Two orthorhombic crystal structures of a galactose-specific lectin from Artocarpus hirsuta in complex with methyl-alpha-D-galactose

First Insights into the Repertoire of Secretory Lectins in Rotifers

Due to their high biodiversity and adaptation to a mutable and challenging environment, aquatic lophotrochozoan animals are regarded as a virtually unlimited source of bioactive molecules. Among these, lectins, i.e., proteins with remarkable carbohydrate-recognition properties involved in immunity, reproduction, self/nonself recognition and several other biological processes, are particularly attractive targets for biotechnological research. To date, lectin research in the Lophotrochozoa has been restricted to the most widespread phyla, which are the usual targets of comparative immunology studies, such as Mollusca and Annelida. Here we provide the first overview of the repertoire of the secretory lectin-like molecules encoded by the genomes of six target rotifer species: Brachionus calyciflorus, Brachionus plicatilis, Proales similis (class Monogononta), Adineta ricciae, Didymodactylos carnosus and Rotaria sordida (class Bdelloidea). Overall, while rotifer secretory lectins display a high molecular diversity and belong to nine different structural classes, their total number is significantly lower than for other groups of lophotrochozoans, with no evidence of lineage-specific expansion events. Considering the high evolutionary divergence between rotifers and the other major sister phyla, their widespread distribution in aquatic environments and the ease of their collection and rearing in laboratory conditions, these organisms may represent interesting targets for glycobiological studies, which may allow the identification of novel carbohydrate-binding proteins with peculiar biological properties.

Structures of jacalin-related lectin PPL3 regulating pearl shell biomineralization

The nacreous layer of pearl oysters is one of the major biominerals of commercial and industrial interest. Jacalin-related lectins, including PPL3 isoforms, are known to regulate biomineralization of the Pteria penguin pearl shell, although the molecular mechanisms are largely unknown. The PPL3 crystal structures were determined partly by utilizing microgravity environments for 3 isoforms, namely, PPL3A, PPL3B, and PPL3C. The structures revealed a tail-to-tail dimer structure established by forming a unique inter-subunit disulfide bond at C-termini. The N-terminal residues were found in pyroglutamate form, and this was partly explained by the post-translational modification of PPL3 isoforms implied from the discrepancy between amino acid and gene sequences. The complex structures with trehalose and isomaltose indicated that the novel specificity originated from the unique α-helix of PPL3 isoforms. Docking simulations of PPL3B to various calcite crystal faces suggested the edge of a β-sheet and the carbohydrate-binding site rich in charged residues were the interface to the biomineral, and implied that the isoforms differed in calcite interactions.

Purification and Characterization of a Novel Hemagglutinin with Inhibitory Activity toward Osteocarcinoma Cells from Northeast China Black Beans

In the present study, we isolated a novel hemagglutinin from an edible legume and explored its growth-inhibitory effect on osteocarcinoma and liver cancer cells. The protein was purified by liquid chromatography techniques which entailed affinity chromatography on Affi-gel blue gel, ion-exchange chromatography on Mono Q, and gel filtration on Superdex 75 with an FPLC system. The hemagglutinating activity of this hemagglutinin was demonstrated to be ion dependent and stable over a wide range of temperature and pH values. Antiproliferative activity was observed in the tumor cell lines MG-63 and HepG2 but not in the normal cell line WRL 68. Osteocarcinoma cells treated with the hemagglutinin underwent obvious cell shrinkage, chromatin condensation, mitochondrial membrane depolarization, and apoptosis. The mRNA expression level of interleukin-2 (IL-2), interleukin-6 (IL-6), interleukin-1 beta (IL-1β), interferon-gamma (IFN-γ), and tumor necrosis factor alpha (TNF-α) were found to be up-regulated to different extents after treatment of this hemagglutinin.

Two jacalin-related lectins from seeds of the African breadfruit (Treculia africana L.)

Two jacalin-related lectins (JRLs) were purified by mannose-agarose and melibiose-agarose from seeds of Treculia africana. One is galactose-recognizing JRL (gJRL), named T. africana agglutinin-G (TAA-G), and another one is mannose-recognizing JRL (mJRL), TAA-M. The yields of the two lectins from the seed flour were approximately 7.0 mg/g for gJRL and 7.2 mg/g for mJRL. The primary structure of TAA-G was determined by protein sequencing of lysyl endopeptic peptides and chymotryptic peptides. The sequence identity of TAA-G to other gJRLs was around 70%. Two-residue insertion was found around the sugar-binding sites, compared with the sequences of other gJRLs. Crystallographic studies on other gJRLs have shown that the primary sugar-binding site of gJRLs can accommodate Gal, GalNAc, and GalNAc residue of T-antigen (Galβ1-3GalNAcα-). However, hemagglutination inhibition and glycan array showed that TAA-G did not recognize GalNAc itself and T-antigen. TAA-G preferred melibiose and core 3 O-glycan.

