Non-carbohydrate binding partners/domains of animal lectins

What is the Sugar Code?

A code is defined by the nature of the symbols, which are used to generate information-storing combinations (e. g. oligo- and polymers). Like nucleic acids and proteins, oligo- and polysaccharides are ubiquitous, and they are a biochemical platform for establishing molecular messages. Of note, the letters of the sugar code system (third alphabet of life) excel in coding capacity by making an unsurpassed versatility for isomer (code word) formation possible by variability in anomery and linkage position of the glycosidic bond, ring size and branching. The enzymatic machinery for glycan biosynthesis (writers) realizes this enormous potential for building a large vocabulary. It includes possibilities for dynamic editing/erasing as known from nucleic acids and proteins. Matching the glycome diversity, a large panel of sugar receptors (lectins) has developed based on more than a dozen folds. Lectins 'read' the glycan-encoded information. Hydrogen/coordination bonding and ionic pairing together with stacking and C-H/π-interactions as well as modes of spatial glycan presentation underlie the selectivity and specificity of glycan-lectin recognition. Modular design of lectins together with glycan display and the nature of the cognate glycoconjugate account for the large number of post-binding events. They give an entry to the glycan vocabulary its functional, often context-dependent meaning(s), hereby building the dictionary of the sugar code.

Proteomimetic surface fragments distinguish targets by function

The fragment-centric design promises a means to develop complex xenobiotic protein surface mimetics, but it is challenging to find locally biomimetic structures. To address this issue, foldameric local surface mimetic (LSM) libraries were constructed. Protein affinity patterns, ligand promiscuity and protein druggability were evaluated using pull-down data for targets with various interaction tendencies and levels of homology. LSM probes based on H14 helices exhibited sufficient binding affinities for the detection of both orthosteric and non-orthosteric spots, and overall binding tendencies correlated with the magnitude of the target interactome. Binding was driven by two proteinogenic side chains and LSM probes could distinguish structurally similar proteins with different functions, indicating limited promiscuity. Binding patterns displayed similar side chain enrichment values to those for native protein-protein interfaces implying locally biomimetic behavior. These analyses suggest that in a fragment-centric approach foldameric LSMs can serve as useful probes and building blocks for undruggable protein interfaces.

An overview on human serum lectins

An extensive literature survey done on the various naturally occurring lectins in human serum upon its salient features such as methods of detection, level and sites of synthesis, binding specificity, cation dependency, modes of isolation, molecular and functional characterization way back from 1930s to till date was presented in a tabulated section. In addition, the generation of lectin and other immune molecules in vertebrates upon treatment with exogenous elicitors has also been framed in a tabular form. Furthermore, ANEW lectin induced in human serum for the very first time by an exogenous elicitor was detected, isolated and characterized by us whose features are also tabulated explicitly.

Gallic acid binding to Spatholobus parviflorus lectin provides insight to its quaternary structure forming

Therapeutic effects of gallic acid (GA) have already been extensively studied. However, its interaction with lectins has not gained much attention. It is of interest to validate the binding profile of GA with Spatholobus parviflorus seed lectin. A combination of Isothermal Titration Calorimetry (ITC), haemagglutination assay and molecular docking was applied on SPL-GA interaction. ITC results showed four binding sites, stoichiometry, n=4, irrespective of the ratio of SPL:GA taken for titration. Difference among the four binding sites of a single molecule of SPL with regard to GA binding kinetic parameters was consistently varying. Similarly, the glide scores obtained for GA in the four different binding clefts of SPL were also conformed to the ITC. The binding of GA on SPL without affecting its sugar binding property could be considered as a boon for glycobiological research. From the presented studies, it could be proposed that the SPL-GA interactions may facilitate drug delivery by specific targeting/attachment by profiling of cell-surface glycans, followed by controlled release of drugs.

