Fungal galectins, sequence and specificity of two isolectins from Coprinus cinereus

Fucose as a nutrient ligand for Dikarya and a building block of early diverging lineages

Fucose is a deoxyhexose sugar present and studied in mammals. The process of fucosylation has been the primary focus in studies relating to fucose in animals due to the presence of fucose in Lewis antigens. Very few studies have reported its presence in Fungi, mostly in Mucoromycotina. The constitution of 25% and 12% of this sugar in the carbohydrates of cell wall in the respective Umbelopsis and Mucorales strains boosts the need to bridge the gap of knowledge on fucose metabolism across the fungal tree of life. In the absence of a network map involving fucose proteins, we carried out an in-silico approach to construct the fucose metabolic map in Fungi. We analyzed the taxonomic distribution of 85 protein families in Fungi including diverse early diverging fungal lineages. The expression of fucose-related protein-coding genes proteins was validated with the help of transcriptomic data originating from representatives of early diverging fungi. We found proteins involved in several metabolic activities apart from fucosylation such as synthesis, transport and binding. Most of the identified protein families are shared with Metazoa suggesting an ancestral origin in Opisthokonta. However, the overall complexity of fucose metabolism is greater in Metazoa than in Fungi. Massive gene loss has shaped the evolutionary history of these metabolic pathways, leading to a repeated reduction of these pathways in most yeast-forming lineages. Our results point to a distinctive mode of utilization of fucose among fungi belonging to Dikarya and the early diverging lineages. We speculate that, while Dikarya used fucose as a source of nutrients for metabolism, the early diverging group of fungi depended on fucose as a building block and signaling compound.

Coprinopsis cinerea Galectin CGL1 Induces Apoptosis and Inhibits Tumor Growth in Colorectal Cancer Cells

Mushroom galectins are promising anticancer agents for their low IC50 values against cancer cells in vitro. In this study, two Coprinopsis cinerea galectins, CGL1 and CGL2, were heterologously expressed, and their biochemistry properties and anticancer effects were evaluated. The purified galectins were thermostable at neutral pH conditions. They both existed as tetramers and shared a high affinity towards lactose. CGL1 and CGL2 strongly inhibited the cell viability of many cancer cell lines, including three colorectal cancer cells, in a dose-dependent manner by inducing mitochondria-mediated caspase-dependent apoptosis. Furthermore, CGL1 exhibited higher apoptosis-inducing ability and cytotoxicity than CGL2. In vivo cell viability experiments based on two xenograft mouse models showed that CGL1 had a more substantial inhibitory effect than CGL2 on HCT116 tumor growth (p < 0.0001), whereas only CGL1 inhibited DLD1 tumor growth (p < 0.01). This is the first study to evaluate the anti-colorectal cancer effect of mushroom lectins in vivo, and our results showed that CGL1 is a potent agent for colorectal cancer treatment.

Purification of Recombinant Galectins from Different Species Using Distinct Affinity Chromatography Methods

Galectins are lectins having the capacity to recognize β-galactose-containing glycan structures and are widely distributed among various taxa. However, the exact physiological and biochemical functions mediated by galectins that necessitate their wide occurrence among diverse species have not yet been delineated in a precise manner. Purification of recombinant galectins in active form is a fundamental requirement to elucidate their biological function. In this chapter, we are describing methods to recombinantly express and purify galectins using three different methods of affinity purification, i.e., lactosyl-Sepharose chromatography for fungal galectin Coprinopsis cinerea galectin 2 (CGL2), nickel-chromatography for histidine-tagged human galectin-7, and glutathione-Sepharose chromatography for Glutathione S-transferase-tagged (GST-tagged) human galectin-7. Step-by-step instructions are provided for obtaining the above-mentioned recombinant galectins that retain carbohydrate-binding activity and are suitable for conducting biochemical experiments.

Utilization of Galectins by Pathogens for Infection

Galectins are glycan-binding proteins which are expressed by many different cell types and secreted extracellularly. These molecules are well-known regulators of immune responses and involved in a broad range of cellular and pathophysiological functions. During infections, host galectins can either avoid or facilitate infections by interacting with host cells- and/or pathogen-derived glycoconjugates and less commonly, with proteins. Some pathogens also express self-produced galectins to interfere with host immune responses. This review summarizes pathogens which take advantage of host- or pathogen-produced galectins to establish the infection.

Biochemical Characterization of Oyster and Clam Galectins: Selective Recognition of Carbohydrate Ligands on Host Hemocytes and Perkinsus Parasites

Both vertebrates and invertebrates display active innate immune mechanisms for defense against microbial infection, including diversified repertoires of soluble and cell-associated lectins that can effect recognition and binding to potential pathogens, and trigger downstream effector pathways that clear them from the host internal milieu. Galectins are widely distributed and highly conserved lectins that have key regulatory effects on both innate and adaptive immune responses. In addition, galectins can bind to exogenous (“non-self”) carbohydrates on the surface of bacteria, enveloped viruses, parasites, and fungi, and function as recognition receptors and effector factors in innate immunity. Like most invertebrates, eastern oysters (Crassostrea virginica) and softshell clams (Mya arenaria) can effectively respond to most immune challenges through soluble and hemocyte-associated lectins. The protozoan parasite Perkinsus marinus, however, can infect eastern oysters and cause “Dermo” disease, which is highly detrimental to both natural and farmed oyster populations. The sympatric Perkinsus chesapeaki, initially isolated from infected M. arenaria clams, can also be present in oysters, and there is little evidence of pathogenicity in either clams or oysters. In this review, we discuss selected observations from our studies on the mechanisms of Perkinsus recognition that are mediated by galectin-carbohydrate interactions. We identified in the oyster two galectins that we designated CvGal1 and CvGal2, which strongly recognize P. marinus trophozoites. In the clam we also identified galectin sequences, and focused on one (that we named MaGal1) that also recognizes Perkinsus species. Here we describe the biochemical characterization of CvGal1, CvGal2, and MaGal1 with focus on the detailed study of the carbohydrate specificity, and the glycosylated moieties on the surfaces of the oyster hemocytes and the two Perkinsus species (P. marinus and P. chesapeaki). Our goal is to gain further understanding of the biochemical basis for the interactions that lead to recognition and opsonization of the Perkinsus trophozoites by the bivalve hemocytes. These basic studies on the biology of host-parasite interactions may contribute to the development of novel intervention strategies for parasitic diseases of biomedical interest.