Characterization of the secondary binding sites of Maclura pomifera agglutinin by glycan array and crystallographic analyses

The Maclura pomifera agglutinin (MPA) recognizes the T-antigen disaccharide Galβ1,3GalNAc mainly through interaction of the α-GalNAc moiety with its primary site, but the interactions of the two flanking subsites A and B with aglycones and substituents other than Gal, respectively, are not well understood. We therefore characterized the specificity of MPA in more detail by glycan microarray analysis and determined the crystal structures of MPA without ligand and in complexes with Galβ1,3GalNAc and p-nitrophenyl α-GalNAc. In both sugar complexes, pairs of ligands created inter-tetramer hydrogen-bond bridging networks. While subsite A showed increased affinity for hydrophobic aglycones, it also accommodated several sugar substituents. Notably, a GalNAc-O-tripeptide, a Tn-antigen mimic, showed lower affinity than these compounds in surface plasmon resonance (SPR) experiments. The glycan array data that showed subsite B accepted compounds in which the O3 position of the GalNAc was substituted with various sugars other than Gal, but substitutions at O6 led to inactivity. Additions to the Gal moiety of the disaccharide also had only small effects on reactivity. These results are all compatible with the features seen in the crystal structures.

Conformational choice and selectivity in singly and multiply hydrated monosaccharides in the gas phase

Factors governing hydration, regioselectivity and conformational choice in hydrated carbohydrates have been examined by determining and reviewing the structures of a systematically varied set of singly and multiply hydrated monosaccharide complexes in the gas phase. This has been achieved through a combination of experiments, including infrared ion-depletion spectroscopy conducted in a supersonic jet expansion, and computation through molecular mechanics, density functional theory (DFT) and ab initio calculations. New spectroscopic and/or computational results obtained for the singly hydrated complexes of phenyl beta-D-mannopyranoside (beta-D-PhMan), methyl alpha-D-gluco- and alpha-D-galactopyranoside (alpha-D-MeGlc and alpha-D-MeGal), when coupled with those reported earlier for the singly hydrated complexes of alpha-D-PhMan, beta-D-PhGlc and beta-D-PhGal, have created a comprehensive data set, which reveals a systematic pattern of conformational preference and binding site selectivity, driven by the provision of optimal, co-operative hydrogen-bonded networks in the hydrated sugars. Their control of conformational choice and structure has been further revealed through spectroscopic and/or computational investigations of a series of multiply hydrated complexes; they include beta-D-PhMan.(H2O)2,3, which has an exocyclic hydroxymethyl group, and the doubly hydrated complex of phenyl alpha-L-fucopyranoside, alpha-L-PhFuc.(H2O)2, which does not. Despite the very large number of potential structures and binding sites, the choice is highly selective with binding invariably "focussed" around the hydroxymethyl group (when present). In beta-D-PhMan.(H2O)2,3, the bound water molecules are located exclusively on its polar face and their orientation is dictated by the (perturbed) conformation of the carbohydrate to which they are attached. The possible operation of similar rules governing the structures of hydrogen-bonded protein-carbohydrate complexes is proposed.

Mosquito larvicidal activity of gluanol acetate, a tetracyclic triterpenes derived from Ficus racemosa Linn

The larvicidal activity of crude hexane, ethyl acetate, petroleum ether, acetone, and methanol extracts of the leaf and bark of Ficus racemosa (Moraceae) was assayed for their toxicity against the early fourth-instar larvae of Culex quinquefasciatus (Diptera: Culicidae). The larval mortality was observed after 24-h exposure. All extracts showed moderate larvicidal effects; however, the highest larval mortality was found in bark acetone extract of F. racemosa. In the present study, bioassay-guided fractionation of acetone extract led to the separation and identification of a tetracyclic triterpenes derivative; gluanol acetate was isolated and identified as new mosquito larvicidal compound. Gluanol acetate was quite potent against fourth-instar larvae of Aedes aegypti L. (LC(50) 14.55 and LC(90) 64.99 ppm), Anopheles stephensi Liston (LC(50) 28.50 and LC(90) 106.50 ppm) and C. quinquefasciatus Say (LC(50) 41.42 and LC(90) 192.77 ppm). The structure was elucidated from infrared, ultraviolet, (1)H-nuclear magnetic resonance (NMR), (13)C-NMR, and mass spectral data. This is the first report on the mosquito larvicidal activity of the reported compound from F. racemosa.

