Sequence specificity of the post-translational proteolytic cleavage of vicilin, a seed storage protein of pea (Pisum sativum L.)

Characterization of mung bean endogenous proteases and globulins and their effects on the production of mung bean protein

Mung bean protein possesses several health benefits, and aqueous processing methods are used for its production. However, mung bean protein yields are different with different methods, which are actually different in conditions (e.g., pH, temperature, and time). Herein, liquid chromatography tandem mass spectrometry identified 28 endopeptidases and exopeptidases in mung bean protein extract, and the positions of 8S and 11S globulins on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) gel were confirmed in our experimental conditions. The SDS-PAGE, trichloroacetic acid-nitrogen solubility index, and free amino acid analysis revealed that (1) 8S globulins showed strong resistance to the endopeptidases (optimal at pH 5 and 50 °C) at pH 3-9, and 11S globulin exhibit strong resistance expect at pH 3-3.5; (2) the exopeptidases (optimal at pH 6 and 50 °C) preferred to liberate methionine and tryptophan. These proteases negatively affected protein yield, and short production time and low temperature were recommended.

Proteases catalyzing vicilin cleavage in developing pea (Pisum sativum L.) seeds

Legume species differ in whether or not the 7S globulins stored in seeds undergo proteolytic processing during seed development, while preserving the bicupin structure and trimeric assembly necessary for accumulation and packing into protein storage vacuoles. Two such cleavage sites have been documented for the vicilins in pea cotyledons: one in the linker region between the two cupin domains, and another in an exposed loop in the C-terminal cupin. In this report, we explain the occurrence of vicilin cleavage in developing pea by showing that the storage vacuoles are already acidified before germination, in contrast to soybean and peanut where acidification occurs only after germination. We also show that the two cleavage reactions are catalyzed by two different proteases. The vicilin cleavage at the linker region was inhibited by AEBSF (4-(2-aminoethyl)benzenesulfonyl fluoride), indicative of a serine protease. The cleavage in the C-terminal cupin domain was sensitive to the sulfhydryl-reactive reagents p-chloromercuriphenylsulfonate and iodoacetate, but not to E-64 (N-[N-(L-3-transcarboxyirane-2-carbonyl)-l-leucyl]-agmatine), characteristic of the legumain class of cysteine proteases. During seed development, we found the predominant vicilin cleavage in this pea cultivar (Knight) to be at the site in the second cupin domain; but after germination, both sites were cleaved at about the same rate.

Structure and function of seed storage proteins in faba bean (Vicia faba L.)

The protein subunit is the most important basic unit of protein, and its study can unravel the structure and function of seed storage proteins in faba bean. In this study, we identified six specific protein subunits in Faba bean (cv. Qinghai 13) combining liquid chromatography (LC), liquid chromatography-electronic spray ionization mass (LC-ESI-MS/MS) and bio-information technology. The results suggested a diversity of seed storage proteins in faba bean, and a total of 16 proteins (four GroEL molecular chaperones and 12 plant-specific proteins) were identified from 97-, 96-, 64-, 47-, 42-, and 38-kD-specific protein subunits in faba bean based on the peptide sequence. We also analyzed the composition and abundance of the amino acids, the physicochemical characteristics, secondary structure, three-dimensional structure, transmembrane domain, and possible subcellular localization of these identified proteins in faba bean seed, and finally predicted function and structure. The three-dimensional structures were generated based on homologous modeling, and the protein function was analyzed based on the annotation from the non-redundant protein database (NR database, NCBI) and function analysis of optimal modeling. The objective of this study was to identify the seed storage proteins in faba bean and confirm the structure and function of these proteins. Our results can be useful for the study of protein nutrition and achieve breeding goals for optimal protein quality in faba bean.

Proteolysis of the peanut allergen Ara h 1 by an endogenous aspartic protease

The 7S and 11S globulins of peanuts are subjected to proteolysis two days after seed imbibition, with Ara h 1 and the arachin acidic chains being among the first storage proteins to be mobilized. Proteolytic activity was greatest at pH 2.6-3 and is inhibited by pepstatin A, characteristic of an aspartic protease. This activity persists in seedling cotyledons up to at least 8 days after imbibition. In vitro proteolysis of Ara h 1 at pH 2.6 by extracts of cotyledons from seedlings harvested 24 h after seed imbibition generates newly appearing bands on SDS-PAGE. Partial sequences of Ara h 1 that were obtained through LC-MS/MS analysis of in-gel trypsin digests of those bands, combined with information on fragment size, suggest that proteolysis begins in the region that links the two cupin domains to produce two 33/34 kD fragments, each one encompassing an intact cupin domain. The later appearance of two 18 and 10/11 kD fragments can be explained by proteolysis within an exposed site in the cupin domains of each of the 33/34 kD fragments. The same or similar proteolytic activity was observed in developing seeds, but Ara h 1 remains intact through seed maturation. This is partly explained by the observation that acidification of the protein storage vacuoles, demonstrated by vacuolar accumulation of acridine orange that was dissipated by a membrane-permeable base, occurs only after germination. These findings suggest a method for use of the seed aspartic protease in reducing peanut allergy due to Ara h 1.

