Ricin B chain and discoidin I share a common primitive protein fold

A Multivalent Marine Lectin from Crenomytilus grayanus Possesses Anti-cancer Activity through Recognizing Globotriose Gb3

In this study, we report the structure and function of a lectin from the sea mollusk Crenomytilus grayanus collected from the sublittoral zone of Peter the Great Bay of the Sea of Japan. The crystal structure of C. grayanus lectin (CGL) was solved to a resolution of 1.08 Å, revealing a β-trefoil fold that dimerizes into a dumbbell-shaped quaternary structure. Analysis of the crystal CGL structures bound to galactose, galactosamine, and globotriose Gb3 indicated that each CGL can bind three ligands through a carbohydrate-binding motif involving an extensive histidine- and water-mediated hydrogen bond network. CGL binding to Gb3 is further enhanced by additional side-chain-mediated hydrogen bonds in each of the three ligand-binding sites. NMR titrations revealed that the three binding sites have distinct microscopic affinities toward galactose and galactosamine. Cell viability assays showed that CGL recognizes Gb3 on the surface of breast cancer cells, leading to cell death. Our findings suggest the use of this lectin in cancer diagnosis and treatment.

Comparative analysis of carbohydrate binding properties of Sambucus nigra lectins and ribosome-inactivating proteins

In the past three decades a lot of research has been done on the extended family of carbohydrate-binding proteins from Sambucus nigra, including several so-called type 2 RIPs as well as hololectins. Although all these proteins have been studied for their carbohydrate-binding properties using hapten inhibition assays, detailed carbohydrate specificity studies have only been performed for a few Sambucus proteins. In particular SNA-I, has been studied extensively. Because of its unique binding characteristics this lectin was developed as an important tool in glycoconjugate research to detect sialic acid containing glycoconjugates. At present much less information is available with respect to the detailed carbohydrate binding specificity of other S. nigra lectins and RIPs, and as a consequence their applications remain limited. In this paper we report a comparative analysis of several lectins from S. nigra using the glycan microarray technology. Ultimately a better understanding of the ligands for each lectin can contribute to new/more applications for these lectins in glycoconjugate research. Furthermore, the data from glycan microarray analyses combined with the previously obtained sequence information can help to explain how evolution within a single lectin family eventually yielded a set of carbohydrate-binding proteins with a very broad specificity range.

Induction of antitumor immunity against cervical cancer by protein HPV-16 E7 in fusion with ricin B chain in tumor-bearing mice

OBJECTIVE

In immunotherapy of HPV-16-associated cervical cancers, the E7 protein is considered as a prime candidate. However, it is a poor inducer of a cytotoxic T-cell response when used as a singular antigen in protein vaccination. Therefore, to design effective cancer vaccines, the best tumor antigens should be combined with the most effective immunogens or drug delivery tools to achieve positive clinical results. In this study, we fused HPV-16 E7 with the lectin subunit of ricin toxin (RTB) from castor plant as a vaccine adjuvant/carrier.

MATERIALS AND METHODS

After reaching the soluble form of the recombinant protein, we designed 2 preventive and inhibition tumor models for investigation of the prevention and rejection of TC-1 cell growth in female C57BL/6 mice, respectively. In each model, mice were immunized with the recombinant protein of E7-RTB or E7 without any adjuvant.

RESULTS

We demonstrated that prophylactic immunization of E7-RTB protected mice against challenge from TC-1 cells. Also in the therapeutic model, E7-RTB could inhibit TC-1 tumor growth in the lung. The results were significant compared with the immunization of E7 singularly.

CONCLUSIONS

We concluded that immunization with E7-RTB protein without any adjuvant could generate antitumor effects in mice challenged with TC-1 cells. This research verifies the clinical applications and the future prospects for development of HPV-16 E7 therapeutic vaccines fused to immunoadjuvants.

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.

