The amino-acid sequence of a lectin from conger eel, Conger myriaster, skin mucus

Comparison of skin mucus lectins among longtooth grouper Epinephelus bruneus and giant grouper E. lanceolatus as well as the hybrid, Kue-Tama, and their binding abilities to the skin fluke Benedeniaepinepheli

Interspecific hybrids of farm-raised fish are becoming popular in aquaculture owing to their advantages over pure species, including improved growth and higher resistance to infectious diseases. Kue-Tama is a recently established hybrid grouper derived from the longtooth grouper Epinephelus bruneus (♀) × giant grouper E. lanceolatus (♂). In our previous study, this hybrid showed significantly higher resistance against the skin fluke Benedenia epinepheli, a problematic parasite in grouper farming, than the longtooth grouper. In the present study, we explored lectins in the skin mucus of hybrids and their parent species. While C-type lectins of approximately 15 kDa were obtained from longtooth groupers, additional C-type lectins with molecular masses of approximately 20 and 30 kDa, as well as 45-kDa F-type lectin, were also detected in Kue-Tama and giant groupers. Semi-quantitative reverse transcript-polymerase chain reaction (RT-PCR) demonstrated that the gene expression levels of both C-type and F-type lectins were significantly higher in the skin of the hybrid and giant groupers than that of the longtooth grouper. In addition, some skin mucus lectins of the hybrid and giant groupers were bound to the fluke, suggesting that these lectins conferred resistance to parasitic infections.

Mucosal galectin genes in all freshwater eels of the genus Anguilla

In this study, we determined the genomic DNA sequences of the mucosal galectin-encoding genes from all 19 species and subspecies of the genus Anguilla. The nucleotide sequences of the galectin genes were c. 2.3-2.5 kb long and the organisation of their four exons and three introns was conserved in all species. An unusual sequence was found in the fourth exon of Anguilla reinhardtii, resulting in a unique deduced amino-acid sequence at the C-terminus. All six amino-acid residues important for β-galactoside binding were conserved in three species, while one residue (R⁷³ ) was substituted to K⁷³ in the other 16 species-subspecies, including Anguilla marmorata. However, this substitution did not appear to affect the sugar-binding ability of galectins because the galectin of A. marmorata was previously shown to bind to lactose. We also discuss the molecular evolution of galectins among Anguilla spp. and the homologues previously identified in Conger myriaster.

Immunological characterization and expression of lily-type lectin in response to environmental stress in turbot (Scophthalmus maximus)

Lectins are a superfamily of carbohydrate-binding proteins that are widely distributed throughout living organisms. In earlier work, we identified lily-type lectin (SmLTL) in the skin mucus of turbot Scophthalmus maximus, and we characterized the protein in the present study. Results from qRT-PCR indicated that SmLTL was expressed highly in skin, intestine and gill tissue. Changes in SmLTL expression occurred in these tissues in response to environmental stressors including ciliate infection, high temperature and salinity. Recombinant SmLTL purified from Escherichia coli was able to haemagglutinate mouse erythrocytes in the absence of calcium, and was inhibited by d-mannose. In addition, SmLTL displayed selective binding to bacterial species including Edwardsiella tarda and Vibrio anguillarum, and exhibited toxicity towards Philasterides dicentrarchi, with a mortality of over 60% after 24 h at a concentration of only 100 μgml^-1. To investigate this toxicity further, we measured binding of SmLTL after incubating the ciliate in FITC-SmLTL solution. Surface fluorescence decreased substantially in the presence of 400 mM d-mannose. Together these results suggest that lily-type lectins serve as the first line of defence against microbial attack and play a pivotal role in the mucosal immune system.

Differential expression of skin mucus C-type lectin in two freshwater eel species, Anguilla marmorata and Anguilla japonica

Two types of lactose-specific lectins, galectin (AJL-1) and C-type lectin (AJL-2), were previously identified in the mucus of adult Anguilla japonica. Here, we compared the expression profiles of these two homologous lectins at the adult and juvenile stages between the tropical eel Anguilla marmorata and the temperate eel A. japonica. Only one lectin, predicted to be an orthologue of AJL-1 by LC-MS/MS, was detected in the mucus of adult A. marmorata. We also found that an orthologous gene to AJL-2 was expressed at very low levels, or not at all, in the skin of adult A. marmorata. However, we detected the gene expression of an AJL-2-orthologue in the skin of juvenile A. marmorata, and a specific antibody also detected the lectin in the juvenile fish epidermis. These findings suggest that expression profiles of mucosal lectins vary during development as well as between species in the Anguilla genus.

Identification and characterization of pufflectin from the grass pufferfish Takifugu niphobles and comparison of its expression with that of Takifugu rubripes

Pufflectin found in Takifugu rubripes (Tr pufflectin) is the first animal lectin reported to show sequence similarity to monocotyledonous plant lectins. In the present study, we identified and characterized an orthologous lectin from Takifugu niphobles (Tn pufflectin), a species closely related to T. rubripes. Tn pufflectin exhibits 86% identity to Tr pufflectin with two conserved mannose-binding domains. Tn pufflectin was mainly expressed in the skin, gills, brain, and muscles; however, it was expressed at a lower level in the other examined tissues. Recombinant Tn pufflectin, expressed by Escherichia coli, exhibited binding activity specific for d-mannose. The expression of pufflectin in the gills was much lower in T. niphobles than in T. rubripes; notably, the former and latter are resistant and susceptible, respectively, to the monogenean parasite Heterobothrium okamotoi, which parasitizes gills. This suggests that pufflectin might be utilized by the parasite for host recognition.