Legumes steam allergy in childhood: Update of the reported cases

In the past few decades, the prevalence of allergic diseases has deeply increased, with a key role played by food allergies. Legumes seem to play a major role towards the overall increase in the scenario of food allergy, since they are an appreciated source, consumed worldwide, due to their high protein content, variable amounts of lipids and for the presence of vitamins. In literature there are numerous descriptions of adverse reactions after ingestion of uncooked and cooked legumes. Nevertheless, cases of allergic reactions induced by inhaling vapours from cooking legumes have rarely been described. Herein the authors report an update of the literature data on allergic reactions caused by legume steam inhalation, underlying the possible pathogenic mechanism of these atopic events and the knowledge of literature data in paediatric age. The importance of this review is the focus on the clinical aspects concerning legume vapour allergy, referring to literature data in childhood.

Isolation and characterization of Korean pine (Pinus koraiensis) convicilin

A vicilin-like globulin seed storage protein, termed convicilin, was isolated for the first time from Korean pine (Pinus koraiensis). SDS-PAGE analysis revealed that Korean pine convicilin was post-translationally processed. The N-terminal peptide sequences of its components were determined. These peptides could be mapped to a protein translated from an embryo abundant transcript isolated in this study. Similar to vicilin, native convicilin appeared to be homotrimeric. Differential scanning calorimetry (DSC) analyses revealed that this protein is less resistant to thermal treatment than Korean pine vicilin. Its transition temperature was 75.57 °C compared with 84.13 °C for vicilin. The urea induced folding-unfolding equilibrium of pine convicilin monitored by intrinsic fluorescence could be interpreted in terms of a two-state model, with a Cm of 4.41 ± 0.15 M.

Urticaria and anaphylaxis in a child after inhalation of lentil vapours: a case report and literature review

BACKGROUND

Among legumes, lentils seem to be the most common legume implicated in pediatric allergic reactions in the Mediterranean area and India, and usually they start early in life, below 4 years of age.

CASE REPORT

A 22 -month-old child was admitted to our Pediatric Department for anaphylaxis and urticaria. At the age of 9 months she presented a first episode of angioedema and laryngeal obstruction, due to a second assumption of lentils in her diet. At admission we performed routine analyses that were all in the normal range, except for the dosage of specific IgE, that revealed a positive result for lentils. Prick tests too were positive for lentils, while they were all negative for other main food allergens. The child also performed a prick by prick that gave the same positive result (with a wheal of 8 mm of diameter). The child had not previously eaten lentils and other legumes, but her pathological anamnesis highlighted that the allergic reaction appeared soon after the inhalation of cooking lentil vapours when the child entered the kitchen Therefore a diagnosis of lentils vapours allergy was made.

CONCLUSIONS

Our case shows the peculiarity of a very early onset. In literature there are no data on episodes of anaphylaxis in so young children, considering that our child was already on lentils exclusion diet. Therefore a diet of exclusion does not absolutely preserve patients from allergic reactions, that can develop also after their cooking steams inhalation.

Identification in lupin seed of a serine-endopeptidase activity cleaving between twin arginine pairs and causing limited proteolysis of seed storage proteins

The occurrence of twin-arginine motifs (-R-R-) in the amino acid sequences of animal pro-proteins frequently defines the cleavage site(s) for their structural/functional maturation. No information is available on the presence and possible biological meaning of these motifs in the seed storage proteins. In this work, a novel endopeptidase activity with cleavage specificity to twin-arginine pairs has been detected in mature dry Lupinus albus seeds. The endopeptidase was tested with a number of endogenous and exogenous protein substrates, which were selected according to the presence of one or more twin-arginine residue motifs in their amino acid sequences. The observed hydrolysis patterns were limited and highly specific. Partial proteolysis led to stable polypeptide fragments that were characterized by 1- and 2-D electrophoresis. Selected polypeptides were submitted to N-terminal amino acid sequencing and mass spectrometry analyses. These approaches, supported by bioinformatic analysis of the available sequences, allowed the conclusion that the polypeptide cleavage events had occurred at the peptide bonds comprised between twin-arginine residue pairs with all tested protein substrates. The endopeptidase activity was inhibited by 4-(2-AminoEthyl)Benzene-Sulphonyl Fluoride hydrochloride (AEBSF), leupeptin, and serine proteinase protein inhibitors, while it was not affected by pepstatin, trans-Epoxysuccinyl-L-leucylamido(4-guanidino)butane (E64), and ethylenediaminetetraacetic acid (EDTA), thus qualifying the Arg-Arg cleaving enzyme as a serine endopeptidase. The structural features of storage proteins from lupin and other legume seeds strongly support the hypothesis that the occurrence of an endopeptidase activity cleaving -R-R- bonds may be functional to facilitate their degradation at germination and possibly generate polypeptide fragments with specific biological activity.

Seed storage proteins of the globulin family are cleaved post-translationally in wheat embryos

Background

The 7S globulins are plant seed storage proteins that have been associated with the development of a number of human diseases, including peanut allergy. Immune reactivity to the wheat seed storage protein globulin-3 (Glo-3) has been associated with the development of the autoimmune disease type 1 diabetes in diabetes-prone rats and mice, as well as in a subset of human patients.