Conformation-dependent high-affinity potent ricin-neutralizing monoclonal antibodies

Ricin is a potential biothreat agent with no approved antidote available for ricin poisoning. The aim of this study was to develop potent antibody-based antiricin antidotes. Four strong ricin resistant hybridoma clones secreting antiricin monoclonal antibodies (mAbs) were developed. All four mAbs are bound to conformational epitopes of ricin toxin B (RTB) with high affinity (KD values from 2.55 to 36.27 nM). RTB not only triggers cellular uptake of ricin, but also facilitates transport of the ricin toxin A (RTA) from the endoplasmic reticulum to the cytosol, where RTA exerts its toxic activity. The four mAbs were found to have potent ricin-neutralizing capacities and synergistic effects among them as determined by an in vitro neutralization assay. In vivo protection assay demonstrated that all four mAbs had strong efficacy against ricin challenges. D9 was found to be exceptionally effective. Intraperitoneal (i.p.) administration of D9, at a dose of 5 μ g, 6 weeks before or 6 hours after an i.p. challenge with 5 × LD50 of ricin was able to protect or rescue 100% of the mice, indicating that mAb D9 is an excellent candidate to be developed as a potent antidote against ricin poisoning for both prophylactic and therapeutic purposes.

Structural basis for sugar recognition, including the Tn carcinoma antigen, by the lectin SNA-II from Sambucus nigra

Bark of elderberry (Sambucus nigra) contains a galactose (Gal)/N-acetylgalactosamine (GalNAc)-specific lectin (SNA-II) corresponding to slightly truncated B-chains of a genuine Type-II ribosome-inactivating protein (Type-II RIPs, SNA-V), found in the same species. The three-dimensional X-ray structure of SNA-II has been determined in two distinct crystal forms, hexagonal and tetragonal, at 1.90 A and 1.35 A, respectively. In both crystal forms, the SNA-II molecule folds into two linked beta-trefoil domains, with an overall conformation similar to that of the B-chains of ricin and other Type-II RIPs. Glycosylation is observed at four sites along the polypeptide chain, accounting for 14 saccharide units. The high-resolution structures of SNA-II in complex with Gal and five Gal-related saccharides (GalNAc, lactose, alpha1-methylgalactose, fucose, and the carcinoma-specific Tn antigen) were determined at 1.55 A resolution or better. Binding is observed in two saccharide-binding sites for most of the sugars: a conserved aspartate residue interacts simultaneously with the O3 and O4 atoms of saccharides. In one of the binding sites, additional interactions with the protein involve the O6 atom. Analytical gel filtration, small angle X-ray scattering studies and crystal packing analysis indicate that, although some oligomeric species are present, the monomeric species predominate in solution.

Carbohydrate recognition factors of the lectin domains present in the Ricinus communis toxic protein (ricin)

Ricin (RCA60) is a potent cytotoxic protein with lectin domains, contained in the seeds of the castor bean Ricinus communis. It is a potential biohazard. To corroborate the biological properties of ricin, it is essential to understand the recognition factors involved in the ricin-glycotope interaction. In previous reports, knowledge of the binding properties of ricin was limited to oligosugars and glycopeptides with different specificities. Here, recognition factors of the lectin domains in ricin were examined by enzyme-linked lectinosorbent (ELLSA) and inhibition assays, using mammalian Gal/GalNAc structural units and corresponding polyvalent forms. Except for blood group GalNAcalpha1-3Gal (A) active and Forssman (GalNAcalpha1-3GalNAc, F) disaccharides, ricin has a broad range of affinity for mammalian disaccharide structural units-Galbeta1-4Glcbeta1-(Lbeta), Galbeta1-4GlcNAc (II), Galbeta1-3GlcNAc (I), Galbeta1-3GalNAcalpha1-(Talpha), Galbeta1-3GalNAcbeta1-(Tbeta), Galalpha1-3Gal (B), Galalpha1-4Gal (E), GalNAcbeta1-3Gal (P), GalNAcalpha1-Ser/Thr (Tn) and GalNAcbeta1-4Gal (S). Among the polyvalent glycotopes tested, ricin reacted best with type II-containing glycoproteins (gps). It also reacted well with several T (Thomsen-Friedenreich), tumor-associated Tn and blood group Sd. (a+)-containing gps. Except for bird nest and Tamm-Horsfall gps (THGP), this lectin reacted weakly or not at all with ABH-blood type and sialylated gps. From the present and previous results, it can be concluded that: (i) the combining sites of these lectin domains should be a shallow-groove type, recognizing Galbeta1-4Glcbeta1- and Galbeta1-3(4)GlcNAcbeta- as the major binding site; (ii) its size may be as large as a tetrasaccharide and most complementary to lacto-N-tetraose (Galbeta1-3GlcNAc beta1-3Galbeta1-4Glc) and lacto-N-neotetraose (Galbeta1-4GlcNAcbeta1-3Galbeta1-4Glc); (iii) the polyvalency of glycotopes, in general, enhances binding; (iv) a hydrophobic interaction in the vicinity of the binding site for sugar accommodation, increases the affinity for Galbeta-. This study should assist in understanding the glyco-recognition factors involved in carbohydrate-toxin interactions in biological processes. The effect of the polyvalent P/S glycotopes on ricin binding should be examined. However, this is hampered by the lack of availability of suitable reagents.