Induction of Apoptosis and Antitumor Activity of Eel Skin Mucus, Containing Lactose-Binding Molecules, on Human Leukemic K562 Cells

For innate immune defense, lower animals such as fish and amphibian are covered with skin mucus, which acts as both a mechanical and biochemical barrier. Although several mucus sources have been isolated and studied for their biochemical and immunological functions, the precise mechanism(s) of action remains unknown. In the present study, we additionally found the eel skin mucus (ESM) to be a promising candidate for use in anti-tumor therapy. Our results showed that the viability of K562 cells was decreased in a dose-dependent manner by treatment with the isolated ESM. The cleaved forms of caspase-9, caspase-3 and poly adenosine diphosphate-ribose polymerase were increased by ESM. The levels of Bax expression and released cytochrome C were also increased after treatment with ESM. Furthermore, during the ESM mediated-apoptosis, phosphorylation levels of ERK1/2 and p38 but not JNK were increased and cell viabilities of the co-treated cells with ESM and inhibitors of ERK 1/2 or p38 were also increased. In addition, treatment with lactose rescued the ESM-mediated decrease in cell viability, indicating lactose-containing glycans in the leukemia cells acted as a counterpart of the ESM for interaction. Taken together, these results suggest that ESM could induce mitochondria-mediated apoptosis through membrane interaction of the K562 human leukemia cells. To the best of our knowledge, this is the first observation that ESM has anti-tumor activity in human cells.

Localization and functional properties of two galectin-1 proteins in Atlantic cod (Gadus morhua) mucosal tissues

Galectin-1 is a β-galactoside binding lectin with multiple immune functions in higher vertebrates. We report the characterization of two galectin-1 proteins from Atlantic cod, with emphasis on mucosal tissues. Tissue distribution of these two ≈14kDa galectin-1 proteins (Codgal1-1 and Codgal1-2) was ascertained by western blotting of one dimensional (1D) and two dimensional (2DE) gels. The two galectin-1 proteins were differentially localized in the mucosal tissues of cod. Codgal1-1 was predominantly localized in the basal cells of skin and this protein was present in all the early developmental stages examined, indicating a likely involvement in developmental processes. The two lectins were also localized in the adherent macrophage-like cells (MLC) from cod head kidney and results gathered indicate their possible secretion during Francisella noatunensis infection, suggesting that they are active components of immune defence. Lactose affinity chromatography coupled with gel filtration co-purified the two cod galectin-1 proteins, which hemagglutinated horse red blood cells in a lactose inhibitable manner. They also could bind and agglutinate both Gram-positive and Gram-negative bacteria. This study suggests multiple functional roles for galectin-1, especially in development and innate immune response of Atlantic cod.

Galectins in the abdominal cavity of the conger eel Conger myriaster participate in the cellular encapsulation of parasitic nematodes by host cells

Congerin is a proto-type galectin distributed on the skin and mucosal epithelia of the upper digestive tract of the Japanese conger eel Conger myriaster. It has at least 2 isotypes, namely, congerin I and II, and plays a role in bio-defense at the body surface. In the current study, we identified both isotypes in the peritoneal fluid and peritoneal cells of C. myriaster by western blot and mass spectrometry (MS)/MS analysis. Cucullanus nematodes parasitize the abdominal cavity of C. myriaster, and immunohistochemical analyses demonstrated that congerins can bind to both the body surface of the encapsulated nematodes and the encapsulating cells. Furthermore, adhesion of the peritoneal cells to Sepharose particles was greatly accelerated when the microspheres were coated with congerin. Indeed, this effect was significantly hampered by the addition of lactose. These results indicate that congerin participates in the cellular encapsulation of the Cucullanus nematode via the induction of cellular adhesion to the parasites depending on lectin-glycoside recognition.

Allosteric regulation of the carbohydrate-binding ability of a novel conger eel galectin by D-mannoside

Conger eel has two galectins, termed congerins I and II (Con I and II), that function in mucus as biodefense molecules. Con I and II have acquired a novel protein fold via domain swapping and a new ligand-binding site by accelerated evolution, which enables recognition of some marine bacteria. In this study, we identified a new congerin isotype, congerin P (Con-P), from the peritoneal cells of conger eel. Although Con-P displayed obvious homology with galectins, we observed substitution of 7 out of 8 amino acid residues in the carbohydrate recognition domain that are conserved in all other known galectins. To understand the structure-function relationships of this unique galectin, recombinant Con-P was successfully expressed in Escherichia coli by using a Con II-tagged fusion protein system and subsequently characterized. In the presence of D-mannose, Con-P displayed 30-fold greater hemagglutinating activity than Con I; however, no activity was observed without mannose, indicating that D-mannoside can act as a modulator of Con-P. Frontal affinity chromatography analysis showed that activated Con-P, allosterically induced by mannose, displayed affinity for oligomannose-type sugars as well as N-acetyllactosamine-type β-galactosides. Thus, Con-P represents a new member of the galectin family with unique properties.