Findings

The present study characterized native wheat Glo-3 in salt-soluble wheat seed protein extracts. Glo-3-like peptides were observed primarily in the wheat embryo. Glo-3-like proteins varied significantly in their molecular masses and isoelectric points, as determined by two dimensional electrophoresis and immunoblotting with anti-Glo-3A antibodies. Five major polypeptide spots were identified by mass spectrometry and N-terminal sequencing as belonging to the Glo-3 family.

Conclusions

These results in combination with our previous findings have allowed for the development of a hypothetical model of the post-translational events contributing to the wheat 7S globulin profile in mature wheat kernels.

Mobilization of seed protein reserves

The mobilization of seed storage proteins upon seed imbibition and germination is a crucial process in the establishment of the seedling. Storage proteins fold compactly, presenting only a few vulnerable regions for initial proteolytic digestion. Evolutionarily related storage proteins have similar three-dimensional structure, and thus tend to be initially cleaved at similar sites. The initial cleavage makes possible subsequent rapid and extensive breakdown catalyzed by endo- and exopeptidases. The proteolytic enzymes that degrade the storage proteins during mobilization identified so far are mostly cysteine proteases, but also include serine, aspartic and metalloproteases. Plants often ensure early initiation of storage protein mobilization by depositing active proteases during seed maturation, in the very compartments where storage proteins are sequestered. Various means are used in such cases to prevent proteolytic attack until after imbibition of the seed with water. This constraint, however, is not always enforced as the dry seeds of some plant species contain proteolytic intermediates as a result of limited proteolysis of some storage proteins. Besides addressing fundamental questions in plant protein metabolism, studies of the mobilization of storage proteins will point out proteolytic events to avoid in large-scale production of cloned products in seeds. Conversely, proteolytic enzymes may be applied toward reduction of food allergens, many of which are seed storage proteins.

Dissecting the proteome of pea mature seeds reveals the phenotypic plasticity of seed protein composition

Pea (Pisum sativum L.) is the most cultivated European pulse crop and the pea seeds mainly serve as a protein source for monogastric animals. Because the seed protein composition impacts on seed nutritional value, we aimed at identifying the determinants of its variability. This paper presents the first pea mature seed proteome reference map, which includes 156 identified proteins (http://www.inra.fr/legumbase/peaseedmap/). This map provides a fine dissection of the pea seed storage protein composition revealing a large diversity of storage proteins resulting both from gene diversity and post-translational processing. It gives new insights into the pea storage protein processing (especially 7S globulins) as a possible adaptation towards progressive mobilization of the proteins during germination. The nonstorage seed proteome revealed the presence of proteins involved in seed defense together with proteins preparing germination. The plasticity of the seed proteome was revealed for seeds produced in three successive years of cultivation, and 30% of the spots were affected by environmental variations. This work pinpoints seed proteins most affected by environment, highlighting new targets to stabilize storage protein composition that should be further analyzed.

Genetic variation in pea seed globulin composition

A quantitative characterization of seeds from 59 pea (Pisum sativum L.) lines and relative taxa with various external characteristics and wide geographical origin was performed to explore the genetic variation of pea concerning its starch and protein contents and globulin composition. Pea lines, which produce round, wrinkled, flat, and round-dimpled seeds, have starch as the major reserve, with an average content of 46%. Protein content varied from 13.7 to 30.7% of the seed dry matter, with an overall average of 22.3%. Densitometric quantification of the individual globulins (legumin, vicilin, convicilin, and globulin-related proteins) based on SDS-PAGE gels showed no lines lacking any particular globulin. Among the lines tested, variation was shown in both their total globulins content and their globulin composition. The total globulin content ranged from 49.2 to 81.8% of the total pea protein extract (TPPE). Legumin content varied between 5.9 and 24.5% of the TPPE. Vicilin was the most abundant protein of pea, and its content varied between 26.3 and 52.0% of the TPPE. Both processed and nonprocessed vicilins occurred. The processed vicilin was the predominant one, with values between 17.8 and 40.8%, whereas the nonprocessed ones constituted between 3.1 and 13.5% of the TPPE. Convicilin was the least abundant globulin, and its content ranged from 3.9 to 8.3%. Finally, the globulin-related proteins were present in amounts ranging from 2.8 to 17.3%. They were less abundant in comparison with legumin and vicilin, but they showed the largest relative variation of the four globulin classes. Correlations between the different external characteristics and globulin composition were determined. Comparison with soybean showed that pea lines show more variety in the abundance of globulin proteins, enabling a wider range of food application.

A multifunctional bicupin serves as precursor for a chromosomal protein of Pisum sativum seeds

The fact that the psp54 gene codes for p16, a seed chromatin protein of Pisum sativum, has been described previously. In the present paper it is shown that p54, the p16 precursor, also exists as a free polypeptide in pea and that it also yields p38, a second polypeptide from the N-terminal region of p54, which is co-localized at a subcellular level with p16. By using antibodies against pea p16 and p38, it was found that these proteins are present in the members of the tribe Viciae examined. Sequence analysis and 3D modelling indicates that p54 proteins belong to the cupin superfamily, and that they are related to sucrose binding proteins and, to a lesser extent, to vicilin-type seed storage proteins. Nevertheless, several distinctive characteristics of psp54 expression have been found: (i) the gene is differentially induced by ABA and several stress situations, in accordance with the presence of putative separate ABA and stress responsive elements in its promoter; (ii) the proteins are present in pods and seed coats, tissues of maternal origin; and (iii) p54 mRNA accumulates in the dry seeds. In view of both the functional properties of p54-derived proteins and the features of the psp54 gene expression, it is concluded that p54 represents a novel class within the cupin superfamily.

