectins, pentraxins, calnexin, calreticulin and rhesus rotavirus Vp4 sialic-acid-binding domain. The lectin structure graphs have been used to obtain clusters of non-covalently interacting amino acid residues at the intersubunit interfaces. The present study, performed along with traditional sequence alignment methods, has provided the signature sequence motifs for different kinds of quaternary association seen in lectins. Furthermore, the network representation of the lectin oligomers has enabled us to detect the residues which make extensive interactions ('hubs') across the oligomeric interfaces that can be targetted for interface-destabilizing mutations. The present review also provides an overview of the methodology involved in representing oligomeric protein structures as connected networks of amino acid residues. Further, it illustrates the potential of such a representation in elucidating the structural determinants of protein-protein association in general and will be of significance to protein chemists and structural biologists.

Crystallization and preliminary X-ray analysis of the Man(alpha1-2)Man-specific lectin from Bowringia mildbraedii in complex with its carbohydrate ligand

The lectin from Bowringia mildbraedii seeds crystallizes in the presence of the disaccharide Man(alpha1-2)Man. The best crystals grow at 293 K within four weeks after a pre-incubation at 277 K to induce nucleation. A complete data set was collected to a resolution of 1.90 A using synchrotron radiation. The crystals belong to space group I222, with unit-cell parameters a = 66.06, b = 86.35, c = 91.76 A, and contain one lectin monomer in the asymmetric unit.

Modeling oligomers with Cn or Dn symmetry: Application to CAPRI target 10

The abundance of oligomeric proteins makes them a frequent target for structure prediction. However, homologous proteins sometimes adopt different oligomerization states, rendering the prediction of structures of whole oligomers beyond the scope of comparative modeling. This obstacle can be overcome by combining comparative modeling of the single subunit of an oligomer with docking techniques, designed for predicting subunit-subunit interfaces. We present here algorithms for predicting the structures of homo-oligomers with C(n) or D(n) (n > 2) symmetry. The prediction procedure includes a symmetry-restricted docking step followed by a C(n) or D(n) oligomer-forming step, in which the dimers from the docking step are assembled to oligomers. The procedure is applied to each of the crystallographically independent subunits in 8 C(n) and 3 D(n) oligomers, producing very accurate predictions. It is further applied to a single monomer of the tick-borne encephalitis virus coat protein E (Target 10 of the CAPRI experiment). The predicted trimer ranked 30, obtained via rigid-body geometric-hydrophobic docking followed by C(n) oligomer formation, is very similar to the experimentally observed trimer formed by domain II of this protein. Furthermore, the predicted trimer formed from the separated domain I is also close to the experimental structure.

Oligomerization endows enormous stability to soybean agglutinin: a comparison of the stability of monomer and tetramer of soybean agglutinin

Soybean agglutinin is a tetrameric legume lectin, each of whose subunits are glycosylated. This protein shows a very high degree of stability when compared to the other proteins of the same family. In a previous work, it was shown that the unusual stability of the protein is due to a high degree of subunit interactions. In this study we present the thermodynamic parameters for the stability of soybean agglutinin monomer. The monomeric species is found at pH 2 and below which it is most populated at pH 1.9, as evident from size-exclusion chromatographic and dynamic light scattering studies. The analyses of circular dichroism and fluorescence spectroscopy suggest that the monomer is well folded, and that it has certain characteristic features when compared to its tetrameric counterpart. The conformational stabilities of the tetramer and the monomer at the temperature of their maximum stabilities (310 K) are 59.2 kcal/mol and 9.8 kcal/mol, respectively, indicating that oligomerization contributes significantly to the stability of the native molecule. Also, the T(g) difference for the two forms of the protein is approximately 40 K, whereas the difference in DeltaC(p) is only 1.6 kcal/mol/K. This suggests that the major hydrophobic core is present in the monomer itself, and that oligomerization involves mainly ionic interactions.

Lectins: tools for the molecular understanding of the glycocode

Recent progress in glycobiology has revealed that cell surface oligosaccharides play an essential role in recognition events. More precisely, these saccharides may be complexed by lectins, carbohydrate-binding proteins other than enzymes and antibodies, able to recognise sugars in a highly specific manner. The ubiquity of lectin-carbohydrate interactions opens enormous potential for their exploitation in medicine. Therefore, extraordinary effort is made into the identification of new lectins as well as into the achievement of a deep understanding of their functions and of the precise mechanism of their association with specific ligands. In this review, a summary of the main features of lectins, particularly those found in legumes, will be presented with a focus on the mechanism of carbohydrate-binding. An overview of lectin-carbohydrate interactions will also be given, together with an insight into their energetics. In addition, therapeutic applications of lectins will be discussed.

Energetics of 5-bromo-4-chloro-3-indolyl-alpha-D-mannose binding to the Parkia platycephala seed lectin and its use for MAD phasing

Parkia platycephala belongs to the most primitive group of Leguminosae plants. Its seed lectin is made up of three homologous beta-prism repeats and exhibits binding specificity for mannose/glucose. The properties of the association between the lectin from P. platycephala seeds and monosaccharide ligands were analysed by isothermal titration calorimetry and surface plasmon resonance. The results are consistent with the lectin bearing three thermodynamically identical binding sites for mannose/glucose per monomer with dissociation constants in the millimolar range. Binding of each ligand by the lectin is enthalpically driven. Crystals have been obtained of the lectin in complex with a brominated derivative of mannose (5-bromo-4-chloro-3-indolyl-alpha-D-mannose), which were suitable for deriving an electron-density map by MAD phasing. In agreement with the thermodynamic data, six Br atoms were found in the asymmetric unit of the monoclinic P2(1) crystals, which contained two P. platycephala lectin molecules. The availability of other Br derivatives of monosaccharides (glucose, galactose, fucose) may make this strategy widely useful for structure elucidation of novel lectins or when (as in the case of the P. platycephala lectin) molecular-replacement methods fail.

Quaternary association and reactivation of dimeric concanavalin A

The reconstitution of dimeric concanavalin A (ConA) in terms of quaternary association and reactivation, after denaturation in urea, has been investigated using intrinsic fluorescence, 8-anilino-1-naphthalenesulfonate (ANS) binding, far-UV circular dichroism (CD), and an activity assay developed through a combination of affinity binding and the o-phthalaldehyde (OPA) procedure of protein estimation. The equilibrium denaturation of dimeric ConA in urea exhibits a biphasic unfolding pathway involving an intermediate with hydrophobic exposure, and the overall free energy of stabilization for the dimeric protein is obtained as 16.3 kcal mol(-1). The time course of reassociation and regain of activity during reconstitution reveals that the reactivation of ConA runs almost parallel to the process of subunit association. The reactivation reaction follows second-order kinetics, with a rate constant (k) of 2.6 x 10(2) M(-1) s(-1). These results may provide insight into the relationship between quaternary association and function of legume lectins.