Diversified carbohydrate-binding lectins from marine resources

Marine bioresources produce a great variety of specific and potent bioactive molecules including natural organic compounds such as fatty acids, polysaccharides, polyether, peptides, proteins, and enzymes. Lectins are also one of the promising candidates for useful therapeutic agents because they can recognize the specific carbohydrate structures such as proteoglycans, glycoproteins, and glycolipids, resulting in the regulation of various cells via glycoconjugates and their physiological and pathological phenomenon through the host-pathogen interactions and cell-cell communications. Here, we review the multiple lectins from marine resources including fishes and sea invertebrate in terms of their structure-activity relationships and molecular evolution. Especially, we focus on the unique structural properties and molecular evolution of C-type lectins, galectin, F-type lectin, and rhamnose-binding lectin families.

A new type of lectin discovered in a fish, flathead (Platycephalus indicus), suggests an alternative functional role for mammalian plasma kallikrein

A skin mucus lectin exhibiting a homodimeric structure and an S-S bond between subunits of ~40 kDa was purified from flathead Platycephalus indicus (Scorpaeniformes). This lectin, named FHL (FlatHead Lectin), exhibited mannose-specific activity in a Ca(2+)-dependent manner. Although FHL showed no homology to any previously reported lectins, it did exhibit ~20% identity to previously discovered plasma kallikreins and coagulation factor XIs of mammals and Xenopus laevis. These known proteins are serine proteases and play pivotal roles in the kinin-generating system or the blood coagulation pathway. However, alignment analysis revealed that while FHL lacked a serine protease domain, it was homologous to the heavy-chain domain of plasma kallikreins and coagulation factor XI therefore suggesting that FHL is not an enzyme but rather a novel animal lectin. On the basis of this finding, we investigated the lectin activity of human plasma kallikrein and revealed that it could indeed act as a lectin. Other genes homologous to FHL were also found in the genome databases of some fish species, but not in mammals. In contrast, plasma kallikreins and coagulation factor XI have yet to be identified in fish. The present findings suggest that these mammalian enzymes may have originally emerged as a lectin and may have evolved into molecules with protease activity after separation from common ancestors.

Protein engineering of conger eel galectins by tracing of molecular evolution using probable ancestral mutants

Background

Conger eel galectins, congerin I (ConI) and congerin II (ConII), show the different molecular characteristics resulting from accelerating evolution. We recently reconstructed a probable ancestral form of congerins, Con-anc. It showed properties similar to those of ConII in terms of thermostability and carbohydrate recognition specificity, although it shares a higher sequence similarity with ConI than ConII.

Results

In this study, we have focused on the different amino acid residues between Con-anc and ConI, and have performed the protein engineering of Con-anc through site-directed mutagenesis, followed by the molecular evolution analysis of the mutants. This approach revealed the functional importance of loop structures of congerins: (1) N- and C-terminal and loop 5 regions that are involved in conferring a high thermostability to ConI; (2) loops 3, 5, and 6 that are responsible for stronger binding of ConI to most sugars; and (3) loops 5 and 6, and Thr38 residue in loop 3 contribute the specificity of ConI toward lacto-N-fucopentaose-containing sugars.

Conclusions

Thus, this methodology, with tracing of the molecular evolution using ancestral mutants, is a powerful tool for the analysis of not only the molecular evolutionary process, but also the structural elements of a protein responsible for its various functions.

A novel recombinant system for functional expression of myonecrotic snake phospholipase A(2) in Escherichia coli using a new fusion affinity tag

A novel recombinant expression system in Escherichia coli was developed using conger eel galectin, namely, congerin II, as an affinity tag. This system was applied for the functional expression of myotoxic lysine-49-phospholipase A(2) ([Lys(49)]PLA(2)), termed BPII and obtained from Protobothrops flavoviridis (Pf) venom. Recombinant Pf BPII fused with a congerin tag has been successfully expressed as a soluble fraction and showed better quantitative yield when folded correctly. The solubility of the recombinant congerin II-tagged BPII increased up to >90% in E. coli strain JM109 when coexpressed with the molecular chaperones GroEL, GroES, and trigger factor (Tf). The tag protein was cleaved by digestion with restriction protease, such as alpha-thrombin or Microbacterium liquefaciens protease (MLP), to obtain completely active recombinant BPII. Thus, the congerin-tagged fusion systems containing the cleavage recognition site for alpha-thrombin or MLP were demonstrated to be highly efficient and useful for producing proteins of desired solubility and activity.

Isolation and characterization of l-rhamnose-binding lectin, which binds to microsporidian Glugea plecoglossi, from ayu (Plecoglossus altivelis) eggs

A rhamnose-binding lectin, named SFL, was isolated from the eggs of ayu (sweet fish, Plecoglossus altivelis) by affinity and ion-exchange chromatographies. SFL revealed 287 amino acid residues with 3 tandemly repeated domains, and contained 8 half-Cys residues in each domain. The lectin was shown to have a highly specific binding affinity to globotriaosylceramide (Gb3) by frontal affinity chromatography using 100 oligosaccharides. SFL was localized in several tissues and serum of both male and female ayu, such as gill, liver, ovary, testis, intestine, stomach, brain, kidney and serum. The lectin agglutinated the spores of the microsporidian Glugea plecoglossi, which is a pathogen of ayu. Although SFL bound to glycoproteins and glycolipids of G. plecoglossi spores, Gb3 could not be detected in either of them. The results suggest that SFL could interact with various glycoconjugates of pathogens to play a role in the adhesion of microorganisms invading in the body.

