Evolutionary relationships among proteins in the phytohemagglutinin-arcelin-alpha-amylase inhibitor family of the common bean and its relatives

Genetic variation in a tepary bean (Phaseolus acutifolius A. Gray) diversity panel reveals loci associated with biotic stress resistance

Tepary bean (Phaseolus acutifolius A. Gray), indigenous to the arid climates of northern Mexico and the Southwest United States, diverged from common bean (Phaseolus vulgaris L.), approximately 2 million years ago and exhibits a wide range of resistance to biotic stressors. The tepary genome is highly syntenic to the common bean genome providing a foundation for discovery and breeding of agronomic traits between these two crop species. Although a limited number of adaptive traits from tepary bean have been introgressed into common bean, hybridization barriers between these two species required the development of bridging lines to alleviate this barrier. Thus, to fully utilize the extant tepary bean germplasm as both a crop and as a donor of adaptive traits, we developed a diversity panel of 422 cultivated, weedy, and wild tepary bean accessions which were then genotyped and phenotyped to enable population genetic analyses and genome-wide association studies for their response to a range of biotic stressors. Population structure analyses of the panel revealed eight subpopulations and the differentiation of botanical varieties within P. acutifolius. Genome-wide association studies revealed loci and candidate genes underlying biotic stress resistance including quantitative trait loci for resistance to weevils, common bacterial blight, Fusarium wilt, and bean common mosaic necrosis virus that can be harnessed not only for tepary bean but also common bean improvement.

Comparative study, homology modelling and molecular docking with cancer associated glycans of two non-fetuin-binding Tepary bean lectins

We present the purification and characterization of the two most abundant isoforms of lectins isolated from Tepary bean (Phaseolus acutifolius) seeds, which have been shown to differentially affect the survival of different cancer cells. They were separated by concanavalin A-affinity chromatography. After purification, to release the N-glycans, they were digested with the endoglycosidases PNGase and Glycanase A. Fractions resulted from the hydrolysis products were analyzed to determine their carbohydrate composition. Mass spectrometry data indicated that both isoforms contained high mannose glycans being mannose 6 the most abundant form. Furthermore, based on sequence Ans-X-Ser/Thr, where X is any amino acid except proline, a glycosylation site was determined on asparagine 36. When their metal requirement to preserve their biological activity was determined, the lectins showed differences. While lectin A (LA) agglutination activity was best in the presence of magnesium, lectin B (LB) was best with calcium. Additionally, only LA exhibited affinity to human type-A erythrocytes. Although both lectins showed small differences in their properties, an identical structure-model for both lectins was generated by the homology modelling process. Also, the analysis of ligand binding sites and in silico glycosylation were achieved. Molecular docking with colon adenocarcinoma associated-N-glycans revealed some highly possible interactions and, on the other hand, that N-glycan interaction zones of Tepary bean lectins is not restricted to the carbohydrate binding domain but to an extended part of their surface, which could lead new strategies to explain their biological activity.

Common bean (Phaseolus vulgaris L.) α-amylase inhibitors as safe nutraceutical strategy against diabetes and obesity: An update review

Overweight and obesity are constantly increasing, not only in Western countries but also in low‐middle‐income ones. The decrease of both the intake of carbohydrates and their assimilation are among the main dietary strategies to counter these conditions. α‐Amylase, a key enzyme involved in the digestion of carbohydrates, is the target enzyme to reduce the absorption rate of carbohydrates. α‐Amylase inhibitors (α‐AIs) can be found in plants. The common bean, Phaseolus vulgaris is of particular interest due to the presence of protein‐based α‐AIs which, through a protein–protein interaction, reduce the activity of this enzyme. Here we describe the nature of the various types of common bean seed extracts, the type of protein inhibitors they contain, reviewing the recent Literature about their molecular structure and mechanism of action. We also explore the existing evidence (clinical trials conducted on both animals and humans) supporting the potential benefits of this protein inhibitors from P. vulgaris, also highlighting the urgent need of further studies to confirm the clinical efficacy of the commercial products. This work could contribute to summarize the knowledge and application of P. vulgaris extract as a nutraceutical strategy for controlling unwanted weight gains, also highlighting the current limitations.

Cloning, expression and characterization of arcelin and its impact on digestive enzymes of the stored product insect pest, Callosobruchus maculatus (F.)

The pulse beetle Callosobruchus maculatus causes potential damage to legume crops by infesting the seeds, leading to a reduction of total protein content. Arcelin found in the wild accessions of the common bean, is an insecticidal protein that has the potency to hamper the metabolism of the bruchid beetle. The arcelin gene from the wild accession of Phaseolus lunatus was isolated and the ORF encoding 158 amino acids was cloned in pET-45b (+) vector. The recombinant clones were transformed in BL21 STAR (DE3) pLysS cells, and the expressed arcelin was purified using Ni-NTA column. The recombinant protein was used in preparing an artificial diet, and the insecticidal activity was elucidated against the bruchid pest C. maculatus. Adult emergence and seed damage were drastically reduced in the treated groups. The response towards ingested diet by digestive enzymes involved in metabolism was elucidated through quantitative gene expression. The highest expression was observed in the aminopeptidase, followed by upregulation of alpha-amylase, glycoside hydrolase family 31 and cathepsin D-like aspartic protease, and downregulation of cathepsin L-like cysteine protease. The recombinant arcelin demonstrates effective insecticidal activity against the bruchid beetle. The changes in digestive enzymes to counteract the anti-nutritional nature of the protein were the strategies of the insect defense mechanism.

Bioaccessibility and In Vitro Intestinal Permeability of a Recombinant Lectin from Tepary Bean (Phaseolus acutifolius) Using the Everted Intestine Assay

Tepary bean (Phaseolus acutifolius) lectins exhibit differential in vitro and in vivo biological effects, but their gastrointestinal interactions and digestion have not yet been assessed. This work aimed to evaluate the changes of a recombinant Tepary bean lectin (rTBL-1) through an in vitro and ex vivo gastrointestinal process. A polyclonal antibody was developed to selectively detect rTBL-1 by Western blot (WB) and immunohistochemical analysis. Everted gut sac viability was confirmed until 60 min, where protein bioaccessibility, apparent permeability coefficient, and efflux ratio showed rTBL-1 partial digestion and absorption. Immunoblot assays suggested rTBL-1 internalization, since the lectin was detected in the digestible fraction. The immunohistochemical assay detected rTBL-1 presence at the apical side of the small intestine, potentially due to the interaction with the intestinal cell membrane. The in silico interactions between rTBL-1 and some saccharides or derivatives showed high binding affinity to sialic acid (-6.70 kcal/mol) and N-acetylglucosamine (-6.10 kcal/mol). The ultra-high-performance liquid chromatography-electron spray ionization-quantitative time-of-flight coupled to mass spectrometry (UHPLC-ESI-QTOF/MS) analysis showed rTBL-1 presence in the gastric content and the non-digestible fraction after intestinal simulation conditions. The results indicated that rTBL-1 partially resisted the digestive conditions and interacted with the intestinal membrane, whereas its digestion allowed the absorption or internalization of the protein or the derivative peptides. Further purification of digestion samples should be conducted to identify intact rTBL-1 protein and digested peptides to assess their physiological effects.

