Membrane binding of zebrafish actinoporin-like protein: AF domains, a novel superfamily of cell membrane binding domains

Actinoporins are potent eukaryotic pore-forming toxins specific for sphingomyelin-containing membranes. They are structurally similar to members of the fungal fruit-body lectin family that bind cell-surface exposed Thomsen-Friedenreich antigen. In the present study we found a number of sequences in public databases with similarity to actinoporins. They originate from three animal and two plant phyla and can be classified in three families according to phylogenetic analysis. The sequence similarity is confined to a region from the C-terminal half of the actinoporin molecule and comprises the membrane binding site with a highly conserved P-[WYF]-D pattern. A member of this novel actinoporin-like protein family from zebrafish was cloned and expressed in Escherichia coli. It displays membrane-binding behaviour but does not have permeabilizing activity or sphingomyelin specificity, two properties typical of actinoporins. We propose that the three families of actinoporin-like proteins and the fungal fruit-body lectin family comprise a novel superfamily of membrane binding proteins, tentatively called AF domains (abbreviated from actinoporin-like proteins and fungal fruit-body lectins).     

Preparation and evaluation of lactose-modified monoliths for the adsorption and decontamination of plant toxins and lectins

A series of sugar-modified porous silica monoliths with different sugar ligands (β-lactoside, β-N-acetyllactosaminide, β-d-galactoside, β-d-N-acetylgalactosaminide and β-d-glucoside) and linkers were prepared and evaluated using plant toxins and lectins including ricin and a Ricinus communis agglutinin (RCA₁₂₀). Among these sugar monoliths, a lactose monolith carrying a triethylene glycol spacer adsorbed ricin and RCA₁₂₀ with the highest efficiency. The monolith showed no binding with albumin, globulin, and lectins from Jack beans, Osage orange, Amur maackia and wheat germ. All these data support the utility of the lactose-modified monolith as a tool for adsorption and decontamination of plant toxins.     

Isolation and characterization of a jacalin-related mannose-binding lectin from salt-stressed rice (Oryza sativa) plants

A novel plant lectin was isolated from salt-stressed rice (Oryzasativa L.) plants and partially characterized. The lectin occurs as a natural mixture of two closely related isoforms consisting of two identical non-covalently linked subunits of 15 kDa. Both isoforms are best inhibited by mannose and exhibit potent mitogenic activity towards T-lymphocytes. Biochemical analyses and sequence comparisons further revealed that the rice lectins belong to the subgroup of mannose-binding jacalin-related lectins. In addition, it could be demonstrated that the lectins described here correspond to the protein products of previously described salt-stress-induced genes. Our results not only identify the rice lectin as a stress protein but also highlight the possible importance of protein-carbohydrate interactions in stress responses in plants. Received: 27 July 1999 / Accepted: 11 November 1999     

Regulation of surfactant phospholipid secretion from isolated rat alveolar type II cells by lectins

The major surfactant-associated protein is a potent inhibitor of surfactant phospholipid secretion from isolated type II cells. Since the major surfactant-associated protein contains a carboxy terminal polypeptide domain which is homologous to the lectin-like liver mannose-binding protein, we tested whether lectins inhibit surfactant phospholipid secretion from rat alveolar type II cells. Concanavalin A, wheat germ agglutinin and Maclura pomifera agglutinin were potent inhibitors of surfactant phospholipid secretion. When adenosine 5'-triphosphate (ATP) was utilized as a secretagogue, the IC50 values for inhibition of surfactant phospholipid secretion were 5.10(-7) (wheat germ agglutinin), 1.10(-6) (concanavalin A) and 2.5.10(-5) M (M. pomifera agglutinin). Similar results were obtained when 12-O-tetradecanoylphorbol 13-acetate was utilized as a secretagogue: IC50 values of 1.10(-6) M for concanavalin A and wheat germ agglutinin and 2.5.10(-5) M for M. pomifera agglutinin. Hapten sugars were utilized to antagonize the inhibitory effect of the lectins. N-Acetyl-D-glucosamine significantly reversed inhibition of phospholipid secretion by wheat germ agglutinin in a dose-dependent fashion and methyl alpha-D-mannoside significantly reversed inhibition of phospholipid secretion by concanavalin A. N-Acetyl-D-galactosamine had no significant effect on inhibition of secretion produced by any of the lectins. The inhibitory effect of the lectins did not appear to be due to cytotoxicity since lactate dehydrogenase was not released above control levels and the inhibition of the surfactant phospholipid secretion by wheat germ agglutinin could be reversed after treatment of cells with wheat germ agglutinin by washing the lectin from the cells followed by treatment of the cells with ATP. These studies demonstrate a direct inhibitory effect of plant lectins on phospholipid secretion from type II cells in vitro.     