Reconstruction of a probable ancestral form of conger eel galectins revealed their rapid adaptive evolution process for specific carbohydrate recognition

Recently, many cases of rapid adaptive evolution, which is characterized by the higher substitution rates of nonsynonymous substitutions to synonymous ones, have been identified in the various genes of venomous and biodefense proteins, including the conger eel galectins, congerins I and II (ConI and ConII). To understand the evolutionary process of congerins, we prepared a probable ancestral form, Con-anc, corresponding to the putative amino acid sequence at the divergence of ConI and ConII in phylogenetic tree with 76% and 61% sequence identities to the current proteins, respectively. Con-anc and ConII had comparable thermostability and similar carbohydrate specificities in general, whereas ConI was more thermostable and showed different carbohydrate specificities. Con-anc showed decreased specificity to oligosaccharides with alpha 2,3-sialyl galactose moieties. These suggest that ConI and ConII have evolved via accelerated evolution under significant selective pressure to increase the thermostability and to acquire the activity to bind to alpha2,3-sialyl galactose present in pathogenic bacteria, respectively. Furthermore, comparative mutagenesis analyses of Con-anc and congerins revealed the structural basis for specific recognition of ConII to alpha2,3-sialyl galactose moiety, and strong binding ability of ConI to oligosaccharides including lacto-N-biosyl (Galbeta1-3GlcNAc) or lacto-N-neobiosyl (Galbeta1-4GlcNAc) residues, respectively. Thus, protein engineering using a probable ancestral form presented here is a powerful approach not only to determine the evolutionary process but also to investigate the structure-activity relationships of proteins.

Possible immune functions of congerin, a mucosal galectin, in the intestinal lumen of Japanese conger eel

Congerin, a mucosal galectin of the Japanese conger eel, provides chemical fortification through its agglutinating and opsonizing activity. Congerin is produced in the epidermis, and the epithelia of the oral cavity to the esophagus, but not in the stomach or intestine. We hypothesized that congerin secreted from the upper digestive tract can reach and function in the intestinal lumen. We found that congerin possessed marked resistance against digestion by gastric and enteric enzymes of conger eel. It was not degraded until 6h of incubation with stomach extract or intestinal digestion juice. Western blotting demonstrated that congerin essentially remained in the intestinal mucus. The mucus agglutinated rabbit erythrocytes, and the agglutination was hampered by anti-congerin antibody. Furthermore, congerin could bind to some enteric bacteria. These results support the above hypothesis.

Structure based studies of the adaptive diversification process of congerins

The isoforms of a fish galectin, congerins I and II, have several features that make them suitable for a study of accelerated process of molecular diversification based on 3D structures: They have been generated by a gene duplication, and still maintain 47% amino acid sequence identity to each other. Their genes show very high K A: /K S: ratio, and are though to be components of fish defense system. The crystal systems for a high-resolution analysis are known for both proteins. A series of works with biochemistry, molecular biology, and X-ray crystallography techniques have suggested that the two proteins might have evolved under differential selection pressures. Congerin I appeared to be a stabilized version of galectin-1. Congerin II was shown to be adapted to a new carbohydrate-ligand. The 3D structures of the wild type and mutant proteins have revealed the probable cause and consequence of the selection pressure responsible for the diversification of congerins.

Characterisation of carbohydrate-binding sites in developmental stages of Myxobolus cerebralis

Glycans and lectins (carbohydrate-binding molecules) form a mutual recognition system, which enables parasitic organisms to attach themselves to the host cells and/or take part in the migration of their developmental stages into the target tissue. The aim of the present study was to identify and characterise the potential binding activity of glycoconjugates in different developmental stages of Myxobolus cerebralis, the causative agent of whirling disease in salmonids. The binding patterns of 13 biotinylated neoglycoconjugates were histochemically examined in thin-sections of infected rainbow trout (Oncorhynchus mykiss) and oligochaetes (Tubifex tubifex), as well as isolated waterborne triactinomyxon spores. A distinct structure-selective and developmental stage-regulated expression of certain classes of carbohydrate binding was observed. In triactinomyxon spores, the expression of carbohydrate binding activity for alpha-l-Fuc-BSA-biotin, alpha-d-GalNAc-BSA-biotin, beta-d-GlcNAc-BSA-biotin, Lac-BSA-biotin and ASF-biotin was up-regulated in the polar capsules; the shell valves showed no activity. In the gut of T. tubifex, polar capsules of the parasite showed strong positive reaction only for beta-d-GlcNAc-BSA-biotin. In fish cartilage, polar capsules were negative, but the spore shell valves showed a broad range of carbohydrate binding activity. No activity was detected for either alpha6- or alpha3-linked N-acetyl-d-neuraminic acid to galactose. An adhesion assay was performed on GlycoWell plates and Myxobolus spores were found to specifically adhere to matrices containing residues of lactose, fucose, galactose, N-acetyl-d-galactosamine and N-acetyl-d-glucosamine. This is the first study to identify lectin activity in a myxozoan parasite; activity that is likely to play a role in the recognition systems involved in host specificity and the processes of spore attachment and invasion.