Comparative genomics reveals unique wood-decay strategies and fruiting body development in the Schizophyllaceae

Agaricomycetes are fruiting body-forming fungi that produce some of the most efficient enzyme systems to degrade wood. Despite decades-long interest in their biology, the evolution and functional diversity of both wood-decay and fruiting body formation are incompletely known. We performed comparative genomic and transcriptomic analyses of wood-decay and fruiting body development in Auriculariopsis ampla and Schizophyllum commune (Schizophyllaceae), species with secondarily simplified morphologies, an enigmatic wood-decay strategy and weak pathogenicity to woody plants. The plant cell wall-degrading enzyme repertoires of Schizophyllaceae are transitional between those of white rot species and less efficient wood-degraders such as brown rot or mycorrhizal fungi. Rich repertoires of suberinase and tannase genes were found in both species, with tannases restricted to Agaricomycetes that preferentially colonize bark-covered wood, suggesting potential complementation of their weaker wood-decaying abilities and adaptations to wood colonization through the bark. Fruiting body transcriptomes revealed a high rate of divergence in developmental gene expression, but also several genes with conserved expression patterns, including novel transcription factors and small-secreted proteins, some of the latter which might represent fruiting body effectors. Taken together, our analyses highlighted novel aspects of wood-decay and fruiting body development in an important family of mushroom-forming fungi.

Remusatia vivipara lectin and Sclerotium rolfsii lectin interfere with the development and gall formation activity of Meloidogyne incognita in transgenic tomato

Root knot nematodes are serious threats to growth and yield of solaneous crops including tomato. In this study, a binary vector carrying Remusatia vivipara (rvl1) and Sclerotium rolfsii (srl1) lectin genes were introduced independently into Lycopersicon esculentum cv. Pusa Ruby via Agrobacterium tumefaciens for resistance against root knot nematode, Meloidogyne incognita. In total, one hundred and one rvl1 and srl1-transformed plants exhibiting kanamycin resistance were confirmed to carry transgenes as detected by polymerase chain reaction (PCR) with 4.59% transformation efficiency. Genetic analysis of T₁ progeny confirmed Mendelian segregation of the introduced genes. Three events each of rvl1 and srl1 transgenic tomato were randomly selected for further confirmation by Southern and TAIL-PCR analyses. All three events of srl1 transgenics showed single copy transgene, whereas two rvl1 transgenic events showed single copy of transgene, while remaining event showed two copies of transgenes. Site of integration obtained for rvl1 and srl1 transgenic events by TAIL-PCR revealed that all the three events of rvl1 and srl1 transgenics differed for their site of integration and insertion sites did not contain any predicted gene. Moreover, expression of the rvl1 and srl1 transgenes was detected by haemagglutination assay in all three events of rvl1 and srl1, but not in non-transgenic tomato plant. Homozygous progenies of these events were grown and inoculated with M. incognita. Development and reproduction of M. incognita was severely affected in transgenic tomato plants expressing RVL1 and SRL1 exhibiting the high levels of resistance compared to non-transgenic plants. Therefore, these transgenic lines demonstrate a promising potential for variety development of tomato lines with enhanced resistance against M. incognita.

Blue light exposure and nutrient conditions influence the expression of genes involved in simultaneous hyphal knot formation in Coprinopsis cinerea

Light and nutrients are crucial environmental factors influencing fungal sexual reproduction. Blue light induces simultaneous hyphal knot formation in Coprinopsis cinerea mycelia grown on low-glucose media but not in mycelia grown on high-glucose media. Many hyphal knots are visible in the arc near the edge of the colony one day after 15 min of blue light stimulation. These findings collectively suggest that blue light accelerates hyphal knot induction in nutrient-limited conditions. Transcriptome analysis revealed that gene expression after light exposure is divided into at least two major stages. In the first stage, genes coding for fasciclin (fas1), cyclopropane-fatty-acyl-phospholipid synthases (cfs1 and cfs2), and putative lipid exporter (nod1) are highly expressed after 1 h of light exposure in the mycelial region where the hyphal knot will be developed. These genes are upregulated by blue light and not influenced by glucose condition and mating. These results suggest that although some of the genes are critical for induction of the hyphal knots, they are not sufficient for hyphal knot development. In the second gene expression stage, genes encoding galectins (cgl1-3), farnesyl cysteine-carboxyl methyltransferases, mating pheromone-containing protein, nucleus protein (ich1), and laccase (lcc1) are specifically upregulated at 10-16 h after blue light exposure when the mycelia are cultivated on low-glucose media. These genes might be involved in the architecture of hyphal knots or signal transduction for further fruiting body development. These results contribute to the understanding of the effect of environmental factors on sexual reproduction in basidiomycetous fungi.

Chicken GRIFIN: Structural characterization in crystals and in solution

Despite its natural abundance in lenses of vertebrates the physiological function(s) of the galectin-related inter-fiber protein (GRIFIN) is (are) still unclear. The same holds true for the significance of the unique interspecies (fish/birds vs mammals) variability in the capacity to bind lactose. In solution, ultracentrifugation and small angle X-ray scattering (at concentrations up to 9 mg/mL) characterize the protein as compact and stable homodimer without evidence for aggregation. The crystal structure of chicken (C-)GRIFIN at seven pH values from 4.2 to 8.5 is reported, revealing compelling stability. Binding of lactose despite the Arg71Val deviation from the sequence signature of galectins matched the otherwise canonical contact pattern with thermodynamics of an enthalpically driven process. Upon lactose accommodation, the side chain of Arg50 is shifted for hydrogen bonding to the 3-hydroxyl of glucose. No evidence for a further ligand-dependent structural alteration was obtained in solution by measuring hydrogen/deuterium exchange mass spectrometrically in peptic fingerprints. The introduction of the Asn48Lys mutation, characteristic for mammalian GRIFINs that have lost lectin activity, lets labeled C-GRIFIN maintain capacity to stain tissue sections. Binding is no longer inhibitable by lactose, as seen for the wild-type protein. These results establish the basis for detailed structure-activity considerations and are a step to complete the structural description of all seven members of the galectin network in chicken.

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First crystal structure of an eye lens GRIFIN defines differences to galectins.