Rhizosecretion of a cisgenic lectin by genetic manipulation of Tepary bean plants (Phaseolus acutifolius)

Tepary bean (Phaseolus acutifolius) lectin fraction (TBLF) has been shown to specifically bind and induce cell death of different types of cancer cells and also has exhibited an effect on early colon tumorigenesis. However, the development of a pharmaceutical formula is not possible yet because the production process is expensive and slow and provides low yields. Therefore, the purpose of the present work was to develop a strategy to produce one bioactive lectin by rhizosecretion through root exudates on genetically modified plants. Amplification of Tepary bean transcripts was performed using degenerate primers, and the products obtained were sequenced. Multiple alignments of sequences led to elucidating one of the lectins present in TBLF. Its coding sequence was flanked by an N-terminal secretion signal peptide and a 6xHis-tail. This construction was introduced into P. acutifolius plants using Agrobacterium tumefaciens to subsequently carry out the in vitro growth of the plants. When roots grew, plants were transferred to hydroponic conditions and root exudates were analyzed. Results showed the presence of a glycosylated cisgenic lectin with biological activity, confirming that the strategy followed provides an alternative for the synthetic production and purification of this lectin.

P-element Somatic Inhibitor Protein Binding a Target Sequence in dsx Pre-mRNA Conserved in Bombyx mori and Spodoptera litura

Bombyx mori doublesex (Bmdsx) functions as a double-switch gene in the final step of the sex-determination cascade in the silkworm Bombyx mori. The P-element somatic inhibitor (PSI) protein in B. mori interacts with Bmdsx pre-mRNA in CE1 as an exonic splicing silencer to promote male-specific splicing of Bmdsx. However, the character of the interaction between BmPSI and Bmdsx pre-mRNA remains unclear. Electrophoretic mobility shift assay (EMSA) results showed that the four KH_1 motifs in BmPSI are all essential for the binding, especially the former two KH_1 motifs. Three active sites (I116, L127, and IGGI) in the KH_1 motif were found to be necessary for the binding through EMSA, circular dichroism (CD) spectroscopy, and isothermal titration calorimetry (ITC). The PSI homologous protein in S. litura (SlPSI) was purified and the binding of SlPSI and CE1 was verified. Compared with BmPSI, the mutant SlPSI proteins of I116 and IGGI lost their ability to bind to CE1. In conclusion, the binding of PSI and dsx pre-mRNA are generally conserved in both B. mori and S. litura. These findings provide clues for sex determination in Lepidoptera.

Isolation, purification and characterization of proteinaceous fungal α-amylase inhibitor from rhizome of Cheilocostus speciosus (J.Koenig) C.D.Specht

As the aim of this present study, a proteinaceous α-amylase inhibitor has been isolated from the rhizome of Cheilocostus specious (C. speciosus) and was purified using DEAE cellulose anion exchange chromatography followed by gel filtration using Sephacryl-S-200 column. The purity and molecular mass of the purified inhibitor was determined by SDS-PAGE and LC-MS respectively. The molecular mass of the purified inhibitor was determined to be 31.18kDa. Protein-protein docking was also carried out as molecular model. Model validation methods such as Ramachandran plot and Z-score plot were adopted to validate the structural description (sequence analysis) of proteins. The inhibitory activity was confirmed using spectrophotometric and reverse zymogram analyses. This 31.18kDa protein from C. speciosus inhibited the activity of fungal α-amylase by 71% at the level of ion exchange chromatography and 96% after gel filtration. The inhibition activity of the α-amylase inhibitor was stable and high at optimum pH6 (52.2%) and temperatures of 30-40°C (72.2%). Thus it was suggested that the main responsible for the versatile biological and pharmacological activities of C. speciosus is due to its primary metabolites (proteins) only.

Transcriptomic and Proteomic Research To Explore Bruchid-Resistant Genes in Mungbean Isogenic Lines

Mungbean (Vigna radiata (L.) Wilczek) is an important rotation legume crop for human nutrition in Asia. Bruchids (Callosobruchus spp.) currently cause heavy damage as pests of grain legumes during storage. We used omics-related technologies to study the mechanisms of bruchid resistance in seeds of the nearly isogenic lines VC1973A (bruchid-susceptible) and VC6089A (bruchid-resistant). A total of 399 differentially expressed genes (DEGs) were identified between the two lines by transcriptome sequencing. Among these DEGs, 251 exhibited high expression levels and 148 expressed low expression levels in seeds of VC6089A. Forty-five differential proteins (DPs) were identified by isobaric tags for relative and absolute quantification (iTRAQ); 21 DPs had higher abundances in VC6089A, and 24 DPs had higher abundances in VC1973A. According to transcriptome and proteome data, only three DEGs/DPs, including resistant-specific protein (g39185), gag/pol polyprotein (g34458), and aspartic proteinase (g5551), were identified and located on chromosomes 5, 1, and 7, respectively. Both g39185 and g34458 genes encode a protein containing a BURP domain. In previous research on bruchid molecular markers, the g39185 gene located close to the molecular markers of major bruchid-resistant locus may be a bruchid-resistant gene.

Plant as a plenteous reserve of lectin

Lectins are clusters of glycoproteins of nonimmune foundation that combine specifically and reversibly to carbohydrates, mainly the sugar moiety of glycoconjugates, resulting in cell agglutination and precipitation of glycoconjugates. They are universally distributed in nature, being established in plants, fungi, viruses, bacteria, crustacea, insects, and animals, but leguminacae plants are rich source of lectins. The present review reveals the structure, biological properties, and application of plant lectins.

QUES, a new Phaseolus vulgaris genotype resistant to common bean weevils, contains the Arcelin-8 allele coding for new lectin-related variants

In common bean (Phaseolus vulgaris L.), the most abundant seed proteins are the storage protein phaseolin and the family of closely related APA proteins (arcelin, phytohemagglutinin and α-amylase inhibitor). High variation in APA protein composition has been described and the presence of arcelin (Arc) has been associated with bean resistance against two bruchid beetles, the bean weevil (Acanthoscelides obtectus Say) and the Mexican bean weevil (Zabrotes subfasciatus Bohemian). So far, seven Arc variants have been identified, all in wild accessions, however, only those containing Arc-4 were reported to be resistant to both species. Although many efforts have been made, a successful breeding of this genetic trait into cultivated genotypes has not yet been achieved. Here, we describe a newly collected wild accession (named QUES) and demonstrate its resistance to both A. obtectus and Z. subfasciatus. Immunological and proteomic analyses of QUES seed protein composition indicated the presence of new Arc and arcelin-like (ARL) polypeptides of about 30 and 27 kDa, respectively. Sequencing of cDNAs coding for QUES APA proteins confirmed that this accession contains new APA variants, here referred to as Arc-8 and ARL-8. Moreover, bioinformatic analysis showed the two proteins are closely related to APA components present in the G12949 wild bean accession, which contains the Arc-4 variant. The presence of these new APA components, combined with the observations that they are poorly digested and remain very abundant in A. obtectus feces, so-called frass, suggest that the QUES APA locus is involved in the bruchid resistance. Moreover, molecular analysis indicated a lower complexity of the locus compared to that of G12949, suggesting that QUES should be considered a valuable source of resistance for further breeding purposes.