Domain-swapped structure of the potent antiviral protein griffithsin and its mode of carbohydrate binding

The crystal structure of griffithsin, an antiviral lectin from the red alga Griffithsia sp., was solved and refined at 1.3 A resolution for the free protein and 0.94 A for a complex with mannose. Griffithsin molecules form a domain-swapped dimer, in which two beta strands of one molecule complete a beta prism consisting of three four-stranded sheets, with an approximate 3-fold axis, of another molecule. The structure of each monomer bears close resemblance to jacalin-related lectins, but its dimeric structure is unique. The structures of complexes of griffithsin with mannose and N-acetylglucosamine defined the locations of three almost identical carbohydrate binding sites on each monomer. We have also shown that griffithsin is a potent inhibitor of the coronavirus responsible for severe acute respiratory syndrome (SARS). Antiviral potency of griffithsin is likely due to the presence of multiple, similar sugar binding sites that provide redundant attachment points for complex carbohydrate molecules present on viral envelopes.

Crystal structure of trehalose-6-phosphate phosphatase-related protein: biochemical and biological implications

We report here the crystal structure of a trehalose-6-phosphate phosphatase-related protein (T6PP) from Thermoplasma acidophilum, TA1209, determined by the dual-wavelength anomalous diffraction (DAD) method. T6PP is a member of the haloacid dehalogenase (HAD) superfamily with significant sequence homology with trehalose-6-phosphate phosphatase, phosphoserine phosphatase, P-type ATPases and other members of the family. T6PP possesses a core domain of known alpha/beta-hydrolase fold, characteristic of the HAD family, and a cap domain, with a tertiary fold consisting of a four-stranded beta-sheet with two alpha-helices on one side of the sheet. An active-site magnesium ion and a glycerol molecule bound at the interface between the two domains provide insight into the mode of substrate binding by T6PP. A trehalose-6-phosphate molecule modeled into a cage formed by the two domains makes favorable interactions with the protein molecule. We have confirmed that T6PP is a trehalose phosphatase from amino acid sequence, three-dimensional structure, and biochemical assays.

Studies on recombinant single chain Jacalin lectin reveal reduced affinity for saccharides despite normal folding like native Jacalin

Sugar binding studies, inactivation, unfolding, and refolding of native Jacalin (nJacalin) from Artocarpus integrifolia and recombinant single-chain Jacalin (rJacalin) expressed in Escherichia coli were studied by intrinsic fluorescence and thermal and chemical denaturation approaches. Interestingly, rJacalin does not undergo any proteolytic processing in an E. coli environment. It has 100fold less affinity for methyl-alpha-galactose (Ka: 2.48 x 10(2)) in comparison to nJacalin (Ka: 1.58 x 10(4)), and it also binds Thomsen-Friedenreich (TF) disaccharide (Galbeta1-3GalNAc) with less affinity. Overall sugar binding characteristics of rJacalin are qualitatively similar to that of nJacalin (Gal<MealphaGal<MealphaTFdisaccharide). Circular dichroism studies at near- and far-UV, thermal, and chemical denaturation studies reveal that the rJacalin behaves like nJacalin. Guanidine hydrochloride-induced denaturation, followed by renaturation, yielded total recovery of sugar binding activity of rJacalin in comparison to partial recovery for nJacalin. This signifies the minor changes in the refolding pathways between native and recombinant lectins. The stability of rJacalin is dramatically reduced in the extreme pH range unlike nJacalin. Both lectins do not bind 1-anilino-8-naphthalene sulfonic acid (ANS) in the pH range of 5 to 12 but they do in the pH range of 1-3. Solute quenching studies of the lectin using acrylamide, KI, and CsCl indicated that the tryptophan residues have full accessibility to the neutral quencher and poor accessibility to ionic quenchers. In summary, biophysical and biochemical studies on the native versus recombinant Jacalin suggest that post-translational modification, i.e., the processing of Jacalin into two chains is probably not a prerequisite for sugar binding but may be required for higher affinity.