Isolation, macromolecular properties, and combining site of a chito-oligosaccharide-specific lectin from the exudate of ridge gourd (Luffa acutangula)

Subunit association, and thermal and chemical unfolding of Cucurbitaceae phloem exudate lectins. A review

Detailed characterization of protein (un)folding intermediates is crucial for understanding the (un)folding pathway, aggregation, stability and their functional properties. In recent years, stress-inducible lectins are being investigated with much interest. In plants phloem proteins PP1 and PP2 are major components of the phloem fluid. While PP1 is a structural protein, PP2 exhibits lectin activity, and was proposed to play key roles in wound sealing, anti-pathogenic activity, and transportation of various molecules including RNA within the plant. Cucurbitaceae fruits contain high concentrations of PP2 lectins, which recognize chitooligosaccharides with high specificity. Although the presence of PP2 lectins in the phloem exudate of Cucurbitaceae species was documented over 40 years ago, so far only a few proteins from this family have been purified and characterized in detail. This review summarizes the results of biophysical studies aimed at investigating the oligomeric status of these lectins, their thermal stability, structural perturbations caused by changes in pH and addition of chaotropic agents and characterization of intermediates observed in the unfolding process. The implications of these results in the functional roles played by PP2 type lectins in their native environment are discussed. Finally, perspectives for future biophysical research on these proteins are given.

Structure and interactions of the phloem lectin (phloem protein 2) Cus17 from Cucumis sativus

Phloem protein 2 (PP2) contributes crucially to phloem-based defense in plants by binding to carbohydrates displayed by pathogens. However, its three-dimensional structure and the sugar binding site remained unexplored. Here, we report the crystal structure of the dimeric PP2 Cus17 from Cucumis sativus in its apo form and complexed with nitrobenzene, N-acetyllactosamine, and chitotriose. Each protomer of Cus17 consists of two antiparallel four-stranded twisted β sheets, a β hairpin, and three short helices forming a β sandwich architectural fold. This structural fold has not been previously observed in other plant lectin families. Structure analysis of the lectin-carbohydrate complexes reveals an extended carbohydrate binding site in Cus17, composed mostly of aromatic amino acids. Our studies suggest a highly conserved tertiary structure and a versatile binding site capable of recognizing motifs common to diverse glycans on plant pathogens/pests, which makes the PP2 family suited for phloem-based plant defense.

Cucurbitaceae phloem exudate lectins: Purification, molecular characterization and carbohydrate binding characteristics

Much of the plant lectin research was focused on these proteins from seeds, whereas lectins from other plant tissues have been less investigated. Although presence of lectins in the phloem exudate of Cucurbitaceae species was reported over 40 years ago, only a few proteins from this family have been purified and characterized with respect to ligand binding properties, primary and secondary structures, while no 3D structure of a member of this family is known so far. Unlike lectins from other plant families and sources (e.g., seeds and tubers), which exhibit specificity towards different carbohydrate structures, all the Cucurbitaceae phloem exudate lectins characterized so far have been shown to recognize only chitooligosaccharides or glycans containing chitooligosaccharides. Interestingly, some of these proteins also bind various types of RNAs, suggesting that they may also play a role in the transport of RNA information molecules in the phloem. The present review gives an overview of the current knowledge of Cucurbitaceae phloem exudate lectins with regard to their purification, determination of primary and secondary structures, elucidation of thermodynamics and kinetics of carbohydrate binding and computational modeling to get information on their 3D structures. Finally, future perspectives of research on this important class of proteins are considered.