Network monitoring of adhesion/growth-regulatory galectins: localization of the five canonical chicken proteins in embryonic and maturing bone and cartilage and their introduction as histochemical tools

Divergence from an ancestral gene leads to a family of homologous proteins. Whether they are physiologically distinct, similar, or even redundant is an open question in each case. Defining profiles of tissue localization is a step toward giving diversity a functional meaning. Due to the significance of endogenous sugar receptors (lectins) as effectors for a wide range of cellular activities we have focused on galectins. The comparatively low level of network complexity constituted by only five canonical proteins makes chicken galectins (CGs) an attractive choice to perform comprehensive analysis, here studied on bone/cartilage as organ system. Galectin expression was monitored by Western blotting and immunohistochemistry using non-cross-reactive antibodies. Overall, three galectins (CG-1B, CG-3, CG-8) were present with individual expression patterns, one was found exclusively in the mesenchyme (CG-1A), the fifth (CG-2) not being detectable. The documented extents of separation are a sign for functional divergence; in cases with overlapping stainings, as for example in the osteoprogenitor layer or periosteum, cooperation may also be possible. Recombinant production enabled the introduction of the endogenous lectins as tools for binding-site localization. Their testing revealed developmental regulation and cell-type-specific staining. Of relevance for research on mammalian galectins, this study illustrates that certain cell types can express more than one galectin, letting functional interrelationships appear likely. Thus, complete network analysis irrespective of its degree of complexity is mandatory.

New Activity of a Protein from Canavalia ensiformis

Concanavalin A is a legume lectin which preferentially agglutinates transformed cells and shows antitumor effects on human breast carcinoma cells in vitro and in vivo. It is considered as a new potential antineoplastic agent targeting apoptosis, autophagy, and anti-angiogenesis in preclinical or clinical trials for cancer therapeutics, which has recently become the object of intensive study. In the present investigation, we show the capacity of the lectin to bind manganese, gold, iron, and zinc porphyrins: all potential anticancer agents. The interaction of the legume lectin with the studied compounds has been investigated by tryptophan fluorescence, showing conformational changes within the quaternary and tertiary structures of the protein. The binding of Con A with manganese, gold, and iron porphyrins, as well as adenine, was studied by fluorescence quenching. In contrast, the interaction of Con A with zinc porphyrin caused an increase in Trp fluorescence and a red shift of 10 nm of the emission maximum position.

However, the binding of Con A to iron porphyrin was accompanied by a 5 nm blue shift of the emission maximum, and a k_D of 0.95 ± 0.13 μM was calculated, respectively. The sigmoidal shape of the curve showed cooperative interactions, which indicated the presence of more than one class of binding site within the Con A molecule for iron porphyrin, confirmed by the Hill slope (h = 1.89±0.46). We have found that the legume lectin interacts with porphyrins and adenine with an affinity (0.14–1.89 µM) similar to that of the non-legume lectin, wheat germ agglutinin. In conclusion, the protein Con A shows new binding activity towards porphyrins with anticancer activities and could find prospective application as a drug delivery molecule that specifically targets cancer cells.

Targeting the glycoproteome

Despite numerous original publications describing the structural complexity of N- and O-linked glycans on glycoproteins, only very few answer the basic question of which particular glycans are linked to which amino acid residues along the polypeptide chain. Such structural information is of fundamental importance for understanding the biological roles of complex glycosylations as well as deciphering their non-template driven biosynthesis. This review focuses on presenting and commenting on recent strategies, specifically aimed at identifying the glycoproteome of cultured cells and biological samples, using targeted and global enrichment procedures and utilizing the high resolution power, high through-put capacity and complementary fragmentation techniques of tandem mass spectrometry. The goal is to give an update of this emerging field of protein and glyco-sciences and suggest routes to bridge the data gap between the two aspects of glycoprotein characteristics, i.e. glycan structures and their attachment sites.