Generation of H2O2 by Human Neutrophils and Changes of Cytosolic Ca2+ and pH of Rat Thymocytes in Response to Galactoside-Binding Proteins (Lectins or Immunoglobulins)

In contrast to plant agglutinins, biological activities of animal/human lectins are not well defined yet. Testing a panel of seven mammalian carbohydrate-binding proteins we have found that the dimeric lectin from chicken liver (CL-16) was a stimulator of H₂O₂ release from human neutrophils as well as effector for induction of cytosolic Ca²⁺ and pH increase in rat thymocytes. Activity of this lectin was comparable to potent galactoside-specific plant lectins such as Viscum album L. agglutinin. The activities of the tested plant lectins depended significantly on their nominal carbohydrate specificity as well as on the source. The results indicate that endogenous lectins may be involved in the regulation of neutrophil and lymphocyte functions by elicitation of selective biosignaling reactions.     

植物凝集素研究进展

植物凝集素广泛分布于植物界,它可以根据不同性质进行分类,按进化及结构相关性可以分为七个家族;豆科凝集素,单子叶植物甘露糖结构凝集素,含橡胶素结构域的几丁质结合凝集素,2型核糖体失活蛋白,葫芦科韧皮部凝集素,木菠萝素相关凝集素和苋科凝集素,在长期的进化过程中,它们形成几种不同的结合模体来识别一些外源多糖,在植物中未发现合适的内源性多糖受体。植物凝集素在生物学研究,农业和医学上有广泛的应用。     

抗虫植物基因工程研究进展

虫害是造成农业减产的主要原因之一。据不完全统计,全世界每年因虫害引起的作物减产达总产量的15％,损失高达数千亿美元。在我国,因虫害水稻减产在lO％以上;小麦减产近20％;棉花减产在     

Assembling an arsenal: origin and evolution of the snake venom proteome inferred from phylogenetic analysis of toxin sequences

We analyzed the origin and evolution of snake venom toxin families represented in both viperid and elapid snakes by means of phylogenetic analysis of the amino acid sequences of the toxins and related nonvenom proteins. Out of eight toxin families analyzed, five provided clear evidence of recruitment into the snake venom proteome before the diversification of the advanced snakes (Kunitz-type protease inhibitors, CRISP toxins, galactose-binding lectins, M12B peptidases, nerve growth factor toxins), and one was equivocal (cystatin toxins). In two others (phospholipase A(2) and natriuretic toxins), the nonmonophyly of venom toxins demonstrates that presence of these proteins in elapids and viperids results from independent recruitment events. The ANP/BNP natriuretic toxins are likely to be basal, whereas the CNP/BPP toxins are Viperidae only. Similarly, the lectins were recruited twice. In contrast to the basal recruitment of the galactose-binding lectins, the C-type lectins were shown to be Viperidae only, with the alpha-chains and beta-chains resulting from an early duplication event. These results provide strong additional evidence that venom evolved once, at the base of the advanced snake radiation, rather than multiple times in different lineages, with these toxins also present in the venoms of the "colubrid" snake families. Moreover, they provide a first insight into the composition of the earliest ophidian venoms and point the way toward a research program that could elucidate the functional context of the evolution of the snake venom proteome.     

Solution structure of drosomycin, the first inducible antifungal protein from insects.

