Lectin and lectin-related proteins in lima bean (Phaseolus lunatus L.) seeds: biochemical and evolutionary studies

Partial Characterization of Lectins Purified from the Surco and Vara (Furrow and Rod) Varieties of Black Phaseolus vulgaris

As they manifest specifically and reversibly, lectins are proteins or glycoproteins with the characteristic of agglutinating erythrocytes. Given that grain legume lectins can represent 10% of protein content and can have various biological functions, they are extensively studied. The objective of this work was to purify and partially characterize the lectins of Phaseolus vulgaris black, var surco and vara (LBBS and LBBV). Both lectin types were purified by affinity chromatography on stroma matrix, which agglutinated human erythrocytes type A, B, and O, as well as rabbit, hamster, pig, and chicken erythrocytes. Native-PAGE was employed for molecular mass determination, yielding 109.36 and 112.68 kDa for BBS and BBV, respectively. Further analyses revealed that these lectins are tetrameric glycoproteins that require Ca⁺², Mn⁺² and Mg⁺² ions for exhibiting their hemagglutinating function, which can be inhibited by fetuin. Moreover, optimal pH was established for both lectins (10.5 for LBBS and 7-9 for LBBV), while their activity was temperature-dependent and ceased above 70 °C. Finally, the observed differences in the biochemical characteristics and bioactive functions were ascribed to the different physiological characteristics of each seed, as well as the protein itself.

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.

Legume Lectins: Proteins with Diverse Applications

Lectins are a diverse class of proteins distributed extensively in nature. Among these proteins; legume lectins display a variety of interesting features including antimicrobial; insecticidal and antitumor activities. Because lectins recognize and bind to specific glycoconjugates present on the surface of cells and intracellular structures; they can serve as potential target molecules for developing practical applications in the fields of food; agriculture; health and pharmaceutical research. This review presents the current knowledge of the main structural characteristics of legume lectins and the relationship of structure to the exhibited specificities; provides an overview of their particular antimicrobial; insecticidal and antitumor biological activities and describes possible applications based on the pattern of recognized glyco-targets.

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.

'Fagiolo a Formella', an Italian lima bean ecotype: biochemical and nutritional characterisation of dry and processed seeds

BACKGROUND

An ecotype of the lima bean, named 'fagiolo a Formella', which, to the best of our knowledge, is the only example of an Italian lima bean (Phaseolus lunatus L.) ecotype, is cultivated in the Campania region of southern Italy. Physical, nutritional and processing traits of dry seeds were evaluated for two consecutive growing seasons (2009 and 2010). The canning quality was also investigated, but only for the harvest of 2010.

RESULTS

Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of total seed proteins allowed the attribution of 'fagiolo a Formella' to the Mesoamerican gene pool and Sieva morphotype. Seeds have a trapezoid shape, white coat and 100-seed weight greater than 42 g. Yield, protein, trypsin inhibitor and phytic acid values were found comparable with those reported for lima bean varieties cultivated in sub-tropical areas. Moreover, we found that this ecotype is devoid of lectin.

CONCLUSIONS

The good adaptation to growing environment is indicated by the fact that 'fagiolo a Formella' seed quality is comparable to that of lima beans grown in America. Overall the canning quality was found satisfactory and canning significantly destroys the main anti-nutritional compounds present in dry seeds.

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.

The nutraceutical role of thePhaseolus vulgarisα-amylase inhibitor

The present review assesses the potential of thePhaseolus vulgarisα-amylase inhibitor isoform 1 (α-AI1) starch blockers as a widely used remedy against obesity and diabetes. Consumption of the α-amylase inhibitor causes marginal intraluminal α-amylase activity facilitated by the inhibitor's appropriate structural, physico-chemical and functional properties. As a result there is decreased postprandial plasma hyperglycaemia and insulin levels, increased resistance of starch to digestion and increased activity of colorectal bacteria. The efficacy and safety of the amylase inhibitor extracts, however, depend on the processing and extraction techniques used. The extracts are potential ingredients in foods for increased carbohydrate tolerance in diabetics, decreased energy intake for reducing obesity and for increased resistant starch. Research developments in the distribution and biosynthesis of the α-amylase inhibitor, relevant physico-chemical properties, the molecular starch-blocking mechanism, anti-obesity and anti-diabetes effects, safety of extracts and the need for research into their potential anti-colorectal cancer effect are 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.

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.

Lunatusin, a trypsin-stable antimicrobial peptide from lima beans (Phaseolus lunatus L.)

An anti-fungal peptide designated as lunatusin, with a molecular mass around 7kDa, was purified from the seeds of Chinese lima bean (Phaseolus lunatus L.). The peptide was isolated using a simple protocol consisting of affinity chromatography on Affi-gel blue gel and gel filtration on Superdex 75. Lunatusin exerted an anti-fungal activity toward fungal species such as Fusarium oxysporum, Mycosphaerella arachidicola and Botrytis cinerea, and an antibacterial action on, Bacillus megaterium, Bacillus subtilis, Proteus vulgaris and Mycobacterium phlei. It also inhibited proliferation in the breast cancer cell line MCF-7. Lunatusin reduced the activity of HIV-1 reverse transcriptase and it also inhibited translation in a cell-free rabbit reticulocyte lysate system. Its anti-fungal activity was retained after incubation with trypsin. Lunatusin elicited a mitogenic response from mouse splenocytes.

Purification and characterization of complex carbohydrate specific isolectins from wild legume seeds: Acacia constricta is (vinorama) highly homologous to Phaseolus vulgaris lectins

Vinorama isolectins (VL2-VL4) were purified from seeds of Acacia constricta (vinorama) using affinity chromatography on a fetuin-fractogel column followed by cationic-exchange chromatography. Each isolectin fraction presented a characteristic isoelectric point range from 5.5 to 8.4. Under native conditions, VL containing fractions migrated as tetramers of 133 kDa, while in SDS-PAGE, in presence of 2-mercaptoethanol, a single subunit band with M(r) of 34 kDa was observed. VL was found to be a glycoprotein with a 7.5% neutral sugar content. Antibodies to Phaseolus vulgaris lectins PHA and other wild legume lectins as Olneya tesota (palo fierro) PF2 and PF3, and Parkinsonia aculeate (palo verde) PV reacted with VL, but not with anti Glycine max agglutinin SBA or anti Lotus tetragonolobus agglutinin LTA. Furthermore, direct analysis of VL peptides showed sequences homologous to those reported in different lectins of the Phaseolus genus. VL2-VL4 did not have ABO serological or simple sugar specificity, but were inhibited by complex carbohydrates from fetuin and thyroglobulin. Asialofetuin carbohydrates strongly interacted with VL4 and VL3. Vinorama isolectins could be classified as "complex lectins".

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.