DSC and FCS Studies Reveal the Mechanism of Thermal and Chemical Unfolding of CIA17, a Polydisperse Oligomeric Protein from Coccinia Indica

The mechanism of thermal and chemical unfolding of Coccinia indica agglutinin (CIA17), a chitooligosacharide-specific phloem exudate lectin, was investigated by biophysical approaches. DSC studies revealed that the unfolding thermogram of CIA17 consists of three components (T_m ∼ 98, 106, and 109 °C), which could be attributed to the dissociation of protein oligomers into constituent dimers, dissociation of the dimers into monomers, and unfolding of the monomers. Intrinsic fluorescence studies on the chemical denaturation by guanidinium thiocyanate and guanidinium chloride indicated the presence of two distinct steps in the unfolding pathway, which could be assigned to dissociation of the dimeric protein into monomers and unfolding of the monomers. Results of fluorescence correlation spectroscopic studies could be interpreted in terms of the following model: CIA17 forms oligomeric structures in a concentration dependent manner, with the protein existing as a monomer below 1 nM concentration but associating to form dimers at higher concentrations (K_D ≈ 2.9 nM). The dimers associate to yield tetramers with a K_D of ∼50 μM, which further associate to form higher oligomers with further increase in concentration. These results are consistent with the proposed role of CIA17 as a key player in the defense response of the plant against microbes and insects.

Purification, biochemical/biophysical characterization and chitooligosaccharide binding to BGL24, a new PP2-type phloem exudate lectin from bottle gourd (Lagenaria siceraria)

Phloem Protein 2 (PP2), highly abundant in the sieve elements of plants, plays a significant role in wound sealing and anti-pathogenic responses. In this study, we report the purification and characterization of a new PP2-type lectin, BGL24 from the phloem exudate of bottle gourd (Lagenaria siceraria). BGL24 is a homodimer with a subunit mass of ~24 kDa and exhibits high specificity for chitooligosaccharides. The isoelectric point of BGL24 was estimated from zeta potential measurements as 5.95. Partial amino acid sequence obtained by mass spectrometric studies indicated that BGL24 exhibits extensive homology with other PP2-type phloem exudate lectins. CD spectroscopic measurements revealed that the lectin contains predominantly β-sheets, with low α-helical content. CD spectroscopic and DSC studies showed that BGL24 exhibits high thermal stability with an unfolding temperature of ~82 °C, and that its secondary structure is essentially unaltered between pH 3.0 and 8.0. Fluorescence titrations employing 4-methylumbelliferyl-β-D-N,N',N″-triacetylchitotrioside as an indicator ligand revealed that the association constants for BGL24-chitooligosaccharide interaction increase considerably when the ligand size is increased from chitotriose to chitotetraose, whereas only marginal increase was observed for chitopentaose and chitohexaose. BGL24 exhibited moderate cytotoxicity against MDA-MB-231 breast cancer cells, whereas its effect on normal splenocytes was marginal.

The interaction of N-trifluoroacetylgalactosamine and its derivatives with winged bean (Psophocarpus tetragonolobus) basic agglutinin reveals differential mechanism of their recognition: a fluorine-19 nuclear magnetic resonance study

Here, we show the binding results of a leguminosae lectin, winged bean basic agglutinin (WBA I) to N-trifluoroacetylgalactosamine (NTFAGalN), methyl-α-N-trifluoroacetylgalactosamine (MeαNTFAGalN) and methyl-β-tifluoroacetylgalactosamine (MeβNTFAGalN) using (19) F NMR spectroscopy. No chemical shift difference between the free and bound states for NTFAGalN and MeβNTFAGalN, and 0.01-ppm chemical shift change for MeαNTFAGalN, demonstrate that the MeαNTFAGalN has a sufficiently long residence time on the protein binding site as compared to MeβNTFAGalN and the free anomers of NTFAGalN. The sugar anomers were found in slow exchange with the binding site of agglutinin. Consequently, we obtained their binding parameters to the protein using line shape analyses. Aforementioned analyses of the activation parameters for the interactions of these saccharides indicate that the binding of α and β anomers of NTFAGalN and MeαNTFAGalN is controlled enthalpically, while that of MeβNTFAGalN is controlled entropically. This asserts the sterically constrained nature of the interaction of the MeβNTFAGalN with WBA I. These studies thus highlight a significant role of the conformation of the monosaccharide ligands for their recognition by WBA I.