Human galectin-3 interacts with two anticancer drugs

Human galectin-3 (hGal-3) is a mammalian lectin involved in regulation of RNA splicing, apoptosis, cell differentiation, and proliferation. Multimerized extracellular hGal-3 is thought to crosslink cells by binding to glycoproteins and glycosylated cancer antigens on the cell surface or extracellular matrix. Fluorescence spectroscopy and circular dichroism were used to study the interaction of hGal-3 with two anticancer agents: bohemine and Zn porphyrin (ZnTPPS(4)). The dissociation constant (k(D)) for binding of bohemine with hGal-3 was k(D) 0.23+/-0.05 microM. The hyperbolic titration curve indicated the presence of a single bohemine binding site. The binding of ZnTPPS(4) to hGal-3 (with and without lactose) is of high affinity having k(D)=0.18-0.20 microM and is not inhibited by lactose, indicating that ZnTPPS(4) and carbohydrate bind different sites. Circular dichroism spectra of the hGal-3 complexes suggested that the binding of the hydrophobic compounds changed the hGal-3 secondary structure. In summary, we show that two compounds with anticancer activity, bohemine and ZnTPPS(4), have high affinity for hGal-3 at a site that is distinct from its carbohydrate site. Since hGal-3 binds to several carbohydrate cancer antigens, the results suggest that it may have utility in the targeted delivery of drugs for cancer.

Multivalent binding of carbohydrates by the human alpha-defensin, HD5

Four of the six human alpha-defensins (human neutrophil peptides 1-3 and human alpha-defensin 5; HD5) have a lectin-like ability to bind glycosylated proteins. Using HD5 as a model, we applied surface plasmon resonance techniques to gain insights into this property. HD5 bound natural glycoproteins > neoglycoproteins based on BSA > nonglycosylated BSA >> free sugars. The affinity of HD5 for simple sugars covalently bound to BSA was orders of magnitude greater than its affinity for the same sugars in solution. The affinity of HD5 for protein-bound carbohydrates resulted from multivalent interactions which may also involve noncarbohydrate residues of the proteins. HD5 showed concentration-dependent self-association that began at submicromolar concentrations and proceeded to dimer and tetramer formation at concentrations below 5 microM. The (R9A, R28A) and (R13A, R32A) analogs of HD5 showed greatly reduced self-association as well as minimal binding to BSA and to BSA-affixed sugars. From this and other evidence, we conclude that the extensive binding of HD5 to (neo)glycoproteins results from multivalent nonspecific interactions of individual HD5 molecules with carbohydrate and noncarbohydrate moieties of the target molecule and that the primary binding events are magnified and enhanced by subsequent in situ assembly and oligomerization of HD5. Self-association and multivalent binding may play integral roles in the ability of HD5 to protect against infections caused by viruses and other infectious agents.

CEL-I, an N-acetylgalactosamine (GalNAc)-specific C-type lectin, induces nitric oxide production in RAW264.7 mouse macrophage cell line

We found that CEL-I, a GalNAc-specific C-type lectin isolated from the marine invertebrate Holothuroidea (Cucumaria echinata), induces inducible nitric oxide synthase (iNOS) expression and NO production in RAW264.7 cells. The NO production was inhibited by an iNOS inhibitor, L-NAME, but was not by a lipopolysaccharide (LPS) inhibitor, polymyxin B. In the presence of 0.1-M GalNAc, increased NO production by CEL-I-treated RAW264.7 cells was observed rather than the inhibition. Bovine serum albumin (BSA) significantly inhibited the CEL-I-induced NO production as well as the binding of FITC-labelled CEL-I on RAW264.7 cells. Three MAP kinase inhibitors (specific to extra-cellular regulated kinase, c-jun NH(2)-terminal kinase and p38 MAP kinase) inhibited CEL-I-induced NO production with different extents. Heat-treatment of CEL-I resulted in a decreased activity of CEL-I depending on the temperature. These results suggest that CEL-I induces NO production in RAW264.7 cells through the protein-cell interaction rather than the binding to the specific carbohydrate chains on the cell surface.