Drosomycin is the first antifungal protein characterized recently among the broad family of inducible peptides and proteins produced by insects to respond to bacterial or septic injuries. It is a small protein of 44 amino acid residues extracted from Drosophila melanogaster that exhibits a potent activity against filamentous fungi. Its three-dimensional structure in aqueous solution was determined using 1H 2D NMR. This structure, involving an alpha-helix and a twisted three-stranded beta-sheet, is stabilized by three disulfide bridges. The corresponding Cysteine Stabilized alpha beta (CS alpha beta) motif, which was found in other defense proteins such as the antibacterial insect defensin A, short- and long-chain scorpion toxins, as well as in plant thionins and potent antifungal plant defensins, appears as remarkably persistent along evolution.     

Protein proteinase inhibitor genes in combat against insects, pests, and pathogens: natural and engineered phytoprotection

The continual need to increase food production necessitates the development and application of novel biotechnologies to enable the provision of improved crop varieties in a timely and cost-effective way. A milestone in this field was the introduction of Bacillus thuringiensis (Bt) entomotoxic proteins into plants. Despite the success of this technology, there is need for development of alternative strategies of phytoprotection. Biotechnology offers sustainable solutions to the problem of pests, pathogens, and plant parasitic nematodes in the form of other insecticidal protein genes. A variety of genes, besides (Bt) toxins that are now available for genetic engineering for pest resistance are genes for vegetative insecticidal proteins, proteinase inhibitors, alpha-amylase inhibitors, and plant lectins. This review presents a comprehensive summary of research efforts that focus on the potential use and advantages of using proteinase inhibitor genes to engineer insect- and pest-resistance. Crop protection by means of PI genes is an important component of Integrated Pest Management programmes.     

Saporin and ricin A chain follow different intracellular routes to enter the cytosol of intoxicated cells

Several protein toxins, such as the potent plant toxin ricin, enter mammalian cells by endocytosis and undergo retrograde transport via the Golgi complex to reach the endoplasmic reticulum (ER). In this compartment the catalytic moieties exploit the ER-associated degradation (ERAD) pathway to reach their cytosolic targets. Bacterial toxins such as cholera toxin or Pseudomonas exotoxin A carry KDEL or KDEL-like C-terminal tetrapeptides for efficient delivery to the ER. Chimeric toxins containing monomeric plant ribosome-inactivating proteins linked to various targeting moieties are highly cytotoxic, but it remains unclear how these molecules travel within the target cell to reach cytosolic ribosomes. We investigated the intracellular pathways of saporin, a monomeric plant ribosome-inactivating protein that can enter cells by receptor-mediated endocytosis. Saporin toxicity was not affected by treatment with Brefeldin A or chloroquine, indicating that this toxin follows a Golgi-independent pathway to the cytosol and does not require a low pH for membrane translocation. In intoxicated Vero or HeLa cells, ricin but not saporin could be clearly visualized in the Golgi complex using immunofluorescence. The saporin signal was not evident in the Golgi, but was found to partially overlap with that of a late endosome/lysosome marker. Consistently, the toxicities of saporin or saporin-based targeted chimeric polypeptides were not enhanced by the addition of ER retrieval sequences. Thus, the intracellular movement of saporin differs from that followed by ricin and other protein toxins that rely on Golgi-mediated retrograde transport to reach their retrotranslocation site.     

Purification and characterization of three toxins and two agglutinins from Abrus precatorius seed by using lactamyl-Sepharose affinity chromatography

Three toxins, abrin-I, -II, and -III, and two agglutinins, APA-I and -II, were purified from the seeds of Abrus precatorius by lactamyl-Sepharose affinity chromatography followed by gel filtration and DEAE-Sephacel column chromatography. Abrin-I did not bind on DEAE-Sephacel column chromatography and the bound abrin-II, abrin-III, APA-I, and APA-II were eluted with a sodium acetate gradient. The identity of each protein was established by sodium dodecylsulfate-polyacrylamide gel electrophoresis and isoelectric focusing. The relative molecular weights are abrin-I, 64,000; abrin-II and abrin-III, 63,000 each: APA-I, 130,000; and APA-II, 128,000. Isoelectric focusing revealed microheterogeneity due to the presence of isoforms in each protein. Toxicity and binding studies further confirmed the differences among the lectins. The time course of inhibition of protein synthesis in thymocytes by the toxins showed lag times of 78, 61, and 72 min with Ki's of 0.55, 0.99, and 0.74 ms-1 at a 0.63 nM concentration of each of abrin-I, -II, and -III, respectively. A Scatchard plot obtained from the equilibrium measurement for the lectins binding to lactamyl-Sepharose beads showed nonlinearity, indicating a cooperative mode of binding which was not observed for APA-I binding to Sepharose 4B beads. Further, by the criterion of the isoelectric focusing profile, it was shown that the least toxic abrin-I and the highly toxic abrin-II isolated by lactamyl-Sepharose chromatography were not retained on a low-affinity Sepharose 4B matrix, which signifies the necessity of using a high-affinity matrix for the purification of the lectins.     