Ribosome-inactivating and related proteins

Ribosome-inactivating proteins (RIPs) are toxins that act as N-glycosidases (EC 3.2.2.22). They are mainly produced by plants and classified as type 1 RIPs and type 2 RIPs. There are also RIPs and RIP related proteins that cannot be grouped into the classical type 1 and type 2 RIPs because of their different sizes, structures or functions. In addition, there is still not a uniform nomenclature or classification existing for RIPs. In this review, we give the current status of all known plant RIPs and we make a suggestion about how to unify those RIPs and RIP related proteins that cannot be classified as type 1 or type 2 RIPs.

Mutational analysis of the carbohydrate binding activity of the tobacco lectin

At present the three-dimensional structure of the tobacco lectin, further referred to as Nictaba, and its carbohydrate-binding site are unresolved. In this paper, we propose a three-dimensional model for the Nictaba domain based on the homology between Nictaba and the carbohydrate-binding module 22 of Clostridium thermocellum Xyn10B. The suggested model nicely fits with results from circular dichroism experiments, indicating that Nictaba consists mainly of β-sheet. In addition, the previously identified nuclear localization signal is located at the top of the protein as a part of a protruding loop. Judging from this model and sequence alignments with closely related proteins, conserved glutamic acid and tryptophan residues in the Nictaba sequence were selected for mutational analysis. The mutant DNA sequences as well as the original Nictaba sequence have been expressed in Pichia pastoris and the recombinant proteins were purified from the culture medium. Subsequently, the recombinant proteins were characterized and their carbohydrate binding properties analyzed with glycan array technology. It was shown that mutation of glutamic acid residues in the C-terminal half of the protein did not alter the carbohydrate-binding activity of the lectin. In contrast, mutation of tryptophan residues in the N-terminal half of the Nictaba domain resulted in a complete loss of carbohydrate binding activity. These results suggest that tryptophan residues play an important role in the carbohydrate binding site of Nictaba.

Rapid affinity-purification and physicochemical characterization of pumpkin (Cucurbita maxima) phloem exudate lectin

The chito-oligosaccharide-specific lectin from pumpkin (Cucurbita maxima) phloem exudate has been purified to homogeneity by affinity chromatography on chitin. After SDS/PAGE in the presence of 2-mercaptoethanol, the pumpkin phloem lectin yielded a single band corresponding to a molecular mass of 23.7 kDa, whereas ESI-MS (electrospray ionization MS) gave the molecular masses of the subunit as 24645 Da. Analysis of the CD spectrum of the protein indicated that the secondary structure of the lectin consists of 9.7% α-helix, 35.8% β-sheet, 22.5% β-turn and 32.3% unordered structure. Saccharide binding did not significantly affect the secondary and tertiary structures of the protein. The haemagglutinating activity of pumpkin phloem lectin was mostly unaffected in the temperature range 4–70 °C, but a sharp decrease was seen between 75 and 85 °C. Differential scanning calorimetric and CD spectroscopic studies suggest that the lectin undergoes a co-operative thermal unfolding process centred at approx. 81.5 °C, indicating that it is a relatively stable protein.