Characterization of pea vicilin. 2. Consequences of compositional heterogeneity on heat-induced gelation behavior

The gelling characteristics of two vicilin fractions from pea (Pisum sativum L.) were compared over a range of pH and salt conditions after preliminary results showed that despite having equal opportunity to unfold, and expose hydrophobic residues, they had different minimum gelling concentrations (at pH 7.6). Furthermore, at this pH one fraction formed turbid gels and the other formed transparent gels. The fraction that formed transparent gels contained a substantial amount of the 70 kDa alpha-subunits of vicilin, and thus it was hypothesized that the highly charged N-terminal extension region on these 70 kDa subunits hinders gelation of this vicilin fraction at pH 7.6 and I = 0.2 due to repulsion of the net negative charge. The experiments designed to test this hypothesis are presented and discussed in this paper and prove that the hypothesis was true, which offers the possibility to control or modify the gelation behavior of vicilin on the basis of information of its subunit composition.

Characterization of pea vicilin. 1. Denoting convicilin as the alpha-subunit of the Pisum vicilin family

Vicilin, a major globulin protein of pea that has been described as "extremely heterogeneous in terms of its polypeptide composition", was extracted from pea flour under alkaline conditions and subsequently fractionated by salt under acid conditions. This procedure induced the separation of vicilin into two fractions, which, after purification, were called vicilin 1 degrees and vicilin 2 degrees. Vicilin 2 degrees was seen on SDS-PAGE to contain the third globulin protein of pea, convicilin (a band at approximately 70 kDa). Vicilin fractions were thus characterized using gel electrophoresis, differential scanning calorimetry, circular dichroism, and pH-dependent solubility in order to determine whether the convicilin should in fact be considered as a third separate globulin protein of pea. On the basis of the results obtained it was concluded that this distinct polypeptide of the Pisum vicilin gene family should be further denoted as a subunit of the salt extractable protein vicilin. The definition of vicilin heterogeneity should therefore be extended to acknowledge the possible oligomeric inclusion of the 70 kDa polypeptide that is here denoted as the alpha-subunit.

Functional properties of purified vicilins from cowpea (Vigna unguiculata) and pea (Pisum sativum) and cowpea protein isolate

The major storage globulins (vicilins) of cowpea (Vigna unguiculata) and pea (Pisum sativum) seeds were purified by ammonium sulfate precipitation, and a semipurified cowpea protein isolate (CPI) was prepared by isoelectric precipitation. Some of the functional properties of these proteins, including solubility, foaming, and emulsifying capacities, were investigated and compared. The solubility of purified cowpea vicilin was reduced at pH 5.0, increasing markedly below and above this value. Pea vicilin exhibited poor solubility between pH 5.0 and pH 6.0, and CPI was little soluble in the pH range from 4.0 to 6.0. At neutral pH, the emulsifying activity indexes (EAI) of purified pea vicilin and CPI were 194 and 291 m(2)/g, respectively, which compare quite favorably to EAIs of 110 and 133 m(2)/g for casein and albumin, respectively. Remarkably, purified cowpea vicilin exhibited an EAI of 490 m(2)/g, indicating a very high emulsifying activity. Purified cowpea and pea vicilins exhibited lower foaming capacities and foam stablity indexes (FSI) than CPI. FSI values of 80 and 260 min were obtained for purified pea and cowpea vicilin, respectively, whereas a FSI value of 380 min was obtained for CPI. These results are discussed in terms of the possible utilization of purified vicilins or protein isolates from pea and cowpea in the food processing industry.

Evidence that the N-terminal extension of the Vicieae convicilin genes evolved by intragenic duplications and trinucleotide expansions

This study was aimed to identify lentil (Lens culinaris subsp. culinaris) convicilin genes and to carry out a comparative analysis of these genes in the tribe Vicieae. Convicilins differ from vicilins, a related group of plant seed storage proteins, mainly by the presence of an additional sequence of amino acids in the sequence corresponding to the first exon, referred as the N-terminal extension. A single gene for convicilin, a component of legume seed storage proteins, was identified in the cultivated lentil. In this species, the N-terminal extension is formed by a stretch of 126 amino acids of which 59.2% are charged amino acids: 29.6% glutamic acid, 3.2% aspartic acid, 14.4% arginine, 8.8% lysine, and 3.2% histidine. This lentil convicilin sequence is similar to the sequence of convicilins in other species of the tribe Vicieae. However, the size of the N-terminal extension clearly differs among convicilins. Sequence comparison and phylogenetic analyses including convicilin and vicilin of Vicieae species indicated that the differentiation between vicilins and convicilins predated the differentiation of the two vicilin gene families (47- and 50-kDa vicilins), and that the N-terminal extension evolved mainly by a series of duplications of short internal sequences and triplet expansions, the predominant one being GAA.Key words: convicilin, evolution by duplications, Lens culinaris Medik., lentil, legumes, trinucleotide expansion.