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pH screening discloses high degree of structural stability in crystals.

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Hydrogen-deuterium exchange reveals unusually rigid structure in solution.

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Lectin histochemical assays identify critical sites for in situ ligand binding.

Galectin CvGal2 from the Eastern Oyster (Crassostrea virginica) Displays Unique Specificity for ABH Blood Group Oligosaccharides and Differentially Recognizes Sympatric Perkinsus Species

Galectins are highly conserved lectins that are key to multiple biological functions, including pathogen recognition and regulation of immune responses. We previously reported that CvGal1, a galectin expressed in phagocytic cells (hemocytes) of the eastern oyster (Crassostrea virginica), is hijacked by the parasite Perkinsus marinus to enter the host, where it causes systemic infection and death. Screening of an oyster hemocyte cDNA library revealed a novel galectin, which we designated CvGal2, with four tandemly arrayed carbohydrate recognition domains (CRDs). Phylogentic analysis of the CvGal2 CRDs suggests close relationships with homologous CRDs from CvGal1. Glycan array analysis, however, revealed that, unlike CvGal1 which preferentially binds to the blood group A tetrasaccharide, CvGal2 recognizes both blood group A and B tetrasaccharides and related structures, suggesting that CvGal2 has broader binding specificity. Furthermore, SPR analysis demonstrated significant differences in the binding kinetics of CvGal1 and CvGal2, and structural modeling revealed substantial differences in their interactions with the oligosaccharide ligands. CvGal2 is homogeneously distributed in the hemocyte cytoplasm, is released to the extracellular space, and binds to the hemocyte surface. CvGal2 binds to P. marinus trophozoites in a dose-dependent and β-galactoside-specific manner. Strikingly, negligible binding of CvGal2 was observed for Perkinsus chesapeaki, a sympatric parasite species mostly prevalent in the clams Mya arenaria and Macoma balthica. The differential recognition of Perkinsus species by the oyster galectins is consistent with their relative prevalence in oyster and clam species and supports their role in facilitating parasite entry and infectivity in a host-preferential manner.

Hitting the sweet spot-glycans as targets of fungal defense effector proteins

Organisms which rely solely on innate defense systems must combat a large number of antagonists with a comparably low number of defense effector molecules. As one solution of this problem, these organisms have evolved effector molecules targeting epitopes that are conserved between different antagonists of a specific taxon or, if possible, even of different taxa. In order to restrict the activity of the defense effector molecules to physiologically relevant taxa, these target epitopes should, on the other hand, be taxon-specific and easily accessible. Glycans fulfill all these requirements and are therefore a preferred target of defense effector molecules, in particular defense proteins. Here, we review this defense strategy using the example of the defense system of multicellular (filamentous) fungi against microbial competitors and animal predators.

Mushroom lectins: specificity, structure and bioactivity relevant to human disease

Lectins are non-immunoglobulin proteins that bind diverse sugar structures with a high degree of selectivity. Lectins play crucial role in various biological processes such as cellular signaling, scavenging of glycoproteins from the circulatory system, cell-cell interactions in the immune system, differentiation and protein targeting to cellular compartments, as well as in host defence mechanisms, inflammation, and cancer. Among all the sources of lectins, plants have been most extensively studied. However, more recently fungal lectins have attracted considerable attention due to their antitumor, antiproliferative and immunomodulatory activities. Given that only 10% of mushroom species are known and have been taxonomically classified, mushrooms represent an enormous unexplored source of potentially useful and novel lectins. In this review we provide an up-to-date summary on the biochemical, molecular and structural properties of mushroom lectins, as well as their versatile applications specifically focusing on mushroom lectin bioactivity.

Comparative transcriptomics of the model mushroom Coprinopsis cinerea reveals tissue-specific armories and a conserved circuitry for sexual development

Background

It is well known that mushrooms produce defense proteins and secondary metabolites against predators and competitors; however, less is known about the correlation between the tissue-specific expression and the target organism (antagonist) specificity of these molecules. In addition, conserved transcriptional circuitries involved in developing sexual organs in fungi are not characterized, despite the growing number of gene expression datasets available from reproductive and vegetative tissue. The aims of this study were: first, to evaluate the tissue specificity of defense gene expression in the model mushroom Coprinopsis cinerea and, second, to assess the degree of conservation in transcriptional regulation during sexual development in basidiomycetes.

Results

In order to characterize the regulation in the expression of defense loci and the transcriptional circuitries controlling sexual reproduction in basidiomycetes, we sequenced the poly (A)-positive transcriptome of stage 1 primordia and vegetative mycelium of C. cinerea A43mutB43mut. Our data show that many genes encoding predicted and already characterized defense proteins are differentially expressed in these tissues. The predicted specificity of these proteins with regard to target organisms suggests that their expression pattern correlates with the type of antagonists these tissues are confronted with. Accordingly, we show that the stage 1 primordium-specific protein CC1G_11805 is toxic to insects and nematodes. Comparison of our data to analogous data from Laccaria bicolor and Schizophyllum commune revealed that the transcriptional regulation of nearly 70 loci is conserved and probably subjected to stabilizing selection. A Velvet domain-containing protein was found to be up-regulated in all three fungi, providing preliminary evidence of a possible role of the Velvet protein family in sexual development of basidiomycetes. The PBS-soluble proteome of C. cinerea primordia and mycelium was analyzed by shotgun LC-MS. This proteome data confirmed the presence of intracellular defense proteins in primordia.

Conclusions

This study shows that the exposure of different tissues in fungi to different types of antagonists shapes the expression pattern of defense loci in a tissue-specific manner. Furthermore, we identify a transcriptional circuitry conserved among basidiomycetes during fruiting body formation that involves, amongst other transcription factors, the up-regulation of a Velvet domain-containing protein.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-492) contains supplementary material, which is available to authorized users.