Fluorescence study of steroid hormone binding activity of Helix pomatia agglutinin

Helix pomatia agglutinin (HPA) is a N-acetylgalactosamine (GalNAc) binding lectin, found in the reproductive gland of a Roman snail. The present study has shown that HPA, in addition to its carbohydrate binding capacity possesses a hydrophobic binding activity. This protein binds with high affinity (k(D)=1.9-2.4 microM) steroid hormones: testosterone and progesterone, identified as putative ligands for the animal lectin HPA. Additionally, we have found that this lectin also interacts with adenine (k(D)=5.4+/-0.5 microM) and arylaminonaphthalene sulfonate TNS (k(D)=12+/-0.3 microM). Binding of HPA to hormones and adenine was accompanied by a significant increase of the intrinsic Trp fluorescence (up to 50%), characterizing the conformational changes in the lectin molecule. The hyperbolic shape of the binding curves indicated one high affinity site for the two steroid hormones and adenine, and more than one hydrophobic site for TNS, showed by the sigmoidal curve fit and Hill coefficient of (n(H)=1.5+/-0.2). Hormones and adenine compete for an identical binding site, suggested to occupy the central hydrophobic cavity of the HPA hexamer. Fluorescence resonance energy transfer (FRET) was applied to calculate the intramolecular distance between TNS and Trp chromophores.

Structural studies on mannose-selective glycoprotein receptors using molecular modeling techniques

Glycoproteins play important roles in various cellular events and their presence in appropriate locations in proper active conformations is essential for many biochemical functions. Recent evidences suggest that some glycoproteins may require sorting receptors for efficient exit from the endoplasmic reticulum. These receptors need the presence of calcium or other metal ions for their native activity. The three-dimensional structure of such a receptor, p58/ERGIC-53, has been recently solved by x-ray crystallography, which is a mannose-selective lectin and contains two Ca(2+) ions. Homology search in the sequence databases indicates a large number of proteins which bear varying degrees of homology in a wide spectrum of species with this receptor. In this study we have systematically searched for such genes which are potential candidates for acting as mannose-mediated glycoprotein receptors in various species as initially inferred from their amino acid sequence homology. Structures of a number of proteins have been predicted using knowledge-based homology modeling, and their ability to act as the glycoprotein receptor has been explored by examining the nature of sugar-binding site. Tetramer of mannose was docked in the binding pockets of the modeled structures followed by energy minimization and molecular dynamics to obtain most probable structures of the complexes. Properties of these modeled complexes were studied to examine the nature of physicochemical forces involved in the complex formation and compared with p58/ERGIC-53-mannose complex.

Immunomodulatory role of native and heat denatured agglutinin from Abrus precatorius

Lectins are known as polyclonal activators of lymphocytes and work through the induction of battery of cytokines, which vary from lectin to lectin. Most widely used biological response modifier Mistletoe lectin (ML-1) in therapy stimulates lymphocytes, macrophages, and natural killer cells and induces both TH1 and TH2 type cytokines. Abrus agglutinin, similar to ML-1 with respect to carbohydrate specificity [gal (beta1-->3) gal/Nac], was studied both in native (NA) and heat denatured (HDA) condition for murine splenocyte proliferation, cytokine secretion, NK-cell activation, and thymocyte proliferation in vitro with a view to assess its potential as an immunomodulator. Both NA and HDA activate splenocytes and induce production of cytokines like IL-2, IFN-gamma and TNF-alphabeta indicating a TH1 type of immune response. Native agglutinin and HDA induced conditioned media of adherent splenocytes could stimulate non-adherent splenocytes and vice versa. Heat denatured agglutinin was able to induce NK-cell activation at much lower concentration than that of NA, but the extent of NK-cell activation was higher for NA. Proliferation of thymocytes by NA and HDA was also observed. This study indicates that Abrus agglutinin could be a potential immunomodulator both in native as well as in heat denatured form.