Identification of three oligosaccharide binding sites in ricin

The galactoside-binding sites of ricin B chain can be blocked by affinity-directed chemical modification using a reactive ligand derived from asialoglycopeptides containing triantennary N-linked oligosaccharides. The terminal galactosyl residue of one branch of the triantennary oligosaccharide is modified to contain a reactive dichlorotriazine moiety. Two separate galactoside-binding sites have been clearly established in the ricin B chain by X-ray crystallography [Rutenber, E., and Robertus, J. D. (1991) Proteins 10, 260-269], and it is necessary to covalently attach two such reactive ligands to the B chain to block its binding to galactoside affinity matrixes. A method was developed using thiol-specific labeling of the ligand combined with subsequent immunoaffinity chromatography which allowed the isolation of ricin B chain peptides covalently linked to the ligand from proteolytic digests of purified blocked ricin. The sites of covalent attachment of the two ligands in blocked ricin were inferred from sequence analysis to be Lys 62 in domain 1 of the B chain and Tyr 148 in domain 2. A minor species of blocked ricin contains a third covalently attached ligand. From the analysis of peptides derived from blocked ricin enriched in this species, it is inferred that Tyr 67 in domain 1 is the specific site on the ricin B chain where a third reactive ligand becomes covalently linked to the protein. These results are interpreted as providing support for the notion that the ricin B chain has three oligosaccharide binding sites.

Complete amino acid sequence of the B chain of mistletoe lectin I

The primary structure of the B chain of mistletoe lectin I, the component of a commercially available extract from Viscum album exhibiting immunomodulatory capacity, was established based on amino acid sequence analysis of the protein and peptides derived from its enzymatic digestion. It is composed of 264 residues, including seven cysteine residues and three N-linked carbohydrate chains. The amino acid sequence of MLB shows a high homology with those from other structurally related galactoside-specific lectins such as ricin and abrin with 169 and 146 identities, respectively. These results are of crucial importance in order to understand the biological activity of ML-1.

The complete primary structure of abrin-a B chain

The complete 267 amino acid sequence of abrin-a B chain was determined by analysis of peptides obtained by digestion with trypsin, chymotrypsin, lysyl endopeptidase, Staphylococcus aureus V8 protease and thermolysin. The sequence is not identical with that predicted previously by nucleotide sequencing, indicating the presence of isoforms of abrin. Comparison of the amino acid sequence of abrin-a B chain with that of ricin-D B chain reveals a high degree of sequence identity (59%). Abrin-a B chain appears to consist of two domains, each domain with subdomains (alpha, beta, gamma) of about 40 amino acid residues.

The galactose-binding sites of the cytotoxic lectin ricin can be chemically blocked in high yield with reactive ligands prepared by chemical modification of glycopeptides containing triantennary N-linked oligosaccharides

A glycopeptide containing a triantennary N-linked oligosaccharide from fetuin was modified by a series of chemical and enzymic reactions to afford a reagent that contained a terminal residue of 6-(N-methylamino)-6-deoxy-D-galactose on one branch of the triantennary structure and terminal galactose residues on the other two branches. Binding assays and gel filtration experiments showed that this modified glycopeptide could bind to the sugar-binding sites of ricin. The ligand was activated at the 6-(N-methylamino)-6-deoxy-D-galactose residue by reaction with cyanuric chloride. The resulting dichlorotriazine derivative of the ligand reacts with ricin, forming a stable covalent linkage. The reaction was confined to the B-chain and was inhibited by lactose. Bovine serum albumin and ovalbumin were not modified by the activated ligand under similar conditions, and we conclude, therefore, that the reaction of the ligand with ricin B-chain was dependent upon specific binding to sugar-binding sites. Ricin that had its galactose-binding sites blocked by the covalent reaction with the activated ligand was purified by affinity chromatography. The major species in this fraction was found to contain 2 covalently linked ligands per ricin B-chain, while a minor species contained 3 ligands per B-chain. The cytotoxicity of blocked ricin was at least 1000-fold less than that of native ricin for cultured cells in vitro, even though the activity of the A-chain in a cell-free system was equal to that from native ricin. Modified ricin that contained only 1 covalently linked ligand was also purified. This fraction retained an ability to bind to galactose affinity columns, although with a lower affinity than ricin, and was only 5- to 20-fold less cytotoxic than native ricin.