Genetically modified α-amylase inhibitor peas are not specifically allergenic in mice

Weevils can devastate food legumes in developing countries, but genetically modified peas (Pisum sativum), chickpeas and cowpeas expressing the gene for alpha-amylase inhibitor-1 (αAI) from the common bean (Phaseolus vulgaris) are completely protected from weevil destruction. αAI is seed-specific, accumulated at high levels and undergoes post-translational modification as it traverses the seed endomembrane system. This modification was thought to be responsible for the reported allergenicity in mice of the transgenic pea but not the bean. Here, we observed that transgenic αAI peas, chickpeas and cowpeas as well as non-transgenic beans were all allergenic in BALB/c mice. Even consuming non-transgenic peas lacking αAI led to an anti-αAI response due to a cross-reactive response to pea lectin. Our data demonstrate that αAI transgenic peas are not more allergenic than beans or non-transgenic peas in mice. This study illustrates the importance of repeat experiments in independent laboratories and the potential for unexpected cross-reactive allergic responses upon consumption of plant products in mice.

Analysis of common bean expressed sequence tags identifies sulfur metabolic pathways active in seed and sulfur-rich proteins highly expressed in the absence of phaseolin and major lectins

BACKGROUND

A deficiency in phaseolin and phytohemagglutinin is associated with a near doubling of sulfur amino acid content in genetically related lines of common bean (Phaseolus vulgaris), particularly cysteine, elevated by 70%, and methionine, elevated by 10%. This mostly takes place at the expense of an abundant non-protein amino acid, S-methyl-cysteine. The deficiency in phaseolin and phytohemagglutinin is mainly compensated by increased levels of the 11S globulin legumin and residual lectins. Legumin, albumin-2, defensin and albumin-1 were previously identified as contributing to the increased sulfur amino acid content in the mutant line, on the basis of similarity to proteins from other legumes.

RESULTS

Profiling of free amino acid in developing seeds of the BAT93 reference genotype revealed a biphasic accumulation of gamma-glutamyl-S-methyl-cysteine, the main soluble form of S-methyl-cysteine, with a lag phase occurring during storage protein accumulation. A collection of 30,147 expressed sequence tags (ESTs) was generated from four developmental stages, corresponding to distinct phases of gamma-glutamyl-S-methyl-cysteine accumulation, and covering the transitions to reserve accumulation and dessication. Analysis of gene ontology categories indicated the occurrence of multiple sulfur metabolic pathways, including all enzymatic activities responsible for sulfate assimilation, de novo cysteine and methionine biosynthesis. Integration of genomic and proteomic data enabled the identification and isolation of cDNAs coding for legumin, albumin-2, defensin D1 and albumin-1A and -B induced in the absence of phaseolin and phytohemagglutinin. Their deduced amino acid sequences have a higher content of cysteine than methionine, providing an explanation for the preferential increase of cysteine in the mutant line.

CONCLUSION

The EST collection provides a foundation to further investigate sulfur metabolism and the differential accumulation of sulfur amino acids in seed of common bean. Identification of sulfur-rich proteins whose levels are elevated in seed lacking phaseolin and phytohemagglutinin and sulfur metabolic genes may assist the improvement of protein quality.

Purification, biochemical characterization, and bioactive properties of a lectin purified from the seeds of white tepary bean (phaseolus acutifolius variety latifolius)

The present work shows the characterization of Phaseolus acutifolius variety latifolius, on which little research has been published, and provides detailed information on the corresponding lectin. This protein was purified from a semi-domesticated line of white tepary beans from Sonora, Mexico, by precipitation of the aqueous extract with ammonium sulfate, followed by affinity chromatography on an immobilized fetuin matrix. MALDI TOF analysis of Phaseolus acutifolius agglutinin (PAA) showed that this lectin is composed of monomers with molecular weights ranging between 28 and 31 kDa. At high salt concentrations, PAA forms a dimer of 63 kDa, but at low salt concentrations, the subunits form a tetramer. Analysis of PAA on 2D-PAGE showed that there are mainly three types of subunits with isoelectric points of 4.2, 4.4, and 4.5. The partial sequence obtained by LC/MS/MS of tryptic fragments from the PAA subunits showed 90-100% identity with subunits from genus Phaseolus lectins in previous reports. The tepary bean lectin showed lower hemagglutination activity than Phaseolus vulgaris hemagglutinin (PHA-E) toward trypsinized human A and O type erythrocytes. The hemagglutination activity was inhibited by N-glycans from glycoproteins. Affinity chromatography with the immobilized PAA showed a high affinity to glycopeptides from thyroglobulin, which also has N-glycans with a high content of N-acetylglucosamine. PAA showed less mitogenic activity toward human lymphocytes than PHA-L and Con A. The cytotoxicity of PAA was determined by employing three clones of the 3T3 cell line, demonstrating variability among the clones as follows: T4 (DI₅₀ 51.5 µg/mL); J20 (DI₅₀ 275 µg/mL), and N5 (DI₅₀ 72.5 µg/mL).

Genetic mapping of microsatellite markers around the arcelin bruchid resistance locus in common bean

The deployment in common beans (Phaseolus vulgaris L.) of arcelin-based bruchid resistance could help reduce post-harvest storage losses to the Mexican bean weevil [(Zabrotes subfasciatus (Boheman)]. Arcelin is a member of the arcelin-phytohemagglutinin-alpha-amylase inhibitor (APA) family of seed proteins, which has been extensively studied but not widely used in bean breeding programs. The purpose of this study was to evaluate microsatellite markers for genetic analysis of arcelin-based bruchid resistance and to determine the orientation of markers and the rate of recombination around the APA locus. A total of 10 previously developed microsatellites and 22 newly developed markers based on a sequenced BAC from the APA locus were screened for polymorphism and of these 15 were mapped with an F(2) population of 157 individuals resulting from a susceptible x resistant cross of SEQ1006 x RAZ106 that segregated for both the arcelin 1 allele and resistance to the bruchid, Z. subfasciatus. Microsatellites derived from APA gene sequences were linked within 0.8 cM of each other and were placed relative to the rest of the b04 linkage group. In a comparison of genetic to physical distance on the BAC sequence, recombination was found to be moderate with a ratio of 125 kb/cM, but repressed within the APA locus itself. Several markers were predicted to be very effective for genetic studies or marker-assisted selection, based on their significant associations with bruchid resistance and on low adult insect emergence and positions flanking the arcelin and phytohemagglutinin genes.

Linkage disequilibrium at the APA insecticidal seed protein locus of common bean (Phaseolus vulgaris L.)

BACKGROUND

An interesting seed protein family with a role in preventing insect herbivory is the multi-gene, APA family encoding the alpha-amylase inhibitor, phytohemagglutinin and arcelin proteins of common bean (Phaseolus vulgaris). Variability for this gene family exists and has been exploited to breed for insect resistance. For example, the arcelin locus has been successfully transferred from wild to cultivated common bean genotypes to provide resistance against the bruchid species Zabrotes subfasciatus although the process has been hampered by a lack of genetic tools for and understanding about the locus. In this study, we analyzed linkage disequilibrium (LD) between microsatellite markers at the APA locus and bruchid resistance in a germplasm survey of 105 resistant and susceptible genotypes and compared this with LD in other parts of the genome.

RESULTS

Microsatellite allele diversity was found to vary with each of the eight APA-linked markers analyzed, and two markers within the APA locus were found to be diagnostic for bruchid resistance or susceptibility and for the different arcelin alleles inherited from the wild accessions. Arc1 was found to provide higher levels of resistance than Arc5 and the markers in the APA locus were highly associated with resistance showing that introgression of this gene-family from wild beans provides resistance in cultivated beans. LD around the APA locus was found to be intermediate compared to other regions of the genome and the highest LD was found within the APA locus itself for example between the markers PV-atct001 and PV-ag004.