Chemical biology of the sugar code

A high-density coding system is essential to allow cells to communicate efficiently and swiftly through complex surface interactions. All the structural requirements for forming a wide array of signals with a system of minimal size are met by oligomers of carbohydrates. These molecules surpass amino acids and nucleotides by far in information-storing capacity and serve as ligands in biorecognition processes for the transfer of information. The results of work aiming to reveal the intricate ways in which oligosaccharide determinants of cellular glycoconjugates interact with tissue lectins and thereby trigger multifarious cellular responses (e.g. in adhesion or growth regulation) are teaching amazing lessons about the range of finely tuned activities involved. The ability of enzymes to generate an enormous diversity of biochemical signals is matched by receptor proteins (lectins), which are equally elaborate. The multiformity of lectins ensures accurate signal decoding and transmission. The exquisite refinement of both sides of the protein-carbohydrate recognition system turns the structural complexity of glycans--a demanding but essentially mastered problem for analytical chemistry--into a biochemical virtue. The emerging medical importance of protein-carbohydrate recognition, for example in combating infection and the spread of tumors or in targeting drugs, also explains why this interaction system is no longer below industrial radarscopes. Our review sketches the concept of the sugar code, with a solid description of the historical background. We also place emphasis on a distinctive feature of the code, that is, the potential of a carbohydrate ligand to adopt various defined shapes, each with its own particular ligand properties (differential conformer selection). Proper consideration of the structure and shape of the ligand enables us to envision the chemical design of potent binding partners for a target (in lectin-mediated drug delivery) or ways to block lectins of medical importance (in infection, tumor spread, or inflammation).

Animal lectins: from initial description to elaborated structural and functional classification

The genetic code connects the two biochemical dimensions of nucleic acids and proteins. Theoretical calculations on coding capacity reveal that oligosaccharides as hardware surpass peptides by more than seven orders of magnitude based on hexamer synthesis. Thus, the sugar code establishes the third dimension of biological information transfer. Using carbohydrate-binding proteins (lectins, enzymes and antibodies) the information content of such epitopes is decoded. Currently, five families of animal lectins are defined in structural terms. i.e. the C-type, I-type and P-type groups, the galectins and the pentraxins. They are involved in intra- and intercellular glycan routing using oligosaccharides as postal-code equivalents and acting as defense molecules homing in on foreign or aberrant glycosignatures, as crosslinking agent in biosignaling and as coordinator of transient or firm cell-cell/cell-matrix contacts. By delineating the driving forces toward complex formation, knowledge about the causes for specificity can be turned into design of custom-made high-affinity ligands for clinical application, e.g. in anti-adhesion therapy, drug targeting or diagnostic histopathology.

Influence of spacer length on interaction of mannosylated liposomes with human phagocytic cells

PURPOSE

To improve target specificity and uptake of liposomes by macrophages, one can improve high-affinity receptor binding to mannose determinants with their 175-kDa mannose receptor (MR), which is mainly influenced by the length and flexibility of the spacer between the carbohydrate head group and liposome surface. Liposomes containing alkylmannosides with hydrophilic spacers 0 to 8 ethyleneoxy units (EO) long (Man0...Man8) were used to investigate systematically the effects of spacer length on liposome-cell interactions.

METHODS

Concanavalin A (ConA)-induced liposome aggregation was studied by turbidity measurement and cell uptake using PMA-induced HL-60 cells or native human macrophages by determining 6-CF after cell lysis or NBD-fluorescence with flow cytometry. Detection of MR in native cell populations was carried out by an antibody assay using flow cytometry; MR-representing cells were selected analytically.

RESULTS

Liposomes containing mannosides with more than one EO spacer length were specifically aggregated by ConA, indicating accessibility of the carbohydrate ligands of these derivatives. Increase in EO spacer units of incorporated mannosides (two or more EO) led to suppression of cellular uptake of mannosylated liposomes by phagocytes lacking MR (HL60, U937). The extent of suppression increased with spacer length. Liposome uptake by native macrophages expressing MR was, on the contrary, improved, particularly by Man6 and Man8.