Ribosome-inhibiting proteins, retroviral reverse transcriptases, and RNase H share common structural elements

Plant ribosome-inhibiting proteins are shown to be homologous at the domain level to RNase H from Escherichia coli and to two regions of the pol gene product of retroviral reverse transcriptases. One of these regions carries the viral integrase or int function, while the other has previously been suggested to contain the viral RNase H exo activity. Several residues conserved among the ribosome inhibitors, E. coli RNase H, and the integrase proteins are seen to occupy a prominent cleft in the tertiary structure of the ribosome inhibitor ricin, suggesting roles in binding or catalysis. It is likely that these homologous sequences represent modern derivatives of an ancient protein-folding unit capable of nucleic acid binding and modification which has been incorporated into a variety of enzyme functions.

Evidence for the involvement of protein-carbohydrate interaction in hematopoietic, multipotential colony-stimulating factor-dependent stem cell proliferation

Immunologically important mediators have been shown to exhibit ability to specifically bind distinct carbohydrates. This type of protein-carbohydrate interaction is one mechanism how to explain involvement of glycochemical interactions in regulatory processes. Interference of certain saccharides with murine multipotential colony-stimulating factor (multi-CSF)-dependent colony formation from progenitor cells in semisolid agar raised evidence for similar potential involvement of protein-carbohydrate interactions. Affinity depletion of conditioned WEHI-3B-medium on resins, bearing saccharides that have been elucidated to be effective inhibitors (mannose and lactose), resulted in preparations with significantly reduced capability to sustain development and proliferation. Sequence comparison of multi-CSF to carbohydrate-binding proteins (lectins) with this specificity failed to uncover extended homologies in diagonal plots. But detailed sequence alignments revealed confined, high-scoring stretches of homology between various lectins and two types of CSF. These results prove the importance of protein-carbohydrate interactions in stem cell proliferation.

Redesigning nature's poisons to create anti-tumor reagents

Immunotoxins are conjugates of cell-reactive antibodies and toxins or their subunits. In this report, the chemistry, biology, pharmacokinetics, and anti-tumor effects of first generation immunotoxins; the preparation of improved second generation immunotoxins that display greater anti-tumor efficacy; and the role of genetic engineering in creating third-generation immunotoxins are discussed.

Characterization of a cDNA encoding ricin E, a hybrid ricin-Ricinus communis agglutinin gene from the castor plant Ricinus communis

Two classes of ricin cDNA clones have been identified and sequenced. The cDNA clone pBL-1 closely matches in nucleotide sequence the ricin genomic clone pAKG previously described by Halling et al., 1985 (Nucl. Acids Res. 13:8019). A second group of cDNA clones, represented by pBL-3, encode a hybrid protein (ricin E), having a ricin-like A chain and N-terminal half of the B chain and an RCA (Ricinus communis agglutinin)-like C-terminal half of the B chain.

Structure and evolution of ricin B chain

Ricin is a dimeric toxin from the castor bean Ricinus communis, which is composed of a sugar-binding subunit (B) that attaches to receptors on the surfaces of target cells and a subunit (A) with enzymatic activity that attacks and inactivates ribosomes. We report here that comparison of amino-acid sequence data with high-resolution structure analysis of the ricin B subunit shows it to be the product of a series of gene duplications. The modern protein has two sugar-binding domains, each of which is composed of three copies of a more ancient galactose-binding peptide of about 40 residues.

The complete amino acid sequence of the B-chain of ricin E isolated from small-grain castor bean seeds. Ricin E is a gene recombination product of ricin D and Ricinus communis agglutinin

The complete amino acid sequence of the B-chain of ricin E has been determined. The reduced and carboxymethylated B-chain was digested with trypsin, followed by separation and purification of the resulting peptides using reverse-phase HPLC. The amino acid sequence of each tryptic peptide was determined employing the DABITC/PITC double-coupling method. The B-chain of ricin E proved to consist of 262 amino acid residues. By comparing the amino acid sequence of the B-chain of ricin E with those of ricin D and of Ricinus communis agglutinin, it was found that the B-chain of ricin E was composed of the N-terminal half of ricin D and C-terminal half R. communis agglutinin. This result suggested that the gene recombination probably occurred at the center region of two B-chain genes of ricin D and R. communis agglutinin.