CONCLUSIONS

We found the APA locus to be an important genetic determinant of bruchid resistance and also found that LD existed mostly within the APA locus but not beyond it. Moderate LD was also found for some other regions of the genome perhaps related to domestication genes. The LD pattern may reflect the introgression of arcelin from the wild into the cultivated background through breeding. LD and association studies for the arcelin gene, linked genes and other members of the APA family are essential for breaking linkage drag while maintaining high levels of bruchid resistance in common bean.

Calcium ions make phytohemagglutinin resistant to trypsin proteolysis

To investigate the mechanism of phytohemagglutinin (PHA) susceptibility or resistance to the action of proteolytic enzymes, its in vitro proteolysis by trypsin was studied. It was found that Ca (2+) gives resistance to the native PHA molecule to trypsin proteolysis. In the absence of Ca (2+) trypsin performs a thorough hydrolysis of PHA. At the first stage of trypsin hydrolysis of PHA the formation of a relatively stable high molecular mass product occurs (PHA-T) as a result of non-co-operative proteolysis. At the second stage, the degradation of PHA-T occurs, and this degradation is performed by parallel co-operative proteolysis. This type of proteolysis differs from the action of trypsin on phaseolin, the main storage protein from common bean ( Phaseolus vulgaris L.). The implications of Ca (2+)influence of PHA hydrolysis by trypsin are discussed.

Activity of alpha-amylase inhibitors from Phaseolus coccineus on digestive alpha-amylases of the coffee berry borer

Seeds of scarlet runner bean ( Phaseolus coccineus L.) were analyzed for alpha-amylase inhibitor (alpha-AI) activity. Through the use of polyclonal antibodies raised against pure alpha-AI-1 from common bean ( Phaseolus vulgaris L.), typical alpha-AlphaIota polypeptides (Mr 14-18 kDa) as well as a large polypeptide of Mr 32000 Da, usually referred to as "amylase inhibitor like", were detected. The inhibitor activity present in four accessions of P. coccineus was examined, both in semiquantitative zymograms allowing the separation of different isoforms and in quantitative assays against human salivary amylase, porcine pancreatic amylase, and coffee berry borer, Hypothenemus hampei Ferrari (Coleoptera: Scolytidae) amylase. Differential inhibition curves lead to the suggestion that the gene encoding one of the inhibitors in P. coccineus (in accession G35590) would be a good candidate for genetic engineering of coffee resistance toward the coffee berry borer. An in vitro proteolytic digestion treatment of pure alpha-AlphaIota-1 resulted in a rapid loss of the inhibitory activity, seriously affecting its natural capacity to interact with mammalian alpha-amylases.

Arcelins from an Indian wild pulse, Lablab purpureus, and insecticidal activity in storage pests

A partially purified protein fraction was isolated from seed flour of the Indian wild bean, Lablab purpureus, by ion exchange and size-exclusion chromatographies. Partially purified L. purpureus proteins had hemagglutination and glycoslyation properties similar to those of lectins or lectin-like proteins from other pulses. Data obtained from two-dimensional gel electrophoresis, MALDI-TOF, and MALDI-TOF/TOF and N-terminal protein sequencing of the isolated polypeptides from L. purpureus demonstrated that the extract contained proteins similar to isoforms of arcelins 3 and 4 and pathogenesis-related protein 1 (PvPR1) of Phaseolus vulgaris. L. purpureus proteins were resistant to degradation by the commercial enzymes trypsin and chymotrypsin and were moderately resistant to pepsin, but were readily hydrolyzed to smaller peptides by papain. Insect feeding bioassays of the extract with the storage pests Rhyzopertha dominica and Oryzaephilus surinamensis, internal and external feeders of grain, respectively, demonstrated that L. purpureus proteins at 2% in the diet resulted in retarded development. However, a 5% dose of the L. purpureus fraction resulted in complete mortality of all larvae in both species. This study has demonstrated that proteins in the partially purified L. purpureus extract have the potential to control storage pests in cereals transformed with L. purpureus defense-related genes, but the need for more studies regarding efficacy and safety is discussed.

Evolutionary analysis of the APA genes in the Phaseolus genus: wild and cultivated bean species as sources of lectin-related resistance factors?

The APA (Arcelin/Phytohemagglutinin/alpha-Amylase inhibitor) gene family is composed of various members, present in Phaseolus species and coding for lectin and lectin-related seed proteins having the double role of storage and defense proteins. Here members of the APA family have been identified by immunological, functional, and molecular analyses and representative genes were sequenced in nine wild species of Phaseolus. All taxa possessed at least one member of the true lectin gene. No arcelin type sequences have been isolated from the species examined. Among the wild species studied, only P. costaricensis contained an alpha-amylase inhibitor (alpha-AI). In addition P. augusti, P. maculatus, P. microcarpus, and P. oligospermus showed the presence of the lectin-related alpha-amylase inhibitor-like (AIL) genes and alpha-AI activity. Data from Southern blot analysis indicated the presence of only one lectin gene in P. parvulus and P. filiformis, while an extensive gene duplication of the APA locus was found in the other Phaseolus species. Phylogenetic analysis carried out on the nucleotide sequences showed the existence of two main clusters and clearly indicated that lectin-related genes originated from a paralogous duplication event preceding the development of the ancestor to the Phaseolus genus. The finding of detectable alpha-AI activity in species containing AIL genes suggests that exploiting APA genes variability in the Phaseolus genus may represent a valuable tool to find new members that may have acquired insecticidal activities.

Molecular identification of four different alpha-amylase inhibitors from baru (Dipteryx alata) seeds with activity toward insect enzymes

The endophytic bruchid pest Callosobruchus maculatus causes severe damage to storage cowpea seeds, leading to economical losses. For this reason the use of alpha-amylase inhibitors to interfere with the pest digestion process has been an interesting alternative to control bruchids. With this aim, alpha-amylase inhibitors from baru seeds (Dipteryx alata) were isolated by affinity chromatographic procedures, causing enhanced inhibition of C. maculatus and Anthonomus grandis alpha-amylases. To attempt further purification, this fraction was applied onto a reversed-phase HPLC column, generating four peaks with remarkable inhibition toward C. maculatus alpha-amylases. SDS-PAGE and MALDI-ToF analysis identified major proteins of approximately 5.0, 11.0, 20.0 and 55 kDa that showed alpha-amylase inhibition. Results of in vivo bioassays using artificial seeds containing 1.0% (w/w) of baru crude extract revealed 40% cowpea weevil larvae mortality. These results provide evidence that several alpha-amylase inhibitors classes, with biotechnological potential, can be isolated from a single plant species.

Characterization of alpha-amylase inhibitor from Palo Fierro seeds

Alpha amylase inhibitor from Palo Fierro seeds (alphaAI-PF) was purified using affinity chromatography on a fetuin-fractogel column followed by anionic exchange chromatography. AlphaAI-PF has a molecular mass of 77kDa with two subunits (15.8 and 17.4 kDa), it is nonglycosylated and has pI of 4.7. AlphaAI-PF inhibited porcine pancreatic alpha-amylase (PPA) (1,4-alpha-D-glucan glucanohydrolase; EC 3.2.1.1), but was almost devoid of inhibitory activity on alpha-amylase extracts from Zabrotes subfasciatus (ZSA). Analysis of alphaAI-PF peptides showed a high homology to alphaAI-1 from Phaseolus vulgaris that also inhibits PPA.