Mungbean [Vigna radiata (L.) Wilczek] globulins: purification and characterization

Vicilin type (8S) and basic 7S globulins and legumin type (11S) globulins were isolated from mungbean [Vigna radiata (L.) Wilczek]. The native molecular weights of the different globulin types were 360000 for legumin, 200000 for vicilin, and 135000 for basic 7S. Some of the 8S globulin apparently complexed and coeluted with the 11S on gel filtration. On SDS-PAGE, 11S was composed of two bands of 40000 and 24000, 8S was composed of 60000, 48000, 32000, and 26000 bands, and basic 7S was composed of 28000 and 16000 bands. The percent composition of total globulins was estimated to be as follow: 8S, 89%; basic 7S, 3.4%; and 11S, 7.6%. The basic 7S and 11S but not the 8S globulins were found to have disulfide bonds. The presence of carbohydrates by conjugated peroxidase reaction was observed in all bands of 8S, the acidic polypeptide of basic 7S, and its complex but not in 11S. The 28000 basic 7S band and its 42000 complex and the first three major bands of 8S cross-reacted with antibodies to all types of soybean conglycinin subunits (alpha, alpha', and beta), whereas the fourth band cross-reacted only with the anti-beta subunit. None of the mungbean globulins cross-reacted with anti-soybean glycinin. Basic 7S was found to be easily extracted with 0.15 M NaCl, 11S was extracted with 0.35 M NaCl,and 8S was extracted over a wide range of NaCl concentrations. The N-terminal sequences of the different subunits/fragments of the globulins were determined and found to have strong homology with storage proteins of other legumes and crops.

Isolation and characterization of relevant allergens from boiled lentils

BACKGROUND

Lentils seem to be the most common legume implicated in pediatric allergic patients in the Mediterranean area. However, no lentil allergen has been isolated and characterized.

OBJECTIVE

We sought to purify and characterize relevant IgE-binding proteins from boiled lentil extracts.

METHODS

IgE-binding proteins from crude and boiled lentil extracts were detected with a pool of sera from patients with lentil allergy. Allergens were isolated by gel-filtration chromatography followed by cation- and anion-exchange chromatography or by reverse-phase HPLC. Their characterization included N-terminal amino acid sequencing, complex asparagine-linked glycan detection, specific IgE immunodetection with 22 individual sera from allergic patients, and immunoblot and CAP inhibition assays.

RESULTS

Heat treatment of lentils produced substantial changes in the SDS-PAGE patterns of whole extracts, mainly a strong increase of 12- to 16-kd bands and a decrease of 25- to 45-kd components. Major IgE-binding proteins from the boiled lentil extract were located in the 12- to 16-kd and 45- to 70-kd ranges. Two allergens of 16 kd, proteins L1 and L2, and another one of 12 kd, protein L3, were purified. N-terminal sequencing indicated that all 3 were related and allowed their identification as gamma-vicilin subunits. Protein L1 was recognized by 68% of the individual sera tested and inhibited 64% of the IgE binding by commercial lentil CAPs. A second type of allergen of 66 kd, named protein H, was also isolated and identified as a seed-specific biotinylated protein. Protein H reacted with 41% of the individual sera and produced 45% inhibition in CAP inhibition assays.

CONCLUSIONS

Two different types of allergens have been identified in boiled lentils. Those of 12 to 16 kd, called Len c 1, correspond to gamma-vicilin subunits, and those of 66 kd, designated Len c 2, correspond to seed-specific biotinylated protein. Homology with proteins from other legume species can explain potential cross-reactions among these foods.

A pea nuclear protein that is induced by dehydration belongs to the vicilin superfamily

The purification to homogeneity of p16, a protein with an electrophoretic mobility compatible with an apparent molecular mass of 16 kDa, from nuclei of ungerminated pea embryonic axes is described. A cDNA clone of its gene, which was designated psp54, was also isolated. The psp54 cDNA contains an open reading frame coding for a 54.4-kDa polypeptide (p54). p16 corresponds to the C-terminal third of p54, although the mechanisms by which the primary polypeptide could be processed are not yet known. The sequence of p54 is 60% identical with that of the precursor of a sucrose-binding soybean protein, and, to a lesser extent (31-34%), it shares homology with some storage proteins. p16 is also 30% homologous with Nhp2p, a yeast nuclear protein. The psp54 gene, present in a single copy in pea genome, starts being expressed during seed desiccation. Soon after rehydration in seed germination, p54 mRNA disappears and is no longer detectable in vegetative tissues, except in response to hydric stress (exposure to abscisic acid, osmolites or desiccation). p16 can be recovered from nuclei cross-linked to histone H3, when the disulfide bridges that occur in vivo are preserved. On the other hand, p16 shares some properties with dehydrins, which are thought to protect cellular structures against desiccation. We propose that the possible precursor polypeptide p54 belongs to the vicilin superfamily, members of which play a variety of roles. The function of p16 may be related to the protection of chromatin structure against desiccation during seed development.