Tandem repeat L-rhamnose-binding lectin from the skin mucus of ponyfish, Leiognathus nuchalis

Two lactose-binding lectins (PFL-1 and -2) were identified in the skin mucus of ponyfish, Leiognathus nuchalis. The molecular masses of PFL-1 and -2 were estimated to be 24 and 30kDa, respectively. Cloning of the PFL-1 cDNA demonstrated its unique tandem repeat structure composed of two homologous domains with 41.7% internal identity. Furthermore, PFL-1 exhibited homology with L-rhamnose-binding lectins previously purified from the eggs of fish and sea urchins. The N-terminal amino acid sequence of PFL-2 was similar to that of PFL-1, suggesting that this protein is an isotype of PFL-1. The PFL-1 gene was expressed in the skin, an important line of defense against pathogens in fish, but was not expressed in any of the other tissues tested here. PFL-1 is the fourth type of fish skin mucus lectin to be identified, suggesting that different species of fish express different types of lectin in their skin mucus.

The speciation of conger eel galectins by rapid adaptive evolution

Many cases of accelerated evolution driven by positive Darwinian selection are identified in the genes of venomous and reproductive proteins. This evolutional phenomenon might have important consequences in their gene-products' functions, such as multiple specific toxins for quick immobilization of the prey and the establishment of barriers to fertilization that might lead to speciation, and in the molecular evolution of novel genes. Recently, we analyzed the molecular evolution of two galectins isolated from the skin mucus of conger eel (Conger myriaster), named congerins I and II, by cDNA cloning and X-ray structural analysis, and we found that they have evolved in the rapid adaptive manner to emergence of a new structure including strand-swapping and a unique new ligand-binding site. In this review article we summarize and discuss the molecular evolution, especially the rapid adaptive evolution, and the structure-function relationships of conger eel galectins.

Characteristics and primary structure of a galectin in the skin mucus of the Japanese eel, Anguilla japonica

The characteristics and primary structure of AJL-1, one of the lectins in the skin mucus of the Japanese eel (Anguilla japonica), were examined. This lectin exhibited beta-galactoside specific activity in a Ca2+ independent manner. We previously reported that its molecular mass was 16,091Da, although it was approximately 30 kDa as determined by gel filtration, indicating that it is a homodimer having non-covalent bonds. This lectin was composed of 142 amino acid residues having no half-cystinyl residues, and showed homology to members of the galectin family, especially to proto-type galectins. Gene expression of this lectin was detected in skin only, and relative expression was high in an individual that exhibited resistance to infectious disease. AJL-1 showed agglutinating activity against pathogenic bacteria, Streptococcus difficile. This suggests that AJL-1 functions as an important defensive factor at the body surface.

Molecular diversity of skin mucus lectins in fish

Among lectins in the skin mucus of fish, primary structures of four different types of lectin have been determined. Congerin from the conger eel Conger myriaster and AJL-1 from the Japanese eel Anguilla japonica were identified as galectin, characterized by its specific binding to beta-galactoside. Eel has additionally a unique lectin, AJL-2, which has a highly conserved sequence of C-type lectins but displays Ca(2+)-independent activity. This is rational because the lectin exerts its function on the cutaneous surface, which is exposed to a Ca(2+) scarce environment when the eel is in fresh water. The third type lectin is pufflectin, a mannose specific lectin in the skin mucus of pufferfish Takifugu rubripes. This lectin showed no sequence similarity with any known animal lectins but, surprisingly, shares sequence homology with mannose-binding lectins of monocotyledonous plants. The fourth lectin was found in the ponyfish Leiognathus nuchalis and exhibits homology with rhamnose-binding lectins known in eggs of some fish species. These lectins, except ponyfish lectin, showed agglutination of certain bacteria. In addition, pufflectin was found to bind to a parasitic trematode, Heterobothrium okamotoi. Taken together, these results demonstrate that skin mucus lectins in fish have wide molecular diversity.

Lectins homologous to those of monocotyledonous plants in the skin mucus and intestine of pufferfish, Fugu rubripes

We have characterized pufflectin, a novel mannose-specific lectin, from the skin mucus of the pufferfish, Fugu rubripes. Molecular mass estimations by gel filtration and matrix-assisted laser desorption ionization time-of-flight mass spectrometry and the SDS-PAGE pattern suggest that pufflectin is a homodimer composed of non-covalently associated subunits of 13 kDa. The full-length pufflectin cDNA consists of 527 bp, with 116 amino acid residues deduced from the open reading frame. The amino acid sequence of pufflectin shows no homology with any known animal lectin. Surprisingly, pufflectin shares sequence homology with mannose-binding lectins of monocotyledonous plants and has conserved two of three carbohydrate recognition domains of these plant lectins. The pufflectin gene is expressed in gills, oral cavity wall, esophagus, and skin. In addition, an isoform occurs exclusively in the intestine. Pufflectin differs from mannose-binding lectins purified from the blood plasma of Fugu. Whereas pufflectin did not agglutinate five bacterial species tested, it was demonstrated to bind to the parasitic trematode, Heterobothrium okamotoi. This finding suggests that pufflectin contributes to the parasite-defense system in Fugu.