Fungal lectins: a growing family

Fungi are members of a large group of eukaryotic organisms that include yeasts and molds, as well as the most familiar member, mushrooms. Fungal lectins with unique specificity and structures have been discovered. In general, fungal lectins are classified into specific families based on their amino acid sequences and three-dimensional structures. In this chapter, we provide an overview of the approximately 80 types of mushroom and fungal lectins that have been isolated and studied to date. In particular, we have focused on ten fungal lectins (Agaricus bisporus, Agrocybe cylindracea, Aleuria aurantia, Aspergillus oryzae, Clitocybe nebularis, Marasmius oreades, Psathyrella velutina, Rhizopus stolonifer, Pholiota squarrosa, Polyporus squamosus), many of which are commercially available and their properties, sugar-binding specificities, structural grouping into families, and applications for biological research being described. The sialic acid-specific lectins (Agrocybe cylindracea and Polyporus squamosus) and fucose-specific lectins (Aleuria aurantia, Aspergillus oryzae, Rhizopus stolonifer, and Pholiota squarrosa) each showed potential for use in identifying sialic acid glycoconjugates and fucose glycoconjugates. Although not much is currently known about fungal lectins compared to animal and plant lectins, the knowledge accumulated thus far shows great promise for several applications in the fields of taxonomy, biomedicine, and molecular and cellular biology.

Fine-tuning of prototype chicken galectins: structure of CG-2 and structure-activity correlations

The comparatively small number of members of the family of adhesion/growth-regulatory galectins in chicken predestines this system as an attractive model to study the divergence of these lectins after gene duplication. Expression profiling of the three homodimeric (prototype) chicken galectins (CG-1A, CG-1B and CG-2) has raised evidence of distinct functionalities, explaining the interest in a detailed crystallographic analysis of CG-2. As revealed here, marked differences are found in the ligand-binding site and in the contact pattern within the homodimer interface, underlying a characteristic orientation of the two subunits. Notably, a distinctive trimer of dimers that is unique in all galectin crystal structures reported to date forms the core unit of the crystallographic assembly. Combination with spectroscopic and thermodynamic measurements, and comparisons with CG-1A and CG-1B, identify differential changes in the circular-dichroism spectra in the presence of lactose, reflecting the far-reaching impact of the ligand on hydrodynamic behaviour, and inter-galectin differences in both the entropy and the enthalpy of binding. This structural information is a salient step to complete the analysis of the full set of galectins from this model organism.

The galectin CvGal1 from the eastern oyster (Crassostrea virginica) binds to blood group A oligosaccharides on the hemocyte surface

The galectin CvGal1 from the eastern oyster (Crassostrea virginica), which possesses four tandemly arrayed carbohydrate recognition domains, was previously shown to display stronger binding to galactosamine and N-acetylgalactosamine relative to d-galactose. CvGal1 expressed by phagocytic cells is "hijacked" by the parasite Perkinsus marinus to enter the host, where it proliferates and causes systemic infection and death. In this study, a detailed glycan array analysis revealed that CvGal1 preferentially recognizes type 2 blood group A oligosaccharides. Homology modeling of the protein and its oligosaccharide ligands supported this preference over type 1 blood group A and B oligosaccharides. The CvGal ligand models were further validated by binding, inhibition, and competitive binding studies of CvGal1 and ABH-specific monoclonal antibodies with intact and deglycosylated glycoproteins, hemocyte extracts, and intact hemocytes and by surface plasmon resonance analysis. A parallel glycomic study carried out on oyster hemocytes (Kurz, S., Jin, C., Hykollari, A., Gregorich, D., Giomarelli, B., Vasta, G. R., Wilson, I. B. H., and Paschinger, K. (2013) J. Biol. Chem. 288) determined the structures of oligosaccharides recognized by CvGal1. Proteomic analysis of the hemocyte glycoproteins identified β-integrin and dominin as CvGal1 "self"-ligands. Despite strong CvGal1 binding to P. marinus trophozoites, no binding of ABH blood group antibodies was observed. Thus, parasite glycans structurally distinct from the blood group A oligosaccharides on the hemocyte surface may function as potentially effective ligands for CvGal1. We hypothesize that carbohydrate-based mimicry resulting from the host/parasite co-evolution facilitates CvGal1-mediated cross-linking to β-integrin, located on the hemocyte surface, leading to cell activation, phagocytosis, and host infection.

5'-Serial Analysis of Gene Expression studies reveal a transcriptomic switch during fruiting body development in Coprinopsis cinerea

Background

The transition from the vegetative mycelium to the primordium during fruiting body development is the most complex and critical developmental event in the life cycle of many basidiomycete fungi. Understanding the molecular mechanisms underlying this process has long been a goal of research on basidiomycetes. Large scale assessment of the expressed transcriptomes of these developmental stages will facilitate the generation of a more comprehensive picture of the mushroom fruiting process. In this study, we coupled 5'-Serial Analysis of Gene Expression (5'-SAGE) to high-throughput pyrosequencing from 454 Life Sciences to analyze the transcriptomes and identify up-regulated genes among vegetative mycelium (Myc) and stage 1 primordium (S1-Pri) of Coprinopsis cinerea during fruiting body development.

Results

We evaluated the expression of >3,000 genes in the two respective growth stages and discovered that almost one-third of these genes were preferentially expressed in either stage. This identified a significant turnover of the transcriptome during the course of fruiting body development. Additionally, we annotated more than 79,000 transcription start sites (TSSs) based on the transcriptomes of the mycelium and stage 1 primoridum stages. Patterns of enrichment based on gene annotations from the GO and KEGG databases indicated that various structural and functional protein families were uniquely employed in either stage and that during primordial growth, cellular metabolism is highly up-regulated. Various signaling pathways such as the cAMP-PKA, MAPK and TOR pathways were also identified as up-regulated, consistent with the model that sensing of nutrient levels and the environment are important in this developmental transition. More than 100 up-regulated genes were also found to be unique to mushroom forming basidiomycetes, highlighting the novelty of fruiting body development in the fungal kingdom.

Conclusions

We implicated a wealth of new candidate genes important to early stages of mushroom fruiting development, though their precise molecular functions and biological roles are not yet fully known. This study serves to advance our understanding of the molecular mechanisms of fruiting body development in the model mushroom C. cinerea.