A family of antimicrobial peptides is produced by processing of a 7S globulin protein in Macadamia integrifolia kernels

A new family of antimicrobial peptides has been discovered in Macadamia integrifolia. The first member of this new family to be purified from nut kernels was a peptide of 45 aa residues, termed MiAMP2c. This peptide inhibited various plant pathogenic fungi in vitro. cDNA clones corresponding to MiAMP2c encoded a 666 aa precursor protein homologous to vicilin 7S globulin proteins. The deduced precursor protein sequence contained a putative hydrophobic N-terminal signal sequence (28 aa), an extremely hydrophilic N-proximal region (212 aa), and a C-terminal region of 426 aa which is represented in all vicilins. The hydrophilic portion of the deduced protein contained the sequence for MiAMP2c as well as three additional segments having the same cysteine spacing pattern as MiAMP2c. Each member of the MiAMP2 family (i.e. MiAMP2a, b, c and d) consisted of approximately 50 amino acids and contained a C-X-X-X-C-(10-12)X-C-X-X-X-C motif. Subsequent isolations from seed exudates led to the purification of the predicted family members MiAMP2b and 2d, both of which also exhibited antimicrobial activity in vitro. These results suggest that some vicilins play a role in defence during seed germination.

Deposition of storage proteins

Plants store amino acids for longer periods in the form of specific storage proteins. These are deposited in seeds, in root and shoot tubers, in the wood and bark parenchyma of trees and in other vegetative organs. Storage proteins are protected against uncontrolled premature degradation by several mechanisms. The major one is to deposit the storage proteins into specialized membrane-bounded storage organelles, called protein bodies (PB). In the endosperm cells of maize and rice prolamins are sequestered into PBs which are derived from the endoplasmic reticulum (ER). Globulins, the typical storage proteins of dicotyledonous plants, and prolamins of some cereals are transported from the ER through the Golgi apparatus and then into protein storage vacuoles (PSV) which later become transformed into PBs. Sorting and targeting of storage proteins begins during their biosynthesis on membrane-bound polysomes where an N-terminal signal peptide mediates their segregation into the lumen of the ER. After cleavage of the signal peptide, the polypeptides are glycosylated and folded with the aid of chaperones. While still in the ER, disulfide bridges are formed which stabilize the structure and several polypeptides are joined to form an oligomer which has the proper conformation to be either deposited in ER-derived PB or to be further transferred to the PSV. At the trans-Golgi cisternae transport vesicles are sequestered which carry the storage proteins to the PSV. Several storage proteins are also processed after arriving in the PSVs in order to generate a conformation that is capable of final deposition. Some storage protein precursors have short N- or C-terminal targeting sequences which are detached after arrival in the PSV. Others have been shown to have internal sequence regions which could act as targeting information. In some cases positive targeting information is known to mediate sorting into the PSV whereas in other cases aggregation and membrane association seem to be major sorting mechanisms.

Deposition of storage proteins

Plants store amino acids for longer periods in the form of specific storage proteins. These are deposited in seeds, in root and shoot tubers, in the wood and bark parenchyma of trees and in other vegetative organs. Storage proteins are protected against uncontrolled premature degradation by several mechanisms. The major one is to deposit the storage proteins into specialized membrane-bounded storage organelles, called protein bodies (PB). In the endosperm cells of maize and rice prolamins are sequestered into PBs which are derived from the endoplasmic reticulum (ER). Globulins, the typical storage proteins of dicotyledonous plants, and prolamins of some cereals are transported from the ER through the Golgi apparatus and then into protein storage vacuoles (PSV) which later become transformed into PBs. Sorting and targeting of storage proteins begins during their biosynthesis on membrane-bound polysomes where an N-terminal signal peptide mediates their segregation into the lumen of the ER. After cleavage of the signal peptide, the polypeptides are glycosylated and folded with the aid of chaperones. While still in the ER, disulfide bridges are formed which stabilize the structure and several polypeptides are joined to form an oligomer which has the proper conformation to be either deposited in ER-derived PB or to be further transferred to the PSV. At the trans-Golgi cisternae transport vesicles are sequestered which carry the storage proteins to the PSV. Several storage proteins are also processed after arriving in the PSVs in order to generate a conformation that is capable of final deposition. Some storage protein precursors have short N- or C-terminal targeting sequences which are detached after arrival in the PSV. Others have been shown to have internal sequence regions which could act as targeting information. In some cases positive targeting information is known to mediate sorting into the PSV whereas in other cases aggregation and membrane association seem to be major sorting mechanisms.