A novel family of lectins evolutionarily related to class V chitinases: an example of neofunctionalization in legumes

A lectin has been identified in black locust (Robinia pseudoacacia) bark that shares approximately 50% sequence identity with plant class V chitinases but is essentially devoid of chitinase activity. Specificity studies indicated that the black locust chitinase-related agglutinin (RobpsCRA) preferentially binds to high-mannose N-glycans comprising the proximal pentasaccharide core structure. Closely related orthologs of RobpsCRA could be identified in the legumes Glycine max, Medicago truncatula, and Lotus japonicus but in no other plant species, suggesting that this novel lectin family most probably evolved in an ancient legume species or possibly an earlier ancestor. This identification of RobpsCRA not only illustrates neofunctionalization in plants, but also provides firm evidence that plants are capable of developing a sugar-binding domain from an existing structural scaffold with a different activity and accordingly sheds new light on the molecular evolution of plant lectins.

Cloning and functional expression of the gene encoding an inhibitor against Aspergillus flavus alpha-amylase, a novel seed lectin from Lablab purpureus (Dolichos lablab)

Maize is one of the more important agricultural crops in the world and, under certain conditions, prone to attack from pathogenic fungi. One of these, Aspergillus flavus, produces toxic and carcinogenic metabolites, called aflatoxins, as byproducts of its infection of maize kernels. The alpha-amylase of A. flavus is known to promote aflatoxin production in the endosperm of these infected kernels, and a 36-kDa protein from the Lablab purpureus, denoted AILP, has been shown to inhibit alpha-amylase production and the growth of A. flavus. Here, we report the isolation of six full-length labAI genes encoding AILP and a detailed analysis of the activities of the encoded proteins. Each of the six labAI genes encoded sequences of 274 amino acids, with the deduced amino acid sequences showing approximately 95-99% identity. The sequences are similar to those of lectin members of a legume lectin-arcelin-alpha-amylase inhibitor family reported to function in plant resistance to insect pests. The labAI genes did not show any of the structures characteristic of conserved structures identified in alpha-amylase inhibitors to date. The recombinant proteins of labAI-1 and labAI-2 agglutinated human red blood cells and inhibited A. flavus alpha-amylase in a manner similar to that shown by AILP. These data indicate that labAI genes are a new class of lectin members in legume seeds and that their proteins have both lectin and alpha-amylase inhibitor activity. These results are a valuable contribution to our knowledge of plant-pathogen interactions and will be applicable for developing protocols aimed at controlling A. flavus infection.

Assessment of the importance of alpha-amylase inhibitor-2 in bruchid resistance of wild common bean

Both alpha-amylase inhibitor-2 (alphaAI-2) and arcelin have been implicated in resistance of wild common bean (Phaseolus vulgaris L.) to the Mexican bean weevil (Zabrotes subfasciatus Boheman). Near isogenic lines (NILs) for arcelin 1-5 were generated by backcrossing wild common bean accessions with a cultivated variety. Whereas seeds of a wild accession (G12953) containing both alphaAI-2 and arcelin 4 were completely resistant to Z. subfasciatus, those of the corresponding NIL were susceptible to infestation, suggesting that the principal determinant of resistance was lost during backcrossing. Three independent lines of transgenic azuki bean [Vigna angularis (Willd.) Ohwi and Ohashi] expressing alphaAI-2 accumulated high levels of this protein in seeds. The expression of alphaAI-2 in these lines conferred protection against the azuki bean weevil (Callosobruchus chinensis L.), likely through inhibition of larval digestive alpha-amylase. However, although the seed content of alphaAI-2 in these transgenic lines was similar to that in a wild accession of common bean (G12953), it did not confer a level of resistance to Z. subfasciatus similar to that of the wild accession. These results suggest that alphaAI-2 alone does not provide a high level of resistance to Z. subfasciatus. However, alphaAI-2 is an effective insecticidal protein with a spectrum of activity distinct from that of alphaAI-1, and it may prove beneficial in genetic engineering of insect resistance in legumes.

Development of four phylogenetically-arrayed BAC libraries and sequence of the APA locus in Phaseolus vulgaris

The APA family of seed proteins consists of three subfamilies, in evolutionary order of hypothesized appearance: phytohaemagglutinins (PHA), alpha-amylase inhibitors (alphaAI), and arcelins (ARL). The APA family plays a defensive role against mammalian and insect seed predation in common bean (Phaseolus vulgaris L.). The main locus (APA) for this gene family is situated on linkage group B4. In order to elucidate the pattern of duplication and diversification at this locus, we developed a BAC library in each of four different Phaseolus genotypes that represent presumptive steps in the evolutionary diversification of the APA family. Specifically, BAC libraries were established in one P. lunatus (cv. 'Henderson: PHA+ alphaAI- ARL-) and three P. vulgaris accessions (presumed ancestral wild G21245 from northern Peru: PHA+ alphaAI+ ARL-; Mesoamerican wild G02771: PHA+ alphaAI+ ARL+; and Mesoamerican breeding line BAT93: PHA+ alphaAI+ ARL-). The libraries were constructed after HindIII digestion of high molecular weight DNA, obtained with a novel nuclei isolation procedure. The frequency of empty or cpDNA-sequence-containing clones in all libraries is low (generally <1%). The Henderson, G21245, and G02771 libraries have a 10x genome coverage, whereas the BAT93 library has a 20x coverage to allow further, more detailed genomic analysis of the bean genome. The complete sequence of a 155 kbp-insert clone of the G02771 library revealed six sequences belonging to the APA gene family, including members of the three subfamilies, as hypothesized. The different subfamilies were interspersed with retrotransposon sequences. In addition, other sequences were identified with similarity to chloroplast DNA, a dehydrin gene, and the Arabidopsis flowering D locus. Linkage between the dehydrin gene and the D1711 RFLP marker identifies a potential syntenic region between parts of common bean linkage group B4 and cowpea linkage group 2.

Identification of a novel bean α-amylase inhibitor with chitinolytic activity

Zabrotes subfasciatus is a devastating starch-dependent storage bean pest. In this study, we attempted to identify novel alpha-amylase inhibitors from wild bean seeds, with efficiency toward pest alpha-amylases. An inhibitor named Phaseolus vulgaris chitinolytic alpha-amylase inhibitor (PvCAI) was purified and mass spectrometry analyses showed a protein with 33330 Da with the ability to form dimers. Purified PvCAI showed significant inhibitory activity against larval Z. subfasciatus alpha-amylases with no activity against mammalian enzymes. N-terminal sequence analyses showed an unexpected high identity to plant chitinases from the glycoside hydrolase family 18. Furthermore, their chitinolytic activity was also detected. Our data provides compelling evidence that PvCAI also possessed chitinolytic activity, indicating the emergence of a novel alpha-amylase inhibitor class.