Identification and characterization of a chitin-binding protein purified from coelomic fluid of the lugworm Arenicola marina defining a novel protein sequence family

We have isolated a novel type of lectin named Arenicola marina lectin-1 (AML-1) from the lugworm A. marina. The lectin was purified from the coelomic fluid by affinity chromatography on a GlcNAc-derivatized column and eluted with GlcNAc. On SDS-PAGE, AML-1 showed an apparent molecular mass of 27 and 31 kDa in the reduced state. The N-terminal amino acid sequences were identical in these two bands. In the unreduced state, a complex band pattern was observed with bands from 35 kDa to more than 200 kDa. Two different full-length clones encoding polypeptides of 241 and 243 amino acids, respectively, were isolated from a coelomocyte cDNA library. The two clones, designated AML-1a and AML-1b, were 92% identical at the protein level and represent a novel type of protein sequence family. Purified AML-1 induced agglutination of rabbit erythrocytes, which could be inhibited by N-acetylated saccharides. Recombinant AML-1b showed the same band pattern as the native protein, whereas recombinant AML-1a in the reduced state lacked a 27 kDa band. AML-1b bound GlcNAc-derivatized columns and chitin, whereas AML-1a did not bind to these matrices. Immunohistochemical analysis revealed that AML-1 is expressed by coelomocytes in the nephridium and in round cells in the epidermis and in eggs. Moreover, AML-1 expression was up-regulated in response to a parasitic infection. We conclude that AML-1 purified from coelomic fluid is encoded by AML-1b and represents a novel type of protein family that binds acetylated components.

Primary structure and specificity of a new member of galectin family from the Amethyst deceiver mushroom Laccaria amethystina

A new galectin was characterized in the Amethyst deceiver mushroom Laccaria amethystina. The complete amino acid (AA) sequence of the lectin, which exhibited β-galactoside specificity, was deduced from its peptide sequences. The AA sequence of L. amethystina galectin (LAG) showed high homology with those of the same genus, at 75.6% identity to Laccaria bicolor, and 35.5-65.0% to galectins of Agrocybe spp. and Coprinopsis cinerea. The AA sequence of LAG contained all but one conserved residue known to be involved in β-galactoside binding, with His at the position 57 residue replaced by Thr in LAG. Analysis of binding specificity by hemagglutination inhibition assay and enzyme-linked lectin-sorbent assay revealed high specificity of LAG towards O-glycoproteins.

A multidomain galectin involved in innate immune response of pearl oyster Pinctada fucata

Galectins could specifically bind to β-galactoside residues and play crucial roles in innate immune responses of vertebrates and invertebrates. In this study, the cDNA of a galectin with multiple carbohydrate-recognition domains (CRDs) was cloned from pearl oyster Pinctada fucata (designated as PoGal). PoGal cDNA was 2138bp long and consisted of a 5'-untranslated region (UTR) of 120bp, a 3'-UTR of 350bp with two cytokine RNA instability motifs (ATTTA), and an open reading frame (ORF) of 1668bp encoding a polypeptide of 555 amino acids with an estimated molecular mass of 63.4kDa and a theoretical isoelectric point of 4.8. PoGal contained four CRDs, each CRD of PoGal all had the conserved carbohydrate-binding motifs H-NPR and WG-ER. PoGal shared 43.7% and 62.9% identity to those of bay scallop and eastern oyster, respectively, which were only two galectins with four CRDs. The phylogenetic analysis revealed that all galectins with four CRDs formed a single clade. PoGal mRNA was constitutively expressed in all detected tissues, and the expression level of PoGal mRNA was significantly up-regulated in digestive gland, mantle, haemocyte, gonad and intestine after Vibrio alginolyticus stimulation. The expression profile analysis showed that the expression level of PoGal mRNA was significantly up-regulated at 4, 8 and 12h after V. alginolyticus stimulation. These results suggested that PoGal was a constitutive and inducible acute-phase protein that perhaps involved in innate immune response of pearl oyster.

Mushroom lectins: current status and future perspectives

Lectins are nonimmune proteins or glycoproteins that bind specifically to cell surface carbohydrates, culminating in cell agglutination. These are known to play key roles in host defense system and also in metastasis. Many new sources have been explored for the occurrence of lectins during the last few years. Numerous novel lectins with unique specificities and exploitable properties have been discovered. Mushrooms have attracted a number of researchers in food and pharmaceuticals. Many species have long been used in traditional Chinese medicines or functional foods in Japan and other Asian countries. A number of bioactive constituents have been isolated from mushrooms including polysaccharides, polysaccharopeptides, polysaccharide-protein complexes, proteases, ribonucleases, ribosome inactivating proteins, antifungal proteins, immunomodulatory proteins, enzymes, lectins, etc. Mushroom lectins are endowed with mitogenic, antiproliferative, antitumor, antiviral, and immune stimulating potential. In this review, an attempt has been made to collate the information on mushroom lectins, their blood group and sugar specificities, with an emphasis on their biomedical potential and future perspectives.

Crystal structure of the GalNAc/Gal-specific agglutinin from the phytopathogenic ascomycete Sclerotinia sclerotiorum reveals novel adaptation of a beta-trefoil domain

A lectin from the phytopathogenic ascomycete Sclerotinia sclerotiorum that shares only weak sequence similarity with characterized fungal lectins has recently been identified. S. sclerotiorum agglutinin (SSA) is a homodimeric protein consisting of two identical subunits of approximately 17 kDa and displays specificity primarily towards Gal/GalNAc. Glycan array screening indicates that SSA readily interacts with Gal/GalNAc-bearing glycan chains. The crystal structures of SSA in the ligand-free form and in complex with the Gal-beta1,3-GalNAc (T-antigen) disaccharide have been determined at 1.6 and 1.97 A resolution, respectively. SSA adopts a beta-trefoil domain as previously identified for other carbohydrate-binding proteins of the ricin B-like lectin superfamily and accommodates terminal non-reducing galactosyl and N-acetylgalactosaminyl glycans. Unlike other structurally related lectins, SSA contains a single carbohydrate-binding site at site alpha. SSA reveals a novel dimeric assembly markedly dissimilar to those described earlier for ricin-type lectins. The present structure exemplifies the adaptability of the beta-trefoil domain in the evolution of fungal lectins.