Cloning of pea storage protein genes

Vicilin and legumin are the major storage proteins of Pisum sativum . Complementary DNAs (cDNAs) have been produced from poly(A) + mRNA isolated from developing seeds and specific storage protein cDNAs cloned into pBR322. The amino acid sequences predicted from the cDNA sequences have been compared with the actual amino acid sequences derived from the purified protein subunits. These comparisons have confirmed that the legumin α and β subunits as initially synthesized are covalently joined together and that a small peptide is subsequently removed by endoproteolysis to give the disulphide linked subunits of the mature seed legumins. Similar comparisons between the predicted amino acid sequence of vicilin cDNA clones and the amino acid sequence determined on the isolated subunits has shown that some of the 50000 M r type subunits are subsequently cleaved to give three subunits as products, i.e. polypeptides of 19000 M r (α), 13500 M r (β) and 12500 M r or 16000 M r if glycosylated (γ). In addition to these three subunits, cleavage at one or other of the two potential cleavage sites, results in a 33000 M r polypeptide (α + β) and a 31000 M r polypeptide tentatively identified as β + γ. The presence of the sequence Lys-Glu-Asn leads to cleavage on the carboxy side of Asn at the β :γ cleavage site whereas the sequence Gly-Leu-Arg does not lead to cleavage. Comparable sequence data for the α: β processing site do not exist. Comparisons of the cDNA and amino acid sequence disclose the presence of a 15 or 16 amino acid residue vicilin leader sequence as well as a 12 amino acid residue C-terminal peptide which is also removed. The codon usage of the messenger RNAs for the storage proteins are similar to those of other plant proteins and differ somewhat from animal messenger RNAs. Complementary DNAs for specific storage proteins when used to probe different restriction enzyme digests of pea genomic DNA reveal the presence of a small number of legumin and vicilin coding sequences (two to five for legumin and three to seven for vicilin) that occur as single copies except for one vicilin sequence present in two to three copies. Genetic mapping experiments using whole plants locate both the main legumin and the vicilin genes on chromosome 7. The main legumin subunits are coded by genes located at a single Mendelian locus Lg-1 located on the short arm of chromosome 7 very close to the rub locus and the vicilin gene is located 16 map units away close to the r locus. Gene libraries prepared with size fractionated partial restriction enzymic digests of pea genomic DNA ligated into both phage λ L47 and phage λ gt wes have led to the isolation of at least three similar but different legumin genomic sequences. Comparison of the λ and cDNA legumin clones suggests the presence of at least one intron in the former. Legumes in general contain two major seed storage protein types, vicilin and legumin (Derbyshire et al. 1976). Seeds of Pisum sativum (L) have significant amounts of both proteins and since a considerable body of knowledge exists about pea physiology and genetics, this species is a good choice for the study of storage protein genes. Peas are also one of the world’s major legume crops. Since the storage proteins are found only in the tissues of the developing seed (Millerd 1975) and then only in significant amounts during the middle and late stages of development, it was suspected from the onset that the genes responsible for the storage protein would belong to the class of developmentally regulated genes, i.e. those that are only switched on in specific tissues over restricted periods of time.

The genetics of legume storage proteins

The seed storage proteins of Pisum (pea) and Phaseolus vulgaris (French bean) exhibit genetic variation for polypeptide structure; genetical studies indicate that most of the major storage protein genes exhibit simple, codominant Mendelian inheritance. Biochemical analysis of the storage protein polypeptides and their messenger RNAs shows that the allelic alternatives are probably small families of closely linked structural genes. Two of these genes - those for the major legumin gene family and for convicilin, both from Pisum sativum - have been assigned to specific sites on linkage groups. Genes affecting the synthesis of legumin in Pisum sativum and of phaseolin in Phaseolus vulgaris have been identified.

Limited proteolysis of beta-conglycinin and glycinin, the 7S and 11S storage globulins from soybean [Glycine max (L.) Merr.]. Structural and evolutionary implications

The G2 (A2B1a) glycinin subunit from soybean (Glycine max L. Merr.) was purified and renatured to the homohexameric holoprotein. This protein along with purified beta-conglycinin were subjected to limited proteolysis by trypsin. The generated polypeptide fragments were separated via SDS/PAGE and the amino acid sequence of the N-terminals was determined. Four cleavage points were detected in the alpha-chain A2 of glycinin as well as in the alpha'-chain of beta-conglycinin. From the known three-dimensional structure of 7S globulin and the hypothetical model of 7S globulin-like 11S globulin structure, it was possible to draw the conclusion that two distinct types of susceptible sites for proteolytic cleavage are characteristic of the subunits of both globulins. The first includes the sequences linking N- and C-terminal domains of both globulins and the sequence of N-terminal extensions of 70-kDa subunits from the vicilin-like 7S globulins. The second type includes the loop between beta-strands E and F of the N-terminal domain of 11S globulins and of the C-terminal domain of 7S globulins. A statistically significant similarity was found between the N-terminal extension of the alpha'-chain of beta-conglycinin and the interdomain linker regions of soybean glycinin and pea legumin. It is proposed that the three sequence regions which form the first type of susceptible sites are of similar structural function and might have evolved from the N-terminal segment of a putative single-domain ancestor.

Biochemical genetics of a 7S globulin-like protein from Pinus pinaster seed

The megagametophytes of seeds of Pinus pinaster Ait. contain two types of oligomeric globulins of approximately 175 and 190 kDa that are comprised of 47-kDa and 27- and 22-kDa, monomers, respectively, joined by weak interactions. The 27- and 22-kDa components were purified and their N-terminal sequences determined. Both polypeptides were inherited as if they were coded by a single unit of recombination. The results obtained suggest that these two polypeptides originate from a single protein that undergoes proteolytic processing. The characteristics of this P. pinaster globulin indicate that it is a member of the 7S globulin family of seed storage proteins.