Solubility-insolubility interconversion of sophoragrin, a mannose/glucose-specific lectin in Sophora japonica (Japanese pagoda tree) bark, regulated by the sugar-specific interaction

Sophoragrin, a mannose/glucose-specific lectin in Sophora japonica (Japanese pagoda tree) bark, was the first lectin found to show self-aggregation that is dependent on the sugar concentration accompanying the interconversion between solubility and insolubility [Ueno, Ogawa, Matsumoto and Seno (1991) J. Biol. Chem. 266, 3146-3153]. The interconversion is regulated by the concentrations of Ca(2+) and specific sugars: mannose, glucose or sucrose. The specific glycotopes for sophoragrin were found in the sophoragrin subunit and an endogenous galactose-specific lectin, B-SJA-I (bark S. japonica agglutinin I), and the lectin subunit that binds to the glycotope was identified by photoaffinity glycan probes. Remarkably, the insoluble polymer of sophoragrin is dissociated by interaction with B-SJA-I into various soluble complexes. Based on these results, self-aggregation of sophoragrin was shown to be a unique homopolymerization due to the sugar-specific interaction. An immunostaining study indicated that sophoragrin localizes mainly in vacuoles of parenchymal cells coincidently with B-SJA-I. These results indicate that sophoragrin can sequester endogenous glycoprotein ligands via sugar-specific interactions, thus providing new insights into the occurrence and significance of the intravacuolar interaction shown by a legume lectin.

Proteinaceous α-amylase inhibitors

Proteins that inhibit alpha-amylases have been isolated from plants and microorganisms. These inhibitors can have natural roles in the control of endogenous alpha-amylase activity or in defence against pathogens and pests; certain inhibitors are reported to be antinutritional factors. The alpha-amylase inhibitors belong to seven different protein structural families, most of which also contain evolutionary related proteins without inhibitory activity. Two families include bifunctional inhibitors acting both on alpha-amylases and proteases. High-resolution structures are available of target alpha-amylases in complex with inhibitors from five families. These structures indicate major diversity but also some similarity in the structural basis of alpha-amylase inhibition. Mutational analysis of the mechanism of inhibition was performed in a few cases and various protein engineering and biotechnological approaches have been outlined for exploitation of the inhibitory function.

Lectin-related resistance factors against bruchids evolved through a number of duplication events

Abundant lectin-related proteins found in common beans ( Phaseolus vulgaris L.) have been shown to confer resistance against the larvae of a number of bruchid species. Genes encoding for these proteins are members of the lectin multigene family, the most representative components being arcelins, phytohemagglutinins and alpha-amylase inhibitors. Arcelins have been described in seven variants, some of which are resistance factors against the Mexican bean weevil ( Zabrotes subfasciatus), a major bean predator. In this study the isolation and sequencing of arcelin genes from wild P. vulgaris genotypes, containing Arc3 and Arc7 variants, is reported, and similarities and evolutionary relationships among the seven known arcelins are described. The evolutionary analysis shows that arcelins 3 and 4 cluster together and are the most-ancient variants. A duplication event gave rise to two additional clusters, one comprising arcelins 1, 2 and 6 and separated from the cluster of arcelins 5 and 7. A multiple number of arcelin genes were found in arcelin 3 and 4 genotypes indicating that more than one type of arcelin gene may be present in the same locus. Some of these sequences are reminiscent of ancient duplication events in arcelin evolution demonstrating that arcelins have evolved through multiple duplications. A further aim of this paper was to better understand and describe the evolution of the entire lectin multigene family. Beside arcelins, a number of other types of sequences, such as putative lectins and sequences not easily classifiable, were found in genotypes containing Arc3 and Arc4. These results, together with the evolutionary analysis, indicate that lectin loci are quite complex and confirm their origin by multiple duplication events.

A new bioinformatic approach to detect common 3D sites in protein structures

An innovative bioinformatic method has been designed and implemented to detect similar three-dimensional (3D) sites in proteins. This approach allows the comparison of protein structures or substructures and detects local spatial similarities: this method is completely independent from the amino acid sequence and from the backbone structure. In contrast to already existing tools, the basis for this method is a representation of the protein structure by a set of stereochemical groups that are defined independently from the notion of amino acid. An efficient heuristic for finding similarities that uses graphs of triangles of chemical groups to represent the protein structures has been developed. The implementation of this heuristic constitutes a software named SuMo (Surfing the Molecules), which allows the dynamic definition of chemical groups, the selection of sites in the proteins, and the management and screening of databases. To show the relevance of this approach, we focused on two extreme examples illustrating convergent and divergent evolution. In two unrelated serine proteases, SuMo detects one common site, which corresponds to the catalytic triad. In the legume lectins family composed of >100 structures that share similar sequences and folds but may have lost their ability to bind a carbohydrate molecule, SuMo discriminates between functional and non-functional lectins with a selectivity of 96%. The time needed for searching a given site in a protein structure is typically 0.1 s on a PIII 800MHz/Linux computer; thus, in further studies, SuMo will be used to screen the PDB.

The Tn antigen-specific lectin from ground ivy is an insecticidal protein with an unusual physiology

Leaves of ground ivy (Glechoma hederacea) contain a lectin (called Gleheda) that is structurally and evolutionary related to the classical legume lectins. Screening of a population of wild plants revealed that Gleheda accounts for more than one-third of the total leaf protein in some clones, whereas it cannot be detected in other clones growing in the same environment. Gleheda is predominantly expressed in the leaves where it accumulates during early leaf maturation. The lectin is not uniformly distributed over the leaves but exhibits a unique localization pattern characterized by an almost exclusive confinement to a single layer of palisade parenchyma cells. Insect feeding trials demonstrated that Gleheda is a potent insecticidal protein for larvae of the Colorado potato beetle (Leptinotarsa decemlineata). Because Gleheda is not cytotoxic, it is suggested that the insecticidal activity is linked to the carbohydrate-binding specificity of the lectin, which as could be demonstrated by agglutination assays with different types of polyagglutinable human erythrocytes is specifically directed against the Tn antigen structure (N-acetylgalactosamine O-linked to serine or threonine residues of proteins).

Identification of putative genes in bean (Phaseolus vulgaris) genomic (Bng) RFLP clones and their conversion to STSs

A set of 79 previously mapped bean (Phaseolus vulgaris) genomic (Bng) clones were partially sequenced. BLAST database searches detected homologies between 59 of these clones and genes from a variety of plants, especially Arabidopsis thaliana. Some matches in the database to the Bng clones included a putative P-glycoprotein-ABC transporter from Arabidopsis, an early nodulin-binding protein (ENBPI) from Medicago truncatula, a lon-protease protein from spinach, a branched-chain amino-acid aminotransferase from Arabidopsis, and a vacuolar sorting receptor (BP-80) from Pisum sativum. Additional matches were found for genes involved in isoprenoid biosynthesis, sulfur metabolism, proline biosynthesis, and floral development. Sequence tagged site (STSs) were produced for 16 of the clones, 2 of which contain simple sequence repeats (SSRs). Polymorphisms were detected for six of the STSs.

Plant toxic proteins with insecticidal properties. A review on their potentialities as bioinsecticides

To meet the demands for food of the expanding world population, there is need of new ways for protecting plant crops against predators and pathogens while avoiding the use of environmentally aggressive chemicals. A milestone in this field was the introduction into crop plants of genes expressing Bacillus thuringiensis entomotoxic proteins. In spite of the success of this new technology, however, there are difficulties for acceptance of these 'anti-natural' products by the consumers and some concerns about its biosafety in mammals. An alternative could be exploring the plant's own defense mechanisms, by manipulating the expression of their endogenous defense proteins, or introducing an insect control gene derived from another plant. This review deals with the biochemical features and mechanisms of actions of plant proteins supposedly involved in defense mechanisms against insects, including lectins, ribosome-inactivating proteins, enzymes inhibitors, arcelins, chitinases, ureases, and modified storage proteins. The potentialities of genetic engineering of plants with increased resistance to insect predation relying on the repertoire of genes found in plants are also discussed. Several different genes encoding plant entomotoxic proteins have been introduced into crop genomes and many of these insect resistant plants are now being tested in field conditions or awaiting commercialization.