[Galectin expression in urological cancer. Diagnostic, prognostic and therapeutic potential]

Galectins are a family of 15 proteins. They interact with glycoproteins in both the extracellular and intracellular milieu and regulate various biological cycles including cell growth, cell differentiation, cell adhesion and apoptosis. In biomolecular studies certain patterns of expression showed a correlation with metastasis and apoptosis. Therefore, galectins could be used as potential markers for early tumour recognition, long-term prognosis and a better-founded choice of therapy. Acknowledging these possibilities this review points out the standing of galectins with all currently available data in the development and progression of renal, bladder and prostatic tumours. The expression patterns of galectin-1 and -3 have been researched extensively. For example, several studies could show a decreased expression of galectin-3 in clear renal cell carcinoma, which correlated with a poor clinical prognosis. On the contrary, patients with a bladder tumour showed an elevated serum level of galectin-3. Furthermore, in analysis of prostatic tumour tissue galectin-1 was found to be an independent predictor of a PSA relapse. In addition, pathological patterns of galectin expression could be detected in non-urological malignancies such as breast cancer. Though all findings indicate a future significance of galectins as markers of urological malignancies with clinical relevance, more research will be necessary to particularly assess the evolutional-dependent function of galectins in the process of tumourigenesis.

Opposing developmental functions of Agrocybe aegerita galectin (AAL) during mycelia differentiation

Mycelia of basidiomycetes differentiating into fruiting body is a controlled developmental process, however the underlying molecular mechanism remains unknown. In previous work, a novel fungal Agrocybe aegerita galectin (AAL) was isolated from A. aegerita in our laboratory. AAL was shown to promote mycelial differentiation in A. aegerita and Auricularia polytricha, indicating that AAL might function as a conserved fruiting initiator during basidiomycete mycelia development. In the current work, we investigate the role of AAL in mycelia differentiation and fruiting body formation. First, the expression and localization of AAL in mycelia, primordium and fruiting body were assessed by Western blotting and immunohistochemistry. AAL was found to be ubiquitously expressed in the primordium and fruiting body but not in the mycelia. AAL facilitated mycelia congregation and promoted fruiting body production when AAL was applied on mycelia. At the same time, when AAL was spread on potato dextrose agar (PDA) medium prior to mycelia inoculation, mycelia exhibited slowed growth rates, resulting in mycelia cords formation and inhibition of fruiting body formation. The 5' regulatory sequence of aal was cloned by 'genome walking'. Here, we show that aal lack introns in the coding region and the upstream 740 bp sequence was characterized by the existence of core promoter elements, which included: two CCAAT boxes (-535/-280), a GC box (-145), a TATA box (-30) and a fungal leader intron within the 5' UTR. The identification of regulatory expression elements may provide an explanation to the stage-specific and high-level expression of aal during fruiting development.

Caenorhabditis elegans N-glycan core beta-galactoside confers sensitivity towards nematotoxic fungal galectin CGL2

The physiological role of fungal galectins has remained elusive. Here, we show that feeding of a mushroom galectin, Coprinopsis cinerea CGL2, to Caenorhabditis elegans inhibited development and reproduction and ultimately resulted in killing of this nematode. The lack of toxicity of a carbohydrate-binding defective CGL2 variant and the resistance of a C. elegans mutant defective in GDP-fucose biosynthesis suggested that CGL2-mediated nematotoxicity depends on the interaction between the galectin and a fucose-containing glycoconjugate. A screen for CGL2-resistant worm mutants identified this glycoconjugate as a Galbeta1,4Fucalpha1,6 modification of C. elegans N-glycan cores. Analysis of N-glycan structures in wild type and CGL2-resistant nematodes confirmed this finding and allowed the identification of a novel putative glycosyltransferase required for the biosynthesis of this glycoepitope. The X-ray crystal structure of a complex between CGL2 and the Galbeta1,4Fucalpha1,6GlcNAc trisaccharide at 1.5 A resolution revealed the biophysical basis for this interaction. Our results suggest that fungal galectins play a role in the defense of fungi against predators by binding to specific glycoconjugates of these organisms.

Molecular basis for galactosylation of core fucose residues in invertebrates: identification of caenorhabditis elegans N-glycan core alpha1,6-fucoside beta1,4-galactosyltransferase GALT-1 as a member of a novel glycosyltransferase family

Galectin CGL2 from the ink cap mushroom Coprinopsis cinerea displays toxicity toward the model nematode Caenorhabditis elegans. A mutation in a putative glycosyltransferase-encoding gene resulted in a CGL2-resistant C. elegans strain characterized by N-glycans lacking the beta1,4-galactoside linked to the alpha1,6-linked core fucose. Expression of the corresponding GALT-1 protein in insect cells was used to demonstrate a manganese-dependent galactosyltransferase activity. In vitro, the GALT-1 enzyme showed strong selectivity for acceptors with alpha1,6-linked N-glycan core fucosides and required Golgi- dependent modifications on the oligosaccharide antennae for optimal synthesis of the Gal-beta1,4-fucose structure. Phylogenetic analysis of the GALT-1 protein sequence identified a novel glycosyltransferase family (GT92) with members widespread among eukarya but absent in mammals.

Importance of nuclear localization for the apoptosis-induced activity of a fungal galectin AAL (Agrocybe aegerita lectin)

Agrocybe aegerita lectin (AAL) was identified previously in our group as a novel galectin from medicinal fungi Agrocybe aegerita, and has been shown to effectively induce cancer cell cycle arrest and apoptosis in vitro and tumor regression in vivo. Here, AAL was observed to translocate into the HeLa cell nucleus and induce cell apoptosis when it was predominantly in the nucleus. The N-terminus and C-terminus of AAL were required for nuclear localization. Site mutated proteins were generated based on AAL structure. Dimer interface mutant I25G, carbohydrate recognition domain (CRD) mutant R63H, and loop region mutant L33A could not enter the nucleus and lost the ability to induce apoptosis. CRD mutant H59Q and loop region mutant I144G maintained nuclear localization activity, and H59Q retained residual bioability but I144G had no activity, indicating that nuclear localization is important but not sufficient for AAL to become apoptotically active. Our findings provide a novel antitumor mechanism of fungal galectin.