Crystal structure of a conger eel galectin (congerin II) at 1.45A resolution: implication for the accelerated evolution of a new ligand-binding site following gene duplication

The crystal structure of congerin II, a galectin family lectin from conger eel, was determined at 1.45A resolution. The previously determined structure of its isoform, congerin I, had revealed a fold evolution via strand swap; however, the structure of congerin II described here resembles other prototype galectins. A comparison of the two congerin genes with that of several other galectins suggests acceralated evolution of both congerin genes following gene duplication. The presence of a Mes (2-[N-morpholino]ethanesulfonic acid) molecule near the carbohydrate-binding site in the crystal structure points to the possibility of an additional binding site in congerin II. The binding site consists of a group of residues that had been replaced following gene duplication suggesting that the binding site was built under selective pressure. Congerin II may be a protein specialized for biological defense with an affinity for target carbohydrates on parasites' cell surface.

Primary structure and characteristics of a lectin from skin mucus of the Japanese eel Anguilla japonica

Two types of lactose-binding lectins, AJL-1 and AJL-2, were purified from the skin mucus extract of the Japanese eel Anguilla japonica by lactose affinity chromatography and subsequent gel filtration. The molecular masses of AJL-1 and AJL-2 were 16,091 and 31,743 Da, respectively. Intact AJL-1 was comprised of two identical 16-kDa subunits having blocked N termini and no disulfide bonds. AJL-2 was a homodimer with disulfide bonds. Based on the N-terminal amino acid sequence of the AJL-2 monomer, the nucleotide sequence of cDNA encoding this lectin was determined by 3'- and 5'-rapid amplification of cDNA ends. The deduced amino acid sequence showed approximately 30% homology with C-type lectins, which bind to carbohydrates in a Ca(2+)-dependent manner. In addition, AJL-2 exhibited highly conserved consensus amino acid residues of the C-type carbohydrate recognition domain, although this lectin showed Ca(2+)-independent activity. Gene expression of AJL-2 was detected only in the skin by Northern blot analysis, and this lectin localization was demonstrated in the club cells by immunohistochemistry. These results indicate that AJL-2 is secreted on the body surface and function as a component of skin mucus. AJL-2 agglutinated Escherichia coli and suppressed its growth, suggesting that this lectin is involved in host defense.

Rhamnose-binding lectins from steelhead trout (Oncorhynchus mykiss) eggs recognize bacterial lipopolysaccharides and lipoteichoic acid

The interaction between bacteria and three L-rham-nose-binding lectins, named STL1, STL2, and STL3, from steelhead trout (Oncorhynchus mykiss) eggs was investigated. Although STLs bound to most Gram-negative and Gram-positive bacteria, they agglutinated only Escherichia coli K-12 and Bacillus subtilis among the bacteria tested. The binding was inhibited by L-rhamnose. STLs bound to distinct serotypes of lipopolysaccharides (LPSs), and showed much higher binding activities to smooth-type LPSs of Escherichia coli K-12 and Shigella flexneri 1A than to their corresponding rough-type LPSs. STLs also bound to lipoteichoic acid (LTA) of Bacillus subtilis. These results indicate that STLs bound to bacteria by recognizing LPSs or LTA on the cell surfaces.

Galectin containing cells in the skin and mucosal tissues in Japanese conger eel, Conger myriaster: an immunohistochemical study

Congerin is a beta-galactoside binding lectin (galectin) purified from the skin mucus of the Japanese conger, Conger myriaster. To clarify its tissue distribution and productive cells, several tissue samples including skin, buccal cavity wall, tang, pharynx, gills, esophagus, stomach, intestine, liver, kidney, spleen and ovary of conger were stained immunohistochemically using polyclonal rabbit anti-congerin serum. In the epidermis, a number of club cells were strongly stained. Because no agglutinating activity was detected in plasma, it appears evident that congerin is produced and secreted into mucus by those cells. In addition, congerin-positive club cells were distributed in the mucosal epithelium lining the digestive tract preceding the stomach and in the gills. These findings suggest that congerin participates in innate immunity on the intra- and the extra-body surface of the conger. The putative functions of club cells in fish and their contained lectin are discussed.

Cloning and characterisation of tandem-repeat type galectin in rainbow trout (Oncorhynchus mykiss)

Fish beta-galactoside binding lectin (galectin) cDNA was cloned from the cDNA library of rainbow trout (Oncorhynchus mykiss) head kidney. The clone contained a single open reading frame encoding 341 amino acids (aa) (38 kDa protein), including the initiator methionine. Significant sequence homology to mammalian galectin-9 (40-55% identity) was observed. Its amino acid sequence showed two distinct N- and C-terminal domains (148 and 130 aa, respectively) connected by a peptide linker (63 aa). The galectin contains two consensus WG-E-R/K motifs thought to play an essential role in sugar-binding, indicating that this lectin is a member of the tandem-repeat type galectins which have not been identified in fish. The 1.6 kDa mRNA of the lectin was found by Northern blot analyses to be widely expressed in the spleen, head kidney, thymus, peritoneal exudate cells, ovary, gills and heart. Southern blot analyses with the probe for C-terminal of the lectin showed the existence of two hybridising genes. These results suggest that rainbow trout has at least one tandem-repeat type galectin as well as proto-type galectin.