Protein structures of common bean (Phaseolus vulgaris) alpha-amylase inhibitors

Two nucleotide sequences for genes that encode alpha-amylase inhibitor 4 (alphaAI-4) from white kidney bean (WKB) cv. 858, designated gene alphaAI-4 (Accession No. ), and alpha-amylase inhibitor 5 (alphaAI-5) from black bean (BB), designated gene alphaAI-5 (Accession No. ), were determined. Genes alphaAI-4 and alphaAI-5 encode 244 amino acid prepro-alphaAI-4 and prepro-alphaAI-5 polypeptides that are 93 and 95% identical with alpha-amylase inhibitor l (alphaAI-l; Hoffman, L. M.; Ma, Y.; Barker, R. F. Nucleic Acids Res. 1982, 10, 7819-7828), 40 and 43% identical with red kidney bean lectin, and 52 and 55% identical with arcelin l of wild-type bean. The high degree of sequence similarity indicates the evolutionary relationship among these genes. PCR analysis of genomic DNA purified from six genotypes of Phaseolus vulgaris showed very similar band patterns in 2% agarose gel, another indication of the conserved size homology among these genes. Proteolytic processing sites were located between Asn77 and Ser78 for pro-alphaAI-4 and pro-alphaAI-5. A bend next to Asn77 in three-dimensional model structures of alphaAI-4 and alphaAI-5 proinhibitors indicates that the proteolytic cleavage is necessary to remove the conformational constraint for activation to the mature protein. Mature WKB alphaAI-4 was composed of four subunits (2alpha2beta) and had a molecular weight of 50000 determined by multiangle laser light scattering and 56714 determined by laser-assisted time-of-flight mass spectrometry.

Allelochemical stress produced by the aqueous leachate of Callicarpa acuminata: effects on roots of bean, maize, and tomato

The in vitro effects of an aqueous leachate (1%) of Callicarpa acuminata Kunth. (Verbenaceae) on radicle growth, protein expression, catalase activity, free radical production and membrane lipid peroxidation in roots of bean, maize, and tomato were examined. Aqueous extract of C. acuminata inhibited the radicle growth of tomato by 47%, but had no effect on root growth of maize and beans. 2D-PAGE and densitometry analysis showed that C. acuminata aqueous leachate modified the expression of various proteins in the roots of all treated plants. In treated bean roots, microsequencing analysis of an 11.3-kDa protein, whose expression was enhanced by leachate treatment, revealed a 99% similarity with subunits of alpha-amylase inhibitor of other beans. A 27.5-kDa protein induced in treated tomato showed 69-95% similarity to glutathione-S-transferases (GST) of other Solanaceae. Spectrophotometric analysis and native gels revealed that catalase activity was increased by 2.2-fold in tomato roots and 1.4-fold in bean roots. No significant changes were observed in treated maize roots. Luminol chemiluminescence levels, a measure of free radicals, increased 3.8-fold in treated tomato roots and 2.1-fold in treated bean roots. Oxidative membrane damage in treated roots was measured by lipid peroxidation rates. In tomato we observed a 2.4-fold increase in peroxidation, however, no effect was observed in maize or beans.

Biochemical characterization of a lectin from Delonix regia seeds

A lectin from Delonix regia (DRL) seeds was purified by gel filtration on Sephadex G-100 followed by ion-exchange chromatography on diethylaminoethyl-Sepharose and reverse-phase high-performance liquid chromatography on a C18 column. Hemagglutinating activity was monitored using rat erythrocytes. DRL showed no specificity for human erythrocytes of ABO blood groups. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) revealed a single protein in the presence of 0.1 M of dithiothreitol (DTT) and in nonreducing conditions. Native-PAGE showed that DRL is a monomer with a molecular mass of about 12 kDa, as determined by denaturing gel electrophoresis and gel filtration chromatography. An amino acid composition revealed the absence of cysteine residues, the presence of 1 mol methionine/mol protein and a high proportion of acidic amino acids and glycine. The N-terminal sequence of DRL was determined by Edman degradation, and up to 16 amino acid residues showed more than 90% homology with other lectins from the Leguminosae family. The optimal pH range for lectin activity was between pH 8.0 and 9.0, and the lectin was active up to 60 degrees C. The lectin required Mn2+ for hemagglutinating activity and remained active after reduction with 0.1 M of DTT, but lost activity in the presence of 8 M of urea. Sodium metaperiodate had no effect on the activity of DRL.

Porcine pancreatic alpha-amylase shows binding activity toward N-linked oligosaccharides of glycoproteins

Porcine pancreatic alpha-amylase was shown by interaction analyses using a resonance mirror detector and alpha-amylase-immobilized Sepharose to bind with glycoproteins possessing N-glycans but not O-linked mucin-type glycans. Direct binding of three types of N-glycans to the alpha-amylase was demonstrated by surface plasmon resonance. Binding with biotin-polymer sugar probes revealed that the alpha-amylase has affinity to alpha-mannose, alpha-N-acetylneuraminic acid, and beta-N-acetyllactosamine, which are components of N-glycans. The binding of glycoproteins or carbohydrates enhanced the enzyme activity, indicating that the recognition site for N-glycans is different from its catalytic site. The binding activity was unique to porcine pancreatic alpha-amylase and was not observed for alpha-amylase from saliva, wheat, and fungus.

Plant alpha-amylase inhibitors and their interaction with insect alpha-amylases

Insect pests and pathogens (fungi, bacteria and viruses) are responsible for severe crop losses. Insects feed directly on the plant tissues, while the pathogens lead to damage or death of the plant. Plants have evolved a certain degree of resistance through the production of defence compounds, which may be aproteic, e.g. antibiotics, alkaloids, terpenes, cyanogenic glucosides or proteic, e.g. chitinases, beta-1,3-glucanases, lectins, arcelins, vicilins, systemins and enzyme inhibitors. The enzyme inhibitors impede digestion through their action on insect gut digestive alpha-amylases and proteinases, which play a key role in the digestion of plant starch and proteins. The natural defences of crop plants may be improved through the use of transgenic technology. Current research in the area focuses particularly on weevils as these are highly dependent on starch for their energy supply. Six different alpha-amylase inhibitor classes, lectin-like, knottin-like, cereal-type, Kunitz-like, gamma-purothionin-like and thaumatin-like could be used in pest control. These classes of inhibitors show remarkable structural variety leading to different modes of inhibition and different specificity profiles against diverse alpha-amylases. Specificity of inhibition is an important issue as the introduced inhibitor must not adversely affect the plant's own alpha-amylases, nor the nutritional value of the crop. Of particular interest are some bifunctional inhibitors with additional favourable properties, such as proteinase inhibitory activity or chitinase activity. The area has benefited from the recent determination of many structures of alpha-amylases, inhibitors and complexes. These structures highlight the remarkable variety in structural modes of alpha-amylase inhibition. The continuing discovery of new classes of alpha-amylase inhibitor ensures that exciting discoveries remain to be made. In this review, we summarize existing knowledge of insect alpha-amylases, plant alpha-amylase inhibitors and their interaction. Positive results recently obtained for transgenic plants and future prospects in the area are reviewed.