Purification and characterization of a novel lectin from the toxic wild mushroom Inocybe umbrinella

From the dried fruiting bodies of the toxic mushroom Inocybe umbrinella, a novel lectin with a molecular mass of 17 kDa has been isolated with about 160-fold purification. The purification protocol comprised ion exchange chromatography on DEAE-cellulose, and CM-cellulose, and gel filtration on Superdex 75. Among the carbohydrates tested, raffinose, d-melibiose, alpha-lactose and d(+)-galactose could inhibit the hemagglutinating activity of the lectin. The hemagglutinating activity was stable between 10 and 60 degrees C, in 12.5-100mM HCl, and in 50mM NaOH. The hemagglutinating activity was inhibited by Ca(2+), Mn(2+)and Mg(2+) ions, but was unaffected by Fe(3+), Zn(2+) and Al(3+) ions. The lectin inhibited HIV-1 reverse transcriptase with an IC(50) of 4.7+/-0.2 microM. Proliferation of tumor cells including hepatoma HepG2 cells and breast cancer MCF7 cells was inhibited by the lectin with an IC(50) of 3.5+/-0.2 microM and 7.4+/-0.3 micoM, respectively. The lectin has a unique N-terminal amino acid sequence, DGVLATNAVA. It did not exhibit antifungal activity. The present report is the first on an Inocybe lectin and represents one of the very few reports on lectins from toxic mushrooms.

Structural Basis for the Carbohydrate Recognition of the Sclerotium rolfsii Lectin

The crystal structure of a novel fungal lectin from Sclerotium rolfsii (SRL) in its free form and in complex with N-acetyl-d-galactosamine (GalNAc) and N-acetyl- d -glucosamine (GlcNAc) has been determined at 1.1 A, 2.0 A, and 1.7 A resolution, respectively. The protein structure is composed of two beta-sheets, which consist of four and six beta-strands, connected by two alpha-helices. Sequence and structural comparisons reveal that SRL is the third member of a newly identified family of fungal lectins, which includes lectins from Agaricus bisporus and Xerocomus chrysenteron that share a high degree of structural similarity and carbohydrate specificity. The data for the free SRL are the highest resolution data for any protein of this family. The crystal structures of the SRL in complex with two carbohydrates, GalNAc and GlcNAc, which differ only in the configuration of a single epimeric hydroxyl group, provide the structural basis for its carbohydrate specificity. SRL has two distinct carbohydrate-binding sites, a primary and a secondary. GalNAc binds at the primary site, whereas GlcNAc binds only at the secondary site. Thus, SRL has the ability to recognize and probably bind at the same time two different carbohydrate structures. Structural comparison to Agaricus bisporus lectin-carbohydrate complexes reveals that the primary site is also able to bind the Thomsen-Friedenreich antigen (Galbeta1-->3GalNAc-alpha- glycan structures) whereas the secondary site cannot. The features of the molecular recognition at the two sites are described in detail.

Characterization of neutral and acidic glycosphingolipids from the lectin-producing mushroom, Polyporus squamosus

The polypore mushroom Polyporus squamosus is the source of a lectin that exhibits a general affinity for terminal beta-galactosides, but appears to have an extended carbohydrate-binding site with high affinity and strict specificity for the nonreducing terminal trisaccharide sequence NeuAcalpha2 --> 6Galbeta1 --> 4Glc/GlcNAc. In considering the possibility that the lectin's in vivo function could involve interaction with an endogenous glycoconjugate, it would clearly be helpful to identify candidate ligands among various classes of carbohydrate-containing materials expressed by P. squamosus. Since evidence has been accumulating that glycosphingolipids (GSLs) may serve as key ligands for some endogenous lectins in animal species, possible similar roles for fungal GSLs could be considered. For this study, total lipids were extracted from mature fruiting body of P. squamosus. Multistep fractionation yielded a major monohexosylceramide (CMH) component and three major glycosylinositol phosphorylceramides (GIPCs) from the neutral and acidic lipids, respectively. These were characterized by a variety of techniques as required, including one- and two-dimensional (1)H- and (13)C-nuclear magnetic resonance (NMR) spectroscopy; electrospray ionization-mass spectrometry (ESI-MS, tandem-MS/collision-induced decay-MS, and ion trap-MS(n)); and component and methylation linkage analysis by gas chromatography-mass spectrometry. The CMH was determined to be glucosylceramide having a typical ceramide consisting of 2-hydroxy fatty-N-acylated (4E,8E)-9-methyl-sphinga-4,8-dienine. The GIPCs were identified as Manalpha1 --> 2Ins1-P-1Cer (Ps-1), Galbeta1 --> 6Manalpha1 --> 2Ins1-P-1Cer (Ps-2), and Manalpha1 --> 3Fucalpha1 --> 2Galalpha1 --> 6Galbeta1 --> 6Manalpha1 -->2Ins1-P-1Cer (Ps-5), respectively (where Ins = myo-inositol, P = phosphodiester, and Cer = ceramide consisting mainly of long-chain 2-hydroxy and 2,3-dihydroxy fatty-N-acylated 4-hydroxy-sphinganines). Of these GSLs, Ps-2 could potentially interact with P. squamosus lectin, and further investigations will focus on determining the binding affinity, if any, of the lectin for the GIPCs isolated from this fungus.

The Sclerotinia sclerotiorum agglutinin represents a novel family of fungal lectins remotely related to the Clostridium botulinum non-toxin haemagglutinin HA33/A

Previous studies indicated that sclerotes of the phytopathogenic Ascomycete Sclerotinia sclerotiorum contain a lectin that based on its molecular structure, specificity and N-terminal amino acid sequence could not be classified yet into any lectin family. Using a combination of molecular cloning, frontal affinity chromatography and molecular modelling the identity of the S. sclerotiorum agglutinin (SSA) was analyzed. Molecular cloning demonstrated that SSA shares no sequence similarity with any known fungal lectin or protein. The lectin is synthesized as a 153 amino acid polypeptide without signal peptide and undergoes apart from the removal of the N-terminal methionine no further processing. Frontal affinity chromatography revealed that the binding site of SSA primarily accommodates a non-reducing terminal GalNAc with a preference for the alpha- over the beta-anomer. SSA also strongly interacts with both glycolipid type glycans with terminal non-reducing Gal or GalNAc and galactosylated N-glycans. SSA shares a residual sequence similarity with part of the non-toxin haemagglutinin HA33/A from Clostridium botulinum. Molecular modeling using the three-dimensional structure of HA33/A as a template indicated that SSA can fold into a similar beta-trefoil domain. Though these results should be interpreted with care it is tempting to speculate that the Sclerotiniaceae lectins thus appear to be structurally related to the ricin-B superfamily. All evidence suggests that SSA represents a novel family of fungal lectins with a unique sequence and sugar-binding properties. Taking into account that orthologues of SSA are fairly common within the family Sclerotiniaceae but could not be identified in any other fungal species one can reasonably conclude that SSA-type lectins are confined to a small taxonomic group of the Ascomycota.