CONCLUSIONS

Uptake of liposomes modified with Man6 or Man8 by native cells was enhanced but did not reach an optimum. Thus, Man6, Man8, and mannosides with even longer spacer arms are of potential use in receptor-mediated targeting.

Identification of human placental wheat germ agglutinin-immunoreactive protein by mass spectrometry

A preparation of polyclonal antibodies to human placental calcium-dependent, carbohydrate-binding glycoprotein, previously identified as wheat germ agglutinin-immunoreactive protein, was applied as the ligand in an immunoaffinity procedure. Following electrophoretic separation of purified material, the specific 66-kDa antigen band was excised and subjected to in-gel protein cleavage. Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) analysis of the tryptic digest yielded 42 isotopically resolved peptide fragments from mass 860 to 2,690. Empirical data from MALDI-MS were analyzed by computer assistance using ProteoMetrics PROFOUND software. Protein candidates reported in the specified identification database search are discussed in relation to the biochemical characteristics of the protein analyzed and its possible antigenic relatedness to wheat germ agglutinin. We speculate that a fibulin-like member of the epidermal growth factor-repeat protein family might be selected as a positive match.

Glycohistochemistry: the why and how of detection and localization of endogenous lectins

The central dogma of molecular biology limits the downstream flow of genetic information to proteins. Progress from the last two decades of research on cellular glycoconjugates justifies adding the enzymatic production of glycan antennae with information-bearing determinants to this famous and basic pathway. An impressive variety of regulatory processes including cell growth and apoptosis, folding and routing of glycoproteins and cell adhesion/migration have been unravelled and found to be mediated or modulated by specific protein (lectin)-carbohydrate interactions. The conclusion has emerged that it would have meant missing manifold opportunities not to recruit the sugar code to cellular information transfer. Currently, the potential for medical applications in anti-adhesion therapy or drug targeting is one of the major driving forces fuelling progress in glycosciences. In histochemistry, this concept has prompted the introduction of carrier-immobilized carbohydrate ligands (neoglycoconjugates) to visualize the cells' capacity to be engaged in oligosaccharide recognition. After their isolation these tissue lectins will be tested for ligand analysis. Since fine specificities of different lectins can differ despite identical monosaccharide binding, the tissue lectins will eventually replace plant agglutinins to move from glycan profiling and localization to functional considerations. Namely, these two marker types, i.e. neoglycoconjugates and tissue lectins, track down accessible binding sites with relevance for involvement in interactions in situ. The documented interplay of synthetic organic chemistry and biochemistry with cyto- and histochemistry nourishes the optimism that the application of this set of innovative custom-prepared tools will provide important insights into the ways in which glycans can act as hardware in transmitting information during normal tissue development and pathological situations.

A specific wheat germ agglutinin-immunoreactive protein in human placenta

In this study we examined human placenta for the presence of molecules antigenically related to a plant lectin, wheat germ agglutinin. The initial results of immunolocalization using polyclonal antibodies against wheat germ agglutinin showed that human placenta contains protein(s) recognized specifically. Staining of syncytiotrophoblast brush border and cytotrophoblast, granular in appearance was observed in first trimester human placenta. Specific binding was also seen in trophoblast-derived JAr and BeWo carcinoma cells. Isolation of wheat germ agglutinin-immunoreactive material from human placenta was achieved by ion-exchange- and affinity-chromatography on anti-wheat germ agglutinin-immunoglobulin G-Sepharose. The placental protein having molecular mass of 66 kD was identified as specific. The protein of 66 kD was characterized as a calcium-dependent, asialofetuin-binding molecule.