Isolation and characterization of a mannan-binding lectin from the freshwater cyanobacterium (blue-green algae) Microcystis viridis

Microcystis viridis NIES-102 strain, a unicellular freshwater bloom-forming cyanobacterium, showed transient hemagglutinating activity in laboratory culture during stationary phase under nonaeration conditions. However, the hemagglutinating activity which was inhibited with yeast mannan could not be observed during culture with aeration. A mannan-binding lectin named MVL was isolated with the assay of the hemagglutinating activity against rabbit erythrocytes from the cyanobacterium by successive hydrophobic and gel filtration chromatography. MVL was composed of a single polypeptide of 13 kDa. The gene (mvl) for MVL was cloned from a genomic DNA of NIES-102 strain as a template, and its sequence was determined. The deduced amino acid sequence showed that MVL consisted of 113 amino acid residues and was composed of two tandemly repeated homologous domains of 54 amino acid residues. MVL showed no sequence homology to any other lectins or proteins.

High-resolution structure of the conger eel galectin, congerin I, in lactose-liganded and ligand-free forms: emergence of a new structure class by accelerated evolution

BACKGROUND

Congerin I is a member of the galectin (animal beta-galactoside-binding lectin) family and is found in the skin mucus of conger eel. The galectin family proteins perform a variety of biological activities. Because of its histological localization and activity against marine bacteria and starfish embryos, congerin I is thought to take part in the eels' biological defense system against parasites.

RESULTS

The crystal structure of congerin I has been determined in both lactose-liganded and ligand-free forms to 1. 5 A and 1.6 A resolution, respectively. The protein is a homodimer of 15 kDa subunits. Congerin I has a beta-sheet topology that is markedly different from those of known relatives. One of the beta-strands is exchanged between two identical subunits. This strand swap might increase the dimer stability. Of the known galectin complexes, congerin I forms the most extensive interaction with lactose molecules. Most of these interactions are substituted by similar interactions with water molecules, including a pi-electron hydrogen bond, in the ligand-free form. This observation indicates an increased affinity of congerin I for the ligand.

CONCLUSIONS

The genes for congerin I and an isoform, congerin II, are known to have evolved under positive selection pressure. The strand swap and the modification in the carbohydrate-binding site might enhance the cross-linking activity, and should be the most apparent consequence of positive selection. The protein has been adapted to functioning in skin mucus that is in direct contact with surrounding environments by an enhancement in cross-linking activity. The structure of congerin I demonstrates the emergence of a new structure class by accelerated evolution under selection pressure.

Functional and structural characterization of multiple galectins from the skin mucus of conger eel, Conger myriaster

The complete amino acid sequence of an isogalectin, named congerin II, isolated from the skin mucus of conger eel, was determined by sequencing of the protein and its peptides generated by enzymatic and chemical cleavages. Congerin II consisted of 135 amino acids residues containing an acetylated N-terminus. Congerin II was found to be only 46% homologous in sequence to congerin I which was previously determined (Muramoto K., Kamiya H., Biochem. Biophys. Acta, 1992;1116:129-136), suggesting that the galectins with diverse molecular properties are present in the skin mucus of conger eel. However, it was confirmed by analysis of the secondary structures using circular dichroism that both congerins I and II shared similar folds characterized by beta structures. Congerins I and II showed different molecular properties such as thermostability, pH dependency for hemagglutinating activity and for binding specificity against the pyridylamino derivative of lactose. Congerin I showed more strict recognition specificity for lactose than did congerin II. Furthermore, the effects of chemical modification on congerins I and II were investigated in order to identify the type of amino acids involved in their different lectin activities. Modification of tyrosine and lysine residues did not affect the carbohydrate-binding activities of congerins. However, modification of tryptophan, arginine, histidine, glutamic acid and aspartic acid residues led to considerable loss of their activities, and a different mode of binding activity was observed between modified congerins I and II. These results suggest that multiple galectins from conger eel with the same scaffold have different biological functions and properties.

Molecular analysis of Physarum haemagglutinin I: lack of a signal sequence, sulphur amino acids and post-translational modifications

The cDNAs encoding haemagglutinin I from plasmodia of Physarum polycephalum have been cloned using PCR protocols. The composite haemagglutinin I cDNA sequence, derived from several overlapping clones from PCR fragments, spans 408 nt and the 315 bp ORF encodes a polypeptide of 104 aa without a typical signal sequence. The putative molecular mass deduced from the amino acid sequence (10,760.76 Da) corresponds exactly to that determined by electrospray ionization MS (10,759.86 +/- 0.15 Da), suggesting that haemagglutinin I is not subject to post-translational modification. Haemagglutinin I lacks sulphur amino acids and has a beta-sheet as the major secondary structure. Expression of the coding sequence in Escherichia coli yielded a product that exhibits the same sugar-binding specificity as natural haemagglutinin I. The deduced amino acid sequence shows little similarity to that of any known lectins and thus apparently represents a novel type of lectin.

Immunohistochemical localisation of a galectin from Bufo arenarum ovary

Galectins are a group of soluble animal lectins that exhibit specificity for beta-galactosides and conserve sequence homology in the carbohydrate-recognition domain. The galectin from Bufo arenarum ovary showed a strong cross-reaction with the lectin of 14.5 kDa purified from embryos at early blastula stage. In this paper, we studied the immunohistochemical localisation of the galectin of 14.5 kDa from ovary of the toad B. arenarum in adult ovary sections. We also analysed the immunohistochemical localisation of the embryonic lectin during early development using the antiserum anti-ovary galectin. In the ovary, oocytes in the previtellogenic stage showed strong reactivity in the nucleus and the cortex but not in the cytoplasm. Oocytes in the stage of primary vitellogenesis exhibited a similar pattern in the nuclear and cortical areas but showed immunostaining in the cytoplasm. Intense nuclear staining was detected in oocytes in the stage of late vitellogenesis and in mature oocytes, which also presented strong reactions in the yolk platelets that completely covered the cytoplasm. In blastula embryos the staining was found in the blastomeres, the yolk platelets and the blastocoele. Each lectin localisation is discussed in relation to potential biological roles in the corresponding tissues.