Comparison of natural and recombinant clitocypins, the fungal cysteine protease inhibitors

A member of the cysteine protease inhibitor clitocypin gene family from basidiomycete Clitocybe nebularis was expressed in Escherichia coli. Following careful optimization of the expression procedure the active inhibitor was purified from inclusion bodies and its properties examined and compared to those of the natural clitocypin. The CD spectrum of recombinant clitocypin was similar to that of natural clitocypin, indicating that protein was properly refolded during purification. In spite of some differences in primary structure, structural, functional and immunological equivalence was established. Kinetic analyses of the natural and recombinant clitocypins were performed. Both clitocypins inhibited a range of cysteine proteases to a similar extent, and demonstrated an unusually broad inhibitory spectrum, including distantly related proteases, such as papain and legumain, belonging to different protease families. The homogenous, biologically active recombinant clitocypin is obtained at levels adequate for further structure-function studies.

Galectin-3 induces death of Candida species expressing specific beta-1,2-linked mannans

Lectins play a critical role in host protection against infection. The galectin family of lectins recognizes saccharide ligands on a variety of microbial pathogens, including viruses, bacteria, and parasites. Galectin-3, a galectin expressed by macrophages, dendritic cells, and epithelial cells, binds bacterial and parasitic pathogens including Leishmania major, Trypanosoma cruzi, and Neisseria gonorrhoeae. However, there have been no reports of galectins having direct effects on microbial viability. We found that galectin-3 bound only to Candida albicans species that bear beta-1,2-linked oligomannans on the cell surface, but did not bind Saccharomyces cerevisiae that lacks beta-1,2-linked oligomannans. Surprisingly, binding directly induced death of Candida species containing specific beta-1,2-linked oligomannosides. Thus, galectin-3 can act as a pattern recognition receptor that recognizes a unique pathogen-specific oligosaccharide sequence. This is the first description of antimicrobial activity for a member of the galectin family of mammalian lectins; unlike other lectins of the innate immune system that promote opsonization and phagocytosis, galectin-3 has direct fungicidal activity against opportunistic fungal pathogens.

Isolation and characterization of a novel lectin from the mushroom Armillaria luteo-virens

From the dried fruiting bodies of the mushroom Armillaria luteo-virens, a dimeric lectin with a molecular mass of 29.4 kDa has been isolated. The purification procedure involved (NH(4))(2)SO(4) precipitation, ion exchange chromatography on DEAE-cellulose, CM-cellulose, and Q-Sepharose, and gel filtration by fast protein liquid chromatography on Superdex 75. The hemagglutinating activity of the lectin could not be inhibited by simple sugars but was inhibited by the polysaccharide inulin. The activity was stable up to 70 degrees C but was acid- and alkali-labile. Salts including FeCl(3), AlCl(3), and ZnCl(2) inhibited the activity whereas MgCl(2), MnCl(2), and CaCl(2) did not. The lectin stimulated mitogenic response of mouse splenocytes with the maximal response achieved by 1microM lectin. Proliferation of tumor cells including MBL2 cells, HeLa cells, and L1210 cells was inhibited by the lectin with an IC(50) of 2.5, 5, and 10 microM, respectively. However, proliferation of HepG2 cells was not affected. The novel aspects of the isolated lectin include a novel N-terminal sequence, fair thermostability, acid stability, and alkali stability, together with potent mitogenic activity toward spleen cells and antiproliferative activity toward tumor cells.

A novel homodimeric lactose-binding lectin from the edible split gill medicinal mushroom Schizophyllum commune

A homodimeric lactose-binding lectin with a molecular mass of 64kDa was isolated from fresh fruiting bodies of the split gill mushroom Schizophyllum commune. The N-terminal sequence of the lectin is similar to a part of the sequence of the cell division protein from Gleobacter violaceus. The lectin was isolated by using a procedure which involved ion exchange chromatography on DEAE-cellulose, CM-cellulose, and Q-Sepharose, and gel filtration by fast protein liquid chromatography on Superdex 75. The hemagglutinating activity of the lectin was stable at temperatures up to 40 degrees C, and in concentrations of NaOH and HCl solution up to 125 and 25mM, respectively. The lectin exhibited potent mitogenic activity toward mouse splenocytes, antiproliferative activity toward tumor cell lines, and inhibitory activity toward HIV-1 reverse transcriptase. It was devoid of antifungal activity.

The gene for a lectin-like protein is transcriptionally activated during sexual development, but is not essential for fruiting body formation in the filamentous fungus Sordaria macrospora

Background

The filamentous fungus Sordaria macrospora forms complex three-dimensional fruiting bodies called perithecia that protect the developing ascospores and ensure their proper discharge. In previous microarray analyses, several genes have been identified that are downregulated in sterile mutants compared to the wild type. Among these genes was tap1 (transcript associated with perithecial development), a gene encoding a putative lectin homolog.

Results

Analysis of tap1 transcript levels in the wild type under conditions allowing only vegetative growth compared to conditions that lead to fruiting body development showed that tap1 is not only downregulated in developmental mutants but is also upregulated in the wild type during fruiting body development. We have cloned and sequenced a 3.2 kb fragment of genomic DNA containing the tap1 open reading frame and adjoining sequences. The genomic region comprising tap1 is syntenic to its homologous region in the closely related filamentous fungus Neurospora crassa. To determine whether tap1 is involved in fruiting body development in S. macrospora, a knockout construct was generated in which the tap1 open reading frame was replaced by the hygromycin B resistance gene hph under the control of fungal regulatory regions. Transformation of the S. macrospora wild type with this construct resulted in a tap1 deletion strain where tap1 had been replaced by the hph cassette. The knockout strain displayed no phenotypic differences under conditions of vegetative growth and sexual development when compared to the wild type. Double mutants carrying the Δtap1 allele in several developmental mutant backgrounds were phenotypically similar to the corresponding developmental mutant strains.

Conclusion

The tap1 transcript is strongly upregulated during sexual development in S. macrospora; however, analysis of a tap1 knockout strain shows that tap1 is not essential for fruiting body formation in S. macrospora.