Synthesis of chitotetraose and chitohexaose based on dimethylmaleoyl protection

tert-Butyldimethylsilyl 3,6-di-O-benzyl-2-deoxy-2-dimethylmaleimido-beta-D-glucopyranoside was readily transformed into the disaccharide glycosyl donor, 3,4,6-tri-O-acetyl-2-deoxy-2-dimethylmaleimido-beta-D-glucopyranosyl-(1 --> 4)-3,6-di-O-benzyl-2-deoxy-2-dimethylmaleimido-alpha/beta-D-glucopyranosyl trichloroacetimidate, and the disaccharide glycosyl acceptor, tert-butyldimethylsilyl 3,6-di-O-benzyl-2-deoxy-2-dimethylmaleimido-beta-D-glucopyranosyl-(1 --> 4)-3,6-di-O-benzyl-2-deoxy-2-dimethylmaleimido-beta-D-glucopyranoside. A TMSOTf-catalysed coupling of the acceptor with the donor afforded the respective tetrasaccharide derivative, which can be transformed to chitotetraose. tert-Butyldimethylsilyl 3,6-di-O-benzyl-2-deoxy-2-dimethylmaleimido-4-O-phenoxyacetyl-beta-D-glucopyranosyl-(1 --> 4)-3,6-di-O-benzyl-2-deoxy-2-dimethylmaleimido-beta-D-glucopyranoside was converted into donor 3,6-di-O-benzyl-2-deoxy-2-dimethylmaleimido-4-O-phenoxyacetyl-beta-D-glucopyranosyl-(1 --> 4)-3,6-di-O-benzyl-2-deoxy-2-dimethylmaleimido-beta-D-glucopyranosyl trichloroacetimidate. Its coupling with benzyl 3,6-di-O-benzyl-2-deoxy-2-dimethylmaleimido-beta-D-glucopyranosyl-(1 --> 4)-3,6-di-O-benzyl-2-deoxy-2-dimethylmaleimido-beta-D-glucopyranoside, followed by dephenoxyacetylation, gave benzyl 3,6-di-O-benzyl-2-deoxy-2-dimethylmaleimido-beta-D-glucopyranosyl-(1 --> 4)-3,6-di-O-benzyl-2-deoxy-2-dimethylmaleimido-beta-D-glucopyranosyl-(1 --> 4)-3,6-di-O-benzyl-2-deoxy-2-dimethylmaleimido-beta-D-glucopyranosyl-(1 --> 4)-3,6-di-O-benzyl-2-deoxy-2-dimethylmaleimido-beta-D-glucopyranoside, whose glycosylation furnished, after replacement of the DMM-group by the acetyl moiety and subsequent deprotection, chitohexaose.

Classification of plant lectins in families of structurally and evolutionary related proteins

The majority of plant lectins can be classified in seven families of structurally and evolutionary related proteins. Within a given lectin family most but not necessarily all members are built up of protomers with a similar primary structure and overall 3-D fold. The overall structure of the native lectins is not only determined by the structure of the protomers but depends also on the degree of oligomerization and in some cases on the post-translational processing of the lectin precursors. In general, lectin families are fairly homogeneous for what concerns the overall specificity of the individual lectins, which illustrates that the 3-D structure of the binding site has been conserved during evolution. In the case of the jacalin-related lectins the occurrence of a mannose- and galactose-binding subfamily can be explained by the fact that a post-translational cleavage of the protomers (of the galactose-binding subfamily) yields a slightly altered binding site. Unlike the other families, the legume lectins display a wide range of specificites, which is clearly reflected in the occurrence of sugar-binding sites with a different 3-D structure.

On the relationship of thermodynamic parameters with the buried surface area in protein-ligand complex formation

Prediction of thermodynamic parameters of protein-protein and antigen-antibody complex formation from high resolution structural parameters has recently received much attention, since an understanding of the contributions of different fundamental processes like hydrophobic interactions, hydrogen bonding, salt bridge formation, solvent reorganization etc. to the overall thermodynamic parameters and their relations with the structural parameters would lead to rational drug design. Using the results of the dissolution of hydrocarbons and other model compounds the changes in heat capacity (delta C(p)), enthalpy (delta H) and entropy (delta S) have been empirically correlated with the polar and apolar surface areas buried during the process of protein folding/unfolding and protein-ligand complex formation. In this regard, the polar and apolar surfaces removed from the solvent in a protein-ligand complex have been calculated from the experimentally observed values of changes in heat capacity (delta C(p)) and enthalpy (delta H) for protein-ligand complexes for which accurate thermodynamic and high resolution structural data are available, and the results have been compared with the x-ray crystallographic observations. Analyses of the available results show poor correlation between the thermodynamic and structural parameters. Probable reasons for this discrepancy are mostly related with the reorganization of water accompanying the reaction which is indeed proven by the analyses of the energetics of the binding of the wheat germ agglutinin to oligosaccharides.