Characterization of a soybean beta-conglycinin-degrading protease cleavage site

Protease C1, an enzyme from soybean (Glycine max [L.] Merrill cv Amsoy 71) seedling cotyledons, was previously determined to be the enzyme responsible for the initial degradation of the alpha' and alpha subunits, but not the beta subunit, of beta-conglycinin storage protein. The sizes of the proteolytic products generated by the action of protease C1 suggest that the cleavage sites on the alpha' and alpha subunits of beta-conglycinin may be located in their N-terminal domain, which is not found in the beta subunit of beta-conglycinin. To check this hypothesis, storage proteins from other plant species that are homologous to either the alpha'/alpha or the beta subunit of beta-conglycinin were tested as substrates. As expected, the convicilin from pea (Pisum sativum), a protein homologous to the alpha' and alpha subunits of beta-conglycinin, was digested by protease C1. The vicilins from pea as well as vicilins from adzuki bean (Vigna angularis), garden bean (Phaseolus vulgaris), black-eyed pea (Vigna unguiculata), and mung bean (Vigna radiata), storage proteins that are homologous to the beta subunit of soybean beta-conglycinin, were not degraded by protease C1. Degradation of soybean beta-conglycinin involves a sequential attack of the alpha subunit at multiple sites, culminating in the formation of a stable intermediate of 53.5 kD and a final product of 48.0 kD. The cleavage sites resulting in this formation of the intermediates and final product were determined by N-terminal analysis. These were compared to the known amino acid sequences of the three beta-conglycinin subunits. Results showed these two polypeptides to be generated by proteolysis of the alpha subunit at regions bearing long strings of acidic amino acid residues.

Characterization of the maize Globulin-2 gene and analysis of two null alleles

The most abundant proteins present in maize (Zea mays L.) embryos are saline-soluble globulins. A Mr 45,000 globulin component, designated GLB2, is encoded by the Glb2 gene. A cDNA clone corresponding to Glb2 was used as radiolabeled probe to examine the expression of Glb2 in developing embryos and other maize tissues. Glb2 transcripts accumulate during embryo development and are not detectable in germinating kernels. Glb2 transcripts are found only in the developing embryo, and not in endosperm, seedling, or unfertilized ears. Analysis of globulin profiles in embryos homozygous for either a previously described null allele, Glb-2-0, or a novel null allele, Glb2-N1, revealed that these embryos lack not only the GLB2 protein but also globulins of lower molecular mass which may represent processed forms of GLB2. Southern blot analysis of DNA from Glb2-0/0 and Glb2-N1/N1 plants in which a Glb2-specific clone is used as probe indicates that the two null alleles are genetically distinct.

Another class of vicilin gene in Pisum

We describe a novel class of vicilin gene in Pisum corresponding to a precursor polypeptide of Mr 68000. The mRNA corresponding to this polypeptide accumulates during seed development in a pattern similar to that of convicilin. Hybridization and sequence analyses show that this vicilin gene class is less homologous to other Pisum vicilin gene classes than these last are to each other. Gene-copy-number estimates and decreased stringency hybridizations of the gene class described here show that the Pisum vicilin gene family is bigger and more complex than hitherto reported.

Pea convicilin: structure and primary sequence of the protein and expression of a gene in the seeds of transgenic tobacco

Convicilin, a trimeric globulin of pea (Pisum sativum L.) seeds, is closely related to vicilin and composed of polypeptides of 68.2 kilodaltons. A partial copy DNA (cDNA) clone encoding convicilin was isolated, sequenced, and used to select a convicilin gene from a pea genomic library. A part of the genomic clone was sequenced to obtain the coding sequences missing in the cDNA clone and a further 1 kilobase 5' to the start of transcription were also obtained. The entire sequence of convicilin was deduced from the combined genomic and cDNA sequences. The complete gene encoding convicilin was transferred to tobacco (Nicotiana tabacum L.) and the characteristics of its expression in the seeds of transgenic plants were studied. An unprocessed polypeptide, which was found only in the seeds of the transgenic plants, was identical in size to pea convicilin, and was recognized by vicilin antibodies. Convicilin, which does not undergo posttranslational cleavage in peas, was partially processed to polypeptides of a relative molecular mass (Mr) of approx. 50000 in transgenic tobacco seeds. There was a twofold variation in the level of convicilin accumulated by the mature seeds of a number of transgenic plants and this was well correlated with the number of gene copies incorporated in the different transformants. In the seeds of tobacco plants that contained a single copy of the transferred gene it was estimated that convicilin comprised up to 2% of the seed protein. Thus, using a combination of gene sequencing and expression in a heterologous host we believe we have characterized the gene corresponding to theCvc locus, whereas the gene described by D. Bown et al. (1988, Biochem J.,251, 717-726) probably encodes a minor convicilin-related protein.

Isolation and expression of a pea vicilin cDNA in the yeast Saccharomyces cerevisiae

A cDNA clone containing the complete coding sequence for vicilin from pea (Pisum sativum L.) was isolated. It specifies a 50,000-Mr protein that in pea is neither post-translationally processed nor glycosylated. The cDNA clone was expressed in yeast from a 2 micron plasmid by using the yeast phosphoglycerate kinase promoter and initiator codon. The resultant fusion protein, which contains the first 16 amino acid residues of phosphoglycerate kinase in addition to the vicilin sequence, was purified and subsequently characterized. It has slightly slower mobility on SDS/polyacrylamide-gel electrophoresis than standard pea vicilin and forms a mixture of multimers, some of which resemble the native protein.