Induction of digestive alpha-amylases in larvae of Zabrotes subfasciatus (Coleoptera: Bruchidae) in response to ingestion of common bean alpha-amylase inhibitor 1

Zabrotes subfasciatus larvae possess three alpha-amylase isoforms determined by in gel assays following SDS-PAGE. Two minor isoforms present lower electrophoretic mobility than the major form. When developed inside Vigna unguiculata (cowpea) seeds, fourth instar larvae have minor quantities of the slow-migrating isoforms, but when reared on seeds of Phaseolus vulgaris (common bean), the two slow-migrating forms are expressed in higher amounts, whilst the quantity of the major constitutive form is independent of the host bean. Larvae at the beginning of the fourth instar were fed on flour or cotyledons of cowpea and common bean and it was observed that the larvae fed on the common bean expressed the two slow-migrating forms in higher amounts when compared to the control larvae fed on cowpea. In order to investigate the possible correlation between the induction of alpha-amylases and the ingestion of the common bean alpha-amylase inhibitor 1 (alphaAI-1), this inhibitor was incorporated into artificial diet. It was observed that larvae fed on diet containing chronic doses of alphaAI-1 during their development, produced the two slow-migrating forms in higher amounts than control larvae, however, fourth-instar larvae fed on the same diet presented less amylase activity than control larvae. The data suggested that alphaAI-1 is involved in amylase induction and that it has inhibitory activity against the constitutive amylase, when starch granules are used as substrate.

Purification and characterization of two alpha-amylase inhibitors from seeds of tepary bean (Phaseolus acutifolius A. Gray)

Two proteinaceous alpha-amylase inhibitors termed alphaAI-Pa1 and alphaAI-Pa2 were purified from seeds of a cultivated tepary bean (Phaseolus acutifolius A. Gray, cv. PI311897). The two inhibitors differed in their specificity towards alpha-amylases of insect pests such as bruchids, although neither showed any inhibitory activity against alpha-amylases of mammalian, bacterial or fungal origin. AlphaAI-Pa2 resembles two common bean inhibitors, alphaAI-1 and alphaAI-2, in several characteristics such as N-terminal amino acid sequences and oligomeric structure being composed of alpha and beta subunits. In contrast alphaAI-Pa1 is composed of a single glycopolypeptide with a molecular mass of 35 kDa, and its N-terminal amino acid sequence resembled that of seed lectins in tepary bean and common bean. The information on the two tepary bean alpha-amylase inhibitors may be useful not only for providing insight into critical structure for the specificity towards different alpha-amylase enzymes but also for enhancing insect resistance in crops.

Inhibition of growth of Aspergillus flavus and fungal alpha-amylases by a lectin-like protein from Lablab purpureus

Aspergillus flavus is a fungal pathogen of maize causing an important ear rot disease when plants are exposed to drought and heat stress. Associated with the disease is the production of aflatoxins, which are a series of structurally related mycotoxins known to be carcinogenic. Previous research has suggested that the alpha-amylase of A. flavus promotes aflatoxin production in the endosperm of infected maize kernels. We report here the isolation and characterization of a 36-kDa alpha-amylase inhibitor from Lablab purpureus (AILP). AILP inhibited the alpha-amylases from several fungi but had little effect on those from animal and plant sources. The protein inhibited conidial germination and hyphal growth of A. flavus. The amino acid sequence indicated that AILP is similar to lectin members of a lectin-arcelin-alpha-amylase inhibitor family described in common bean and shown to be a component of plant resistance to insect pests. AILP also agglutinated papain-treated red blood cells from human and rabbit. These data indicate that AILP represents a novel variant in the lectin-arcelin-alpha-amylase inhibitor family of proteins having lectin-like and alpha-amylase inhibitory activity.

One- and two-dimensional gel electrophoresic identification of African yam bean seed proteins

Seed proteins were extracted from the African yam bean (AYB; Sphenostylis stenocarpa), an underutilized West African food legume. One- and two-dimensional polyacrylamide gel electrophoresis was then used to analyze the albumin fraction, galactose-specific lectins purified on immobilized galactose-Sepharose 4B, and abundant non-lectin seed proteins left over following affinity chromatography. N-terminal sequencing of prominently resolved polypetide bands led to identification of proteins having sequence homology with characterized legume seed proteins, namely, mung bean seed albumin, pea alpha-fucosidase, soybean Kunitz-type trypsin inhibitor, an endochitinase, pea pathogenesis-related protein, and/or cowpea seed storage proteins. Minor lectin-like proteins lacking hemagglutinating activity against rabbit and human erythrocytes were also identified. Because proteins such as protease inhibitors, chitinases, pathogenesis-related proteins, and lectins are known to have antimetabolic effects, the findings from this study may have relevance in the acceptability, adoption, and utilization of AYB as human food.

Bean alpha-amylase inhibitor 1 in transgenic peas (Pisum sativum) provides complete protection from pea weevil (Bruchus pisorum) under field conditions

Two alpha-amylase inhibitors, called alphaAI-1 and alphaAI-2, that share 78% amino acid sequence identity and have a differential specificity toward mammalian and insect alpha-amylases are present in different accessions of the common bean (Phaseolus vulgaris). Using greenhouse-grown transgenic peas (Pisum sativum), we have shown previously that expression of alphaAI-1 in pea seeds can provide complete protection against the pea weevil (Bruchus pisorum). Here, we report that alphaAI-1 also protects peas from the weevil under field conditions. The high degree of protection is explained by our finding that alphaAI-1 inhibits pea bruchid alpha-amylase by 80% over a broad pH range (pH 4.5-6.5). alphaAI-2, on the other hand, is a much less effective inhibitor of pea bruchid alpha-amylase, inhibiting the enzyme by only 40%, and only in the pH 4.0-4.5 range. Nevertheless, this inhibitor was still partially effective in protecting field-grown transgenic peas against pea weevils. The primary effect of alphaAI-2 appeared to be a delay in the maturation of the larvae. This contrasts with the effect of alphaAI-1, which results in larval mortality at the first or second instar. These results are discussed in relationship to the use of amylase inhibitors with different specificities to bring about protection of crops from their insect pests or to decrease insect pest populations below the economic injury level.

Molecular characterization of a new arcelin-5 gene

Arcelins are insecticidal proteins found in some wild accessions of the common bean, Phaseolus vulgaris. They are grouped in six allelic variants and arcelin-5 is the variant with the highest inhibitory effect on the development of Zabrotes subfasciatus larvae. Characterization of the protein and its genes resulted in the identification of three polypeptides and the isolation of two genes that encode the Arc5a and Arc5b polypeptides. Here we describe a new gene, Arc5-III. The protein it encodes has 81% amino acid identity with the derived amino acid sequences of Arc5-I and Arc5-II. The Arc5-III gene is highly expressed in developing seeds and at a much lower level in roots. Data obtained by a combination of two-dimensional gel electrophoresis, protein sequencing and MALDI-TOF mass spectrometry analysis support the conclusion that Arc5-III encodes a polypeptide present in Arc5c band. Using ion-exchange chromatography, three fractions containing arcelin-5 polypeptides were eluted by increasing the salt concentration. The three fractions contain various amounts of the three arc-5 polypeptides and inhibit the growth of Zabrotes subfasciatus larvae differentially, suggesting differences in insecticidal activity among the arcelin-5 isoforms.