Glycosidase-catalysed oligosaccharide synthesis: preparation of N-acetylchitooligosaccharides using the beta-N-acetylhexosaminidase of Aspergillus oryzae

The beta-N-acetylhexosaminidase of Aspergillus oryzae catalyses the formation of 2-acetamido-4-O-(2-acetamido-2-deoxy-beta-D-glucopyranosyl)-2-deoxy-D- glucopyranose (di-N-acetylchitobiose) and 2-acetamido-6-O-(2-acetamido-2-deoxy-beta-D-glucopyranosyl)-2-deoxy-D- glucopyranose from p-nitrophenyl 2-acetamido-2-deoxy-beta-D-glucopyranoside and 2-acetamido-2-deoxy-D-glucopyranose. The ratio of the two disaccharides is time-dependent. The ratio of (1-->4)- to (1-->6)-isomers is a maximum (approximately 9:1) at the point of disappearance of the glycosyl donor. If left to evolve, the ratio changes to 92:8 in favour of the (1-->6)-isomer. Either the (1-->4)- or the (1-->6)-isomer can be isolated by treating the appropriately enriched dissaccharide mixture with the beta-N-acetylhexosaminidase of Jack bean (Canavalia ensiformis) or the beta-N-acetylhexosaminidase of A. oryzae, respectively. Di-N-acetylchitobiose [GlcNAc(beta 1-4)GlcNAc] is an efficient donor of 2-acetamido-2-deoxy-D-glucopyranosyl units in reactions catalysed by the N-acetylhexosaminidase of A. oryzae. Di-N-acetylchitobiose itself acts as acceptor to give tri-N-acetylchitotriose [GlcNAc(beta 1-4)GlcNAc(beta 1-4)GlcNAc]. As the trisaccharide accumulates it, in turn, acts as acceptor giving tetra-N-acetylchitotetraose [GlcNAc(beta 1-4)GlcNAc(beta 1-4)GlcNAc(beta 1-4)GlcNAc]. The product mixture consisting of mono-, di-, tri-, and tetrasaccharides is conveniently separated by charcoal-Celite chromatography.

Organization and characterization of Cucurbita phloem lectin genes

The phloem of pumpkin and squash contains a dimeric chitin-binding lectin called PP2 (phloem protein 2). We have isolated three genomic clones from pumpkin (Cucurbita maxima Duch.) that encoded PP2. One clone, lambda gPC13-1, contained two PP2 genes that were 99.8% identical over a region of 3055 nucleotides. This conserved region included 1922 bp of 5' non-coding sequence, 844 bp of protein coding sequence (including two introns), and 289 bp of 3' non-coding sequence. To examine the conservation of the phloem lectin within the genus Cucurbita, we analyzed nine different species for PP2, its mRNA, and the genes that encode PP2. DNA blot analysis indicated that each species contained genes that encoded PP2, however, there was considerable restriction fragment length polymorphism (RFLP) among the species. PP2 gene copy number reconstructions indicated that PP2 is encoded by a small gene family (two to eight genes). Although a high level of PP2 DNA polymorphism existed among species, a single mRNA (ca. 1 kb) was detected in each species. PP2, affinity-purified from the vascular exudate of each species, reacted with PP2-specific antibodies; five species contained a single PP2 polypeptide while four species contained two PP2 polypeptides.

Purification, characterisation, and carbohydrate specificity of the lectin of Ficus cunia

A lectin, isolated from the seeds of Ficus cunia and purified by affinity chromatography on fetuin-Sepharose, was homogeneous in PAGE, GPC, HPLC, and immunodiffusion, and had mol wt of 3200-3500. In SDS-PAGE and HPLC in the absence and presence of 2-mercaptoethanol, the lectin gave a single band or peak corresponding to M(r) 3300-3500, thus indicating it to be a monomer. The lectin agglutinated human erythrocytes regardless of blood group, bound to Ehrlich ascites cells and to human rat spermatozoa, and was thermally stable; its activity was enhanced by Ca2+. The lectin is a metalloprotein that was inactivated by dialysis with EDTA followed by acetic acid, but reactivated by the addition of Ca2+. The lectin contained 2.0% of carbohydrates, large proportions of acidic amino acids, but little methionine. In hapten-inhibition assays, chitin oligosaccharides [(1-->4)-linked beta-GlcNAc] and N-acetyl-lactosamine were inhibitors of which N,N',N",N"'-tetra-acetylchitotetraose was the most potent. Among the macromolecules tested that contain either multiple N-acetyl-lactosamine and/or (1-->4)/(1-->6)-linked beta-GlcNAc, asialofetuin glycopeptide was the most potent inhibitor. Thus, an N-acetyl group and substitution at C-1 of D-GlcN are necessary for binding.