A new lectin family with structure similarity to actinoporins revealed by the crystal structure of Xerocomus chrysenteron lectin XCL

A newly defined family of fungal lectins displays no significant sequence similarity to any protein in the databases. These proteins, made of about 140 amino acid residues, have sequence identities ranging from 38% to 65% and share binding specificity to N-acetyl galactosamine. One member of this family, the lectin XCL from Xerocomus chrysenteron, induces drastic changes in the actin cytoskeleton after sugar binding at the cell surface and internalization, and has potent insecticidal activity. The crystal structure of XCL to 1.4 A resolution reveals the architecture of this new lectin family. The fold of the protein is not related to any of the several lectin folds documented so far. Unexpectedly, the structure similarity is significant with actinoporins, a family of pore-forming toxins. The specific structural features and sequence signatures in each protein family suggest a potential sugar binding site in XCL and a possible evolutionary relationship between these proteins. Finally, the tetrameric assembly of XCL reveals a complex network of protomer-protomer interfaces and generates a large, hydrated cavity of 1000 A3, which may become accessible to larger solutes after a small conformational change of the protein.     

Amino sugars in the glycoprotein toxin from Bacillus thuringiensis subsp. israelensis.

The carbohydrate content of purified Bacillus thuriniensis subsp. israelensis crystal toxin was determined by six biochemical tests, column chromatography on an amino acid analyzer, and the binding of 11 fluorescent lectins. The crystals contained approximately 1.0% neutral sugars and 1.7% amino sugars. The amino sugars consisted of 70% glucosamine and 30% galactosamine. No N-acetylneuraminic acid (sialic acid) was detected. The presence of amino sugars was confirmed by the strong binding of fluorescent wheat germ agglutinin and the weak binding of fluorescent soybean agglutinin. These lectins recognize N-acetyl-D-glucosamine and N-acetyl-D-galactosamine, respectively. The lectin-binding sites appeared evenly distributed among the protein subunits of the crystal. The sugars were covalently attached to the crystal toxin because wheat germ agglutinin still bound alkali-solubilized toxin which had been boiled in sodium dodecyl sulfate, separate by polyacrylamide gel electrophoresis, and transferred to nitrocellulose membranes. This study demonstrates the covalent attachment of amino sugars and indicates that the B. thuringiensis subsp. israelensis protein toxins should be viewed as glycoprotein toxins. The crystals used in the present study were purified on sodium bromide density gradients. Studies employing crystals purified on Renografin density gradients can give artificially high values for the anthrone test for neutral sugars.     

Entry of lethal doses of abrin, ricin and modeccin into the cytosol of HeLa cells

In attempts to assess how many molecules of the toxic lectins abrin, ricin and modeccin are needed in the cytosol to kill HeLa cells the effect of these toxins on protein synthesis and plating efficiency was studied. The incubation time of the cells after a 1 h exposure to the toxins influenced strongly the extent of inhibition of protein synthesis. The full toxic effect was expressed about 20 h of incubation after the exposure. On further incubation, protein synthesis again increased at a rate comparable to that in the control cells. After exposure to increasing concentrations of toxins the inhibition of cellular protein synthesis measured after 20 h showed excellent agreement with the inhibition of plating efficiency, indicating that the inhibition of protein synthesis can be used as a measure of cell killing. The inhibition of protein synthesis by toxins was found to follow first order kinetics, indicating that the cells are killed by an all- or none-effect. Autoradiographic studies indicated that after exposure to intermediate toxin concentrations protein synthesis was completely abolished in some cells, whereas it appeared to proceed at a normal rate in the remaining cells. The results provide evidence that penetration of one molecule of abrin, ricin or modeccin into cytosol is lethal to HeLa cells and that the efficiency of toxin entry into the cytoplasm is very low compared to the rate of bulk toxin uptake.     