Sequence and expression of the monkey homologue of the ER-golgi intermediate compartment lectin, ERGIC-53

We obtained the cDNA sequence of the monkey homologue of the intermediate compartment protein ERGIC-53 by both cDNA library screening and RT-PCR amplification. The final sequence of 2422 nts of the monkey ERGIC-53 cDNA is 96.2% identical to the human ERGIC-53 cDNA and 87% and 67% identical to the rat and amphibian cDNA, respectively. The translated CV1 ERGIC-53 protein is 96.47% identical to the human ERGIC-53, 87% identical to the rat p58 and 66. 98% to the Xenopus laevis protein. Southern blot analysis of multiple genomic DNAs shows the presence of sequences similar to ERGIC-53 in different species. ERGIC-53 is expressed as a major transcript of about 5.5 kb in either monkey CV1 or in human CaCo2. A shorter transcript of 2.3 kb was detected in both cell lines and in mRNAs derived from human pancreas and placenta.

Glycoaffinity chromatography and biological recognition

The potential of bioaffinity chromatography as a tool for study of biological recognition mechanisms is gaining increasing recognition. Biochromatographic methods allow the separation of proteins according to both the structure of their polypeptidic chain and their post-translational modifications. Among the various post-translational modifications which proteins undergo, glycosylation has conducted to the development of original methods (glycotechnologies). This review discusses the applications of glycotechnologies in bioaffinity chromatography, and particularly the use of biochromatography to elucidate mechanisms involved in glycobiology.

Animal lectins

Protein and lipid glycosylation is no longer considered as a topic whose appeal is restricted to a limited number of analytical experts perseveringly pursuing the comprehensive cataloguing of structural variants. It is in fact arousing curiosity in various areas of basic and applied bioscience. Well founded by the conspicuous coding potential of the sugar part of cellular glycoconjugates which surpasses the storage capacity of oligonucleotide- or oligopeptide-based code systems, recognition of distinct oligosaccharide ligands by endogenous receptors, i.e. lectins and sugar-binding enzymes or antibodies, is increasingly being discovered to play salient roles in animal physiology. Having inevitably started with a descriptive stage, research on animal lectins has now undubitably reached maturity. Besides listing the current categories for lectin classification and providing presentations of the individual families and their presently delineated physiological significance, this review places special emphasis on tracing common structural and functional themes which appear to reverberate in nominally separated lectin and animal categories as well as lines of research which may come to fruition for medical sciences.

Myrosinase-binding proteins are derived from a large wound-inducible and repetitive transcript

Several non-myrosinase proteins have been found in association with some of the myrosinases extracted from rape (Brassica napus) seed. Most of these proteins seemed to belong to a large family of proteins ranging in size over approximately 30-110 kDa, namely the myrosinase-binding protein (MBP) family. Potentially all of these MBPs might be derived from a single large precursor, encoded by a 3.3-kb transcript. This transcript coded for a 99-kDa glycine-rich protein with a highly repetitive structure. The mature 50-kDa and 52-kDa MBP amino-terminal was located 255 amino acids from the putative initiation methionine. Also, a more divergently related transcript, the protein product of which was unknown, has been cloned. However, the largest open reading frame suggested a proline-rich protein. While this transcript seemed to be expressed predominantly in seeds, the MBP transcripts were expressed in several tissues and also exhibited a responsiveness to wounding and methyl jasmonate. Both proteins exhibited significant similarities to lectins from Artocarpus integer and from Maclura pomifera.

Sperm-binding proteins

Gamete recognition and binding are mediated by specific proteins on the surface of the sperm and egg. Identification and characterization of some of these proteins from several model systems, particularly mouse and sea urchin, have focused interest on the general properties and functions of gamete recognition proteins. Sperm-binding proteins located in egg extracellular coats as well as sperm-binding proteins that are localized to the egg plasma membrane are presented in the context of their structure and function in gamete binding. Unifying and disparate characteristics are discussed in light of the diverse biology of fertilization among species. Outstanding questions, alternative mechanisms and models, and strategies for future work are presented.