Proteins with abortifacient, ribosome inactivating, immunomodulatory, antitumor and anti-AIDS activities from Cucurbitaceae plants

1. The biochemical characteristics and biological activities of eight Cucurbitaceae plant proteins designated trichosanthin (isolated from tubers of Trichosanthes kirilowii), beta-trichosanthin (isolated from tubers of Trichosanthes cucumeroides), alpha- and beta-momorcharins (isolated from seeds of Momordica charantia), momorchochin (isolated from tubers of Momordica cochinchinensis), luffaculin (isolated from seeds of Luffa acutangula) and luffin-a and luffin-b (isolated from seeds of Luffa cylindrica), were reviewed. 2. The isolation procedures for all eight proteins are based on aqueous extraction, acetone fractionation and ion exchange chromatography. Ammonium sulfate precipitation and gel filtration are steps which may be included to improve purification. 3. The proteins are basic in nature and possess a molecular weight of approx. 30,000. All except trichosanthin are glycoproteins. The content of Asx and Glx residues is high. The N-terminal amino acid residue is Asp. Their amino acid compositions and N-terminal amino acid sequences are similar. 4. Circular dichroism spectroscopic studies revealed that trichosanthin, alpha- and beta-momorcharins possess similar secondary but different tertiary structures. 5. Most of the proteins are immunologically distinct. 6. The proteins exhibit abortifacient, antitumor, ribosome inactivating and immunomodulatory activities. Trichosanthin manifests anti-human immunodeficiency virus activity.

Isolation of a ribosome-inactivating and abortifacient protein from seeds of Luffa acutangula

A glycoprotein with a molecular weight of 28,000 as estimated by SDS-polyacrylamide gel electrophoresis was isolated from seeds of Luffa acutangula using a procedure that involved acetone precipitation, ion exchange chromatography on CM Sepharose CL-6B and gel filtration on Sephadex G-50. In immunodiffusion studies it was found to be immunologically distinct from abortifacient proteins isolated from other members of the Cucurbitaceae family including Momordica charantia, Momordica cochinchinensis, Trichosanthes kirilowii and Trichosanthes cucumeroides. There were some differences in amino acid composition among the proteins although there was a gross similarity. The protein from L. acutangula was capable of inducing mid-term abortion in mice and inhibiting protein synthesis in a cell-free system.

The binding of N-trifluoroacetyl chito-oligosaccharides to wheat-germ agglutinin: a fluorescence investigation

We describe the synthesis of N-trifluoroacetyl chito-oligosaccharides and their use as ligands to probe the binding sites of wheat-germ agglutinin, a lectin specific for N-acetylglucosamine. The binding is monitored using intrinsic protein fluorescence, which is due to tryptophan side-chains. We present arguments purporting to show the presence of a fluorophore close to each of the four sites. The binding of chito-oligosaccharides to wheat-germ agglutinin is complex and can only be approximately described by an independent and equivalent sites model. This model applies when the ligand concentration range is restricted to higher values. The possible role of ligand-mediated protein aggregation and of site inequivalence is discussed. We find that the affinity of trifluoroacetylated chito-oligosaccharides for wheat-germ agglutinin is higher than that of the N-acetylated parent compounds, the difference increasing with chain length. Our results are in agreement with a model of the binding site previously proposed by Clegg et al. (Biochemistry 22 (1983) 4797-4804).