Molecular mechanisms of protective reactions of the body: time scale, specificity scale

Heterogeneous conformation transition (HCT) peculiar to proteins is the earliest protection of the organism from poisons, toxins and pathological metabolites, HCT is conjugated with the biosynthesis of heat shock proteins (HSP), glycosylation of modified albumin and with an increase in the biosynthesis of soluble lectins, acute phase reactants (APR). All phases except for HCT are regulated at different levels of the protein biosynthesis. Their individual time corresponds to the evolution stages.     

植物凝集素的分子生物学研究

植物凝集素是一类具有高度特异性糖结合活性的蛋白,含有一个或多个可与单糖或寡聚糖特异可逆结合的非催化结构域。它的糖结合特异性主要针对外源寡糖,主要生理功能是介异植物的防御反应。到目前为止已克隆了２２２个植物凝集素基因。作者就植物凝集素的分类、性质、功能、凝集素基因的克隆和凝集素的翻译后加工过程作一综述。     

Studies on the structure and properties of the lectins from Abrus precatorius and Ricinus communis.

The amino acid composition of the isolated A- and B-chains of the toxic lectins abrin and ricin was determined and compared. Even though the two toxins originate from widely different plants, statistical analysis of the amino acid content indicates extensive homologies in the amino acid sequence of the 4 chains. The intact lectins contain no free SH-groups whereas the isolated A- and B-chains contain close to one free SH-group each. The results indicate that in both toxins the A- and B-chains are connected by a single S-S bond. The B-chains of abrin and ricin contain similar amounts of mannose and glucosamine. The A-chain of ricin also contains some carbohydrate, whereas the A-chain of abrin appears not to be a glycoprotein. The non-toxic abrus and ricinus agglutinins contain more carbohydrate than abrin and ricin. The isoelectric points of the different lectin preparations were measured by isoelectrofocusing. The intact lectins are much more resistant to heat, freezing and chemical treatments than the isolated A- and B-chains. The intact lectins are also very resistant to treatment with proteolytic enzymes, whereas the isolated chains are easily digested. Evidence indicating that the toxins and their chains undergo extensive conformational changes upon reduction of the S-S bond is discussed.     

植物凝集素在植物体内的生理作用

现有研究表明,植物种子中的凝集素是植物体内的储存蛋白;扁豆和稻胚凝集素对胚胎的分裂和分化有促进作用;在豆科植物和根瘤菌之间的共生作用中,凝集素起着高度专一的识别作用;麦胚凝集素在种胚萌发时,起着抗真菌的作用;体外实验也证明凝集素对危害玉米的主要害虫的发育有阻碍作用;还发现植物凝集素具有酶的活性和酶抑制剂的作用,从而调节植物体的生理活动。     

Detection of natural products that induce aberrations of the mitotic spindle

A number of natural products which inhibit the synthesis and functioning of the mitotic spindle have been shown to be potent antitumour agents. As an aid to the detection and evaluation of these chemicals that produce aberrations of mitotic cell division we have utilized a model screening system based upon the simultaneous visualisation of both the mitotic spindle and the chromosomes.This assay allows the detection of morphological aberrations of both spindle synthesis and function produced by exposure to chemicals active upon the mitotic spindle. This assay has been evaluated for its ability to detect natural spindle damaging agents. Extracts of Catharanthus roseus, which produces the potent spindle toxins vinblastine and vincristine, derived from both leaves and tissue culture samples were assayed for their ability to induce spindle aberrations in human fibroblasts. Samples of extract which lacked natural vinblastine were spiked with pure vinblastine sulphate to estimate the sensitivity of the assay to detect low concentrations of known spindle toxins.This study indicates that spindle toxins may be identified in specific plant extracts at high levels of sensitivity thus providing an effective screening technique for the identification of potentially valuable plant materials for their use as sources of antitumour chemicals.