A rainbow trout lectin with multimeric structure

A novel lectin has been identified in rainbow trout serum and plasma. The lectin binds to Sepharose (an agarose polymer) in a calcium-dependent manner. Glucose, N-acetyl-glucosamine, mannose, N-acetyl-mannosamine, L-fucose, maltose and alpha-methyl-mannoside are good inhibitors of this binding, whereas glucosamine and D-fucose inhibits to a lesser degree and mannosamine and galactose do not inhibit the binding to Sepharose. When analysed by SDS-PAGE under non-reducing conditions, the lectin appears as a characteristic ladder of bands with approximately 16 kDa between consecutive bands. Upon reduction, the lectin appears as a 16-kDa band. On size-exclusion chromatography of trout serum and plasma, the protein emerges over a broad range corresponding to sizes from about 2000 kDa to less than 200 kDa. The NH2-terminal sequence (AAENRNQXPPG) shows no significant homology with known proteins. Because of the characteristic appearance in non-reducing SDS-PAGE and the lectin activity, we propose to name the protein "ladderlectin."

The primary structure and carbohydrate specificity of a beta-galactosyl-binding lectin from toad (Bufo arenarum Hensel) ovary reveal closer similarities to the mammalian galectin-1 than to the galectin from the clawed frog Xenopus laevis

The detailed characterization of a galectin from the toad (Bufo arenarum Hensel) ovary in its primary structure, carbohydrate specificity, and overall biochemical properties has provided novel information pertaining to structural and evolutionary aspects of the galectin family. The lectin consists of identical single-chain polypeptide subunits composed of 134 amino acids (calculated mass, 14,797 daltons), and its N-terminal residue, alanine, is N-acetylated. When compared to the sequences of known galectins, the B. arenarum galectin exhibited the highest identity (48% for the whole molecule and 77% for the carbohydrate recognition domain (CRD)) with the bovine spleen galectin-1, but surprisingly less identity (38% for the whole molecule and 47% for the CRD) with a galectin from Xenopus laevis skin (Marschal, P., Herrmann, J., Leffler, H., Barondes, S. H., and Cooper, D. N. W. (1992) J. Biol. Chem. 267, 12942-12949). Unlike the X. laevis galectin, the binding activity of the B. arenarum galectin for N-acetyllactosamine, the human blood group A tetrasaccharide and Galbeta1,3GalNAc relative to lactose, was in agreement with that observed for the galectin-1 subgroup and those galectins having "conserved" (type I) CRDs (Ahmed, H., and Vasta, G. R. (1994) Glycobiology 4, 545-549). Moreover, the toad galectin shares three of the six cysteine residues that are conserved in all mammalian galectins-1, but not in the galectins from X. laevis, fish, and invertebrates described so far. Based on the homologies of the B. arenarum galectin with the bovine spleen galectin-1 and X. laevis skin galectin, it should be concluded that within the galectin family the correlation between conservation of primary structure and phylogenetic distances among the source species may not be a direct one as proposed elsewhere (Hirabayashi, J., and Kasai, K. (1993) Glycobiology 3, 297-304). Furthermore, galectins with conserved (type I) CRDs, represented by the B. arenarum ovary galectin, and those with "variable" (type II) CRDs, represented by the X. laevis 16-kDa galectin, clearly constitute distinct subgroups in the extant amphibian taxa and may have diverged early in the evolution of chordate lineages.

Purification and partial biochemical characterization of an S-type lectin from blastula embryos of Bufo arenarum

1. S-type lectin from Bufo arenarum embryos at blastula stage was purified by affinity chromatography. The molecule is a dimer with equal-sized monomers and the apparent subunit molecular weight was found to be 14.5 kDa. 2. Analytical isoelectric focusing of the pure lectin showed an acidic pI of 4.7. 3. Inhibition of the hemagglutination by mono- and oligosaccharides revealed a specificity for sugars bearing a beta-galactoside configuration. 4. Crossreactivity studies between the blastula lectin and the one purified earlier from adult ovary performed by immunodotting, ELISA and immunoblotting showed that these lectins share many epitopes.

Molecular cloning of the defense factor in the albumen gland of the sea hare Aplysia kurodai

Aplysianin-A, an antibacterial glycoprotein in the albumen gland of the sea hare Aplysia kurodai, inhibited the growth of both Gram-positive and Gram-negative bacteria. Aplysianin-A cDNA clones were isolated from an albumen gland cDNA library. Sequence analysis reveals that aplysianin-A is produced as a precursor protein of 556 amino acid residues with a signal peptide of 19 amino acid residues and contains 6 potential N-glycosylation sites. Aplysianin-A mRNA was expressed tissue-specifically in the albumen gland. Homology search reveals that aplysianin-A has a 50% overall amino acid sequence homology to achacin, an antibacterial glycoprotein of the giant African snail Achatina fulica.